Built-in-polarization and thermal conductivity of InxGa1-xN/GaN heterostructures

The effect of built-in-polarization (BIP) field on thermal properties of In_xGa_1−xN/GaN heterostructure has been investigated. The thermal conductivity k of In_xGa_1−xN alloy has been estimated using Callaway's formula including the BIP field for In content x = 0, 0.1, 0.3, 0.5 and 0.9. This study reports that irrespective of In content, the room temperature k of In_xGa_1−xN/GaN heterostructure is enhanced by BIP field. The result predicts the existence of a characteristic temperature T_p at which both thermal conductivities (including and excluding BIP field) show a crossover. This gives signature of pyroelectric nature of In_xGa_1−xN alloy which arises due to variation of polarization with temperature indicating that thermal conductivity measurement can reveal pyroelectric nature. The pyroelectric transition temperature of In_xGa_1−xN alloy has been predicted for various x. The composition dependent nature of room temperature k for x = 0.1 and 0.5 are in line with prior experimental studies. The result can be used to minimize the self heating effect in In_xGa_1−xN/GaN heterostructures.     

Ga填充n型方钴矿化合物的结构及热电性能

用熔融退火结合放电等离子烧结(SPS)技术制备了具有不同Ga填充含量的Ga_xCo₄Sb₁₂方钴矿化合物,研究了不同Ga含量对其热电传输特性的影响规律. Rietveld结构解析表明,Ga占据晶体学2a空洞位置,Ga填充上限约为0.22,当Ga的名义组成x≤0.25时,样品的电导率、室温载流子浓度N_p随Ga含量的增加而增加,Seebeck系数随Ga含量的增加而减小. 室温下霍尔测试表明,每一个Ga授予框架0.9个电子,比Ga的氧化价态Ga³⁺小得多. 由于Ga离子半径相对较小,致使Ga填充方钴矿化合物的热导率κ及晶格热导率κ_L较其他元素填充的方钴矿化合物低. 当x＝0.22时对应的样品在300K时的热导率和晶格热导率分别为3.05Wm^-1·K^-1和 2.86Wm^-1·K^-1.在600K下Ga_0.22Co_4.0Sb_12.0样品晶格热导率达到最小,为1.83Wm^-1·K^-1,最大热电优值Z,在560K处达1.31×10^-3K^-1. 关键词： skutterudite化合物 Ga原子填充 结构 热电性能     

Spatial fluctuations of carrier density in n type GaxIn1−xSb determined by SH-DH effect

Variation of carrier concentration through n-type Ga_xIn_1?xSb single crystals, interpreted as due to vacancies behaving as acceptors, is deduced from Shubnikov-de Haas measurements. A quantitative agreement between calculated and experimental values of the mobility at 4.2°K is obtained with this model as well as some information about the influence of growing conditions on the quality of the crystal.     

ZnO高掺杂Ga的浓度对导电性能和红移效应影响的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法,在相同环境条件下建立了浓度不同的由Ga原子取代Zn原子的Zn_1-xGa_xO模型.对低温高掺杂Ga原子的Zn_1-xGa_xO半导体的能带结构、态密度和吸收光谱进行了计算.结果表明:Ga原子浓度越大,进入导带的相对电子数越多,但是电子迁移率反而减小.通过对掺杂和未掺杂ZnO的电导率以及最小间隙带宽度分别进行了比较 关键词： ZnO高掺杂Ga 电导率 红移 第一性原理     

Zn掺杂n型笼合物Ba8Ga16-2xZnxGe30+x的热电传输特性

用高温熔融结合放电等离子烧结法制备了Zn掺杂单相n型Ba₈Ga_16-2xZn_xGe_30+x笼合物,探索了Zn对Ga的取代对其热电传输特性的影响规律.研究结果表明,n型Ba₈Ga_16-2xZn_xGe_30+x化合物的电导率随着x的增加逐渐增 关键词： 热电传输性能 n型笼合物 框架取代     

Different temperature dependence of carrier transport properties between AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures

The temperature dependence of carrier transport properties of AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures has been investigated.It is shown that the Hall mobility in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures is higher than that in Al0.25Ga0.75N/GaN heterostructures at temperatures above 500 K,even the mobility in the former is much lower than that in the latter at 300 K.More importantly,the electron sheet density in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures decreases slightly,whereas the electron sheet density in Al0.25Ga0.75N/GaN heterostructures gradually increases with increasing temperature above 500 K.It is believed that an electron depletion layer is formed due to the negative polarization charges at the InyGa1-yN/GaN heterointerface induced by the compressive strain in the InyGa1-yN channel,which e-ectively suppresses the parallel conductivity originating from the thermal excitation in the underlying GaN layer at high temperatures.     

