Estimates of the spontaneous polarization and permittivities of AlN,GaN, InN,and SiC crystals

Physics of the Solid State - The values of spontaneous polarization and permittivities of aluminum, gallium, and indium nitrides, as well as silicon carbide, with a wurtzite structure are...     

Effect of electron-phonon interaction on surface states in zinc-blende GaN,AlN, and InN under pressure

A variational approach is used to study the surface states of electrons in a semi-infinite polar semiconductor under hydrostatic pressure. The effective Hamiltonian and the surface-state levels are derived including the effects of electron-optical phonon interaction and pressure. The numerical computation has been performed for the surface-state energies versus pressure for zinc-blende GaN, AlN, and InN. The results show that the effect of electron-optical phonon interaction lowers the surface-state energy. It is also found that the effect of electron-surface optical phonon interaction is much bigger than the effect of electron-half space longitudinal optical phonon interaction for surface-state levels. It indicates that the surface-state energies and the influence of electron-phonon interaction increase with pressure obviously.Received: 12 June 2003, Published online: 22 September 2003PACS: 63.20.Kr Phonon electron and phonon-phonon interactions - 71.38.-k Polarons and electron-phonon interactions - 73.20.At Surface states, band structure, electron density of states     

The dielectric and dynamical properties of zinc-blende BN, AlN and GaN from first-principle calculation

The ab initio calculations of the electronic structural, dielectric and lattice-dynamical properties of zinc-blende BN, AlN and GaN were presented. The ground-state properties, i.e., the lattice constant, the bulk modulus and band gap, were calculated using a plane-wave-pseudopotential method within the density-function theory. A linear-response approach to the density-function perturb theory was used to derive the Born effective charge, the high-frequency dielectric constants and interatomic force constants for these materials. The interatomic force contants (IFCs) are useful for interpolating the dynamical matrices through the whole Brillouin zone. Phonon frequencies along high-symmetry lines were also obtained by interpolating the dynamical matrices using the interatomic force constants. In this paper, we discussed the difference of dielectric and dynamical properties among zinc-blende BN, AlN and GaN, and meanwhile, also compared these properties with other experimental data available and theoretical values. Generally, the calculations were in good agreement with the other existing experimental data and theoretical values. Supported by the Hunan Provincial Natural Science Foundation of China (Grant No. 05JJ40135) and Xiangnan University Important Science Foundation (Grant No. 2007Z010)     

Surface reconstructions on InN and GaN polar and nonpolar surfaces

We report a systematic and comprehensive computational study of surface reconstructions on GaN and InN surfaces in various orientations, including the polar c plane as well as the nonpolar a and m planes. For GaN we have identified several new metallic reconstructions under highly Ga-rich conditions on the nonpolar planes. For InN we find several distinct differences from the GaN case: the absence of a nitrogen-adatom reconstruction on the (0 0 0 1) plane; the presence of a single, metallic reconstruction over the entire stability range on the plane; and In-adlayer reconstructions on the (m) plane. An interesting “inverted polarity” defect structure on the (m) plane is also revealed.     

The charge neutrality level and the fermi level pinning in A<Superscript>3</Superscript>N (BN,AlN, GaN,InN) nitrides

The electronic band structure and position of the charge neutrality level (CNL) in BN, AlN, GaN, and InN compounds with cubic and hexagonal lattices are calculated within the density functional theory (DFT-GGA). It is shown that the charge neutrality level is shifted from the middle of the BN and AlN forbidden band to the upper half of the GaN forbidden band and to the allowed energy region in the InN conduction band as the cation atomic weight increases. This determines semiinsulating properties of BN and AlN, n-type conductivity of GaN, and n ⁺-type conductivity of InN upon saturation of these materials by intrinsic lattice defects due to hard radiation. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 24–31, December, 2008.     

AlN, GaN and InN (0 0 1) surface electronic band structure

We have studied the electronic band structure of the ideal (0 0 1) surface of AlN, GaN and InN in the zinc-blende phase. We have employed an empirical sp³s^∗d⁵ Hamiltonian with nearest-neighbor interactions including spin-orbit coupling and the surface Green function matching method. We have obtained the different surface states together with their corresponding orbital character and localization in the different layers. A similar physical picture is obtained for the three materials.     

