Design of graphene-like gallium nitride and WS_2/WSe_2 nanocomposites for photocatalyst applications

The geometric,electronic and optical properties of the graphene-like gallium nitride(GaN) monolayer paired with WS_2 or WSe_2 were studied systematically using the first-principles calculations.GaN interacts with WS2 or WSe_2 via van der Waals interaction and all the most stable configurations of these two nanocomposites exhibit direct band gap characteristics.Meanwhile,the type-Ⅱ heterojunctions are formed because the conduction band minimums and valence band maximums are respectively contributed by WS_2(or WSe_2) and GaN.The imaginary parts of the dielectric function and the absorption spectra of the heterostructures were also calculated and the relatively improved optical properties were observed because of the new interband transitions.In addition,the band offsets as well as the intrinsic electric fields resulting from the interlayer charge transfer indicate that the electron-hole pairs recombination can be effectively inhibited,which is conducive for the photocatalysis process.Moreover,the band gaps of the heterostructures can be modulated by applying biaxial strains and even shift away the conduction band edge potential from the H^+/H_2potential in a certain range,which further enhances the photocatalyst performance.The results indicate that GaN/WS2 or GaN/WSe_2 nanocomposites are good candidate materials for photocatalyst or photoelectronic applications.

Layer-Contacted Graphene-Like BN/Ultrathin Bi_(3)O_(4)Br Stacking for Boosting Photocatalytic Molecular Oxygen Activation

Novel graphene-like boron nitride(BN)/Bi_(3)O_(4)Br photocatalysts have been controllably synthesized through a facile solvothermal method for the first time. Layer contact stacking between graphene-like BN and ultrathin Bi_(3)O_(4)Br was achieved with strong interaction. Dehalogenation is designed to harvest more visible light, and the ultrathin structure of Bi_(3)O_(4)Br is designed to accelerate charge transfer from inside to the surface. After graphene-like BN was engineered, photocatalytic performance greatly improved under visible light irradiation. Graphene-like BN can act as a surface electron-withdrawing center and adsorption center, facilitating molecular oxygen activation. O_(2)^(·-)was determined to be the main active species during the degradation process through analyses of electron spin resonance and XPS valence band spectra.

Hydride vapor phase epitaxy for gallium nitride substrate

Due to the remarkable growth rate compared to another growth methods for gallium nitride(GaN)growth,hydride vapor phase epitaxy(HVPE)is now the only method for mass product GaN substrates.In this review,commercial HVPE systems and the GaN crystals grown by them are demonstrated.This article also illustrates some innovative attempts to develop homebuilt HVPE systems.Finally,the prospects for the further development of HVPE for GaN crystal growth in the future are also discussed.

Direct growth of graphene on gallium nitride by using chemical vapor deposition without extra catalyst

Graphene on gallium nitride （GaN） will be quite useful when the graphene is used as transparent electrodes to improve the performance of gallium nitride devices. In this work, we report the direct synthesis of graphene on GaN without an extra catalyst by chemical vapor deposition. Raman spectra indicate that the graphene films are uniform and about 5-6 layers in thickness. Meanwhile, the effects of growth temperatures on the growth of graphene films are systematically studied, of which 950 ℃ is found to be the optimum growth temperature. The sheet resistance of the grown graphene is 41.1 Ω/square, which is close to the lowest sheet resistance of transferred graphene reported. The mechanism of graphene growth on GaN is proposed and discussed in detail. XRD spectra and photoluminescence spectra indicate that the quality of GaN epi-layers will not be affected after the growth of graphene.

In-situ multi-information measurement system for preparing gallium nitride photocathode

We introduce the first domestic in-situ multi-information measurement system for a gallium nitride （GaN） photo- cathode. This system can successfully fulfill heat cleaning and activation for GaN in an ultrahigh vacuum environment and produce a GaN photocathode with a negative electron affinity （NEA） status. Information including the heat clean- ing temperature, vacuum degree, photocurrent, electric current of cesium source, oxygen source, and the most important information about the spectral response, or equivalently, the quantum efficiency （QE） can be obtained during prepa- ration. The preparation of a GaN photocathode with this system indicates that the optimal heating temperature in a vacuum is about 700 ~C. We also develop a method of quickly evaluating the atomically clean surface with the vacuum degree versus wavelength curve to prevent possible secondary contamination when the atomic level cleaning surface is tested with X-ray photoelectron spectroscopy. The photocurrent shows a quick enhancement when the current ratio between the cesium source and oxygen source is 1.025. The spectral response of the GaN photocathode is flat in a wavelength range from 240 nm to 365 nm, and an abrupt decline is observed at 365 nm, which demonstrates that with the in-si$u multi-information measurement system the NEA GaN photocathode can be successfully prepared.

