Zinc-blende Gallium Nitride Research Articles

Study of subsurface damage during nano-grinding of B3-GaN using molecular dynamics simulations

The mechanism of subsurface damage formation during zinc blende gallium nitride (B3-GaN) nano-grinding was investigated using molecular dynamics (MD). The effects of grinding depth and abrasive radius on the grinding force, temperature, potential energy, phase transition, dislocations, the average coefficient of friction (COF), material removal rate (MRR) and subsurface damage were systematically investigated. The results show that dislocations, amorphization and wurtzite (B4) phase transitions are the primary defects present in the subsurface damage layer. Dislocation nucleation and proliferation are the main mechanisms aggravating the subsurface damage, and the 1/2<110> perfect dislocation along the <110> slip direction predominates in the dislocation. Grinding forces, temperature, potential energy, amorphous atoms, B4 phase atoms and dislocations increase with increasing grinding depth, exacerbating the subsurface damage of B3-GaN. As the abrasive radius increases, the amorphous atoms increase and then decrease, with the B4 phase atoms gradually reducing. The rest of the above parameters increase gradually, exacerbating the subsurface damage of B3-GaN. Furthermore, a smaller grinding depth and abrasive radius result in better machining accuracy, but decrease MRR. A technical reference for the ultra-precise and low-damage processing of B3-GaN can be provided by this study.

Time-resolved pump–probe spectroscopic ellipsometry of cubic GaN. I. Determination of the dielectric function

An ultra-fast change of the absorption onset for zincblende gallium-nitride (zb-GaN) (fundamental bandgap: 3.23 eV) is observed by investigating the imaginary part of the dielectric function using time-dependent femtosecond pump–probe spectroscopic ellipsometry between 2.9 and 3.7 eV. The 266 nm (4.66 eV) pump pulses induce a large electron–hole pair concentration up to 4×1020cm−3, which shift the transition energy between conduction and valence bands due to many-body effects up to ≈500 meV. Here, the absorption onset increases due to band filling while the bandgap renormalization at the same time decreases the bandgap. Additionally, the absorption of the pump-beam creates a free-carrier profile within the 605 nm zb-GaN layer with high free-carrier concentrations at the surface, and low concentrations at the interface to the substrate. This leads to varying optical properties from the sample surface (high transition energy) to substrate (low transition energy), which are taken into account by grading analysis for an accurate description of the experimental data. For this, a model describing the time- and position-dependent free-carrier concentration is formulated by considering the relaxation, recombination, and diffusion of those carriers. We provide a quantitative analysis of optical experimental data (ellipsometric angles Ψ and Δ) as well as a plot for the time-dependent change of the imaginary part of the dielectric function.

Influence of gallium surface saturation on GaN nanowire polytype selection during molecular-beam epitaxy

We have examined the influence of Ga surface saturation on gallium nitride (GaN) nanowire (NW) polytype selection during molecular-beam epitaxy. The Ga surface saturation in the absence and presence of nitrogen determines the GaN polytype and morphology (i.e., films vs NW) selection, respectively. We discuss the interplay between surface and step-edge diffusion barriers governing the NW-to-film-transition and the influence of SixNy interlayer formation on zinc blende (ZB) vs wurtzite (WZ) polytype selection of GaN. In addition, distinct exciton emissions associated with ZB and WZ GaN are observed, suggesting a type-I WZ/ZB GaN band-offset. This work provides a crucial step toward the realization of polarization-free, CMOS-compatible GaN-based optoelectronics.

Photoluminescence Line‐Shape Analysis of Highly n‐Type Doped Zincblende GaN

An investigation of different n‐type doped zincblende gallium nitride thin films measured by photoluminescence from 7 K to room temperature is presented. The spectra change with increasing free‐carrier concentration due to many‐body effects such as the Burstein–Moss shift and band‐gap renormalization. The samples are grown by molecular beam epitaxy on a 3C‐SiC/Si (001) substrate, and a free‐carrier concentration above 1020 cm−3 is achieved by introducing germanium as a donor. The analysis of the measured spectra by a line‐shape fit yields different transition processes for different doping concentrations and temperatures, such as a band–band transition and a band–acceptor transition. The conduction band dispersion of Kane's model is perfectly suited to explain the experimental data quantitatively.