Elastic waves at the (0 0 1) and (1 1 0) surfaces of AlN, GaN and InN

We have studied the acoustic waves in the (0 0 1) and (1 1 0) surfaces of AlN, GaN and InN. We have employed the surface Green function matching method and different sets of calculated elastic constants available in the literature for these materials, because no experimental values are available. Important differences are found for the velocities of the bulk and surface acoustic waves coming from these sets of elastic constants, in such a way that they could be easily measured by ultrasonic and Brillouin techniques. These results together with the expressions obtained here for the velocities of the acoustic waves in high symmetry directions could be used to determine the elastic coefficients of these materials.     

GaN and InN nanowires grown by MBE: A comparison

Morphological, optical and transport properties of GaN and InN nanowires grown by molecular beam epitaxy (MBE) have been studied. The differences between the two materials in respect to growth parameters and optimization procedure was stressed. The nanowires crystalline quality has been investigated by means of their optical properties. A comparison of the transport characteristics was given. For each material a band schema was shown, which takes into account transport and optical features and is based on Fermi level pinning at the surface. PACS 73.30.+y; 73.21.Hb; 73.22.-f     

Chemical shielding properties for BN,BP, AlN,and AlP nanocones: DFT studies

The properties of boron nitride (BN), boron phosphide (BP), aluminum nitride (AlN), and aluminum phosphide (AlP) nanocones were investigated by density functional theory (DFT) calculations. The investigated structures were optimized and chemical shielding (CS) properties including isotropic and anisotropic CS parameters were calculated for the atoms of the optimized structures. The magnitudes of CS parameters were observed to be mainly dependent on the bond lengths of considered atoms. The results indicated that the atoms could be divided into atomic layers due to the similarities of their CS properties for the atoms of each layer. The trend means that the atoms of each layer detect almost similar electronic environments. Moreover, the atoms at the apex and mouth of nanocones exhibit different properties with respect to the other atomic layers.     

Vibrational spectra of AlN/GaN superlattices: Theory and experiment

Computer simulation of the dynamics of layered AlN/GaN superlattices is performed to elucidate the microscopic nature of the vibrational states corresponding to the strongest bands in the Raman spectra. Experimental Raman spectra are shown to consist of two groups of lines, one of which exhibits a two-mode behavior and the other shows a one-mode behavior as the relative layer thicknesses are varied. The results of computer simulation and calculations within the dielectric-continuum approximation suggest that the behavior of the observed vibrational modes is dictated by the degree of their localization and that the interlayer coupling is due to long-range dipole-dipole interactions. It is shown that the delocalized modes, which exhibit one-mode behavior, can be used as a sensitive probe of the structure and composition of superlattices.     

Theoretical study of CO adsorption on the surface of BN,AlN, BP and AlP nanotubes

Behavior of CO adsorption on the surface of BN, AlN, BP, and AlP nanotubes was investigated using density functional theory calculations, by means of B3LYP and B97D functionals. It was found that energetic feasibility of this process depends on several factors including LUMO energy level of tubes, electron density, and length of the surrounding bonds of adsorbing atoms plus their hybridization. These factors compete against each other to specify the adsorption behavior of the tubes. Frontier molecular orbital theory (FMO) and structural analyses show that high energy level of LUMO and short bond lengths of the tube surfaces prevent the adsorption of CO on BN nanotubes. The results suggest that the AlN nanotubes are energetically the most favorable cases toward the CO adsorption. It was found that B97D functional changes the absolute energy values of B3LYP results, but it doesn't change their relative-order of magnitudes.     

AlN and GaN Nanostructures: Analytical Estimations of the Characteristics of the Electronic Spectrum

Physics of the Solid State - Analytical expressions for the band gaps and characteristic velocities and effective masses of carriers are obtained for infinite flat sheets, free and decorated...     