Self-screening of the polarized electric field in wurtzite gallium nitride along[0001]direction

The strong polarization effect of GaN-based materials is widely used in high-performance devices such as white-lightemitting diodes(white LEDs),high electron mobility transistors(HEMTs),and GaN polarization superjunctions.However,the current researches on the polarization mechanism of GaN-based materials are not sufficient.In this paper,we studied the influence of polarization on electric field and energy band characteristics of Ga-face GaN bulk materials by using a combination of theoretical analysis and semiconductor technology computer-aided design(TCAD) simulation.The selfscreening effect in Ga-face bulk GaN under ideal and non-ideal conditions is studied respectively.We believe that the formation of high-density two-dimensional electron gas(2 DEG) in GaN is the accumulation of screening charges.We also clarify the source and accumulation of the screening charges caused by the GaN self-screening effect in this paper and aim to guide the design and optimization of high-performance GaN-based devices.

Analysis and Characterization of Normally-Off Gallium Nitride High Electron Mobility Transistors

High electron mobility transistor(HEMT)based on gallium nitride(GaN)is one of the most promising candidates for the future generation of high frequencies and high-power electronic applications.This research work aims at designing and characterization of enhancement-mode or normally-off GaN HEMT.The impact of variations in gate length,mole concentration,barrier variations and other important design parameters on the performance of normally-off GaN HEMT is thoroughly investigated.An increase in the gate length causes a decrease in the drain current and transconductance,while an increase in drain current and transconductance can be achieved by increasing the concentration of aluminium(Al).For Al mole fractions of 23%,25%,and 27%,within Al gallium nitride(AlGaN)barrier,the GaN HEMT devices provide a maximum drain current of 347,408 and 474 mA/μm and a transconductance of 19,20.2,21.5 mS/μm,respectively.Whereas,for Al mole fraction of 10%and 15%,within AlGaN buffer,these devices are observed to provide a drain current of 329 and 283 mA/μm,respectively.Furthermore,for a gate length of 2.4,3.4,and 4.4μm,the device is observed to exhibit a maximum drain current of 272,235,and 221 mA/μm and the transconductance of 16.2,14,and 12.3 mS/μm,respectively.It is established that a maximum drain current of 997 mA/μm can be achieved with an Al concentration of 23%,and the device exhibits a steady drain current with enhanced transconductance.These observations demonstrate tremendous potential for two-dimensional electron gas(2DEG)for securing of the normally-off mode operation.A suitable setting of gate length and other design parameters is critical in preserving the normally-off mode operation while also enhancing the critical performance parameters at the same time.Due to the normallyon depletion-mode nature of GaN HEMT,it is usually not considered as suitable for high power levels,frequencies,and temperature.In such settings,a negative bias is required to enter the blocking condition;however,in the before-mentioned normally-off devices,the negative bias can be avoided and the channel can be depleted without applying a negative bias.

Modeling,simulations,and optimizations of gallium oxide on gallium-nitride Schottky barrier diodes

With technology computer-aided design(TCAD)simulation software,we design a new structure of gallium oxide on gallium-nitride Schottky barrier diode(SBD).The parameters of gallium oxide are defined as new material parameters in the material library,and the SBD turn-on and breakdown behavior are simulated.The simulation results reveal that this new structure has a larger turn-on current than Ga2O3 SBD and a larger breakdown voltage than Ga N SBD.Also,to solve the lattice mismatch problem in the real epitaxy,we add a Zn O layer as a transition layer.The simulations show that the device still has good properties after adding this layer.

Structural,Morphological and Electrical Properties of In-Doped Zinc Oxide Nanostructure Thin Films Grown on p-Type Gallium Nitride by Simultaneous Radio-Frequency Direct-Current Magnetron Co-Sputtering

Zinc oxide （ZnO） is one of the most promising and frequently used semiconductor materials. In-doped nanos- tructure ZnO thin films are grown on p-type gallium nitride substrates by employing the simultaneous rf and dc magnetron co-sputtering technique. The effect of In-doping on structural, morphological and electrical properties is studied. The different dopant concentrations are accomplished by varying the direct current power of the In target while keeping the fixed radio frequency power of the ZnO target through the co-sputtering deposition technique by using argon as the sputtering gas at ambient temperature. The structural analysis confirms that all the grown thin films preferentially orientate along the c-axis with the wurtzite hexagonal crystal structure without having any kind of In oxide phases. The presenting Zn, 0 and In elements＇ chemical compositions are identified with EDX mapping analysis of the deposited thin films and the calculated M ratio has been found to decrease with the increasing In power. The surface topographies of the grown thin films are examined with the atomic force microscope technique. The obtained results reveal that the grown film roughness increases with the In power. The Hall measurements ascertain that all the grown films have n-type conductivity and also the other electrical parameters such as resistivity,mobility and carrier concentration are analyzed.

