Download Impact of Ionizing Radiation on 4H-SiC Devices

Impact of Ionizing Radiation on
4H-SiC Devices
Doctoral Thesis
by
Muhammad Usman
Microelectronics and Applied Physics
School of Information and Communication Technology (ICT)
KTH Royal Institute of Technology
Stockholm, Sweden 2012
Impact of Ionizing Radiation on 4H-SiC Devices
A dissertation submitted to KTH Royal Institute of Technology, Stockholm, Sweden, in
partial fulfillment of the requirements for the degree of Teknologie Doktor (Doctor of
Philosophy)
©2012 Muhammad Usman
KTH Royal Institute of Technology,
School of Information and Communication Technology (ICT),
Electrum 229,
Kista, SE-16440
Sweden
TRITA-ICT/MAP AVH Report 2012:02
ISSN 1653-7610
ISRN KTH/ICT-MAP/AVH-2012:02-SE
ISBN 978-91-7501-225-4
To my family
Muhammad Usman: Impact of Ionizing Radiation on 4H-SiC Devices, Microelectronics and
Applied Physics, School of Information and Communication Technology (ICT), KTH Royal
Institute of Technology, Stockholm 2012.
TRITA-ICT/MAP AVH Report 2012:02, ISSN 1653-7610, ISRN KTH/ICT-MAP/AVH2012:02-SE, ISBN 978-91-7501-225-4
Abstract
Electronic components, based on current semiconductor technologies and operating in
radiation rich environments, suffer degradation of their performance as a result of
radiation exposure. Silicon carbide (SiC) provides an alternate solution as a radiation
hard material, because of its wide bandgap and higher atomic displacement energies,
for devices intended for radiation environment applications. However, the radiation
tolerance and reliability of SiC-based devices needs to be understood by testing devices
under controlled radiation environments. These kinds of studies have been previously
performed on diodes and MESFETs, but multilayer devices such as bipolar junction
transistors (BJT) have not yet been studied.
In this thesis, SiC material, BJTs fabricated from SiC, and various dielectrics for
SiC passivation are studied by exposure to high energy ion beams with selected
energies and fluences. The studies reveal that the implantation induced crystal damage
in SiC material can be partly recovered at relatively low temperatures, for damage
levels much lower than needed for amorphization. The implantation experiments
performed on BJTs in the bulk of devices show that the degradation in device
performance produced by low dose ion implantations can be recovered at 420 oC,
however, higher doses produce more resistant damage. Ion induced damage at the
interface of passivation layer and SiC in BJT has also been examined in this thesis. It is
found that damaging of the interface by ionizing radiation reduces the current gain as
well. However, for this type of damage, annealing at low temperatures further reduces
the gain.
Silicon dioxide (SiO2) is today the dielectric material most often used for gate
dielectric or passivation layers, also for SiC. However, in this thesis several alternate
passivation materials are investigated, such as, AlN, Al2O3 and Ta2O5. These materials
are deposited by atomic layer deposition (ALD) both as single layers and in stacks,
combining several different layers. Al2O3 is further investigated with respect to thermal
stability and radiation hardness. It is observed that high temperature treatment of Al2O3
can substantially improve the performance of the dielectric film. A radiation hardness
study furthermore reveals that Al2O3 is more resistant to ionizing radiation than
currently used SiO2 and it is a suitable candidate for devices in radiation rich
applications.
Key words:
Silicon carbide, ionizing radiation, bipolar junction transistors, reliability, surface
passivation, high-k dielectrics, MIS, radiation hardness
Acknowledgements
I would like to start my acknowledgements with all praise and thanks to the Almighty,
who gave me the power and courage to accomplish this dissertation.
I would like to express my gratitude to my supervisor Prof. Anders Hallén, who
kindly agreed to consider me as his PhD student. I feel very lucky to have him as a
supervisor. His positive attitude and continuous support kept me motivated throughout
my stay at KTH. During all these years, his door was always open to discuss anything.
I do not remember any day when he was at KTH and we did not have time to talk about
something.
Thanks to the groups in the University of Helsinki for ALD depositions, and to
Prof. Bengt G. Svensson’s along with his group in Oslo University for providing the
opportunity for me to complete TDRC measurements in his lab. I also want to say
thanks to Accelerator laboratory staff at the Ångström laboratory for their help to
perform ion beam analysis. Dr. Muhammad Nawaz is also thanked for his continuous
support and encouragement, and introducing me to new dimensions of the field.
I would like to thank our department secretaries in the first year Marianne Widing
and in later years Gunilla Gabrielsson for their valuable help in administrative matters.
I am obliged to everybody in EKT group for providing a nice working atmosphere.
Furthermore, I must acknowledge all who have helped me during all these years.
Specifically to; Dr. Martin Domeij for his help regarding BJT studies, and Dr. Henry
Radamson for discussions on all issues. For their support in different scientific matters,
Prof. Mikael Östling, Prof. Carl-Mikael Zetterling, Prof. Gunnar Malm, Dr. Per-Erik
Hellström, Dr. Margareta Linarsson are also thanked. Special thank goes to Dr.
Jiantong Li, my office mate during the last three and half years, I learned a lot from
him. To Benedetto Buono, Luigia Lanni, Eugenio Dentoni Litta, Oscar Gustafsson, Dr.
Christoph Henkel, Dr. Valur Guðmundsson, and many others......, who have been part
of my life at KTH, I also give thanks.
My special thanks for Mohsin Saleemi for the very nice time we spent together at
KTH and outside. That was a part of my life, which I will never forget. Terrance
Burks, Naeem Shahid and Shagufta Naureen are also thanked for sharing time and
thoughts especially at lunch. I would also like to say thanks to my friends outside KTH
for helping me to relax during my stay in Sweden.
I also want to thank Dr. Reza Ghandi, Dr. Mohammadreza Kalahdouz Esfahani and
Dr. Jun Luo for all scientific and non-scientific discussions. I spent very good time
with them.
Lastly, I want to send my gratitude to my parents and sweet sister Hina, for their
prayers, continuous support and encouragement throughout my stay in Sweden. They
were always with me, even though there were large distances between us.
Muhammad Usman
Stockholm, January 2012
!
!
Appended Papers
I.
M. Usman, M. Nour, A. Yu. Azarov, and A. Hallén, “Annealing of ion
implanted 4H-SiC in the temperature range of 100-800 oC analysed by ion
beam techniques”, Nuclear Instruments and Methods in Physics Research B,
vol. 268, no. 11-12, p. 2083-2085, 2010
II.
M. Usman, M. Nawaz, and A. Hallén, “Position dependent traps and
carrier compensation in 4H-SiC bipolar junction transistors”, IEEE
Transactions on Electron Devices (submitted)
III.
M. Usman, A. Hallén, R. Ghandi, and M. Domeij, “Effect of 3.0 MeV
helium implantation on electrical characteristics of 4H-SiC BJTs”, Physica
Scripta, vol. T141, p. 014012, 2010
IV.
A. Hallén, M. Nawaz, C. Zaring, M. Usman, M. Domeij, and M. Östling,
“Low-temperature annealing of radiation-induced degradation of 4H-SiC
bipolar transistors”, IEEE Electron Device Letters, vol. 31, no. 7, p. 707709, 2010
V.
M. Usman, B. Buono, and A. Hallén, “Impact of ionizing radiation on the
SiO2/SiC interface in 4H-SiC BJTs”, IEEE Transactions on Electron Devices
(submitted)
VI.
M. Usman, T. Pilvi, M. Leskelä, A. Schöner, and A. Hallén,
“Characterization of Al-based high-k stacked dielectric layers deposited
on 4H-SiC by atomic layer deposition”, Materials Science Forum, vols.
679-680, p. 441-444, 2011
VII.
M. Usman, A. Hallén, T. Pilvi, A. Schöner, and M. Leskelä, “Toward the
understanding of stacked Al-based high-k dielectrics for passivation of 4HSiC”, Journal of Electrochemical Society, vol. 158, no. 1, p. H75-H79, 2011
VIII.
M. Usman, and A. Hallén, “Radiation-hard dielectrics for 4H-SiC: A
comparison between SiO2 and Al2O3”, IEEE Electron Device Letters, vol.
32, no. 12, p. 1653-1655, 2011
IX.
M. Usman, A. Hallén, K. Gulbinas, and V. Grivickas, “Effect of nuclear
scattering damage at the SiO2/SiC and Al2O3/SiC interface – A radiation
hardness study of dielectrics”, Materials Science Forum, To be published in
2012 (accepted)
Other papers, not appended
X.
M. Usman, A. Nazir, T. Aggerstam, M. K. Linnarsson, and A. Hallén,
“Electrical and structural characterization of ion implanted GaN”, Nuclear
Instruments and Methods in Physics Research B, vol. 267, no. 8-9, p. 15611563, 2009
XI.
A. Pinos, S. Marcinkevičius, M. Usman, and A. Hallén, “Time-resolved
luminescence studies of proton-implanted GaN”, Applied Physics Letters,
vol. 95, no. 11, p. 112108, 2009
XII.
K. Gulbinas, V. Grivickas, H. P. Mahabadi, M. Usman, and A. Hallén,
“Surface recombination investigation in thin 4H-SiC layers”, Materials
Science (MEDŽIAGOTYRA), vol. 17, no. 2, p. 119-124, 2011
Conference contributions
i)
M. Usman, A. Nazir, T. Aggerstam, M. K. Linnarsson, and A. Hallén,
“Electrical and structural characterization of ion implanted GaN”, 16th
International conference on ion beam modification of materials (IBMM),
Dresden, Germany, September 2008 (Poster)
ii)
A. Hallén, M. Nawaz, C. Zaring, M. Usman, M. Domeij, and M. Östling,
“Radiation hardness of 4H-SiC bipolar junction transistors”, 7th European
conference on silicon carbide and related materials (ECSCRM), Barcelona,
Spain, September 2008 (Poster)
iii)
M. Usman, A. Hallén, R. Ghandi, and M. Domeij, “Effect of 3.0 MeV helium
implantation on electrical characteristics of 4H-SiC BJTs”, 23rd Nordic
semiconductor meeting (NSM), Reykjavik, Iceland, June 2009 (Oral
presentation)
iv)
A. Pinos, M. Usman, S. Marcinkevičius, and A. Hallén, “Time-resolved
photoluminescence studies of proton implanted GaN”, 15th Semiconductor and
insulating materials conference (SIMC-XV), Vilnius, Lithuania, June 2009 (Oral
presentation)
v)
V. Venkatachalapathy, M. Usman, A. Galeckas, A. Hallén, and A. Yu.
Kuznetsov, “Intrinsic defect evolution resulted from II/VI ratio variation in
MOVPE ZnO”, 25th International conference on defects in semiconductors
(ICDS), St. Petersburg, Russia, July 2009 (Poster)
vi)
M. Usman, M. Nour, A. Yu. Azarov, and A. Hallén, “Annealing of ion
implanted 4H-SiC in the temperature range of 100-800 oC analyzed by ion beam
techniques”, 19th International conference on ion beam analysis (IBA),
Cambridge, UK, September 2009 (Poster)
vii)
M. Usman, T. Pilvi, M. Leskelä, A. Schöner, and A. Hallén,
“Characterization of Al-based high-k stacked dielectric layers on 4H-SiC
by Atomic Layer Deposition”, 8 th European conference on silicon carbide and
related materials (ECSCRM), Oslo, Norway, September 2010 (Poster)
viii)
M. Usman, and A. Hallén, “Ion implantation studies on SiC BJTs”, 1st SPIRIT
Workshop on new detector technologies for advanced materials research using
ion beam analysis, Croatia, October 2010 (Oral presentation)
ix)
M. Usman, M. Nawaz, and A. Hallén, “Position dependent traps investigation
in 4H-SiC bipolar junction transistors”, 24th Nordic semiconductor meeting
(NSM), Aarhus, Denmark, June 2011 (Oral presentation)
x)
M. Usman, A. Hallén, K. Gulbinas, and V. Grivickas, “Radiation hard
dielectrics for 4H-SiC”, 14th International conference on silicon carbide and
related materials (ICSCRM), Cleveland, USA, September 2011 (Poster)
Summary of Appended Papers and Author’s contribution
PAPER I
The main focus of the paper is the damage production and annealing in 4H-SiC as a
result of relatively high doses of argon implantation. The analysis of the implanted and
annealed SiC has been conducted with ion beam techniques (RBS and NRA) by
studying Si and C sublattices individually. It is observed that a considerable amount of
relatively low concentration of damage in SiC can be annealed up to 800 oC
temperature treatment. However, the damage in the range of atomic density, i.e.
amorphization level, is not possible to recover in this temperature range.
AUTHOR’S CONTRIBUTION
The author was involved in the planning of experiments and sample preparation,
RBS measurements on some samples and all NRA measurements. Also responsible
for the data analysis and manuscript writing.
PAPER II
The degradation of the electrical properties of SiC-based bipolar junction transistors
due to carrier traps at different positions in the device has been investigated in this
paper. The study has been conducted by using TCAD device simulator and some
results have been qualitatively compared with the experimental results obtained after
ion implantation in the collector region of BJTs. The results show that the base region
in BJT is very sensitive to the traps, whereas, similar concentrations of traps in emitter
and collector regions produce less degradation in the device properties.
AUTHOR’S CONTRIBUTION
The study was planned on author’s own initiative, and the author conducted all
TCAD simulations, sample preparation, conducted all experimental measurements
and data analysis, and also wrote the manuscript.
PAPER III
The paper deals with the collector damage due to high energy and high fluence helium
implantations in BJTs. A clear degradation of the electrical properties of the device has
been observed. In addition, it has been observed that ion implantation in the bulk of the
device produce a layer of damaged material, which has negative implications on device
output characteristics. An annealing of these devices does not result in full recovery of
the devices for such high damage levels, however, a partial improvement due to low
temperature damage recovery of the material is observed.
AUTHOR’S CONTRIBUTION
The author was involved in planning the study, samples preparation, performing all
measurements and data analysis, and also responsible for the manuscript writing.
PAPER IV
Low temperature annealing and recovery of the electrical properties of BJTs after low
fluence ion implantation has been studied in this paper. It has been observed that a
temperature treatment as low as 420 oC can fully recover the ion implantation induced
degradation in the device due to relatively low concentration of damage. The annealing
behavior has been correlated with previous DLTS studies as well.
AUTHOR’S CONTRIBUTION
The author participated in sample preparation, ion implantation, and data analysis.
PAPER V
In this paper, the SiO2 interface with SiC, between the emitter and base contact region
and beyond, in SiC-based BJTs has been bombarded with helium ions and the
degradation of the device is studied. The results indicate that roughly 15% degradation
in device characteristics appears after high doses of ionizing radiation. The primary
cause of the degradation, analyzed by TCAD device simulations, is the production of
ionization-induced positive charge in the oxide. The annealing of ion implantation
induced defects further degrades the performance of the device.
AUTHOR’S CONTRIBUTION
The study was planned on author’s own initiative. The author prepared all samples,
performed all electrical measurements and data analysis, participated in simulations,
and wrote the manuscript.
PAPER VI
The paper deals with different types of Al-based stacked dielectrics for 4H-SiC, with a
main focus on AlN. It has been found that ALD-deposited AlN forms a polycrystalline
structure that leads to current leakage through the dielectric, however, the introduction
of thin SiO2 layer prevents this leakage to a certain extent. In this stack, addition of
Al2O3 layers further improves the CV and IV characteristics of the dielectric by
forming a multi-layer dielectric stack.
AUTHOR’S CONTRIBUTION
The author was fully involved in planning, sample preparation and processing, all
measurements, data analysis, and manuscript writing.
PAPER VII
Aluminum-based multi-layer dielectric stacks with alternating layers of AlN and Al2O3
on top of a thin layer of SiO2 have been investigated and compared in this paper. A
current leakage through AlN layer, due to its polycrystalline structure, is prevented by
adding an amorphous layer of Al2O3. The high temperature stability and annealing
behavior are also investigated. It has been observed that a significant difference in the
electrical properties of the whole stacked dielectrics appears by altering the sequence of
deposited layers. However, both types of the stacked dielectrics are reliable and can
present an alternate solution for the passivation of 4H-SiC.
AUTHOR’S CONTRIBUTION
The author was fully involved in planning, sample preparation and processing, all
measurements, data analysis, and manuscript writing.
PAPER VIII
In this paper, ion implantation induced degradation of the dielectric/SiC interface has
been investigated for the first time. A comparison between existing dielectric, i.e. SiO2,
and a potential replacement Al2O3, has been studied with respect to their radiation
hardness. It has been revealed that the interface of Al2O3 with SiC shows less
degradation than the interface of SiO2 in terms of interface trap density as a function of
ionizing radiation. A comparison of same dielectrics on Si has also been presented. In
addition to that, it has been observed that the current leakage and breakdown of the
dielectrics are not sensitive to the ionizing radiation damage and remain relatively
unaffected even after high doses.
AUTHOR’S CONTRIBUTION
The study was planned on the author’s own initiative. Sample preparation and
processing were done by the author, as well as all measurements, data analysis, and
responsible for manuscript writing.
PAPER IX
The influence of nuclear scattering damage on the interface of SiO2 and Al2O3 with
SiC has been studied with the help of capacitance-voltage (CV) and optical free carrier
absorption measurements (FCA). It has been observed that the defects induced by ion
implantation at the interface between the dielectric and SiC result in degrading the
average lifetime in the epi-layer due to enhanced surface recombination. The trend
observed for the average lifetime of carriers in the epi-layer for different fluences of
ions follows the same trend as obtained from CV measurements.
