Comparison between wurtzite phase and zincblende phase GaN MESFETsusing a full band Monte Carlo simulation

Cutoff frequency, breakdown voltage, and the transconductance of wurtzite and zincblende phase GaN MESFETs have been calculated using a self-consistent, full band Monte Carlo simulation. The effect of interface states on the device performance is modeled by including uniformly depleted regions at the device surface under the passivation layers. It is found that the drain current increases gradually with increasing drain-source voltage at the onset of breakdown for both phases. The calculated breakdown voltage for the wurtzite device is considerably higher than the breakdown voltage calculated for the zincblende device. On the other hand, the zincblende device is calculated to have higher transconductance and cutoff frequency than the wurtzite device. The higher breakdown voltage of the wurtzite phase device is attributed to the higher density of electronic states for this phase compared to the zincblende phase. The higher cutoff frequency and transconductance of the zincblende phase device is apparently due to the greater electron velocity overshoot for this phase compared to that for the wurtzite phase. The maximum cutoff frequency and transconductance of a 0.1 μm gate-length zincblende GaN MESFET are calculated to be 220 GHz and 210 mS/mm, respectively. The corresponding quantities for the wurtzite GaN device are calculated to be 160 GHz and 158 mS/mm     

Cr掺杂纤锌矿GaN的磁性和光学性质

采用自旋极化密度泛函理论方法,对Cr掺杂纤锌矿GaN的能带结构和态密度进行计算,通过计算的能带结构和光学线性响应函数,系统讨论了Cr掺杂对纤锌矿GaN电子结构、磁学和光学属性的影响。计算结果显示:Cr掺杂纤锌矿GaN的反铁磁态具有半金属特征,铁磁态具有金属特性和磁性,磁矩主要源于费米能级附近的Cr 3d和N 2p能带发生劈裂导致自旋向上和自旋向下的电子的态密度不同所致。光学性质计算结果显示,Cr掺杂纤锌矿GaN的费米能级附近出现极大的介电峰,具有优异的发光性能,峰值对应于紫外波段,在高能区吸收峰发生了红移现象,在低能区红外波吸收增强。理论计算表明,Cr掺杂纤锌矿GaN可能是一种性能优异的磁光材料。     

Optical gain of strained wurtzite GaN quantum-well lasers

A theory for the electronic band structure and the free-carrier optical gain of wurtzite-strained quantum-well lasers is presented. We take into account the strain-induced band-edge shifts and the realistic band structures of the GaN wurtzite crystals. The effective-mass Hamiltonian, the basis functions, the valence band structures, the interband momentum matrix elements, and the optical gain are presented with analytical expressions and numerical results for GaN-AlGaN strained quantum-well lasers. This theoretical model provides a foundation for investigating the electronic and optical properties of wurtzite-strained quantum-well lasers.     

Full band Monte Carlo simulation of zincblende GaN MESFET'sincluding realistic impact ionization rates

In this paper, we present the first theoretical study of the breakdown properties of zincblende phase GaN MESFET devices. The calculations are made using a full band, ensemble Monte Carlo simulation that includes a numerical formulation of the impact ionization transition rates. The breakdown voltage, transconductance and cutoff frequency are calculated for the GaN MESFET under two different conditions, with and without semiconductor-oxide interface states. Uniform surface depletion regions model the effect of the interface states. It is found that the breakdown voltage of the zincblende GaN MESFET is less dependent upon the surface depletion conditions than a corresponding GaAs MESFET. It is also found that the drain current increases more gradually with increasing drain-source voltage at the onset of breakdown and that the breakdown voltage of the zincblende GaN MESFET is predicted to be several times larger than that of a comparable GaAs MESFET. The maximum current gain cutoff frequency of a 0.1 μm gate length GaN MESFET is calculated to be 230 and 220 GHz, for the non-surface-depleted and the surface depleted devices respectively     

Synthesis of wurtzite GaN thin film via spin coating method

In this research, hexagonal wurtzite structure gallium nitride (GaN) thin film was grown on silicon (Si) substrate by using spin coating deposition method. Simple ethanol-based precursor with the addition of diethanolamine solution was used. High resolution X-ray diffraction results revealed that wurtzite structure GaN thin film with (002) preferred orientation was deposited on Si substrate. Flied-emission scanning electron microscopy and atomic force microscopy results showed that crack free GaN thin film with uniform and dense grains of GaN was formed. Finally, lattice vibrational characterization by p-polarized infrared reflectance technique revealed a strong reststrahlen feature of crystalline wurtzite GaN, and the transverse and longitudinal phonon modes of wurtzite GaN were clearly identified.     

