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.     

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.     

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.     

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.     

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.     

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.     

MOCVD生长温度对氧化锌薄膜结构及发光性能的影响

利用甲醇做氧源,采用金属有机物化学气相沉积(MOCVD)工艺在硅(111)衬底上生长了一系列的氧化锌薄膜,生长温度为400～600 ℃.薄膜的表面形貌及晶体质量分别利用场发射扫描电镜及X射线衍射仪进行了测量.研究表明:随着生长温度的降低,在X射线衍射图谱中氧化锌(101)峰取代了(002)峰成为了主峰.这可能是由于温度过低使得甲醇未完全分解,而甲醇分子抑制了氧化锌沿c轴极性过快的生长所致.室温光致发光光谱结果表明在较高生长温度下获得的样品具有良好的光学性质,发光强度随着温度的降低而降低.     

Numerical analysis for the growth of GaN layer in MOCVD reactor

The axi-symmetric vertical reactor is a classical reactor configuration for the growth of compound semiconductors by MOCVD. In the present study, the modified reactor is developed to produce uniform and large-volume epitaxial deposition of gallium nitride (GaN). A comprehensive knowledge of the flow, thermal and concentration fields, as well as gas surface reaction, is necessary to develop a CVD reactor. The full elliptic governing equations for continuity, momentum, energy and chemical species are solved numerically. It is investigated how thermal characteristics, reactor geometry, and the operating parameters affect flow fields, mass fraction of each reactant, and deposition rate uniformity. As results, inlet flow rate, inclination angle of wall and inlet design are proposed for optimum operational conditions.     

Growth and annealing effect of high-quality ZnSe:N/ZnSe by MOCVD

The ZnSe : N epitaxial layers were grown on (1 1 0) ZnSe substrates in a low-pressure metalorganic chemical vapor deposition (MOCVD) system using hydrogen as a carrier gas, and using ammonia as a dopant source. In order to obtain highly doped ZnSe : N epitaxial layers, the optimum growth and doping conditions were determined by studying the photoluminescence (PL) spectra from the ZnSe epitaxial layers grown at different ammonia flux and VI/II flux ratio. Furthermore, in order to enhance the concentration of active nitrogen in ZnSe epitaxial layer, a rapid thermal anneal technique was used for post-heat-treating. The results show that the annealing temperature of over 1023 K is necessary. Beside, a novel treatment method to obtain a smooth substrate surface for growing high quality ZnSe epitaxial layers is also described.     

缓冲层厚度对MOCVD法生长GaN外延薄膜性能的影响

本文研究了低温GaN(LT-GaN)缓冲层表面形貌,其随厚度的变化规律及对随后生长GaN外延膜各项性能的影响.用场发射扫描电镜(SEM)和原子力显微镜(AFM)研究LT-GaN缓冲层表面形貌,发现随着厚度的增加,其表面由疏松、粗糙变得致密、平整,六角GaN小晶粒的数量减少,且取向较为一致.用X光双晶衍射(XRD)、AFM和Hall测量研究1μm厚本征GaN外延薄膜的结晶质量、表面粗糙度、背底载流子浓度和迁移率等性能,发现随着LT-GaN缓冲层厚度的增加:XRD的半高宽FWHMs增大,表面粗糙度先减小后又略有增大,背底载流子浓度则随之减少,而迁移率的变化则不明显.通过分析进一步确认LT-GaN缓冲层的最优生长时间.     

