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BRAZIL-MRS
2nd Brazilian MRS Meeting
October 26-29, 2003
Workshop:
Growth, Characterization and Device
Applications of Semiconductor Nanostructures
Based on Group III Nitrides
Symposium Organizers:____________________________________________________
José Roberto Leite (USP)
Eronides F. da Silva Jr. (UFPe)
1
INVITED PRESENTATIONS
W-I1
TRENDS IN SURFACE AND INTERFACE ELECTRONIC PROPERTIES OF III-NITRIDE
MATERIALS
Robert J. Nemanich, North Carolina State University, Raleigh NC 27695-8202, USA
The properties of clean III-nitride surfaces will serve as a starting point for understanding the
development of the electronic states at metal, dielectric, and heterojunction interfaces. Through a
variety of surface spectroscopies and surface imaging approaches, it is established that the polar
nature of the (0001) surfaces significantly affects the electronic states of the clean surfaces. The
properties of metal and dielectric interfaces are then described and related to the electronic states of
the clean surfaces. General trends of band bending and interface dipoles are presented for both nand p-type surfaces. The extension of these properties to non-polar surfaces is then described.
W-I2
DEVELOPMENT AND PRODUCTION OF HIGH PERFORMANCE GaInNAs 1.3 μm
VERTICAL CAVITY SURFACE EMITTING LASER
A. P. Lima, G. Ebbinghaus, A. Ramakrishnan, D. Supper, G. Kristen, M. Popp, H. Riechert, and G.
Steinle, Infineon Technologies AG, D-81730 Munich, Germany
Long-wavelength lasers emitting in the 1.3 to 1.6 μm range form the cornerstone of the
telecommunications optoelectronic devices, mainly due to the dispersion minimum of the standard
single-mode fiber at this minimum wavelength region, what allow maximum transmission rates and
much longer transmission distance. For vertical-cavity surface emitting lasers (VCSELs) gain
structures that emit in this region and can be epitaxially grown on GaAs substrates are particularly
interesting, because they allow the use of mature and efficient AlGaAs/GaAs distributed Bragg
reflectors (DBR) mirrors technology. An successful approach to achieve this goal utilizes the
alloying of InGaAs with low concentrations of GaN, forming the GaInNAs, what lead to a strong
bowing of the bandgap. Since the lattice constant of the cubic GaN is 20% smaller than that of
GaAs, in principle all of the compressive strain in InGaAs can be compensated. In this work we
will discuss the epitaxy of GaInNAs by Molecular Beam Epitaxy (MBE). It will also be presented
the realisation, development and production of 1.3 μm GaInNAs-based VCSEL with datacom
suitable performance. These low cost laser diodes are well suited for optical interconnect
aplications with transmission distances up to 15 Km. The possibilities as well as the advantages and
limits of shifting the wavelength from commercially available VCSEL emitting at 850 nm to 1300
nm are discussed.
2
W-I3
OPTICAL PROPERTIES OF CUBIC GAN WITH AND WITHOUT CARBON DOPING
J. R. L. Fernandez, F. Cerdeira, E. A Meneses, Instituto de Física “Gleb Wataghin”, Universidade
Estadual de Campinas, Caixa Postal 6165, 13.083-970 Campinas, SP, Brazil; J.A.N.T. Soares,
A.M. Santos, O.C. Noriega, J.R. Leite, Instituto de Física, Universidade de São Paulo, Caixa Postal
66318, São Paulo, SP, Brazil; D. J. As, U. Köhler, D.G.P. Salazar, D. Schikora and K. Lischka,
Universität Paderborn, FB-6 Physik, D-33095 Paderborn, Germany
The fabrication of the first c-InGaN/GaN double-heterostructure LED grown by MOCVD has been
recently reported [1]. Contrary to their hexagonal counterparts, the cubic structures can be grown
free from modulation due to spontaneous polarization and strain-induced piezoelectric fields. Thus,
the spatial separation of the carriers wave function, induced by the quantum-confined Stark effect
in the hexagonal phase, is avoided in the cubic structure. For this reason the cubic polytype is
expected to increase the optical recombination efficiency in nitride-based quantum wells. Hence,
the growth and characterization of high-quality c-GaN epitaxial layers is an essential first step on
the road to high performance devices fabricated with this material. Also, for the fabrication of
devices with this material, it is essential to be able to introduce p- and n-type doping in a controlled
manner. This involves introducing dopant impurities, which produce shallow acceptor or donor
levels. Understanding how these impurities enter into the GaN lattice is of fundamental importance
for achieving useful doping.
