Influence of Al Composition on Transport Properties of Two-Dimensional Electron Gas in Al_xGa_(1-x)N/GaN Heterostructures

Magnetotransport properties of two-dimensional electron gases （2DEG） in AlxGa1-x N/GaN heterostructures with different Al compositions are investigated by magnetotransport measurements at low temperatures and in high magnetic fields. It is found that heterostructures with a lower Al composition in the barrier have lower 2DEG concentration and higher 2DEG mobility.

Fabrication of GaN-Based Heterostructures with an InA1GaN/AlGaN Composite Barrier

CaN-based heterostructures with an InAlCaN/AlCaN composite barrier on sapphire （0001） substrates are grown by a low-pressure metal organic chemical vapor deposition system. Compositions of the InAiGaN layer are determined by x-ray photoelectron spectroscopy, structure and crystal quality of the heterostruetures are identified by high resolution x-ray diffraction, surface morphology of the samples are examined by an atomic force microscope, and Hall effect and capacitance-voltage measurements are performed at room temperature to evaluate the electrical properties of heterostructures. The Al/In ratio of the InAlGaN layer is 4.43, which indicates that the InAlCaN quaternary layer is nearly lattice-matched to the CaN channel. Capacitance-voltage results show that there is no parasitic channel formed between the InAIGaN layer and the AlCaN layer. Compared with the InAl- CaN/CaN heterostructure, the electrical properties of the InAlCaN/AlGaN/GaN heterostructure are improved obviously. Influences of the thickness of the AlGaN layer on the electrical properties of the heterostructures are studied. With the optimal thickness of the AlGaN layer to be 5 nm, the 2DEG mobility, sheet density and the sheet resistance of the sample is 1889.61 cm2/V.s, 1.44 × 10^13 cm-2 and as low as 201.1 Ω/sq, respectively.

Influence of the channel electric field distribution on the polarization Coulomb field scattering in In_(0.18) Al_(0.82) N/AlN/GaN heterostructure field-effect transistors

By making use of the quasi-two-dimensional （quasi-2D） model, the current-voltage （l-V） characteristics of In0AsA10.82N/A1N/GaN heterostructure field-effect transistors （HFETs） with different gate lengths are simulated based on the measured capacitance-voltage （C-V） characteristics and I-V characteristics. By analyzing the variation of the electron mobility for the two-dimensional electron gas （2DEG） with electric field, it is found that the different polarization charge distributions generated by the different channel electric field distributions can result in different polarization Coulomb field scatterings. The difference between the electron mobilities primarily caused by the polarization Coulomb field scatterings can reach up to 1522.9 cm2/V.s for the prepared In0.38AI0.82N/A1N/GaN HFETs. In addition, when the 2DEG sheet density is modulated by the drain-source bias, the electron mobility presents a peak with the variation of the 2DEG sheet density, the gate length is smaller, and the 2DEG sheet density corresponding to the peak point is higher.

多孔n-GaN/p-Zn_(x)Cu_(1-x)S异质结的制备及紫外探测性能研究

本文首先采用紫外光辅助电化学刻蚀(UV-EC)方法制备出了孔隙密度为1.51×10^(10)cm^(-2)、平均孔径为38 nm的多孔n-GaN薄膜;随后在其上通过水浴法沉积了一系列Zn_(x)Cu_(1-x)S复合薄膜,x为0.0、0.2、0.4、0.6、0.8、1.0,形成的多孔n-GaN/p-Zn_(x)Cu_(1-x)S异质结带隙在2.34~3.51 eV调控;最后基于这些异质结构建出p-n结型紫外探测器。I-V曲线结果表明这些探测器均具有良好的整流特性,特别是n-GaN/p-Zn_(0.4)Cu_(0.6)S探测器性能最优。在暗态下,I_(+3 V)/I_(-3 V)约为1.78×10^(5);在偏压为-3 V、光功率密度为432μW/cm^(2)(365 nm)的条件下,光暗电流比超过10^(3),上升/下降时间为0.09/39.8 ms,响应度(R)为0.352 A/W,外量子效率(EQE)为119.6%,探测率(D^(*))为3.21×10^(12)Jones。I-t曲线结果表明,多孔n-GaN/p-Zn_(x)Cu_(1-x)S异质结紫外探测器在连续开-关光循环过程中拥有稳定的光电流响应。该研究为制备异质结紫外探测器提供了一定的理论指导和实验数据。

