GaAs(111)衬底上生长的GaN的极性与生长方法和生长条件的关系

研究了在GaAs(111)衬底上生长的六角相GaN的极性的相关关系.在高Ⅴ/Ⅲ比的条件下用MOVPE和MOMBE方法生长的GaN的极性和GaAs衬底的极性一致;在(111)A-Ga表面上的生长层呈现Ga的极性,而在(111)B-As表面上的生长层呈现N的极性.然而,在低的Ⅴ/Ⅲ比,或采用一个AIN中间层的条件下,用HVPE和MOMBE方法在GaAs(111)B表面上生长的GaN呈现出Ga的极性.目前,其原因尚不清楚,但是这些结果表明采用HVPE生长方法或用一高温AlN阻挡层可以得到高质量的GaN.     

高厚度n型β-Ga2O3薄膜的MOCVD制备

高厚度的Ga2 O3薄膜能够提高器件的击穿电压,这种高厚度Ga2 O3薄膜往往是通过HVPE法制备的.然而HVPE法存在着成本高、设备少等缺点.本文通过金属有机化学气相沉积(MOCVD)工艺,以SiH4为n型掺杂源,在Ga2 O3衬底上生长了高厚度的n型β-Ga2 O3薄膜,并且研究了SiH4流量对β-Ga2 O3性质...     

HVPE反应器的环形分隔进口数对生长均匀性的影响

利用FLUENT软件对3种环形分隔进口(4环、8环、12环)的氢化物气相外延(HVPE)反应器的生长均匀性进行了二维数值模拟研究。分别考虑输运模型和输运-生长模型,模拟过程保持相同的GaCl、NH3及N2气体进口流量。结果显示:在只考虑输运的模型中,反应室流动均匀性随着进口环数的增多而改善。8环进口时,衬底上方温度分布最均匀;4环进口时,衬底上方的GaCl浓度较高,但均匀性最差,Ⅴ/Ⅲ比也较低;8环及12环进口可得到均匀的GaCl浓度分布及较高的Ⅴ/Ⅲ比。在包括输运和GaN生长的模型中,尽管8环进口反应器衬底上方的GaCl浓度低于12环进口反应器,却因其较薄的边界层厚度而导致较高的生长速率,并且生长均匀性较高。因此,8环进口反应室更有利于GaN的HVPE生长。     

侧向外延生长GaN的结构特性

研究了在刻有图形的GaN"衬底”上用HVPE方法侧向外延生长(ELO)GaN的结构特性.在SiO2衬底上侧向外延生长GaN已经实现,并得到了平面的ELO GaN薄膜.采用扫描电子显微镜、透射电微镜和原子力显微镜技术研究了这种ELO GaN材料的结构和表面形貌.原子力显微镜图像表明:在ELO范围中的4μm2面积上不存在明显的阶状形貌.透射电子显微镜的观测表明在ELO范围内位错密度很低.在接合的界面上没观察到有空隙存在.但观测到晶格的弯曲高达3.3°,这被归因为由GaN层下的"籽层”和接合界面处的水平倾料和猝灭所产生的螺旋位错的积聚.     

Effect of Reactor Pressure on Qualities of GaN Layers Grown by Hydride Vapour Phase Epitaxy

The influence of reactor pressure on GaN layers grown by hydride vapour phase epitaxy （HVPE） is investigated. By decreasing the reactor pressure from0. 7 to 0.5 mm, the GaN layer growth mode changes from the island-like one to the step flow. The improvements in structural and optical properties and surface morphology of GaN layers are observed in the step flow growth mode. The results clearly indicate that the reactor pressure, similarly to the growth temperature, is One of the important parameters to influence the qualities of GaN epilayers grown by HVPE, due to the change of growth mode.     

Growth Kinetics of Silicon Carbide Film Prepared by HeatingPolystyrene/Si(111)

在不同的温度下热处理用sol-gel法制的PS/Si(111)叠层凝胶膜制备出了SiC薄膜．用XRD、SEM、XPS、FTIR等分析方法研究了SiC薄膜的结构、组成和表面形貌等．根据FTIR光谱计算了不同温度下得到的SiC薄膜的厚度,并研究了PS/Si(111)热解法生长SiC薄膜的生长动力学．结果表明,随着生长温度的增加,SiC薄膜生长速率变化趋势为：1200～1250 oC生长速率增加缓慢是2D生长机制,1250～1270 oC生长速率快速增加是3D生长机制,1270～1300 oC生长速率为负增长是由于SiC薄膜生长与Si和C原子的挥发共同作用所致．由速率变化求得各段表观生长激活能分别是122.5、522.5、-127.5 J/mol．     