In the present work we report the results of a study of the optical and x-ray diffraction experiments
performed on cubic GaN thin films, deposited by plasma-assisted molecular beam epitaxy on GaAs
(001) substrates, with and without intentional doping with carbon atoms (p-type doping). We used
photoluminescence (PL), photoluminescence excitation spectroscopy (PLE) and photoreflectance
(PR) to study the spectral region near the fundamental absorption edge of these samples and Raman
scattering by phonons to obtain structural information. The PLE experiments, carried out for the
first time on these type of samples, allow the identification of the optical transitions taking place in
the layers. We observe a clear step-like absorption edge, resulting from the merging of the free
exciton with the continuum. Quantitative values for the absorption edge energy and lifetime
broadening are obtained. The temperature dependence of the absorption edge of the cubic material
is also measured and is analyzed with standard theoretical models. The evolution of the optical
spectra of the C-doped samples is consistent with a picture in which carbon enters into N-vacancies
at low concentrations, thus producing a marked improvement in the crystalline properties of the
material. At higher concentrations it begins to form complexes, possibly due to interstitial
occupation. This view is reinforced by the observation of a line (at 584 cm-1) in the Raman
spectrum of the samples with relatively large amounts of carbon and best crystalline quality. A
valence force field calculation attributes this line to a resonance vibration of C-atoms occupying Nsites in the cubic GaN lattice. Finally, x-ray diffraction data support these conclusions based on
optical experiments
[1] Y. Taniyasu et. A, Phys. Status Solidi (a) 180, 241 (2000).
3
W-I4
CONFINEMENT OF POLAR OPTICAL PHONONS IN AlN/GaN MULTILAYER
STRUCTURES
E. L. Albuquerque, Departamento de Física, Universidade Federal do Rio Grande do Norte, NatalRN, Brazil
Complex structures, with dimensions of nanometers, composed by semiconductor materials, have
been a topic of a lot of theoretical and experimental research, due to their technological potential.
Among its applications, we stress the enhancement of semiconductor lasers performance through a
better understanding of the confinement of polar optical phonons in the structure. Investigations
about optical phonons in semiconductors with wurtzite (eg. Nitrites) crystalline structures have
been done due to a large gap characteristic featured by these semiconductors. Since the wurtzite
crystals have a different unit cell structure (4 atoms by unit cell) as well as a lower symmetry, when
compared to materials like “zinc-blend”, the optical phonon confinement in these materials has
important physical properties, not completely understood yet. In this work we study the optical
phonon spectra in multilayer structures composed of AlN/GaN layers arranged in a periodical
fashion. We make use of a model based on the Frölich Hamiltonian, taking into account the
macroscopic theory developed by Loudon. Despite GaN and AlN nitrites usually have nine optical
phonons, we will consider in this work only the active phonons in the infra-red region. Therefore,
on the Γ-point of the electronic structure, there will be an activation of optical phonons polarized
parallel to the crystal easy axis - A1(Z) modes, as well as the orthogonally polarized tothis axis E1(XY) modes. Numerical results of the confined optical phonon spectra are presented and their
main aspects are discussed.
W-I5
SPIN-POLARIZED CHARGE DENSITY versus MAGNETIC ORDER IN (Ga,Mn)N
I. C. da Cunha Lima, Instituto de Física, Universidade do Estado do Rio de Janeiro, Rua São
Francisco Xavier, 524, Maracanã 22.500-013 Rio de Janeiro, RJ, Brazil
The realization of ferromagnetic (Ga,Mn)N thin films with high transition temperatures has
attracted much attention during the last few years. In (Ga,Mn)As, a substitutional Mn binds a hole,
acting as a shallow acceptor. It also introduces a localized magnetic moment due to the half-filled
3d orbital. In (Ga,Mn)N, the hole binding energy is large, and it is hard to justify the observed high
density free carrier sea. The sensitivity of the transition temperature on the annealing processes is
an indication that sources of carriers and magnetic moments are different. Once a high density
carrier system exists, there is no reason to assume the mechanism leading to the magnetic order to
be different from that of (Ga,Mn)As. But mean field explanations seems to be excluded, since
transition temperatures have been obtained within this approximation near 300K. Here I review
some recent theoretical results concerning the electronic structure of very thin (Ga,Mn)N layers. I
distinguish those containing a homogeneous concentration of Mn, which comes out of a growth by
MBE technique, for instance, from those with a Gaussian-like distribution, in ion-implanted
samples. I also comment on the magnetic order which has been obtained by Monte Carlo
simulations in such samples.
