湿法腐蚀研究PVT法生长的SiC单晶中的位错

采用湿法腐蚀对不同电阻率的4H-SiC衬底进行腐蚀。观察发现,对于同一类型缺陷,低电阻率衬底的缺陷腐蚀坑尺寸比高电阻率衬底的小1~2倍,证明氮元素掺杂的低电阻率SiC衬底的腐蚀速率更低。通过显微镜观察腐蚀后的衬底,对六边形的螺位错(TSD)、近圆形的刃位错(TED)和贝壳形的基平面位错(BPD)腐蚀坑进行了分析。对比了TSD与微管腐蚀坑形貌的区别,虽然两种缺陷腐蚀坑都具有六边形形貌,但微管腐蚀坑尺寸比TSD腐蚀坑大1.4倍左右。通过对位错密度的统计,发现目前4H-SiC衬底中主要的位错为TED和BPD,而TSD密度相对较低,仅为420 cm-2。     

MOCVD法氧化锌单晶薄膜生长

介绍了氧化锌材料的一些突出特性以及生长氧化锌的方法。并通过金属有机化学气相沉积 (MOCVD)方法制备了优良的氧化锌薄膜。使用X射线衍射 (XRD)谱和室温光致发光(PL)光谱对所生长氧化锌薄膜的晶体质量和光学特性进行了研究。X射线衍射谱图显示仅在2θ =34.72°处有一个很陡峭的ZnO (0 0 2 )晶面衍射峰 ,说明所制备的氧化锌薄膜c轴取向高度一致。此衍射峰的半高宽为 0 .2 82° ,显示出较好的晶体质量。在室温光致发光谱中 ,薄膜的紫外发光强度与深能级复合发光的强度比超过 10∶1,表明薄膜的光学质量较高     

PVT法碳化硅单晶生长系统的热场模拟

为了分析物理气相传输法碳化硅单晶生长系统中的温度分布,采用Matlab软件对PVT工艺中的热场进行了模拟。以能量方程为基础分析了由传导和辐射这两种传热方式所决定的系统的热量分配;采用有限元素法对所建立的描述连续函数的偏微分方程进行数值化离散;应用迦辽金加权残值法对由近似函数表征的离散方程转化为矩阵方程的形式;设计了平均算法计算出了系统的温度分布。更好地了解了碳化硅晶体生长过程的物理实质,以便更有效地改进生长系统,优化工艺参数。     

物理升华法生长ZnO晶体的研究

Zinc oxide（ZnO） has a wide band gap, high stability and a high thermal operating range that makes it a suitable material as a semiconductor for fabricating light emitting diodes（LEDs） and laser diodes, photodiodes, power diodes and other semiconductor devices. Recently, a new crystal growth for producing ZnO crystal boules was developed, which was physical vapor transport（PVT）, at temperatures exceeding 1500 ？C under a certain system pressure. ZnO crystal wafers in sizes up to 50 mm in diameter were produced. The conditions of ZnO crystal growth, growth rate and the quality of ZnO crystal were analyzed. Results from crystal growth and material characterization are presented and discussed. Our research results suggest that the novel crystal growth technique is a viable production technique for producing ZnO crystals and substrates for semiconductor device applications.     

PVT生长掺V SiC单晶的扫描电镜二次电子像衬度

在用SEM观察沿单晶生长方向切割掺V SiC晶片时,发现其二次电子像存在衬度。表现为先生长部分较明亮,后生长部分较暗淡,中间存在明显突变。在用PVT生长掺V SiC单晶时,SiC单晶中同时含有浅施主N和深受主杂质V是补偿半导体。从补偿半导体载流子浓度计算出发,建立了二次电子像衬度与载流子浓度的对应关系,很好解释了这一实验现象。结果表明,SiC单晶生长过程中随着浅施主N的减少,n型载流子的浓度逐步减少;当其浓度与V相当时,载流子浓度突变,可瞬间减少10个量级,此后又缓慢减少。正是这种载流子的突变引发了扫描电镜二次电子像衬度。     

基于PVT法自发形核生长AlN晶体的研究

根据物理气相传输法(PVT) AlN晶体生长特点及工艺要求,自主设计了AlN晶体生长炉及其配套热场.FEMAG软件热场模拟结果表明,自主设计的晶体生长炉及其配套热场可以达到AlN晶体生长所需坩埚内部温度梯度要求.基于设计的PVT生长炉,开展了在2 250℃生长温度、40 h长晶时间条件下的自发形核生长实验.实验研究结果表明,在该工艺条件下,通过自发形核可生长得到典型长度为3～Smm、直径为2 mm的高质量AlN单晶;AlN晶体的c-plane(0001)生长速率最快,易形成尖锥形晶体结构,不利于晶体的扩径;Raman表征图谱中AlN晶体的E2 (high)半峰宽仅为5.65 cm-1,表明AlN晶体质量非常高;SEM、EDS分析得出晶体内部质量较为均匀,c-plane和m-plane腐蚀形貌特征明显.     

