PVT法同质外延制备大尺寸Al极性氮化铝晶体生长工艺及其表征分析研究

本文基于自主设计的氮化铝生长炉,开展了四组不同工艺条件下Al极性面氮化铝籽晶同质外延生长氮化铝单晶的生长特征及其结晶质量表征研究。研究发现:不同工艺条件下生长的晶体的拉曼图谱E₂(high)特征峰峰位表明,晶体内部均存在较小的拉应力;在坩埚顶部在相对较高温度2 210 ℃、坩埚底部与顶部温差42 ℃的低过饱和度生长条件下,晶体表面光滑,呈现阶梯流生长形貌,并具有典型的氮化铝单晶生长习性面,晶体初始扩张角大于40°,高分辨率X射线衍射(HRXRD)测得0002、1012反射摇摆曲线及拉曼光谱检测结果表明,该条件下生长的氮化铝晶体结晶质量优异,并可实现快速扩径。基于该生长条件,通过外延生长后成功获得尺寸ϕ45~47 mm的氮化铝单晶锭,相关表征结果表明生长的氮化铝晶体具有优越的结晶性能。     

静态溶液中KDP籽晶长大的浮力对流模拟研究

采用有限容积法,对KDP籽晶在静态溶液中的生长过程进行了数值模拟,研究了籽晶长大过程中的形状变化,考察了体过饱和度和籽晶特征尺寸对籽晶表面过饱和度及剪切力的影响,重点分析了籽晶长大过程中的尺寸效应.结果表明,当籽晶的特征长度小于临界尺寸时,籽晶的生长速率随着晶体尺寸的增大而加快;而当籽晶的特征长度大于临界尺寸时,籽晶的生长速率随着晶体尺寸的增大而减慢.     

PVT法AlN单晶生长技术研究进展及其面临挑战

氮化铝(AlN)具有超宽禁带宽度(6.2 eV)、高热导率(340 W/(m·℃))、高击穿场强(11.7 MV/cm)、良好的紫外透过率、高化学和热稳定性等优异性能,是氮化镓基(GaN)高温、高频、高功率电子器件以及高Al组分深紫外光电器件的理想衬底材料.物理气相传输(PVT)法是制备大尺寸高质量AlN单晶最有前途的方法.本文介绍了AlN单晶的晶体结构、基本性质及PVT法生长AlN晶体的原理与生长习性.基于AlN单晶PVT生长策略,综述了自发形核工艺、同质外延工艺及异质外延工艺的研究历程,各生长策略的优缺点及其最新进展.最后对PVT法生长AlN单晶的发展趋势及其面临的挑战进行了简要展望.     

气相生长氮化铝单晶的新方法

通过在钨坩埚盖开小孔的方法改变氮化铝结晶衬底上的温度场分布,在开孔处形成局部低温区;由于孔的几何尺寸的限制和氮化铝晶体生长的各向异性,开孔处的氮化铝晶体单晶化;随后,开孔处的单晶起籽晶的作用,逐渐长成较大尺寸、较高质量的氮化铝单晶.目前用该方法已经制备出直径大于2mm的氮化铝单晶体.     

AlN单晶生长行为研究

本文采用物理气相传输法对不同衬底温度和温差下制备的氮化铝(AlN)晶体形貌进行研究,研究结果表明AlN晶体生长受到AlN晶面表面能、Al基元平均动能和AlN晶体表面极性的共同影响.当温差为60℃时,AlN晶体(0001)面生长速率小于(10-10)面,AlN以带状形式生长.将该工艺应用于AlN同质生长中,研究结果表明:温差为60℃时AlN晶体(0001)面呈现畴生长模式,该晶体质量最差;温差为35℃时AlN晶体(0001)面呈现台阶流生长模式,该晶体质量最优;温差为20℃时AlN晶体(0001)面呈现台阶簇生长模式,该晶体容易开裂.通过工艺优化最终获得了直径为40 mm AlN单晶衬底,完全满足器件制备需求.     

