Zinc-blende GaN Research Articles

First principles calculation of effective mass parameters of GaN

First principles electronic band calculations are performed for the wurtzite and zincblende GaN, by using a full-potential linearized augmented plane wave method. In order to clarify the difference of electronic properties between two kinds of crystal structures, and to give important information on the characteristic analysis of GaN-based quantum well devices, the Luttinger-like valence band parameters, as well as the electron and hole effective masses and splitting energies, are derived from the calculated band structures with the assistance of the k·p theory, considering the cubic and hexagonal symmetries. The small spin-orbit coupling of a nitrogen makes us use the 6 × 6 k·p Hamiltonians for both structures in the analysis of the valence band states.

Simulation of Vacancy Pairs in GaN Using Tight-Binding Molecular Dynamics

AbstractThe electronic structure, geometry and energetics of Ga vacancy pairs and N vacancy pairs in both wurtzite and zincblende GaN are investigated via molecular dynamics (MD) simulations using an empirical tight-binding (TB) model with total energy capabilities and supercells containing up to 216 atoms. Our calculations suggest that, by pairing, N vacancies, which in isolation act as shallow donors, can lower their collective formation energy by about 5 eV. In doing so, however, these N vacancies lose their shallow-donor character as the lattice relaxes in response to this aggregation. Contrasting with the N vacancies, the Ga vacancies are found to retain their isolated shallow acceptor behavior and do not gain significant energy upon aggregation. The possible implications for larger aggregate defects are discussed.

Acceptor Binding Energies in GaN and AIN

AbstractWe present binding energy calculations for Mg, Zn, and C substitutional shallow acceptors in GaN and AIN for both, wurtzite (WZ) and zincblende (ZB) crystal phases. The calculations are performed within the effective mass theory through the 6 × 6 Rashba-Sheka-Pikus and the Luttinger- Kohn matrix Hamiltonians for WZ and ZB bulk crystals, respectively. An analytic representation for the pseudopotential is used to introduce the nature of the impurity atom. The energy shift due to polaron effects is also considered in this approach. The estimated ionization energies are in good agreement with those reported experimentally and those reported theoretically employing other methods. We find that the binding energies for ZB GaN acceptors are shallower than the corresponding impurities in the WZ crystalline phase. The binding energy dependence upon the crystal field splitting in the WZ compounds is analyzed.

Inheritance of zinc-blende structure from [formula omitted] substrate in growth of GaN by MOCVD

GaN has been grown on 3 C SiC Si(001) at temperatures from 600 to 800°C by low pressure MOCVD using TMGa and NH 3. The grown films were characterized by X-ray diffraction signals from the zinc-blende GaN(002) and the wurtzite GaN(0002) planes. An X-ray diffraction signal detected at 36.8° indicated the wurtzite GaN(101̄1) plane. The growth of the wurtzite GaN was suppressed by increasing the growth temperature. The annealing of the thick polycrystalline GaN in NH 3 ambient at 800°C was found to cause the atomic displacement and sublimation. Zinc-blende GaN with suppressed wurtzite domain was obtained on 3 C SiC Si(001) at 800°C by inserting a thin polycrystalline GaN buffer layer deposited at 600°C.

Electronic Structure of GaP 1-xNx Alloys Determined Using Pseudopotentials and Gaussian Orbitals

A simple model based on empirical pseudopotentials, Gaussian orbitals, and the virtual-crystal approximation is employed to study the electronic structure of GaP1-xNx alloys. First, the pseudopotential form factors and the scaling parameters of the Gaussian orbitals are determined for the zinc-blende GaN constituent, and then the pseudopotential form factors are adjusted for GaP, by fitting to experimental data and the results of more accurate calculations. The band structure of the alloys is then calculated using the parameters obtained. Large nonlinear dependences of the direct Γ–Γ and indirect Γ–X gaps on nitrogen concentration are predicted. We found that the bowing parameters of these energy gaps are large, but not large enough to yield a negative band-gap energy.