Vertical Gallium Nitride Power Devices：Current Status and Prospects

A comprehensive review on the current status and prospects of vertical gallium nitride(GaN) power devices is presented.The paper starts with an introduction of the market potential for GaN power devices, and presents a comparison between lateral and vertical GaN power devices.Then, different high-performance vertical GaN power devices are introduced, including diodes and transistors on free-standing GaN substrates and low-cost Si substrates.The paper is concluded by elucidating the research and commercialization prospects in developing several key components of vertical GaN power devices.This relatively new area has only been explored for 3-5 years, but already seen the demonstration of a series of successful vertical power devices that outperformed lateral GaN power devices and conventional Si power devices.There are tremendous research opportunities regarding materials, physics, devices and system-level integrations of vertical GaN power devices.The vertical GaN power devices show great potential for >600 V level high-current,high-voltage and high-power applications.

Ga-N熔体热力学性质及钠助熔剂法制备GaN单晶的研究进展

作为第三代半导体的关键材料之一,Ⅲ族氮化物在过去几十年中因其应用于光电子和微电子器件而得到了广泛的研究,如发光二极管(LED)、激光二极管(LD)和高电子迁移率晶体管(HEMT)。Na助熔剂法已成为生长高质量GaN晶体的重要技术之一。综述和讨论了Na助熔剂法GaN结晶的发展历程与最新技术,包括Ga-N的结晶热力学性质、熔体结构、助熔剂选择、单点籽晶技术、多点籽晶技术、孔隙与位错控制、形貌演化与生长条件优化等。最后,对比其他体GaN技术,展望了Na助熔剂法的挑战与机遇。

低成本高结晶GaN纳米线柔性薄膜制备及其场发射性能

旨在探究非贵金属Cu替代贵金属Au作为催化剂在柔性碳膜上制备高结晶的GaN纳米线的可行性,并研究其场发射特性及机理。采用非贵金属Cu替代贵金属Au作为催化剂,在柔性碳膜上制备了直径为20~100 nm、长度为3~15μm的高结晶的GaN纳米线,并通过工艺参数对其结构与尺寸进行调控,得到GaN纳米线薄膜的催化生长机制。通过对其场发射特性进行研究,发现其场发射性能与其纳米结构紧密相关,催化剂厚度以及薄膜弯曲状态可显著影响其场发射性能。结果表明,采用Cu作为催化剂所制备的GaN纳米线柔性薄膜的场发射电流具有较好的稳定性。该研究为GaN纳米线的低成本制备方法提供了可借鉴思路,同时也为场发射柔性器件的制作提供了可行的技术手段。

基于GaN的开关线性复合高速随动脉冲负载直流变换器

有源相控阵雷达发射机收发组件工作在高频脉冲负载模式,这对其供电电源的负载动态性能提出了很高要求。该文针对高速负载切换的二次DC-DC电源,提出开关线性复合并联结构的脉冲负载变换器及其相应的控制策略。基于器件级环路建模的方法有效提升了线性稳压电路的环路带宽,使其进行快速功率跟踪,并设计基于GaN的低阻抗交错并联Buck变换器,使其以较快的速度高效地提供主要脉冲功率。针对重复频率50 kHz、峰值功率120 W的高频模拟雷达脉冲负载,该文搭建脉冲电流上升下降时间50 ns以内的脉冲电源原理样机,输出电压跌落小于5%。实验结果表明所提方法能够有效改善脉冲电源的动态性能。

NH_(3)/N_(2)复合热退火技术改善高浓度Mg掺杂GaN材料性能

研究了NH_(3)/N_(2)复合热退火技术对高浓度Mg掺杂GaN材料晶体质量、发光性质及导电性能的影响。实验结果表明,相较于传统N_(2)氛围高温热退火后处理工艺而言,NH_(3)氛围高温热退火后处理工艺可以改善高浓度Mg掺杂GaN材料的晶体质量,同时可以增进Mg受主原子的有效掺杂,使得其光致发光谱中蓝光峰强度增强。采用NH_(3)氛围高温热退火结合N_(2)氛围低温热退火后处理工艺复合技术制备得到的高浓度Mg掺杂GaN材料内部背景电子浓度显著降低。这是由于在NH_(3)氛围高温热退火后处理工艺中,NH_(3)的热分解产物能够有效降低材料内N空位和间隙Ga原子等浅施主型缺陷浓度,最终改善高浓度Mg掺杂GaN材料的导电性能。