CdS薄膜的制备、结构和光致发光性能

CdS薄膜是一种n型半导体材料,用于CdTe多晶薄膜太阳电池的窗口层,其质量直接影响太阳电池的光电转换效率和寿命。用磁控溅射法制备了CdS薄膜,通过对薄膜进行X射线衍射(XRD)、扫描电子显微镜(SEM)、光致发光和紫外-可见光谱等测试,研究了CdS薄膜的制备工艺对薄膜结晶度、表面形貌和禁带宽度的影响关系。研究发现,随着CdS薄膜溅射功率的升高,薄膜的结晶度变好,晶粒增大,薄膜增厚,光致发光峰强度增加,禁带宽度减小;随着溅射气压减小,薄膜厚度增大,光致发光峰强度减小,禁带宽度减小。     

生长条件及退火处理对磷化铟单晶结构完整性的影响

利用X射线双晶衍射(XRD)技术研究了原生及退火处理后的磷化铟单晶的晶格完整性.原生磷化铟单晶中由于存在着大量的位错和高的残留热应力,导致晶格产生很大的畸变,表现为XRD半峰宽的值较高并且分布不均匀,甚至有些原生的磷化铟单晶片出现XRD双峰等.通过降低晶体生长过程的温度梯度,降低位错密度并减小晶体中的残留热应力可以提高晶体的完整性.利用高温退火处理也可有效地降低磷化铟晶体中的残留热应力.对磷化铟晶体生长过程中熔体的配比、掺杂浓度等条件对结构完整性的影响进行了分析.     

Growth of CdS crystals by the physical vapor transport method

Based on the physical vapor transport (PVT) method, the growth of large-size CdS crystals inside a vertical semi-closed tube is studied. Firstly, in order to ensure 1D diffusion-advection transport, multi-thin tubes are used in the growth tube. The XRD spectra of the CdS crystal grown in this configuration indicates that the crystal quality has clearly been improved, where the FWHM is 58.5 arcsec. Secondly, theoretical and experimental growth rates under different total pressures are compared; the results show that the experiential growth rate equation is valid for our semi-tube growth, and it could be used to estimate the growth rate and maximum growth time under different total pressures.     

Growth of CdGeAs2 single crystals by the chemical vapor transport method

We have grown CdGeAs₂ single crystals by chemical vapor transport (CVT), a method not previously applied for this compound. The crystallographic data of this chalcopyrite (cell parametersa ₀ = 5.9456 ± 0.0001Å, c₀ = 11.2131 ± 0.0007Å) and its electrical transport properties are reported. Predominantly n-type crystals are obtained (at RTn = 1 · 10¹⁷cm?3, μⁿ = 2000 cm²(Vs)^?1). Vacuum heat treatment at 500° C yields a type conversion fromn- to p-type. In all p-type samples the minority carrier mobility is calculated to be larger than 10000 cm²(Vs)^?1.     

Effects of Growth Conditions on the Microstructure Characteristics of CdS Thin Films by AP-MOCVD

Growth of cadmium sulfide(CdS)thin films on glass substrates was carried out by atmospheric pressure metal-organic chemical vapor deposition(AP-MOCVD)using Cd(S2CNEt2)2 as the single precursor.Changes in the surface morphology of the deposited CdS thin films were investigated by atomic force microscope(AFM)as the function of substrate temperature(Ts),vaporizing temperature(Tv),and Ar flow rate.With the increase of Tv,CdS thin films evolved from pyramidal structure with fine grains to columnar structure with large grains.X-ray diffraction(XRD)patterns indicated that the CdS films had random orientation at the lower T,and preferred orientation at the higher Tv.In addition,Ts had a great effect on the surface roughness of the CdS films,and a quantum dot-like structural CdS films were obtained in a narrow range of T,with high Ar flow rate.Furthermore,the optical properties of the CdS films were measured using ultraviolet-visible(UV/VIS)spectrometer.     