Effective phonon scattering and enhancement of thermoelectric performance in Ga-excess Bi0.4Sb1.6Te3 compounds

We investigate the thermoelectric properties on Ga-excess p-type Ga_xBi_0.4Sb_1.6Te₃ compounds. Even though the random distribution of Ga-doping increases electrical resistivity giving rise to the decrease of power factor, the significant decrease of lattice thermal conductivity by the excess Ga-doping induces significant enhancement of ZT value (1.13 at 350 K) for the Ga x = 0.03 doped compound. From the X-ray diffraction and elemental mapping by energy dispersive X-ray spectroscopy, we observed Sb and Ga phase separation leading to the phonon scattering. The Sb precipitation implies atomic defect in the matrix which can induce short wavelength phonon scattering. The synergetic phonon scatterings from various scattering sources such as point defect, alloy scattering, and grain boundary phonon scattering have an important role in the enhancement of thermoelectric performance.     

InAs/InxGa1－xSb二类超晶格红外探测器的吸收波长与电子-空穴波函数交叠的研究

运用包络函数模型和传输矩阵方法计算了二类超晶格的能级结构.考虑到InAs与In_xGa_1-xSb结合存在应变,在计算中InAs/In_xGa_1-xSb二类超晶格的带阶采用模型固体理论处理.因为吸收系数与电子-空穴波函数交叠成正比,所以研究了InAs/In_xGa_1-xSb二类超晶格红外探测器的吸收波 关键词： 二类超晶格 红外探测器 波函数的交叠 传输矩阵     

In0.3Co4Sb12-xSex 方钴矿热电材料的制备和热电性能

用熔融退火结合放电等离子烧结法制备了In_0.3Co₄Sb_12-xSe_x(x=0—0.3)方钴矿热电材料,探讨了In的存在形式,系统研究了Se掺杂量对结构和热电性能的影响.结果表明:In可以填充到方钴矿二十面体空洞处,过量In在晶界处形成InSb第二相,Se对Sb的置换使晶格常数减小,In填充上限降低;In_0.3Co₄Sb_12-xSe_x样品呈n型传导,随着Se掺杂量的增大,载流子浓度降低,电导率下降,Seebeck系数增大,功率因子有所降低;由于在结构中引入了质量波动及晶格畸变,适量的Se掺杂可以大幅降低材料晶格热导率;样品In_0.3Co₄Sb₁₂和In_0.3Co₄Sb_11.95Se_0.05的最大ZT值均达到1.0以上. 关键词： 掺杂 填充式方钴矿 热电性能     

Cu addition and its role in thermoelectric properties and nanostructuring in the series compounds (InSb)nCum

We have performed a comparative investigation of the series compounds (InSb)_nCu_m to assess the roles of Cu addition on the thermoelectric properties and nanostructuring in bulk InSb. Detailed temperature dependent transport properties including electrical conductivity, the Seebeck coefficient, and thermal conductivity are presented. The Seebeck coefficients of In₂₀Sb₂₀Cu (m:n = 1:20) are increased by 13 percent in magnitude if compared to those of InSb, which is responsible for the 22 percent enhancement in the highest ZT value at 687 K. Although the magnitudes of κ_L are larger than those of InSb over the entire temperature range, a remarkable reduction in lattice thermal conductivities (κ_L) was observed with measuring temperature elevation. Such changes are mainly due to the precipitation of a large number of Cu₉In₄ nanoparticles with the size of smaller than 5 nm, dispersed in the matrix observed using high resolution transmission electron microscopy (HRTEM) images.     

