Download Light emitting devices based on silicon nanostructures

UNIVERSITA’ DEGLI STUDI DI CATANIA
DOTTORATO DI RICERCA IN SCIENZA DEI MATERIALI
XVI CICLO
Alessia Irrera
Light emitting devices based on silicon
nanostructures
Tutor: Prof. Francesco Priolo
Supervisor: Dott. Fabio Iacona
Coordinatore: Prof. Francesco Priolo
Tesi per il conseguimento del titolo
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(da Ode alla vita di Pablo Neruda)
Contents
Chapter 1: Towards a silicon-based optoelectronics
1.1 Silicon-based optoelectronics.........................................................2
1.2 Silicon-based light source state of art.............................................8
Scheme of the thesis .................................................................17
Chapter 2: Structural and optical properties of Si nc
and Er-doped Si nc
2.1 Recombination processes in crystalline silicon............................20
2.2 Silicon nanocrystals......................................................................24
2.3 Si nanocrystals synthesis ..............................................................31
2.4 Structure of SiOx films and growth of Si nc.................................38
2.5 Structural and photoluminescence properties Si nc......................41
2.6 Excitation and De-excitation properties .......................................52
2.7 Synthesis of Er doped Si nc..........................................................62
2.8 Photoluminescence properties of Er-doped Si nc.........................65
2.9 Excitation properties of Er-doped Si nc .......................................70
2.10 Conclusions ..............................................................................72
I
Chapter 3: Light emitting devices based on silicon nc
3.1
Realization of the MOS-devices...............................................78
3.2
Characteristics of the active layer.............................................84
3.3
Electrical properties..................................................................89
3.4
Electroluminescence (EL) measurements.................................99
3.5
Reliability test.........................................................................111
3.6
Excitation and de-excitation properties .................................112
3.7
Effects of the trapped charge ..................................................115
3.8
Calaculation of the excitation cross section............................120
3.9
EL properties as a function of the temperature.......................122
3.10 Determination of the single-triplet
splitting of excitonic levels.....................................................127
3.11 Quantum efficiency measurements .......................................132
3.12 Conclusions ...........................................................................135
Chapter 4: Light emitting devices based on
rare-earth doped Si nanoclusters
4.1
Realization of the MOS-devices.............................................138
4.2
Characteristics of the active layer...........................................144
4.3
Electrical properties ...............................................................147
4.4
Electroluminescence (EL) properties .....................................149
4.5
Excitation and De-excitation properties .................................153
4.6
Influence of the temperature on the EL properties .................155
4.7
Electroluminescent devices based on Tm doped Si nc...........162
4.8
Conclusions ............................................................................166
II
References ............................................................................... 169
List of publications
III
Chapter 1: Towards a silicon-based optoelectronics
Chapter 1
Towards a silicon-based optoelectronics
This chapter discusses the importance to develop an optoelectronics fully based
on silicon. In fact, the use of optical signals instead of the present metallic lines
for the inter- and intra-chip connections could remarkably increase the speed
and the reliability of the signal processing in microelectronic devices.
While Si-based waveguides and detectors are already available, the big
obstacle to develop a Si-based optoelectronics is the possibility to fabricate a
light source based on silicon, since its indirect bandgap implies that radiative
recombination process are very inefficient.
A review of the most significant light emitting devices (LEDs) based on silicon
that have been proposed in literature is presented. The different strategies that
have been studied (rare earth doping of silicon, silicon nanostructures,
semiconducting silicides and so on), as well as the limits and the advances of
each approach are discussed. Finally, the scheme of the thesis is presented.
1
Chapter 1: Towards a silicon-based optoelectronics
1.1 Silicon-based optoelectronics
Digital processing of information requires particular devices and circuits for
logical functions and storage, and also interconnections to carry the
informations from one place to another. At the same time the enormous
progress of communication technologies in the last few years has increased the
demand for faster communication systems able to convey a larger amount of
data, and urge the industrial companies involved in the communication market
to satisfy these requests by innovative and sophisticated low-cost technologies.
The communication technology will soon require data transfer rates of more
than 1 Gbit/s for channel; in the next future, data transfer rates of the order of
the Tbit/s, for the inter-chip connections of devices, will be also requested.