金刚石衬底上GaN HEMT沟道温度建模:各向异性且非均匀热导率的影响

为了改善GaN HEMT的自热效应,集成高热导率的金刚石衬底有助于增强器件有源区的热量耗散。然而,化学气相淀积(CVD)生长的多晶金刚石(PCD)具有柱状晶粒结构,导致了各向异性的材料热导率,且其热导率值与生长厚度有关。为此,通过建模金刚石生长过程中晶粒尺寸的演变过程,计算了金刚石沿面内和截面方向的热导率。基于该PCD热导率模型,利用计入材料非线性热导率的GaN器件热阻解析模型,计算得到了GaN HEMT沟道温度的波动范围,并分析了其与器件结构(栅长、栅宽、栅间距、衬底厚度)和功耗的依赖关系。最后,通过与有限元(FEM)仿真结果对比,分区域提取了GaN HEMT器件中PCD衬底的有效热导率,分别为260~310 W/(m·K)和1 250~1 450 W/(m·K)。本文的计算为预测金刚石衬底上GaN HEMT器件的沟道温度提供了快速、有效的方法。     

Fractal slice model analysis for effective thermal conductivity and temperature distribution of porous crystal layer via layer crystallization

A fractal slice model was established based on the reported model to predict the effective thermal conductivity of a porous crystal layer via layer crystallization. The temperature distribution of the crystal layer was obtained by the fractal slice method. The simulation results agreed with the experimental data better than the other theoretical models. The results were helpful to enhance the thermal transport by mitigating the thermal resistance effectively.     

Experimental measurement of temperature distribution across a loop-like heater

A new method for in situ measuring the temperature distribution of small loop-like heater for crystal growth in space is proposed. The existence of thermocouple and other attachment may cause the non-uniformity of the heater temperature distribution.     

The influence of the growth temperature and interruption time on the crystal quality of InGaAs/GaAs QW structures grown by MBE and MOCVD methods

The impact of two technological parameters, i.e., the growth temperature and the interface growth interruption, on the crystal quality of strained InGaAs/GaAs quantum well (QW) structures was studied. The investigated heterostructures were grown by molecular beam epitaxy (MBE) and metalorganic chemical vapour deposition (MOCVD) under As-rich conditions. Photoluminescence (PL), reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) were adopted for the evaluation of specified interfaces smoothness and the quality of layers. Comparison between both epitaxial techniques allowed us to find, that the growth temperature plays more significant role in the case of structures grown by MBE technique, whereas the quality of MOCVD grown structures is more sensitive to the growth interruption. Optimum values of the investigated parameters of QW crystallization were obtained for both growth techniques.     

Well parameters of two‐dimensional electron gas in Al0.88In0.12N/AlN/GaN/AlN heterostructures grown by MOCVD

Resistivity and Hall effect measurements were carried out as a function of magnetic field (0‐1.5 T) and temperature (30‐300 K) for Al_0.88In_0.12N/AlN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Magnetic field dependent Hall data were analyzed by using the quantitative mobility spectrum analysis (QMSA). A two‐dimensional electron gas (2DEG) channel located at the Al_0.88In_0.12N/GaN interface with an AlN interlayer and a two‐dimensional hole gas (2DHG) channel located at the GaN/AlN interface were determined for Al_0.88In_0.12N/AlN/GaN/AlN heterostructures. The interface parameters, such as quantum well width, the deformation potential constant and correlation length as well as the dominant scattering mechanisms for the Al_0.88In_0.12N/GaN interface with an AlN interlayer were determined from scattering analyses based on the exact 2DEG carrier density and mobility obtained with QMSA. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Precipitation kinetics of sodium bicarbonate in an industrial bubble column crystallizer

Sodium bicarbonate is a substance which is produced in the middle stages of the soda ash production process. In this precipitation process, carbon dioxide gas is continuously injected into the bubble column reactor which contains carbonate and bicarbonate solutions. To elucidate the key parameters affecting precipitation kinetics, an experimental study was conducted to understand nucleation and growth in conditions of industrial reactor. The composition of the solution is followed during the crystallization process by titration. Magma density is also monitored and crystal size distribution (CSD) is obtained by sieving. The method of moments was used to determine nucleation and growth rates of crystals. The nucleation and growth rate correlations for sodium bicarbonate precipitation in industrial scale were correlated by empirical power laws as B = 26.685M_T^0.42Δw^1.31 and G = 1.381×10^–4Δw^1.53. The nucleation and growth rate correlations obtained in this study can be used to simulate the crystallization of sodium bicarbonate plants. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Room,low, and high temperature dehydration and phase transitions of kernite in vacuum and in air

