极化效应对AlGaN/GaN异质结p-i-n光探测器的影响

基于等效薄层电荷近似模拟表征极化电荷的作用, 通过自洽求解Poisson-Schrödinger方程以及求解载流子连续性方程, 计算并且讨论了p-AlGaN层掺杂浓度和界面极化电荷对AlGaN/GaN异质结p-i-n紫外探测器能带结构和电场分布以及光电响应的影响. 结果表明, 极化效应与p-AlGaN层掺杂浓度相互作用对探测器性能有较大影响. 其中, 在完全极化条件下, p-AlGaN层掺杂浓度越大, p-AlGaN层的耗尽区越窄, i-GaN层越容易被耗尽, 器件光电流越小. 在一定掺杂浓度条件下, 极化效应越强, p-AlGaN层的耗尽区越宽, 器件的光电流越大. 最后还分析了该结构在不同温度下的探测性能, 证明了该结构可以在高温下正常工作.     

Neutron irradiation effects on AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) were exposed to 1 MeV neutron irradiation at a neutron fluence of 1 × 10¹⁵ cm^-2. The dc characteristics of the devices, such as the drain saturation current and the maximum transconductance, decreased after neutron irradiation. The gate leakage currents increased obviously after neutron irradiation. However, the rf characteristics, such as the cut-off frequency and the maximum frequency, were hardly affected by neutron irradiation. The AlGaN/GaN heterojunctions have been employed for the better understanding of the degradation mechanism. It is shown in the Hall measurements and capacitance-voltage tests that the mobility and concentration of two-dimensional electron gas (2DEG) decreased after neutron irradiation. There was no evidence of the full-width at half-maximum of X-ray diffraction (XRD) rocking curve changing after irradiation, so the dislocation was not influenced by neutron irradiation. It is concluded that the point defects induced in AlGaN and GaN by neutron irradiation are the dominant mechanisms responsible for performance degradations of AlGaN/GaN HEMT devices.     

基于量子微扰的AlGaN/GaN异质结波函数半解析求解

基于量子力学微扰理论的分析，得到AlGaN/GaN异质结波函数的半解析模型.给出了模型的理论分析和计算结果.对于相同问题，给出了与差分算法的对照结果.与传统的差分方法相比，半解析方法具有收敛性强、大规模问题计算效率高的特点，更适合作为AlGaN/GaN异质结量子阱的求解算法. 关键词： AlGaN GaN 量子阱 薛定谔方程     

Effects of donor density and temperature on electron systems in AlGaN/AlN/GaN and AlGaN/GaN structures

It was reported by Shen et al that the two-dimensional electron gas (2DEG) in an AlGaN/AlN/GaN structure showed high density and improved mobility compared with an AlGaN/GaN structure, but the potential of the AlGaN/AlN/GaN structure needs further exploration. By the self-consistent solving of one-dimensional Schr\"{o}dinger--Poisson equations, theoretical investigation is carried out about the effects of donor density (0--1\times 10¹⁹cm^-3 and temperature (50--500K) on the electron systems in the AlGaN/AlN/GaN and AlGaN/GaN structures. It is found that in the former structure, since the effective \Delta E_c is larger, the efficiency with which the 2DEG absorbs the electrons originating from donor ionization is higher, the resistance to parallel conduction is stronger, and the deterioration of 2DEG mobility is slower as the donor density rises. When temperature rises, the three-dimensional properties of the whole electron system become prominent for both of the structures, but the stability of 2DEG is higher in the former structure, which is also ascribed to the larger effective \Delta E_c. The Capacitance--Voltage (C-V) carrier density profiles at different temperatures are measured for two Schottky diodes on the considered heterostructure samples separately, showing obviously different 2DEG densities. And the temperature-dependent tendency of the experimental curves agrees well with our calculations.     