Fig.1.1 The growing complexity of integrated circuits (by Moore’s law [1])
For what concerns the intra-chip connections, in order to give an estimation of
the growing complexity of integrated circuits, it is predicted by Moore’s law
[1] (see fig.1.1) that by the year 2005 logic chips with 200 millions transistors
2
Chapter 1: Towards a silicon-based optoelectronics
will be necessary, where as the total length of interconnecting conductors
within a chip reach as 20 Km [2] (see fig.1.2).
Fig.1.2 Interconnection lenght per chip versus year. Date are from international Semiconductor
Roadmap 2000.
Therefore, communication inside a chip will become a serious obstacle to the
continuation of the growth of the integrated circuits. For intra-chip
interconnections, the “bottleneck” can be traced back to a combined effect of
two problems: the spatial density problems (leading to a higher integration
density) and the signal speed problem (requesting the use of higher frequency).
There are several possible approaches to such interconnection scaling
problems, and likely all of these will be used to some degree.
Architectures could be changed to minimize interconnections. Design
approaches could put increasing emphasis on the interconnection layout.
Signaling on wires could be significantly improved through the use of a variety
of techniques [3-4]. Most important for this discussion is the approach of
changing the physical means of interconnections. Optics is arguably a very
3
Chapter 1: Towards a silicon-based optoelectronics
interesting and different physical approach to interconnection that can in
principle address most, if not all, of the problems encountered in electrical
interconnections.
The difficulties of electrical interconnections are not simply ones of scaling of
the raw capacity of the interconnection system. There are a variety of other
difficulties that are readily quantified but that in the end could be dominant
reasons for changing to a radical solution like optics. This includes issues such
as voltage isolation, timing accuracy, and overall ease of design. There are
other possible physical approaches for improving electrical interconnections,
including cooling the chip and/or circuits, e.g. to 77 K, or using
superconducting lines. Cooling below room temperature is already used in
some computers. Superconducting materials are still not available for room
temperature operations, and so their use would also require significant cooling;
unless temperatures < 77 K are used, relatively novel, practical, hightemperature superconductor materials would have to be developed.
The most used approach is currently the continuous increase in the number of
metal levels, with seven levels currently in production (see fig.1.3), and larger
numbers of levels under development. There is significant cost to developing
further levels, and it is not clear that this is a scalable solution to interconnect
problems in the long run. In fact, the problem with this approach is that (as
device dimensions shrink to less than 0.13 µm) propagation delay, cross-talk
noise, and power dissipation due to resistance-capacitance (RC) coupling
become significant due to increased wiring capacitance, especially interline
capacitance between the metal lines on the same metal level. The smaller line
dimensions increase the resistivity (R) of the metal line, and the narrower
interline spacing increases the capacitance (C) between the lines. To address
these problems, new materials for use as metal lines and interlayer dielectric as
4
Chapter 1: Towards a silicon-based optoelectronics
well as alternative architectures have been proposed to replace the Al and SiO2
interconnect technology. These alternative architectures require the use of low
dielectric constant materials (k < 3) and low-resistivity conductors such as Cu
[5].
Fig.1.3 (a) Micrographs of the lateral section of the interconnections inside a chip. (b) top
view micrograph showing the complexity of the interconnections.
Another interesting approach is to stack chips in a three-dimensional (3-D)
structure with appropriate vertical connections. This approach may well help,
though power dissipation can become a problem because the surface area is not
increasing significantly as chip area stacked.
All of the above approaches do not fully address issues like voltage isolation,
timing accuracy, and ease of design. Clearly, a long-term solution will require a
radical change in the present technology to definitely solve the intra and interchips connections problem. Many findings prove that the substitution of
present electrical signals with optical signals to process and transmit
information can definitely solve power dispersion and signal delay time
5
Chapter 1: Towards a silicon-based optoelectronics
problems. In fact, as it is known, using light in optical fibres for
communication systems would allow us to speed up the information flux, since
optical signals travel at the light speed, without any reciprocal influences, while
losses as low as 0.2 dB/km can be reached in well manufactured silica optical
fibers. As a consequence the interest of the present technological research
strongly focused on optical approaches. In particular, optoelectronics is
dedicating great efforts to the study of optical interconnects, since, according to
the 1998 Technology Roadmap for Optoelectronics Interconnects for Integrated
Circuits edited by the European Commission [1], optical interconnected
components are suited for massive parallel data communications, both in
optical fibers and waveguides, suggesting their strong diffusion in the
communication sector.
Optoelectronics is currently involving a vast group of people in the scientific
community working for the advancement in materials science technology.
Indeed silicon is the leading semiconductor in the microelectronics industry.
