Effect of reactor geometry on GaN in a vertical MOCVD reactor

A simulation of the GaN growth in a vertical MOCVD reactor is presented. The results show that the gas phase transfer process in GaN growth and material quality of GaN are all affected by the reactor height and the rotation speed of the substrate. With the increasing rotation speed of the substrate, the flow field distribution in the MOCVD reactor becomes more uniform and stable, and the gas flow rate on the substrate surface and the GaN growth rate increase. The uniformity of the GaN is improved at the same time. But the growth rate will decrease when the rotation speed becomes too high. Under the same conditions, the flow field becomes more uniform and stable when the reactor height increases, which is helpful to improving the uniformity of the GaN material. And the growth rate decreases first and then increases in the same process; the gas phase reaction is enhanced at the same time.     

Effect of growth parameters on GaN in a vertical MOCVD reactor

A simulation of reactants in the transfer and reaction process during the GaN growth in a vertical MOCVD reactor is presented. The results show that the GaN growth rate and thickness uniformity are all affected by the chamber pressure and the velocity of reactants into the chamber. With the increasing velocity of reactants into the chamber, pre-reaction will be enhanced, GaN growth rate will be increased and thickness uniformity decreased. With the inlet velocity remaining the same and chamber pressure decreasing, the growth rate is improved within a certain scope, but the thickness uniformity may be increased at the same time with the thickness of the central region of the substrate increased.     

Structural and optical characteristics of Al_xGa_(1-x)N/AlN superlattice

AlN/Al0.3Ga0.7N superlattices were grown on (0001) sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The superlattice period varies from 6 to 30. The layer thickness of different period stack was designed. GaN or AlGaN template was employed for growing AlN/AlGaN superlattices. Reflectivity, SEM, AFM and XRD data of the AlxGa1-xN/AlN superlattices are presented. It is found that the templates used have an intensive impact on surface roughness and interfacial properties of following AlN/AlGaN superlattices. The result of atomic force microscopy indicates that AlN/AlGaN superlattices grown on GaN template exhibit quasi-two-dimensional growth mode. The resulting superlattice has a smooth surface morphology and distinct interface. No crack is observed in the area of a 2-inch wafer.     

Role of lateral growth on the structural properties of high temperature GaN layer

The role of lateral growth on the structural properties of high temperature(HT) GaN epitaxial layer has been investigated by means of transmission electron microscopy(TEM) and X-ray diffraction(XRD).Variations of the lateral growth rate of HT GaN in metal-organic chemical vapor deposition(MOCVD) can be obtained by changing the V/Ⅲ ratio.It is found that under higher lateral growth rate,dislocation is easier to bend into subgrains away from c axis,and the position where bend occurs is closer to the buffer la...     

Influence of growth pressure of a GaN buffer layer on the properties of MOCVD GaN

GaN based III-nitrides are one of the most promising wide band gap semiconductors for the fabrication of optoelectronic devices emitting in the green-ultra-violet range and high-temperature, high-speed electronic devices, because these compounds have wide and direct energy band-gap[1]. Although a lattice-matched substrate is difficult to obtain, -Al2O3 has been successfully used as the substrate to grow GaN film by metalorganic chemical vapor deposition (MOCVD). GaN grown on sapphire cont…     

MOCVD生长GaN薄膜的实时干涉曲线分析

金属有机化学气相沉积（MOCVD）生长系统中通常用激光干涉曲线对材料生长进行实时监控.根据薄膜干涉原理,通过建立反射模型,对GaN薄膜的生长干涉曲线振幅的变化与其生长模式及表面粗糙度的关系进行了详细的阐述和分析.结果表明：干涉曲线中的振幅会随薄膜表面粗糙度的增大而衰减,其衰减程度与粗糙的变化快慢有关,根据材料的实时干涉曲线可揭示其在不同生长过程中的生长模式和定量分析样品表面粗糙度.     