氢化物气相外延生长的GaN膜中的应力分析

对在c面蓝宝石上用氢化物气相外延法(HVPE)生长的六方相纤锌矿结构的GaN膜中的应力进行了分析。高分辨X射线衍射(002)面和(102)面摇摆曲线扫描(半高宽数值分别为317和358角秒)表明生长的GaN膜具有较好的晶体质量。利用高分辨X射线衍射技术准确测量了制备的GaN膜的晶格常数,并计算得到GaN膜中面内双轴应变和面外双轴应变分别为3.37×10-4和-8.52×10-4,等静压应变为-7.61×10-5。拉曼光谱和激光光致发光谱测试表明HVPE-GaN外延膜具有较好的光学特性,利用拉曼光谱的E₂模式特征峰和激光光致发光谱中近带边发射峰的频移定量计算了外延膜中的面内双轴压应力和等静压应力。两种方法得到的面内双轴压应力较为相符。     

一种测量纤锌矿n-GaN位错密度的新方法

采用点缺陷线性分布模型,利用能量弛豫方法得到了基于van der Pauw变温霍尔效应测量来确定纤锌矿n-GaN位错密度的新方法.用高分辨率X射线衍射仪测试了两个分别用MOCVD方法和用HVPE方法生长的n-GaN样品,用Srikant方法拟合得到了位错密度.结果表明两种方法高度一致.进一步的研究表明,新方法和化学腐蚀方法的测试结果基本一致,相关拟合参数与采用Rode迭代法精确求解Boltzmann输运方程的理论结果也基本一致.研究还表明,新方法能有效消除施主杂质带和界面简并层对测试结果的影响,测试剔除界面层影响后的整个外延层的刃、螺位错密度,而不是穿透位错密度.该方法适合霍尔迁移率曲线峰位在200 K左右及以下并且峰位明确的各种生长工艺、各种厚度、各种质量层次的薄膜和体材料,具有对迁移率曲线高度拟合,材料参数精确,计算简便、收敛速度快等优点.     

Gradual variation method for thick GaN heteroepitaxy by hydride vapour phase epitaxy

Two strain-state samples of GaN, labelled the strain-relief sample and the quality-improved sample, were grown by hydride vapour phase epitaxy (HVPE), and then characterized by high-resolution X-ray diffraction, photoluminescence and optical microscopy. Two strain states of GaN in HVPE, like 3D and 2D growth modes in metal-organic chemical vapour deposition (MOCVD), provide an effective way to solve the heteroepitaxial problems of both strain relief and quality improvement. The gradual variation method (GVM), developed based on the two strain states, is characterized by growth parameters' gradual variation alternating between the strain-relief growth conditions and the quality-improved growth conditions. In GVM, the introduction of the strain-relief amplitude, which is defined by the range from the quality-improved growth conditions to the strain-relief growth conditions, makes the strain-relief control concise and effective. The 300-μm thick bright and crack-free GaN film grown on a two-inch sapphire proves the effectiveness of GVM.     

Si1-xGex/Si异质结构中热应力对临界厚度的影响

系统分析了Si_1-xGe_x/Si外延生长中热应力对外延生长的影响．假设Si Ge的线膨胀系数是随Ge的组分线性变化的，由此计算出热应变和应变能密度．根据能量平衡原理，当失配应变能密度加热应变能密度等于位错能密度时，外延层达到临界厚度，在People的基础上得出了改进的临界厚度计算公式．理论计算值与People的实验值更相近． 关键词：     

Effect of nanoporous GaN templates with different pore diameters on the subsequent thick GaN layers by HVPE

A set of GaN films were overgrown by hydride vapor phase epitaxy (HVPE) on nanoporous GaN templates with different pore diameters. These samples have various properties as seen from the measurements of X-ray diffraction (XRD) and photoluminescence (PL). Cross-sectional observations under a scanning electron microscopy (SEM) reveal that the overgrowth mechanism and process are strongly related to the dimension of nanopores, indicating that an optimum diameter exists for the properties of subsequent HVPE–GaN layers. When the diameters of nanopores are less than the optimum value, the pores on top of GaN templates can be left, and the properties of HVPE–GaN films show significant improvement. In contrast, the pores are almost stuffed with HVPE–GaN films, which obviously limit the improvement degree of HVPE–GaN films.     

Progress in bulk GaN growth

Three main technologies for bulk GaN growth,i.e.,hydride vapor phase epitaxy(HVPE),Na-flux method,and ammonothermal method,are discussed.We report our recent work in HVPE growth of GaN substrate,including dislocation reduction,strain control,separation,and doping of GaN film.The growth mechanisms of GaN by Na-flux and ammonothermal methods are compared with those of HVPE.The mechanical behaviors of dislocation in bulk GaN are investigated through nano-indentation and high-space resolution surface photo-voltage spectroscopy.In the last part,the progress in growing some devices on GaN substrate by homo-epitaxy is introduced.     