Hole transport properties of bulk zinc-blende and wurtzite phases of GaN based on an ensemble Monte Carlo calculation including a full zone band structure

In this paper, we present calculations of the hole transport properties of bulk zinc-blende and wurtzite phase GaN at field strengths at which impact ionization does not occur significantly. The calculations are made using an ensemble Monte Carlo simulator, including the full details of the band structure and a numerically determined phonon scattering rate based on an empirical pseudopotential method. Band intersection points—including band crossings and band mixings—are treated by carefully evaluating the overlap integral between the initial and possible final drift states. In this way, the hole trajectories in phase space can be accurately traced. It is found that the average hole energies are significantly lower than the corresponding electron energies for the field strengths examined. This result is most probably due to the drastic difference in curvature between the uppermost valence bands and the lowest conduction band. The relatively flat valence bands impede hole-heating, leading to low average hole energy.

Surface reconstruction of zinc-blende GaN

We propose a model of the surface reconstruction for cubic GaN which accounts both for short- and long-range forces. Our analysis shows that short-range covalent contributions to the bond energies cannot by themselves explain the difference in stability of different surface configurations. Our analysis, which includes long-range ionic contributions, shows that at all typical molecular beam epitaxy (MBE) epitaxy temperatures the cubic GaN film with a free-carrier concentration larger than 1016 cm−3 should have a tendency to develop the surface reconstruction. The qualitative explanation of the reconstruction effects on Ga surface of (001) surface of cubic GaN can be linked to a large difference in atomic sizes of Ga and N.

Optical properties of doped GaN grown by a modified molecular beam epitaxial (MBE) process on GaAs substrates

Both zinc-blende and wurtzite GaN layers were grown by a modified molecular beam epitaxy (MBE) method. These layers were grown on semi-insulating GaAs(100) substrates and doped with silicon and beryllium as n- and p-type dopants. The structure of the materials was confirmed using X-ray diffraction (XRD). Photoluminescence (PL) measurement at 77 K shows a single sharp emission at 3.473 eV for the wurtzite GaN doped with silicon, and this peak is shifted downwards in energy by about 200 meV in the zinc-blende GaN doped with silicon. The donor ionisation energy is estimated to be ∼ 30 meV. For beryllium in GaN, PL shows a broad emission at about 2.55 eV for the wurtzite material, while two lines at 2.90 eV and about 2.2 eV appear in the zinc-blende sample. An acceptor ionisation energy for zinc-blende GaN of 200 meV is estimated for the first time.

Lattice dynamics of zinc-blende GaN and AlN: I. Bulk phonons

The lattice dynamics of zinc-blende GaN and AlN were studied theoretically using a two-parameter Keating potential together with the long-range Coulomb interactions. Phonon frequencies transformed from those of the wurtzite counterparts were used to determine the two Keating parameters and the effective charge. We present for the first time the phonon dispersion curves for zinc-blende GaN and AlN. An interesting feature was found for the phonons propagating along the [110] direction, which does not exist in other III - V semiconductors.

Electronic structure of cubic gallium nitride films grown on GaAs

The composition, surface structure, and electronic structure of zinc blende–GaN films grown on GaAs (100) and (110) by plasma-assisted molecular beam epitaxy were investigated by means of core and valence level photoemission. Angle-resolved photoelectron spectra (photon energy 30–110 eV) exhibited emission from the Ga 3d and N 2s levels, as well as a clear peak structure in the valence band region. These peaks were found to shift with photon energy, indicative of direct transitions between occupied and unoccupied GaN bands. By using a free electron final band, we are able to derive the course of the bands along the Γ-X and Γ-K-X directions of the Brillouin zone and to determine the energy of critical points at the X point. The relative energies of the Ga 3d and nitrogen 2s bands were also studied, and a small amount of dispersion was detected in the latter. The resulting band structure is discussed in relation to existing band structure calculations.