Influence of the free-electron concentration on the optical properties of zincblende GaN up to 1×1020cm−3

We analyze the optical properties of zincblende gallium-nitride in the infrared and ultraviolet spectral range ($\ensuremath{\approx}27\phantom{\rule{0.16em}{0ex}}\mathrm{meV}--6.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$) experimentally by spectroscopic ellipsometry and provide a quantitative description of these results by $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ perturbation theory. Free-electron concentrations above ${10}^{20}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ are achieved by introducing germanium as a donor. We determine the dielectric function as well as band filling effects like the Burstein-Moss shift and band gap renormalization. The Kane model for the band structure of semiconductors near the $\mathrm{\ensuremath{\Gamma}}$-point allows to calculate the effective electron mass and to determine the nonparabolicity of the conduction band. At the same time, these results can be used to derive the free-electron concentration all-optically. The combination of Kane's model, Burstein-Moss shift, and band-gap renormalization can be used to expertly describe the measured transition energies up to $\ensuremath{\approx}3.7\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ dependent on the carrier concentration, yielding an averaged hole mass of $\ensuremath{\approx}0.61\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}_{\mathrm{e}}$ for the contributing valence bands.

Crystal Structures of GaN Nanodots by Nitrogen Plasma Treatment on Ga Metal Droplets

Gallium nitride (GaN) is one of important functional materials for optoelectronics and electronics. GaN exists both in equilibrium wurtzite and metastable zinc-blende structural phases. The zinc-blende GaN has superior electronic and optical properties over wurtzite one. In this report, GaN nanodots can be fabricated by Ga metal droplets in ultra-high vacuum and then nitridation by nitrogen plasma. The size, shape, density, and crystal structure of GaN nanodots can be characterized by transmission electron microscopy. The growth parameters, such as pre-nitridation treatment on Si surface, substrate temperature, and plasma nitridation time, affect the crystal structure of GaN nanodots. Higher thermal energy could provide the driving force for the phase transformation of GaN nanodots from zinc-blende to wurtzite structures. Metastable zinc-blende GaN nanodots can be synthesized by the surface modification of Si (111) by nitrogen plasma, i.e., the pre-nitridation treatment is done at a lower growth temperature. This is because the pre-nitridation process can provide a nitrogen-terminal surface for the following Ga droplet formation and a nitrogen-rich condition for the formation of GaN nanodots during droplet epitaxy. The pre-nitridation of Si substrates, the formation of a thin SiNx layer, could inhibit the phase transformation of GaN nanodots from zinc-blende to wurtzite phases. The pre-nitridation treatment also affects the dot size, density, and surface roughness of samples.

The sensitivity of the electron transport within bulk zinc-blende gallium nitride to variations in the crystal temperature, the doping concentration, and the non-parabolicity coefficient associated with the lowest energy conduction band valley

Within the framework of a semi-classical three-valley Monte Carlo simulation approach, we analyze the steady-state and transient electron transport that occurs within bulk zinc-blende gallium nitride. In particular, we examine how the steady-state and transient electron transport that occurs within this material changes in response to variations in the crystal temperature, the doping concentration, and the non-parabolicity coefficient associated with the lowest energy conduction band valley. These results are then contrasted with those corresponding to a number of other compound semiconductors of interest.

Synthesis of Single Crystal GaN Nanowires

The straight and curved gallium nitride (GaN) nanowires were successfully synthesized by controlling the gallium/nitrogen reactant ratio via a chemical vapour deposition method. The structure and morphology of nanowires were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM), field emission scanning electron microscopy (FESEM), selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). The straight and curved GaN nanowires are composed of wurtzite and a zinc blende structure, respectively. Photoluminescence (PL) spectra of zinc blende GaN nanowires showed a strong UV emission band at 400 nm, indicating potential application in optoelectronic devices.

