AlGaN/GaN异质结构中二维电子气的高温输运性质

采用高温Hall测量仪对一个全应变和一个部分应变弛豫的AlGaN/GaN异质结构中2DEG的高温输运特性进行了研究,温度变化范围从室温到680K.研究结果表明:在高温段2DEG的迁移率主要受LO声子散射限制; 在室温,异质界面处的非均匀压电极化场对2DEG迁移率的散射也是一个主要的散射机制.同时,计算结果显示,随着温度升高,更多的电子跃迁到更高的子带,在更高的子带,其波函数逐渐扩展到AlGaN层内部以及GaN体内更深的位置,导致LO声子散射的屏蔽效应减弱且来自AlGaN层内的合金无序散射增强.     

量子点场效应单光子探测器二维电子气电子迁移率分析

设计了一种基于场效应晶体管的量子点场效应单光子探测器,利用二维弛豫时间的近似理论建立了二维电子气电子迁移率的散射模型,通过求解量子点场效应单光子探测器GaAs/AlxGa1-xAs二维电子气系统电子和声子相互作用的Hamiltonian函数,得到了不同温度、不同Al组分以及不同二维电子气电子面密度条件下晶格振动散射对探测器二维电子气电子迁移率的影响.仿真结果显示,提高二维电子气的电子面密度浓度和适当增大Al组分,并降低工作温度,有助于探测器获得更高的二维电子气电子迁移率.     

AlxGa1-xAs/GaAs异质结中电子迁移率的压力效应

对Al_xGa_1-xAs/GaAs半导体单异质结系统,引入有限高势垒与考虑导带弯曲的真实势，同时计入电子对异质结势垒的隧穿，利用变分法和记忆函数方法讨论在界面光学声子和体纵光学声子的散射下，异质结界面附近电子迁移率随温度的变化关系及其压力效应.结果显示：电子迁移率随温度、压力的增加而减小；且两种声子的散射作用均随压力增强，界面光学声子的变化幅度更大.因此，在讨论压力的情形下，界面光学声子的作用不容忽略.     

Electron mobility in two-dimensional electron gas in AIGaN/GaN heterostructures and in bulk GaN

We report on temperature dependencies of the electron mobility in the two-dimensional electron gas (2DEG) in AIGaN/GaN heterostructures and in doped bulk GaN. Calculations and experimental data show that the polar optical scattering and ionized impurity scattering are the two dominant scattering mechanisms in bulk GaN for temperatures between 77 and 500K. In the 2DEG in AIGaN/GaN heterostructures, the piezoelectric scattering also plays an important role. Even for doped GaN, with a significant concentration of ionized impurities, a large volume electron concentration in the 2DEG significantly enhances the electron mobility, and the mobility values close to 1700 cm²/Vs may be obtained in the GaN 2DEG at room temperature. The maximum measured Hall mobility at 80K is nearly 5000 cm²/Vs compared to approximately 1200 cm²/Vs in a bulk GaN layer. With a change in temperature from 300 to 80K, the 2DEG in our samples changes from nondegenerate and weakly degenerate to degenerate. Therefore, in order to interpret the experimental data, we propose a new interpolation formula for low field mobility limited by the ionized impurity scattering. This formula is valid for an arbitrary degree of the electron gas degeneracy. Based on our theory, we show that the mobility enhancement in the 2DEG is related to a much higher volume electron concentration in the 2DEG, and, hence, to a more effective screening.     

Comparative studies on electronic transport due to the reduced dimensionality at the heterojunctions of GaAs/A1xGa(1−x)As and GaxIn(1−x)As/InP systems at low temperatures

Comparative studies on ac/dc mobility due to the reduced dimensionality of spatially confined low dimensional systems, at the heterojunctions of GaAs/A1_xGa_(1−x)As and Ga_xIn_(1−x)As/InP forming quasi-two dimensional (Q2D) and quasi-one dimensional (Q1D) systems have been made. The effect of various low temperature nonphonon scattering mechanisms such as ionized impurity, alloy disorder scattering and surface roughness scattering mechanisms; and phonon scattering mechanisms such as acoustic phonon via deformation potential and piezoelectric scattering mechanisms on the systems has been studied. It is found that the surface roughness scattering mechanism dominates in Q2D system whereas acoustic phonon scattering mechanism dominates in Q1D system due to which the nature and magnitude of the temperature dependent dc/ac mobility curves shows significant variation. Whereas, it is observed that the confinement does not change the nature of the frequency dependent real and imaginary parts of ac mobility curves. However, the mobility is found to be enhanced with effective mass and also due to the confinement, i.e. the mobility for Q1D system is higher than that for Q2D system.     