Study of buffer and epitaxy technology in two-step growth of aluminium nitride

AlN (aluminium nitride) films were prepared by metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire (α-Al₂O₃) substrates. By means of studying the traces of in-situ optical reflectivity, it was found that the AlN nucleation layer showed a specific (0001) lattice orientation, which differed from the GaN nucleation layer on (0001) sapphire substrates. The AlN buffer suffered the compressive stress at the initial stage of nucleation. And the compressive strain was relaxed gradually along with the thickness of buffer increasing and consequently annealing. Optical transmission spectra revealed that in the process of growing AlN epilayers, higher V/III ratio could improve the crystal quality but reduce the growth rates. In addition, proper doping of silane (SiH₄) could improve the surface morphology of AlN film.     

双AlN插入层法在Si图形衬底上进行AlGaN/GaN HEMT的MOCVD生长

双AlN插入层方法被用来在Si（111）图形衬底上进行AlGaN/GaN高迁移率晶体管（HEMT）的金属有机物化学气相沉积（MOCVD）外延生长。Si图形衬底采用SiO2掩膜和湿法腐蚀（无掩膜）两种方法进行制备。高温生长双AlN插入层用来释放GaN外延层和Si衬底之间由于晶格失配和热失配而产生的张应力。AlGaN/GaN HEMT的生长特性被讨论和分析。在使用优化的双AlN插入层之前,可以在图形[1-100]方向观察到比[11-20]方向更多的由于应力而引起的裂纹。这是由于GaN在（1-100）面比（11-20）更稳定。建议在图形设计中,长边应沿着[11-20]方向进行制备。拉曼测试显示在图形凹角处比凸角处有更大的拉曼频移,证明在图形凹角处有更大的张应力。     

Structural and optical characteristics of Al x Ga1- x N/AlN superlattice

AlN/Al_0.3Ga_0.7N superlattices were grown on (0001) sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The superlattice period varies from 6 to 30. The layer thickness of different period stack was designed. GaN or AlGaN template was employed for growing AlN/AlGaN superlattices. Reflectivity, SEM, AFM and XRD data of the Al _x Ga_1-x N/AlN superlattices are presented. It is found that the templates used have an intensive impact on surface roughness and interfacial properties of following AlN/AlGaN superlattices. The result of atomic force microscopy indicates that AlN/AlGaN superlattices grown on GaN template exhibit quasi-two-dimensional growth mode. The resulting superlattice has a smooth surface morphology and distinct interface. No crack is observed in the area of a 2-inch wafer. Supported by the Special Funds for Major State Basic Research Project (973 Project) (Grant No. 2006CB6049), the Hi-tech Research Project (Grant Nos. 2006AA03A103, 2006AA03A118, and 2006AA03A142), the National Natural Science Foundation of China (Grant No. 60676057), and the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20050284004)     

不同衬底上低温生长GaN形貌的AFM观察

　Aim　To study the relationship between the substrate temperature and the morphology and properties of GaN. Methods Applying the hydride chemical vapor deposition method, GaN films were deposited on different kinds of substrates, including sapphire, Si(111),Si(100),GaAs and GaP(111) both on the P face and the Ga face. The growth was performed at low temperatures of below 700℃. XRD, Hall measurement, cathodoluminescence (CL) and atomic force microscopy (AFM) were used to characterise the film properties. Results It was found that the temperature and the nature of substrate materials influence the layer morphology. Conclusion The analysis shows that no apparent relationship exists between the optical properties and layer morphology.     

Improved GaN grown on Si(111) substrate using ammonia flow modulation on SiN x mask layer by MOCVD

In this paper, 1 μm n-GaN was grown by using varied and fixed ammonia flow (NH₃) on SiN _x mask layer on Si(111) substrate using metal organic chemical vapor deposition (MOCVD). In-situ optical reflectivity traces of GaN growth show that the three- to two-dimensional process has been prolonged by using varied ammonia flow on SiN _x mask layer method compared with that grown by fixing ammonia flow. Structural and optical properties were characterized by high-resolution X-ray diffraction and photoluminescence, and compared with the sample grown by fixing ammonia flow, GaN grown using the varied ammonia flow on SiNx mask layer showed better structure and optical quality. It was assumed that the low NH₃ flow in the initial growth stage considerably increased the GaN island density on the nano-porous SiN _x layer by enhancing vertical growth. Lateral growth was significantly favored by high NH₃ flow in the subsequent step. As a result, the improved crystal and optical quality was achieved utilizing NH₃ flow modulation for GaN buffer growth on Si(111) substrate.

