化学气相法生长ZnO单晶的热动特性研究

研究了ZnO-C体系化学气相法生长ZnO单晶的Zn、CO、O2、CO2、ZnO气体热力学平衡过程,计算发现主要气相物种为Zn,CO.分析了Zn和CO在N2,Ar,He气体中的扩散系数,发现Ar气既可抑制Zn物质流传输,又可减缓Zn扩散系数随温度升高而递减的趋势,是较适宜的外加气体.建立了ZnO单晶气相生长一维传输模型,揭示了Zn的动力学输运过程,获取了三个温度梯度下(氩气背景压力分别为0 atm、0.5 atm、1 atm)Zn的物质流密度,以及温度梯度、背景压力对最大生长速率的影响.     

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.     

稻壳灰制备Si3N4/SiC复合粉体的研究

本文主要研究不同的SiO2∶C比例对所制备Si3N4/SiC复合粉体组成的影响.采用生物质能电厂所产生的工业废弃物稻壳灰和炭黑为原料,以稻壳灰中的SiO2为标准与炭黑配成不同比例的粉料,在氮气气氛下经1550℃保温3h热处理.采用X射线衍射(XRD)和扫描电子显微镜(SEM)分析烧后试样的物相组成和显微结构.结果表明:当SiO2∶C为5∶6时得到的产物为Si3N4/SiC复合粉体;当SiO2∶C为5∶2和5∶3时,得到产物的组成为SiC、O'-Sialon和β-Si3N4.     

温度对Al2O3-C材料中β-Sialon晶须形成的影响研究

本文研究了温度对Al3O3-C耐火材料中β-Si3Al3O3N5晶须形成的影响规律.研究发现:在弱还原性气氛下,Al2O3-Si-Al-C稳定存在的非氧化物物相为Al4C3 、AlN、SiC和p-Sialon相.其中,催化作用下,Al2O3-C耐火材料中的金属Al在1000℃时转化为AlN,金属Si在1200℃时转化成SiC,在1400℃时有β-Sialon相生成;而无催化剂存在时,生成的物相仅为SiC和AlN相.催化作用下,AlN形貌呈短柱状;SiC呈晶须状,直径为亚微米级,且晶须有液点存在;β-Sialon相呈纳米晶须状.SiC和β-Sialon晶须的生长都符合气-液-固机理.     

SiC晶体生长中气相组分输运特性

针对物理气相传输(PVT)法生长碳化硅(SiC)晶体,建立了一个二维生长动力学模型研究SiC生长腔内气相组分输运特性,该模型考虑了氩气与气相组分之间的流动耦合,Stefan流和浮力影响.研究表明:在压力较低的情况下,自然对流对气相组分的输运过程影响很小,可以忽略,而当压力增高时,自然对流强度显著增大,不可忽略.其次,随着生长温度升高对流的作用增强,生长腔内输运过程由扩散向对流转变,最终对流主导组分的输运过程.随着压力升高对流作用减弱,扩散为气相组分主要输运方式.     

多孔石墨对碳化硅晶体生长影响的数值模拟研究

SiC是新一代射频器件和功率器件的理想材料，电阻式物理气相传输法由于具有温度均匀性，成为生长大尺寸SiC单晶的有效方法。近年来，多孔石墨等的使用提高了SiC晶体的质量和产量，而关于其机理的研究却相对较少。本文使用数值模拟的方法系统研究了多孔石墨对SiC晶体生长的影响，并进行了晶体生长验证。模拟结果表明：多孔石墨的使用提高了原料区域的温度及温度均匀性，增大了坩埚内轴向温差，对减弱原料表层的重结晶也具有一定作用；在生长腔内，多孔石墨改善了物质流动在整个生长过程中的稳定性，提高了生长区域的C/Si比，有助于减小相变发生概率，同时多孔石墨对晶体界面也起到改善作用。晶体生长结果实际验证了多孔石墨在提高传质均匀性、降低相变发生率和改善晶体外形上的作用。本文结果对于理解多孔石墨的作用机理以及改善SiC晶体生长条件具有实际意义。     

