Band transitions in wurtzite GaN and InN determined by valence electron energy loss spectroscopy

Valence electron energy loss spectroscopy (VEELS) was applied to determine band transitions in wurtzite InN, deposited by molecular beam epitaxy on (0001) sapphire substrates or GaN buffer layers. The GaN buffer layer was used as VEELS reference. At room temperature a band transition for wurtzite InN was found at (1.7±0.2 eV) and for wurtzite GaN at (3.3±0.2 eV) that are ascribed to the fundamental bandgap. Additional band transitions could be identified at higher and lower energy losses. The latter may be related to transitions involving defect bands. In InN, neither oxygen related crystal phases nor indium metal clusters were observed in the areas of the epilayers investigated by VEELS. Consequently, the obtained results mainly describe the properties of the InN host crystal.     

Influence of Grain Size on Electrical and Optical Properties of InP Films

InP film samples were prepared by spray pyrolysis technique using aqueous solutions of InCl₃ and Na₂HPO₄, which were atomized with compressed air as carrier gas onto glass substrates at 500°C with different thicknesses of the films. It is found that the resistivity of the polycrystalline films strongly depends on the grain size. It is observed that the grain size of the films increase with the decrease of the energy band gap and strain of the film. The changes observed in the energy band gap and strain related to the film grain size of the films are discussed in detail.     

Influence of magnetic quantization on the Einstein relation in non-linear optical, optoelectronic and related materials: Simplified theory, relative comparison and suggestion for experimental determination

In this paper, we have investigated the Einstein relation for the diffusivity-to-mobility ratio (DMR) under magnetic quantization in non-linear optical materials on the basis of a newly formulated electron dispersion law by considering the crystal field constant, the anisotropies of the momentum-matrix element and the spin-orbit splitting constant, respectively, within the frame work of k·p formalism. The corresponding result for the three-band model of Kane (the conduction electrons of III-V, ternary and quaternary compounds obey this model) forms a special case of our generalized analysis. The DMR under magnetic quantization has also been investigated for II-VI (on the basis of Hopfield model), bismuth (using the models of McClure and Choi, Cohen, Lax and parabolic ellipsoidal, respectively), and stressed materials (on the basis of model of Seiler et al.) by formulating the respective electron statistics under magnetic quantization incorporating the respective energy band constants. It has been found, taking n-CdGeAs₂, n-Hg_1−xCd_xTe, p-CdS, and stressed n-InSb as examples of the aforementioned compounds, that the DMR exhibits oscillatory dependence with the inverse quantizing magnetic field due to Subhnikov de Haas (SdH) effect with different numerical values. The DMR also increases with increasing carrier degeneracy and the nature of oscillations are totally dependent on their respective band structures in various cases. The classical expression of the DMR has been obtained as a special case from the results of all the materials as considered here under certain limiting conditions, and this compatibility is the indirect test of our generalized formalism. In addition, we have suggested an experimental method of determining the DMR for degenerate materials under magnetic quantization having arbitrary dispersion laws. The three applications of our results in the presence of magneto-transport have further been suggested.     

Optical phonon modes in graded III-V nitride quantum wells

The dispersion relation for optical phonon modes in graded wurtzite AlN/GaN and AlN/InN quantum wells is calculated taking into account the existence of interfacial transition regions. We make use of a model based on the macroscopic theory developed by Loudon, known as the continuum dielectric model. The optical phonon modes are modelled considering only the electrostatic boundary conditions (neglecting retardation effects), in the absence of charge transfer between ions. We show that the graded interfaces strongly shift the frequencies of the phonon modes of the otherwise abrupt nitrides quantum wells.     

Demonstration of GaN/InGaN Light Emitting Diodes on (100) β-Ga2O3 Substrates by Metalorganic Chemical Vapour Deposition

The growth and fabrication of GaN/InGaN multiple quantum well (MQW) light emitting diodes (LEDs) on (100) β-Ga₂O₃ single crystal substrates by metal-organic chemical vapour deposition (MOCVD) technique are reported. x-ray diffraction (XRD) θ-2θ scan spectroscopy is carried out on the GaN buffer layer grown on a (100) β-Ga₂O₃ substrate. The spectrum presents several sharppeaks corresponding to the (100) β-Ga₂O₃ and (004) GaN. High-quality (0002) GaN material is obtained. The emission characteristics of the GaN/InGaN MQW LED are measurement. The first green LED on β-Ga₂O₃ with vertical current injection is demonstrated.     