氮化镓籽晶的表面损伤处理及氨热生长研究

籽晶的表面损伤会导致后续生长的晶体位错增多.为了降低籽晶表面的损伤,通常采用粗磨-精磨-抛光的多步过程处理的晶片作为籽晶,工艺步骤多、复杂,成本高.本文采用磷酸去除表面损伤层的粗磨GaN与化学机械抛光的GaN分别作为籽晶,对比了两种籽晶氨热生长后晶体表面、生长速率、结晶质量、应力状况.光学显微镜表明两种籽晶生长后晶体的表面具有相似的丘状表面.氨热法生长速率较慢,化学机械抛光籽晶生长速率略高于粗磨籽晶.X射线单晶衍射(XRD) (002)和(102)的摇摆曲线半高宽显示抛光籽晶与粗磨籽晶生长得到GaN结晶质量基本一致.Raman E2(high)频移表明抛光籽晶生长的GaN晶体接近无应力状态,粗磨籽晶生长的晶体存在较小的压应力.     

6H-SiC单晶生长温度场优化及多型控制

本文模拟了升华法生长6H-SiC单晶的不同温度场,并进行了相应的生长实验.结果表明:改变石墨坩埚和感应线圈的相对位置,可以改变温度场形状;下移石墨坩埚;可以增大温度场径向温度梯度.在不同的径向温度梯度下,6H-SiC晶体分别以凹界面、平界面和凸界面生长.晶体生长界面的形状和速率影响晶体多型的产生,在平界面,生长速率小于300μm/h的晶体生长条件下,可获得无多型的高质量6H-SiC单晶.     

高质量N型SiC单晶生长及其器件应用

采用物理气相传输法在(0001)面偏向<11-20>方向4°的籽晶上生长了掺氮低电阻率碳化硅(SiC)单晶.结合碳化硅邻位面生长机制,通过优化温场设计,在近平温场下生长出了晶型稳定、微管密度低、高结晶质量的低电阻率4H-SiC单晶.在加工的“epi-ready”SiC衬底上进行了同质外延,获得了光滑的外延层表面.利用该外延材料研制了600V/10 A SiC肖特基二极管,器件的直流性能与进口衬底结果相当,反向漏电成品率高达67;.另外研制了600 V/50 A SiC肖特基二极管,器件的直流性能也达到了进口衬底水平.     

电光晶体硒酸氢铷(RHSe)的生长动力学

本文采用动态循环观察法测量了RHSe晶体在水溶液中不同过饱和度下主要显露晶面的生长速率.结果表明,RHSe晶体主要显露晶面的生长机制是多二维成核生长机制,在饱和点为38.00℃的溶液中,RHSe晶体的{111}和{101}晶面的临界过饱和度分别为1.01;和1.16;.实验过程中还发现,当过饱和度大于1.76;时,晶体生长速率太快而出现宏观缺陷,因而在生长RHSe单晶时,过饱和度应控制在1.76;以下.     

物理气相传输法生长1英寸AlN单晶及其表征分析

本文采用物理气相传输法(PVT)及同质外延工艺,在自发生长的6 mm×7 mm AlN籽晶片上,通过4次迭代,成功生长出高质量1英寸AlN单晶锭.将生长出的单晶锭经过切片、研磨和抛光工艺加工成1英寸低表面粗糙度的单晶片,并采用拉曼光谱仪、扫描电子显微镜、高分辨率X射线衍射仪、分光光度计对籽晶片与外延晶片进行结晶质量、位错密度以及紫外透光率等性能表征.结果 表明:外延晶片的拉曼E2(high)半高宽为2.86 cm-1,(002)面XRD摇摆曲线半高宽为241 arcsec,说明晶片具有很高的结晶质量;经过同质外延4次迭代后的晶片较初始籽晶片相比质量有所下降,说明生长过程中由于非平衡生长存在缺陷的增殖;外延晶片具有极其优异的紫外透光率,深紫外265～280 nm波段下的吸收系数低至19～21.5 cm-1.     