4
W-I6
OPTICAL AND ELECTRONIC PROPERTIES OF UNDOPED AND DOPED GAN FILMS
J. A. Freitas, Jr., Naval Research Laboratory, Washington, DC 20375 (USA)
The remarkable progress observed in the last two decades in heteroepitaxial growth of GaN has
been achieved as a result of a better understanding of the GaN nucleation process and the
introduction of a two-step growth method. It was demonstrated that by adding a thin layer of GaN
or AlN deposited at lower temperature ( 500C) between the substrate and the active layers
(deposited at high-temperature, ~ 1000C), a considerable improvement of the structural and
electronic properties of the top layers was achieved. A number of researchers across the world
have contributed to the optimization of both the nucleation layer and the high-temperature film,
resulting in a substantial reduction (102-103) of the unknown n-type background carrier
concentration. Unintentionally doped films with room temperature free-carrier concentrations
typically between 5x1016 and 1x1017 electrons/cm-3 were successfully doped with Mg to achieve ptype conductivity. These accomplishments led to the fabrication and commercialization of a
number of optical devices, despite the high concentration of dislocations (typically between 10 9 and
1010 cm-2), the limitation on the hole concentration, and the lack of identification and further
reduction of the background donor concentration.
We report here on a systematic investigation carried out on freestanding unintentional doped (UID)
HVPE-GaN, and doped and UID-homoepitaxial layers to identify the chemical nature of the
shallow donors. Results from high-resolution, variable temperature photoluminescence (PL)
experiments performed in the spectral region associated with recombination processes involving the
ground and excited states of the neutral donor bound excitons and their two-electron-satellites in
GaN substrates will be presented. High-resolution PL studies of UID and doped homoepitaxial
GaN MOCVD-films confirm unambiguously previous impurity identifications.
CW and pulsed variable-temperature PL studies carried out on heteroepitaxial GaN MOCVD-films
doped with different levels of Mg shown evidences of complex formation and local potential
fluctuation. Both mechanisms affect devices performance.
If the time allows comparison between the properties of the state of the art cubic and wurtzite GaN
films will be presented.
This work partially supported by the Department of the Navy Grant by ONR (Contract
N00014WR20015 monitored by Dr. C.E.C. Wood)
W-I7
A NEW X-RAY DIFFRACTION METHOD FOR THE INSITU DIAGNOSTICS OF EPITAXIAL
FILMS DURING MOCVD
A. Kharchenko and K. Lischka, University of Paderborn, Department of Physics, D- 33098
Paderborn, Germany; K. Schmidegg, A. Bonanni and H. Sitter, Institute for Semiconductor and
Solid State Physics, Johannes Kepler University, A-4040 Linz, Austria; J. Bethke, PANalytical
B.V., 7602 EA Almelo, The Netherlands
We will present a new method of diffraction x-ray which allows the investigation of semiconductor
structures during metal-organic chemical vapor deposition (MOCVD). Our setup does not use a
goniometer stage and has a conventional x-ray tube. It does not need precise adjustment of the
samples before measurement and allows x-ray diffraction measurements within a few seconds.
The new apparatus has been tested with different material systems which are important in
semiconductor industry today, namely AlGaN-, InGaN- based materials and SiGe semiconductors.
Information about the composition of layers, the period of multiquantum well structures as well as
the thickness of nm thin layers was obtained. All results perfectly agree with the results obtained by
conventional high resolution x-ray diffraction.
We employed our new method to measure the x-ray diffraction during the MOCVD of cubic IIInitride layers. The experiments demonstrated that the growth rate and the composition of the
growing layers can be monitored with high accuracy. These are worldwide the first insitu x-ray
diffraction measurements performed without the use of a synchrotron source.
5
W-I8
MICROSTRUCTURE AND ELECTRONIC PROPERTIES OF InGaN QUANTUM WELLS
F. A. Ponce, Department of Physics and Astronomy, Arizona State University, Tempe, Arizona,
U.S.A.
This talk will present an overview of the properties of In xGa1-xN epilayers based on a systematic
study of thick layers and of quantum well structures. We find that the microstructure of thick films
varies significantly with indium composition. For x<0.08, the alloy is uniform and unperturbed by
dislocations. For 0.10<x<0.20, secondary phases nucleate at threading dislocations. For x>0.20,
spontaneous phase separation occurs resulting in a polycrystalline, inhomogeneous layer. A
correlation between optical properties and microstructure will be presented. It is observed that the
misfit strain is affected by threading dislocations. Mechanisms of misfit strain relaxation have been
identified for InxGa1-xN layers grown on standard GaN on sapphire and on epitaxial-lateralovergrowth GaN layers. In addition, we have studied the properties of quantum well structures
using several novel techniques. The electrostatic fields across the wells have been profiled using
electron holography in the TEM. The effect of well thickness on the strength of the fields will be
reported. The effects of localization by compositional fluctuations and of internal field screening
have been studied using time-resolved cathodoluminescence spectroscopy.