Optical gain in zinc-blende [formula omitted] strained quantum well laser

Based on the valence subbands of the zinc-blende GaN Ga 0.85Al 0.15N strained quantum wells obtained by a 6×6 Hamiltonian (including heavy hole, light hole and spin-orbit splitting band), optical gain and radiative current density are calculated for the strained quantum well laser structures. The compressive strain in the GaN well region strongly depresses the TM mode optical gain and enhances the TE mode optical gain.

Low-temperature luminescence study of GaN films grown by MBE

We report the results of low-temperature photoluminescence measurements on GaN films grown by molecular beam epitaxy on (0001) sapphire substrates. Samples were either nominally undoped or doped with Si. The spectra are generally dominated by a sharp peak at 3.47 eV which is attributed to excitons bound to neutral donors. A much weaker peak (or shoulder) near 3.45 eV probably arises from excitons bound to neutral acceptors. On raising the temperature to 50 K, in some samples free exciton peaks can be partially resolved on the high-energy side of the main line. In others we believe that these free excitons are recaptured onto neutral acceptors, thus enhancing the low-energy side of the line. A broader emission line appears in many samples at an energy near 3.42 eV which shows significant variation in position between samples. Our data show that it represents a free-to-bound, probably a free hole-to-donor, transition. This donor has previously been associated with oxygen. Of particular interest is the fact that some samples show a second sharp peak at 3.27 eV, together with a second broader peak at about 3.17 eV (also variable in energy). The sharp peak is energetically consistent with its being either a donor - acceptor or a free electron-to-bound hole transition, but subsidiary measurements rule out both these possibilities. We suggest that it may represent an exciton bound to a deep donor or a shallow donor-bound exciton in zinc blende GaN inclusions contained within the mainly wurtzite material. We tentatively interpret the 3.17 eV line as a phonon replica of this zinc blende line, the phonon energy being perturbed by the small size of the inclusions and by strain effects within these inclusions.

Band structure parameters of zinc-blende GaN, AlN and their alloys Ga 1−xAl xN

The electronic properties of wide energy gap zinc-blende structure GaN, AlN and their alloys Ga 1−xAl xN are investigated using the empirical pseudopotential method. Electron and hole Effective mass parameters, hydrostatic and shear deformation potential constants of the valence band at Γ and those of the conduction band at Γ and X are obtained. The energies of Γ, X, L conduction valleys of Ga 1−xAl xN alloy versus Al fraction x are also calculated. The information will be useful for the design of lattice mismatched heterostructure optoelectronic devices in the blue light range.

TEM and PL characterisation of MBE-grown epitaxial GaN/GaAs

AbstractMBE-grown epitaxial GaN deposited at 700°C on {001}, {111}A and B GaAs has been characterised using the combined techniques of transmission electron microscopy (TEM) and photoluminescence (PL). On both { 111 } A and 111 B GaAs substrates, single crystal wurtzite GaN was formed, but with very high densities of threading defects. Best epitaxy occurred on 111B GaAs in accordance with PL measurements. An amorphous phase was identified at the GaN/{ 111 }A GaAs interface and the GaN epilayer evolved in this instance with the same Nterminated growth surface as for the case of growth on 111 B GaAs, as determined by convergent beam electron diffraction (CBED). Growth on {001} GaAs produced highly faulted columnar grains of zincblende GaN. Conversely, growth on {001} GaAs under an additional arsenic flux at 700°C resulted in the deposition of single crystal zincblende GaN with a high density of stacking faults and microtwins. Thus, the microstructure of epitaxial GaN depends very much on the detailed growth conditions and substrate orientations used.

