Valence band mixing of cubic GaN/AlN quantum dots

We study the spin purity of the hole ground state in nearly axially symmetric GaN/AlN quantum dots (QDs). To this end, we develop a six-band Burt-Foreman Hamiltonian describing the valence band structure of zinc blende nanostructures with cylindrical symmetry and calculate the effects of eccentricity variationally. We show that the aspect ratio is a key factor for spin purity. In typical QDs with small aspect ratio the ground state is essentially a heavy hole (HH) whose spin purity is even higher than that of InGaAs QDs of similar sizes. When the aspect ratio increases, mixing with light-hole (LH) and split-off (SO) subbands becomes important and, additionally, the ground state becomes sensitive to QD anisotropy, which further enhances the mixing. We finally show that, despite the large GaN hole effective mass, an efficient magnetic modulation is feasible in QDs with aspect ratio ~1, which can be used to modify the ground state symmetry and hence the optical spectrum properties.     

Study of isolated cubic GaN quantum dots by low-temperature cathodoluminescence

We report single dot spectroscopy of cubic GaN/AlN self-assembled quantum dots. Typical linewidths of the zero-phonon line between 2 and 8 meV are observed and interpreted in terms of charge fluctuations around a given quantum dot. The phonon sideband contribution in this emission, even at low temperature, reveals the importance of the acoustic phonon broadening mechanism which controls the exciton dephasing and may impose the real limits to the optical properties of GaN single QDs emission.     

Hexagonal (wurtzite) GaN inclusions as a defect in cubic (zinc-blende) GaN

The dependence of the hexagonal fraction with thickness in MBE-grown bulk cubic (c-) GaN epilayer is presented in this paper. A number of c-GaN epilayers with different thicknesses were characterized via PL and XRD measurements. From the PL spectra, the signal due to h-GaN inclusions increases as the thickness of the c-GaN increases. On the contrary, in the XRD diffractogram, c-GaN shows a dominant signal at all thicknesses, and only a weak peak at ∼35° is observed in the diffractogram, implying the existence of a small amount of h-GaN in the c-GaN layer. The best quality of c-GaN is observed in the first 10 μm of GaN on the top of GaAs substrate. Even though the hexagonal content increases with the thickness, the average content remains below 20% in c-GaN layers up to 50 μm thick. The surface morphology of thick c-GaN is also presented.     

Anharmonic stabilization of the high-pressure simple cubic phase of calcium

The phonon spectrum of the high-pressure simple cubic phase of calcium, in the harmonic approximation, shows imaginary branches that make it mechanically unstable. In this Letter, the phonon spectrum is recalculated by using density-functional theory ab initio methods fully including anharmonic effects up to fourth order at 50 GPa. Considering that the perturbation theory cannot be employed with imaginary harmonic frequencies, a variational procedure based on the Gibbs-Bogoliubov inequality is used to estimate the renormalized phonon frequencies. The results show that strong quantum anharmonic effects make the imaginary phonons become positive even at zero temperature so that the simple cubic phase becomes mechanically stable, as experiments suggest. Moreover, our calculations find a superconducting T(c) in agreement with experiments and predict an anomalous behavior of the specific heat.     

Selected growth of cubic and hexagonal GaN epitaxial films on polar MgO(111)

Selected molecular beam epitaxy of zinc blende (111) or wurtzite (0001) GaN films on polar MgO(111) is achieved depending on whether N or Ga is deposited first. The cubic stacking is enabled by nitrogen-induced polar surface stabilization, which yields a metallic MgO(111)-(1 x 1)-ON surface. High-resolution transmission electron microscopy and density functional theory studies indicate that the atomically abrupt semiconducting GaN(111)/MgO(111) interface has a Mg-O-N-Ga stacking, where the N atom is bonded to O at a top site. This specific atomic arrangement at the interface allows the cubic stacking to more effectively screen the substrate and film electric dipole moment than the hexagonal stacking, thus stabilizing the zinc blende phase even though the wurtzite phase is the ground state in the bulk.     

Acoustic phonon dynamics in strained cubic and hexagonal GaN/Al2O3 superlattices

We study the acoustic-phonon spectra in periodic and quasiperiodic (Fibonacci type) superlattices made up from III–V nitride materials (GaN) intercalated by sapphire (Al₂O₃). Due to the misalignments between the sapphire and the GaN layers that can lead to threading dislocation densities as high as 10⁸-10¹⁰ cm^-1, and a significant lattice mismatch (~14%), the phonon dynamics is described beyond the continuum elastic model using coupled elastic and electromagnetic equations, stressing the importance of the piezoelectric polarization field in a strained condition. We use a transfer-matrix treatment to simplify the algebra, which would be otherwise quite complicated, allowing a neat analytical expressions for the phonon dispersion relation. Furthermore, a quantitative analysis of the localization and magnitude of the allowed band widths in the phonon's spectra, as well as their scale law and the parametric spectrum of singularities f(α), are presented and discussed.     