TEM和CL准原位表征GaN单根纳米线中WZ/ZB结构及其发光特性

纤锌矿（WZ）/闪锌矿（ZB）结构以及WZ/ZB超结构广泛存在于一维Ⅲ-Ⅴ族半导体材料中。本文采用TEM和CL（阴极荧光谱）准原位表征的方法,对GaN纳米线的各种结构进行了系统的表征,并且建立了材料纳米结构与对应发光性质之间的直接联系。研究发现相对于纯WZ结构的GaN单根纳米线,具有WZ/ZB结构纳米线的CL出现了蓝移。通过对CL发光谱的蓝移现象的分析和研究表明,蓝移是由于高密度的WZ/ZB结构单元及其各种超结构所引起的特殊效应。     

Ab initio study on physical properties of wurtzite,zincblende, and rocksalt structures of zinc oxide using revised functionals

To provide a more comprehensive understanding of zinc oxide (ZnO), the structural, optical, dielectric and thermodynamic properties of wurtzite (B4), zincblende (B3) and rocksalt (B1) phases are studied based on density functional theory with various revised functionals. It is found that the equilibrium volume, bulk modulus, phase transition point and covalent degree of B3-ZnO are close to those of B4-ZnO. Similar behaviors can be observed from optical properties, dielectric constants and Born effective charges. Moreover, thermodynamic parameters including isothermal bulk modulus, thermal expansion coefficients, Grüneisen parameter, and heat capacity with temperature of B3-ZnO are also in accordance with B4-ZnO. In contrast, B1-ZnO obviously undergoes dissimilar physical properties. Thus, it can be concluded that B3 phase may be the substitute of B4 phase in case of growing on the closely cubic lattice-matched substrates.     

Self-consistent calculation of mode spectra and modulation responsein wurtzite GaN quantum-well lasers

The photon rate-equation formalism is used to evaluate the multimode photon density, the output lasing power, and the modulation frequency response in pseudomorphically strained wurtzite GaN quantum-well lasers. The formalism is based on a self-consistent methodology that couples an envelope function (or k·p) Hamiltonian with Poisson's equation. From this approach, we consider (1) the band structure under the influence of large piezoelectric fields and with many-body effect; and (2) the stimulated and spontaneous emissions for each Fabry-Perot mode. Our calculations predict a threshold current density of approximately 1 kA/cm² and an intrinsic 3-dB modulation bandwidth of 11.7 GHz at 40-mW output power for a 50-Å pseudomorphically strained GaN-Al_0.2Ga_0.8N single-quantum-well (SQW) laser. Our estimation of the threshold current density represents the theoretical limit and is compatible with recent experimental results in InGaN multiple-quantum-well (MQW) lasers     

Refractive indices of liquid crystals for display applications

This paper reviews the extended Cauchy model and the four-parameter model for describing the wavelength and temperature effects of liquid crystal (LC) refractive indices. The refractive indices of nine commercial LCs, MLC-9200-000, MLC-9200-100, MLC-6608, MLC-6241-000, 5PCH, 5CB, TL-216, E7, and E44 are measured by the Multi-wavelength Abbe Refractometer. These experimental data are used to validate the theoretical models. Excellent agreement between experiment and theory is obtained.     

Non-equilibrium electron distributions and electron-longitudinal optical phonon scattering rates in wurtzite GaN

Non-equilibrium electron distributions and electron-longitudinal optical phonon scattering rates in wurtzite GaN have been studied by subpicosecond time-resolved Raman spectroscopy. Our experimental results show that for electron densities n≥5×10¹⁷cm⁻³, the non-equilibrium electron distributions in wurtzite GaN can be very well described by Fermi-Dirac distribution functions with the effective electron temperature much higher than the lattice temperature. In addition, we find that the total electron-longitudinal optical phonon scattering rate in GaN is about one order of magnitude larger than that in GaAs. We attribute this enormous increase in the electron-longitudinal optical phonon scattering rate to the much larger ionicity in GaN.     

Exciton states in wurtzite and zinc-blende InGaN/GaN coupled quantum dots

Based on the effective-mass approximation, exciton states in wurtzite (WZ) and zinc-blende (ZB) InGaN/GaN coupled quantum dots (QDs) are studied by means of a variational method. Numerical results show clearly that both the sizes and In content of QDs have a significant influence on exciton states in WZ and ZB InGaN/GaN coupled QDs. Moreover, the ground-state exciton binding energy decreases when the interdot barrier layer thickness increases in the WZ InGaN/GaN coupled QDs. However, the ground-state exciton binding energy has a minimum if the interdot barrier layer thickness increases in the ZB InGaN/GaN coupled QDs.     

Property comparison of polarons in zinc-blende and wurtzite GaN/AlN quantum wells

本文详细比较闪锌矿和纤锌矿GaN/AlN量子阱中Fr?hlich电声子相互作用哈密顿量及极化子性质。采用LLP变分方法计算由界面、局域和半空间声子模导致的基态极化子能移。结果表明,若忽略z方向和x-y平面之间异性的影响,闪锌矿和纤锌矿哈密顿量是一致的。各向异性在窄阱情形对界面声子能移的影响较为明显,而在略宽阱时对局域声子能移影响明显。此外,内建电场对各支光学声子引起的能移的影响较大。     

Property comparison of polarons in zinc-blende and wurtzite GaN/AlN quantum wells

The properties of polarons in zinc-blende and wurtzite GaN/AlN quantum wells with Fr(o|¨)hlich interaction Hamiltonians are compared in detail.The energy shifts of polarons at ground state due to the interface(IF), confined(CO) and half-space phonon modes are calculated by a finite-difference computation combined with a modified LLP variational method.It is found that the two Fr(o|¨)hlich interaction Hamiltonians are consistent with each other when the anisotropic effect from the z-direction and the x-y plane is neglected.The influence of the anisotropy on the polaron energy shifts due to the IF phonon modes for a smaller well width or due to the CO phonon modes for a moderate well width is obvious.In addition,the built-in electric field has a remarkable effect on the polaron energy shifts contributed by the various phonon modes.     