Light-intensity dependence of slow-relaxation phenomena in semi-insulating GaAs

Quenching-only and quenching-enhancement phenomena in low-temperature photoconductivity (PC) of SI GaAs have been studied as a function of light intensity for photons in 1.0–1.2 eV energy range. Quenching-only of PC occurs only at high light intensities (above 10¹⁴ photons/cm² s) and reflects well-known bleaching of EL2 defects. On the contrary, the quenching-enhancement effect can be observed only for several orders of magnitude lower light intensities and neither the quenching nor the enhancement part of low-temperature evolution of PC is directly connected with EL2 defects, but reflects the time evolution of the occupancy of deep traps other than EL2. It was also found that bleaching of EL2 is quite an unefficient process.     

Temperature and Thickness Effects on Electrical Properties of InP Films Deposited by Spray Pyrolysis

InP film samples are prepared by spray pyrolysis technique using aqueous solutions of InCl₃ and Na₂HPO₄, which are atomized with compressed air as carrier gas onto glass substrates at 500°C with different thicknesses of the films. The structural properties of the samples are determined by x-ray diffraction (XRD). It is found that the crystal structure of the InP films is polycrystalline hexagonal. The orientations of all the obtained films are along the c-axis perpendicular to the substrate. The electrical measurements of the samples are obtained by dc four-probe technique on rectangular-shape samples. The effects of temperature on the electrical properties of the InP films are studied in detail.     

Curie temperature in InMnP and the mechanism of phase transition

We present the phase transition of high Curie temperature InMnP system grown by liquid phase epitaxy. InMnP system has a distribution of uniform islands on the surface layer which has a width of 40 nm and height of 30 nm. Two kinds of phase transition in the magnetization were observed. The origin of these structures and their influence for the magnetization are discussed from the view of two-dimensional Ising model.     

Electronic structure of GaAs1−xNx under pressure

The electronic band structures of GaAs_1−xN_x for x=0.009, 0.016, 0.031 and 0.062 are calculated ab initio using a supercell approach in connection with the full-potential linear muffin-tin orbital method. Corrections for the ‘LDA gap errors’ are made by adding external potentials which are adjusted to yield correct gaps in pure GaAs. Even small amounts of nitrogen modify significantly the conduction bands, which become strongly non-parabolic. The effective mass in the lowest conduction band thus exhibits strong k-vector dependence. Calculated variations of gaps and effective masses with x and externally applied pressure are presented and compared to a variety of experimental data. There are significant error bars on our results due to the use of the supercell approach. These are estimated by examining the effects of varying the geometrical arrangement of the N-atoms substituting As. However, the calculations show that the electron mass for x>0.009 is much larger than that of pure GaAs, and that it decreases with x.     

Surface passivation of III-V semiconductors for future CMOS devices—Past research, present status and key issues for future

Currently, III-V metal-insulator-semiconductor field effect transistors (MISFETs) are considered to be promising device candidates for the so-called “More Moore Approach” to continue scaling CMOS transistors on the silicon platform. Strong interest also exists in III-V nanowire MISFETs as a possible candidate for a “Beyond CMOS”-type device. III-V sensors using insulator-semiconductor interfaces are good candidates for “More Moore”-type of devices on the Si platform. The success of these new approaches for future electronics depends on the availability of a surface passivation technology which can realize pinning-free, high-quality interfaces between insulator and III-V semiconductors.This paper reviews the past history, present status and key issues of the research on the surface passivation technology for III-V semiconductors. First, a brief survey of previous research on surface passivation and MISFETs is made, and Fermi level pinning at insulator-semiconductor interface is discussed. Then, a brief review is made on recent approaches of interface control for high-k III-V MIS structures. Subsequently, as an actual example of interface control, latest results on the authors’ surface passivation approach using a silicon interface control layer (Si ICL) are discussed. Finally, a photoluminescence (PL) method to characterize the interface quality is presented as an efficient contactless and non-destructive method which can be applied at each step of interface formation process without fabrication of MIS capacitors and MISFETs.     

Electronic Structures of Wurtzite GaN with Ga and N Vacancies

The electronic band structures of wurtzite GaN with Ga and N vacancy defects are investigated by means of the first-principles total energy calculations in the neutral charge state. Our results show that the band structures can be significantly modified by the Ga and N vacancies in the GaN samples. Generally, the width of the valence band is reduced and the band gap is enlarged. The defect-induced bands can be introduced in the band gap of GMV due to the Ga and N vacancies. Moreover, the GaN with high density of N vacancies becomes an indirect gap semiconductor. Three defect bands due to Ga vacancy defects are created within the band gap and near the top of the valence band. In contrast, the N vacancies introduce four defect bands within the band gap. One is in the vicinity of the top of the valence band, and the others are near the bottom of the conduction band. The physical origin of the defect bands and modification of the band structures due to the Ga and N vacancies are analysed in depth.     