Modeling of band discontinuity using modified Harrison model

Modeling of the valence band discontinuity was proposed with Harrison model modified by introducing the effective bond length for AlX (X = P, As, Sb). The valence band discontinuity of heterostructures including AlX as constituent, for example, GaAs/Al_xGa_1−xAs and Al_xIn1_1−xAs/ In_xGa_1−xAs, was predicted. The predicted values were in good agreement with the experimental values, different from ordinary Harrison model. Prediction of the valence band discontinuity of quarternary alloy (Al_xGa_1−x)_y In_1−yP lattice matched to GaAs was also attempted.     

Band-to-band recombination in GaxIn1−xSb

The carrier lifetimes in Ga_xIn_1−xSb for radiative and Auger recombination are calculated for the temperature range 77–300 K and the composition range 0 ⩽ x ⩽ 1. The possible band-to-band Auger recombination mechanisms in direct-gap semiconductors are investigated. The Auger rates are calculated, including Boltzmann statistics and nonparabolic bands, using the Kane-band model. In the low temperature range, for lightly doped material, the carrier lifetime is determined by radiative recombination. At higher temperatures the CHCC process is dominant in n-type Ga_xIn_1−xSb but in p-type material the CHLH process is dominant. The influence of CHSH processes on the carrier lifetime is appreciable in p-type GaSb. The calculations are compared with experimental data reported by other authors.     

Optimization of growth parameters for MOVPE-grown GaSb and Ga1−xInxSb

The triethylgallium/trimethylantimony (TEGa/TMSb) precursor combination was used for the metal-organic vapour phase epitaxial growth of GaSb at a growth temperature of 520 °C at atmospheric pressure. Trimethylindium was added in the case of Ga_1−xIn_xSb growth. The effects of group V flux to group III flux ratio (V/III ratio) on the crystallinity and optical properties of GaSb layers are reported. It has been observed from the crystalline quality and optical properties that nominal V/III ratios of values greater than unity are required for GaSb epitaxial layers grown at this temperature. It has also been shown that Ga_1−xIn_xSb can be grown using TEGa as a source of gallium species at atmospheric pressure. The relationship between Ga_1−xIn_xSb vapour composition and solid composition has been studied at a V/III ratio of 0.78.     

Effect of lattice constant on band-gap energy and optimization and stabilization of high-temperature In x Ga1?x N quantum-dot lasers

We analyze the effect of the lattice constant on the band-gap energy of In _x Ga_1?x N and optimize the structure of the device with a separate-confinement heterostructure. To vary the lattice constants, we change the In molar fraction, which permits us to investigate a wide range of the band gap of the active material employed in diode lasers. In _x Ga_1?x N is a promising active material for high-performance 1.55???m quantum-dot lasers due to its excellent band-gap-energy stability with respect to temperature variations. The band gap of In _x Ga_1?x N decreases from 3.4 to 0.7?eV, and the necessary band gap can be achieved by changing the lattice parameters depending on the device application. It has been found that In_0.86Ga_0.14N can be a promising material for emitting light at a wavelength of 1.55???m.     

Theoretical investigation of electronic structure and optical response in relation to the transport properties of Ga1−xInxN (x = 0, 0.25, 0.50, 0.75)

The state-of-the-art all-electron FLPAW method and the BoltzTrap software package based on semi-classical theory were adopted to explore the electronic structure and the optical and thermoelectric properties of Ga_1−xIn_xN. Ga_1−xIn_xN is predicted to be a direct band gap material for all values of x. Moreover, the band gap varies between 2.99 eV and 1.95 eV as x changes. Optical parameters such as the dielectric constant, absorption coefficient, reflectivity and refractive index are calculated and discussed in detail. The doping of In plays an important role in the modulation of the optical constants. The static dielectric constant ɛ(0) of Ga_1−xIn_xN was calculated as 3.95, 3.99, 3.99 and 4.03 at x = 0.00, 0.25, 0.50 and 0.75, respectively. The static refractive index is 2.0 for pure Ga_1−xIn_xN at x = 0.00. The thermal properties varied greatly as x fluctuated. The ternary alloy has large values for the Seebeck coefficient and figure of merit at high temperatures and is thus suitable for thermoelectric applications. Pure Ga_1−xIn_xN at x = 0 exhibited ZT = 0.80 at room temperature, and at higher temperatures, the thermal conductivity decreased with increased In doping.     