Optimization of cubic GaN/AlGaN quantum cascade structures for negative refraction in the THz spectral range

In this work we theoretically investigate a possibility to use cubic nitride based multi-layer periodic nanostructure as a semiconductor metamaterial. The structure design is based on an active region of a quantum cascade laser optimized to achieve optical gain in the Terahertz (THz) spectral range. In particular, we test the GaN/AlGaN quantum well configurations, which should exhibit important advantages compared to GaAs-based structures, namely room temperature operation without the assistance of magnetic field and lower doping densities. Our numerical rate-equations model is solved self-consistently and it takes into account electron-longitudinal optical phonon scattering between all the relevant states among the adjacent periods of the structure. A global optimization routine, specifically genetic algorithm is then used to generate new gain-optimized structures. This work confirms the advantages of cubic GaN designs over GaAs ones, namely feasibility of negative refraction at room temperature without the assistance of magnetic field while keeping the doping densities of the same order of magnitude.     

Effect of stabilization processes on NMR signals from 180° domain boundaries in cubic ferrite-garnet crystals

Numerical methods were used to study the effect of stabilization processes on the line shape of NMR signals from 180° domain walls in cubic ferrite-garnet crystals, taking into account the structural change of the boundaries as they shift quasistatically in a magnetic field. The dependences of the amplitude of absorption maxima on the induced anisotropy constants and the amplitude of the external magnetic field are analyzed. Fiz. Tverd. Tela (St. Petersburg) 40, 694–698 (April 1998)     

Pressure dependence of elastic constants in zinc-blende GaN and InN and their influence on the pressure coefficients of the light emission in cubic InGaN/GaN quantum wells

We have studied the influence of nonlinear elastic effects on the pressure coefficients of light emission, dE_E/dP, in cubic InGaN/GaN quantum wells. By means of ab-initio calculations, we have determined the pressure dependences of the elastic constants, C₁₁, C₁₂ and C₄₄ in zinc-blende InN and GaN. Further, we show that the pressure dependence of the elastic constants results in significant reduction of dE_E/dP in cubic InGaN/GaN quantum wells and essentially improves the agreement between experimental and theoretical values.     

GaN nanotweezers

A new form of GaN nanomaterial (nanotweezers) has been obtained by chemical vapor deposition on an etched cubic MgO (100) plane. The nanotweezers consist of a bottom rod and two arms. The bottom rods have diameters of about 100–150 nm and lengths of about 200–500 nm, on which two arms grow out. The bottoms of the arms are about 40–70 nm and the tops are about 15–30 nm in diameter, and 0.8–1.5 μm in length. X-ray and electron diffractions indicate the nanotweezers are zinc blende gallium nitride. We infer that the fabrication of the GaN nanotweezers is associated with small convex hillocks on the surface of the etched cubic MgO (100) single-crystal substrates and that the nanotweezers grow by a growth mechanism that is similar to vapor-phase heteroepitaxy. Received: 23 April 2002 / Accepted: 25 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/8264-9531, E-mail: xlchen@aphy.iphy.ac.cn     

Properties of Si-doped GaN and AlGaN/GaN heterostructures grown by RF-MBE on high resistivity Fe-doped GaN

Silicon-doped GaN epilayers and AlGaN/GaN heterostructures were developed by nitrogen plasma-assisted molecular beam epitaxy on high resistivity iron-doped GaN (0001) templates and their properties were investigated by atomic force microscopy, x-ray diffraction and Hall effect measurements. In the case of high electron mobility transistors heterostructures, the AlN mole fraction and the thickness of the AlGaN barrier employed were in the range of from 0.17 to 0.36 and from 7.5 to 30 nm, respectively. All structures were capped with a 2 nm GaN layer.Despite the absence of Ga droplets formation on the surface, growth of both GaN and AlGaN by RF-MBE on the GaN (0001) surfaces followed a step-flow growth mode resulting in low surface roughness and very abrupt heterointerfaces, as revealed by XRD. Reciprocal space maps around the reciprocal space point reveal that the AlGaN barriers are fully coherent with the GaN layer.GaN layers, n-doped with silicon in the range from 10¹⁵ to 10¹⁹ cm⁻³ exhibited state of the art electrical properties, consistent with a low unintentional background doping level and low compensation ratio. The carrier concentration versus silicon cell temperatures followed an Arhenius behaviour in the whole investigated doping range. The degenerate 2DEG, at the AlGaN/GaN heteroiterfaces, exhibited high Hall mobilities reaching 1860 cm²/V s at 300 K and 10 220 cm²/V s at 77 K for a sheet carrier density of 9.6E12 cm⁻².The two dimensional degenerate electron gas concentration in the GaN capped AlGaN/GaN structures was also calculated by self-consistent solving the Schrödinger–Poisson equations. Comparison with the experimental measured values reveals a Fermi level pinning of the GaN (0001) surface at about 0.8 eV below the GaN conduction band.     

