HVPE生长室气流分布模拟及GaCl载气流量对GaN单晶生长的影响

采用计算流体力学软件Fluent对HVPE反应室进行了数值模拟,研究了GaCl载气流量对HVPE反应室气流分布的影响,发现GaCl载气流量是影响GaCl和NH3在衬底上均匀分布的重要因素.采用HVPE方法在不同GaCl载气流量下生长GaN单晶,研究了GaCl载气流量对GaN单晶质量的影响,得到了与模拟一致的结果.     

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.     

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.     

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.     

垂直转盘式MOCVD反应器中GaN化学反应路径的影响研究

对垂直转盘式MOCVD反应器生长GaN的气相化学反应路径进行研究.结合反应动力学模型,分别采用预混合进口但改变反应腔高度,以及采用环形分隔进口,对反应器的温场、流场和浓度场进行CFD数值模拟,由此确定反应器结构参数对化学反应路径的影响.通过观察主要含Ga粒子的浓度分布以及不同反应路径对生长速率的贡献,判断该反应器可能采取何种反应路径.研究发现,RDR反应器的主要反应路径是TMG热解为DMG,DMG为薄膜沉积的主要前体.反应腔高度变化对反应路径影响较小,但生长速率略有增大;当从预混合进口改为环形分隔进口时,生长更倾向于TMG热解路径,同时生长速率增大,但均匀性变差.     

氮化镓生长反应模型与数值模拟研究(Ⅱ)

本文运用提出的MOCVD生长GaN的核心反应模型,耦合化学反应动力学和输运过程,对新型切向喷射式MOCVD反应器,进行了三维数值模拟。得到反应器内部温场、流场、反应产物以及GaN的生长速率的分布。数值模拟得到的生长速率与文献中的实验值较为吻合,验证了模型的可行性。与传统垂直喷射式进行对比,发现优化后的新型反应器对反应源的利用率较高,在一定区域内沉积速率更为均匀。此外,文中还发现新型反应器适应生长预反应强烈的薄膜材料。     

MOCVD水平式反应器中热泳力对沉积过程中反应前体浓度分布的影响分析及数值模拟

在MOCVD反应器中,针对GaN生长中的TMGa分子,推导出热泳力、热泳速度以及扩散速度的计算公式.在低温区,热泳速度大于扩散速度;在高温区则相反.影响热泳力的主要因素为温度梯度和分子直径.水平式反应器内,粒子同时受到热泳速度和扩散速度的影响.在只考虑组分输运以及包括化学反应等两种情况下,通过改变反应器上壁面温度,模拟得到水平式反应器中热泳力对沉积速率以及反应物粒子浓度分布的影响.并与文献中的实验数据对比,验证了模拟结果的正确性.结果显示,由于热泳力的影响,在相同操作条件下高温区H2等小直径粒子的质量分数增大、TMGa和NH3等大分子粒子的质量分数减小.从提高生长速率的角度,需减小上下壁面温度梯度;从沉积均匀性的角度,应使到达下游的反应粒子数增多,故需增大上下壁面温度梯度.     

工艺参数对CVD ZnS沉积速率的影响

本文报道了CVD法制备ZnS晶体的制备工艺,系统地研究了气体流量、工作气压、基板温度等主要工艺参数对沉积速率的影响规律.实验表明,随着H2S(s)/Zn摩尔流量比的增加,沉积速率逐渐增大;基板温度升高,沉积速率加快;工作气压增大,沉积速率变化不大.在本实验研究的条件下认为,采用合适的H2S(s)/Zn摩尔流量比和沉积温度,在较低的工作气压下生长晶体,能保证较稳定的沉积速率,生长出高质量的晶体.     