AUTHOR’S CONTRIBUTION
The author was involved in planning, sample preparation and processing, all
electrical measurements, data analysis, and responsible for manuscript writing
Contents
1!
1 Introduction
2 Ionizing radiation and SiC
5!
2.1 Radiation environments ...................................................................................... 5!
2.2 Ion implantation and irradiation.......................................................................... 8!
2.3 Radiation effects in microelectronics.................................................................. 9!
2.3.1 Bipolar junction transistor ...................................................................... 10!
2.3.2 MOS-based devices ................................................................................ 10!
2.4 Silicon versus silicon carbide............................................................................ 11!
2.5 Implantation-induced defects and damage in SiC ............................................ 11!
2.6 Radiation hardness studies of SiC devices and device passivation .................. 15!
3 Characterization techniques and simulation tools
17!
3.1 Structural analysis techniques ........................................................................... 17!
3.1.1 Ion beam analysis.................................................................................... 17!
3.1.2 X-ray diffraction ..................................................................................... 19!
3.1.3 Scanning probe microscopy (SPM) ........................................................ 20!
3.2 Free carrier absorption (FCA) ........................................................................... 21!
3.3 Electrical measurements for BJTs .................................................................... 22!
i
3.4 Electrical characterization of dielectrics ........................................................... 22!
3.4.1 Capacitance–Voltage (CV) ..................................................................... 22!
3.4.2 Current–Voltage (IV) .............................................................................. 24!
3.4.3 Thermal dielectric relaxation current (TDRC) ....................................... 25!
3.5 Simulation tools ................................................................................................ 25!
3.5.1 SRIM ....................................................................................................... 25!
3.5.2 TCAD simulation tool ............................................................................ 26!
4 4H-SiC BJTs subjected to ionizing radiation
29!
4.1 Device structure ................................................................................................ 29!
4.2 Device simulations ............................................................................................ 30!
4.2.1 Ionization studies .................................................................................... 30!
4.2.2 Bulk traps ................................................................................................ 32!
4.3 Ion irradiation of BJTs – Experimental results ................................................. 37!
4.3.1 Implantation in dielectric passivation layer ............................................ 37!
4.3.2 Ion induced damage in the collector ....................................................... 40!
5 Dielectrics for 4H-SiC – Passivation and radiation hardness
45!
5.1 Processing of MIS structures ............................................................................ 46!
5.1.1 Dielectric formation ................................................................................ 47!
5.2 Characterization of the MIS structures ............................................................. 48!
5.2.1 AlN.......................................................................................................... 48!
5.2.2 Al2O3 ....................................................................................................... 49!
5.2.3 Ta2O5 ....................................................................................................... 51!
5.2.4 Dielectric stacks ...................................................................................... 51!
5.3 Thermal stability ............................................................................................... 55!
5.4 Radiation hardness ............................................................................................ 57!
61!
6 Conclusions and future outlook
65!
References
ii
Symbols and Acronyms
MIS
MOS
MOSFET
MISFET
MESFET
BJT
IGBT
RBS
NRA
XRD
AFM
SPM
SSRM
FCA
ALD
PECVD
CMP
SRIM
TRIM
TCAD
SRV
CV
IV
Cox
Qeff
Dit
Lt
Ic
Ib
Vbe
Vce
β
εr
ΔEc
Ec
τ
Metal insulator semiconductor
Metal oxide semiconductor
Metal oxide semiconductor field effect transistor
Metal insulator semiconductor field effect transistor
Metal semiconductor field effect transistor
Bipolar junction transistor
Integrated gate bipolar junction transistor
Rutherford backscattering
Nuclear reaction analysis
X-ray diffraction
Atomic force microscopy
Scanning probe microscopy
Scanning spreading resistance microscopy
Free carrier absorption
Atomic layer deposition
Plasma enhanced chemical vapor deposition
Chemical mechanical polishing
Stopping and range of ions in matter
Transport of ions in matter
Technology computer aided design
Surface recombination velocity
Capacitance voltage
Current voltage
Oxide capacitance
Effective oxide charge
Interface trap density
Traps layer
Collector current
Base current
Base-emitter voltage
Collector-emitter voltage
Current gain
Dielectric constant
Conduction band offset
Critical field
Carrier lifetime
iii
iv
Chapter 1
Introduction
Electronic components are often exposed to various types of radiation. The radiation
can alter the electrical performance, or even lead to permanent failure of electronic
components deployed in radiation environments. The effect of radiation on devices
depends on the type of radiation; for example, gamma radiation mostly induce
ionization in the dielectric layers of the device, whereas energetic particles mainly
create crystal damage in the semiconductor by the process of elastic scattering.
In the present world, silicon is a dominant electronic material for device
manufacturing. However, it is not an ideal material for high power, high temperature,
and radiation rich environments due to its relatively small bandgap and low thermal
conductivity [1, 2]. In many applications, such as radiation environments, very high
electric power applications, or very high temperature usage, larger bandgap, higher
thermal conductivity and higher displacement energy are required. Wide bandgap
semiconductor materials provide an alternate solution with a capability to perform
better in such extreme environments. These materials include Silicon carbide (SiC),
Gallium nitride (GaN), Zinc oxide (ZnO) and a few others. Silicon carbide is
particularly interesting because it has been studied and developed to a larger extent
than its competitors. Improvements in SiC material have provided an opportunity to
offer a viable replacement for Si in power devices, and devices for harsh environment
applications.
The effect of ion implantation or irradiation in SiC material has been extensively
studied both electrically and structurally [3-5]. However, studies for SiC devices,
subjected to ion implantation or irradiation, are also needed to understand the
mechanisms involved in the degradation of the electrical performance. Such studies
provide information on device tolerance and threshold radiation doses, and they are
important for rating devices and to improve the device design for specific applications.
Previously, the effects of radiation on SiC diodes [6, 7], MESFETs [8] and radiation
detectors [9] have been studied, and it has been observed that SiC-based devices are
highly resistant to radiation and generally give higher threshold for their degradation in
comparison to conventionally available Si devices. The SiC bipolar junction transistor
2
Introduction
(BJT) presents itself as a contender for next generation of high voltage switches for
high power and harsh environment applications. Successful BJTs have already been
demonstrated with high current gain values, and they are available commercially [10].
However, reliability issues, such as radiation hardness for BJTs, are now becoming
important.
In the present thesis, the effects of ionizing radiation on SiC material, dielectrics
and SiC-based BJTs have been studied by using ion implantation, or irradiation at
multiple energies and doses in a controlled accelerator environment. The ion species
chosen for these studies are hydrogen (protons), helium (α-particles), carbon and argon.
Ions in the high energy range have high non-linear energy transfer both for elastic and
non-elastic events, but the main effect of these ions comes from the accumulation of
point defects, particularly around the end of the ion path. Therefore, it has a high value
of interest to understand the effects of ion radiation on the overall performance of the
device due to the radiation damage effects in different areas of a BJT. It has been
observed that higher doses of radiation produce more resistant damage in the device,
which is not recoverable at low temperatures. However, the degradation produced by
low doses shows full recovery of device parameters already up to 420 oC annealing
temperature.
Dielectrics are an essential part of electronic devices in the form of passivating
material, or gate dielectric in MOSFETs. For semiconductor power devices,
passivation of surfaces and sharp edges plays a critical role in determining the overall
performance and long-term stability of the device. Dielectrics are also vulnerable to the
radiation effects and therefore, special attention is required for this part of the device.
Silicon dioxide (SiO2), which is a natural oxide of SiC, is currently used as a
passivation material in SiC-based devices. It has a relatively low dielectric constant and
limits the performance of devices and thereby it needs to be replaced with some other
high-k material, which is compatible with SiC. Several studies have been conducted in
this area, in an effort to find an alternate passivating dielectric for SiC [11]. Potential
high-k dielectrics for SiC such as Al2O3, AlN and Ta2O5, are studied in this thesis, both
individually and also as alternate layers in a stacking geometry. Based on promising
initial results, Al2O3 is further investigated with respect to its thermal stability and
radiation tolerance in comparison to existing SiO2. The results of this thesis indicate
that the Al2O3 is stable at higher temperatures compared to SiO2, both structurally and
electrically, and is also able to resist higher fluences of ionizing radiation.
To summarize, the main focus of this thesis is the effect of ionizing radiation on
SiC-based devices, mainly BJTs. In addition, MIS structures are prepared on SiC and
the radiation hardness of the dielectrics for passivation and gate dielectrics is analyzed.
The thesis is organized in five main chapters. In Chapter 2, a description of different
forms of natural radiation present in the cosmic ray spectrum, as well as in man-made
environments is given. The fundamental properties of SiC in comparison to Si are
briefly touched upon, and the concept of electronic and nuclear stopping of energetic
ions in matter is described. Additionally, the defect production and defect annealing in
relatively low temperature ranges in SiC is presented. Chapter 3 deals with
characterization techniques used during the whole work. In addition to that, a brief
description of the SRIM [12] simulation tool for ion implantation studies and Sentaurus
TCAD device simulator [13] is provided. In Chapter 4, along with an introduction to
Introduction
BJT, TCAD-based simulation studies of traps in different regions of a SiC-based BJT
are presented. Key results from BJT ion implantation and irradiation studies in
comparison to unimplanted device results, are also presented. The impact of low
temperature annealing on these implanted devices is also presented in this chapter.
Chapter 5 focuses on different high-k dielectrics for 4H-SiC. The fabrication processes
of MIS structures for different dielectrics are described. The studies are extended from
individual dielectric to stacked dielectrics to improve the overall performance. The
thermal stability for these dielectrics is also examined. Moreover, the defects produced
at the dielectric and SiC interface due to ionizing radiation are investigated to find
better and radiation hard dielectric. Finally, some concluding remarks from the thesis
and a future outlook of the current work are given in Chapter 6.
3
4
Introduction
Chapter 2
Ionizing radiation and SiC
The term ionizing radiation refers to the type of radiation that have enough energy to
remove the electrons from the lattice atoms and break the chemical bonds between
atoms of a material. This radiation consists of charged particles such as heavy ions,
alpha particles (helium nuclei), protons, neutrons, electrons (beta particles), gamma
radiation and other more exotic particles. These types of radiation affect the
performance of electronic devices working in environments where such radiation exist.
A description of these environments and the abundance of various types of radiation
are given in next section. In the following sections of this chapter, the effect of
radiation on material and devices with a special focus on SiC will be discussed in
comparison to Si. Our main interest is on radiation effects caused by our processing,
i.e., ion implantation and controlled experiments with ions.
2.1 Radiation environments
The spectrum of radiation may vary significantly in different environments. The
radiation environments can be classified as Upper Space, Aviation, Natural
environment, and some artificial environments such as, High energy physics
experiments, Nuclear industry and Device processing [14].
In space environment, ionizing radiation exist in the form of high energy charged
particles. The main sources for these radiation are the trapped protons and electrons in
the so called Van Allen belts, heavy ions trapped in the magnetosphere, and protons
and heavy ions from solar flares. Further away from the earth, various types of ionizing
radiation ubiquitously exist in cosmic background radiation, which consist of about
87% protons, 12% helium nuclei, and 1% of electrons and other heavier ions [15]. The
energies of these cosmic rays range from 107 eV to 1018 eV. The cosmic radiation can
be classified according to the origin as solar, galactic, or extra galactic radiation and
have different energy ranges. The energy of solar cosmic radiation lies in the range of
107 to 1010 eV and they have the highest flux. Galactic cosmic radiation have typical
energy in the range from 1010 to 1015 eV and the extra galactic cosmic radiation lies in
6
Ionizing radiation and SiC
higher energy range i.e. up to 1018 eV [16]. A typical cosmic radiation spectrum is
shown in Figure 2.1, which summarizes the overall intensities of the cosmic rays with
particle energies. The intensity of cosmic rays outside the Earth’s atmosphere is much
larger and this fact needs to be considered in the design of electronics for satellites and
aircrafts. However, the lifetime of such devices is still a critical issue.
Figure 2.1: Cosmic ray flux versus particle energy [17].
The cosmic rays present in space are primary cosmic rays, which are converted to
secondary cosmic rays when they collide with the particles in earth’s atmosphere,
generating cosmic ray showers. When cosmic ray particles enter the earth’s
atmosphere, the probability for their collision with molecules, such as oxygen and
nitrogen, is increased and ionization occurs. An initial collision produces pions, which
quickly decay into muons and some of them decay into electrons, positrons and
neutrinos. A typical cosmic shower and its reactions are shown in Figure 2.2. For being
highly interacting particles, electrons and positrons produce further ionization into the
atmosphere and normally annihilate before reaching the surface of the earth. On the
contrary, muons are highly non-interacting particles and can reach the surface of the
earth by losing part of their energy in more ionization processes. The resulting form of
cosmic rays that appears at the surface of earth is mostly muons. A rough estimate for
an average flux of muons at sea level is as 1 muon/cm2-min.
2.1 Radiation environments
Figure 2.2: Development of cosmic ray shower in the earth's atmosphere [18].
In Natural environment, which is typical for the electronics used in our everyday
life, a major source of radiation affecting the electronic devices comes from the decay
of radioactive materials, which consists of mainly alpha and beta particles. In addition,
decay products of cosmic rays exist abundantly in natural environment.
In high energy physics experiments, high intensity and rate of radiation are present,
which affect the detectors around the reaction point negatively. Typical examples are
huge particle detectors installed at the Large Hadron Collider (LHC) in CERN, such as
CMS, ATLAS, ALICE and LHC-b. These detectors suffer very high doses of multiple
types of radiation, which is referred as luminosity. The luminosity level for particle
detectors at LHC is 1033-1034 cm-2s-1, which will be upgraded to 1035 cm-2s-1 for super
LHC in near future [19]. In these experiments, highly energetic protons, electrons, or
heavy particles are used to study elementary particles, e.g. hadrons, bosons, muons and
quarks. Electromagnetic radiation are also produced as a result of particle interactions,
which affect the performance of semiconductor detectors.
Nuclear reactors are a major source of energy these days. The types of radiation
generated in these reactors are mostly gammas and neutrons, but also heavy and light
ions. The by-products of nuclear reactions consist of energetic ions; however, their
penetration in the surroundings is relatively low. In comparison to other radiation
environments, radiation flux in nuclear applications is enormously high, and can easily
lead to the degradation of electronic devices placed at, or near such installations.
Semiconductor device fabrication involves multiple processing steps, which can
induce radiation damage at different locations in the device. The important processing
procedures that can be harmful for state of the art device, from a radiation viewpoint,
involves ion implantation, dry etching, photo-resist stripping, sputtering of metals,
lithography, plasma enhanced chemical vapor deposition (PECVD), high temperature
treatment etc. The effect of these processes on devices is considerable and therefore
special annealing and cleaning processes must be developed to limit the effects. One
7
8
Ionizing radiation and SiC
example is the interface between the dielectric and the semiconductor surface, which
needs to have low density of electronic states and defects. A number of etching
processes, involved in the processing, severely damage the surface of the
semiconductor leading to a high level of surface states [20], and a degradation of the
device performance follows.
Summing up the natural and artificial radiation environments, it is clear that the
semiconductor devices are constantly subjected to some kind of radiation. The
radiation can alter the performance of a device by affecting its different regions. In this
thesis, we have studied the effects of ionizing radiation on 4H-SiC material and in
different regions of its devices. In this effort, a controlled ion beam with different
energies and ions is used to implant or irradiate the material, or devices.
2.2 Ion implantation and irradiation
In laboratory environments, the ion radiation effects on electronic devices are studied
by using ion implantation with different energies to produce defects in different regions
of the device. Ions in the keV to MeV range produce ionization and crystal damage in
semiconductor materials and devices, and different fluences of ions can simulate the
effect of radiation flux. For this purpose, an ion accelerator is used in a controlled
environment, which provides an opportunity to probe different depths in a material. In
a semiconductor device, consisting of dielectrics or semiconducting multilayer of
material, the damaging effect of ionizing radiation on the performance and reliability of
the device can be understood with the help of these kinds of experiments.
Ion implantation is performed by directing a focused mono-energetic beam of
accelerated ions onto a target and is normally used to alter the properties of the target
by introducing other elemental species in the target, i.e. doping. The word implantation
refers to the process when ions stop in the target at a certain depth depending on the
energy of impinging ions. However, when the energy of the implanting ions is
sufficiently high so they pass through the target, the process is named irradiation. In
this thesis, the term, ion irradiation, is used when the projected range of ions is beyond
the region of interest, i.e., when ion implantation is performed on a device structure
with adequately higher energy so the ions pass through the active area of a particular
device, and stop in the substrate.