Fast growth of Cd1−xZnxTe single crystals by physical vapor transport

Good quality single crystals of Cd_1−xZn_xTe (x=0.04, 0.08, and 0.115) were obtained by the very fast vapor growth technique developed in this laboratory. The abundance of lamellar twins seems to be less than that in CdTe grown under the same conditions. Chemical etching results indicate a similar quality of the vapor grown crystals to those obtained from melt growth. The overall compositions of the crystals are uniform and approach those of the initial source material with increasing amount transported.     

Seeded growth of AlN crystals on nonpolar seeds via physical vapor transport

Seeded growth of AlN single crystals was demonstrated in an induction-heated, high-temperature reactor via a physical vapor transport (PVT) process. AlN seeds were prepared from a self-seeded boule containing large single-crystalline grains. Seeded growth was interrupted several times in order to refill the AlN powder source, and a dedicated process scheme was used to ensure epitaxial growth on the seed surface, after prior exposure to air. The growth temperatures were in the range of 2200–2300°C, and the reactor pressure was in the range of 500–900 torr of UHP-grade nitrogen during each growth run. Under these growth conditions, a seed (10 mm diameter) expanded at an angle of 45°, and a larger single crystal up to 18 mm in diameter was obtained. The as-grown surface had three facets, of which facet (1120) was smooth and featureless while the other two, (4150) and (2570), showed serrated morphologies. The double-crystal x-ray rocking curve and glow discharge mass spectroscopy analysis confirmed that the grown crystal was of high crystalline quality with low impurity incorporation.     

Identification of prismatic slip bands in 4H SiC boules grown by physical vapor transport

Transmission electron microscopy (TEM) and KOH etching have been used to study the dislocation structure of 4H SiC wafers grown by physical vapor transport. A new type of threading dislocation arrays was observed. Rows of etch pits corresponding to dislocation arrays were observed in vicinity of micropipes, misoriented grains and polytypic inclusions at the periphery of the boules and extended along the directions. Plan view conventional and high resolution TEM showed that the arrays consisted of dislocations threading along the c-axis with Burgers vectors having edge components of the a/3 type. The Burgers vectors were parallel to the corresponding arrays. The dislocation arrays were interpreted as slip bands formed by dislocation glide in the prismatic slip system of hexagonal SiC during post-growth cooling.     

Growth of undoped and chromium-doped CdS<Subscript>x</Subscript>Se<Subscript>1−x</Subscript> crystals by the physical vapor transport method

Chromium-doped CdSe is one of the host materials being considered for solid-state tunable mid-infrared (IR) lasers. Alloying CdSe with CdS allows the increase of the thermal conductivity of the crystal (for CdS the thermal conductivity is a factor of 4 larger than CdSe), which is a favorable parameter as a laser host. In this study, we have grown CdS_xSe_1−x (x≅0.8) single crystals by the physical vapor transport (PVT) method. Crystals with dimensions of 1.2 cm in diameter and 5 cm in length, free of precipitates and inclusions, have been grown. Chromium ions were diffused into the crystal by a postgrowth-diffusion technique at 900°C. Incorporation of Cr ion gives rise to a broad absorption peak at 1.87 μm.     

Halide-hydrogen vapor transport for growth of ZnO single crystals with controllable electrical parameters

The advantages of HCl+H₂ gas mixture as a chemical vapor transport agent for ZnO single crystals growth in the closed growth chambers are shown in comparison with Cl₂, HCl and H₂ by the thermodynamic analysis. The influence of the growth temperature, density of HCl+H₂ transport agent and undercooling were investigated experimentally on the rate of ZnO mass transport. It was shown that HCl+H₂ gas mixture provides (i) a rather high growth rate (up to 1 mm per day), (ii) a minimization of wall adhesion effect and deformations during a post-growth cooling, (iii) stable and reproduced seeded growth of the void-free single crystals with controllable conductivity and charge carrier concentration varied in the range of 2–22 (Ω cm)⁻¹ and (1–31)·10¹⁷ cm⁻³, respectively. The characterization by the photoluminescence spectra, the transmission spectra and the electrical properties, as well as energy spectra of stable Cl-containing defects are analyzed.