反向流动垂直喷淋式MOCVD反应器生长GaN的化学反应数值模拟

本文提出了MOCVD生长GaN的表面循环反应模型,将该反应模型应用于作者新近提出的反向流动垂直喷淋式反应器,进行三维数值模拟.得出反应器内流速、温度和TMGa浓度分布,以及GaN的生长速率分布.将此计算结果与传统的反应器情况进行对比,发现在相同参数情况下,两种反应器的衬底上方温度分布都比较均匀,近衬底处温度梯度较大,高温区域被压制在离衬底较近的区域,流线均比较平滑,在衬底上方没有明显的旋涡;新型反应器内反应气体在近衬底处的浓度均匀性以及GaN在基片表面的沉积均匀性都优于传统反应器,但沉积速率小于后者,大约只有后者的1/2.     

切向喷射式MOCVD反应器的设计与数值模拟

本文提出一种新的切向喷射式MOCVD反应器,反应气体从均匀分布于内壁的切向进口喷管喷入反应器,尾气从位于反应器中心的上方或下方出口排出.通过切向喷射,使气体发生人工可控的螺旋流,在水平方向逐渐旋转与加速,从而补偿反应物浓度从边缘进口到中心出口的沿程损失,以便获得均匀的薄膜沉积.针对新的反应器设计,结合GaN的MOCVD生长进行了三维数值模拟,确定了喷管夹角、喷管数目和反应器高度对生长区的温场、流场和浓度场的影响,优化了参数组合,并与传统的垂直喷射式反应器作了对比.此外,这款新型反应器能够摆脱复杂的托盘旋转系统.     

径向流动MOCVD输运过程的数值模拟和反应器优化

针对三重进口径向流动行星式MOCVD反应器的输运过程进行二维数值模拟研究,探讨有关行星式反应器流道高度和托盘直径能否继续扩大,如何控制基片上方温场和浓度场为最佳分布这样一些本质问题,同时寻找反应器的优化条件.模拟结果发现:(1)通过对反应器形状进行优化,使进口处流道趋向于流线的形状,可以大大地削弱甚至消除由流道扩张引起的涡旋;(2)在影响对流涡旋的几何参数中,反应腔高度起主要作用,而反应腔直径影响较小.对于优化后的反应器,发生对流涡旋的临界高度提高到2～2.5cm,对应的反应器直径增加到40cm;(3)在相同温差、不同衬底温度的条件下,反应器内的流动形态不同.衬底温度高,对流涡旋较弱;衬底温度低,对流涡旋较强.其原因在于气体的粘滞力随温度升高从而抑制了浮升力的作用;(4)衬底上方均匀的流场对应均匀的温场和较高的反应物浓度,热扩散则使TMGa在衬底处的浓度降低.     

径向三重流MOCVD反应器生长GaN的数值模拟

采用计算流体力学方法对生长半导体材料GaN的重要设备三重进口行星式MOCVD (金属有机物化学气相沉积) 反应室中的输运过程进行了二维数值模拟.从浓度场的角度分析反应器内衬底上方NH3和TMGa的浓度影响因素.根据对模拟结果的分析,发现较均匀的流场对应衬底上方的反应物浓度较高,降低反应器内压强,也可获得衬底上方较高的反应物浓度,由于MOCVD反应器内有较大的温差,因此热扩散效应不能忽视.     

多片式热壁 MOCVD 反应器的设计与数值模拟分析

本文提出一种多反应腔并联的水平热壁 MOCVD 反应器,反应器上下(左右)壁面都采用高温,减少了热泳力的排斥作用,提高了衬底上方的 TMG 浓度.由于取消了传统反应器的冷壁,减少了寄生产物的凝结,提高了反应前体的利用率和 GaN 的生长速率.可以多个反应腔并联生长,从而实现反应器的扩容.针对这种热壁式反应器,结合 GaN 的 MOCVD 生长进行了二维数值模拟,计算了不同流速、高度、长度和压力时反应器内流场、温场、浓度场分布以及生长速率,发现存在一个最佳的气体流速、反应器高度和长度条件,在此条件下,反应前体的产生与沉积达到平衡,从而有效抵消反应前体的沿程损耗,实现均匀的 GaN 生长.     

一种多喷淋头式MOCVD反应器的设计与数值模拟

提出了一种多喷淋头式MOCVD反应器.针对新型反应器,对GaN生长的MOCVD过程进行了数值模拟,模拟考虑了热辐射和化学反应,计算了反应器内流场、温场和浓度场,导流(筒)壁面的寄生沉积以及GaN生长速率,并分析了反应室几何因素对生长均匀性的影响.模拟结果显示,衬底表面大部分区域具有均匀的温场和良好的滞止流.通过对浓度场和GaN生长速率的分析,得出MMGa是薄膜生长的主要反应前体.通过对反应器高度H、导流筒与托盘间距h、导流筒半径R等参数的优化,给出了提高薄膜生长速率和均匀性的条件.     