Empirical expression for the composition and temperature dependence of the energy gap in InAlSb

An empirical expression for the energy bandgap as a function of alloy composition x and temperature for In_1−xAl_xSb was reported. The In_1−xAl_xSb epitaxial layers were grown by molecular beam epitaxy (MBE) on InSb(1 0 0) substrate, utilizing a p⁺–p⁺–n–n⁺ structure. High resolution X-ray diffraction was used to characterize the epitaxial layers. The Al composition of 2.8% was obtained by assuming the Bragg’s formula and Vegard’s law. Spectral response measurement of the diodes has been employed to investigate the temperature dependence of the band gap of In_1−xAl_xSb alloys in the range between 77 K and 260 K. The calculated results for energy gap of InAlSb were in good agreement with the available data and our experimental observation.     

Emission and elastic strain in InAs dot-in-a well InGaAs/GaAs structures

The paper presents the photoluminescence (PL) study of InAs quantum dots (QDs) embedded in the asymmetric GaAs/In_xGa_1?xAs/In_0.15Ga_0.85As/GaAs quantum wells (QWs) with the different compositions of capping In_xGa_1?xAs layers. The composition of the buffer In_0.15Ga_0.85As layer was the same in all studied QD structures, but the In content (parameter x) in the capping In_xGa_1?xAs layers varied within the range 0.10–0.25. The In concentration (x) increase in the In_xGa_1?xAs capping layers is accompanied by the variation non-monotonously of InAs QD emission: PL intensity and peak positions. To understand the reasons of PL variation, the PL temperature dependences and X ray diffraction (XRD) have been investigated. It was revealed that the level of elastic deformation (elastic strain) and the Ga/In interdiffusion at the In_xGa_1?xAs/InAs QD interface are characterized by the non-monotonous dependences versus parameter x. The physical reasons for the non-monotonous variation of the elastic strains and PL parameters in studied QD structures have been discussed.     

Exciton absorption,photoluminescence and band structure of N-Free and N-DOPED In1−xGaxP

Optical absorption data on In_1?xGa_xP are reported showing the first observation of the exciton absorption peak in a III-V ternary alloy. This allows the most accurate determination to date of both the composition dependence of the Γ band gap and the position of the Γ ? X crossover (x_c = 0.72, 77°K; x_c = 0.73, 300°K). Absorptio and photoluminescence spectra are compared for both direct and indirect In_1?xGa_xP. In addition, absorption, photoluminescence, and luminescence lifetimes of In_1?xGa_xP:N are examined. These data suggest that the A-line is perturbed by alloy disorder and forms a broad luminescence band for x ? 0.94. The observed Stokes shift for the N-trap in the alloy is characteristic of an impurity strongly coupled to the lattice. The temperature dependence of the N luminescence band is found to be well described by the conventional configuration-coordinate model for phonon-coupled impurity luminescence. Implications of these results for light emitting devices are mentioned.     

Ballistic transport in InSb quantum wells at high temperature

Measurements were made on a 0.2 μm four-terminal device, fabricated from an InSb/Al_0.15In_0.85Sb quantum well structure, at temperatures from 1.5 to 300 K. Negative bend resistance, a signature of ballistic transport, was observed at temperatures up to 205 K. The disappearance of the negative bend resistance at higher temperatures was accompanied by a non-linear dependence of the Hall voltage on magnetic field. The non-linearity indicates multiple-carrier conduction, which we characterize using quantitative mobility spectrum analysis.     

Determination of the critical indium composition corresponding to the metal–insulator transition in InxGa1−xN (0.06 ⩽ x ⩽ 0.135) layers

The low-temperature conductivity of In_xGa_1−xN alloys (0.06 ⩽ x ⩽ 0.135) is analyzed as a function of indium composition (x). Although our In_xGa_1−xN alloys were on the metallic side of the metal–insulator transition, neither the Kubo-Greenwood nor Born approach were able to describe the transport properties of the In_xGa_1−xN alloys. In addition, all of the In_xGa_1−xN alloys took place below the Ioeffe–Regel regime with their low conductivities. The observed behavior is discussed in the framework of the scaling theory. With decreasing indium composition, a decrease in thermal activation energy is observed. For the metal–insulator transition, the critical indium composition is obtained as x_c = 0.0543 for In_xGa_1−xN alloys.