During the process of ion implantation, each ion undergoes scattering events with
electrons and nucleus of the atoms in the target material and loses its energy due to
electronic and nuclear stopping interactions, and as a result, several types of defects are
introduced in the target. In electronic stopping, the traversing ions interact with the
electronic system of the target, i.e. electrons orbiting around the nucleus of the crystal
atoms, and consequently, electrons are knocked out from the atoms producing
ionization damage. Meanwhile, energetic ions also scatter from the nucleus and lose
their energy in elastic collisions. This process is known as nuclear stopping and it
becomes the dominant process around the end of ion range. The nuclear stopping
results in displacement damage, when the energy transferred to the target atoms is
larger than its binding energy. The electronic and nuclear stopping powers or the
energy loss of the incoming particles per unit path length determine the depth of
implanted ions in a particular material. The average depth of implanted ions in a target
2.3 Radiation effects in microelectronics
is referred as projected range of ions, Rp, and it depends on the angle of incidence of
ions in a crystalline material, ion type and its energy. Due to the stochastic nature of
the collision events, the stopped ions are distributed around their projected range and
the phenomenon is called straggling, which is the standard deviation of the projected
ion range, ΔRp.
2.3 Radiation effects in microelectronics
Radiation induced effects play an important role in limiting the lifetime and reliability
of microelectronic devices. The semiconductor components typically consist of a
semiconductor material, dielectrics/insulators, various metallization and packaging
materials. The operation of these components in the radiation environments, described
previously, is affected by energetic particles and electromagnetic radiation with a wide
range of energies. These energetic particles, or photons, lose their kinetic energy by
interacting with the atoms of the material in different ways, while traversing through it.
The influence of radiation on insulating and conductive parts of the device is very
different. The general description of known effects on these parts is summarized in
Table 2.1.
Table 2.1: A comparison of radiation effects in different materials, commonly used in a
semiconductor device, due to electronic and nuclear stopping.
Material
Insulators
Semiconductors
Metals
Electronic stopping
(ionization)
Nuclear stopping
(crystal damage)
Charge build up [21]
Not so problematic
Could affect devices under
operation, not important
Crystal damage leading to
leakage, compensation and
lifetime alteration
No effect
No effects unless very high
doses
Radiation induced errors in semiconductor devices can be classified in the
following way:
1) Single event upset (SEU), create soft errors
2) Single event latchup (SEL), single event gate rupture (SER), and single event
burnout (SEB), create permanent damaging effects resulting in catastrophic
failure of the device.
In many space applications, high power and high voltage devices are required. In
silicon technology, large area and bias conditions in power devices make them more
susceptible to radiation effects. For such applications, power metal oxide field effect
transistor (MOSFET), isolated gate bipolar transistor (IGBT) and bipolar junction
9
10
Ionizing radiation and SiC
transistor (BJT) are often employed for high speed, high frequency and very large
power or current applications. Some of these commercially available Si-based power
devices have been tested with respect to their radiation response under the influence of
neutrons, protons and heavy ions’ fluxes [22, 23]. In this thesis, the focus is on ion
radiation effects on the performance of BJTs and the passivating dielectrics, and a full
description of radiation effects in microelectronics is beyond the scope. In the context
of the present work, general effects of ionizing radiation in BJTs and MOS structures
are discussed in following sections.
2.3.1 Bipolar junction transistor
Bipolar junction transistors are prone to degradation of device properties, caused by
ionization and displacement damage in the passivating oxide and the semiconductor
material. The BJT properties that are mainly affected are current gain (β=Ic/Ib) and the
leakage current. Current gain is reduced due to the introduction of recombination
centers in the semiconductor material due to displacement damage, but also from
ionizing effects in the dielectrics, and the interface between semiconductor and
dielectric. The effects of radiation on Si-based BJTs have been studied widely in last
few decades [24, 25]. Radiation induced recombination centers in the emitter, the
emitter-base space charge region, the base, or in the collector can all increase the base
current, resulting in degradation of the current gain. Displacement damage in the
semiconductor can increase the number of recombination centers and increase the
reverse leakage through the device. The surface of the semiconductors, and the
interface properties are also important for controlling current gain values of the device.
Ionizing radiation can induce charges in the oxide [26] and/or increase the number of
interface states, resulting in an increased base current. In addition to these effects,
radiation such as electrons or heavy ions can produce doping compensation, affect the
mobility (in case of high damage), increase the leakage current or alter the lifetime of
carriers in different regions of the device. In the present work, SiC-based BJTs are
studied with respect to their radiation tolerance.
2.3.2 MOS-based devices
Dielectrics such as SiO2 are used as passivation in most devices, or a gate dielectric in
MOSFET applications. Ionizing radiation induce trapped charges in the
oxide/dielectric layers in semiconductor devices. This creates a major concern for the
reliability of the devices. The induced charges in the oxides degrade the performance of
the device by shifting the threshold voltage, increasing the noise level and affecting the
channel mobility in MOSFETs. The induced charges are stored in trap sites (preexisting or radiation induced) for some time, and released subsequently under bias
conditions of the device, affecting its operation. The radiation affects in dielectrics are
mostly studied by fabricating metal oxide semiconductor (MOS) structures. In the
present thesis, MOSFET is not discussed; however, a central part of these devices,
which has strong influence on device performance, i.e. MOS, has been studied with
respect to ionizing radiation affects due to heavy ions.
2.4 Silicon versus silicon carbide
2.4 Silicon versus silicon carbide
Silicon carbide is considered to be a promising candidate material for high temperature,
high power, high frequency, and radiation rich environment applications [27]. This is
due to its large bandgap, high thermal conductivity, high breakdown field, and higher
radiation resistance. With its potential use in radiation rich environments, wide
bandgap and high displacement energy for both Si and C lattice atoms are particularly
interesting properties that offer a significant advantage over conventional
semiconductor materials and classify it as a radiation hard semiconductor material.
Some basic properties of SiC, which are of interest in radiation applications, in
comparison to Si, are listed in Table 2.2.
Table 2.2: A comparison of basic properties of Si and 4H-SiC [26-30].
Property
Si
4H-SiC
Bandgap (eV)
1.12
3.26
Dielectric constant (εr)
11.9
9.7
Critical field Ec (MV/cm)
0.3
2.2
13-20
Si=35, C=20
Atomic density (g/cm )
2.33
3.21
Thermal Conductivity (W/cm-s)
1.5
3.7
Electron-hole pair production energy (eV)
3.6
7.8-9
Atomic displacement Energy (eV)
3
2.5 Implantation-induced defects and damage in SiC
Various types of point defects normally exist in as-grown semiconductor materials, but
they can also be introduced externally by various processes of irradiation or ion
implantation. Also larger structural defects, such as micropipes or stacking faults are
introduced during bulk or epitaxial growth. However, with improving growth
technology, the material quality has improved considerably. The total amount of point
defects produced by irradiation or implantation is referred as damage. Point defects
have significant influence on electrical and optical properties of the material depending
on their position in bandgap, capture cross-section, and their concentration. The
interaction of these defects results in reduced charge carrier lifetimes, carrier
compensation or deactivation of dopants, and also elevated diffusion of dopants, which
has strong impact on the fabricated device performance.
By the process of ion implantation, mainly vacancies, interstitials, di-vacancies and
their combinations thereof are introduced. The defects and their effect on the material
can be characterized with the help of different electrical or structural techniques such
as, deep level transient spectroscopy (DLTS), ion beam techniques (RBS, NRA) and
scanning spreading resistance microscopy (SSRM). The analysis techniques are
normally chosen by considering the implantation doses and the resultant defect
11
Ionizing radiation and SiC
concentrations in the material. For very low doses DLTS is more favorable and for
higher doses RBS, NRA and SSRM are employed.
Most commonly discussed defects in SiC are the Z1/Z2 defects, which exist both in
as-grown and irradiated material. DLTS studies on these defects suggest that the Z1/Z2
defects are positioned at Ec-0.68 eV level in the bandgap [31] and are thermally stable
at temperatures even beyond 1500 oC [32], which is a typical defect annealing
temperature for SiC [33]. In addition, the EH6/EH7 [34] defect complex at Ec-1.5 eV in
4H-SiC, are also important to mention. These defects have relatively large charge
carrier capture cross-sections and have been pointed out as two major recombination
and generation centers in 4H-SiC [35]. In addition, it has been shown by DLTS that a
handful of electrically active defects exist in the upper part of the bandgap after low
dose irradiation of 4H-SiC [36]. Many more defects [37-39] have been reported but the
origin and identity of most of the defects is still unknown and a lot of research is on the
way for their understanding.
In SiC, with high displacement energies, even higher fluences (ions/cm2) of
implantation ions produce relatively less amount of damage. However, the higher
implantation doses produce considerable damage in the material, which can be
conveniently studied with the help of ion beam techniques. A detailed description of
these techniques is provided in Chapter 3 of this thesis. The studies of the damage
accumulation and its annealing in SiC are conducted for relatively high doses of Ar
ions in SiC. Figure 2.3 shows Rutherford backscattering analysis of SiC epitaxial
layers implanted with different fluences of argon ions with energy of 200 keV. It is
clear from the figure that the low fluence (4×1014 cm-2) produces a damage peak
around the projected range of ions whereas higher fluence (2×1015 cm-2) amorphize the
whole region, where the ions have passed, and produce higher concentration of damage
in this regions. The spectra show only Si damage in the material.
2.0
1.5
Depth (µm)
1.0
0.5
0.0
1.0
Relative disorder
12
0.8
Surface
0.6
0.4
15
-2
2x10 cm
14
-2
4x10 cm
0.2
240
250
260
270
280
290
Channel No.
Figure 2.3: RBS damage profile of 200 keV Ar+ implanted SiC. The profiles have been
extracted using Schmid algorithm [40].
6
Si
5
40
C
30
4
3
20
2
10
1
0
200
400
600
Integrated Yield (x1000)
17
2
Integrated Damage (x10 atoms/cm )
2.5 Implantation-induced defects and damage in SiC
800
o
Temperature ( C)
Figure 2.4: Damage annealing of Si and C sublattices [Paper I].
Since SiC contains 50% of carbon atoms, it is also important to understand the
damage formation in the carbon sublattice. The carbon signal in RBS spectra of SiC is
normally buried under the silicon signal. To enhance signals from carbon atoms in SiC,
carbon resonance reactions are considered. In the present study, 12C(α,α)12C at 4.26
MeV is used to enhance the yield from the carbon sublattice in implanted SiC by using
high energy helium ion beam.
Annealing of this damage, studied with the help of RBS (Si sublattice) and NRA (C
sublattice), reveals that the low concentration of damage is possible to anneal at
relatively low temperatures both for Si and C sublattices, however, the highly damaged
SiC crystals are not possible to recover with low temperature annealing, up to 800 oC.
Figure 2.4 shows that the sublattices of Si and C in SiC, implanted with lower fluence
of ions, can be annealed at relatively low temperatures. However, full recovery of this
implantation-induced damage requires higher temperatures [33].
The electrical impact of high radiation dose induced damage in SiC has been widely
discussed at material level by using SSRM [4, 41]. These studies suggest that the
irradiation or implantation induces crystal defects, which reduce the carrier flow
Figure 2.5: SSRM image of 330 keV He+ implanted SiC. An average resistance profile
extracted from the image is also presented [Paper II].
13
Ionizing radiation and SiC
24
10
22
10
20
10
18
10
16
10
14
12
Atomic density of SiC
10
Vacancies in SiC
15
-2
+
2x10 cm Ar
14
-2
+
4x10 cm Ar
10
-2
+
3x10 cm He
6
4
200 keV Ar
distribution
0.0
8
0.2
0.4
+
330 keV He
distribution
0.6
0.8
+
4
10
Ions x10 /cm
3
through the damaged region, and consequently create a highly resistive layer in the
material. An example of a cross-sectional SSRM scan on a SiC epitaxial layer,
implanted with 330 keV helium ions with a dose of 3×1010 cm-2, is presented in Figure
2.5. An extracted averaged resistance profile is overlaid on the SSRM image. The
analysis suggests that this fluence of ion implantation creates a localized damage
region resulting in an increased resistance around Rp. The annealing behavior of the
electrical properties of ion implanted SiC in low temperature range has been previously
studied by SSRM [4].
The main difference of the Ar+ and He+ implantation (discussed in this section) is
the shape of damage profile and depth of the damage under the surface of the sample.
In the case of helium, the damage is more localized and more deeply located due to
higher energy, whereas Ar ions, with higher mass, create more scattering damage along
its entire track inside the material. Figure 2.6 shows a SRIM simulation [12] analysis of
vacancies produced by the Ar and He ions in SiC. The distribution of implanted ions as
a function of depth is also presented. Based on the results presented before (Figure
2.3), it is obvious that the high dose of Ar ions produces amorphous layers, and the
vacancies produced in this case are of the order of atomic density of SiC. This amount
of damage is not possible to recover at low temperatures, whereas low fluence Ar+
implanted SiC is recoverable in the same range of temperature. On the other hand, even
four orders of magnitude lower concentration of vacancies in SiC show electrical
effects, with an increased resistance around the end of ion range (Figure 2.5).
Vacancies/cm
14
2
1.0
1.2
0
Depth (µm)
Figure 2.6: SRIM vacancy and implanted ion's profile distribution versus depth in SiC.
The lattice damage produced by ion fluences results in point defects. It is well know
that the elastic energy deposition forms primary defects, such as Si and C vacancies
and interstitials. A substantial part of these defects, however, recombines or forms
electrically neutral defects. Only 5–10% form electrically active stable defects [5] that
influence the electrical properties of SiC, and is clearly visible in SSRM studies.
2.6 Radiation hardness studies of SiC devices and device passivation
2.6 Radiation hardness studies of SiC devices and
device passivation
Silicon carbide based devices such as radiation detectors, Schottky diodes and
MESFETs have been previously studied with respect to their radiation hardness. The
studies of radiation detectors i.e. diodes have been performed by the irradiation of
neutrons [42], protons, gamma rays [43], electrons [44], light ions [45] and pions [46],
and Schottky diodes by proton, neutron [47] and heavy ion [48] irradiation. In addition,
studies on MESFETs have been carried out by energetic electron irradiation [49] and
high dose irradiation of gamma rays [8]. All these studies provide a clue to the superior
properties of SiC-based basic devices in radiation environments. For more complex
devices such as bipolar junction transistors and MOSFETs there is no literature
available.
The growth, or deposition of passivation material plays an important role in the
radiation response of the device that can be different for differently grown oxides [26].
Therefore, for radiation point of view, passivation is one of the important issues to be
addressed. In this respect, different studies have been reported to enhance the radiation
hardness of the devices [50]. In addition, the choice of dielectric material for
passivation, its deposition, processing and post processing treatments along with
appropriate geometry requires special attention to improve the passivation quality and
make the devices radiation hard.
In the present thesis, the main focus has been on the radiation tolerance of BJTs in
Chapter 4, and MOS capacitors (a basic building block of MOSFET) structures in
Chapter 5, to understand the impact of ion radiation on the performance of the device.
15
16
Ionizing radiation and SiC
Chapter 3
Characterization techniques and
simulation tools
In this thesis, several characterization techniques have been employed that can be
grouped in two categories, i.e. structural and electrical analysis techniques. The main
techniques considered for structural analysis are ion beam analysis techniques, such as
Rutherford backscattering (RBS) and nuclear reaction analysis (NRA), x-ray
diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy
(AFM). The electrical analysis has been performed by using mainly capacitancevoltage (CV) and current-voltage (IV) measurements, but also thermal dielectric
relaxation current (TDRC) and scanning spreading resistance microscopy (SSRM)
have been used. In addition to that, novel optical analysis method by using free carrier
absorption (FCA) has also been introduced to perform surface recombination analysis
at oxide/semiconductor interfaces.
Simulations provide detailed information about the materials and devices in
operation. Therefore, some important simulation tools are also used, to understand the
ion implantation in materials (SRIM) [12], RBS data analysis (SIMNRA) [51] and
device simulations (Sentaurus TCAD) [13]. The most important techniques and
simulation tools for this thesis are described in detail in the following sections.
3.1 Structural analysis techniques
3.1.1 Ion beam analysis
Ion beam analysis (IBA) techniques present a powerful tool for material analysis.
These techniques are mainly based on energetic ions’ interaction with material. In IBA
techniques, energetic ions are directed on a target material; when a charged particle
with high speed strikes the target, it interacts and loses energy both with the electronic
system of the material and also with nuclei of the material atoms. The interactions lead
to scattering of the incident ions and the emission of secondary particles, where the
Characterization techniques and simulation tools
emitted particles, their energy and direction reflect the characteristics of the target
material. In particular, by studying backscattered particles, information about the
elemental composition (stoichiometry) of the material and depth distribution in near
surface layers in the material is obtained. Ion beam analysis is a broad term used for
several different techniques named as Rutherford backscattering (RBS), nuclear
reaction analysis (NRA), elastic recoil detection analysis (ERDA) and particle induced
x-ray emission (PIXE). In the present work, RBS and NRA have been utilized
extensively. The measurements were performed at the Ion Technology Center in
Uppsala, Sweden using 5 MV pelletron accelerator.
a) Rutherford backscattering
Rutherford backscattering spectroscopy (RBS) is a commonly used technique for
material analysis. It is performed by using light ions such as proton or helium for the
analysis of a material containing heavier elements. It is a well-suited analytic tool to
determine the trace elements present in the target. The main principle of RBS involves
the elastic collision of light energetic ions with heavier target elements. The energy of
backscattered particles provides information about the mass and depth of the elements
present in the material under investigation.
A specialized form of RBS is called channeling RBS, which is used to determine
the crystal damage distribution in a crystalline material. In channeling mode, the
incoming ion beam is aligned with the crystal orientation to reduce the scattering
events with the lattice. In this way, information regarding amount and depth
distribution of lattice disorder is obtained.
