Smooth and Vertical Facet Formation for AlGaN-Based Deep-UV Laser Diodes

Using a two-step method of plasma and wet chemical etching, we demonstrate smooth, vertical facets for use in Al _x Ga_1−x N-based deep-ultraviolet laser-diode heterostructures where x = 0 to 0.5. Optimization of plasma-etching conditions included increasing both temperature and radiofrequency (RF) power to achieve a facet angle of 5 deg from vertical. Subsequent etching in AZ400K developer was investigated to reduce the facet surface roughness and improve facet verticality. The resulting combined processes produced improved facet sidewalls with an average angle of 0.7 deg from vertical and less than 2-nm root-mean-square (RMS) roughness, yielding an estimated reflectivity greater than 95% of that of a perfectly smooth and vertical facet.     

Structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm Si (111) by ammonia molecular beam epitaxy

The structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm diameter Si (111) substrate by ammonia molecular beam epitaxy have been reported. High resolution X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy and secondary ion mass spectroscopy have been used to study the influence of AlN thickness and AlGaN growth temperature on the quality of GaN. GaN grown on thicker AlN showed reduced dislocation density and lesser tensile strain. Three-dimensional growth regime was observed for GaN grown at lower AlGaN growth temperature while higher AlGaN growth temperature resulted in two-dimensional growth mode. The dislocation bending and looping at the AlGaN/AlN interface was found to have significant influence on the dislocation density and strain in the GaN layer. The evolution and interaction of threading dislocations play a major role in determining the quality and the strain states of GaN.     

氧气等离子体处理对AlGaN肖特基接触的影响

研究了不同的干法刻蚀以及氧气等离子体处理条件对AlGaN表面特性的影响。在合适的条件下,氧气等离子体处理可以使AlGaN表面发生氧化,并使肖特基接触的反向漏电流降低两个数量级,反向击穿电压也有显著提高。该方法简单易行,可应用于制备高性能的AlGaN／GaN HEMT器件。     

半导体紫外探测器技术进展

介绍了几种新型半导体紫外探测器。列出了这类探测器的各项用途及优点，并对其结构，制作方法以及性能作出了简要说明。N化物半导体紫外探测器具有探测波长范围可调，可靠性好等优点，可以认为是半导体紫外探测器发展的主要方向。     

Transmission-line-matrix (TLM) modeling of self-heating in AlGaN/GaN transistor structures

Self-heating in AlGaN/GaN (GaN—gallium nitride) heterostructures is an important issue for a large use of these devices in high-density power telecommunication applications. The equation of heat associated with this type of problem does not admit an analytical solution. Hence, we propose a numerical solution based on the use of a transmission line matrix (TLM). The method is easy to program and gives insights on temperature distribution throughout the device. It allows a better understanding of heat behavior and management at each layer that forms the structure. Some TLM simulation results have been compared with those obtained experimentally using integrated micro-Raman/infrared (IR) thermography methods, and have been found to agree within the bounds set by the resolution of the meshes used. The TLM has also the advantage upon other numerical methods of being unconditionally stable, one step and can adapt to complex geometries such as devices with several fingers.     

p-GaN-i-GaN/AlGaN multiple-quantum well n-AlGaN back-illuminated ultraviolet detectors

The spectral response of back-surface-illuminated p-GaN-i-GaN/AlGaN multiplequantum well (MQW)-n-AlGaN ultraviolet (UV) photodetector is reported. The structure was grown by molecular-beam epitaxy on a c-plane sapphire substrate. A MQW is introduced into the active region of the device to enhance the quantum efficiency caused by the high absorption coefficient of the two-dimensional (2-D) system. Another advantage of using MQW in the active region is the ability to tune the cutoff wavelength of the photodetector by adjusting the well width, well composition, and barrier height. The zero-bias peak responsivity was found to be 0.095 A/W at 330 nm, which corresponds to 36% quantum efficiency from as-grown p-i-n GaN/AlGaN MQW devices. An anomalous effect, occurring in responsivity as a negative photoresponse in the spectra peaked at 362 nm because of poor ohmic contact to p-type GaN, was also observed. Etching the sample in KOH for 30 sec before fabrication removed the surface contaminants and improved the surface smoothness of the as-grown sample, resulting in significant improvement in the device performance, giving a peak responsivity of 0.12 A/W. The device has a quantum efficiency of 45% at 330 nm without the anomalous negative photocurrent.     