Influence of thermal stress on the relative permittivity of the AlGaN barrier layer in an AlGaN/GaN heterostructure Schottky contacts

Ni Schottky contacts on A1GaN/CaN heterostructures were fabricated. Some samples were thermally treated in a furnace with N2 ambience at 600 ~C for different times （0.5 h, 4.5 h, 10.5 h, 18 h, 33 h, 48 h, and 72 h）, the others were thermally treated for 0.5 h at different temperatures （500 ~C, 600 ~C, 700 ~C, and 800 ~C）. With the measured current-voltage （I-V） and capacitance-voltage （C V） curves and by self-consistently solving Schrodinger＇s and Poisson＇s equations, we found that the relative permittivity of the A1GaN barrier layer was related to the piezoelectric and the spontaneous polarization of the A1GaN barrier layer. The relative permittivity was in proportion to the strain of the A1GaN barrier layer. The relative permittivity and the strain reduced with the increased thermal stress time until the A1GaN barrier totally relaxed （after 18 h at 600 ~C in the current study）, and then the relative permittivity was almost a constant with the increased thermal strcss time. When the sample was treated at 800 ~C for 0.5 h, the relative permittivity was less than the constant due to the huge diffusion of the contact metal atoms. Considering the relation between the relative permittivity of the A1GaN barrier layer and the converse piezoelectric effect, the conclusion can be made that a moderate thermal stress can restrain the converse piezoelectric effect and can improve the stability of A1GaN/GaN heterostructure devices.

A simple method of extracting the polarization charge density in the AlGaN/GaN heterostructure from current-voltage and capacitance-voltage characteristics

An Ni Schottky contact on the A1GaN/GaN heterostructure is fabricated. The flat-band voltage for the Schottky contact on the A1GaN/GaN heterostructure is obtained from the forward current-voltage characteristics. With the measured capacitance-voltage curve and the flat-band voltage, the polarization charge density in the A1GaN/GaN heterostructure is investigated, and a simple formula for calculating the polarization charge density is obtained and analyzed. With the approach described in this paper, the obtained polarization charge density agrees well with the one calculated by self-consistently solving Schrodinger＇s and Poisson＇s equations.

GaN极性和Al组份对AlcGa1−cN/GaN双异质结IMPATTD性能的影响

GaN基碰撞雪崩渡越时间二极管(IMPATTD)是前景光明的太赫兹波源,具有输出功率高、转换效率高、体积小、集成方便等优点.构建了GaN/AlcGa1-cN/GaN和AlcGa1-cN/GaN/AlcGa1-cN双异质结构(DHS)n+/n/i/p/p+型IMPATTD,通过求解半导体器件基本方程(包括泊松方程、电流密度方程和载流子连续性方程),仿真了工作于大气低损耗窗口频率0.22 THz处的IMPATTD,计算了所设计器件的直流参数(如电场分布、归一化电流密度、击穿电压等)、大信号参数(如端电压、雪崩电流密度、端电流密度、导纳-频率关系、输出功率、转换效率等)和噪声特性参数(如噪声场分布、噪声谱密度和噪声测度等),分析了GaN极性和Al组份对AlcGa1-cN/GaN DHS IMPATTD性能的影响.本文提出了一种优化AlcGa1-cN/GaN DHS IMPATT二极管结构和性能的方法.