For future applications on optoelectronic interconnects it is necessary to have a
material in which light can be generated, guided, modulated, amplified and
detected. Silicon is characterized by an indirect bandgap, it is therefore not
suitable in its bulk form for the implementation of fundamental optical devices
such as light sources and modulators, while Si-based waveguides and detectors
are already available.
As a consequence, this sector is still dominated by III-V direct band gap
semiconductors, whose technology deeply differs from the VLSI integration
process used for silicon. The hybrid approach, i.e. the integration process of
III-V semiconductors into silicon chips, is quite difficult and very expensive for
the microelectronic industry. The goal is to find new technological solutions to
overcome the integration problems. Many of these difficulties could be solved
6
Chapter 1: Towards a silicon-based optoelectronics
if it would be possible to use silicon for optoelectronic applications, i.e. to
develop a silicon-based optoelectronics.
Several approaches need therefore to be investigated. In particular, after the
1990 discovery by Canham of strong light emission from porous silicon, the
opinion on the inadequacy of silicon for optoelectronics applications has
definitely
changed.
Consequently,
research
interests
focused
on
Si
nanostructured materials and in particular, for their major stability with respect
to porous silicon, on silicon nanocrystals embedded into a SiO2 matrix. Silicon
nanocrystals exhibit a strong light emission at room temperature and can be
prepared by means of techniques fully compatible with Si VLSI technology.
Furthemore, silicon nanocrystals are also a very versatile material and by
doping this system with rare earth ions it is possible to change their
luminescence properties.
7
Chapter 1: Towards a silicon-based optoelectronics
1.2 Silicon-based light source state of art
As discussed in the previous paragraph to develop a Si-based optoelevtronics
we have to solve a material problem to circumvent the physical inability of
bulk silicon to emit light.
In this paragraph we will show a brief review of some significant light emitting
devices (LEDs) based on silicon that have been proposed in the literature.
Several strategies have been considered and explored.
A first approach is based on light emitting impurities; in particular we present
here the most promising case, represented by the introduction of erbium ions in
silicon. A relevant advantage here is that standard Si technology (ion
implantation) can be used to introduce erbium as a dopant and to process the
material. One important point is that the wavelength of the emission is ∼1.5
µm, that is a strategic wavelength for telecommunication because the loss of
optical fibers in silica has a minimum at 1.5 µm. Room temperature operating
diodes were fabricated by incorporating Er ions in the depletion layer of a p(+)n(+) junction [6]. More in detail, the diode is a p(+) - n(+) junction fabricated by
implanting Er and O ions ( thus forming the n(+) side of the junction) and B (in
order to realize the p(+) side of the junction). Oxygen atoms have a fundamental
role in Er emission from crystalline silicon, since they inhibit Er segregation by
forming Er-O complexes during the annealing processes needed to promote
crystal regrowth after the implantation steps.
By using Er and O coimplantation in a ratio of 1:10 it is possible to incorporate
up to ∼ 1020 Er/cm3 in crystalline Si. In fig.1.4 the EL spectra under reverse and
forward bias of the device are reported in comparison with the PL spectrum. As
suggested by the different shape of the two EL spectra., the mechanism leading
8
Chapter 1: Towards a silicon-based optoelectronics
to light emission changes by changing the kind of polarization. In particular,
the mechanism leading to Er excitation is e-h recombination at an Er-related
level under forward bias and impact excitation by hot carriers of Er ions under
reverse bias. An internal quantum efficiency of 0.0015% under forward bias
and of 0.05% under reverse bias was measured at room temperature.
Fig.1.4 Room temperature EL spectra under reverse and forward bias in comparison with the PL
spectrum of Er-doped crystalline silicon [6].
Another approach, based on low dimensional systems (porous silicon, silicon
nanocrystals and Si/SiO2 quantum wells), is currently attracting a strong
interest. In all of these cases nanometer-sized silicon is embedded within an
insulating host. The quantum confinement has several effects: it increases the
9
Chapter 1: Towards a silicon-based optoelectronics
probability of radiative recombination, it decreases the non-radiative
recombination routes, it increases the energy of emission. Indeed intense room
temperature photoluminescence can be achieved [7-9]. The main problem here
is the carrier injection to achieve electroluminescence. Several routes have been
followed and devices operating at room temperature were fabricated, but up to
now, it has been never reported the fabrication of devices simultaneously
satisfying main different requirements such as full compatibility with Si VLSI
technology, low operating voltage, high stabilityand quantum efficiencies
comparable with those typical of III-V semiconductors.