AlGaN表面坑状缺陷及GaN缓冲层位错缺陷对AlGaN/GaN HEMT电流崩塌效应的影响

利用金属有机气相外延(MOVPE)技术生长了具有不同AlGaN表面坑状缺陷和GaN缓冲层位错缺陷密度的AlGaN/GaN 高电子迁移率晶体管(HEMT)样品,并对比研究了两种缺陷对器件栅、漏延迟电流崩塌效应的影响.栅延迟测试表明,AlGaN表面坑状缺陷会引起栅延迟电流崩塌效应和源漏电阻的增加,而且表面坑状缺陷越多,栅延迟电流崩塌程度和源漏电阻的增加越明显.漏延迟测试显示,AlGaN表面坑状缺陷对漏延迟电流崩塌影响不大,而GaN缓冲层位错缺陷主要影响漏延迟电流崩塌.研究结果表明,AlGaN表面坑状缺陷和Ga 关键词： AlGaN/GaN HEMT 电流崩塌 坑状缺陷 位错缺陷     

Normally-off AlGaN/GaN heterojunction field-effect transistors with in-situ AlN gate insulator

AlGaN/GaN heterojunction field-effect transistors (HFETs) with p-GaN cap layer are developed for normally-off operation, in which an in-situ grown AlN layer is utilized as the gate insulator. Compared with the SiN_x gate insulator, the AlN/p-GaN interface presents a more obvious energy band bending and a wider depletion region, which helps to positively shift the threshold voltage. In addition, the relatively large conduction band offset of AlN/p-GaN is beneficial to suppress the gate leakage current and enhance the gate breakdown voltage. Owing to the introduction of AlN layer, normally-off p-GaN capped AlGaN/GaN HFET with a threshold voltage of 4 V and a gate swing of 13 V is realized. Furthermore, the field-effect mobility is approximately 1500 cm²·V^-1·s^-1 in the 2DEG channel, implying a good device performance.     

Improved mobility of AlGaN channel heterojunction material using an AlGaN/GaN composite buffer layer

The quality of an AlGaN channel heterojunction on a sapphire substrate is massively improved by using an AlGaN/GaN composite buffer layer. We demonstrate an Al0.4Ga0.6N/Al0.18Ga0.82N heterojunction with a state-of-the-art mobility of 815 cm2/(V·s) and a sheet resistance of 890Ω/ under room temperature. The crystalline quality and the electrical properties of the AlGaN heterojunction material are analyzed by atomic force microscopy, high-resolution X-ray diffraction, and van der Pauw Hall and capacitance–voltage(C–V) measurements. The results indicate that the improved electrical properties should derive from the reduced surface roughness and low dislocation density.     

Low-resistance Ohmic contact on polarization-doped AlGaN/GaN heterojunction

High electronic density is achieved by polarization doping without an impurity dopant in graded AlGaN films. Low specific contact resistance is studied on the polarization-doped AlGaN/GaN heterojunctions by using the transmission line method(TLM). The sheet density of polarization-doped AlGaN/GaN heterojunction is 6×1014cm-2at room temperature.The linearly graded material structure is demonstrated by X-ray diffraction. The carrier concentration and mobility are characterized by a temperature-dependent Hall measurement. Multiple-layer metal(Ti/Al/Ti/Au) is deposited and annealed at 650°C to realize the Ohmic contacts on the graded AlGaN/GaN heterojunctions.     

Numerical simulation of transconductance of AlGaN/GaN heterojunction field effect transistors at high temperatures

Based on the investigation of the influence of temperatures on parameters, including polarization, electron mobility, thermal conductivity, and conduction band discontinuity at the interface between AlGaN and GaN, the temperature dependence of transconductance for AlGaN/GaN heterojunction field effect transistors (HFETs) has been obtained by using a quasi-two-dimensional approach, and the calculated results are in good agreement with the experimental data. The reduction in transconductance at high temperatures is primarily due to the decrease in electron mobility in the channel. Calculations also demonstrate that the self-heating effect becomes serious as environment temperature increases.     