The low-temperature mobility of two-dimensional electron gas in AlGaN/GaN heterostructures

To reveal the internal physics of the low-temperature mobility of two-dimensional electron gas （2DEG） in Al- GaN/GaN heterostructures, we present a theoretical study of the strong dependence of 2DEG mobility on Al content and thickness of AlGaN barrier layer. The theoretical results are compared with one of the highest measured of 2DEG mobility reported for AlGaN/GaN heterostructures. The 2DEG mobility is modelled as a combined effect of the scat- tering mechanisms including acoustic deformation-potential, piezoelectric, ionized background donor, surface donor, dislocation, alloy disorder and interface roughness scattering. The analyses of the individual scattering processes show that the dominant scattering mechanisms are the alloy disorder scattering and the interface roughness scattering at low temperatures. The variation of 2DEG mobility with the barrier layer parameters results mainly from the change of 2DEG density and distribution. It is suggested that in AlGaN/GaN samples with a high Al content or a thick AlGaN layer, the interface roughness scattering may restrict the 2DEG mobility significantly, for the AlGaN/GaN interface roughness increases due to the stress accumulation in AlGaN layer.

Electronic and optical properties of GaN–MoS2 heterostructure from first-principles calculations

Heterostructures(HSs)have attracted significant attention because of their interlayer van der Waals interactions.The electronic structures and optical properties of stacked GaN-MoS2 HSs under strain have been explored in this work using density functional theory.The results indicate that the direct band gap(1.95 e V)of the Ga N-MoS2 HS is lower than the individual band gaps of both the GaN layer(3.48 e V)and the MoS2 layer(2.03 eV)based on HSE06 hybrid functional calculations.Specifically,the GaN-MoS2 HS is a typical type-II band HS semiconductor that provides an effective approach to enhance the charge separation efficiency for improved photocatalytic degradation activity and water splitting efficiency.Under tensile or compressive strain,the direct band gap of the GaN-MoS2 HS undergoes redshifts.Additionally,the GaN-MoS2 HS maintains its direct band gap semiconductor behavior even when the tensile or compressive strain reaches 5%or-5%.Therefore,the results reported above can be used to expand the application of Ga N-MoS2 HSs to photovoltaic cells and photocatalysts.

GaN-based heterostructures： electric-static equilibrium and boundary conditions

In the GaN-based heterostructures, this paper reports that the strong electric fields induced by polarization effects at the structure boundaries complicate the electric-static equilibrium and the boundary conditions. The basic requirements of electric-static equilibrium for the heterostructure systems are discussed first, and it is deduced that in the application of the coupled Schroedinger-Poisson model to the heterostructures of electric static equilibrium state, zero external electric field guarantees the overall electric neutrality, and there is no need to introduce the charge balance equation. Then the relation between the screening of the polar charges in GaN-based heterostructures and the possible boundary conditions of the Poisson equation is analysed, it is shown that the various boundary conditions are equivalent to each other, and the surface charge, which can be used in studying the screening of the polar charges, can be precisely solved even if only the conduction band energy is correctly known at the surface. Finally, through the calculations on an AlGaN/GaN heterostructure with typical structure parameters by the coupled Schroedinger-Poisson model under the various boundary conditions, the correctness of the above analyses are validated.

Magnetotransport properties of two-dimensional electron gas in AlGaN/AlN/GaN heterostructures

Magnetotransport measurements are carried out on the A1GaN/A1N/GaN in an SiC heterostructure, which demon- strates the existence of the high-quality two-dimensional electron gas （2DGE） at the A1N/GaN interface. While the carrier concentration reaches 1.32×10^13 cm^-2 and stays relatively unchanged with the decreasing temperature, the mobility of the 2DEG increases to 1.21 × 10^4 cm2/（V.s） at 2 K. The Shubnikov-de Haas （SdH） oscillations are observed in a magnetic field as low as 2.5 T at 2 K. By the measurements and the analyses of the temperature-dependent SdH oscillations, the effective mass of the 2DEC is determined. The ratio of the transport lifetime to the quantum scattering time is 9 in our sample, indicating that small-angle scattering is predominant.