太赫兹GaN SBD外延材料的应力调控及低缺陷密度控制技术研究

采用金属有机物化学气相沉积(Metal organic chemical vapor deposition,MOCVD)技术在101.6 mm(4英寸)半绝缘SiC衬底上开展太赫兹用GaN肖特基势垒二极管(Schottky barrier diode,SBD)外延材料应力演进及缺陷密度控制的研究。提出了一种基于AlGaN过渡层的应力调控方案,实现了外延材料的应力调控;采用低温脉冲式掺杂技术生长n+-GaN层,降低了外延材料的缺陷密度,提升了晶体质量。研制的101.6 mm GaN SBD外延材料的弯曲度(Bow)/翘曲度(Warp)为-12/18μm,(002)/(102)面半高宽为148/239 arcsec,方阻9.2Ω/□,方阻片内不均匀性1.1%,并基于自研材料实现了截止频率为1.12 THz的GaN SBD器件的研制。

4H-SiC衬底上生长参数对AlGaN基深紫外多量子阱受激辐射特性的影响

SiC衬底是制备高性能AlGaN基深紫外激光器的良好候选衬底。采用金属有机化学气相沉积方法在4H-SiC衬底上生长了深紫外AlGaN/AlN多量子阱结构(MQWs),系统研讨了MQWs生长参数对深紫外激光结构自发辐射和受激辐射特性的影响规律。综合分析MQWs的表面形貌和发光性能后发现,随着NH_(3)流量增加和生长温度升高,MQWs表面粗糙度降低,内量子效率提升。经优化,MQWs的内量子效率达到74.1%,室温下的激射阈值光功率密度和线宽分别为1.03 MW/cm~2和1.82 nm,发光波长为248.8 nm。这主要归因于高的NH3流量和生长温度抑制了有源区中的碳杂质并入,载流子辐射复合效率和材料增益增加;同时生长速率降低,改善了MQWs结构的表面形貌,降低了界面散射损耗。采用干法刻蚀和湿法腐蚀的复合工艺制备了光滑陡直的谐振腔,激光器的腔面损耗降低,阈值光功率密度和线宽进一步降低至889 kW/cm~2和1.39 nm。

基于氮化镓的反激同步整流DC/DC变换器设计

给出了一种适合低轨商用航天的中小功率DC/DC变换器的设计方法。该方法采用了反激同步整流拓扑和表贴器件,可以覆盖常见的航天用中小功率单路及多路输出DC/DC变换器需求,具备低成本、高性能、高可靠、可批量化生产的优点。为了进一步提高转换效率和功率密度,还应用了氮化镓GaN(gallium nitride)功率开关。并且对不同工作条件下拓扑中主要功率器件的损耗进行了计算和对比分析。最后,以一款宽输入电压范围(23~47 V),输出5 V@30 W的电源变换器为例对所提方法进行了验证。

GZO@TiN纳米流体光学性能探究

近年来,纳米流体在太阳能光热利用方面极具前景。核壳结构纳米颗粒可以耦合多种材料的不同波段效应,调控光谱吸收波段,实现太阳能的全光谱捕获吸收。将在长波具有高吸收的氧化锌掺杂镓(GZO)和在可见吸收好的氮化钛(TiN)形成GZO@TiN核壳结构复合纳米颗粒,可以发挥两种材料的优点。进一步,从调节材料的尺寸和核壳材料出发,探究GZO@TiN复合结构纳米流体的光谱特性变化规律。结果表明,当TiN为核、GZO为壳时,可以显现两种材料的共振效应;当GZO为核、TiN为壳时,长波区域的吸收峰消失。当浓度为10^(-4),光学厚度为10 mm时,TiN(壳)/GZO(核)纳米流体的平均吸收率高达100%。此工作对核壳结构复合纳米颗粒的光谱调控、吸收以及外壳、核心材料的选择有着一定的帮助。

开态应力下氮化镓HEMT器件的退化机理研究

当前,氮化镓(gallium nitride,GaN)高电子迁移率晶体管(high electron mobility transistors,HEMTs)器件已逐渐被广泛应用。然而,退化问题仍然是困扰其高可靠应用的重要因素。特别是开态应力下,器件的退化机理值得深入研究。文章基于实验测试及仿真,重点研究了氮化镓射频HEMT器件在开态应力下的退化现象与机理。研究结果表明,单一的高漏压应力并不会对器件带来明显退化,而高漏压与大的漏极电流结合则会对器件产生明显退化,这一影响重点集中在栅极与漏极之间的有源区。需要注意的是,栅极偏置电压在沟道电子进入栅下区域的过程中也起到了重要作用。开态应力下,栅极偏压形成的垂直电场会使得器件栅下区域损伤更加严重。文章的研究成果可以为氮化镓射频器件在复杂环境下的高可靠性应用提供重要支撑。