Electronic Structure and Optical Properties of Semiconducting Orthorhombic BaSi2

Full potential linearized augmented plane wave （FPLAPW） method calculations are carried out for semiconducting orthorhombic BaSi2. The optical properties and the origin of the different optical transitions are investigated. Our calculated band gap of 1.0918eV is indirect, which is in good agreement with the experimental result. The bonds between Ba and Si are considered to be electrovalent bond. The anlsotropy in the imaginary part ε2（w） and real part εl（w） of the optical dielectric tensor are analysed. The contributions of various transition peaks are explained from the imagnary part of the dielectric function.     

Thermally stimulated currents in semi-insulating GaAs Schottky diodes and their simulation

We report the investigation of the non-irradiated and irradiated-with-pions Schottky diodes made on semi-insulating GaAs. Thermally stimulated currents have been measured experimentally and modeled numerically. To reveal the influence of the single levels, we used the thermal emptying of the traps by fractional heating. Attention is paid to the comparative analysis of the distribution of the parameters of different samples produced and processed by the same technique, contrary to the usual approach of the analysis of a few different samples. The following main conclusions are drawn. First of all, many different levels (from 8 to 12) have been found in the temperature range from 90 K to 300 K in all samples. Their activation energies range from 0.07 up to 0.55 eV, their capture cross-sections are 10^-22–10^-14 cm², and initial occupation is 2×10¹¹–5×10¹⁴ cm^-3. The irradiation with pions does not influence the density of most levels significantly. On the other hand, levels with activation energies of about 0.07–0.11 eV, 0.33–0.36 eV, 0.4–0.42 eV, and 0.48–0.55 eV have been found only in the irradiated samples. Irradiation also increases the inhomogeneity of the crystals, which causes the scattering of the activation energies obtained by fractional heating technique. Received: 13 November 1998 / Accepted: 16 April 1999 / Published online: 4 August 1999     

Surface passivation of InGaP/GaAs HBT using silicon-nitride film deposited by ECR-CVD plasma

In this paper we have developed a passivation technique with silicon-nitride (SiN_X) film that requires no surface pre-treatment, and is fully compatible to monolithic microwave integrated circuits (MMICs). The nitride depositions were carried out by ECR-CVD (electron cyclotron resonance-chemical vapor deposition) directly over InGaP/GaAs heterojunction structures, which are used for heterojunction bipolar transistors (HBTs). Optical emission spectrometry (OES) was used for plasma characterization, and low formation of H and NH molecules in the gas phase was detected at pressure of 2.5 mTorr. These molecules can degrade III-V semiconductor surfaces due to the preferential loss of As or P and hydrogen incorporation at the substrate. The substrates were cleaned with organic solvents using a Sox-let distillate. The ECR depositions were carried out at a fixed substrate temperature of 20 °C, SiH₄/N₂ flow ratio of 1, Ar flow of 5 sccm pressure of 2.5 mTorr and microwave (2.45 GHz) power of 250 W and RF (13.56 MHz) power of 4 W. We have applied this film for InGaP/GaAs HBT fabrication process with excellent results, where two major contribuiton is related to this passivation technique, the enhancement in the transistor dc gain β and the improvement in the signal-to-noise ratio when compared unpassivated and passivated devices.     

Surface passivation technology for III-V semiconductor nanoelectronics

The present status and key issues of surface passivation technology for III-V surfaces are discussed in view of applications to emerging novel III-V nanoelectronics. First, necessities of passivation and currently available surface passivation technologies for GaAs, InGaAs and AlGaAs are reviewed. Then, the principle of the Si interface control layer (ICL)-based passivation scheme by the authors’ group is introduced and its basic characterization is presented. Ths Si ICL is a molecular beam epitaxy (MBE)-grown ultrathin Si layer inserted between III-V semiconductor and passivation dielectric. Finally, applications of the Si ICL method to passivation of GaAs nanowires and GaAs nanowire transistors and to realization of pinning-free high-k dielectric/GaAs MOS gate stacks are presented.     