Different LEDs based on porous Si have been fabricated. Devices based on
porous Si exhibit room temperature EL spectra in the visible range. However
porous Si has a series of disadvantages: poor stability due to the high porosity
and the difficulty of
integration of the electrochemical etching needed to
produce porous Si with the Si VLSI processing technology. Different groups
studied the possibility to exceed these problems. Values junctions of EL
external efficiency of about 1% [10]have been reported at room temperature
and at low operating voltage (5 V) from n+-type silicon / electrochemically
oxdized thin porous silicon /
indium-tin oxide. Furthemore, in fig.1.5 a
schematic of an integrated bipolar transistor/ porous silicon LED device in
which the light emitting layer consists of a thermally oxidized layer of porous
Si is reported. This solution has allowed to increase the stability of the layer, by
simultaneously obtaining encouraging characteristic in terms of operating
voltage and quantum efficiency. A proposed solution to solve the problem of
the mechanical stability of the porous layer has been the use of multilayered
structures, in which the high-porosity (> 70 %) active layer is sandwhiched
10
Chapter 1: Towards a silicon-based optoelectronics
between two low-porosity (≤ 30 %) supporting layers. This structure shows
also improved luminescence stability, but a lower quantum efficiency [11].
Fig.1.5 A schematic of an integrated bipolar transistor/ porous silicon LED structure in which
the light emitting layer consists of a thermally oxidized layer of porous Si [11].
11
Chapter 1: Towards a silicon-based optoelectronics
In fig.1.6 the case of a LED based on an ultrathin amorphous silicon layer
containing silicon nanocrystals [12] is presented. The use of a single ultrathin
layer of nanocrystals allows to obtain low operating voltages, but this leads
also to a reduced efficiency, due to the decreased number of emitting centers.
The figure shows the cross section TEM image of the structure and the
corresponding EL spectrum obtained at room temperature under reverse bias of
5 V and current of 60 mA that is centred a wavelength of about 650 nm. The
mechanism leading to the EL is however unclear, so that some points, like the
absence of EL signal under forward bias, have to be better explored.
Fig.1.6 LED based on an ultrathin amorphous silicon layer containing silicon nanocrystals [12]
In fig.1.7 the PL and EL spectra for devices based on Si nc obtained by ion
implantation in thin SiO2 layers are reported [13]. Another approach is to
fabricate LED based on Si/SiO2 multilayers [14] where the formation of Si nc
is due to an annealing process. In fig. 1.8 the EL and PL spectra for devices
with different size of Si nc that are isolated between two SiO2 layers are
reported. It is visible that the PL and EL spectra are centred at different
wavelengths and in particular the EL spectra are centred at lower wavelengths.
This suggests that the emitting centers under photo and electroluminescence are
12
Chapter 1: Towards a silicon-based optoelectronics
different. These last three examples, although representing significant advances
in the field, demonstrate that a lot of work is still needed to fully understand the
mechanism leading to light emission from Si nanostructures and to improve
device performances. In particular, the role of radiative defects needs to be
better explored, since it is possible that the low wavelength signals shown in
ref. [13] are not really due to the presence of Si nc.
Fig.1.7 PL and EL spectra for devices based on Si nc obtained by ion implantation in thin SiO2
layers [13].
13
Chapter 1: Towards a silicon-based optoelectronics
Fig.1.8 EL and PL spectra for devices with different size of Si nc that are isolated between two
SiO2 layers are reported [14].
A third approach, namely that of semiconducting silicides, like for instance βFeSi2 is based on the observation that β-FeSi2 should have a direct bandgap of
14
Chapter 1: Towards a silicon-based optoelectronics
∼0.85 eV. Indeed, different luminescent devices have been fabricated with βFeSi2 precipitates formed by Fe ion implantation in a Si diode [15-16].
Fig.1.9 The cross section TEM image of β-FeSi2 precipitates [15].
These devices emit at 1.5 µm however only values of
external quantum
-3
efficiency at 80 K of about 5×10 % have been reported. In fig.1.9 is shown the
cross section TEM image of β-FeSi2 precipitates [15]. In the figure it is clearly
visible that Fe implant produces a very complex situation; indeed at least two
different kinds of FeSi2 precipitates (dish shaped and spherical) and extended
defects (dislocation loops) are detected. Note also that has recently been
demonstrated that only the dish shaped precipitates exhibit a PL signal [15].
