用大面积立式旋转盘MOVPE反应器生长Ⅲ族氮化物材料

为了降低Ⅲ族氮化物材料和器件的成本,必须开发大尺寸的Ⅲ族氮化物MOVPE生长的反应器.本文报道了EMCORE公司的一种带有旋转盘的立式反应器,其中的衬底片载盘直径为325mm.每次沉积过程中,载盘上可以放置21片2英寸直径的衬底片.反应器的内壁通过水冷,其温度可保持在50～60℃之间.衬底片可用电热丝加热,沉积中衬底载盘的旋转速度可达1 000～1 500rpm.本文给出了用这种325mm GaN生长系统生长的未掺杂GaN,InGaN和p-GaN外延层的相关结果.这些结果表明,这种旋转盘的反应器是一种很适合大尺寸载盘的系统,完全满足Ⅲ族氮化物材料体系生长的要求.与在小反应器中生长的材料相比,用这种大反应器生长的材料具有相同的质量.这种325mm的GaN反应器将使制备蓝光和绿光LED所用的Ⅲ族氮化外延材料的生产成本得到显著的降低.     

高Al组分AlGaN多量子阱结构材料发光机制探讨

紫外LED的发光功率和效率还远不能令人们满意,波长短于300 nm的深紫外LED的发光效率普遍较低。厘清高Al组分Al Ga N多量子阱结构的发光机制将有利于探索改善深紫外LED的发光效率的新途径、新方法。为此,本文通过金属有机气相外延技术外延生长了表面平整、界面清晰可辨且陡峭的高Al组分AlGa N多量子阱结构材料,并对其进行变温光致发光谱测试,结合数值计算,深入探讨了Al Ga N量子阱的发光机制。研究表明,量子阱中具有很强的局域化效应,其发光和局域激子的跳跃息息相关,而发光的猝灭则与局域激子的解局域以及位错引起的非辐射复合有关。     

用于Ⅲ族氮化物生长的紧配合喷淋头反应器

在紫外和可见光电子器件、高温电子学器件、冷阴极和太阳防护探测器的应用中,Ⅲ族氮化物是一类重要的材料.近年来,氮化物基的LED的制备成功,具有提供白光照明代替白炽灯和荧光灯的潜在能力.人们对用MOCVD方法生长GaN基材料的兴趣日益高涨,特别是对多片、均匀生长的大尺寸反应器的要求日益迫切.本文概述了紧配合喷淋头反应器的设计思想和其特性.结合Ⅲ族氮化物生长对设备的相关要求,给出了这种设备运行的一些结果.这些结果表明,这种紧配合喷淋头反应器很适合在研究和产品生产中的GaN基材料结构的生长.     

Ⅲ族氮化物异质结构二维电子气研究进展

本文总结了近年来Ⅲ族氮化物半导体异质结构二维电子气的研究进展。从Ⅲ族氮化物材料晶格结构和特有的极化性质出发,重点讨论了AlGaN/GaN异质结构中二维电子气的性质,总结分析了异质结构中Al组分、势垒层厚度、应变弛豫度、掺杂等对二维电子气浓度和迁移率的影响,同时还涉及AlGaN/GaN/AlGaN,AlGaN/AlN/GaN和AlGaN/InGaN/GaN等异质结构二维电子气性质。     

Ⅲ族氮化物异质结构二维电子气研究进展

本文总结了近年来Ⅲ族氮化物半导体异质结构二维电子气的研究进展。从Ⅲ族氮化物材料晶格结构和特有的极化性质出发,重点讨论了AlGaN/GaN异质结构中二维电子气的性质,总结分析了异质结构中Al组分、势垒层厚度、应变弛豫度、掺杂等对二维电子气浓度和迁移率的影响,同时还涉及AlGaN/GaN/AlGaN,AlGaN/AlN/GaN和AlGaN/InGaN/GaN等异质结构二维电子气性质。     

用于紫外探测器DBR结构的高质量AlGaN材料MOCVD生长及其特性研究

利用MOCVD方法在(0001)取向的蓝宝石衬底上实现了不同工艺条件下的高质量AlGaN材料的制备.得到了无裂纹的全组分Al_xGa_1-xN(0<x<1)薄膜.通过XRD，SEM，AFM等测量分析方法系统研究了生长工艺参数对材料的结构质量、组分、厚度和表面形貌的影响.分析了不同生长工艺对AlGaN材料特性的影响.研制的高质量AlGaN材料在紫外探测器的DBR结构应用中得到比较好的特性.     