In the present thesis, RBS is utilized to characterize the ion implantation damage in
channeling geometry for implanted SiC samples, and in standard (random) mode to
characterize the ALD deposited high-k dielectrics. Figure 3.1 shows an RBS spectrum
of 4H-SiC in random direction, fitted with SIMNRA simulations. In addition,
channeling spectra of as-implanted and unimplanted 4H-SiC is added. A beam of 2
MeV He ions is mostly used for RBS analysis presented in this thesis.
0.4
Energy (MeV)
0.6
0.8
1.0
1.2
Random spectra
Simulated spectra
14
-2
+
4x10 cm Ar
Virgin
600
C
400
Si
Yield
18
Damage peak
200
Channeled spectra
0
50
100
150
200
250
Channel No.
300
Figure 3.1: RBS analysis of 4H-SiC in random and channeling geometry. Channeling RBS on
200 keV Ar+ implanted 4H-SiC with 4×1014 cm-2 fluence of ions is also added.
3.1 Structural analysis techniques
b) Nuclear reaction analysis
Nuclear reaction analysis (NRA) is a specialized technique used to enhance the yield
from lighter elements in a heavier matrix. It can provide the concentration and depth
information of the target element. This technique is based on specific nuclear reactions
between the incoming ions and target nuclei, occurring at resonance energy. The
incoming ion interacts with the target nuclei in a non-Rutherford event with enhanced
cross-section. The nuclei are excited and then decay to a stable form by emitting
secondary particles, which provide characteristics of the target elements. Figure 3.2
demonstrates a resonance reaction 12C(α,α)12C at 4.26 MeV [52], which is utilized in
the present thesis to examine the carbon lattice damage and recovery at low
temperatures in 4H-SiC (Figure 2.4).
4000
12
3000
Yield
12
C(α,α) C resonance peak
@ 4.26 MeV
2000
Non-Rutherford effects
in high energy RBS
Si
1000
0
50
100
150
200
250
300
Channel No.
Figure 3.2: Resonance reaction
carbon atoms in SiC.
12
C(α,α)12C at 4.26 MeV showing an enhanced yield from
c) SIMNRA
SIMNRA is a software package to simulate the backscattering spectra for Rutherford
and non-Rutherford cross-sections [51]. The experimental data obtained from RBS or
NRA is fitted with simulated spectra by providing the experimental conditions i.e.
scattering geometry, incident particle and target information. It offers a quantitative
analysis of the material properties. The program has been used to analyze and simulate
the RBS/NRA spectra in the present thesis. An example of such kind of analysis is
already provided in Figure 3.1.
3.1.2 X-ray diffraction
X-ray diffraction (XRD) is a versatile non-destructive analytical method to determine
the crystallographic structure, chemical composition of materials, lattice dimension,
crystal orientation in crystalline films, and grain size determination in polycrystalline
samples. The basic principle of XRD is based on the interference of scattered x-rays
from different planes of the material under investigation as a function of incident and
19
Characterization techniques and simulation tools
scattered angle, wavelength, and polarization of the incident beam. The output signal
from the diffracted beam is obtained when Bragg’s condition is satisfied, which is:
nλ = 2d sin θ
where n is an integer, λ is wavelength of the incident beam, d is the distance between
crystal planes and θ is the angle of incidence between the incident beam and crystal
surface.
In the present thesis, the technique is utilized to analyze the crystallographic
structure of the ALD deposited thin films. In Figure 3.3, an XRD spectrum for AlN
film is presented. A dominant peak of AlN in [002] direction is observed with
relatively low intensity, which depicts the polycrystallinity of AlN film. The grain size
for this particular polycrystalline film is calculated to be 32 nm using Scherrer formula
[53].
400
[002] AlN
300
Intensity
20
200
100
35.0
35.5
36.0
36.5
37.0
2θ−ω (deg)
Figure 3.3: XRD spectrum for AlN deposited by ALD on SiC. AlN shows a polycrystallinity
with a dominant peak at [002]. The low intensity of the peak represents a polycrystalline
structure of the deposited film [Paper VII].
3.1.3 Scanning probe microscopy (SPM)
Surface quality of deposited or grown films is very important in microelectronics.
Scanning probe microscopy (SPM) is a unique tool that can provide multiple types of
surface analysis. The technique uses a small probe tip at the end of a cantilever to get
high resolution three-dimensional surface analysis. The tip is brought into close
proximity of the surface of the sample while monitoring the tip deflection, which gives
surface roughness, and measuring the current-voltage between tip and the sample
provides information about the electrical properties of the surface. SPM operates in
different modes depending on the application. Atomic force microscopy (AFM) is
based on monitoring tip deflection due to the force between tip and the sample surface
atoms, whereas scanning spreading resistance microscopy (SSRM) measures the
resistance variation on the surface using a conductive tip. In the present thesis, tapping
mode AFM is utilized for surface roughness analysis of epitaxial layers prior to the
3.2 Free carrier absorption (FCA)
Figure 3.4: AFM topographic image of SiC epi-layer grown on an unpolished wafer. The step
bunching in the growth direction can also be seen. The RMS value for the surface is 3 nm.
oxide depositions, and layer surfaces after ALD depositions. The surface roughness of
the SiC epitaxial layer on chemical-mechanical polished (CMP) and unpolished
surfaces are also studied. Figure 3.4 shows an AFM scan on an unpolished SiC surface.
SSRM is considered for cross-sectional analysis of the implanted SiC epitaxial
layers to assess the effects of implantation damage on the resistance variation in the
material. An example of such analysis has been shown in Figure 2.5. The equipment
used for these analyses was Nanoscope dimension 3100 equipped with an SSRM
module.
3.2 Free carrier absorption (FCA)
Optical free carrier absorption (FCA) has been used in this thesis as a non-destructive
coaxial pump-probe technique for carrier lifetime mapping in semiconductors. The
technique is based on the detection of free carrier absorption transients by a low energy
probe laser beam following the excitation of electron-hole pairs, generated by a high
energy pulsed laser beam. In semiconductors, the average lifetime of the excited
carriers depends on the bulk recombination lifetime in the epi-layer, the diffusion
coefficient for carriers and the recombination at surfaces and interfaces. In our samples,
consisting of SiC substrate, SiC epitaxial layer, and a dielectric; the bulk lifetime in
epi-layer and diffusion coefficient are roughly the same, therefore the average lifetime
is mostly affected by the surface recombination velocity (SRV). In the present thesis,
the technique is utilized to analyze the effects of average lifetime in SiC due to surface
defects. A typical experimental carrier transient is shown in Figure 3.5a, which is used
to measure the effective epi-layer lifetime. The bulk lifetime values in SiC have
improved due to improvement in SiC growth technology and high quality epi-layers;
however, the surface recombination directly influences the recombination in SiC and
the effect is strongly dependent on the epi-layer thickness. The measured effective
lifetime provides a way to extract SRVs from these measurements. The dependence of
the effective carrier lifetime on epi-layer thickness is shown in Figure 3.5b. In our
samples, the epi-layer thickness is in a range where the surface recombination on both
sides of the layer, i.e., interface with dielectric (S1) and interface with substrate (S2),
influence the lifetime in epi-layer. This provides an opportunity to explore the SRV at
21
Characterization techniques and simulation tools
the dielectric interface by establishing the other parameters, i.e. τn,p, Dn,p, S2. The area
of interest for such kind of analysis using FCA is highlighted in Figure 3.5b.
(b)
(a)
0
10
τbulk
τsubstrate
epi
3
substrate
10
S1=S2=S
3
S1 S2
-1
10
τepi
S=2×10
4
S=2×10
6
S=1×10
6
S1=1×10 > S2
dielectric
2
τ (ns)
-1
Δα (cm )
22
10
1
10
-2
10
Epi-layers
used
0
0.0
0.2
0.4
0.6
Time (µs)
0.8
1.0
10
1
S2=1×10
4
S2=3×10
4
S2=1×10
5
10
100
Epi-layer thickness (µm)
Figure 3.5: (a) Experimental carrier transient with characteristic lifetime values shown for the
sample with surface passivated with SiO2. Exponential lifetime in the substrate and the effective
lifetime in the epi-layer are also indicated. (b) Simulated monograms for various SRV values are
shown for effective carrier lifetime as a function of epi-layer thickness. The values for S1 and S2
are given in cm/s [54].
3.3 Electrical measurements for BJTs
In the present thesis, a major part of the work is devoted to the radiation effects in
bipolar junction transistors (BJTs). The main characteristics that are studied for BJTs
are collector current versus collector-emitter voltage. It provides information about the
on-state resistance and collector saturation current. Current gain (β), which is the ratio
of collector current to the base current, is an important parameter for BJTs and is
studied by so called Gummel measurements [55]. The measurements are performed by
sweeping base-emitter voltage (0–5 V) at a fixed collector voltage of 6 V. The aim
with a higher collector voltage is to put the collector-base diode in reverse bias. These
types of measurements provide an insight into the individual current behavior for all
three terminals. All electrical measurements for BJTs have been performed by using
HP 4156 parameter analyzer and a probe station.
3.4 Electrical characterization of dielectrics
Electrical characterization of dielectrics for passivation of SiC is extensively conducted
in this thesis. The dielectrics are characterized mainly by using CV, IV, but also TDRC
techniques. A brief description of these techniques is given in following section.
3.4.1 Capacitance–Voltage (CV)
The quality and reliability of dielectrics strongly influence the electronic device output.
CV measurement technique is by far most common method to characterize the
electrical performance of dielectrics and their interface with semiconductors. It can
provide information about the dielectric thickness, semiconductor doping
3.4 Electrical characterization of dielectrics
concentration, density of interface states (Dit), effective fixed oxide charge density
(Qeff), and work function or barrier height. The method is based on capacitance
monitoring against varying applied voltage. The measurements utilize a small AC bias
signal superimposed on the DC bias. The magnitude and frequency of the AC voltage
is kept constant while DC voltage is swept in the time domain. The purpose of the AC
voltage is to allow sampling of charge variation in the device, whereas sweeping of the
DC voltage allows the capacitor to form accumulation, depletion and inversion in the
semiconductor. The oxide charges are estimated by the voltage shift of the CV curve in
relation to an ideal behavior, and the slope of the curve during depletion indicates the
presence of interface states.
In the present thesis, the CV method is used to characterize all dielectrics, and the
Dit values are extracted from high frequency CV curve by using Terman method [56].
This method utilizes a comparison of ideal CV to the experimentally measured CV and
the trap density is calculated by using following relation:
Dit =
Cox ' dVG
$ C
%%
− 1"" − 2s
q & dϕ s
# q
2
Normalized capacitance (C/Cox)
where Cox and Cs are oxide and semiconductor capacitances respectively, VG is applied
DC bias, ϕs is the surface potential for the ideal CV and q is the charge.
All CV measurements, presented in this thesis have been performed on a probe
station by using HP 4284A LCR meter in darkness and the sweep rate was set to 0.1
V/s. Figure 3.6 shows a comparison of an ideal CV curve with a SiO2 grown on SiC by
using N2O oxidation and a PECVD deposited oxide with post-oxide anneal in N2O.
PECVD+N2O anneal
1.0
N2O grown
0.8
0.6
Ideal
Positive charge
in oxide
Negative charge
in oxide
0.4
0.2
0.0
-4
-2
0
2
4
Gate voltage (V)
Figure 3.6: CV curve for SiO2 grown (i) in N2O at 1250 oC and (ii) PECVD deposited and N2O
post-oxide annealed at 1150 oC. Ideal CV curve calculated by Sentaurus TCAD is also presented
for comparison. Shift in the CV curve towards positive voltage shows negative oxide charges,
whereas positive oxide charges shift CV curve towards negative voltage.
23
Characterization techniques and simulation tools
3.4.2 Current–Voltage (IV)
The leakage current through dielectrics and estimation of their breakdown field is
highly desirable in reliability studies of dielectrics. For this purpose, current-voltage
(IV) characterization allows a simple way of judging the quality of the dielectric. The
shape of the typical IV characteristics provides details about the leakage phenomenon
happening inside the dielectric, prior to final breakdown. The tunneling of electrons
through the oxide layer, known as Fowler-Nordheim tunneling [57] and the current
conduction through Frenkel-Poole emission of electron, which is associated with
electron trapping at defect sites in the dielectric [58], can be characterized by utilizing
this method. After applying a strong electric field across the dielectric, the breakdown
value for the dielectric can be assessed, although this is a destructive measurement.
Figure 3.7 shows a typical breakdown curve of SiO2 on SiC, highlighting different
phenomena that can be studied using IV measurements. The breakdown field of
dielectrics can be different for different devices on the same chip area. The main cause
for this non-uniformity is the quality of the dielectric layer, but also the surface
properties of the epitaxial layer. In addition, the contact metal [59] and its formation
may also influence the breakdown characteristics of a dielectric film. To evaluate the
quality and reliability of the dielectrics, statistical methods using Weibull distribution
function are utilized. With this, it is possible to distinguish between the extrinsic or
intrinsic breakdown in a cluster of several measured device results. In the distribution,
the sharpness of the slope gives information about the oxide reliability and type of
breakdown (Figure 5.11).
In the present thesis, SiO2 is grown or deposited by PECVD and the high-k
dielectrics are deposited by ALD. The leakage and breakdown studies have been
conducted by using HP 4156, and Weibull statistical distribution is employed to some
of the dielectrics.
10
2
10
0
Oxide Breakdown
2
Current Density J (A/cm )
24
10
-2
10
-4
10
-6
10
-8
10
Frenkel-Poole
emission of electrons
Fowler-Nordheim
tunneling region
-10
0
2
4
6
8
Electric Field (MV/cm)
Figure 3.7: Typical breakdown (IV) characteristics of SiO2 on SiC.
10
3.5 Simulation tools
3.4.3 Thermal dielectric relaxation current (TDRC)
The thermal dielectric relaxation current (TDRC) is a unique technique used to study
the interface traps’ energy level distribution close to the majority carrier band edge.
The method is based on monitoring the current produced by the majority carriers that
are thermally emitted from interface traps. During the measurement of MIS structures,
the capacitor is brought to accumulation region (charging bias) at room temperature to
fill the traps with majority carriers [60]. The capacitor is then cooled under charging
bias conditions. At minimum cooling temperature, the applied bias is shifted from
accumulation to inversion, i.e. discharging bias, and the temperature is slowly raised at
a constant rate. At sufficiently high temperatures, electrons are released from the filled
interface traps and move to the conduction band in the presence of an electric field
(discharging bias). The emitted electrons are then monitored as a current signal. The
emission of electrons from the filled traps can depend on heating rate (Figure 3.8). The
current versus temperature is measured while heating, which gives the small current
signal due to the emission of carriers from the traps. The temperature provides the
information about the energy levels. Finally, the interface traps at different energy
levels are determined. In the present thesis, TDRC measurements are performed on a
limited number of MIS structures. The measurements have been performed at the
Center for Materials Science and Nanotechnology, Oslo University.
Current (pA)
1.5
1.0
12 K/sec
10 K/sec
8 K/sec
0.5
0.0
50
100
150
200
250
300
Temperature (K)
Figure 3.8: TDRC spectrum for Al2O3 at 22 V of charging voltage at different heating rates.
3.5 Simulation tools
3.5.1 SRIM
Stopping and range of ions in matter (SRIM) [12] is a well-known software package
used to calculate the stopping and range of energetic ions into matter. It is based on
Monte Carlo simulations by taking into account the binary collision approximations.
SRIM mainly uses transport of ions in matter (TRIM) code to calculate the stopping
powers for different ions in a target material [12]. The target material can be chosen
25
Characterization techniques and simulation tools
from a single layer to a combination of several target layers. The main information that
one can get from these simulations is as under:
1) Ion range and straggling in materials.
2) Three-dimensional distributions of implanted ions in the target.
3) Lattice damage distribution in the form of vacancies produced by ions and
recoils.
4) Ionization produced due to traversing ions through the material.
In the present thesis, SRIM is widely utilized to estimate the depth of implanted
ions in SiC and dielectrics, and ionization and vacancies produced by these ions. The
output of the program for vacancies produced by the ions and recoils is in units of
vacancies/Å-ion, which is converted to vacancies/cm3 by multiplying with ion fluence
(ion/cm2), used in experiments. An example of such analysis is provided in Figure 3.9
for 3 MeV helium ions in SiC for three different doses
3
11
17
Vacancies(x10 )/cm
26
-2
1x10 cm
10
-2
3x10 cm
10
-2
1x10 cm
1.6
1.2
0.8
Beam direction
0.4
0
2
4
6
8
10
Depth (µm)
Figure 3.9: Vacancies calculated by using SRIM for different fluences of 3 MeV He+ in SiC.
3.5.2 TCAD simulation tool
Several physical device simulation tools have been designed to understand the device
operation. Synopsis Sentaurus technology computer aided design (TCAD) is one of the
commercially available tools used for process and device simulations of semiconductor
devices [13]. It utilizes physical models based on basic semiconductor theory and
simulates the electrical behavior of semiconductor devices. In this thesis, TCAD is
used to calculate the ideal CV curve, to extract dielectric parameters from CV curves,
and full BJT device simulations to understand the influence of bulk traps at different
positions in the device. In general, BJT simulations are performed by using a device
with geometry and doping of the actual BJT used in experiments, and considering
room temperature conditions. Steady state simulations are executed by including
Shockley-Read-Hall (SRH) [61, 62] and Auger recombination models for the bulk and
interface. Doping and field dependent mobility, incomplete ionization and bandgap
narrowing models were also incorporated in the simulations [63]. Fermi-Dirac statistics
are considered for the carrier concentrations. All simulations for BJTs are based on a
3.5 Simulation tools
model presented in Ref. [64]. Figure 3.10 presents an example of simulated
electrostatic potential distribution within the device at 5 V as a bias voltage between
the base and emitter contact, and collector voltage at 6 V.