Theoretical Prediction of Zinc Blende Phase GaN Avalanche Photodiode Performance Based on Numerically Calculated Electron and Hole Impact Ionization Rate Ratio

AbstractIn this paper, we present the first calculations of the electron and hole initiated interband impact ionization rate in zinc blende phase GaN as a function of the applied electric field strength. The calculations are performed using an ensemble Monte Carlo simulator including the full details of the conduction and valence bands along with a numerically determined, wave-vector dependent interband ionization transition rate determined from an empirical pseudopotential calculation. The first four conduction bands and first three valence bands, which fully comprise the energy range of interest for device simulation, are included in the analysis. It is found that the electron and hole ionization rates are comparable over the full range of applied electric field strengths examined. Based on these calculations an avalanche photodiode, APD, made from bulk zinc blende GaN then would exhibit poor noise and bandwidth performance. It should be noted however, that the accuracy of the band structure employed and the scattering rates is presently unknown since little experimental information is available for comparison. Therefore, due to these uncertainties, it is difficult to unequivocally conclude that the ionization rates are comparable.

Shallow Impurity States in Wurtzite and Zinc Blende Structure GaN

ABSTRACTCalculations of shallow donor states for wurtzite and zincblende structure GaN, and acceptor states for zincblende GaN in an effective mass approximation are presented. Band parameters were taken from experiment or determined from band structure calculations. The effect of wavevector dependent screening is examined, based on dielectric functions calculated from empirical pseudopotential band structures. For donor states, the effects of electron-phonon coupling and free carrier screening in the Thomas-Fermi and Debye approximations are discussed briefly.

Structure, Electronic Properties, and Defects of GaN Using a Self-Consistent Molecular-Dynamics Method

ABSTRACTMolecular dynamics simulations are employed to study defects in GaN. We use local basis density functional theory within the local density approximation where charge transfer between the ions is included in an approximate fashion. We find good agreement for the band structure of wurtzite and zincblende GaN compared to other recent calculations, suggesting the suitability of our method to describe GaN. A 96 atom GaN supercell is used to study the relaxations and electronic properties of common defects in the crystal structure, including Ga and N vacancies and antisites. We analyze the electronic signatures of these defects.

Strain Effects on the Band Structures of GaN

AbstractThe strain effects on the band structure of zincblende GaN are studied by using the ab initio linear muffin‐tin orbital (LMTO) method. The variations of energy splitting with the strain component are calculated for some of the important eigenvalues. The strain effects on the band shape and band gap are discussed. It is found that the compressive (expansive) strain parallel to the (100) surface gives a narrower (wider) Xc1‐Γv15 band gap and a wider (narrower) Xc1‐Γv15 band gap compared with that of the perfect zincblende GaN crystal. The band width along the Δ direction increases when the lattice parallel to the (100) surface a∥ is compressed and decreases when a∥ is expanded. However, the band width along the A direction decreases for both compressive and expansive strain parallel to the (100) surface.

Electronic transport studies of bulk zincblende and wurtzite phases of GaN based on an ensemble Monte Carlo calculation including a full zone band structure

The ensemble Monte Carlo technique including the details of the first four conduction bands within the full Brillouin zone is used to calculate the basic electronic transport properties for both zincblende and wurtzite crystal phases of bulk gallium nitride. The band structure throughout the Brillouin zone is determined using the empirical pseudopotential method. Calculations of the electron steady-state drift velocity, average energy, valley occupancy and band occupancy in the range of electric fields up to 500 kV/cm are presented. It is found that the threshold electric field for intervalley transfer is greater and that the second conduction band is more readily occupied in wurtzite than in zincblende GaN over the range of electric fields examined here.

Surface reconstructions of zinc-blende GaN/GaAs(001) in plasma-assisted molecular-beam epitaxy.

We identify the growth conditions required for the synthesis of purely cubic GaN films on GaAs(001) by means of plasma-assisted molecular-beam epitaxy. It is shown that it is the surface stoichiometry which governs the phase composition and which thus has to be tightly controlled in order to avoid nucleation of hexagonal grains. Such control over the surface stoichiometry is achieved by investigating the surface reconstructions of zinc-blende GaN under both static and dynamic conditions by in situ reflection high-energy electron diffraction.