W-I9
HIGH POWER AlGaN-BASED DEEP ULTRAVIOLET LIGHT-EMITTING DIODES BY
METALORGANIC CHEMICAL VAPOR DEPOSITION
M. Razeghi, Center for Quantum Devices, Department of Electrical and Computer Engineering,
Northwestern University, Evanston, IL 60208
We demonstrate high power deep ultraviolet light-emitting diodes based on AlGaN grown by
MOCVD at wavelengths of 265 nm and shorter. The structure consists of a single quantum well
active region confined between n- and p-type AlGaN confining layers grown on high-quality
AlGaN/AlN template layer. Very narrow X-ray rocking curve with a FWHM of 40 arc.sec
demonstrates the high quality of the AlN epilayer. A smooth surface with well-ordered atomic steps
was observed after growing an Al0.85Ga0.15N/AlN superlattice on top of the AlN layer. A second
AlN layer was grown on top of the superlattice for strain relief. Highly conducting n+-AlInGaN
layer was used for n-type contact layer, with a mobility of 55 cm2/V.s and a carrier concentration of
1.7×1018 cm-3. The active region consists of an asymmetric single quantum well topped by a high
Al-content AlGaN current blocking layer for better carrier confinement. The structure was
terminated by a thin p-GaN contact layer for ohmic contact. The surface of the sample, examined
by optical and scanning electron microscopes, was found to be smooth and crack free. In order to
help dissipate the generated heat during operation of the devices, the LEDs were flip-chip bonded
on AlN submounts. The output power of the LEDs reaches a value of 1.1 mW at an injection
current of 1 A in pulsed operation mode. Due to heating effects, the output power is lower under
continuous-wave (CW) injection (less than 100 W) and saturates at currents of ~300 mA.
Packaging an array of four diodes in parallel can yield higher performance by distributing the heat
between four mesas. A record high output power of 4.5 mW under pulsed injection was achieved,
while the CW operation results in 160 W of light output power.
6
W-I10
InAlGaN QUATERNARY ALLOYS: THE ENERGETICS AND THERMODYNAMIC
PROPERTIES
L. M. R. Scolfaro, Instituto de Física, Universidade de São Paulo, CP 66318, 05315-970, São
Paulo, SP, Brazil
Recently, the group-III nitride quaternary alloys have been object of intense investigations, due to
their importance in the production of efficient UV light emitting diodes and laser diodes. The
possibility of alloying with Indium, besides of using Aluminum, makes the InAlGaN alloy more
flexible since lattice matched materials can be easily achieved. Although a lot of experimental
investigations have been carried out on these quaternary alloys, very few theoretical attempts were
conducted towards a deeper understanding of the energetics and the thermodynamic properties of
InAlGaN alloys.
In this work, the In1-x-yAlxGayN alloys are studied through ab initio total energy calculations,
together with the cluster expansion method within the generalized quasi-chemical approach as well
as via Monte Carlo simulations. Results for structural and electronic properties, such as the lattice
parameter and the energy bandgap of the alloy are shown, as a function of the alloy contents, x and
y. The effects of phase separation in the InAlGaN alloys are discussed, and their relation with the
emission mechanisms taking place in these materials is clarified.
W-I11
QUANTUM WIRES AND QUANTUM DOTS IN III-NITRIDES: AN OVERVIEW
V. N. Freire, E. W. S. Caetano, and G. A. Farias, Departamento de Física, Universidade Federal do
Ceará, Caixa Postal 6030, Campus do Pici, 60455-900 Fortaleza, Ceará, Brazil; E. F. da Silva Jr.,
Departamento de Física, Universidade Federal de Pernambuco, Cidade Universitária, 50760-901
Recife, Pernambuco, Brazil
In the race to reduce the threshold current and voltage of III-nitride laser emitting diodes, a solution
is to reduce the active layer structures from two dimensions to one or zero dimension, i.e. to use
quantum wires and dots. Recent developments as the ultraviolet-blue laser action in single
monocrystalline GaN wires [1] are the result of intensive research focused on III-nitride based
quantum dots and wires in the last few years. The confinement of excitons in InGaN quantum dots
has been confirmed by photoluminescense [2]. The purpose of this talk is to present an overview of
the progress on the multitude of growth techniques, characterization, device applications, and
theoretical investigations concerning III-nitrides based quantum wires and dots, focusing both cubic
and wurtzite structures.
[1] J. C. Johnson, H.-J. Choi, K. P. Knutsen, R. D. Schaller, P. Yang, R. J. Saykally, Nature
Materials, 106 (2002). Published on line 15 September 2002, www.nature.com/naturematerials.
[2] O. Moriwaki, T. Someya, K. Tachibana, S. Ishida, and Y. Arakawa, Appl. Phys. Lett. 76, 2361
(2000).
7
POSTER PRESENTATIONS
W-P1
THEORETICAL STUDY OF III-NITRIDE-BASED TERNARY AND QUATERNARY ALLOYS
USING VALENCE FORCE FIELD MODEL
C. de Oliveira Depto. de Física, ICEX, UFMG, Caixa Postal 702, 30.123-970 Belo Horizonte, MG,
Brazil; J. L. A. Alves Depto. Ciências Naturais, UFSJ, Caixa Postal 110, 36.300-000 São João del
Rei, MG, Brazil.