15
Chapter 1: Towards a silicon-based optoelectronics
Also for β-FeSi2 there are still some open questions, such as the role of defects
in the light emission processes, that need to be solved, to improve device
performances.
Another interesting example of a Si-based LED present in literature has been
fabricated by implanting boron ions in crystalline Si [17]. According to the
authors the implant introduces also dislocations loops that produce a local
strain field which modifies the band structures providing spatial confinement of
the charge carriers. This spatial confinement allows room temperature
electroluminescence at the band edge. The EL signals were observed only
under forward bias at very low voltages ( ≤ 3 V ); moreover the EL signal
increases by increasing the temperature, confirming a strong spatial
confinement of the radiative carriers. For these devices an external quantum
efficiency of 2 × 10-4 at room temperature was measured.
We have shown a brief overview of some LEDs based on Si present in the
literature. Other relevant papers in this field are listed in the reference section
[18-22]. A clear conclusion we can draw is that different strategies have been
studied to find for the problem of the fabrication of a Si-based light source a
solution that is compatible with Si VLSI technology, in fact this is a crucial
limit for devices based on porous Si. For devices based on Si nc it is exceeded
the problem due to the compatibility with VLSI technology, but important open
points are to obtain devices operating at low voltages, in a reproducible and
fully understood way (in fact it is noted that in many papers present in literature
the EL and PL spectra are different for wavelength and shape) and exhibiting
efficiencies high enough for industrial applications.
16
Chapter 1: Towards a silicon-based optoelectronics
Scheme of the thesis
The aim of my thesis is the realization and characterization of light sources
operating at room temperature and at different wavelengths based on silicon
nanocrystals or rare earth doped silicon nanocrystals. The scheme of the thesis
includes:
Chapter 2: after a brief overview on the main physical properties of bulk Si
and Si nanostructures, this chapter is divided into two main sections. The first
section is focused on the preparation of Si nc by PECVD, and on their
structural and photoluminescence properties. The second section is focused on
the preparation and on the photoluminescence properties of Er-doped Si nc.
Chapter 3: This chapter is focused on the fabrication and characterization of
light emitting devices based on Si nc. The main points are the study of the
structural properties of the active layer and of the electrical and
electroluminescence properties of the devices.
Chapter 4: This chapter is focused on the fabrication and characterization of
light emitting devices based on Er-doped Si nanoclusters; a particular attention
has been devoted to the study of the electrical and electroluminescence
properties of the devices.
17
Chapter 2: Structural and optical properties of Si nanocrystals
Chapter 2
Structural and optical properties of
Si nc and Er-doped Si nc
In this chapter we present a detailed study of the structural and optical
properties of Si nc and Er-doped Si nc. Strong room temperature
photoluminescence (PL) in the wavelength range 650-1100 nm has been
observed in high temperature (1000 - 1300 °C) annealed substoichiometric
silicon oxide (SiOx) thin films prepared by plasma enhanced chemical vapour
deposition. A marked redshift of the luminescence peak has been detected by
increasing the Si concentration of the SiOx films, as well as the annealing
temperature. Transmission electron microscopy analyses have demonstrated
that Si nanocrystals (nc), having a mean radius ranging between 0.7 and 2.1
nm, are present in the annealed samples. Each sample is characterized by a
peculiar Si nc size distribution, by increasing the Si content and/or the
annealing temperature of the SiOx samples the distributions become wider and
their mean value increases. The strong correlation between structural and
optical data indicates that they can be qualitatively interpreted in terms of
carrier quantum confinement. The possible reasons for the quantitative
discrepancy between the experimentally measured luminescence energy values
and the theoretical calculations for the enlargement of the bandgap with
decreasing the crystal size are also discussed. Furthermore, it is shown that the
PL time-decay follows a stretched exponential function whose shape tends
towards a single exponential for almost isolated nc. This suggests that
stretched exponential decays are related to the energy transfer from smaller
towards larger nc. Excitation cross sections have been measured, yielding a
value of about 1.8x10-16 cm2 for isolated nc excited with 2.54 eV photons. When
Er ions are introduced in these samples a strong nc–Er interaction sets in and
the energy is preferentially transferred from the nc to the Er ions. The ncrelated luminescence is quenched and the Er-related luminescence at 1.54 µm
appears. The effective excitation cross section of Er ions through Si nc has
been determined to be about 1.1x10-16 cm2. This number resembles the
excitation cross section of nc themselves demonstrating that the coupling is
extremely strong.