量子阱层和垒层具有不同Al组分的270/290/330nm AlGaN基深紫外LED光电性能

为了研究AlGaN量子阱层和垒层中Al组分不同对AlGaN基深紫外发光二极管(LED)光电性能的影响,本文利用MOCVD生长、光刻和干法刻蚀工艺制备了AlGaN量子阱层和垒层具有不同Al组分的270/290/330nm深紫外LED,通过实验和数值模拟计算方法发现,量子阱层和垒层中具有低Al组分紫外LED的AlGaN材料具有较低的位错密度、较高的光输出功率和外量子效率。通过电流-电压(I-V)曲线拟合出的较大的理想因子(3.5)和能带结构图表明,AlGaN深紫外LED的电流产生是隧穿机制占据主导作用,这是因为高Al组分AlGaN量子阱中强极化场造成了有源层区域较大的能带弯曲和电势降。     

使用AlN/GaN超晶格势垒层生长高Al组分AlGaN/GaN HEMT结构

在蓝宝石衬底上生长了以AlN/GaN超晶格准AlGaN合金作为势垒的HEMT结构材料,并与传统AlGaN合金势垒样品进行了对比.在高Al组分(≥40%)情况下,超晶格势垒样品的表面形貌明显改进,电学性能特别是2DEG面电子浓度也有所改进.对超晶格势垒生长参数进行了初步优化,使得HEMT结构薄层电阻进一步降低,最后获得了251 Ω/□的薄层电阻. 关键词： AlGaN/GaN 结构 AlN/GaN超晶格 二维电子气 高电子迁移率晶体管     

KBBF族非线性光学晶体的发现及其深紫外谐波输出能力

回顾了在20世纪90年代,面对200 nm壁垒的科学难题,KBBF族非线性光学晶体的发现过程,系统阐述了该晶体的线性和非线性光学性质,详细介绍了其深紫外谐波输出能力的最新研究成果.     

Pressure studies of band structure,defects and impurities in group III nitrides

Abstract

This paper reviews high pressure investigations of the physical properties of group III nitrides. After presenting the most important results of high pressure research in this field, we focus on the problem of donors and acceptors in GaN and AlGaN with the special emphasis on the highly localized electronic states. Oxygen and silicon donors and their resonant localized states are discussed in detail. Finally the anomaly of the pressure coefficient of the energy gap of InGaN, will be considered in the relation to the peculiarities of these mixed crystals band structure.     

Characteristics of high Al content AlGaN grown by pulsed atomic layer epitaxy

Al_0.91Ga_0.09N epilayers have been obtained by pulsed atomic layer epitaxy (PALE) technique on sapphire (0 0 0 1) substrates. Deep ultraviolet (DUV) photoluminescence (PL) spectroscopy and Raman scattering spectrum have been employed to study the optical transitions in Al_0.91Ga_0.09N epilayers. We found the exciton-phonon interaction by fitting the asymmetric PL peak, in which the transverse optical phonon (TO) and the longitudinal optical (LO) phonon are the main contributor. The abnormal S-shaped temperature dependence of the PL band peak is less pronounced or has disappeared. Further analysis shows that there possibly exists a high density of deeper localized state (∼90 meV) in Al_0.91Ga_0.09N. The formation of these localized states provides a favorable condition for efficient light emission.     

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.     

Investigation of structural stability and electronic properties of group III nitrides: a first principles study

The high-pressure structural phase transition, electronic, superconducting and elastic properties of group III nitrides (ScN, YN and LaN) are investigated by first principles calculation with the density functional theory. The calculated lattice parameters are in good agreement with the experimental and other theoretical values. Electronic structure reveals that these materials are semiconductors with an indirect band gap of 1.4, 0.87 and 0.65?eV for ScN, YN and LaN, respectively. The obtained cubic NaCl structure is energetically the most stable structure at ambient pressure. A pressure-induced structural phase transition from NaCl to CsCl structure is predicted. The structural phase transition of ScN, YN and LaN occurs at a pressure of 158, 132 and 26.5?GPa, respectively. On further increase in the pressure, semiconductor-to-metallic transition and superconductivity is observed in these nitrides. The estimated T _c values as a function of pressure for ScN, YN and LaN are 31.79, 15.50 and 12.84?K, respectively.     