水基流延成型和热压烧结制备碳化硼陶瓷及性能研究

以工业碳化硼粉末为原料、采用Si3N4磨球磨损法引入Si3N4烧结助剂,采用水基流延成型和热压烧结方法制备了碳化硼陶瓷.研究了氧含量、分散剂、pH值等因素对B4C陶瓷浆料分散性能的影响,采用XRD、SEM等对碳化硼陶瓷的物相、显微结构和第二相分布进行了表征,并测试了样品的维氏硬度、断裂韧性、抗弯强度和弹性模量.结果表明:经醇洗后的碳化硼粉末中氧化硼含量降低,有利于B4C陶瓷浆料的分散稳定.采用球磨磨损引入了Si3N4粉,在B4C基体中通过原位反应形成第二相SiC和BN,SiC和BN第二相颗粒在B4C基体中弥散分布均匀.在2100 ℃热压烧结样品的维氏硬度、抗弯强度、断裂韧性和弹性模量分别达到30.2 GPa、596.5 MPa、3.36 MPa·m1/2和362.3 GPa.     

Ar气氛下退火处理对6H-SiC晶片表面结构的影响

本文对物理气相传输法生长的三片2英寸掺氮6H-SiC晶片,分别在不同温度下进行退火处理.采用原子力显微镜(AFM)对SiC晶片表面结构进行表征,研究了不同温度和偏角度对SiC晶片表面结构的影响.发现Ar气氛下高温退火处理可以在晶片表面形成规则的台阶条纹,说明Ar气氛下的高温退火处理对SiC晶片表面有一定的刻蚀作用.     

MEMS用Si台面及SiO2/Si衬底上3C-SiC的LPCVD生长

本文报道用在Si台面及热氧化SiO2衬底上3C-SiC薄膜的LPCVD生长,反应生长使用的气体为SiH4和C2H4,载气为H2,采用光学显微镜、X射线衍射（XRD）、X射线光电子能谱（XPS）、扫描电镜（SEM）、以及室温Hall测试对所生长的3C—SiC材料进行了测试与分析,结果表明在3C-SiC和SiO2之间没有明显的坑洞形成。     

添加多晶硅废料制备SiC/Si3N4复相结合SiC耐火材料

以碳化硅、多晶硅废料、金属硅粉为原料,纸浆废液为结合剂,采用反应烧结工艺制备SiC/Si3N4复相结合SiC耐火材料.运用热力学分析了利用多晶硅废料代替部分工业金属硅粉和碳化硅细粉制备SiC/Si3N4复相结合SiC耐火材料的理论可行性.系统地分析了单质硅氮化机理,提出Si首先与N2中的微量氧反应形成气态SiO,至体系氧分压降至P(02)/pθ＜1 × 10-18.9,Si直接氮化.研究了多晶硅废料对材料物相组成和微观结构的影响.结果表明:利用多晶硅废料制备的SiC/Si3 N4复相结合SiC耐火材料性能优异;多晶硅废料的添加使反应生成的结合相由原先单一的Si3 N4变为Si3 N4和β-Sic,两结合相发挥各自的性能优势;多晶硅废料中的硅粉粒径小,活性大,与工业金属硅粉共存时能发生逐级氮化作用,增加了纤维状Si3N4含量,优化了材料结构.     

Investigation of 3C-SiC growth on Si(111) by vapor–liquid–solid transport using a SiGe liquid phase

The crystal growth of 3C-SiC onto silicon substrate by Vapor–Liquid–Solid (VLS) transport, where a SiGe liquid phase is fed with propane, has been investigated. Three sample configurations were used. In a preliminary approach, the VLS growth of SiC was conducted directly onto Si substrate using a Ge film as liquid catalyst. It led to the growth of a thick continuous SiC polycrystalline layer which was floating over a SiGe alloy located between the silicon substrate and the topping SiC layer. In the second configuration, a thin seeding layer of 3C-SiC grown by chemical vapor deposition (CVD) was used and the VLS growth was localized using a SiO₂ mask. The liquid phase was a CVD deposited SiGe alloy. The growth of a few hundred nanometers thick 3C-SiC epitaxial layer was demonstrated but the process was apparently affected by the presence of the oxide which was dramatically etched at the end. In the last configuration, the silicon substrate was patterned down to 10 μm and a thin seeding layer of 3C-SiC was grown by CVD onto this patterned substrate. The liquid phase was again a CVD deposited SiGe alloy. In this last configuration, the presence of epitaxial SiC was evidenced but it grew as trapezoidal islands instead of an uniform layer.     