The effects of vicinal sapphire substrates on the properties of AlGaN/GaN heterostructures

A1GaN/GaN heterostructures on vicinal sapphire substrates and just-oriented sapphire substrates （0001） are grown by the metalorganic chemical vapor deposition method. Samples are studied by high-resolution x-ray diffraction, atomic force microscopy, capacitance-voltage measurement and the Van der Panw Hall-effect technique. The investigation reveals that better crystal quality and surface morphology of the sample are obtained on the vicinal substrate. Fur- thermore, the electrical properties are also improved when the sample is grown on the vicinal substrate. This is due to the fact that the use of vicinal substrate can promote the step-flow mode of crystal growth, so many macro-steps are formed during crystal growth, which causes a reduction of threading dislocations in the crystal and an improvement in the electrical properties of the AlGaN/GaN heterostructure.

Influence of drain bias on the electron mobility in AlGaN/AlN/GaN heterostructure field-effect transistors

Using measured capacitance-voltage curves and current-voltage characteristics for the AlGaN/AlN/GaN heterostructure field-effect transistors with different gate lengths and drain-to-source distances, the influence of drain bias on the electron mobility is investigated. It is found that below the knee voltage the longitudinal optical （LO） phonon scattering and interface roughness scattering are dominant for the sample with a large ratio of gate length to drain-to-source distance （here 4/5）, and the polarization Coulomb field scattering is dominant for the sample with a small ratio （here 1/5）. However, the above polarization Coulomb field scattering is weakened in the sample with a small drain-to-source distance （here 20 μm） compared with the one with a large distance （here 100 μm）. This is due to the induced strain in the AlGaN layer caused by the drain bias.

Si and Mg pair-doped interlayers for improving performance of AlGaN/GaN heterostructure field effect transistors grown on Si substrate

We report a novel structure of A1GaN/GaN heterostructure field effect transistors （HFETs） with a Si and Mg pair- doped interlayer grown on Si substrate. By optimizing the doping concentrations of the pair-doped interlayers, the mobility of 2DEG increases by twice for the conventional structure under 5 K due to the improved crystalline quality of the conduction channel. The proposed HFET shows a four orders lower off-state leakage current, resulting in a much higher on/off ratio （ - 10^9）. Further temperature-dependent performance of Schottky diodes revealed that the inhibition of shallow surface traps in proposed HFETs should be the main reason for the suppression of leakage current.

Determination of the relative permittivity of the AlGaN barrier layer in strained AlGaN/GaN heterostructures

Using the measured capacitance voltage curves and the photocurrent spectrum obtained from the Ni Schottky contact on a strained Al0.3Ga0.7N/GaN heterostructure, the value of the relative permittivity of the AlGaN barrier layer was analysed and calculated by self-consistently solving SchrSdinger＇s and Poisson＇s equations. It is shown that the calculated values of the relative permittivity are different from those formerly reported, and reverse biasing the Ni Schottky contact has an influence on the value of the relative permittivity. As the reverse bias increases from 0 V to -3 V, the value of the relative permittivity decreases from 7.184 to 7.093.

Influence of thermal stress on the characteristic parameters of AlGaN/GaN heterostructure Schottky contacts

Ni Schottky contacts on A1GaN/GaN heterostructures have been fabricated. The samples are then thermally treated in a furnace with N2 ambient at 600℃ for different times （0.5, 4.5, 10.5, 18, 33, 48 and 72 h）. Current-voltage （I-V） and capacitance-voltage （C-V） relationships are measured, and SchrSdinger＇s and Poisson＇s equations are self- consistently solved to obtain the characteristic parameters related to A1GaN/GaN heterostructure $chottky contacts： the two-dimensional electron gas （2DEG） sheet density, the polarization sheet charge density, the 2DEG distribution in the triangle quantum well and the Schottky barrier height for each thermal stressing time. Most of the above parameters reduce with the increase of stressing time, only the parameter of the average distance of the 2DEG from the A1CaN/GaN interface increases with the increase of thermal stressing time. The changes of the characteristic parameters can be divided into two stages. In the first stage the strain in the A1GaN barrier layer is present. In this stage the characteristic parameters change rapidly compared with those in the second stage in which the AlGaN barrier layer is relaxed and no strain is present.