Optical and structural investigation of a-plane GaN layers on r-plane sapphire with nucleation layer optimization

Nonpolar a-plane GaN epilayers are grown on several r-plane sapphire substrates by metal organic chemical vapour deposition using different nucleation layers:(A) a GaN nucleation layer deposited at low temperature(LT);(B) an AlN nucleation layer deposited at high temperature;or(C) an LT thin AlN nucleation layer with an AlN layer and an AlN/AlGaN superlattice both subsequently deposited at high temperature.The samples have been characterized by Xray diffraction(XRD),atomic force microscopy and photoluminescence.The GaN layers grown using nucleation layers B and C show narrower XRD rocking curves than that using nucleation layer A,indicating a reduction in crystal defect density.Furthermore,the GaN layer grown using nucleation layer C exhibits a surface morphology with triangular defect pits eliminated completely.The improved optical property,corresponding to the enhanced crystal quality,is also confirmed by temperature-dependent and excitation power-dependent photoluminescence measurements.     

Effect of Al mole fraction on structural and electrical properties of AlxGa1−xN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy

The effect of Al mole fractions on the structural and electrical properties of Al_xGa_1−xN/GaN thin films grown by plasma-assisted molecular beam epitaxy (PA-MBE) on Si (1 1 1) substrates has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and current-voltage (I-V) measurements. X-ray results revealed that the AlGaN/GaN/AlN was epitaxially grown on Si substrate. By applying Vegard's law, the Al mole fractions of Al_xGa_1−xN samples were found to be 0.11, 0.24, 0.30 and 0.43, respectively. The structural and morphology results indicated that there is a relatively larger tensile strain for the sample with the smallest Al mole fraction; while a smaller compressive strain and larger grain size appear with Al mole fraction equal to 0.30. The strain gets relaxed with the highest Al mole fraction sample. Finally, the linear relationship between the barrier height and Al mole fraction was obtained.     

Electronic and structural properties of N-vacancy in AlN nanowires: A first-principles study

The stability and electronic structures of AlN nanowires with and without N-vacancy are investigated using firstprinciples calculations.We find that there is an inverse correlation between formation energy and diameter in ideal AlN nanowires.After calculating the formation energies of N-vacancy at different sites in AlN nanowires with different diameters,we find that the N-vacancy prefers to stay at the surface of the nanowires and it is easier to fabricate them under Al-rich conditions.Through studying the electronic properties of AlN nanowires with N-vacancies,we further find that there are two isolated bands in the deep part of the band gap,one of them is fully occupied and the other is half occupied.The charge density indicates that the half-fully occupied band arises from the Al at the surface,and this atom becomes an active centre.     

Modeling and analysis of fully strained and partially relaxed lattice mismatched AlGaN/GaN HEMT for high temperature applications

The paper presents an accurate charge control model for a fully strained (FS), and partially relaxed (PR) lattice mismatched AlGaN/GaN HEMT, taking into consideration the effect of spontaneous and piezoelectric polarization. The model also accounts for the mobility degradation with increase in temperature, which is one of the major causes in deterioration of the driving current. By using the variation of band gap with temperature, the temperature dependence on threshold voltage, sheet carrier concentration and drain current is studied. Further, the temperature variation shows the applicability of the device in a variable thermal environment. A close agreement of calculated data with simulated/experimental data proves the validity of the model.     

磁性纳米生物材料研究进展及其应用

具有磁导向性、小尺寸效应和活性基团的磁性纳米生物材料在靶向给药、固定化酶、细胞分离和免疫分析以及基因治疗等生物医学领域都有一定的研究.本文综述了磁性纳米生物材料的制备与检测,及其在生物医学中的应用.