MPCVD法在基片边缘生长大颗粒金刚石的研究

本研究在自制的5 kW大功率MPCVD装置中,利用边缘效应成功的在基片边缘处以50μm/h的沉积速率沉积出品粒尺寸达500 μm左右的大颗粒金刚石并以70μm/h沉积速率同质外延修复长大了一颗天然的单晶金刚石.在实验中,利用SEM和Raman光谱对基片边缘区域和中央区域所沉积的金刚石颗粒进行了表征.结果表明,边缘处沉积的金刚石颗粒与中央区域沉积的金刚石颗粒相比,具有更大的晶粒尺寸和更好的质量.通过仔细观察实验条件,对边缘效应产生的原因进行了分析,发现由于基片边缘放电,使得基片表面的电场强度和温度分布发生变化,从而导致基片边缘区域的等离子体密度和温度高于中央区域,高等离子体密度和温度的综合作用是使得在基片边缘能以较高的沉积速率沉积出大尺寸金刚石颗粒的主要原因.     

大尺寸HVPE反应器托盘温度的数值模拟研究

对大尺寸氢化物气相外延(HVPE)反应器的流场和温场进行二维数值模拟研究,旨在提高托盘表面温度和温度分布均匀性.基准模拟显示,靠近喷头的加热器对托盘温度的影响大于底部加热器,随着加热器功率增大,温度分布均匀性变差.在基准模拟的基础上,提出在反应器底部设置隔热钼屏的托盘升温方法.优化后的模拟显示,托盘温度升高约48 K,而温度均匀性变化不大.在使用4层钼屏的基础上,通过在石墨托盘内部开圆柱槽,显著提高了托盘温度分布均匀性,并使温度进一步提升约5K.     

Systematic study of epitaxy growth uniformity in a specific MOCVD reactor

Gallium nitride (GaN) is a direct bandgap semiconductor widely used in bright light‐emitting diodes (LEDs). Thin‐film GaN is grown by metal‐organic chemical vapour deposition (MOCVD) technique. Reliability, efficiency and durability of LEDs are influenced critically by the quality of GaN films. In this report, a systematic study has been performed to investigate and optimize the growth process. Fluid flow, heat transfer and chemical reactions are calculated for a specific close‐coupled showerhead (CCS) MOCVD reactor. Influences of reactor dimensions and growth parameters have been examined after introducing the new conceptions of growth uniformity and growth efficiency. It is found that GaN growth rate is mainly affected by the concentration of (CH3)3Ga:NH3 on the susceptor, while growth uniformity is mainly influenced by the recirculating flows above the susceptor caused by natural convection. Effect of gas inlet temperature and the susceptor temperature over the growth rate can be explained by two competing mechanisms. High growth efficiency can be achieved by optimizing the reactor design.     

Hydrogen and nitrogen ambient effects on epitaxial growth of GaN by hydride vapour phase epitaxy

This study presents the influence of the composition of the carrier gas on the growth of GaN by HVPE. Since no hydrogen is introduced in the vapour phase, the deposition is expected to be controlled by Cl desorption in the form of GaCl₃, as has been proposed for GaAs. However, our published model predicts much lower growth rates than those observed. We can account for both the observed parasitic deposition and GaN growth rate if we assume that GaCl₃ is not at its equilibrium pressure in the deposition zone and where nucleation takes place on the walls as well as on the substrate. This yields a high rate of parasitic nucleation even though the nominal supersaturation is vanishing small. Very little growth takes place on the substrate where the equilibrium pressure of GaCl₃ is reached. We describe similar experiments performed with a H₂/N₂ mixture as the carrier gas. In this case, we expect GaN deposition to be controlled by desorption of Cl as HCl, which is known as the H₂ mechanism. It is speculated that the results show the existence of a new growth mechanism.     

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.     

Heat transfer and mass transport in a multiwafer MOVPE reactor: modelling and experimental studies

An improved detailed model for the calculation of the temperature distribution in a multiwafer Planetary Reactor™ has been developed. The temperature field of the reactor has been calculated in dependence of the reactor parameters for (Al,Ga)As growth as well as on the kind and the thickness of the wall and susceptor deposits. The amount of parasitic wall deposits can be minimized by a proper tuning of the reactor temperature distribution. Calculated GaAs growth rate profiles on 3 inch wafers show a strong dependence on the temperature field in the reactor and the amount of parasitic deposits. These predicted relationships have been used to optimize the reactor temperature distribution in order to minimize parasitic wall depositions. By this procedure a growth rate uniformity of < 1% on 3 inch wafers can be reproducibly achieved.     