Comparison of GaN/AlGaN/AlN/GaN HEMTs Grown on Sapphire with Fe-Modulation-Doped and Unintentionally Doped GaN Buffer:Material Growth and Device Fabrication

AlGaN/GaN high electron mobility transistors(HEMTs) grown on Fe-modulation-doped(AID) and unintentionally doped(UID) GaN buffer layers are investigated and compared.Highly resistive GaN buffers(10~9 Ω·cm) are induced by individual mechanisms for the electron traps' formation:the Fe AID buffer(sample A) and the UID buffer with high density of edge-type dislocations(7.24 × 10~9 cm~(-2),sample B).The 300 K Hall test indicates that the mobility of sample A with Fe doping(2503 cm~2 V~(-1) s~(-1)) is much higher than sample B(1926cm~2V~(-1)s~(-1))due to the decreased scattering effect on the two-dimensional electron gas.HEMT devices are fabricated on the two samples and pulsed Ⅰ-Ⅴ measurements are conducted.Device A shows better gate pinch-off characteristics and a higher threshold voltage(-2.63 V) compared with device B(-3.71 V).Lower gate leakage current Igs of device A(3.32 × 10~(-7) A) is present compared with that of device B(8.29 × 10~(-7) A).When the off-state quiescent points Q_2(V_(GQ2) =-8 V,V_(DQ2) = 0 V) are on,V_(th) hardly shifts for device A while device B shows +0.21 V positive threshold voltage shift,resulting from the existence of electron traps associated with the dislocations in the UID-GaN buffer layer under the gate.Under pulsed Ⅰ-Ⅴ and transconductance G_m-V_(GS) measurement,the device with the Fe MD-dopcd buffer shows more potential in improving reliability upon off-state stress.     

Half-metallicity and efficient spin injection in AlN/GaN:Cr (0001) heterostructure

First-principles investigations of the structural, electronic, and magnetic properties of Cr-doped AlN/GaN (0001) heterostructures reveal the possibility of efficient spin injection from a ferromagnetic GaN:Cr electrode through an AlN tunnel barrier. We demonstrate that Cr atoms segregate into the GaN region and that these interfaces retain their half-metallic behavior leading to a complete, i.e., 100%, spin polarization of the conduction electrons. This property makes the wide band-gap nitrides doped with Cr to be excellent candidates for high-efficiency magnetoelectronic devices.     

Influence of Al Preflow Time on Surface Morphology and Quality of AlN and GaN on Si(111) Grown by MOCVD

We investigate the influence of Al preflow time on surface morphology and quality of AIN and GaN. The AIN and GaN layers are grown on a Si(111) substrate by metal organic chemical vapor deposition. Scanning electron microscopy, atomic force microscopy, x-ray diffraction and optical microscopy are used for analysis. Consequently,we find significant differences in the epitaxial properties of AIN buffer and the GaN layer, which are dependent on the Al preflow time. Al preflow layers act as nucleation sites in the case of AIN growth. Compact and uniform AIN nucleation sites are observed with optimizing Al preflow at an early nucleation stage, which will lead to a smooth AIN surface. Trenches and AIN grain clusters appear on the AIN surface while melt-back etching occurs on the GaN surface with excessive Al preflow. The GaN quality variation keeps a similar trend with the AIN quality, which is influenced by Al preflow. With an optimized duration of Al preflow, crystal quality and surface morphology of AIN and GaN could be improved.     

Diffusion and deformations in heterosystems with GaN/AlN superlattices, according to data from EXAFS spectroscopy

Multilayered samples with extremely narrow GaN quantum wells in an AlN host are synthesized via ammonia MBE. The parameters of the microstructure are determined by means of EXAFS spectroscopy, high-resolution electron microscopy, and low-angle scattering. Their relationship to the morphology of GaN/AlN superlattices is established. The influence of growth conditions and the thickness of superlattices on their optical properties and mixing in the near-boundary layers is established.     

A New Method of Growing AlN,GaN, and AlGaN Bulk Crystals Using Hybrid SiC/Si Substrates

Physics of the Solid State - The main principles of a new method of growing bulk single-crystal AlN, AlGaN, and GaN films with thickness from 100 μm and more on silicon substrates with a...