Figure 3.10: Simulated electrostatic potential distribution in BJT. The data presented here is in
log scale.
27
28
Characterization techniques and simulation tools
Chapter 4
4H-SiC BJTs subjected to ionizing
radiation
Bipolar junction transistors (BJTs) have a potential for operating at high current
densities with low power losses and high voltage capability. However, in many power
electronic applications, BJTs have been replaced by insulated gate bipolar junction
transistors (IGBTs) and power metal oxide semiconductor field effect transistors
(MOSFETs), due to relatively low current gain at typical operating current levels. With
the introduction of SiC as a wide bandgap semiconductor and suitable for harsh
environments, SiC-based BJTs are considered for high power applications due to fast
switching capabilities and low on-state resistance.
In this chapter, the degradation of BJTs due to the formation of crystal defects as a
result of interactions with energetic particles is investigated. The crystal defects result
in deep states in the SiC forbidden energy gap where carriers recombine, thereby
affecting the electrical properties of the device. The impact of ionizing radiation on the
SiC BJT performance is studied by implanting ions of various energies and doses on
fully functional devices. Due to the different stopping mechanisms for ions in matter
(described in Chapter 2) it has been possible to introduce a certain amount of defects at
a specific location in the device. By analyzing the degradation of the electrical
performance of the devices exposed to protons, helium and carbon ions, with selected
energies and doses, it is possible to identify different mechanisms involved in the
degradation.
4.1 Device structure
The devices used in the present work are epitaxially grown npn BJTs designed for
2700 V breakdown voltage and an active area of 0.04 mm2 [65]. Figure 4.1 shows a
cross-sectional view of the BJT. The devices are composed of N+ emitter with
thickness of 1.33 µm and doped with nitrogen at 1×1019 cm-3 level. An additional 200
nm layer with higher doping level (Nd = 4×1019 cm-3) is used for low resistive emitter
30
4H-SiC BJTs subjected to ionizing radiation
contact. The aluminum doped epitaxial base layer is 730 nm thick with Na = 3×1017
cm-3. The collector epitaxial layer with N- doping (Nd = 3×1015 cm-3) is grown on a
highly doped (Nd = 1×1018 cm-3) substrate. A SiO2 passivation layer was used to cover
the area between the emitter and base contact. The same passivation was used for the
edges beyond the emitter and base contacts to avoid early breakdown of the device.
The SiO2 passivation is processed by growing ~50 nm high quality SiO2 in N2O
environment, and then 2 µm SiO2 is deposited by PECVD to protect the thin oxide
layer. The devices were fabricated in house with a highest current gain value of 50.
Figure 4.1: Schematic cross-sectional view of 4H-SiC bipolar junction transistor.
4.2 Device simulations
A theoretical analysis using Sentaurus TCAD is conducted to understand the effects of
ionizing particle radiation on SiC-based BJTs. As explained in detail in Chapter 2 of
this thesis, the ionizing particles lose energy due to electronic stopping, that induce
ionization and excitation in the electronic system, and nuclear stopping, which leads to
non-uniform generation of crystal defects. The devices are modeled both with respect
to the ionization effects in the passivating oxide by considering surface concentrations
of positive charges, and nuclear scattering damage by introducing bulk traps in
different regions of the device.
4.2.1 Ionization studies
Dielectric passivation in BJT is a critical area that directly affects the device
performance. Ionizing radiation affect the passivating oxide by producing positive
charge [66], which leads to the degradation of current gain. SRIM simulations show
that the electronic stopping process in the energy range where ions stop at, or around
the SiO2/SiC interface, produce ionization in the oxide that results in increased positive
charge concentration inside the dielectric and leads to the degradation of the device.
The stopping processes and ionization effects in SiO2/SiC layers are shown in Figure
4.2. The effect of positive charge on device performance is studied with the help of
device simulations to understand the degradation of current gain (β=Ic/Ib), i.e. increase
of base current. The effects of trapped oxide charge are modeled by varying the
positive surface charge density from 5×1011 to 1×1012 cm-2 at the SiO2/SiC interface.
4.2 Device simulations
Depth (µm)
10
1
10
0
10
0.5
1.0
1.5
2.0
Electronic Stopping
Nuclear Stopping
Ionization
Beam direction
-1
SiO2 SiC
600
400
200
0
10
4
10
2
10
0
Number of Helium ions
dE/dx (eV/Å)
0.0
Ion Energy (keV)
Figure 4.2: Electronic and nuclear stopping calculated by TRIM, and numerically simulated
ionization produced in the material (left y-axis). The statistical distribution of ions is also
simulated by SRIM (right y-axis) [Paper V].
Device simulations reveal that the positive charge in the oxide can strongly influence
the device performance. The simulation results show that a low concentration of
charges, such as 5×1011 cm-2, slightly increase the current gain, but a further increase in
the amount of surface charge starts reducing the maximum current gain of the device.
The results are presented in Figure 4.3a. Further investigation into the device
characteristics by simulating Gummel plots uncovers the fact that the drop in the
current gain is due to a large increase in the base current while the collector current is
less influenced. The increase in the base current can be understood by analyzing
surface recombination. Lower surface recombination, due to lower hole concentration
caused by the positive fixed charge, increases the current gain for a concentration up to
5×1011 cm-2. But, when the charge concentration is increased to 1×1012 cm-2, although
the surface recombination decreases, the electron concentration becomes larger than
the ionized acceptor concentration, and this creates an inversion layer between the
-3
40
20
10
(a)
0
2.6
2.8
3.0
Vbe (V)
No charge
11
-2
5x10 cm
11
-2
7x10 cm
12
-2
1x10 cm
-4
10
Current (A/µm)
30
Gain (β)
10
No charge
11
-2
5x10 cm
11
-2
7x10 cm
12
-2
1x10 cm
3.2
-5
10
-6
10
IC
IB
-7
10
-8
10
-9
(b)
10
2.6
2.8
3.0
3.2
Vbe (V)
Figure 4.3: (a) Current gain versus base-emitter voltage with no charge and with three different
concentrations. (b) Simulated base and collector currents after introducing surface charge at the
SiO2/SiC interface [Paper V].
31
32
4H-SiC BJTs subjected to ionizing radiation
emitter region and the base contact [67]. Therefore, the base current largely increases
(Figure 4.3b), leading to gain degradation due to positive charges in the passivating
oxide.
4.2.2 Bulk traps
In addition to ionization in the oxide, ionizing particles produces material defects
mainly around their end of range due to the dominance of nuclear scattering processes.
These defects lead to deep states in the forbidden energy gap where carriers recombine.
It has also been shown that nitrogen donors in SiC are deactivated [68], as a result of
energetic ions’ induced defects. Both of these effects lead to a reduction of charge
carrier densities. Higher concentration of defects may also lead to reduced carrier
mobility. As a result, a highly resistive layer in the material is produced, which has also
been confirmed by SSRM analysis in Figure 2.5.
The material defects can also be introduced during device fabrication (described in
Section 2.1). These defects also have a negative influence on the device performance.
In the device simulator, material defects can be modeled by introducing traps in the
device to obtain an insight into the device behavior in the presence of defects/traps.
Additionally, bipolar degradation could be understood from this type of study, which is
mainly caused by stacking faults already present in the material. Stacking faults can
also be induced due to ion irradiation and lead to the degradation of the device. A
theoretical investigation is conducted to quantify the effect of traps located at different
positions in a SiC BJT. The study is specified by introducing a 500 nm thick layer of
traps (Lt) at different positions in:
1) emitter at different distances from base-emitter junction.
2) collector at different distances from base-collector junction.
3) base.
The study is conducted for different values of capture cross-sections for electrons
and holes (σn,p = 1×10-14, 1×10-15 and 1×10-16 cm2), and traps concentrations in Lt,
ranging from 1×1015 to 9×1015 cm-3. The capture cross-section values are taken from
previous experimental studies [34, 69, 70]. The results from simulations for different
regions of BJT are summarized in the following sections.
a) Emitter
Degradation due to traps in the emitter is dependent on their position in the emitter
region. A maximum reduction in current gain occurs when the traps are introduced near
the emitter-base interface. In the present study, Lt was positioned at 5, 20, 100 and 500
nm distance from the base interface in the emitter. A maximum current gain drop, i.e.
20%, appears when the traps are in 5 nm proximity to the interface. The effect reduces
when the position of traps is moved away from the emitter-base interface, i.e. towards
the emitter contact. Figure 4.4a shows IV characteristics of the emitter with trap layers
(Lt) at different locations in the emitter for acceptor type trap concentration of 5×1015
cm-3 with capture cross-section of 1×10-15 cm2. The IV characteristics show that the
traps in the emitter region do not strongly affect the on-state resistance. Simulations are
performed for acceptor and donor type traps, but the degradation in device performance
4.2 Device simulations
appears only due to acceptor type traps. The concentration of traps has also been
observed as an important parameter that controls the device output. Moreover, the
impact of electron and hole capture cross-sections on the collector saturation is very
evident, and the gain drops more for larger cross-section values, shown in Figure 4.4b.
The excess carriers (electrons) flowing through the emitter at a certain bias voltage are
trapped in Lt, giving rise to recombination rate in the trapped region (Figure 4.8e) and
increasing the local concentration of electrons. As long as the trapped electrons are
close to the base, the increased recombination rate in Lt gives rise to the base current,
which leads to the degradation of current gain. In a similar way, when the
concentration of traps is increased, i.e. more trapped electrons, a drop in the current
gain occurs due to an increased rate of recombination in Lt. An important factor for the
low degradation in gain is the emitter doping level which is of the order of ~1019 cm-3.
Therefore, it can be deduced that the emitter region is less sensitive for the degradation
of the BJT due to the presence of traps.
0.08
60
(a)
(b)
50
0.06
0.04
Reference
500 nm
100 nm
20 nm
5 nm
0.02
0.00
Gain (β)
Ic (A/µm)
40
0
1
2
Vce (V)
30
Reference
-16
2
1x10 cm
-15
2
1x10 cm
-14
2
1x10 cm
20
10
3
2
3
4
5
Vbe (V)
Figure 4.4: (a) IV characteristics for different distances of Lt in emitter. 4 mA of base current
used in these simulations. (b) Gain versus base-emitter voltage for 5×1015 cm-3 concentration of
traps for different cross-section values for Lt at 5 nm in emitter [Paper II].
b) Collector
The collector layer in a BJT plays an important role for the current-voltage properties
of the device, for instance, for determining the on-state resistance. Therefore, carrier
traps in this layer can influence the overall characteristics of the device. The traps at
different location in the collector have different influence, specifically the effect of
traps near the base-collector junction is important to understand. For this purpose, Lt
with different concentrations is studied at 5, 10, 20, 100, 500 nm and 1, 2 and 5 µm
distance from the base-collector interface in the collector. The study reveals that the
traps near the base-collector interface do not strongly affect the device characteristics
by trapping electrons. However, as Lt is moved away from the interface, a region with
increased resistance appears before saturation of the collector current at a certain base
current appears, and the effect becomes more prominent at the depth of 2 µm (Figure
4.5a). Figure 4.5b shows the effect of concentration of traps at 1 µm distance from
collector-base junction. The main reason for this behavior is the barrier in the flow of
electrons due to trapped electrons in accepter type traps in Lt, which increases the local
33
4H-SiC BJTs subjected to ionizing radiation
recombination rate (Figure 4.8e). Again the effect of donor traps is negligible, since the
excess carrier are only electrons. The gain of the BJT is not affected in this case by the
location of traps, however, the trap concentration have a considerable effect on the
forward voltage drop.
0.08
0.08
Reference
20 nm
100 nm
500 nm
1 µm
2 µm
5 µm
0.04
0.06
Ic (A/µm)
0.06
Ic (A/µm)
0.02
0.04
0.00
Reference
15
-3
1x10 cm
15
-3
3x10 cm
15
-3
5x10 cm
15
-3
7x10 cm
15
-3
9x10 cm
Increase in
resistance
0.02
(a)
0
1
2
(b)
3
0.00
0
1
2
Vce (V)
3
4
5
6
Vce (V)
Figure 4.5: (a) Traps at different depths in collector. (b) Effect of increasing concentration of
traps at 1 µm deep in collector with σn,p=1×10-15 cm2 (capture cross-section for electrons and
holes has been considered to be same in all simulations). The base current used in these
simulations is 4 mA [Paper II].
For comparison with experimental results (discussed in Section 4.3.2) of implantation
in the collector, two separate simulations are performed:
1) Introducing a 2 µm thick lowly doped n-type region (5×1013 cm-3) in the
collector around the projected range, as estimated by SRIM [12]. This
simulates the deactivation mechanism of nitrogen donors [68].
2) Introducing various trap concentrations in the same region with different
electron capture cross-sections. This simulates the compensation mechanism.
25
Reference
20
Ic (µA)
34
σn,p= 5x10
15
-15
cm
15
Nt = 4x10 cm
10
-3
Reduced doping in collector
13
-3
(5x10 cm )
5
0
2
0
1
2
3
Vce (V)
Figure 4.6: Comparison of simulation 1 and 2. Simulation 2 shows results for the capture crosssection of 5×10-15 cm2 and traps concentration of 4×1015 cm-3. The base current used in all
simulations is 1 µA [Paper II].
4.2 Device simulations
The results from simulations, presented in Figure 4.6 indicate that the trap
concentration of 4×1015 cm-3 with electron and hole capture cross-section of 5×10-15
cm2 [34] follow the same trend as it can be obtained by introducing a lowly doped
(5×1013 cm-3) layer in the collector. This comparison suggests that the carrier
concentration in the highly damaged region in the collector is reduced after introducing
traps, although it is not possible to say if the lower carrier concentration is due to the
deactivation of nitrogen dopants, or a high concentration of traps resulting in increased
trapped electrons in Lt and producing compensation of carriers. It has been seen in the
simulations that the region, where traps are introduced (or the doping is reduced), the
flow of current through the collector is reduced and limits it to a certain level. As a
result, a higher resistance is observed in the transistor characteristics.
c) Base
A lightly doped thin base layer is a very sensitive region of the BJT and it has direct
influence on the gain of the device. In addition to the surface affects, which have been
studied previously [71], the bulk defects also have a strong influence on the overall
properties of the device. When there are some intrinsic defects such as material defects,
introduced during growth, or extrinsic defects produced in the base during device
processing or as a result of radiation exposure, the current gain of the device is
negatively affected. In the present work, the effects of both acceptor and donor type
traps with the same parameters as used for the emitter and collector study, are studied
by introducing traps in the whole base region. Unlike the emitter and collector, donor
and acceptor type traps show approximately the same degradation effect for the gain of
the device. The concentration of traps (shown in Figure 4.7) and capture cross-section
of the traps in base region strongly affect the output. The influence of the capture
cross-section for the carriers is even stronger in the base compared to the emitter and
collector, and a strong degradation appears for smaller cross-sections. The results show
that the highest concentration of the traps (9×1015 cm-3) reduces the gain down to about
67%. In general, when the base is filled with traps, the trapped carriers increase the
recombination rate in the base (Figure 4.8e). As a result, the flow of current is reduced,
which results in the degradation of the current gain.
60
Reference
15
-3
1x10 cm
15
-3
3x10 cm
15
-3
5x10 cm
15
-3
7x10 cm
15
-3
9x10 cm
50
Gain (β)
40
30
20
10
0 -6
10
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
Ic (A)
Figure 4.7: Gain reduction due to increasing traps concentration in base. The value for capture
cross-section (σn,p) for the present simulation is chosen to be 1×10-15 cm2 [Paper II].
35
36
4H-SiC BJTs subjected to ionizing radiation
Recombination due to traps at different locations
a)
b)
Lt at 5 nm in emitter
Traps in base
c)
d)
Lt at 10 nm in collector
Lt at 2 µm in collector
(e)
Figure 4.8: (a)-(d) Total recombination in BJTs by introducing Lt in different regions of the
device. The recombination rate, shown in (e), has been extracted by taking vertical cut for
simulated structures, at a position mentioned in the inset of (e) for a reference device. The
concentration of traps was fixed at 5×1015 cm-3 and the capture cross-section at 1×10-15 cm2
[Paper II].
4.3 Ion irradiation of BJTs – Experimental results
4.3 Ion irradiation of BJTs – Experimental results
The bipolar junction transistor (BJT), as shown in Figure 4.1 (schematics), comprises
of at least three epitaxial layers, where the top n-epi is etched down to the p-base,
leaving emitter mesas. There are also various metallization and passivation layers
involved in the BJT fabrication. These different layers will have different stopping
powers for the ionizing particles, which limit the precision in controlled ion beam
experiments. However, the effect of radiation on passivation layer can be understood
with the help of such experiments, since dielectrics are the top most layers in BJTs and
the energy of the implantation ions can be tuned with the help of SRIM simulations
[12], to produce damage in the passivation or at the interface. However, since the
collector is relatively thick (20 µm), the effects of ionizing radiation damage can also
be studied in this region of the device, although with some loss in precision due to the
different thickness of top layers.