Steady-state and transient electron transport within the wide energy gap compound semiconductors gallium nitride and zinc oxide: an updated and critical review

The wide energy gap compound semiconductors, gallium nitride and zinc oxide, are widely recognized as promising materials for novel electronic and optoelectronic device applications. As informed device design requires a firm grasp of the material properties of the underlying electronic materials, the electron transport that occurs within these wide energy gap compound semiconductors has been the focus of considerable study over the years. In an effort to provide some perspective on this rapidly evolving field, in this paper we review analyzes of the electron transport within the wide energy gap compound semiconductors, gallium nitride and zinc oxide. In particular, we discuss the evolution of the field, compare and contrast results determined by different researchers, and survey the current literature. In order to narrow the scope of this review, we will primarily focus on the electron transport within bulk wurtzite gallium nitride, zinc-blende gallium nitride, and wurtzite zinc oxide. The electron transport that occurs within bulk zinc-blende gallium arsenide will also be considered, albeit primarily for bench-marking purposes. Most of our discussion will focus on results obtained from our ensemble semi-classical three-valley Monte Carlo simulations of the electron transport within these materials, our results conforming with state-of-the-art wide energy gap compound semiconductor orthodoxy. A brief tutorial on the Monte Carlo electron transport simulation approach, this approach being used to generate the results presented herein, will also be featured. Steady-state and transient electron transport results are presented. We conclude our discussion by presenting some recent developments on the electron transport within these materials. The wurtzite gallium nitride and zinc-blende gallium arsenide results, being presented in a previous review article of ours (O’Leary et al. in J Mater Sci Mater Electron 17:87, 2006), are also presented herein for the sake of completeness.

A Comparative Study on the Influence of Elastic Scattering Mechanisms on the Electron Mobility in Wurtzite and Zincblende Gallium Nitride

The electron mobility of zincblende (ZB) and wurtzite (WZ) structures of n-type GaN is investigated as a function of both temperature and donor concentration. Numerical calculations of the mobility are carried out in a temperature range from 10 K up to 400 K and donor doping concentrations from 10 19 m -3 to 10 27 m -3 . Elastic types of scattering mechanisms are considered. These types include ionized impurity scattering, neutral impurity scattering, deformation potential acoustic phonons scattering and piezoelectric potential scattering. The electron drift mobility versus temperature shows peak behavior for both zincblende and wurtzite GaN structures. Below approximately 120/140 K (for ZB and WZ GaN respectively), the mobility increases when the temperature is increased. In this temperature range, the ionized impurity scattering is assumed to be the dominant scattering mechanism. Above 120/140 K, the mobility is lowered down by raising the temperature. In this regime, the lattice scattering is considered to be the dominant mechanism. The variation of the electron drift mobility with donor concentration at room temperature shows a continuous decrease with the increase of impurity doping concentration. This is probably due to Coulomb scattering present particularly at low temperatures.

Ab initiostudy of the unusual thermal transport properties of boron arsenide and related materials

Recently, using a first principles approach, we predicted that zinc blende boron arsenide (BAs) will have an ultrahigh lattice thermal conductivity, \ensuremath{\kappa}, of over 2000 Wm${}^{\ensuremath{-}1}$K${}^{\ensuremath{-}1}$ at room temperature (RT), comparable to that of diamond. Here, we provide a detailed ab initio examination of phonon thermal transport in boron arsenide, contrasting its unconventional behavior with that of other related materials, including the zinc blende crystals boron nitride (BN), boron phosphide, boron antimonide, and gallium nitride (GaN). The unusual vibrational properties of BAs contribute to its weak phonon-phonon scattering and phonon-isotope scattering, which are responsible for its exceptionally high \ensuremath{\kappa}. The thermal conductivity of BAs has contributions from phonons with anomalously large mean free paths (\ensuremath{\sim}2 \ensuremath{\mu}m), two to three times those of diamond and BN. This makes \ensuremath{\kappa} in BAs sensitive to phonon scattering from crystal boundaries. An order of magnitude smaller RT thermal conductivity in a similar material, zinc blende GaN, is connected to more separated acoustic phonon branches, larger anharmonic force constants, and a large isotope mixture on the heavy rather than the light constituent atom. The striking difference in \ensuremath{\kappa} for BAs and GaN demonstrates the importance of using a microscopic first principles thermal transport approach for calculating \ensuremath{\kappa}. BAs also has an advantageous RT coefficient of thermal expansion, which, combined with the high \ensuremath{\kappa} value, suggests that it is a promising material for use in thermal management applications.