The effects of GaN capping layer thickness on two-dimensional electron mobility in GaN/AlGaN/GaN heterostructures

In this paper we present a study of the effect of GaN capping layer thickness on the two-dimensional (2D)-electron mobility and the two-dimensional electron gas (2DEG) sheet density which is formed near the AlGaN barrier/buffer GaN layer. This study is undertaken using a fully numerical calculation for GaN/Al_xGa_1−xN/GaN heterostructures with different Al mole fraction in the Al_xGa_1−xN barrier, and for various values of barrier layer thickness. The results of our analysis clearly indicate that increasing the GaN capping layer thickness leads to a decrease in the 2DEG density. Furthermore, it is found that the room-temperature 2D-electron mobility reaches a maximum value of approximately 1.8×10³ cm² /Vs⁻¹ for GaN capping layer thickness grater than 100 Å with an Al_0.32Ga_0.68N barrier layer of 200 Å thick. In contrast, for same structure, the 2DEG density decreases monotonically with GaN capping layer thickness, and eventually saturates at approximately 6×10¹² cm⁻² for capping layer thickness greater than 500 Å. A comparison between our calculated results with published experimental data is shown to be in good agreement for GaN capping layers up to 500 Å thickness.     

高场应力及栅应力下AlGaN/GaN HEMT器件退化研究

采用不同的高场应力和栅应力对AlGaN/GaN HEMT器件进行直流应力测试,实验发现：应力后器件主要参数如饱和漏电流,跨导峰值和阈值电压等均发生了明显退化,而且这些退化还是可以完全恢复的;高场应力下,器件特性的退化随高场应力偏置电压的增加和应力时间的累积而增大;对于不同的栅应力,相对来说,脉冲栅应力和开态栅应力下器件特性的退化比关态栅应力下的退化大.对不同应力前后器件饱和漏电流,跨导峰值和阈值电压的分析表明,AlGaN势垒层陷阱俘获沟道热电子以及栅极电子在栅漏间电场的作用下填充虚栅中的表面态是这些不同应 关键词： AlGaN/GaN HEMT器件 表面态(虚栅) 势垒层陷阱 应力     

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

AlGaN/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 Pauw Hall-effect technique. The investigation reveals that better crystal quality and surface morphology of the sample are obtained on the vicinal substrate. Furthermore, 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.     

AlGaN/GaN槽栅HEMT模拟与实验研究

分析了栅槽深度对AlGaN/GaN HEMT特性的影响,并对不同栅槽深度的器件特性进行了模拟,得到了器件饱和电流、最大跨导和阈值电压随栅槽深度的变化规律.当槽栅深度增大,器件饱和电流逐渐下降,而最大跨导逐渐增大,阈值电压向X轴正方向移动.研制出不同栅槽深度的蓝宝石衬底AlGaN/GaN HEMT,用实验数据验证了得到的不同栅槽深度器件特性变化规律.从刻蚀损伤和刻蚀引入界面态的角度分析了模拟与实验规律产生差别的原因. 关键词： 高电子迁移率晶体管 AlGaN/GaN 槽栅器件     

AlGaN/GaN异质结Ni/Au肖特基表面处理及退火研究

采用O₂等离子体及HF溶液对AlGaN/GaN异质结材料进行表面处理后，Ni/Au肖特基接触特性比未处理有了明显改善，反向泄漏电流减小3个数量级.对制备的肖特基接触进行200—600℃ 5min的N₂气氛退火，发现退火冷却后肖特基反向泄漏电流随退火温度增大进一步减小.N₂气中600℃退火后肖特基二极管C-V特性曲线在不同频率下一致性变好，这表明退火中Ni向材料表面扩散减小了表面陷阱密度；C-V特性曲线随退火温度增大向右移动，从二维电子气耗尽电压绝对值减小反映了肖特基势垒的提高. 关键词： AlGaN/GaN 肖特基接触 表面处理 退火     

Optimization of two-dimensional electron gases and – characteristics for AlGaN/GaN HEMT devices

In this paper, we take account of the spontaneous and piezoelectric polarization effect at the heterointerface in the AlGaN/GaN HEMT device, and one-dimensional Schrödinger–Poisson equations are solved self-consistently using a nonuniform mesh; using our findings, the AlGaN/GaN heterostructure conduction band and the two-dimensional electron gas (2DEG) density are investigated. The dependences of the 2DEG characteristics on the Al fraction, the thickness of each layer, the donor concentration and the gate voltage are investigated through simulation. The output characteristics are simulated using a quasi-2D model; a saturation voltage and threshold voltage are also shown. The influence of the spacer layer width on the 2DEG density is calculated for the first time. An explanation and analyses are given.     