In these experiments, different chips processed on a same wafer are used for
different doses, and each chip contained different geometry of devices. However, the
measurements are performed on the same type of devices on the chip to acquire better
statistics and to see the real impact of ionizing radiation on BJTs. The results from
these experiments are summarized in the following sections.
4.3.1 Implantation in dielectric passivation layer
Ionizing radiation effects in SiO2 passivation layers of BJT are studied by implanting
600 keV helium ions in fully fabricated devices on different chips. A wide range of ion
fluences, 1×1012 to 1×1016 cm-2, is used to see the dose effect. The energy of the ions is
chosen after an analysis by SRIM [12]. In these experiments, helium ions (α-particle)
are chosen for being inert gas ions and having no contribution to electrical properties of
the device. To understand the depth of the ions in the device, SRIM simulations are
performed by considering the thickness of the oxide layer, covering the area between
emitter and base contacts and beyond.
1.2
1.0
β/βmax
0.8
0.6
Unimplanted
15
-2
1x10 cm
15
-2
5x10 cm
16
-2
1x10 cm
0.4
0.2
2.50
2.75
3.00
3.25
3.50
Vbe (V)
Figure 4.9: Normalized current gain (at 3 V) versus base-emitter voltage for different
implantation fluences in comparison to the unimplanted device gain [Paper V].
37
Current (A)
4H-SiC BJTs subjected to ionizing radiation
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
Unimplanted
12
-2
1x10 cm
13
-2
1x10 cm
15
-2
1x10 cm
15
-2
5x10 cm
16
-2
1x10 cm
Collector
15
5x10 cm
-2
Base
2.50
2.55
Before radiation
After radiation
2.60
2.0
2.5
3.0
3.5
Vce (V)
Figure 4.10: Base and collector current before and after implantation of 5×1015 cm-2 ion fluence.
Inset shows an increase in base current after implantation at different fluences [Paper V].
The helium ions with this energy stop near, or at the interface, as shown in Figure
4.2, producing ionization in the oxide and nuclear damage at the interface, which
results in the degradation of the current gain of the device depending on the fluence of
ions (shown in Figure 4.9). As it has been observed in simulation studies for positive
charge at the interface, the rise in the base current is the primary cause for degradation
of the gain. An increase in collector current also occurs, however, it is two orders of
magnitude less than the increase in base current. Furthermore, the increase in the base
current is also fluence dependent. The results are shown in Figure 4.10. An analysis
based on the collector current versus collector-emitter voltage reveals that the collector
saturation current also reduces as a result of positive charges in the oxide.
The annealing of these devices in vacuum for 30 mins at the temperatures of 300,
420 and 500 oC do not show any recovery. Instead, it affects device properties
negatively. The collector saturation current also drops as the annealing temperature is
raised, as shown in Figure 4.11.
Normalized collector saturation current
38
1.0
0.9
As-implanted
o
300 C anneal
o
420 C anneal
o
500 C anneal
0.8
0.7
0
10
12
10
+
13
10
14
10
15
10
16
-2
He fluence (cm )
Figure 4.11: Collector saturation current at 5 V for different ion fluences in comparison to
different annealing temperatures. Base current used here is 2 mA [Paper V].
4.3 Ion irradiation of BJTs – Experimental results
In addition to that, the Gummel measurements show that the base current is also
significantly affected as a result of the temperature treatment. The base current at low
voltage is increased as a function of temperature. The phenomenon is also fluence
dependent (Figure 4.12). The observation made here, in the case of ion implantation at
the interface, that the annealing in a low temperature range further degrades the device
instead of healing it, which is contrary to the observation of M. Nawaz et.al. [72],
where gamma rays were used as a source of ionization in the oxide. This fact refers to
the key difference in the radiation environments. Gamma rays produce ionization only
in the oxide and at the interface, which can be cured even with mild treatment of 420
o
C. On the other hand, the ions produce ionization in the oxide and the atomic
displacements leading to bond breaking at, or near the interface. The annealing of the
displacement damage at the interface further contributes as a source of positive charge
and leads to more reduction in gain, indicating the involvement of other types of traps
that are formed around the interface. Therefore, it is not possible to recover the device
properties with similar treatments. Additionally, the on-state resistance does not show
any significant fluence, or temperature dependence.
-5
Ib (A)
10
-3
Unimplanted
As-implanted
o
300 C anneal
o
420 C anneal
o
500 C anneal
10
14
-2
1x10 cm
-5
10
Ib (A)
-3
10
-7
10
-9
15
-2
5x10 cm
-7
10
-9
10
10
(a)
1.0
Unimplanted
As-implanted
o
300 C anneal
o
420 C anneal
o
500 C anneal
1.5
2.0
2.5
Vbe (V)
3.0
3.5
(b)
1.0
1.5
2.0
2.5
3.0
3.5
Vbe (V)
Figure 4.12: Base current versus base-emitter voltage for 1×1014 and 5×1015 cm-2 fluence of
ions at 300, 420 and 500 oC annealing in comparison to unimplanted and as-implanted devices.
A clear increase in base current in low voltage region (< 2.5 V) is observed and it is dependent
on the ion fluence [Paper V].
Low energy implantation (330 keV) of helium ions results in an accumulation of
defects in the oxide layer, i.e. away from the SiO2/SiC interface. These kinds of defects
temporarily distort the device characteristics, which are recoverable in few hours at
room temperature. The temporary effect is probably due to the self-heating produced in
the oxide because of implantation, which dissipates after certain time. Therefore, it can
be concluded that the defects produced in the oxide passivation do not lead to a
permanent damage of the device, as occurs for ion induced interface defects in the BJT.
39
40
4H-SiC BJTs subjected to ionizing radiation
4.3.2 Ion induced damage in the collector
In a BJT, the collector, or a drift layer, plays an important role in determining on-state
resistance of the device as shown by simulation studies in Section 4.2. The scattering
damage produced in this layer can directly influence the device output characteristics
by trapping the majority carriers or by deactivating the dopants in damaged region. In
the present work, an investigation is carried out by implanting high energy ions in the
collector region of BJTs with different fluences. The objective of the study is to test the
device tolerance and to understand the mechanisms involved in the degradation. For
this purpose, high energy helium ions (3 MeV) are implanted in BJTs so that they stop
in the collector, producing maximum damage in this region. With this energy,
implanted ions are expected to stop around 5 µm below the base-collector interface
with a spread of ~2 µm in depth. The devices are exposed to 1×1010, 3×1010 and 1×1011
cm-2 fluences of ions. The physical analysis of vacancies performed for the ion
fluences in SiC, by using SRIM, suggests that these numbers of ions produce a peak
concentration of 1.8×1016, 5.3×1016 and 1.8×1017 vacancies/cm3 at the ion end of range
respectively (shown in Figure 3.9). Clearly, the amount of vacancies is at least one
order higher than the doping level in collector (3×1015 cm-3). The electrically active
defects (5–10% of the total defects produced [5]), due to these vacancies, provide deep
levels in the bandgap that compensate the doping at high enough concentrations and
reduce the free carrier concentration. In nitrogen doped layers, it has been shown that
implantation induced defects can deactivate the nitrogen dopants, most likely by
forming secondary defects involving nitrogen, but having no shallow donor states [68].
The net result of both the compensation and deactivation mechanisms described above
is a reduction of free electrons in the damaged region and they have similar effect on
device characteristics as it has been studied and shown in Figure 4.6.
The damage in the collector causes degradation in the output characteristics of
BJTs. The drop in collector saturation current due to these fluences is strongly nonlinear. In addition to that, an extra resistance slope in the collector current appears,
which refers to an increased resistance of the collector. The Gummel plots acquired for
these devices further explain the behavior of current through the base and collector.
Generally, the gain degradation of such devices is caused by bulk material degradation
as well as by ionization in the passivation oxide layer, produced due to traversing of
ions through these regions of the device. The oxide layer or interface defects mainly
affect the base current, as it has been explained in the previous section. In these
experiments, the base current is fluence dependent and is increased by increasing the
ion fluence. Figure 4.13a shows that the base current increases both in low voltage
(recombination at the oxide interface) and in high voltage range (probably due to
defects in base). This fact indicates that the interface of the base-emitter diode and
passivation layer is affected along with lesser number of defects produced in the bulk
regions of the base and the emitter. An investigation of the collector current reveals
that the collector current as a function of base-emitter voltage (Vbe) is limited to a
certain saturation level depending on the ion fluence (Figure 4.13b). Finally, the gain is
reduced due to a decreased collector current and an increased base current.
4.3 Ion irradiation of BJTs – Experimental results
0
0
10
-2
10
-4
-2
10
-4
10
-6
Ic(A)
Ib(A)
10
10
Unimplanted
10
-2
1x10 cm
10
-2
3x10 cm
11
-2
1x10 cm
10
-8
10
-10
10
-10
10
(a)
-12
1.5
-6
10
-8
10
10
Unimplanted
10
-2
1x10 cm
10
-2
3x10 cm
11
-2
1x10 cm
(b)
-12
2.0
2.5
3.0
3.5
10
1.5
2.0
Vbe(V)
2.5
3.0
3.5
Vbe(V)
Figure 4.13: Fluence dependence of (a) base current and (b) collector current, versus baseemitter voltage at a 6 V collector-emitter voltage [Paper III].
Temperature treatment of these devices at 400, 430 and 500 oC results in some
recovery of the device characteristics but a full recovery could not be achieved. The
annealing temperature for these devices is limited to 500 oC due to the metallization
involved in the device. Higher annealing temperatures may alter the properties of the
ohmic contacts of the device. The improvement in device properties is attributed to the
reduction in an extra slope in IV characteristics due to the damage recovery of Si and C
lattice at low temperatures, as it has been discussed in Section 2.5. While looking at the
base current after annealing, the same behavior is observed as for ionization damage
annealing (Figure 4.12). The base current in the low voltage bias region (Vbe = 1.5–2.5
V) is increased after annealing and it is dependent on annealing temperature as shown
in Figure 4.14a. The increased base current in low voltage region is recognized as
increased recombination at the interface. On the other hand, for the highest fluence
implanted device, the collector current shows a slight recovery (Figure 4.14b),
however, it still limits the current saturation level.
0
0
10
-2
10
-4
-2
10
-4
10
-6
Ic (A)
Ib (A)
10
10
As-implanted
o
400 C anneal
o
430 C anneal
o
500 C anneal
10
-8
10
-10
10
-10
10
(a)
-12
1.5
-6
10
-8
10
10
As-implanted
o
400 C anneal
o
430 C anneal
o
500 C anneal
(b)
-12
2.0
2.5
Vbe (V)
3.0
3.5
10
1.5
2.0
2.5
3.0
3.5
Vbe (V)
Figure 4.14: Annealing behavior of 1×1011 cm-2 fluence implanted BJTs (a) base current and
(b) collector current, versus base-emitter voltage at a 6 V collector-emitter voltage.
41
4H-SiC BJTs subjected to ionizing radiation
As-implanted
o
500 C Annealed
Vacancies
0.6
1x10
1.5
3
0.8
1.8
1.2
17
Unimplanted
10
0.9
0.4
3x10
0.2
10
0.6
1x10
0.0
0 1010
-2
10
11
0.3
Vacancies(x10 )/cm
1.0
Normalized Gain
42
11
Implantation Dose (cm )
Figure 4.15: Vacancies produced in comparison to gain degradation and improvement after
annealing at 500 oC versus implantation doses [Paper III].
A comparison of the ion-induced vacancies, estimated by SRIM [12], and the gain
degradation for the as-implanted devices with annealed devices’ gain is shown in
Figure 4.15, which establishes that the degradation in gain is nonlinear and it is not
possible to recover the device current gain to original values at these low temperatures,
although, a small improvement is observed. One possibility is that the vacancies of the
order of doping level increase the complexity of the defects and produce stable defects,
which are not possible to recover at these temperatures.
In addition to above experiments, another study is conducted by using
commercially available 4H-SiC BJTs manufactured by TranSiC, Sweden [10]. A
complete description of the device has been provided in Ref. [73]. In these
experiments, the un-encapsulated devices are exposed to 2.3 MeV protons, with and
without an absorbing Al foil, and 5, 10, or 15 MeV 12C ions. According to SRIM
simulations [12], protons without Al foil would stop in the substrate leaving mostly
ionization damage in the whole device. With absorbing foil, the range of ions reduces
to 10 µm in SiC and they end up in collector region of the device. The 5 MeV 12C ions
would stop in the metallization or passivation layers, while the 10 and 15 MeV 12C ions
would penetrate 2–3 and 6–7 µm, respectively, and end up in the upper part of the
collector. The fluence range chosen for these experiments is relatively low in
comparison to the previously discussed experiments. The fluences for protons are
varied between 1×109 and 5×1012 cm-2, while the carbon fluences range from 5×106 to
5×108 cm-2.
The results from these experiments suggest that the output characteristics (Ic versus
Vce) are very sensitive to implantation induced damage in the device, and even at these
extremely low fluences, there is a clear reduction in gain after the implantation
compared to the unimplanted devices and the reduction is fluence dependent. The
highest fluence of 10 MeV 12C (1×108 cm-2) reduces ~25% of the collector saturation
current. Other characteristics of transistors such as leakage, breakdown and opencollector characteristics are not affected by these implantations. The damage produced
deep in the devices (away from base-collector interface), due to high energy ions, i.e.
4.3 Ion irradiation of BJTs – Experimental results
15 MeV 12C and proton implantation show similar results, however, higher fluences are
required for these beams to reach a similar gain reduction level. The devices implanted
with 5 MeV 12C do not show any significant reduction in device properties. Some
results from these experiments are summarized in Figure 4.16a. The proton
implantations with Al foil show strong degradation for fluences higher than 1010 cm-2
since the damage stays in the collector; nevertheless, implantations without Al foil
(damage in substrate) require higher fluences to degrade the devices. The results
obtained from this study, for low fluences, are attributed to increased number of defects
in the base region and increased density of surface states at the interface between the
passivation layer and the base. On the other hand, higher fluences create compensating
defect concentrations in the collector.
An annealing of some implanted devices show full recovery already at 420 oC.
Figure 4.16b shows an annealing behavior of 10 MeV 12C implanted BJTs with 1×108
cm-2 (1.1×1015 vacancies/cm3), which can be due to the reduction of defects at low
temperature annealing, and the remaining defects have less contribution to the
electrical properties of the device.
6
2.5
5
Ic (A)
4
7
5x10 cm
8
1x10 cm
2
1
-2
(a)
1
2
Vce (V)
-2
2.0
-2
3
0
8
1x10 cm
Before
After
-2
Ic (A)
7
1x10 cm
3
4
1.5
Unimplanted
As-implanted
o
350 C anneal
o
400 C anneal
o
420 C anneal
1.0
0.5
(b)
0
1
2
3
4
Vce (V)
Figure 4.16: (a) IV characteristics of 10 MeV carbon implanted BJTs with different fluences.
The curves shown here correspond to different base currents (b) Annealing behavior of 10 MeV
12
C implanted BJTs with an ion fluence of 1×108 cm-2. The devices were annealed for 30 min at
300, 400, and 420 oC. The statistical variations on a chip are about 5% and around 10% between
different chips [Paper IV].
In summary, the scattering damage introduced due to high energy ionizing radiation
in bulk SiC in BJT reduces the performance of the device. The degradation of device
characteristics is ascribed to the electrically active defects in SiC. The low
concentration of defects can be healed at low temperatures, however, higher
concentrations (of the order of doping level or more) create high concentration of
defects/traps that compensate the doping or deactivate the dopants and results in more
thermally stable damage in the device. The degradation produced due to the ionizing
radiation in the passivation may possibly be reduced by replacing the passivation with
high-k dielectrics, such as Al2O3, since the interface of such dielectrics is far better than
SiO2 due to reduced interface states [74]. A detailed investigation of the dielectric
layers for 4H-SiC in the perspective of passivation and their radiation hardness is
presented in Chapter 5.
43
44
4H-SiC BJTs subjected to ionizing radiation
Chapter 5
Dielectrics for 4H-SiC – Passivation
and radiation hardness
Dielectrics are an essential part of electronic devices, both as passivation and insulation
materials for most of the devices, or as gate dielectrics in MISFETs. Passivation is one
of the most critical issues for the reliability of semiconductor devices and it becomes
even more important in high power, high temperature and radiation rich environment
applications. The choice of dielectric for SiC strongly influences the performance of
devices [75, 76]. In current SiC technology, SiO2 is an attractive candidate for
passivation and gate dielectric, but SiO2 has ~2.5 times lower dielectric constant than
SiC itself, and therefore it may limit the performance of SiC devices. In order to utilize
the full potential of SiC devices, a suitable dielectric material to passivate surfaces and
edges in the device structures is required. The basic properties that are required for a
dielectric to be an alternative choice are:
1)
2)
3)
4)
low interface state density.
high dielectric breakdown strength.
large conduction and valance band offsets compared to the semiconductor.
high thermal stability.
In addition to that, the dielectric material should have a high dielectric constant and
be easy to deposit, for the passivation of SiC devices [75]. Several different materials
have been studied for SiC passivation [58, 77-79], however, aluminum-based materials
have shown promising results regarding interface quality [74] and thermal stability.