Property comparison of polarons in zinc-blende and wurtzite GaN/AIN quantum wells

The properties ofpolarons in zinc-blende and wurtzite GaN/AlN quantum wells with Fr(o)hlich interaction Hamiltonians are compared in detail.The energy shifts of polarons at ground state due to the interface (IF),confined (CO) and half-space phonon modes are calculated by a finite-difference computation combined with a modified LLP variational method.It is found that the two Fr(o)hlich interaction Hamiltonians are consistent with each other when the anisotropic effect from the z-direction and the x-y plane is neglected.The influence of the anisotropy on the polaron energy shifts due to the IF phonon modes for a smaller well width or due to the CO phonon modes for a moderate well width is obvious.In addition,the built-in electric field has a remarkable effect on the polaron energy shifts contributed by the various phonon modes.     

Refractive indices variation with temperature and humidity of optical adhesive

Refractive index and birefringence of optical adhesives is studied under different environmental exposures, such as temperature, humidity, and annealing. Refractive index of the adhesive decreases at a higher rate for the temperature treatment compared to the humidity exposure. A sudden decrease in the rate of refractive index (thermo-optic coefficient, dn/dT), from −2×10⁻⁴°C⁻¹ to −4×10⁻⁴°C⁻¹ was observed near the glass transition temperature. The adhesive’s refractive index (n) was studied during the moisture desorption cycle. Refractive index of the adhesive was found to increase at a constant rate with moisture. The change in refractive index of the adhesive upon moisture absorption was found to be irreversible. Birefringence (T_E — T_M) decreases with higher annealing temperature treatment of the adhesive, because the rate of increase in refractive indices for T_E and T_M modes is not similar. The refractive index in T_M mode increases at a higher rate compared to that in the T_E mode. This suggests that the material has become more isotropic due to annealing. The environmental changes in optical properties of the adhesive are discussed in light of Lorenz-Lorentz equations.     

Refractive indices of SrTiO3 in the infrared region

We measured transmittance and reflectance spectra of SrTiO₃ crystals in the infrared region, at 200 and 100 K. These data and our previous 300 and 20 K data are analyzed in order to derive frequency and temperature dependence of the refractive indices n and k. We thus provide detailed n(v) and k(v) data at 300, 200, 100, and 20 K, in the frequency range 25–16000 cm^?1.     

纤锌矿GaN/AlxGa1-xN量子阱中激子能量

考虑了纤锌矿GaN/Al<,x>Ga<,1-x>N量子阱(QW)材料中空穴带质量和光学声子模的各向异性以及声子频率随波矢变化的效应,采用改进的LLP变分法计算了纤锌矿氮化物QW中激子的基态能量和结合能.给出了激子的基态能量和结合能随着QW宽度和Al组分变化的函数关系,并对闪锌矿和纤锌矿GaN/Al<0.3>Ga<,0....     

Properties of wurtzite GaN MESFETs studied by two-dimensional full band Monte Carlo approach

The properties of a GaN Metal-Semiconductor Field Effect Transistor were studied based on two-dimensional full band Monte Carlo simulations. The dependences of the distributions of the electric potential, electron concentration, electric field, drift velocity, and average electron energy on the gate-source voltage (V_GS) and drain-source voltage (V_DS) were obtained, then the characteristics of the drain current (I_DS) versus the drain-source voltage (V_DS) and the transconductance (G_m) versus V_GS characteristics were determined. At and I_DS is 5.03 A/cm, which is higher value. The G_m−V_GS curve shows bell shaped and the maximum G_m is 112 ms/mm at and The current gain cutoff frequency (f_T) is 98 GHz at and      

In-Plane Optical Anisotropy of GaN Refractive Index in Visible Light Region

The optical properties of metal–organic chemical vapor deposition gallium nitride layers were measured with the use of a grating-assisted optical coupler. The application of a waveguide-based grating structure for in-coupling of radiated modes was the basis of the work presented here. A set of six grating couplers, providing different propagation angles $Phi$ with respect to the $a$-axis of GaN, was fabricated in a multimode GaN planar waveguide layer grown directly on $c$-axis (0001) sapphire. Measurements of the refractive index were carried out for laser wavelength $lambda = 632.8$ nm and both transverse-electric and transverse-magnetic polarized light. It was found that the refractive indices were dependent not only on the polarization state but also on the propagation direction of the excited optical mode in the $a{-}b$ plane. The 60 $^{circ}$ periodic in-plane anisotropy of GaN optical properties in the visible spectrum was clearly observed with maximum refractive index difference $Delta n = 0.018$ for both polarizations.