Group-III (B, Al, Gan and In)-nitride quaternary alloys and group-III (Al, Ga, and In)- nitridebased mixed anion (As, P, and Sb) quaternary alloys are usefull for blue and green light emitting
devices and high-temperature, high-power, and high-frequency electronic devices. Following
Takayama et al. [J. Appl. Phys. vol. 90, 2358-2369, (2001).]. The thermodynamical stability of
these alloys we stydy with respect to an unstable two-phase region in the phase field. The unstable
two-phase region is predicted based on a strictly regular solution model. The interaction parameter
used in our model is obtained using the valence force field (VFF) model for both zinc-blende and
wurtzite structures. We use the VFF model also for predidting the microscopic crystal structure,
such as first neighbor anion-cation bond lengths and angle distributions. The unstable two-phase
regions for A1-x-yBxCyD type group-III-nitride quaternary alloys and A1-xBxC1-yDy type group-IIInitride mixed anion quaternary alloys are calculated. The predicted unstable two-phase regions
agree well with experimetally observed of phase separation in ternary alloy, which suggests that the
model calculations can provide useful guidance in ternary and quaternary systems where there is no
experimetal data. The electronic structure calculations for free-standing quantum dots of these
materials are performed using the semiempirical tight-binding method. Following Saito et al. [Phys.
Rev. B 57, 13016-13019, (1998)], we analize the density of states for the inside states and surface
states.
W-P2
OPTICAL PROPERTIES OF n-TYPE CUBIC GaN:Si
A. Ferreira da Silva, I. Pepe, N. Souza Dantas, Instituto de Física, Laboratório de Propriedades
Óticas, Universidade Federal da Bahia, Campus Universitário de Ondina, 40210 340 Salvador,
Bahia, Brazil; C. Persson, R. Ahuja, Condensed Matter Theory Group, Department of Physics,
Uppsala University, Box 530, SE-751 21 Uppsala, Sweden; H. Arwin, O. P. A. Lindquist, B.
Sernelius, Department of Physics and Measurement Technology, Linkoping University, SE-581 83
Linkoping, Sweden; D. J. As and K. Lischka, University of Paderborn, 33098 Paderborn, Germany
The optical and reduced bandgap energies and the dielectric functions of Si donor cubic GaN have
been investigated experimentally and theoretically. Experimentally by transmission (TR),
photoluminescence (PL) and spectroscopic ellipsometry (SE). Theoretically by an ab initio fullpotential linear augmented plane wave method as well as within a framework of the many-particle
random phase approximation with the Hubbard local-field correction.
The cubic GaN:Si epilayers were grown by an rf plasma-assisted MBE on semi-insulating GaAs
(001) substrates.
We have observed a shift of the optical and reduced bandgaps to higher energies with increasing Sidoping for PL and TR spectroscopies and calculation as well. The obtained MNM transition is
found at concentration of Si about 1E18cm-3 for GaN. The calculated dielectric functions were
found to be in good agreement with the SE measurements.
8
W-P3
(Ga,Mn)N-BASED FERROMAGNETIC SUPERLATTICES: ELECTRONIC PROPERTIES
S. C. P. Rodrigues1, L. M. R. Scolfaro1, J. R. Leite1, G. M. Sipahi2, I. C. da Cunha Lima 1,3, M. A.
Boselli3
1
Instituto de Física, Universidade de São Paulo, CP 66318, 05315-970, São Paulo, SP, Brazil;
2
Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970, São Carlos, SP,
Brazil; 3Instituto de Física, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier
524, 20.500-013 Rio de Janeiro, R.J., Brazil
(Ga,Mn)N has been determined to be a Diluted Magnetic Semiconductor (DMS) alloy with high
Curie temperature. In a high Mn concentration it is allowed to approximate the density of the
magnetic moments by a continuous distributiion when treating the magnetic interaction between
holes and the localized moment on the Mn++ sites. In this regime, the DMS layer is assumed to be
in their metallic and ferromagnetic phase. The electronic structure depends directly on the
orientation of the average magnetization being in-plane or perpendicular-to-the-plane. The final
spin and charge configuration contains a complete information about the composition of heavy
holes and light holes. We use a self-consistent electronic structure calculation based on the
Luttinger-Kohn model to obtain the properties of superlattices made by alternating layers
(Ga,Mn)N and GaN.
W-P4
PRESSURE-INDUCED PHASE TRANSITIONS AND THERMAL PROPERTIES OF GaN AND
InN.