Improvement of switching characteristics by substrate bias in AlGaN/AlN/GaN heterostructure field effect transistors

A simple and effective approach to improve the switching characteristics of AlGaN/AlN/GaN heterostructure field effect transistors （HFETs） by applying a voltage bias on the substrate is presented. With the increase of the substrate bias, the OFF-state drain current is much reduced and the ON-state current keeps constant. Both the ON/OFF current ratio and the subthreshold swing are demonstrated to be greatly improved. With the thinned substrate, the improvement of the switching characteristics with the substrate bias is found to be even greater. The above improvements of the switching characteristics are attributed to the interaction between the substrate bias induced electrical field and the bulk traps in the GaN buffer layer, which reduces the conductivity of the GaN buffer layer.

Degradation mechanism of two-dimensional electron gas density in high Al-content AlGaN/GaN heterostructures

This paper finds that the two-dimensional electron gas density in high Al-content A1GaN/GaN heterostructures exhibits an obvious time-dependent degradation after the epitaxial growth. The degradation mechanism was investigated in depth using Hall effect measurements,high resolution x-ray diffraction,scanning electron microscopy,x-ray photoelectron spectroscopy and energy dispersive x-ray spectroscopy.The results reveal that the formation of surface oxide is the main reason for the degradation,and the surface oxidation always occurs within the surface hexagonal defects for high Al-content AlGaN/GaN heterostructures.

Electron mobility in the linear region of an AlGaN/AlN/GaN heterostructure field-effect transistor

We simulate the current-voltage （I-V） characteristics of AlGaN/AlN/GaN heterostructure field-effect transistors （HFETs） with different gate lengths using the quasi-two-dimensional （quasi-2D） model. The calculation results obtained using the modified mobility model are found to accord well with the experimental data. By analyzing the variation of the electron mobility for the two-dimensional electron gas （213EG） with the electric field in the linear region of the AlGaN/AlN/GaN HFET I-V output characteristics, it is found that the polarization Coulomb field scattering still plays an important role in the electron mobility of AlGaN/AlN/GaN HFETs at the higher drain voltage and channel electric field. As drain voltage and channel electric field increase, the 2DEG density reduces and the polarization Coulomb field scattering increases, as a result, the 2DEG electron mobility decreases.

Effects of the strain relaxation of an AlGaN barrier layer induced by various cap layers on the transport properties in AlGaN/GaN heterostructures

The strain relaxation of an A1GaN barrier layer may be influenced by a thin cap layer above, and affects the transport properties of A1GaN/GaN heterostructures. Compared with the slight strain relaxation found in A1GaN barrier layer without cap layer, it is found that a thin cap layer can induce considerable changes of strain state in the A1GaN barrier layer. The degree of relaxation of the A1GaN layer significantly influences the transport properties of the two-dimensional electron gas （2DEG） in A1GaN/GaN heterostructures. It is observed that electron mobility decreases with the increasing degree of relaxation of the A1GaN barrier, which is believed to be the main cause of the deterioration of crystalline quality and morphology on the A1GaN/GaN interface. On the other hand, both GaN and A1N cap layers lead to a decrease in 2DEC density. The reduction of 2DEG caused by the GaN cap layer may be attributed to the additional negative polarization charges formed at the interface between CaN and A1GaN, while the reduction of the piezoelectric effect in the A1GaN layer results in the decrease of 2DEC density in the case of A1N cap layer.