垂直转盘式MOCVD反应器进口温度对GaN生长的影响

针对垂直转盘式MOCVD反应器进口温度对GaN生长的影响进行数值模拟研究,分别考虑预混合进口和分隔进口两种情况.通过对包含主要化学反应路径的气体输运过程的模拟,对比不同进口温度下衬底前沿的反应前体浓度及其对应的生长速率的变化,从而确定进口温度对化学反应路径及生长速率的影响关系.结果表明,两种进口情况下,随着进口温度的升高,生长速率均呈现先增大后减小的趋势.预混合时,进口温度约500 K时生长速率最大;分隔进口时,进口温度约800 K时生长速率最大.这主要是由于,生长速率取决于衬底上方边界层内含Ga粒子的浓度梯度.预混合时,衬底前沿的含Ga粒子主要为MMGa,其浓度随进口温度的变化趋势与生长速率的变化趋势一致.分隔进口时,衬底前沿的含Ga粒子MMGa和DMGaNH2浓度处于同一数量级.随进口温度的升高,前者略有增加,而后者明显增大.当预混合的进口温度超过500 K、分隔进口的进口温度超过800 K时,衬底前沿的MMGa和DMGaNH2的峰值或明显下降、或明显离开衬底,使得含Ga粒子的浓度梯度显著下降,造成生长速率下降.     

硅基片上异质外延SiC的机理研究

本文利用低压高温MOCVD系统,成功地在Si(111)基片上外延出了具有高质量的SiC薄膜,并对其反应机理做了一些初步的研究.大部分观点认为,SiC/Si的异质外延,其最初的状态应该为Si衬底中Si的扩散.但是,本文通过在不同流量比的条件下,SiC薄膜在Si基片以及Al2O3基片上外延的比较,发现在SiC/Si的异质外延过程中起重大作用的并非Si衬底中Si的扩散,而是很大程度上作用于C向Si衬底的扩散.同时,还发现反应速率的快慢受SiH4流量所限制.当SiH4流量增加时,反应速率会明显加快,但是结晶质量会相对变差.     

垂直式MOCVD反应器中热泳力对浓度分布的影响分析

从分子动力学理论出发,推导出垂直式MOCVD反应器中热泳力和热泳速度与温度、温度梯度、压强、粒子直径的关系式,以及热泳速度与扩散速度、动量速度平衡时的关系式.在典型的生长条件下,计算得到在温度T=605K时,热泳速度与扩散速度、动量速度动量平衡,TMGa浓度达到最大.然后在不考虑化学反应和考虑化学反应两种情况下,针对垂直式MOCVD反应器内的热泳力对粒子浓度分布和沉积的影响进行数值模拟,模拟给出反应粒子在反应器不同进口温度、衬底温度时的温度分布、浓度分布和反应速率.并与文献中的实验值进行对比,模拟结果与实验值有很好的吻合.     

不同衬底表面上大晶粒多晶Si薄膜的制备

利用高频感应加热化学气相沉积(HFCVD)工艺,以H2稀释的SiH4作为反应气体源,分别在n-(111)Si衬底上常规热生长的SiO2层、织构的SiO2层和纳米晶粒多晶Si薄膜表面上,制备了具有均匀分布的大晶粒多晶Si膜.采用扫描电子显微镜(SEM)和X射线衍射(XRD)等检测手段,测量和分析了沉积膜层的表面形貌、晶粒尺寸、密度分布与择优取向等结构特征.结果表明,多晶Si膜中Si晶粒的尺寸大小和密度分布不仅与衬底温度、SiH4浓度与反应气压等工艺参数有关,而且强烈依赖于衬底的表面状态.本实验获得的最好的薄膜中,Si晶粒平均尺寸约为2.3 μm,密度分布约为3.8×107/cm2.对薄膜的沉积机理分析表明,衬底表面上Si原子基团的吸附、迁移、成核与融合等热力学过程支配着大晶粒多晶Si膜的生长.     

氮化物MOCVD反应室流场的仿真与分析

在立式MOCVD反应室中,通过对生长氮化镓(GaN)薄膜材料的仿真,发现衬底表面反应物三甲基镓物质的量的浓度分布与实际生长的GaN薄膜的厚度分布一致,同时,仿真结果表明,薄膜的厚度分布与反应室内涡旋的分布相关.通过分析涡旋产生的原因,对反应条件和反应室的几何条件作了进一步优化,发现在较低的反应室压强、较低的壁面温度和较大的气体入口半径条件下,能使涡旋明显减小,提高薄膜生长的均匀性.     