Ab initio study of electronic structures and magnetism in ZnMnTe and CdMnTe diluted magnetic semiconductors

In this work, we aimed to examine the spin-polarized electronic band structures, the local densities of states as well as the magnetism of ZnMnTe- and CdMnTe-diluted magnetic semiconductors (DMSs) in the ferromagnetic phase, and with 25% of Mn. The calculations are performed by the recent ab initio full potential augmented plane waves plus local orbitals (FP−L/APW+lo) method within the spin-polarized density-functional theory and the local spin density approximation. We have determined the exchange splittings produced by the Mn d states: Δ_x(d) and Δ_x(pd), and we found that the effective potential for the minority spin is more attractive than that for the majority spin. Also, we show the nature of the bonding from the charge spin-densities calculations, and we calculate the exchange constants N₀α and N₀β, which mimics a typical magneto-optical experiment. The calculated total magnetic moment is found to be equal to 5μ_B for both DMSs. This value indicates that every Mn impurity adds no hole carriers to the perfect ZnTe and CdTe crystals. Furthermore, we found that p–d hybridization reduces the local magnetic moment of Mn and produces small local magnetic moments on the nonmagnetic Te, Zn and Cd sites.     

磁控溅射制备铬薄膜的结构和光电学性质

用直流磁控溅射技术在石英基片上制备不同厚度(5 nm~114 nm之间)的铬膜.使用X射线衍射仪和分光光度计分别检测薄膜的结构和光学性质,利用德鲁特模型和薄膜的透射、反射光谱计算铬膜的厚度和光学常量,并采用Van der Pauw方法测量薄膜电学性质.结果表明:制备的铬薄膜为体心立方的多晶态,随着膜厚的增加,薄膜的结晶性能提高,晶粒尺寸增大;在可见光区域,当膜厚小于32 nm时,随着膜厚的增加,折射率快速减小,消光系数快速增大,当膜厚大于32 nm时,折射率和消光系数均缓慢减小并逐渐趋于稳定;薄膜电阻率随膜厚的增加为一次指数衰减.     

磁控溅射制备铬薄膜的结构和光电学性质(英文)

用直流磁控溅射技术在石英基片上制备不同厚度(5nm～114nm之间)的铬膜.使用X射线衍射仪和分光光度计分别检测薄膜的结构和光学性质,利用德鲁特模型和薄膜的透射、反射光谱计算铬膜的厚度和光学常量,并采用Van der Pauw方法测量薄膜电学性质.结果表明:制备的铬薄膜为体心立方的多晶态,随着膜厚的增加,薄膜的结晶性能提高,晶粒尺寸增大;在可见光区域,当膜厚小于32nm时,随着膜厚的增加,折射率快速减小,消光系数快速增大,当膜厚大于32nm时,折射率和消光系数均缓慢减小并逐渐趋于稳定;薄膜电阻率随膜厚的增加为一次指数衰减.     

Structural and Thermodynamic Properties of Gallium Arsenide with Hexagonal Wurtzite Structure from First-Principles Analysis

A first-principles plane wave method with the ultrasoft pseudopotential scheme in the frame of the generalized gradient approximation （GGA） is performed to calculate the lattice parameters, the bulk modulus Bo and its pressure derivative B~o of the hexagonal wurtzite GaAs （w-GaAs） by the Cambridge serial total energy package （CASTEP）. Our calculations show that the most stable structure of the w-GaAs corresponds to the axial ratio c/α = 1.651 and the internal parameter u = 0.374, consistent with other theoretical results. Also, the thermodynamic properties of the w-GaAs are investigated from the quasi-harmonic Debye model. The dependences of the normalized lattice parameters α/α0, c/c0, the axial ratio c/α, the normalized volume V/V0, the heat capacity Cv and the thermal expansion α on pressure P and temperature T are also obtained successfully.     

Effect of Annealing Temperature on Structural and Optical Properties of N-Doped ZnO Films

Nitrogen-doped ZnO （ZnO：N） films are prepared by thermal oxidation of sputtered Zn3N2 layers on A1203 substrates. The correlation between the structural and optical properties of ZnO：N films and annealing temperatures is investigated. X-ray diffraction result demonstrates that the as-sputtered Zn3N2 films are transformed into ZnO：N films after annealing above 600℃. X-ray photoelectron spectroscopy reveals that nitrogen has two chemical states in the ZnO：N films： the No acceptor and the double donor （N2）o. Due to the No acceptor, the hole concentration in the film annealed at 700℃ is predicted to be highest, which is also confirmed by Hall effect measurement. In addition, the temperature dependent photoluminescence spectra allow to calculate the nitrogen acceptor binding energy.