The impact of pressure and temperature on growth rate and layer uniformity in the sublimation growth of AlN crystals

To effectively design a large furnace for producing large-size AlN crystals, a fully coupled compressible flow solver was developed to study the sublimation and mass transport processes in AlN crystal growth. Compressible effect, buoyancy effects, flow coupling between aluminum gas and nitrogen gas, and Stefan effect are included. Two sets of experimental data were used to validate the present solver. Simulation results showed that the distributions of Al and N₂ partial pressures are opposite along the axial direction due to constant total pressure and Stefan effect, with the Al and nitrogen partial pressures being highest at the source and seed crystals positions, respectively. The distributions of species inside the growth chamber are obviously two-dimensional, which can curve a flat crystal surface. Simulation results also showed that AlN crystal growth rate can be increased by reducing total pressure or by increasing seed temperature or by increasing source-seed temperature difference. High nitrogen pressure causes decrease in growth rate, but it is beneficial for obtaining uniform growth rate in the radial direction. Results of simulation also showed that there is an optimized temperature difference (40 °C) in the present furnace for obtaining good homogeneity of growth rate.     

Numerical study of radial temperature distribution in the AlN sublimation growth system

A large radial temperature gradient in the AlN sublimation growth system would lead to non‐uniform growth rate along the radial direction and introduce thermal stress in the as grown crystal. In this paper, we have numerically studied the radial thermal uniformity in the crucible of a AlN sublimation growth system. The temperature difference on the source top surface is insignificant while the radial temperature gradient on the lid surface is too large to be neglected. The simulation results showed that the crucible material with a large thermal conductivity is beneficial to obtain a uniform temperature distribution on the lid surface. Moreover, it was found that the temperature gradient on the lid surface decreases with increased lid thickness and decreased top window size.     

AlGaN epitaxial lateral overgrowth on Ti-evaporated GaN/sapphire substrate

AlGaN growth using epitaxial lateral overgrowth (ELO) by metalorganic chemical vapor deposition on striped Ti, evaporated GaN on sapphire, has been investigated. AlGaN/AlN films growth on GaN/AlGaN superlattices (SLs) structure on the Ti masks, with various SLs growth temperature (1030, 1060 and 1090 °C) were grown. With increasing the growth temperature, AlGaN surface became flat. The AlGaN film had a cathodoluminescence peak around 345 nm. However, in secondary ion mass spectrometry (SIMS) measurement, Ti signal was detected on the top of AlGaN surface when GaN/AlGaN SLs was grown on Ti striped masks. By inserting the AlN blocking layer on SLs, Ti diffusion was stopped at the AlN layer, and the AlGaN crystalline quality was improved.     

KDP晶体生长的溶液流动和物质输运计算

本文采用有限容积法,对KDP晶体生长过程中溶液的流动和物质输运进行了数值模拟.结果表明:随着入口溶液流动速度的增大,籽晶的上表面因自然对流而引起的抽吸作用减小,表面过饱和度的最小值沿x正向发生右移,其上表面的剪切力先减小后增大.随着入口溶液过饱和度的增大,籽晶上表面剪切力增大.不同尺寸的籽晶表面过饱和度的分布差异较大.籽晶的生长边界层厚度与溶液流动密切相关,入口溶液流动速度越大,厚度越小,但其受入口溶液过饱和度的影响较小.     

Measurement of three‐dimensional concentration gradients around a crystal growing from its aqueous solution using laser schlieren

Schlieren measurements of the gradients of the concentration field around a KDP crystal growing from its aqueous solution are reported. The measurement of the concentration gradient field is important for crystal growth because it controls the rate of solute transport from the bulk of the solution to the crystal surface. In the crystal vicinity, the concentration gradients have a three dimensional distribution. The concentration gradient field has been imaged using monochrome schlieren technique. Four view angles, namely 0, 45, 90 and 135° have been utilized. By interpreting the schlieren images as projection data of solute concentration gradient, the three‐dimensional concentration gradient field around the crystal has been determined using an algebraic reconstruction technique. At low supersaturation levels, the growth process is accompanied by weak fluid movement during which diffusion effects are significant. At higher levels of supersaturation and large crystal size, a well‐defined convective plume around the growing crystal is observed. Reconstruction of concentration gradients around the crystal explains the preferential growth rates of various faces of the crystal. The non‐circular shape of the crystal is seen to affect the symmetry of the distribution of concentration gradients in its vicinity. The effect of crystal morphology on the orientation of convection currents rising from the crystal surface has also been brought out on the basis of the reconstructed concentration gradients distribution in the growth chamber. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The mechanism of biological mineralization