C. C. Silva, J. L. A. Alves and H. W. Leite Alves, Depto. Ciências Naturais, UFSJ, Caixa Postal
110, 36.300-000 São João del Rei, MG, Brazil.
The group III-Nitrides(AlN, GaN, InN) and the corresponding alloys have attracted great interest
due to their succesful applications in the electronic and optoelectronic device technology. However,
their growth in the zinc-blende structure has been a hard task to the experimentalists, once the most
stable structure for these compounds is the wurtzite one. From the theoretical side, while the
structural, dynamical and electronic properties of III-Nitrides were extensively studied in the last
years, the amount of knowledge on their thermal properties and how to induce structural phase
transitions on these materials are rather scarce. So, in this work, we have calculated ab initio the
equation of state for the rocksalt(rs), zincblende (zb) and wurtzite(w) modifications of GaN and
InN, by using the density-functional theory within the local density approximation (LDA), plane
wave expansions and the pseudopotential method. From the obtained results, we evaluated the
following pressure-induced structural phase transitions: rs  zb and rs  w. Also, we have
calculated the temperature dependence for the Gibbs free energy, entropy and specific heat. A good
agreement with the experiment is obtained, whenever a comparison is possible. From our results,
we speculate whether these systems undergo, by a second-order phase transition, from the wurtzite
structure to the zincblende one.
W-P5
INFLUENCE OF MN DISTRIBUTION ON THE SHOTTKY BARRIER OF ION IMPLANTED
(Ga,Mn)N THIN FILMS
M. A. Boselli1, I. C. da Cunha Lima1,2, Angela M. Ortiz de Zevallos2, R. C. Pinto2, J. A. N. T.
Soares2, V. A. Chitta2, and J. R. Leite2; 1Instituto de Física, Universidade do Estado do Rio de
Janeiro, Rua São Francisco Xavier 524, 20.500-013 Rio de Janeiro, R.J., Brazil; 2Instituto de Física,
Universidade de São Paulo, CP 66318, 05315-970 São Paulo, SP, Brazil.
Experience has shown that good metallic contacts are crucial for reliable Hall measurements in
GaN, due to the appearance of Shottky barriers. In (Ga,Mn)N samples grown by ion implantation,
an inhomogeneous distribution of Mn in depth occurs. A simulation using TRIM has shown that the
density of Mn ions is well fitted by a Gaussian, whose center and width depend on the beam energy
and incidence angle. Treating the dose and the carrier density as independent variables we
calculated self-consistently the eigenstates of the Mn-carriers system. The interaction between
carriers and ionized atoms, and the carrier-carrier interaction are considered. The magnetic
interaction is assumed through a Kondo-like term. The converged potential shows an effective
barrier separating the carriers from the surface of incidence. This potential has to be added to the
regular Shottky barrier, when metallic contacts are grown. Our calculation is performed for several
doses, beam energy, incidence angle, and ratio between the dose and carrier concentration.
9
W-P7
FERROMAGNETIC ORDER IN THE CUBIC METALIC PHASE OF A (Ga,Mn)N: MONTE
CARLO SIMULATION
M. A. Boselli1, I. C. da Cunha Lima1,2, J. R. Leite2, A. Troper3, and A. Ghazali4; 1Instituto de
Física, Universidade do Estado do Rio de Janeiro, Brazil; 2Instituto de Física, Universidade de São
Paulo, Brazil; 3Centro Brasileiro de Pesquisas Físicas, Brazil; 4Groupe de Physique des Solides,
Universités Paris 6 et Paris 7, France
In (Ga,Mn)N there is a large discrepance on the reported measurements concerning the carrier
concentration, as well as in the transition temperatures. In this work we assume that, at least in the
metallic phase of (Ga,Mn)N, the same mechanism leading to the ferromagnetic phase of
(Ga,Mn)As applies. We assume the samples as type-p, disregarding the origin of these charges. The
concentration of Mn sites and the density of holes are taken as independent parameters. A Monte
Carlo simulation is used, the interaction between magnetic moments being due to an indirect
exchange mechanism provided by the local interaction of the localized magnetic moments with the
spin-polarized hole system. A self-consistent electronic structure calculation, which takes into
account the existence of an average magnetization of the Mn ions, is used to obtain the exchange
terms which enters the simulation. Therefore, electronic and magnetic properties are entangled. The
resulting transitions temperatures observed on the (Ga,Mn)N system is around 500 K, for a system
were p = 2. 1020cm-3, and two 20 Å (Ga,Mn)N layers separated by a 5Å GaN layer. This
temperature is aproximately 10 times that registered on a (Ga,Mn)As similar system.