Nanoporous gallium nitride square microtubes

Porous gallium nitride microtubes were self-fabricated from gallium nitride submicron irregular structures. The microtubes were of square cross-section. Electron diffractions indicated that the microtubes were composed of zincblende gallium nitride. Electron energy-loss spectrum and photoluminescence spectrum of the microtubes were collected and compared with that of single crystals.

Rare-earth impurities in gallium nitride: The role of the Hubbard potential

We performed a first principles investigation on the electronic properties of 4f-rare earth substitutional impurities in zincblende gallium nitride (GaN:REGa, with RE=Eu, Gd, Tb, Dy, Ho, Er and Tm). The calculations were performed within the all electron methodology and the density functional theory. We investigated how the introduction of the on-site Hubbard U potential (GGA+U) corrects the electronic properties of those impurities. We showed that a self-consistent procedure to compute the Hubbard potential provides a reliable description on the position of the 4f-related energy levels with respect of the GaN valence band top. The results were compared to available data coming from a recent phenomenological model.

Formation mechanisms of spatially-directed zincblende gallium nitride nanocrystals

We report on the spatially selective formation of GaN nanocrystals embedded in GaAs. Broad-area N+ implantation followed by rapid thermal annealing leads to the formation of nanocrystals at the depth of maximum ion damage. With additional irradiation using a Ga+ focused ion beam, selective lateral positioning of the nanocrystals within the GaAs matrix is observed in isolated regions of increased vacancy concentration. Following rapid thermal annealing, the formation of zincblende GaN is observed in the regions of highest vacancy concentration. The nucleation of zincblende nanocrystals over the wurtzite phase of bulk GaN is consistent with the predictions of a thermodynamic model for the nanoscale size-dependence of GaN nucleation.

Growth of epitaxial iron nitride ultrathin film on zinc-blende gallium nitride

The authors report the growth of iron nitride on zinc-blende gallium nitride using molecular beam epitaxy. First, zinc-blende GaN is grown on a magnesium oxide substrate having (001) orientation; second, an ultrathin layer of FeN is grown on top of the GaN layer. In situ reflection high-energy electron diffraction is used to monitor the surface during growth, and a well-defined epitaxial relationship is observed. Cross-sectional transmission electron microscopy is used to reveal the epitaxial continuity at the gallium nitride-iron nitride interface. Surface morphology of the iron nitride, similar to yet different from that of the GaN substrate, can be described as plateau valley. The FeN chemical stoichiometry is probed using both bulk and surface sensitive methods, and the magnetic properties of the sample are revealed.

Electron and phonon dynamics in zincblende gallium nitride

AbstractThis report presents new Monte Carlo simulation results revealing the ballistic stage of growth of the phonon number, electron velocity and energy upon the switching‐on electric field, the shape of electron and phonon stationary distributions in high electric fields, as well as electron cooling and phonon number relaxation rates after switching‐off the field in zincblende gallium nitride crystals.LO phonon band population inversion is feasible here up to the room temperature relative to the TO phonons, and below T &lt; 80 K – relative to the LA‐phonon band provided that the phonon lifetimes satisfy the conditions of ≥ 2 ps (LO) and ≥ 2 ns (LA). Phonon decay scenarios with the stimulated emission of infrared‐range photons are discussed including phonon difference transitions. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of growth conditions on the GaN thin film by sputtering deposition

The phase transition between thermodynamically stable hexagonal wurtzite (h-WZ) gallium nitride (GaN) and metastable cubic zinc-blende (c-ZB) GaN during growth by radio-frequency planar magnetron sputtering is studied. GaN films grown on substrates with lower mismatches tend to have a h-WZ structure, but when grown on substrates with higher mismatches, a c-ZB structure is preferred. GaN films grown under high nitrogen pressure also tend to have a h-WZ structure, whereas a c-ZB structure is preferred when grown under low nitrogen pressure. In addition, low target-power growth not only helps to improve hexagonal GaN (h-GaN) crystalline quality at high nitrogen pressure on low-mismatch substrates, but also enhances cubic GaN (c-GaN) quality at low nitrogen pressure on high-mismatch substrates.