The control of supersaturation and the nucleation and growth of crystals in calcium phosphate systems are important in relation to the physiological deposition of bone and tooth. Other calcium salts such as the carbonate and oxalate hydrates are significant components of pathological mineral deposits. The use of a highly reproducible seeded growth technique has enabled kinetic studies to be made of the crystal growth of these minerals. Under conditions of relatively high supersaturation, secondary nucleation may be induced upon the surface of the seed crystals. In the case of the calcium phosphates, temperature, supersaturation, surface concentration, pH, ionic strength and presence of foreign ions are very important in determining the nature of the phase which grows upon the added seed crystals. Kinetic considerations are of overriding importance in determining the course of the reactions. It is not possible to predict the phase which forms purely on the basis of thermodynamic solubility data. Thus, in solutions appreciably supersaturated with respect to both dicalcium phosphate dihydrate (DCPD) and hydroxyapatite (HAP), the addition of low concentrations of HAP seed results in the exclusive formation of DCPD whereas this phase is absent when the seed concentration is increased. The kinetic results for calcium oxalate and phosphates are discussed in terms of the important problems relating to tooth mineralization and the origin and growth of renal calculi.     

Improvement of crystal quality of AlN grown on sapphire substrate by MOCVD

The crystalline quality of aluminum nitride (AlN) epilayers grown on sapphire substrates by MOCVD was improved by increasing hydrogen flow rate during the high temperature growth process. The AlN epilayer exhibited a root mean square (rms) of roughness was 1.944 nm from the 2×2 µm² size atomic force microscopy (AFM) images. Full widths at half maximum (FWHMs) of (002) and (102) rocking curves of triple‐axis high resolution X‐ray diffraction (HRXRD) measurements were as narrow as 28.8 arc sec and 868 arc sec, respectively. The optical transmittance spectra showed a sharp absorption edge at a wavelength of 200 nm and strong Fabry‐Perot (FP) oscillations. It is proposed that the improvement in crystalline quality is due to the surface in the low‐temperature aluminum nitride (LT‐AlN) buffer layer is promoted to be stable Al‐polarity by the conditions of increasing hydrogen flow rate and ramping up the growth temperature. Addtionally, the parasitic reactions are effectively suppressed by increasing the hydrogen flow rate during the growth process of high temperature. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of SiC crystal sublimation growth by fully coupled compressible multi-phase flow simulation

A fully coupled compressible multi-phase flow solver was developed to effectively design a large furnace for producing large-size SiC crystals. Compressible effect, convection and buoyancy effects, flow coupling between argon gas and species, and the Stefan effect are included. A small and experimental furnace is used to validate the solver. First, the essentiality of 2D flow calculation and the significance of incorporating buoyancy effect and gas convection, the Stefan effect, and flow interaction between argon gas and species were investigated by numerical results. Then the effects of argon gas on deposition rate, growth rate, graphitization on the powder source, and supersaturation and stoichiometry on the seed were analyzed. Finally, the advantages of an extra chamber design were explained, and improvement of growth rate was validated by the present solver.     

Influence of off-cut angle of r-plane sapphire on the crystal quality of nonpolar a-plane AlN by LP-HVPE

The effect of the off-cut angle of an r-plane sapphire substrate has been investigated on the growth of a-plane AlN thick layer by low-pressure hydride vapor phase epitaxy (LP-HVPE). The off-cut angle (θ) was changed from +5.0° (close to c-axis) to −5.0° (close to m-axis). Results show that the crystalline quality and surface morphology are very sensitive to the sign of θ off-angle. The plus θ off-angle is found to be dramatically reduce the full-widths at half-maximum (FWHM) of X-ray rocking curves (XRC), compared with the minus θ off-angle. In-plane FWHM anisotropic feature marked as M- or W-shape dependence on azimuth angle was observed for a-plane AlN. The shape and degree of anisotropy depend on the sign of θ off-angle, while the plus of θ off-angle will leads to the W-shape and the decreased anisotropy. The minimum crystal tilts and twists of the films are observed for the vicinal sapphires with the plus off-angles of +0.2° to +1.0°.