W-P8
DILUTED MAGNETIC Ga1-XMnXN AND Al1-XMnXN ALLOYS: A FIRST-PRINCIPLES
STUDY
R. de Paiva, Depto.de Física, ICEX, UFMG, C.P. 702, 30123970 Belo Horizonte, MG, Brazil; R.
A. Nogueira, Depto. de Física, ICEX, UFMG, C.P. 702, 30123970 Belo Horizonte, MG, Brazil; J.
L. A. Alves, DCNAT, UFSJ, C.P. 110, 36300000 São João del Rei, MG, Brazil
The utilization of the quantum properties of the electron spin wave function will allow the
development a new class of devices. The problem is still controversial and unsettled, even
qualitatively, especially for concentrated spin systems such as 3d metals and their alloys. In this
work the properties of diluted Ga1-xMnxN and Al1-xMnxN (x = 0.0630; 0.0315) alloys are
calculated in the zinc-blende phase, within the framework of the density functional theory, using
the full-potential linearized augmented plane wave (FLAPW) method and the local density
approximation (LDA). The alloys are simulated using 32-atom and 64-atom large unit cells,
containing one substitutional Mn atom for a Ga or an Al atom. The calculations are spin-polarized
and we analyze band structures, density of states and total magnetizations. A half-metallic state is
predicted for both, Ga and Al, cases. The majority-spin band has a rather sharp peak, characteristic
of a narrow band, while the minority-spin has a gap. The total magnetization of the cell is 4.00B
which does not change with the Mn concentration. The valence band is ferromagnetically coupled
with the Mn atoms, and the spin splitting is not linearly dependent on the Mn concentration.
W-P9
DILUTED MAGNETIC SEMICONDUCTORS BASED ON 4D-TRANSITION METAL DOPED
GaN
R. de Paiva, Depto. de Física, ICEX, UFMG, C.P. 702, 30123970 Belo Horizonte, MG, Brazil; R.
A. Nogueira, Depto. de Física, ICEX, UFMG, C.P. 702, 30123970 Belo Horizonte, MG, Brazil; J.
L. A. Alves, DCNAT, UFSJ, C.P. 110, 36301160 São João del Rei, MG, Brazil
The electronic structures of diluted magnetic semiconductors (DMS) based on GaN and 4dtrasition metals (4d-TMs) were analyzed in a systematic ab initio study by means of the fullpotential linearized augmented plane wave method (FP-LAPW). In order to investigate the
magnetic state of the GaN-based DMSs, we suppose that the Ga atoms are substituted by the
transition metals TM = Zr, Nb, Mo, Tc, Ru, Rh, Pd building up the alloys Ga 0.94TM0.06N. We carry
out self-consistent electronic structure spin-polarized calculations based on the Density Functional
Theory, the Local Density Approximation and the large unit cell approach (32 atoms) for
generating a zinc-blende lattice. The relativistic effects are fully considered for the core states and
in the scalar approximation for the valence states. We analyze the total density of states per unit
cell, the local density of d states at the transition metal site, the magnetic moments per unit cell,
and the systematic behavior of the magnetic states of the 6 % TM-doped GaN-based DMSs.
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W-P10
THEORETICAL X-RAY SPECTRA OF III-NITRIDE COMPOUNDS
V. D. Falcão Depto. Ciências Naturais, UFSJ, Caixa Postal 110, 36.300-000 São João del Rei, MG,
Brazil; J. L. A. Alves Depto. Ciências Naturais, UFSJ, Caixa Postal 110, 36.300-000 São João del
Rei, MG, Brazil; R. de Paiva Depto. de Física, ICEX, UFMG, Caixa Postal 702, 30.123-970 Belo
Horizonte, MG, Brazil
The techniques of x-ray spectroscopy such as x-ray absorption spectroscopy (XAS), x-ray
emission spectroscopy (XES) and x-ray photoemission spectroscopy (XPS) have been used in the
study of semiconductors. The soft absorption spectroscopy and soft emission spectroscopy are
convenient techniques to study thin films of semiconductors, which are characterized by wide
energy gap. These techniques provide direct measurements of the density of occupied and
unoccupied states and, therefore, a direct comparison with the spectra obtained by electronic
structure calculations is possible. This fact permits the interpretation of the measured spectra in
terms of occupied states (absorption) and unoccupied states (emission). Since the core states are
associated to the chemical elements existing in the compound, these spectroscopies are also specific
for each element. In the present work we calculate the energy band structures of III-nitrides (AlN,
BN, GaN, InN) compounds and obtain their emission and absorption X-ray spectra. For each
semiconductor we study the K level (Al, B, Ga and In) and L3 level (Ga and In). The calculations
of the theoretical spectra are carried out by means of the model suggested by Neckel et al.
[Microchim. Acta, Suppl. 6, 257 (1975)] as implemented in the code Wien97, based on the ab inito
Linear Augmented Plane Waves Method. As far as we know, most of the results are original and
constitute an anticipation of experiments.