Hydride vapor phase epitaxy of GaN boules using high growth rates

The boule-like growth of GaN in a vertical AIXTRON HVPE reactor was studied. Extrinsic factors like properties of the starting substrate and fundamental growth parameters especially the vapor gas composition at the surface have crucial impact on the formation of inverse pyramidal defects. The partial pressure of GaCl strongly affects defect formation, in-plane strain, and crystalline quality. Optimized growth conditions resulted in growth rates of 300–500 μm/h. GaN layers with thicknesses of 2.6 and of 5.8 mm were grown at rates above 300 μm/h. The threading dislocation density reduces with an inverse proportionality to the GaN layer thickness. Thus, it is demonstrated that growth rates above 300 μm/h are promising for GaN boule growth.     

Computational modeling of SiC epitaxial growth in a hot wall reactor

A computational model for chemical vapor deposition (CVD) of silicon carbide (SiC) in a hot-wall reactor is developed, where the susceptor is tapered with a rectangular cross-section. The present work focuses on the advection–diffusion-reaction process in the susceptor. The precursors are propane and silane, and the carrier gas is hydrogen with mass fraction higher than 99%. Computed growth rates under different system pressures and precursor concentrations are compared to the experimental data measured on samples grown in the Linköping CVD reactor. The gas composition distribution in the susceptor and the growth rate profile on the susceptor floor are shown and analyzed. Dependence of the growth rate on precursor concentrations is investigated. It is demonstrated that the growth rate of SiC may either be carbon transport limited or silicon controlled, depending on the input carbon-to-silicon ratio.     

Novel reactor for high volume low-cost silicon epitaxy

The chemical vapor deposition of epitaxial layers of silicon is a widely used process in the electronic industry. It is a batch process and the relatively small capacity (i.e., 20–30 wafers) of epitaxial reactors significantly contributes to the expense of the process. We thus embarked on a research project aimed at a significant expansion of the reactor capacity. The first step was to conduct a complete characterization of the presently used reactors by means of flow visualization, temperature measurements and mass spectrometric studies; results of these studies will be briefly presented and discussed. The main conclusion from these studies was that up-scaling of present reactors is not economical. We thus designed and constructed a novel epitaxial reactor, radically different from current types. In this reactor the susceptor structure consists of parallel graphite discs. Wafers are fastened to one or both sides of these discs. The nutrient gaseous mixture is injected into spaces between discs by a specially designed gas distributor, which delivers the same amount of the mixture to all interdisc spacings, thus insuring the wafer-to-wafer thickness uniformity. A combination of the rotation of the susceptor discs with the gas distributor motion insures the on-the-wafer thickness uniformity. The above described parallel packing allows much higher reactor capacities (e.g., 50–100 wafers). It also results in a more economical reactor in terms of consumption of energy and chemicals. We shall illustrate the application of the novel reactor (known as the “RCA Rotary Disc Reactor”) to epitaxial deposition of silicon from SiCl₂H₂.     

Effects of temperature and carrier gas flow amount on the formation of GaN nanorods by the HVPE method

We fabricated one-dimensional GaN nanorods on AlN/Si (1 1 1) substrates at various temperatures, and carrier gas flow amount, using the hydride vapor phase epitaxy (HVPE) method. An AlN buffer layer of 50 nm thickness was deposited by RF sputtering for 25 min. Stalagmite-like GaN nanorods formed at a growth temperature of 650 °C. The diameters and lengths of GaN nanorods increase with growth time, whereas the density of nanorods decreases. And we performed the experiments by changing the carrier gas flow amount at a growth temperature of 650 °C and HCl:NH₃ flow ratio of 1:40. GaN nanorods, with an average diameter of 50 nm, were obtained at a carrier gas flow amount of 1340 sccm. The shape, structures, and optical characteristics of the nanorods were investigated by field-emission scanning electron microscopy, X-ray diffraction, and photoluminescence.