19
Chapter 2: Structural and optical properties of Si nanocrystals
2.1 Recombination processes in crystalline silicon
The luminescence from a semiconductor is generally the result of an electronhole pair radiative recombination. The photon energy is equal to the energy
released in the recombination. Since photon momentum is negligible, its
emission requires either direct recombination or generation (absorption) of a
phonon for momentum conservation. Semiconductors are therefore commonly
divided into two distinct categories depending on the nature of their energy
band-gap (fig.2.1).
Fig.2.1 Schematic of the direct and indirect band-gap semiconductors.
In direct band-gap semiconductor the minimum of the conduction band and the
maximum of the valence band are coincident in the k-space of the crystalline
momentum (e.g.GaAs). On the other hand, silicon has an indirect band-gap and
20
Chapter 2: Structural and optical properties of Si nanocrystals
the maximum of the valence band and the minimum of the conduction band
appear not to coincide; its band structure is shown in fig.2.2.
Fig.2.2 The band structure of silicon along the X(100) direction and along L(111) direction.
The valence band edge is at the Γ-point (k=0), the botton of the conduction band is along the ∆line at 0.85 of the way to the Χ-point.
Thus, radiative recombination in silicon requires a phonon, which significantly
reduces the probability and rate of these processes, resulting in a radiative
lifetime of about 10 ms above 20 K [23-24] to be compared with radiative
lifetimes for direct band-gap semiconductors of the order of 100 ns. The τrad is
given by:
τrad=
1
B(n0 + ∆n )
(2.1)
21
Chapter 2: Structural and optical properties of Si nanocrystals
where B is the coefficient for radiative recombination, n0 the dopant density for
p-type or n-type material and ∆n the injected carrier density. At the same time,
there are competing non radiative processes, such as Shockley-Hall-Read
recombination through deep traps due to the presence of defects or impurities
in the crystal (see fig.2.3) [25-26], characterized by minority carrier lifetime in
the low-injection regime given by:
τSHR=
1
(2.2)
N T vthσ T
where NT is the density of traps, vth is the thermal velocity of the minority
carriers and σT is the capture cross-section for the minority carriers. The
lifetime for the SHR recombination is at room temperature often of the order of
100 µs. This is much shorter than the radiative recombination lifetime for e-h
pairs in silicon, indicating that non-radiative recombinations are much more
probable.
Fig.2.3 Recombination / generation processes of e-h pairs in crystalline silicon by means of
intermediate trap centers.
22
Chapter 2: Structural and optical properties of Si nanocrystals
Furthermore in heavily doped p-type and n-type silicon e-h pairs can
recombine through Auger processes (see fig.2.4) where the energy of the e-h
pair is given to a third particle such as an electron (eeh Auger recombination
for n-type Si) or a hole (ehh Auger recombination for p-type Si). Non-radiative
Auger recombination provides the fastest deexcitation path [27].
The lifetime τAuger for an Auger process is determined by:
τAuger=
1
(2.3)
C p p02
or
τAuger=
1
(2.4)
C n n02
where Cp and Cn are the Auger coefficients for the ehh (recombination of a e-h
pair and excitation of a hole) and eeh (e-h recombination and excitation of an
electron) and p0 and n0 are the dopant concentrations. Typical values for the
Auger coefficients are Cp~10-31 cm6s-1 and Cn~2.8·10-31 cm6s-1 respectively for p
and n type silicon. As an example, typical values for the lifetime for the Auger
recombination are τAuger ≈ 10 µs for a dopant concentration of 1018 cm-3.
Fig.2.4 Auger recombination process involving free carriers (eeh or ehh).
23
Chapter 2: Structural and optical properties of Si nanocrystals
Since all the above described different recombination processes act in parallel,
the total lifetime is given by
1
τ
=
1
τ rad
+
1
τ SRH
+
1
τ Auger
(2.5)
and the quantum efficiency is:
η=
τ nonrad
τ nonrad + τ rad
(2.6)
where τnonrad is the lifetime for the non radiative processes.
The efficiency of light emission from crystalline Si is low (10-6-10-7) at room
temperature, since the non-radiative recombination processes are faster. It can
be increased either by reducing the radiative lifetime or by increasing the nonradiative lifetime (i.e. suppressing non-radiative processes).