Characterization of thick HVPE GaN films

The morphology of the top surface for HVPE GaN layers grown on a MOCVD GaN template with a thin LT-AlN interlayer was investigated. This surface is characterized by the formation of numerous hillocks associated with screw dislocations or nanopipes. Their size is large and may reach more than 1 mm. The rocking curves of the 002 and 102 reflections correspond to a relaxed layer. The HREM images of the as-deposited and annealed interlayers show a perfect atomic structure with a very abrupt AlN/HVPE GaN interface. Thus, the deposition of the LT-AlN layer has promoted the growth of an HVPE layer with an excellent crystalline quality.     

GaN substrate and GaN homo-epitaxy for LEDs:Progress and challenges

After a brief review on the progresses in GaN substrates by ammonothermal method and Na-flux method and hydride vapor phase epitaxy(HVPE) technology,our research results of growing GaN thick layer by a gas flow-modulated HVPE,removing the GaN layer through an efficient self-separation process from sapphire substrate,and modifying the uniformity of multiple wafer growth are presented.The effects of surface morphology and defect behaviors on the GaN homo-epitaxial growth on free standing substrate are also discussed,and followed by the advances of LEDs on GaN substrates and prospects of their applications in solid state lighting.     

Carrier recombination under one-photon and two-photon excitation in GaN epilayers

Luminescence properties of 100-mum thick GaN epilayers grown by hydride vapor phase epitaxy (HVPE) over three different substrates: high-pressure grown n-type bulk GaN (HP-n-GaN), high-pressure bulk GaN doped with magnesium (HP-GaN:Mg), and free-standing HVPE lifted-off from sapphire (FS-HVPE-GaN), were compared by means of one-photon and two-photon excitations. The contribution of carrier capture to nonradiative traps was estimated by the analysis of luminescence transients with carrier diffusion taken into account. The estimated values of carrier lifetime of about 3ns and diffusion coefficient of 1cm(2)/s indicate the highest quality of GaN epilayers on FS-HVPE-GaN substrates.     

Surface morphology of GaN nanorods grown by catalyst-free hydride vapor phase epitaxy

GaN nanorods were grown on c-plane sapphire substrates by using catalyst-free hydride vapor phase epitaxy (HVPE). The effects of substrate temperature, Ga boat temperature, and Ga pretreatment on the surface morphology of GaN nanorods were investigated. From the dependence of a radial and axial growth rate on the substrate temperature, the kinetically limited process was found to be a rate determining step in the growth of GaN nanorods in HVPE. In addition, the activation energy of the growth along the both axial and radial directions were estimated. The dependence of a Ga boat temperature and the Ga pretreatment effect revealed that the density of nanorods were dependent on the flux of Ga species on the substrate.     

Simulation of growing GaN in vertical HVPE reactor

The paper reports the setting up of a model of fluid dynamic for GaN HVPE system and the simulation. It is found that when the direction of gravity is opposite to the direction of GaCl flow inlet,there exits a distance at which the uniformity of the deposition is optimal. Here the good uniformity of the deposition is obtained when the distance between the substrate and GaCl inlet is 5 cm. The parameters of gas flow used in growing GaN are also optimized. In addition, the influence of gravity and buoyancy on...     

Electrochemical liftoff of freestanding GaN by a thick highly conductive sacrificial layer grown by HVPE

Separation technology is an indispensable step in the preparation of freestanding GaN substrate. In this paper, a largearea freestanding GaN layer was separated from the substrate by an electrochemical liftoff process on a sandwich structure composed of an Fe-doped GaN substrate, a highly conductive Si-doped sacrificial layer and a top Fe-doped layer grown by hydride vapor phase epitaxy(HVPE). The large difference between the resistivity in the Si-doped layer and Fe-doped layer resulted in a sharp interface between the etched and unetched layer. It was found that the etching rate increased linearly with the applied voltage, while it continuously decreased with the electrochemical etching process as a result of the mass transport limitation. Flaky GaN pieces and nitrogen gas generated from the sacrificial layer by electrochemical etching were recognized as the main factors responsible for the blocking of the etching channel. Hence, a thick Si-doped layer grown by HVPE was used as the sacrificial layer to alleviate this problem. Moreover, high temperature and ultrasonic oscillation were also found to increase the etching rate. Based on the results above, we succeeded in the liftoff of ～ 1.5 inch GaN layer. This work could help reduce the cost of freestanding GaN substrate and identifies a new way for mass production.