W-P11
EXCITONS IN CUBIC NON-ABRUPT GaN NANOSTRUCTURES
E. W. S. Caetano, V. N. Freire, G. A. Farias, Departamento de Física, Universidade Federal do
Ceará, Centro de Ciências Exatas, Campus do Pici, Caixa Postal 6030, 60455-900 Fortaleza, Ceará,
Brazil, E. F. da Silva Jr., Departamento de Física, Universidade Federal de Pernambuco, Cidade
Universitária, 50670-901 Recife, Pernambuco, Brazil
III-V nitride semiconductors have been investigated extensively in recent years. GaN is a direct and
wide band-gap semiconductor and when alloyed with InN and AlN, the range of electromagnetic
spectrum between visible to ultraviolet can be covered. GaN heterostructures and quantum wells
are of special importance in the active region of nitride- based light emitting diodes and laser
diodes. Advances in growth technology have stimulated investigations in the field of quantum dot
and quantum wire structures. Potential advantages over quantum wells due to the density of states
makes the nitride-based quantum wires and dots a very promising field of research. It has been
demonstrated the existence of graded interfaces 1-1.5 nm thick AlGaN/InGaN/GaN
heterostructures, which makes reasonable the assumption of non-abrupt interfaces for GaN/AlGaN
heterostructures in general. In this work, we investigate how non-abrupt interfaces affect the carrier
and exciton confinement properties in cubic GaN/AlGaN wells, wires and dots. The effects of
nanostructure dimensionality on the exciton energy are studied. When the existence of graded
interfaces is taken into account, a strong blue shift in the exciton energy is obtained in comparison
to the abrupt case. This blue shift is more pronounced for low-dimensional nanostructures.
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W-P12
IMPROVING OF THE P-TYPE DOPING BY CARBON IN CUBIC GAN SAMPLES UNDER
GA-RICH GROWTH CONDITIONS
D.G. Pacheco-Salazar and J.R. Leite. Institute of Physics, University of São Paulo P.O. Box 66318,
05315-970 São Paulo-SP, Brazil.
D.J. As, S. Potthast and K. Lischka. University of Paderborn, Department of Physics, D-33095,
Paderborn, Germany
The most common impurity to achieve p-type doping in molecular beam epitaxy grown GaN is Mg.
However Mg has a large acceptor ionization energy and shows severe compensation effects.
Therefore we investigate carbon as an alternative p-type dopant, due to its similarity to nitrogen in
atomic radius and electronegativity. Recently it was shown that carbon doped cubic (c-) GaN
samples grown under stoichiometric growth conditions give a maximum hole concentration of
~6x1017 cm-3.
We have grown c-GaN:C on a semi-insulating GaAs wafer (3 inches). Morphological
characterization showed that the density of Ga-droplets increases from the center to the edge. This
implicates a gradient of the Ga-flux over the wafer and ensures Ga-rich growth conditions.
Electrical characterization on selected positions of the c-GaN:C wafer at room temperature (RT),
gave hole concentrations and mobilities as high as 6.1x10 18 cm-3 and 23.5 cm2V-1s-1, respectively.
RT photoluminescence measurements showed a clear near band edge luminescence at 3.2 eV and
an orange band at about 2.2 eV. These results lead to the conclusion that high p-type doping can be
achieved under Ga-rich growth conditions.
W-P13
INFLUENCE OF PLASMA ETCHING AND THERMAL ANNEALING CONDITIONS ON THE
ELECTROLUMINESCE OF GaN pn JUNCTIONS
Angela M.Ortiz de Zevallos, M. Ribeiro, and J.R. Leitte, Instituto de Física, Universidade de São
Paulo, CP66318, São Paulo, SP, Brazil, A.Tabata, Universidade Estadual Paulista, J. A. Freitas, Jr.,
Naval Research Laboratory, Washington, D.C. 20375-5347
We have studied the electroluminescence efficiency from GaN pn junctions as a function of post
growth etching and thermal treatments. The samples were grown by HVPE on sapphire substrate.
The pn junctions were formed by growing 2.2m of Si-doped GaN followed by a 0.3m of Mgdoped GaN. In order to active the magnesium dopant, we annealed the sample using two different
methods: (a) conventional annealing during 30 minutes at the temperature of 950 C under a
nitrogen pressure of 65 atm, and (b) a Rapid Thermal Annealing of 30 seconds at temperature of
950 C. To access the p side of the junction, we have etched the n part of the sample using a
plasma etching technique. Two different plasmas were used: CF4 and Argon. Our experimental
results showed that the activation of the Mg doping were more efficient on samples treated by
conventional annealing and the better etching for this kind of samples was the CF 4 where flat
surface were obtained. For this optimized samples, clear blue electroluminescence was obtained.
For other samples we have observed just a very diffuse electroluminescence.
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