Aluminum-based dielectrics have also been demonstrated for the passivation of
Schottky [76] and PiN diodes [11].
In this chapter, Al2O3, AlN and Ta2O5 are studied as possible alternate dielectrics
for SiC. The dielectrics are studied as single layers but also in stacked arrangement.
The radiation hardness of Al2O3 in comparison to SiO2 is also demonstrated under the
influence of ionizing radiation. A comparison of bandgap and band offsets for different
materials used in the present thesis is given in Table 5.1.
46
Dielectrics for 4H-SiC – Passivation and radiation hardness
Table 5.1: Properties of the high-k dielectric materials, studied for the passivation, in
comparison to Si and 4H-SiC [27,75,80-84].
Material
εr
Bandgap
(eV)
ΔEc
Breakdown
Field (MeV/cm)
Si
11.7
1.12
-
0.3
4H-SiC
9.66
3.23
-
3
SiO2
3.9
9.0
2.7
10
Al2O3
8
8.8
1.7
>5
AlN
9.14
6.03
1.7
1.2-1.8
Ta2O5
25
4.4
-0.4
2-4.5
5.1 Processing of MIS structures
In this thesis, the dielectrics are studied by fabricating metal insulator semiconductor
(MIS) structures. The basic device structure is shown in Figure 5.1. The 4H-SiC
substrates used in all investigations are highly doped n-type wafers purchased from
SiCrystal AG. Epitaxial layers on these wafers were grown at ACREO AB with
thickness and doping level as mentioned below:
1) 15 µm, 3-5×1015 cm-3 n-type on chemical-mechanical polished (CMP) epiready surface
2) 8-10 µm, 5×1015 cm-3 n-type on Si-face polished (no CMP)
3) 5-7 µm, 1×1017 cm-3 p-type on Si-face polished (no CMP)
In addition to 4H-SiC, an n-type Si wafer is used with highly conductive substrate
(0.025-0.05 Ω-cm) and an n-type epi-layer of 8-10 µm of thickness doped with
phosphorus. The carrier concentration in the Si epi-layer was about ~1014 cm-3.
Figure 5.1: Schematic of MIS structure.
For the dielectric studies, the wafers were cut into small 1 cm2 pieces. In the case of
SiC, the back side contact for SiC is provided by nickel silicide formed by sputter
deposition of Ni (200-300 nm) followed by RTA at 950 oC for 60 secs in Ar
environment, whereas for Si samples, Al with a thin intermediate layer of Ti/W
deposition is used as a backside contact.
5.1 Processing of MIS structures
The top contact on the dielectrics was initially formed by evaporating a 100 nm
layer of Ni using a lift-off process for patterning. A positive mask was used in this
process. However, the outline of the contacts was found to be irregular in microscopic
analysis and some of the contacts were removed during the lift-off process. Therefore,
in later experiments, a negative mask was designed and the top contact material was
replaced with Al. Finally, a wet-etch for aluminum was used to pattern the contacts
from the evaporated Al layer. Figure 5.2 shows a difference in the top contacts between
the two processes. The masks prepared for the lithography process contained several
sizes of circular openings for contact pattering. The sizes of the openings were 50, 100,
150, 200, 250, 300, 350, 400 and 500 µm.
Figure 5.2: (left) Lift-off process for Ni as top contact. (right) Improved process by using Al as
top contact and wet-etch, blank part of the sample is spared for optical measurements (FCA).
5.1.1 Dielectric formation
a) SiO2
Two different processes were used to form SiO2 on SiC. At first, a 50 nm thick SiO2
was prepared by oxidizing SiC at 1250 °C in N2O for 8 hours. Secondly, a better
process with low Dit values was adapted by using PECVD deposition of SiO2 at 300 oC
followed by a post-oxide anneal in N2O for 60 mins at 1150 oC [85]. In some
experiments, prior to high-k dielectric deposition, a thin layer of SiO2 (8 nm) on SiC
was grown by oxidizing SiC in dry O2 at 1100 oC followed by 950 °C post-oxidation
annealing in wet O2. SiO2 layer on Si was obtained by dry oxidation in O2 at 1100 oC
for 18 min. The oxide thickness obtained after this process was 53 nm.
b) High-k dielectrics using Atomic Layer Deposition
The method of deposition of dielectric layers is an issue of critical importance. In this
respect, many different techniques have been studied for high-k dielectric deposition on
SiC, for instance, reactive radio frequency magnetron sputtering [86], ultrasonic spray
pyrolysis, and atomic layer deposition (ALD) [78]. Among all of them, ALD has
proved to give a superior layer quality and low content of interface states between the
high-k dielectric and the semiconductor. However, while depositing dielectric layers
using ALD, the process parameters such as, temperature in the chamber, pulse rate for
precursors and surface preparation play an important role in the quality of the layers
[87]. In the present work, thermal ALD process is used for all high-k dielectric film
depositions, and the pulse rate and temperature of the chamber is optimized to obtain
good quality layers.
47
Dielectrics for 4H-SiC – Passivation and radiation hardness
For Al2O3 depositions on SiC, trimethyl alumina (TMA) is used as a main source
for aluminum, and H2O or ozone was used as an oxygen source. The process
temperature was set to 250 oC or 300 oC in different depositions. Ozone pre-treatment
is mostly used while depositing Al2O3 by using ozone as a source of oxygen.
AlN films on SiC were deposited by using AlCl3 and ammonia at 500 °C, and the
multilayer structures combining Al2O3 and AlN were formed by using AlCl3 and
varying water or ammonia, as needed for the chemical reaction to form Al2O3 or AlN
respectively.
The high-k dielectrics were initially deposited at the Department of Chemistry,
University of Helsinki in Finland, and at later stage of the work, a newly installed ALD
facility at KTH was utilized for Al2O3 depositions. A process optimization at KTH was
carried out by depositing similar thickness of Al2O3 on SiC to see the contamination
level and the thickness of the deposited films. The analysis was carried out by using
RBS on both films of same thicknesses. Figure 5.3 shows a comparison between two
films deposited at different places using the same precursors, and it has been seen that
the deposition at new ALD facility at KTH is as good as it is in Helsinki regarding the
presence of unwanted elements in the deposited film.
4000
Helsinki
KTH
3000
Yield
48
C
2000
O
Si
Al
1000
0
100
200
300
Channel No.
Figure 5.3: 50 nm Al2O3 deposited in Helsinki and KTH. A perfect match in the spectra is seen.
5.2 Characterization of the MIS structures
5.2.1 AlN
In the present thesis, aluminum nitride (AlN) is investigated as one of the alternate
passivation candidates for 4H-SiC [88]. AlN normally forms a polycrystalline structure
when it is deposited by ALD [77], which unfortunately enables current leakage through
the dielectric. In order to suppress the phenomenon and to find a reliable and superior
quality AlN-based dielectric, some efforts have previously been made, such as adding
hydrogen in AlN [89], or adding oxygen to AlN to make AlON compound [79],
however, these solutions complicate the process. Therefore, the direct deposition of
AlN on 4H-SiC is investigated by using ALD process. Figure 3.3 shows an x-ray
5.2 Characterization of the MIS structures
diffraction pattern of one of the ALD-deposited AlN films on SiC with a dominant
peak in the [002] direction. The MIS structures prepared with direct deposition do not
even show any capacitance in the measurements due to high current leakage through
grain boundaries of AlN. A thin intermediate layer of SiO2 with 8 nm thickness is
introduced in some samples to improve the band offset between SiC and AlN and to
prevent the leakage. However, this thickness is too thin and does not work effectively,
although the samples at least show some CV behavior with current leakage in
accumulation region, as it can be seen in Figure 5.4. A thicker layer (40 nm) of SiO2
[86] instead, is more effective and even raises the breakdown values by preventing
leakage through the whole stack of dielectrics. Therefore, it can be concluded that, in
comparison to the samples with direct deposition of AlN on SiC, the presence of an
intermediate thin SiO2 presents a significant difference in capacitive behavior of the
dielectric. In these experiments, thicknesses of 50 and 150 nm of the dielectrics are
also examined, but it does not influence the overall behavior of the electrical
characteristics.
2.0
Leakage in accumulation
Capacitance (x10
-11
F)
2.5
1.5
1 kHz
10 kHz
100 kHz
1 MHz
1.0
0.5
-4
-2
0
2
4
6
8
10
Voltage (V)
Figure 5.4: 50 nm AlN on SiC. The measurements were performed in darkness from
accumulation to inversion. The sweep rate was set at 0.1 V/sec [Paper VI].
5.2.2 Al2O3
Aluminum oxide (Al2O3) is one of the strong candidates for SiC passivation and is
normally considered to be an amorphous material. One serious problem that often
appears in Al2O3 depositions is the charge in the oxide layer, which originates from the
local variation of aluminum and oxygen content during the ALD process and also
depends on the thickness of the deposited Al2O3 [90]. The surface preparation prior to
the deposition affects the amount of fixed charge, as well as the interface states, and
results in a flat band voltage shift. In the present work, two major approaches are
followed; firstly, by depositing Al2O3 directly on the SiC right after removing the
native oxide by HF treatment, and secondly, growing an intermediate thin layer (8 nm)
of SiO2 prior to the deposition of Al2O3. Figure 5.5 shows a comparison between the
results obtained for both types of dielectric layers. For these samples the Al2O3
deposition is performed simultaneously at 300 oC.
49
Dielectrics for 4H-SiC – Passivation and radiation hardness
1.5
1.5
-2
12
1 kHz
10 kHz
100 kHz
κ = 7.71
0.9
-11
1.2
F)
Qeff = 5.4x10 cm
Capacitance (x10
-11
F)
10
Capacitance (x10
0.6
0.3
(a)
-8
-4
0
4
-2
Qeff = 1.4x10 cm
1.2
1 kHz
10 kHz
100 kHz
0.9
κ = 7.21
0.6
0.3
8
(b)
-8
-4
Voltage (V)
0
4
8
Voltage (V)
Figure 5.5: (a) No intermediate oxide. (b) With an intermediate layer of SiO2. The values for
effective fixed oxide charge and dielectric constant, extracted from the measurements, are also
presented. The depositions are performed at the University of Helsinki.
These results clearly show that the dielectric film deposited directly on SiC shows
better electrical properties due to less amount of fixed charge. In addition, the
hysteresis in direct deposition is relatively less than the other deposition, indicating a
lower number of near interface states. The RBS analysis of one of these deposited
films, shown in Figure 5.6, reveals that the film is not contaminated with any other
element from ALD precursors. The thickness of the film is also confirmed by this
analysis. It should also be noted that the Ni, seen in the spectrum is from the top
contact of the film, which is hard to avoid due to the size of the probing helium ion
beam.
6000
C
Yield
50
O
4000
Si
2000
Al
Ni
0
100
200
300
400
Channel No.
Figure 5.6: RBS analysis of 150 nm of Al2O3 on SiC. Ni peak observed in the spectra is from
the top contact of MIS.
The current-voltage (IV) measurements on 50 nm thick Al2O3 reveal that the
current leakage prior to breakdown of this oxide is relatively different in comparison to
typical SiO2 characteristics (Figure 3.7). The leakage behavior of Al2O3 is explained in
5.2 Characterization of the MIS structures
Figure 5.7, showing different phenomena happening during the breakdown of Al2O3.
At relatively low electric field (2.5 to 4 MV/cm), Fowler-Nordheim tunneling is a
dominating leakage mechanism, then a plateau appear which is referred as the phonon
assisted tunneling [91]. In the later stage, i.e., above 8 MV/cm, Frenkel-Poole emission
of electrons dominates and leads to the final breakdown of the dielectric. This stepped
breakdown of Al2O3 could be a reliability concern in the long-term operation.
Therefore, other techniques should be implemented to suppress this breakdown
phenomenon.
10
3
10
1
2
Current Density J (A/cm )
Breakdown
10
-1
10
-3
10
-5
10
-7
10
-9
Frenkel-Poole emission
Phonon-assisted
tunneling
Blocking
0
2
Fowler-Nordheim tunneling
4
6
8
10
Electric Field (MV/cm)
Figure 5.7: Leakage and breakdown of Al2O3, deposited at KTH [Paper VIII].
5.2.3 Ta2O5
Out of many high-k dielectric materials, tantalum oxide (Ta2O5) has also been
considered for Si CMOS devices [92], but so far it has not been tested for SiC. One
problem with this material is the negative offset with SiC (presented in Table 5.1).
However, some reports for Si suggest that there is always a thin layer of SiO2 present
when Ta2O5 is deposited on Si [93], which can add an additional band offset. With the
same assumption, Ta2O5 is experimented with SiC, and electrical characterizations are
performed. As it has been seen for AlN, this material also does not show any promising
results regarding capacitive behavior of the dielectric. Therefore, it can be concluded
that Ta2O5 is not an appropriate choice for SiC, as a single dielectric.
5.2.4 Dielectric stacks
A stacked geometry for the dielectrics has been previously tried for Si, combining the
properties of different materials [92]. The same approach is adapted for SiC in the
present work. A stacked dielectric is formed by adjoining Al2O3 and AlN, using a
controlled ALD process. By combining these layers in a stack, the leakage from the
polycrystalline AlN, which is demonstrated separately (Figure 5.4), is prevented by
inserting amorphous layers of Al2O3, while the total passivation dielectric will have a
high-k value. The layers are deposited alternately on two separate samples to find a
better combination. In these stacked Al-based dielectrics, a thin layer (8 nm) of SiO2 is
51
52
Dielectrics for 4H-SiC – Passivation and radiation hardness
used to add an extra band offset as well. Scanning electron microscopy (SEM)
performed on one of the samples (Figure 5.8) confirms the polycrystallinity of the AlN
in the stack. The details of these stacked samples are presented in Table 5.2.
Table 5.2: Dielectric stacks on SiC. The thickness of each layer is given in parentheses. All
thicknesses mentioned here are in nm.
Stack 1
Stack 2
Stack 3
Stack 4
SiO2 (8)–AlN (40)–Al2O3 (20)–AlN (41)–Al2O3 (20)
SiO2 (8)–Al2O3 (17)–AlN (34)–Al2O3 (18)–AlN (37)
Ta2O5(20)–Al2O3 (20)–Ta2O5(20)–Al2O3 (20)
Al2O3 (20)–Ta2O5(20)–Al2O3 (20)–Ta2O5(20)
Figure 5.8: SEM image of stack 1, showing different layers of dielectrics with their respective
thicknesses [Paper VI].
Dielectric characteristics for the Al-based stacked dielectrics show a significant
difference by changing the order of AlN and Al2O3. Figure 5.9 shows a comparison of
the dielectric properties of both stacks. Capacitance-voltage analysis of these samples
reveals a high amount of negative charge, accumulated in the dielectric in both stacks,
evidenced by flatband shift towards a positive voltage. However, the hysteresis in both
of these stacked samples is less than 0.5 V, indicating a low concentration of near
interface traps. An important observation that can be made from the CV characteristics
of the two stacks is the inversion capacitance, which is only observed in the sample
with the first layer being AlN but for all frequencies. This suggests that the
polycrystalline AlN provides a path for the charges that results in a change in the
amount of interface charges at higher frequencies, and therefore a higher capacitance in
the inversion region is observed [94]. In these stacks, a difference of 25% in the values
of the effective dielectric constant is also observed, which is clearly related to the
sequence of layers in the stacks. Annealing of stack 1 at 100, 200 and 300 oC results in
a temporary reduction of fixed oxide charges, but they return to the original values
after few days, whereas for stack 2, the total amount of charge is increased as a result
of annealing.
5.2 Characterization of the MIS structures
Ideal
1 kHz
10 kHz
100 kHz
2
Capacitance (x10 F/cm )
2
Capacitance (x10 F/cm )
5
6
-8
-8
4
3
2
1
Ideal
1 kHz
10 kHz
100 kHz
(a)
-4
0
4
8
4
2
(b)
-8
-4
Voltage (V)
0
4
8
Voltage (V)
Figure 5.9: CV measurements at different frequencies of (a) stack 1, (b) stack 2, with ideal CV
curve (solid line). The CV measurements were obtained at 1, 10, and 100 kHz from
accumulation to inversion and back again [Paper VII].
2
Current Density J (A/cm )
The electrical breakdown measurements of these dielectrics also show very
different behavior. Typical breakdown curves for stacks 1 and 2 are presented in Figure
5.10. In a statistical analysis of the breakdown field by using the Weibull functions,
shown in Figure 5.11, it is observed that the stack with a first layer of Al2O3 is
relatively more reliable even though its breakdown field is less than the other stack.
This kind of stacking geometry also prevents the early leakage in Al2O3 (discussed in
the previous section, Figure 5.7). Optical analysis by a microscope on the postbreakdown devices (Figure 5.11) shows that the step bunching from the SiC surface is
transferred to the top surface if the dielectric has some kind of crystalline structure, and
it can lead to early breakdown as seen in the case of stack 2.
10
2
10
0
10
-2
10
-4
10
-6
10
-8
Stack 1
Stack 2
Frenkel-Poole
10
-10
0
2
4
6
Electric Field (MV/cm)
Figure 5.10: Breakdown curves for stacks 1 and 2 [Paper VII].