2.2 Silicon nanocrystals
The above considerations demonstrate that crystalline silicon is unsuitable for
optoelectronic applications, because of the many non radiative phenomena that
suppress the radiative recombination of e-h pairs. In order to increase the
quantum efficiency, we may, for example, act by limiting the movement of an
exciton by confining it in a limited space, so reducing the probability to
encounter a defect, and consequently, the incidence of the non-radiative
phenomena. If we
confine the excitons in a very small region, having
dimensions of the orders of a few nanometers, we may also increase the
radiative recombination rate and, consequently, the quantum efficiency. In fact,
for this range of the size, the properties of matter completely change. Since the
order of magnitude of the electron and hole De Broglie’s wavelength (∼1 nm)
24
Chapter 2: Structural and optical properties of Si nanocrystals
is comparable with the confinement dimension, they behave as particles in a
box and the problem can be solved by quantum mechanisms. As a
consequence, the confinement effect in a region of nanometric size is referred
to as quantum confinement effect, and the physical structure in which the
quantum confinement of excitons occurs is termed as a nanostructured
material. In order to circumvent the inability of crystalline silicon for
optoelectronic applications we can follow the route of the quantum
confinement of excitons in Si-nanostructures. Quantum confinement of
excitons may occur in only one dimension, in two dimensions or in all the three
dimensions, depending on the shape of nanostructures.
At each confinement direction corresponds a radical change in the wave
function of the particle and, as a consequence, a series of discrete levels
appears. By considering that in bulk crystal a carrier is free to move in every
directions, in a 2-D structure there are only two directions for the movement,
while the third direction determines the quantum confinement direction. In a 1D structure the free movement is possible in only one of the three directions,
while in the two remaining directions the quantum confinement occurs. In a 0D structure there is a total confinement in each direction and the considered
particle cannot move freely anymore.
2-D nanostructures, also termed as quantum wells, can be obtained by
depositing layers of different materials having nanometric dimensions (see
fig.2.5); for example, in Si/SiO2 multilayers SiO2 layers act as a barrier, while
excitons are confined in the Si layers. 1-D structures are also refereed as
quantum wires and can be obtained by lithographic processes (see fig.2.5). 0-D
structures are named quantum dots and can be obtained by means of different
techniques such as chemical vapor deposition, ion implantation, sputter
25
Chapter 2: Structural and optical properties of Si nanocrystals
deposition etc. Si nanocrystals embedded in SiO2 are an example of 0-D
nanostructures (see fig.2.5).
Fig.2.5 Classification of low-dimensional structures.
As a results of quantum confinement in the different directions there is a
change in the wave function describing the behaviour of electrons and holes,
and consequently also the number of states per unit energy, i.e. the density of
state (DOS), changes as a function of the energy E of the particle, as illustrated
in fig.2.6. In the case of a bulk material, the density of states increases with the
energy of the particle following a parabolic law, being the DOS proportional to
E1/2. Quantum confinement in one direction determines a step-like increase of
the DOS function with the energy. Totally different is the DOS function in the
case of a 1-D nanostructure, where the number of state decreases following the
26
Chapter 2: Structural and optical properties of Si nanocrystals
E-1/2 law as the energy increases. Finally, in the case of a 0-D structure the
function, in the ideal case, is formed by deltas marking the presence of discrete
levels.
Fig.2.6 Schematic of the low-dimensional structure and the density of states (DOS) as a
function of the energy for a particle constrained to move in the bulk, in a q-well, in q-wire and in
a q-dot.
The first experimental evidence that Si nanostructures can really constitute a
fundamental step towards the development of a Si based optoelectronics was
the observation by Canham and co-workers of a strong room temperature
27
Chapter 2: Structural and optical properties of Si nanocrystals
luminescence from porous silicon. Porous silicon is made by silicon crystallites
having nanometric dimensions, where non-radiative processes are almost
ineffective since the e-h pairs move in a limited space. As a result, a quantum
efficiency of more than 10% at room temperature and 50% at low temperatures
[28] can be reached.
Porous silicon gives a photoluminescence emission in the visible or near
infrared region, with a photoluminescence peak shifting in the blue region as
the porosity is increased. The use of porous silicon has several drawbacks,
determined by its brittleness and by the instability connected to the aging
process. In fact, an oxidation process due to air exposure
reduces the Si
nanocrystals dimensions and blue shifts the PL peak, reducing also drastically
the PL intensity. As a consequence the research has moved towards more
stable Si nanostructures and in particular to Si nanocrystals (nc) embedded in a
SiO2 matrix.