真空退火处理对纳米氮化硅电子自旋共振谱的影响

用微米级SiO2、Si和碳黑混合粉末为原料, 以氮气为反应气,采用碳热还原法合成了氮化硅纳米粉体.测量了300～1273K退火温度下纳米氮化硅的电子自旋共振(ESR)谱,研究了测量温度、纳米氮化硅退火温度对ESR谱线型、g因子、线宽的影响.结果证实退火影响纳米氮化硅的磁性.     

Effect of AlN doping on the growth morphology of SiC

AlN doped SiC films were deposited on on‐axis Si‐face 4H‐SiC (0001) substrates by the physical vapor transport (PVT) method. Thick film in the range of 20 μm range was grown and morphology was characterized. Films were grown by physical vapor deposition (PVD) in a vertical geometry in the nitrogen atmosphere. We observed that nucleation occurred in the form of discs and growth occurred in hexagonal geometry. The X‐ray studies showed (001) orientation and full width of half maxima (FWHM) was less than 0.1° indicating good crystallinity. We also observed that film deposited on the carbon crucible had long needles with anisotropic growth very similar to that of pure AlN. Some of the needles grew up to sizes of 200 μm in length and 40 to 50 μm in width. It is clear that annealing of SiC‐AlN powder or high temperature physical vapor deposition produces similar crystal structure for producing AlN‐SiC solid solution. SEM studies indicated that facetted hexagons grew on the top of each other and coarsened and merged to form cm size grains on the substrate. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

大尺寸电阻加热式碳化硅晶体生长热场设计与优化

大尺寸低缺陷碳化硅(SiC)单晶体是功率器件和射频(RF)器件的重要基础材料,物理气相传输(physical vapor transport, PVT)法是目前生长大尺寸SiC单晶体的主要方法。获得大尺寸高品质晶体的核心是通过调节组分、温度、压力实现气相组分在晶体生长界面均匀定向结晶,同时尽可能减小晶体的热应力。本文对电阻加热式8英寸(1英寸=2.54 cm)碳化硅大尺寸晶体生长系统展开热场设计研究。首先建立描述碳化硅原料受热分解热质输运及其多孔结构演变、系统热输运的物理和数学模型,进而使用数值模拟方法研究加热器位置、加热器功率和辐射孔径对温度分布的影响及其规律,并优化热场结构。数值模拟结果显示,通过优化散热孔形状、保温棉的结构等设计参数,电阻加热式大尺寸晶体生长系统在晶锭厚度变化、多孔介质原料消耗的情况下均能达到较低的晶体横向温度梯度和较高的纵向温度梯度。     

Nonstoichiometry and nanocrystallization of silicon-rich silicon carbide deposited by chemical vapor deposition

Silicon carbide (SiC) deposited by chemical vapor deposition (CVD) from methyltrichlorosilane (MTS)/H₂ was often found to be silicon-rich and micro- or nanocrystalline. We address here the question whether the excess silicon would be localized inside the small SiC crystals contained in the deposit or not. We performed semi-empirical total energy calculations and free energy estimations which show that excess Si atoms should not be localized inside a finite SiC crystal. This is essentially due to the difference between the Si---Si and Si---C σ overlaps of sp³ hybrids for a given nearest-neighbor distance. Comparison of a crystallographic equation with experimental results from CVD materials suggests that the SiC nanocrystals be of maximum size allowing all excess Si atoms to be localized at the border of the crystals, provided that there are no Si crystals in the deposited material.     