8
53
Dielectrics for 4H-SiC – Passivation and radiation hardness
Stack 2
Weibull Function ln[ln(1/(1-F))]
1
0
-1
-2
1120
-3
Stack 1
2
4
6
8
2
4
6
8
Electric Field (MV/cm)
Figure 5.11: Weibull plots for the breakdown of stacks 1 and 2. Linear fit shows the reliability
of the dielectric stacks. Post breakdown optical image of the contact for each stack are also
presented in the inset of each plot [Paper VII].
Another scheme of stacks is tried by using Al2O3 and Ta2O5 to raise the effective
dielectric constant. Table 5.2 presents the order and thickness of the stacks used in
these experiments. In these stacks, the thickness of each dielectric layer is kept 20 nm
and the sequence of the dielectrics is altered in two separate samples. The final
thickness of the overall dielectric film is 80 nm without any interfacial SiO2.
In this type of stack, Al2O3 provides good offset values between SiC and Ta2O5. An
RBS analysis of the stacked dielectric shows no contamination in the film from
precursors (Figure 5.12). In addition, two layers of tantalum (Ta) can be separately
observed in the spectrum.
Ta (layer 1)
Experimental
Simulated
6000
Ta (layer 3)
Yield
54
4000
2000
C
O
Si
Al (from layer 2 and 4)
0
100
200
300
400
500
Channel No.
Figure 5.12: RBS spectrum for stack 4. No unwanted element is detected in the spectrum and
the thicknesses of the films are confirmed.
1 kHz
10 kHz
100 kHz
500 kHz
4
Capacitance (x10
-11
F)
5.3 Thermal stability
3
2
1
0
4
8
12
16
20
Voltage (V)
Figure 5.13: CV results for stack 4 showing a shift in the curve towards positive voltage. A
frequency dependence of the CV behavior is also observed.
The CV measurements performed on these stacks show a high concentration of
fixed negative oxide charge, revealed by a shift of the CV curve from an ideal
behavior, as can be seen in Figure 5.13, for stack 4. These results indicate a poor
quality of the whole dielectric. The same stack on p-type substrate does not even
produce any capacitive behavior at all.
Based on the stacked dielectric study, it can be concluded that the approach to stack
different high-k materials can be useful in changing the value for the overall dielectric
constant and improving the reliability. This can block higher field strengths on SiC
device surfaces, and also result in a better high-k dielectric for SiC. However, more
experiments are required to optimize this technique.
5.3 Thermal stability
As discussed in previous chapters, 4H-SiC is a good candidate for high temperature
applications. In such applications, the passivation of the devices must also be able to
withstand high temperatures without significant change in dielectric properties, in order
not to affect the overall device characteristics. In the present work, a few experiments
are performed on stacked dielectrics and Al2O3 with respect to their high temperature
stability.
Capacitance measurements performed on Al-based stacked dielectrics (Al2O3 and
AlN) at raised temperatures (not shown) reveal that the stack 2 is very stable at the
temperatures of 100, 200 and 300 oC. On the other hand, stack 1 shows a flat band shift
for these temperatures and the charge from the interface is reduced at high
temperatures due to the leaky AlN film, just above thin SiO2 film.
The structural stability of Al2O3 films was also tested by using atomic force
microscopy (AFM) topographic analysis to see the change in surface roughness of the
samples after high temperature treatments. For this purpose, Al2O3 was deposited by
ALD at KTH setup on the epi-layer grown on an unpolished (no CMP) wafer with 3
nm surface roughness and step bunching in the growth direction, as measured by AFM,
and treated at 950 oC in Ar environment by rapid thermal anneal (RTA) for 60 secs,
and also long term annealing for 60 mins in a furnace. The AFM analysis, presented in
Figure 5.14 for these samples, shows that the surface roughness of Al2O3 is hardly
affected after these temperature treatments.
55
Dielectrics for 4H-SiC – Passivation and radiation hardness
Figure 5.14: (left) As-deposited Al2O3. (right) Annealed at 950 oC for 60 mins. The Al2O3 films
are deposited on unpolished wafer epi-layer, shown in Figure 3.4. The annealing procedure did
not change the surface roughness of the film significantly.
It has previously been demonstrated that high temperature annealing of ALD
deposited Al2O3 results in a better interface with 4H-SiC [95]. This has been verified
by comparing CV measurements on as-deposited sample and after annealing. For this
study, two samples are prepared simultaneously to make a comparison and to see the
real impact of annealing on dielectrics. The results obtained from this analysis for ntype samples are presented in Figure 5.15, which show a remarkable flatband shift in
CV curve by reducing the concentration of charge in the oxide. The temperature for
these annealing experiments is chosen to be 950 oC, which is less than the
crystallization temperature of Al2O3. X-ray diffraction analysis performed on the
sample before and after annealing shows that the structure of the film is not changed as
well and no extra crystallographic planes appear after annealing at these temperatures.
Therefore, it can be concluded that the temperature treatment can strongly improve the
electrical properties of ALD deposited Al2O3, while the structural properties are
unaffected. These experiments are performed both on n- and p-type 4H-SiC, however,
the results on n-type 4H-SiC are more promising.
1.0
As-deposited
o
Annealed at 950 C in Ar
0.8
C/Cox
56
0.6
0.4
0.2
0.0
0
5
10
15
20
25
Voltage (V)
Figure 5.15: CV measurements of as-deposited Al2O3 and annealed at 950 oC in Ar
environment for n-type 4H-SiC. A large flatband voltage shift can be observed after annealing.
5.4 Radiation hardness
5.4 Radiation hardness
In order to operate SiC-based devices in radiation rich environments, the dielectric
passivation of the devices play a crucial role. For this purpose, a separate study has
been conducted to testify the radiation hardness of Al2O3 in comparison to currently
used passivation material, i.e. SiO2. Typical defects that affect the device performance
and reliability are the surface defects at the interface between the dielectric and SiC.
These defects can affect the carrier mobility in SiC MISFETs, and also lead to leakage
and early breakdown in other device configurations.
In the present work, the effect of ion radiation at the interface of the dielectric and
4H-SiC is studied by employing specific ions in an energy range where the
displacement damage in the material is peaked near the interface. MIS structures of
similar thickness are prepared by depositing SiO2 and Al2O3 on 4H-SiC. SiO2 was
deposited by PECVD and post-oxide anneal was performed to get the best possible
interface with SiC (process described in Section 5.1.1). After optimizing the deposition
parameters for the oxide layer, the Al2O3 was deposited by ALD. The analysis of the
dielectric parameters revealed a large amount of negative charge in the oxide with
relatively low concentration of interface states. An RBS analysis confirms the
thickness and stoichiometry of the deposited films.
The samples were exposed to 50 keV Ar ions producing damage at, or near the
interface inside the oxide. The stopping analysis for Ar ions at these energies reflects
that the ratio between the nuclear and the electronic stoppings is ~11, which results in
high concentration of displacement damage around the projected range (Rp), and it is
dependent on the fluence of ions. The fluence of ions used in these experiments was
varied from 1×109 to 1×1013 cm-2 to find threshold damage (fluence) that can affect the
electrical characteristics of the dielectrics. The depth of implanted ions is estimated by
SRIM simulations [12], which shows that the Ar ions with 50 keV stop around the
interface both in SiO2 and Al2O3. For these simulations, the values for the densities
used are 2.6 g/cm3 for SiO2 and 2.7 g/cm3 for ALD-Al2O3 [96]. However, these could
be significantly different from actual values because of the growth or deposition
conditions, and consequently affect the ions’ projected range.
The dielectrics are characterized after radiation exposure by electrical
measurements, and a comparison is made to non-radiated MIS structures. CV
measurements on these samples show that the SiO2 interface starts to get affected after
a fluence of 1×1011 cm-2 due to an increase in interface states. However, the Al2O3
samples show a considerably higher resistance to these radiation fluences. A
comparison of the density of interface states is shown in Figure 5.16. Similar studies
are also conducted on Si substrate, where the thickness of both oxides (SiO2 and Al2O3)
was kept similar to the 4H-SiC samples. These studies resulted in the same trend in the
CV measurements versus Ar ion fluence, as observed for 4H-SiC substrate.
57
Dielectrics for 4H-SiC – Passivation and radiation hardness
30
SiO2/SiC
-2
25
20
11
-1
Dit (x10 cm eV )
58
15
Al2O3/SiC
Unimplanted
10
5
0 109
10
10
10
11
10
12
-2
Argon fluence (cm )
Figure 5.16: Interface trap densities, estimated from high frequency CV measurements by using
Terman method, for different argon fluences on SiO2/SiC and Al2O3/SiC MOS structures. The
distribution of Dit at different trap energies is nearly flat in the investigated interval. Therefore,
the average Dit over the trap energies is presented [Paper VIII].
Current leakage and breakdown of these implanted devices have also been studied
independently to ensure the radiation damage effects on the dielectrics. The analysis
suggests that neither the breakdown nor the leakage behavior of the dielectrics is
affected for any implantation fluences. Therefore, it can be concluded that this kind of
nuclear damage does not directly affect the quality of the dielectric and do not produce
any leakage paths in the amorphous dielectric materials.
Thermal dielectric relaxation current (TDRC) measurements have also been
conducted on these samples to understand the distribution of interface states in the
bandgap [60]. Measurements have been performed for SiO2/SiC, however, due to high
leakage through the oxide, interface states could not be determined. Although the
interface of SiO2 is very good for PECVD deposited oxide with post-oxidation
annealing, the TDRC measurements showed a large amount of leakage in unimplanted
samples, which could be due to the poor quality of the oxide away from the interface.
However, it has been observed that the leakage is reduced for increasing fluence of
ions by passivating the leakage paths in the oxide. This could be a consequence of the
atomic disorder produced by implanted Ar+ in the oxide, which has increased the
amorphization to give a better signal for such measurements. Similar measurements on
implanted Al2O3 resulted in relatively different fluence dependence. Figure 5.17 shows
a comparison of the unimplanted and as-implanted MIS structures for different
fluences of Ar+ on Al2O3/SiC structures. It has been observed that the low
concentration of damage produced by ion implantation can shift the interface states
away from the conduction band (low temperature region) edge towards midgap region
(high temperature region). The intensity of the current is dependent on the amount of
interface states. Nevertheless, the total amount of interface states does not change.
5.4 Radiation hardness
2.0
Current (pA)
1.6
1.2
Unimplanted
11
-2
1x10 cm
11
-2
5x10 cm
12
-2
1x10 cm
0.8
0.4
0.0
50
100
150
200
250
300
Temperature (K)
Figure 5.17: TDRC measurements for Al2O3 samples implanted with different fluences of ions
at a charging bias of 20 V (accumulation). The discharging voltage of -10 V is used to let the
carriers emit from trapped states. The measurements have been performed from 50 K to 300 K
and the rate for increasing temperature was set at 0.2 K/sec.
Average lifetime in the epitaxial layer after Ar+ implantations has also been
investigated on these implanted oxides, by using free carrier absorption technique
(FCA) [54], described in Chapter 3. The average lifetime extracted from these
measurements depends on the bulk lifetime in the epi-layer, the diffusion coefficient
for carriers, and the surface recombination velocity (SRV), which is an important
parameter in MOSFET applications. Figure 5.18 shows an average lifetimes for a
SiO2– and Al2O3–covered SiC epi-layers, after producing damage near the interface by
50 keV Ar+ implantation. It can be observed that the lifetime in SiO2 samples drops to
a certain minimum level (one third of the lifetime value before implantation) at a
fluence of 1×1011 cm-2 and then it saturates, showing the dependence of the carrier
lifetime on the concentration of surface defects. On the other hand, for similar fluences
of Ar ions on Al2O3 samples, the effect on lifetime is negligible. These results are in
qualitative agreement with the electrical measurements obtained from CV technique.
Moreover, FCA measurements are used to extract the SRV values for these Ar+
implanted structures. The results indicate that the SRV values for the implanted doses
follow the same trend for both dielectrics, as it is observed in CV-based Dit analysis
(Figure 5.16). This indicates that the increasing fluence of ions increase the SRV as
well.
In the light of CV and FCA results, it can be concluded that the Al2O3 can present
more resistance to the nuclear damage induced by ionizing radiation at the interface, in
comparison to SiO2.
59
Dielectrics for 4H-SiC – Passivation and radiation hardness
(a)
160
60
τ (ns)
80
(b)
80
120
τ (ns)
60
Ref.
40
40
Ref.
20
0
9
10
10
10
+
11
10
-2
Ar fluence (cm )
12
10
13
10
0
10
11
10
10
+
-2
Ar fluence (cm )
12
10
Figure 5.18: Average lifetime variation in epi-layer after Ar+ implantation for (a) SiO2/SiC and
(b) Al2O3/SiC. Error bars indicate standard deviation in the data [Paper IX].
Chapter 6
Conclusions and future outlook
The thesis focuses on selected ion beam radiation effects on 4H-SiC material properties
and device characteristics. Plain epitaxial 4H-SiC and more complex device structures
such as bipolar junction transistors (BJTs) have been used, as well as dielectric layers
on the same material to study device passivation and interface properties with these
controlled ion beam experiments. It has been possible to separate ionization effects
from lattice damage, and also to study radiation effects in highly localized regions, for
instance, the collector of a BJT.
Ion implantation induced damage studies of plain epitaxial material show that most
of the crystal damage induced by relatively low dose ion implantation can be recovered
in a low temperature range, i.e. up to 800 oC. However, higher concentrations of the
crystal damage obtained for higher dose requires considerably higher temperatures to
anneal. The damage produced through ion implantation is mostly concentrated around
end of the ion range, and it has a clear impact on the resistivity of the material, as
examined by scanning spreading resistance microscopy (SSRM).
Ion implantation experiments in BJTs have been performed by producing a
damaged layer in the collector. The degradation produced by this type of implantation
is not recoverable after annealing at low temperatures. However, the degradation of the
device characteristics produced by low dose implantations can be recovered up to 420
o
C. The degradation of BJT as a result of the damage at the interface between
passivating oxide and SiC has also been investigated by implanting helium ions around
the dielectric and semiconductor interface. The analysis is performed with the help of
device simulations and it reveals that implantation of this kind induces positive charges
in the oxide, causing a surface charge at the interface, and thereby degrading the
performance of the device. Simulation studies have also been performed on BJTs in the
presence of bulk traps at different levels in all regions of the device. These studies
reveal that bulk traps in the base region have a strong degrading effect on the device
performance, whereas the emitter is relatively resistant. In the case of the collector, the
traps introduced away from the base-collector junction produce an additional resistance
62
Conclusions and future outlook
curve in the collector output current, thereby confirming the observations made in
experimental studies.
Device passivation is one of the most critical issues for the reliability and
performance of the device. The area between the base and emitter contacts in a BJT is
normally passivated, and it directly influences the current gain of the device. In this
thesis, an investigation has been made for high-k dielectric passivation for 4H-SiC
devices, by using atomic layer deposition (ALD). Electrical and structural
characterizations have been performed mainly on Al2O3 and AlN, as compared to the
SiO2. The layers have been tried individually, but also in combination, in order to
obtain sufficiently high dielectric properties. Thermal stability tests have also been
conducted and the results show that the high temperature annealing of as-deposited
Al2O3 can dramatically improve its interface with 4H-SiC, instead of degrading the
overall electrical properties.
Radiation tolerance of the Al2O3 passivation material in comparison to SiO2 has
been tested by introducing ionization in the dielectric and nuclear scattering damage
around the interfaces with 4H-SiC. This kind of study is conducted for the first time
and it is proposed that Al2O3 can provide higher radiation resistance to such kind of
damaging events. In these studies, it has also been observed that, at a certain dose of
ion implantation on the Al2O3/SiC structure, the interface states can shift from the
conduction band edge to the valance band. This is a very important result with
implications in SiC-based MOSFET devices, where the flow of carriers in the channel
is affected by the interface states near the conduction band. In this thesis, an initial
study to devise a new approach to extract dielectric and semiconductor interface
parameters by using free carrier absorption (FCA) method is proposed and
investigated.
With the help of ion beams in a selected accelerator environment, it has been
possible to produce damage in different regions of the device to study the impact on
device performance. In addition, these kinds of experiments can be useful in studying
radiation tolerance of the dielectric layers for passivation of SiC-based devices.
The studies conducted in this thesis require more work and can be expanded. Some
suggestions for the future work are as follows:
1) Free carrier absorption technique (FCA) is introduced to analyze the interface
between the dielectric layers and 4H-SiC epi-layer surface. With more precise
determination of bulk lifetime and carrier diffusion constants, the technique
can be a useful complement and provide quantitative information on surface
recombination states.
2) Al2O3 deposition by ALD results in a large amount of charges in the oxide.
Methods should be devised to improve the quality of this oxide. One way is to
find a better surface preparation prior to deposition and optimization of the
ALD process. Annealing of deposited oxides results in a reduced amount of
charge, and this behavior should be investigated further in detail to find
optimum parameters.
3) Al2O3 has shown significant tolerance for radiation. Further study is required
to improve and test it against other types of radiation. Stacked dielectrics may
also be interesting candidates from the radiation resistance point of view, and
Conclusions and future outlook
should be further investigation by combining other high-k dielectric materials,
such as HfO2 with Al2O3.
4) The observation that the ion implantation in the dielectric around interface can
lead to the improvements in electrical characteristics needs more investigation.
Pre-implantation of different atoms such as carbon or nitrogen, at or near the
surface of the semiconductor, prior to the deposition of Al2O3 can also be a
useful processing tool for improving the interface properties.
63
64
Conclusions and future outlook
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