Design and operating considerations for a water-cooled close-spaced reactant injector in a production scale MOCVD reactor

In a previous paper, we described experimental results of the growth of a wide range of III–V materials using a water-cooled close-spaced injector that permits organometallic and hydride reactants to be injected 1 cm above the hot wafer. In this paper, we describe important design and operating considerations for this new metalorganic chemical vapor deposition (MOCVD) injector, including construction details, deposition efficiency and process window considerations. Our theoretical modeling indicates that the gas flow patterns are very different from those in a typical rotating disk reactor. By making analytical approximations for the velocity and temperature profiles, we have developed a flow stability parameter and find that the thermal stability depends on the square of the injector-to-substrate gap.     

Impact of the gas flow ratio on the physical properties of GaN grown by vertical flow metalorganic chemical vapour deposition

We studied the effect of gas flow ratio of the H₂ carrier gas to the NH₃ precursor on the physical and crystal properties of GaN. GaN was grown by vertical reactor metalorganic chemical vapour deposition (MOCVD) on a low-temperature-deposited GaN buffer layer. A (0 0 0 1) sapphire substrate was used. The impact of the gas flow ratio as it was varied from 0.25 to 1 was investigated and discussed. With increase in flow ratio, the concentrations of magnesium and carbon impurities in GaN increased. The flow ratio of 0.5 is the optimum value to minimise the background electron concentration and to maintain crystal quality. The decrease in the background electron concentration is due to the compensation mechanism of acceptor-like magnesium and carbon impurities.     

Uniformity investigation of MOCVD‐grown LED layers

Doped or undoped gallium nitride compounds (GaN/InGaN), usually grown by metal‐organic chemical vapor deposition (MOCVD) method, are at the heart of blue and green light emitting diodes (LEDs). Growth uniformities, such as the excited wavelength, luminous intensity and film thickness, critically influence their application in LED devices. In this paper, growth of GaN compounds in a MOCVD reactor, capable of a one‐time production of 36 × 2” wafers of nitrides, has been investigated. To examine growth uniformity across the wafer and from wafer to wafer, the reactor is divided into Zone A, Zone B and Zone C according to distance to the center of the graphite susceptor. Comparative analysis of each zone offers a straightforward view of the mean excitation wavelength, luminous intensity, film thickness and their standard deviations. Conformity of the growth uniformity in each zone is further checked comprehensively through averaging across‐wafer and wafer‐to‐wafer variables and their standard deviations. Zone B is found to retain excellent wavelength uniformity, since it is located at the middle of the susceptor with weaker effects of the susceptor edge and of the inlet gas flow. Zone A, at the center of the reactor, has the best mean intensity and thickness uniformities due to a well control of the infrared temperature measurement during the growth. And Zone C is worst in all uniformities and should be the main focus when optimizing the reactor. The above experimental analysis reveals the principles common to the MOCVD technique, and provides a basic for further optimization of the process window to improve the cycles with considerable reduction of the costs.     

Modelling of Cu thin film growth by MOCVD process in a vertical reactor

Transport phenomena in a vertical reactor for metalorganic chemical vapor deposition (MOCVD) of copper thin films have been analyzed by numerical simulation of the process. The equations of the mathematical model were solved numerically using the Galerkin finite element method, Newton-Raphson iterations and the frontal algorithm for the gas flow structure, temperature distribution and concentration distribution of the reacting species. Deposition rates of copper thin films using Cu(hfac)VTMS as a precursor were estimated from numerical solutions. Standard process conditions were selected as: a reactor pressure of 1 Torr, a substrate temperature and inlet gas temperature of 200°C and 70°C, respectively, and an inlet gas flow rate of 50 sccm. Under standard conditions, the deposition rates of copper were in the range of 160–230 Å/min. The effects of the process conditions, reactor geometry and shower head structure on the deposition rate and thickness uniformity were examined. It has been demonstrated that numerical simulation can be used for improving the film thickness uniformity and the utilization of source gas.     

Uniformity analysis of temperature distribution in an industrial MOCVD reactor

Three‐dimensional models, coupling fluid flow and heat transfer, have been adopted to analyze influences of the process parameters on the temperature uniformity in an industrial MOCVD reactor. Important factors, such as the inlet gas flow, the susceptor rotation, the heater power, the distance between the heat shield and the susceptor (d₁), as well as the distance between the heater and the susceptor (d₂), have been investigated carefully. The system heating condition is characterized by temperature uniformity denoted as the standard deviation of temperature, and by thermal efficiency expressed as a combination parameter of the dissipated energy. The results reveal that decrease of the gas flow and the rotation rate, as well as increase of the distance d₁, could monotonically enhance the temperature uniformity. The results also show that decrease of the above three parameters could improve the thermal efficiency. Furthermore, increase of the distance d₂ enhances the temperature uniformity, and reduces the thermal efficiency slightly. The influences of the parameters on the uniformity vary at the different locations of the susceptor as divided into Zone A, Zone B and Zone C. The conclusions help the growth engineer optimize the system design and process conditions of the reactor.