Al2O3绝缘栅AlGaN/GaN MOS-HEMT器件温度特性研究

采用原子层淀积(ALD)实现了10 nm Al2O3为栅介质的高性能AlGaN/GaN金属氧化物半导体高电子迁移率晶体管(MOS-HEMT).通过对MOS-HEMT器件和传统MES-HEMT器件室温特性的对比,验证了新型MOS-HEMT器件饱和电流和泄漏电流的优势.通过分析MOS-HEMT器件在30-180℃之间特性的变化规律,与国内报道的传统MES-HEMT器件随温度退化程度对比,得出了器件饱和电流和跨导的退化主要是由于输运特性退化造成的,证明栅介质减小了引入AlGaN界面的表面态是提高特性的重要原因.     

Influence of Schottky drain contacts on the strained AlGaN barrier layer of AlGaN/AlN/GaN heterostructure field-effect transistors

Rectangular Schottky drain AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) with different gate contact areas and conventional AlGaN/AlN/GaN HFETs as control were both fabricated with same size. It was found there is a significant difference between Schottky drain AlGaN/AlN/GaN HFETs and the control group both in drain series resistance and in two-dimensional electron gas (2DEG) electron mobility in the gate-drain channel. We attribute this to the different influence of Ohmic drain contacts and Schottky drain contacts on the strained AlGaN barrier layer. For conventional AlGaN/AlN/GaN HFETs, annealing drain Ohmic contacts gives rise to a strain variation in the AlGaN barrier layer between the gate contacts and the drain contacts, and results in strong polarization Coulomb field scattering in this region. In Schottky drain AlGaN/AlN/GaN HFETs, the strain in the AlGaN barrier layer is distributed more regularly.     

N极性GaN/AlGaN异质结二维电子气模拟

通过自洽求解薛定谔方程和泊松方程,较系统地研究了GaN沟道层、AlGaN背势垒层、Si掺杂和AlN插入层对N极性GaN/AlGaN异质结中二维电子气(2DEG)的影响,分析表明,GaN沟道层厚度、AlGaN背势垒层厚度及Al组分变大都能一定程度上提高二维电子气面密度,AlGaN背势垒层的厚度和Al组分变大也可提高二维电子气限阈性,且不同的Si掺杂形式对二维电子气的影响也有差异,而AlN插入层在提高器件二维电子气面密度、限阈性等方面表现都较为突出,在模拟中GaN沟道层厚度小于5nm时无法形成二维电子气,超过20nm后二维电子气面密度趋于饱和,而AlGaN背势垒厚度超过40nm后二维电子气也有饱和趋势,对均匀掺杂和delta掺杂而言AlGaN背势垒层Si掺杂浓度超过5×10~(19)cm~(-3)后2DEG面密度开始饱和,而厚度为2nmAlN插入层的引入会使2DEG面密度从无AlN插入层时的0.93×10~(13)cm~(-2)提高到1.17×10~(13)cm~(-2)。     

用逆压电极化模型对AlGaN/GaN 高电子迁移率晶体管电流崩塌现象的研究

通过自洽求解一维Poisson-Schrdinger方程,模拟了AlGaN/GaN高电子迁移率晶体管在工作时等效外电场对AlGaN/GaN异质结沟道处二维电子气(2DEG)浓度的影响.分析了逆压电极化效应的作用,从正-逆压电极化现象出发,提出了逆压电极化模型.计算结果显示：逆压电极化明显影响2DEG性质,当Al组分x=0.3,AlGaN层厚度为20 nm时,不考虑逆压电极化,2DEG浓度为1.53×10¹³cm^-2;当等效外电压分别为10和15V 关键词： AlGaN/GaN高电子迁移率晶体管 Poisson-Schrdinger方程 逆压电极化模型 电流崩塌     

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 Al_0.3Ga_0.7N/GaN heterostructure, the value of the relative permittivity of the AlGaN barrier layer was analysed and calculated by self-consistently solving Schr?dinger'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.