     

Improved GaN grown on Si(111) substrate using ammonia flow modulation on SiNx mask layer by MOCVD

In this paper, 1 μm n-GaN was grown by using varied and fixed ammonia flow (NH₃) on SiN _x mask layer on Si(111) substrate using metal organic chemical vapor deposition (MOCVD). In-situ optical reflectivity traces of GaN growth show that the three- to two-dimensional process has been prolonged by using varied ammonia flow on SiN _x mask layer method compared with that grown by fixing ammonia flow. Structural and optical properties were characterized by high-resolution X-ray diffraction and photoluminescence, and compared with the sample grown by fixing ammonia flow, GaN grown using the varied ammonia flow on SiNx mask layer showed better structure and optical quality. It was assumed that the low NH₃ flow in the initial growth stage considerably increased the GaN island density on the nano-porous SiN _x layer by enhancing vertical growth. Lateral growth was significantly favored by high NH₃ flow in the subsequent step. As a result, the improved crystal and optical quality was achieved utilizing NH₃ flow modulation for GaN buffer growth on Si(111) substrate.     

水泥与偏高岭土地聚物砂浆碱骨料反应对比研究

为减少环境污染,近年来对碱激发胶凝材料的研究逐渐增加,而目前研究大多集中在地质聚合物机理方面,较少涉及其碱骨料反应。本文试图探究高温高碱环境下,碱激发胶凝材料砂浆与传统水泥砂浆碱骨料反应的不同,以推进碱激发胶凝材料的工程应用。通过测量试件不同龄期长度变化,研究掺不同骨料的水泥砂浆和偏高岭土基地聚物砂浆在高温高碱溶液中的变形行为,同时采用XRD、SEM等微观手段分析二者不同龄期产物的组成和微观结构。研究结果表明：地聚物与水泥的碱骨料反应历程存在明显区别,地聚物中不会发生严重的碱骨料反应,工程中能使用碱活性强的骨料;地聚物浆体最终形成(类)沸石结构,其笼式结构能吸附和固溶大量有害碱,能适应海工等强腐蚀性环境。     

介孔分子筛SBA-15-HSO3合成ETBE反应本征动力学研究

用水热法直接合成了含磺酸基的介孔分子筛SBA -15 -HSO₃ 。采用X 射线衍射、N₂ 吸附-脱附分析方法对试样进行了表征。表征结果显示, 制得的SBA -15 -HSO₃ 具有高度有序的介孔二维六角结构, 并且有较大的比表面积、孔容和孔径。以乙醇和叔丁醇为原料, SBA -15-HSO₃ 为催化剂合成乙基叔丁基醚(ETBE), 建立了反应动力学模型。反应过程在钢密封间歇反应釜中进行, 且消除了内外扩散的影响。改变原料浓度和反应温度得到了醚化反应本征动力学实验数据。线性回归得动力学方程r=kC^1.5_A C^-0.5_B , 求得频率因子为1 .3 ×10⁷ h^-1 , 活化能为52 .86 kJ/ mo l。根据机理近似推导出的动力学方程r=k′C^1.5_A C^-0.5_B , 频率因子为1 .2×10⁷ h^-1 , 活化能为52 .56kJ/ mol。因此, 可近似认为表面反应是反应的速率控制步骤。     

CaSO4于换热面上结垢过程的控制机理分析

为了掌握结垢过程的控制机理,通过分析CasO4在换热面上的结垢过程,得到控制结垢过程的阻力关系式．计算结果表明,流速越大,过饱和度越小,结垢过程越易为表面反应所控制．流速越小,过饱和度越大,结垢过程越易为对流传质所控制,且往往发生在液壁温差较大,壁温较高的情况下．垢层生长过程中,在恒壁温条件下,由对流传质与表面反应共同控制的结垢过程转变为只由表面反应控制结垢过程．在恒热流条件下,控制机理没有变化．     