Kernite Na₂B₄O₆(OH)₂·3H₂O dehydration in air at high temperature and in vacuum at room temperature has been studied. It was found that kernite easily dehydrates forming a new phase‐I both on heating and in vacuum. The chemical formula Na₂B₄O₆(OH)₂·1.5H₂O of the new phase‐I has been estimated on the basis of thermogravity analysis. It is triclinic with the unit cell parameters a = 7.047(8), b = 8.76(1), c = 13.08(2) Å, α = 93.40(9), β = 95.32(9), γ = 90.28(9)° changing slightly on pressure reduction. Due to the relatively low temperature (353 K) and reversibility of the kernite ⟷ phase‐I transition an anion of the new phase‐I likely consists of the same chains [B₄O₆(OH)₂]^2– like in kernite structure. The high anisotropy of kernite thermal expansion was explained by approaching of NaO chains due to the initial removing of water molecules from kernite crystal structure. The behaviour of the new phase‐I at low temperatures in vacuum was also investigated. A formation of an additional new phase II has been detected at the temperature of 93 K. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High-temperature X-ray analysis of phase evolution in Sr-doped zinc-silicate glasses

Bone grafts are required in many clinical situations. Autografts are the traditional gold standard for treating conditions requiring bone grafts. However autografts have inherent drawbacks such as donor site morbidity, pain and increased operative time. An alternative for autografts are synthetic grafts. A series of strontium doped zinc silicate glasses were developed which were investigated using high temperature X-ray diffraction (HT-XRD) in order to establish phase transformations, which occur up to the first crystallization temperature, (T_p1), thus identifying the composition-structure relationships which arise during this thermal processing. In analysing BT110 it was observed that all glass material crystallised into 4 phases including strontium zinc silicate, sodium calcium silicate, calcium silicate and strontium silicate, leaving no residual glass phase. BT111 and BT112 were shown to contain a residual glassy phase alongside for BT111, sodium zinc silicate, larnite and silicon oxide and for BT112 strontium silicate, calcium silicate, sodium silicate and silicon oxide. In the case of BT111 the residual glass phase appears to be rich in strontium. The residual glass phase being Sr enriched with respect to the glass-ceramic may offer increased release of Sr²⁺ from the material; important for the regulation of osteoblastic and osteoclastic activity. BT113 crystallized to form strontium silicate, sodium silicate, and strontium zinc silicate. BT114 crystallized to form strontium silicate and sodium silicate. The biocompatibility of phases formed in BT113 and BT114 is as yet unknown. Further knowledge will be generated by later work examining the biocompatibility of these phases identified in this research. However, on the basis of these results, the materials (BT110-BT112) exhibit potential as a bone graft substitutes, whilst BT113-BT114 give rise to phases with unknown biocompatibility and so warrant further investigation.     

Application of Rosin–Rammler model for analysis of CSD in sugar crystallization

Crystal‐size distribution (CSD) is one of the most important parameters in sugar production. The objective is to grow crystals of uniform sizes or narrow CSD. CSD appears to be determined by the growth‐rate history of the crystals and the relative supersaturation (SS) of the solution from which crystals growth takes place. Three methods for preparation of nucleation seeds were described and used for industrial crystallization of raw and white sugars; these are wet milling filtered sugar (ML), agitating saturated solution (AS) and powdered sugars (PD). Rosin–Rammler (RR) and mathematical models were adopted to investigate CSD and the uniformity of the produced crystals. Higher uniformity coefficients were reported for the AS seeded crystals than the other two seeding methods. Furthermore, higher crystal contents were obtained for the AS seeded white sugar batches in comparison.