A Raman scattering-based method to probe the carrier drift velocity in semiconductors: Application to gallium nitride

A single expression relating the carrier drift velocity in semiconductors under an electric field to Raman scattering data is derived resorting to a full nonequilibrium picture for electrons and holes. It allows one to probe with high optical precision both the ultrafast transient as well as the steady state carriers' drift velocity in semiconductor systems. This is achieved by simply modifying the experimental geometry, thus changing the angle between the transferred wave vector Q and the applied electric field E, and measuring the frequency shift promoted by the presence of the field to be observed in the single-particle and plasmon scattering spectra. An application to zinc-blende gallium nitride is presented to highlight the power of the method.

Wurtzite to zinc-blende phase transition in gallium nitride thin films

The phase transition in gallium nitride (GaN) thin films at room temperature from the thermodynamically stable wurtzite to the metastable zinc-blende structure is studied. The GaN films were grown at room temperature on (100)-oriented silicon substrates by radio-frequency planar magnetron sputtering of a GaN target in pure nitrogen atmosphere. As the nitrogen gas pressure during the film growth is decreased from 30 to 10 mTorr, the crystalline phase of GaN films changed from a hexagonal wurtzite to a cubic zinc-blende structure with (111) crystallographic planes oriented preferentially parallel to the film surface. The mechanisms responsible for the formation of the nonequilibrium cubic zinc-blende GaN phase at room temperature are suggested, in which the impact of the hyperthermal species impinging on the surface of growing films plays a key role in stabilizing the zinc-blende phase of GaN.

Substrates for gallium nitride epitaxy

In this review, the structural, mechanical, thermal, and chemical properties of substrates used for gallium nitride (GaN) epitaxy are compiled, and the properties of GaN films deposited on these substrates are reviewed. Among semiconductors, GaN is unique; most of its applications uses thin GaN films deposited on foreign substrates (materials other than GaN); that is, heteroepitaxial thin films. As a consequence of heteroepitaxy, the quality of the GaN films is very dependent on the properties of the substrate—both the inherent properties such as lattice constants and thermal expansion coefficients, and process induced properties such as surface roughness, step height and terrace width, and wetting behavior. The consequences of heteroepitaxy are discussed, including the crystallographic orientation and polarity, surface morphology, and inherent and thermally induced stress in the GaN films. Defects such as threading dislocations, inversion domains, and the unintentional incorporation of impurities into the epitaxial GaN layer resulting from heteroepitaxy are presented along with their effect on device processing and performance. A summary of the structure and lattice constants for many semiconductors, metals, metal nitrides, and oxides used or considered for GaN epitaxy is presented. The properties, synthesis, advantages and disadvantages of the six most commonly employed substrates (sapphire, 6H-SiC, Si, GaAs, LiGaO2, and AlN) are presented. Useful substrate properties such as lattice constants, defect densities, elastic moduli, thermal expansion coefficients, thermal conductivities, etching characteristics, and reactivities under deposition conditions are presented. Efforts to reduce the defect densities and to optimize the electrical and optical properties of the GaN epitaxial film by substrate etching, nitridation, and slight misorientation from the (0 0 0 1) crystal plane are reviewed. The requirements, the obstacles, and the results to date to produce zincblende GaN on 3C-SiC/Si(0 0 1) and GaAs are discussed. Tables summarizing measures of the GaN quality such as XRD rocking curve FWHM, photoluminescence peak position and FWHM, and electron mobilities for GaN epitaxial layers produced by MOCVD, MBE, and HVPE for each substrate are given. The initial results using GaN substrates, prepared as bulk crystals and as free-standing epitaxial films, are reviewed. Finally, the promise and the directions of research on new potential substrates, such as compliant and porous substrates are described.