Structure of “cubic graphite”: Simple cubic fullerite C24

Physics of the Solid State - The structure of an intermediate carbon phase, the so-called “cubic graphite,” is determined. Close agreement between the calculated and experimental...     

MOCVD生长的GaN和GaN:Mg薄膜的拉曼散射

通过显微拉曼散射对用金属有机化学气相沉积(MOCVD)法在Al2O3衬底上生长的六方相CaN和掺Mg的P型GaN薄膜进行了研究。在两个样品的拉曼散射谱中同时观察到位于640,660cm^-1附近的两个峰。640cm^-1的峰归因于布里渊区边界(L点)最高声学声子的二倍频,而660cm^-1的峰为布里渊区边界的光学声子支或缺陷诱导的局域振动模。掺Mg的GaN在该处的峰型变宽是Mg诱导的缺陷引起的加宽或Mg的局域模与上述两峰叠加的结果。在掺Mg的样品中还观察到276,376cm^-1几个局域模并给予了解释。同时掺Mg的GaN中出现了应力弛豫的现象,掺Mg引起的失配位错和电子-声子相互作用都有可能对E2模的频率产生影响。     

DLTS study of n-type GaN grown by MOCVD on GaN substrates

Electron traps in n-type GaN layers grown homoepitaxially by MOCVD on free-standing GaN substrates have been characterized using DLTS for vertical Schottky diodes. Two free-standing HVPE GaN substrates (A and B), obtained from two different sources, are used. The Si-doped GaN layers with the thickness of 5 μm are grown on an area of 0.9×0.9 cm² of substrate A and on an area of 1×1 cm² of substrate B. Two traps labeled B1 and B2 are observed with trap B2 dominant in GaN on both substrates. There exist no dislocation-related traps which have been previously observed in MOCVD GaN on sapphire. This might be correlated to the reduction in dislocation density due to the homoepitaxial growth. However, it is found that there is a large variation, more than an order of magnitude, in trap B2 concentration and that the B2 spatial distributions are different between the two substrates used.     

The effects of GaN capping layer thickness on two-dimensional electron mobility in GaN/AlGaN/GaN heterostructures

In this paper we present a study of the effect of GaN capping layer thickness on the two-dimensional (2D)-electron mobility and the two-dimensional electron gas (2DEG) sheet density which is formed near the AlGaN barrier/buffer GaN layer. This study is undertaken using a fully numerical calculation for GaN/Al_xGa_1−xN/GaN heterostructures with different Al mole fraction in the Al_xGa_1−xN barrier, and for various values of barrier layer thickness. The results of our analysis clearly indicate that increasing the GaN capping layer thickness leads to a decrease in the 2DEG density. Furthermore, it is found that the room-temperature 2D-electron mobility reaches a maximum value of approximately 1.8×10³ cm² /Vs⁻¹ for GaN capping layer thickness grater than 100 Å with an Al_0.32Ga_0.68N barrier layer of 200 Å thick. In contrast, for same structure, the 2DEG density decreases monotonically with GaN capping layer thickness, and eventually saturates at approximately 6×10¹² cm⁻² for capping layer thickness greater than 500 Å. A comparison between our calculated results with published experimental data is shown to be in good agreement for GaN capping layers up to 500 Å thickness.     

Photoconductivity of Zn-doped GaN

GaN has been grown by the epitaxial technique from the gas phase, being doped by Zn during the growth. Highly compensated, high resistivity(>10⁸Ωcm) samples have been obtained. The photoconductivity has been studied between 2.9 and 3.6 eV. A structure ascribed to the presence of Zn is seen at about 3.2 eV and a theoretical curve for the photo-ionization cross section, using the theory of Lucovsky, has been fitted. This gives a low temperature value of 3.12 eV for the position of the Zn-associated acceptor below the conduction band. The temperature shift indicates that the level is pinned to the valence band.     

SiC衬底上外延GaN:Mg材料特性研究

利用扫描电子显微镜、拉曼散射光谱和光致发光谱对在SiC衬底上采用MOCVD异质外延的GaN:Mg薄膜特性进行研究发现：除了一部分Mg原子替代Ga原子呈现受主性外，大部分Mg原子以间隙原子状态（Mg_i）存在，并且在缺陷或者位错处大量聚集引起薄膜张力应力减小，薄膜在降温过程中由于应力不均匀会在部分区域内出现大量的裂纹；Mg的掺杂会加剧GaN无序化程度，致使薄膜质量变差；而室温下的PL谱测量表明蓝带发光由DAP（深施主—浅受主）复合引起，其中D为Mg_GaV_{N 关键词： GaN∶Mg 异质外延 扫描电子显微镜 拉曼散射 光致发光谱}     

Analysis of GaN epilayers on sapphire substrates with GaN and AlN sublayers

The features and results of X-ray diffraction analysis of GaN films are presented. The films are grown by metalorganic vapor-phase epitaxy on c-plane sapphire substrates using GaN or AlN nucleation layers deposited at a low temperature. Measurements of the twist angle and concentrations of Al _x Ga_1-x N solid solutions are discussed in detail.