Tuning Electrical and Thermal Transport in AlGaN/GaN Heterostructures via Buffer Layer Engineering

Progress in wide bandgap, III–V material systems based on gallium nitride (GaN) has enabled the realization of high‐power and high‐frequency electronics. Since the highly conductive, 2D electron gas (2DEG) at the aluminum gallium nitride (AlGaN)/GaN interface is based on built‐in polarization fields and is confined to nanoscale thicknesses, its charge carriers exhibit much higher mobilities compared to their doped counterparts. This study shows that such 2DEGs also offer the unique ability to manipulate electrical transport separately from thermal transport, through the examination of fully suspended AlGaN/GaN diaphragms of varied GaN buffer layer thickness. Notably, ≈100 nm thin GaN layers can considerably impede heat flow without electrical transport degradation. These achieve 4× improvement in the thermoelectric figure of merit (zT) over externally doped GaN, with state‐of‐the‐art power factors of 4–7 mW m^‐1 K^‐2. The remarkable tuning behavior and thermoelectric enhancement, elucidated here for the first time in a polarization‐based heterostructure, are achieved because electrons are at the heterostructured interface, while phonons are within the material system. These results highlight the potential for using 2DEGs in III–V materials for on‐chip thermal sensing and energy harvesting.     

氧等离子体表面处理对AlGaN/GaN HEMT欧姆接触的影响

采用电感耦合等离子体(ICP)刻蚀系统,研究了氧等离子体表面处理对AlGaN/GaN HEMT欧姆接触电阻的影响。利用能量色散X射线光谱仪、光致发光谱和原子力显微镜以及电学测试设备对处理前后样品进行表征分析。结果表明,在最佳的氧等离子体处理条件(ICP功率250 W,射频功率60 W,压强0.8 Pa,氧气流量30 cm³/min,时间5 min)下,欧姆接触电阻为0.41Ω·mm,比参照样品接触电阻降低了约69%。分析认为经过氧等离子体处理后,在近表面处产生了一定数量的N空位缺陷,这些N空位表现为浅能级施主掺杂,有利于欧姆接触的形成。通过采用氧等离子体表面处理工艺制备的AlGaN/GaN HEMT,在+2 V的栅极偏压下获得了0.77 A/mm的最大漏极饱和电流。     

Influence of growth conditions of oxide on electrical properties of AlGaN/GaN metal–insulator–semiconductor transistors

AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors(MIS-HEMTs) on a silicon substrate were fabricated with silicon oxide as a gate dielectric by sputtering deposition and electron-beam(EB) evaporation. It was found that the oxide deposition method and conditions have great influences on the electrical properties of HEMTs. The low sputtering temperature or oxygen introduction at higher temperature results in a positive equivalent charge density at the oxide/AlGaN interface(Nequ), which induces a negative shift of threshold voltage and an increase in both sheet electron density(ns) and drain current density(ID). Contrarily, EB deposition makes a negative Nequ, resulting in reduced ns and ID. Besides, the maximum transconductance(gm-max) decreases and the off-state gate current density(I_G-off) increases for oxides at lower sputtering temperature compared with that at higher temperature, possibly due to a more serious sputter-induced damage and much larger Nequ at lower sputtering temperature. At high sputtering temperature, I_G-off decreases by two orders of magnitude compared to that without oxygen, which indicates that oxygen introduction and partial pressure depression of argon decreases the sputter-induced damage significantly. I_G-off for EB-evaporated samples is lower by orders of magnitude than that of sputtered ones, possibly attributed to the lower damage of EB evaporation to the barrier layer surface.