Si nc are complex systems and several models have been proposed in order to
explain their optical properties and the band structure; however a general
disagreement
remains
between
experimental
findings
and
theoretical
calculations, though all models lead to an increase of the band-gap with
decreasing crystallite size due to the quantum confinement, as schematically
shown in fig.2.7. The effect is observable as the nanocrystal size becomes
smaller than 10 nm. The simplest model we can consider to explain the
quantum confinement effect recurs to the effective mass approximation (EMA).
According to this approximation, electrons and holes in a semiconductor are
described as independent particles with an effective mass, me and mh
respectively, determined by the convexity of the band edge structure.
28
Chapter 2: Structural and optical properties of Si nanocrystals
Fig.2.7 The increase of the band-gap with decreasing crystallite size due to the quantum
confinement.
By using this model a Si nc is thought as a spherical potential well that bounds
an exciton by means of a high and wide potential barrier determined by the
SiO2 matrix.
In the EMA approximation, a characteristic length of the system, used to
establish the presence of quantum confinement effects, is the Bohr radius of the
exciton, defined as:
ab =
4πh 2ε r ε 0
µe 2
(2.7)
where µ is the reduced mass of the electron-hole pair, e is the electric charge of
the electron, εr is the relative dielectric constant of Si, εo the dielectric constant
of vacuum and h is the Planck’s constant.
29
Chapter 2: Structural and optical properties of Si nanocrystals
Quantum confinement effects occur if the dimension of a Si nc is comparable
or smaller than the Bohr radius of the exciton, that is equal to 4.3 nm for bulk
silicon. We can distinguish a weak confinement for Si nc whose size is larger
than ab and a strong confinement occuring in Si nc having dimensions smaller
than ab.
Effective mass approximation can be illustrated by a simple case of
confinement in a nanocrystal with linear dimension Lx, Ly, Lz, where the bandgap is given by the expression:
E nc = E Si −bulk +
h 2π 2
2
 1
1  1
1
1 
 ∗ + ∗  2 + 2 + 2 
m

L y L z 
 e m h  L x
(2.8)
where ESi-bulk=1.1 eV, while me∗ e mh∗ are respectively the effective mass of
electron and hole. By this expression it is evident that the energy necessary to
generate an electron-hole pair increases by decreasing the size of the system
[29-31].
Theoretical calculations of the band-gap widening have been confirmed
experimentally by absorption measurements, while PL measurements often
show a large Stokes shift (see fig.2.8). To explain the Stokes shift existing
between the absorption and the emission many authors [29-34] have suggested
the emission could be due to a radiative centre located at the Si-nc/SiO2
interface. In particular the double bonds Si=O present at the Si-nc/SiO2
interface may introduce a deep level in the energy gap, explaining the
divergence between absorption and PL spectra.
30
Chapter 2: Structural and optical properties of Si nanocrystals
Fig.2.8 Optical bandgaps of silicon nanocrystals and porous silicon samples obtained from
optical absorption and luminescence. Dashed and continous lines are calculated values with and
without excitonic correction, respectively. From Delerue et al.[31].
2.3 Si nanocrystals synthesis
Si nc embedded in SiO2 can be obtained by using several preparation
techniques, such as laser ablation [35], aerosol techniques [36], sputter
deposition [37], ion implantation [32,38-41], chemical vapor deposition (CVD)
[33-34], or plasma enhanced chemical vapor deposition (PECVD) [34]. A
schematic picture of the different techniques is reported in fig.2.9. In the laser
ablation technique an energetic laser flux evaporates the silicon surface and Si
nc are formed by condensation in a gas cell, whose pressure controls, along
31
Chapter 2: Structural and optical properties of Si nanocrystals
with the power of the laser pulse the dimensions of Si nc. In the aerosol
technique Si nc are formed and dispersed in a carrier gas flow where they may
undergo oxidation processes or also size selection by precipitation. In the
sputtering technique Ar+ ions are used to hit a target made of Si and SiO2. The
Si and O atoms so obtained are then deposited on a substrate in order to obtain
SiOx films, from where Si nc are obtained by a subsequent thermal annealing.
In the ion implantation technique Si nc can be produced by implanting Si ions
at high doses into silicon oxide film, followed by high temperature annealing to
induce precipitation and form the Si nc.
Fig.2.9 Schematic of the different techniques used to prepare Si nc.
In this thesis the Si nc were prepared by using the PECVD technique. The
PECVD technique is fully compatible with the Si VLSI technology and
32
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