Experimental investigation of different configurations for the seeded growth of SiC crystals via a VLS mechanism

The growth of SiC crystals or epilayers from the liquid phase has already been reported for many years. Even if the resulting material can be of very high structural quality and the possibility to close micropipes was demonstrated, handling the liquid phase still is a challenge. Moreover, it is highly difficult to stabilize the C dissolution front and then to stabilize the growth front over a long growth time. Based on the Vapour‐Liquid‐Solid (VLS) mechanism, we present a new configuration for the growth of SiC single crystal which should allow first to simplify the liquid handling at high temperature and second to precisely control the crystal growth front. The process consists in a modified top and bottom seeded solution growth method, in which the liquid is held under electromagnetic levitation and fed from the gas phase. 3C‐SiC crystals exhibiting well‐faceted morphology were successfully obtained at 1100‐1200 °C with exceptional growth rates, varying from 1 to 1.5 mm/h in Ti‐Si melt. It was shown that the nucleation density decreases simultaneously with increasing propane partial pressure. At 1200‐1400 °C, thick homoepitaxial 6H‐SiC layers were successfully obtained in Co‐Si and Ti‐Si melts, with growth rate up to 200 µm/h. Large terraces with smooth surfaces are observed suggesting a layer by layer growth mode, and the influence of the system pressure was demonstrated. It was shown that the terrace size decrease simultaneously with increasing propane partial pressure which suggests the beginning of a two dimensional to three dimensional growth mode transition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defect passivation by hydrogen reincorporation for silicon quantum dots in SiC/SiOx hetero-superlattice

The SiC/SiO_x hetero-superlattice (HSL) consisting of alternating near-stoichiometric SiC barrier layers for the electrical transport and silicon rich SiO_x matrix layers for the quantum dot formation is a promising approach to the realization of silicon quantum dot (Si–QD) absorbers for 3rd generation solar cells. However, additional defect states are generated during post deposition annealing needed for the Si–QD formation causing an increase in sub-band gap absorption and a decrease in PL intensity. Proper passivation of excess defects is of major importance for both the optical and electrical properties of the SiC/SiO_x HSL Si–QD absorber. In this work, we investigate the effectiveness of the hydrogen reincorporation achieved with hydrogen plasma in a plasma-enhanced chemical vapor deposition (PECVD) reactor, hydrogen dissociation catalysis in hot-wire chemical vapor deposition (HWCVD) reactor and annealing in forming gas atmosphere (FGA). Both the HSL samples and single layer reference samples are tested. The passivation quality of the hydrogen reincorporation was examined by comparing electrical and optical properties measured after deposition, after annealing and after passivation. In addition, the formation of Si–QDs in SiC/SiO_x HSL was evaluated using high resolution transmission electron microscopy. We demonstrated that hydrogen can be successfully reincorporated into the annealed HSL sample and its single layer reference samples. FGA passivation is most effective for SiO_1.2 single layers and HSL samples. Passivation with PECVD appeared to be only effective for SiC single layers.     

Rate-determining process in chemical vapor deposition of SiC on off-axis -SiC

Homoepitaxial growth on off-axis α-SiC at reduced pressures in a horizontal cold-wall chemical vapor deposition (CVD) system operating at has been investigated. The growth rate was found inversely proportional to the square root of total pressure or the partial pressure of H₂, a carrier gas. A model to explain the experimental results is proposed, where the rate-determining process in CVD is competition between Si species and hydrogen atoms for C (carbon) dangling bonds at SiC step edges.     

Propagation behavior of threading dislocations during physical vapor transport growth of silicon carbide (SiC) single crystals

The dislocation formation and propagation processes in physical vapor transport (PVT) grown 4H silicon carbide (4H–SiC) single crystals have been investigated using defect selective etching and transmission electron microscopy (TEM). It was found that while the growth initiation process generally increased the density of threading dislocations in the grown crystal, for certain areas of the crystal, threading dislocations were terminated at the growth initiation. Foreign polytype inclusions also introduced a high density of dislocations at the polytype boundary. In the polytype-transformed areas of the crystal, almost no medium size hexagonal etch pits due to threading screw dislocations were observed, indicating that the foreign polytype inclusions had ceased the propagation of threading screw dislocations. Based on these results, we argued the formation and propagation of the threading dislocations in PVT grown SiC crystals, and proposed the dislocation conversion process as a plausible cause of the density reduction of threading dislocations during the PVT growth of SiC single crystals.