Morphology tuning of mono-disperse silver nanoparticles by reaction temperature adjustment

Mono-disperse silver nanoparticles with tunable morphologies have been fabricated by reducing AgNO3 in the presence of N-dimethylformamide (DMF) and larger molecular weight poly(vinylpyrrolidone)(PVP). By adjusting the reaction temperature, the conversion of the morphology can be easily and effectively controlled. The crystal structures and growth mechanism of mono-disperse silver nanoparticles were studied by using TEM, HR-TEM, FFT, XRD and UV-Vis spectra data. The results show that the morphologies of nanoparticles with spherical shape can be adjusted to a truncated triangle/hexagon along with the change of reaction temperature from 80 to 120 ℃. It is found that the shape transformation from sphere to truncated triangle is caused by the difference in surface energy and the selective adsorption of PVP on silver atom.     

Atomic scale KMC simulation of {100} oriented CVD diamond film growth under low substrate temperature—Part Ⅰ Simulation of CVD diamond film growth under Joe-Badgwell-Hauge model

The growth of {100} oriented CVD (Chemical Vapor Deposition)diamond film under Joe-Badgwell-Hauge (J-B-H) model is simulated at atomic scale by using revised KMC (Kinetic Monte Carlo) method. The results show that: (1) under Joe's model, the growth mechanism from single carbon species is suitable for the growth of {100} oriented CVD diamond film in low temperature; (2) the deposition rate and surface roughness () under Joe's model are influenced intensively by temperature ()and not evident bymass fraction of atom chlorine; (3)the surface roughness increases with the deposition rate, i.e. the film quality becomes worse with elevated temperature, in agreement with Grujicic's prediction; (4) the simulation results cannot make sure the role of single carbon insertion.     

磨球直径对机械合金化合成TiC反应过程的影响

使用机械合金化方法在卧式行星式球磨机上制备了TiC。利用X射线衍射、扫描电子显微镜对粉末的微观结构和形貌的变化进行了分析。讨论了在机械合金化制备TiC过程中磨球直径对于球磨效率、球磨能量的传递以及Ti+C→TiC发生机理的影响,对于TiC的不同形成机理作出了解释。结果表明:在机械合金化制备TiC的过程中,磨球直径对于反应发生的速率和反应机理有着明显的影响,即当磨球直径较小时,Ti+C→TiC反应是以机械诱发扩散反应(MRD)的形式逐步进行的;当磨球直径较大时,反应是以自蔓燃反应(SHS)的形式快速进行的。     

GaN PIN betavoltaic nuclear batteries

GaN PIN betavoltaic nuclear batteries are demonstrated in this work.GaN epitaxial layers were grown on 2-inch sapphire sub-strates by MOCVD,and then the GaN PIN nuclear batteries were fabricated.Current-voltage(I-V)characteristic shows that the small leakage currents are 0.12 nA at 0 V and 1.76 nA at 10 V,respectively.With 147Pm the irradiation source,the maximum open circuit voltage and maximum short circuit current are 1.07 V and 0.554 nA,respectively.The fill factor(FF)of24.7%for the battery was been obtained.The limited performance of the devices is mainly due to the low energy deposition in the microbatteries.Therefore,the GaN nuclear microbatteries are expected to be optimized by growing high quality GaN films,thin dead layer and so on.     

Atomic scale KMC simulation of {100} oriented CVD diamond film growth under low substrate temperature—part Ⅱ Simulation of CVD diamond film growth in C—H system and in Cl—containing systems

The growth of {100}-oriented CVD diamond film under two modifications of J-B-H model at low substrate temperatures was simulated by using a revised KMC method at atomic scale,The results were compared both in Cl-containing systems and in C-H system as follows:(1) Substrate temperature can produce an important effect both on film deposition rate and on surface roughness;(2) Aomic Cl takes an active role for the growth of diamond film at low temperatues;(3){100}-oriented diamond film cannot deposit under single carbon insertion mechanism,which disagrees with the predictions before;(4) The explanation of the exact role of atomic Cl is not provided in the simulation results.