应变Si/(101)Si1-xGex的谷间声子散射机制

Inter valley scattering has a great impact on carrier mobility of strained Si materials,so based on Fermi’s golden rule and the theory of Boltzmann collision term approximation,inter valley phonon scattering mechanism of electrons in nano scale strained Si（101） materials is established under the influence of both energy and stress. It shows that inter valley phonon f₂,f₃,g₃ scattering rates decrease markedly in nano scale strained Si（101） materials with increasing stress.The quantized models can provide valuable references to the understanding of strained Si materials and the research on electron carrier mobility.     

Electron transport in two dimensional electron gas formed at the heterojunction of AlxGa(1−x)N/GaN at microwave frequencies

Transport properties of the two dimensional electron gas (2DEG) formed at the interface of AlGaN/GaN heterostructure ranging from 20 K to 300 K has been investigated theoretically considering the various scattering mechanisms like the acoustic, the piezo, the surface roughness, the alloy, the polar optical phonon and the dislocation scattering. The dc mobility is found to remain constant up to 150 K and then it decrease sharply with further increase in temperature. The ac mobility is also found to decrease with increase in the temperature. The real part of ac mobility, i.e. μ_r decreases with the increase in the frequency very fast initially and then gradually attains a steady value. The imaginary part of the ac mobility μ_im initially increases with the increase in the frequency and then decreases after reaching the maximum value. The value of the ac mobility reduces quite reasonably as the 2D carrier concentration increases at lower range of the frequency. At the carrier concentration of 5 × 10¹⁷ m⁻², the ac mobility remains constant through a wide range of frequencies. With the increase in the dislocation densities, the values of the ac mobility are found to decrease at the lower range of frequencies. The thermo electric power is positive at the 2D carrier concentration of 5 × 10¹⁶ m⁻², the value of which increases with the increase in the temperature and gradually attains a steady value. But the thermoelectric power at n_2D of 10¹⁷ m⁻² is found to be negative in the value. The value of ZT is found to increase with the temperature and attains the maximum value of 0.007 at 150 K and the value of ZT then decreases with increase in the temperature.     

Hooge parameter of InGaAs bulk material and InGaAs 2DEG quantum well structures based on InP substrates

The 1/f noise of various InGaAs layers lattice matched to InP is investigated systematically at room temperature. To elucidate the origin of the 1/f noise in this type of III–V compound semiconductor, thick n-type doped InGaAs layers, typical InP based InAlAs/InGaAs or InP/InGaAs heterostrucuture field-effect transistor (HFET) structures, respectively, as well as InP based InAlAs/InGaAs quantum well structures with doped InGaAs two-dimensional electron gas (2DEG) channel are analysed. From the experiments it is found that mobility fluctuation is the only origin for the 1/f noise observed both in InGaAs bulk material and in 2DEG heterostructures of high quality. A Hooge parameter attributed to phonon scattering α_Hphon in the bulk material is found to be about 7×10⁻⁶ and agrees with those obtained from HFET structures with the highest mobilities (α_H≈1.5×10⁻⁵). Furthermore, the Hooge parameter of 2DEG structures strongly depends on the channel design and on the doping concentration in the n-type doped 2DEG channels.     

Heat-transfer phenomena in alloys

The phonon heat conductivity k _ph in Bi_{1 − x} Sb _x (x = 0.04–0.12) alloys is investigated in the temperature range of 6–60 K. The results are compared with the theory of solids at low temperatures, and the basic sources of phonon scattering are revealed. It is shown that phonon scattering at local mass changes dominates over other sources. The dependences of k _ph on composition are considered at temperatures of 60 and 90 K, and it is found that the normal N processes substantially affect the phonon scattering under these conditions. The donor-impurity effect on heat conductivity of Bi_0.88Sb_0.12 is considered, and the heat resistance caused by the phonon scattering at impurity centers is singled out.     

Photoluminescence and raman scattering characterization of silicon-doped In0.52Al0.48As grown on InP (100) substrates by molecular beam epitaxy

Growth of In_0.52Al_0.48As epilayers on InP (100) substrates by molecular beam epitaxy at different silicon doping levels is carried out. The doped samples show an inverted S-shaped dependence of the PL peak energy variation with the temperature which weakens at high doping levels due to a possible reduction in the donor binding energy. There is a reduction in both the AlAs-like and InAs-like longitudinal-optic (LO) phonon frequencies and a broadening of the LO phonon line shape as the doping level is increased. The PL intensity also showed in increasing degrees at higher doping levels, a temperature dependence which is characteristic of disordered and amorphous materials.