Misoriented domain formation in 6H-SiC single crystal

6H-SiC single crystals were grown by the physical vapor transport (PVT) technique. Misoriented domains (MDs) were observed in as-grown crystals. Raman spectra and X-ray diffraction indicated that the MDs are 4H polytype with either (1 0 1¯ 2) or (1 0 1¯ 6) growth plane. Formation probability of MDs increased continuously as the thermal insulator had been repeatedly used. Simulations based on heat transfer demonstrated that the changes of the temperature and the temperature axial gradient at the center of the growth front were responsible for the phenomenon. The formation mechanism was put forward in terms of atomic structure of various crystal planes.     

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.     

A model for the growth of anomalous polytype structures in vapour grown SiC

A number of polytype structures observed in vapour grown SiC crystals have a unit-cell which is an integral multiple of the unit-cell of the basic 6H, 15R or 4H structure. The growth of such anomalous structures cannot be understood in terms of spiral growth round a single screw dislocation in a basic matrix. However many of these polytype crystals display a single growth spiral on their (0001) face indicating that they have resulted from spiral growth round a single screw dislocation. It is shown that this anomaly can be resolved if the basic matrix is assumed to contain stacking faults near the surface at the time of the origin of the screw dislocation ledge. This possibility, overlooked in the earlier deduction of polytype structures, must be taken into consideration since vapour grown SiC crystals frequently contain a high concentration of random stacking faults, producing continuous streaks on their X-ray diffraction photographs. The most probable fault configurations that can occur in 6H, 15R and 4H structures of SiC have been deduced from a calculation of their stacking fault energy. These fault configurations are then considered to lie at different distances from the surface at the time of the origin of a screw dislocation ledge. Such a faulted ledge gives rise to polytype structures during subsequent spiral growth even if the screw dislocation has an integral Burgers vector. The most probable series of polytype structures that can result from such a faulted matrix model are deduced. It is shown that nearly all the polytype structures of SiC hitherto regarded as anomalous (such as 36H, 54H, 66H, 45R, 90R etc.) are among the expected structures and there is no need to postulate a complicated configuration of cooperating dislocations to account for their growth.     

PVT法生长4H-SiC晶体及多型夹杂缺陷研究进展

作为第三代半导体材料的典型代表,碳化硅因具备宽的带隙、高的热导率、高的击穿电场以及大的电子迁移速率等性能优势,被认为是制作高温、高频、高功率以及高压器件的理想材料之一,可有效突破传统硅基功率半导体器件的物理极限,并被誉为带动“新能源革命”的绿色能源器件。作为制造功率器件的核心材料,碳化硅单晶衬底的生长是关键,尤其是单一4H-SiC晶型制备。各晶型体结构之间有着良好的结晶学相容性和接近的形成自由能,导致所生长的碳化硅晶体容易形成多型夹杂缺陷并严重影响器件性能。为此,本文首先概述了物理气相传输(PVT)法制备碳化硅晶体的基本原理、生长过程以及存在的问题,然后针对多型夹杂缺陷的产生给出了可能的诱导因素并对相关机理进行解释,进一步介绍了常见的碳化硅晶型结构鉴别方式,最后对碳化硅晶体研究作出展望。     

Total pressure‐controlled PVT SiC growth for polytype stability during using 2D nucleation theory

A total pressure‐controlled physical vapor transport growth method that stabilizes SiC polytype is proposed. The supersaturation of carbon during SiC growth changed as a function of the growth time due to changes in the temperature difference between the surfaces of the source and the grown crystal. Supersaturation also varied as a function of the pressure inside the furnace. Therefore, modification of the pressure as a function of growth time allowed for constant supersaturation during growth. The supersaturation was calculated based on classical thermodynamic nucleation theory using data for heat and species of Si₂C and SiC₂ transfer in a furnace obtained from a global model. Based on this analysis, a method for polytype‐stabilized SiC growth was proposed that involves decreasing the pressure as a function of growth time. The 4H‐SiC prepared using this pressure‐controlled method was more stable than that of 4H‐SiC formed using the conventional constant‐pressure method.     

Homoepitaxial growth of 6H–SiC thin films by metal-organic chemical vapor deposition using bis-trimethylsilylmethane precursor

Homoepitaxial silicon carbide (SiC) films were grown on 3.5° off-oriented (0 0 0 1) 6H–SiC by metal-organic chemical vapor deposition (MOCVD) using bis-trimethylsilylmethane (BTMSM, C₇H₂₀Si₂). A pronounced effect of the growth conditions such as source flow rate and growth temperature on the polytype formation and structural imperfection of the epilayer was observed. The growth behavior was explained by a step controlled epitaxy model. It was demonstrated by high-resolution X-ray diffractometry and transmission electron microscopy that high-quality 6H–SiC thin films were successfully grown at the optimized growth condition of substrate temperature 1440°C with the carrier gas flow rate of 10 sccm.     

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)     

Observation and interpretation of eccentric growth spirals

Eccentric growth spirals were observed on the (0001) face of SiC crystals synthesized by the Lely method at the Toshiba Central Research Laboratories. Originating from one screw dislocation, mono-molecular spiral layers (step height 15 Å, 6H polytype), show increasingly wider step separation in the direction towards the inner portion of the furnance where temperature is lower, and hence supersaturation is higher. Contrary to this, step separations become increasingly narrower approaching the base of the crystal which is more close to the wall of the furnace. This eccentricity is interpreted as due to a supersaturation gradient over the surface. In addition, an observed eccentric growth spiral with a hollow core will be presented and interpreted.     

4H-SiC晶体表面形貌和多型结构变化研究

利用光学显微镜、显微拉曼光谱仪研究了4H-SiC晶体表面形貌和多型分布.显微镜观察结果显示4H-SiC小面生长螺蜷线呈圆形,沿<11(2-)0>方向容易出现裂缝.裂缝两侧有不同的生长形貌.拉曼光谱结果显示缺陷两侧为不同的晶型,裂缝实际为晶型转化的标志.纵切片观察发现,在4H-SiC和15R-SiC多型交界处产生平行于<11(2-)0>方向裂缝;15R-SiC多型一旦出现,其径向生长方向平行于<11(2-)0>方向,轴向生长方向平行于<000(1-)>方向.     

高分辨X射线衍射法研究碳化硅单晶片中的多型结构

我们采用高分辨X射线衍射法对SiC单晶片中的多型结构进行了研究,研究发现在以4H-SiC为籽晶的晶体生长过程中,4H-SiC、6H-SiC、15R-SiC出现两相共存或三相共存现象.在单相、两相或三相共存区,X射线摇摆曲线具有明显不同的特征.根据多型结构,可以对摇摆曲线中的衍射峰进行鉴定.     

Working Model for Explanation of Polytype formation Based on Superlattice Ordering of Point Defects in Close Packing Structures

The model is an attempt to give an explanation of polytype orderings in the crystals. Basic assumptions of the model are: the point defects play the important part in the polytype formation; these defects influence the configuration (hexagonal or cubic) of individual layers. The defects have a tendency to ordering themselves into superlattices. It acts selectively on particular stacking of the layer sequences. The results of model operation are shown in examples of experimental data of polytypic materials; they are SiC and A^IIB_VI compounds.     

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。     

Cr doping influence in ZnS single crystals on the complex disordered polytypical structure

The results of X-ray investigation of real structures of ZnS:Cr single crystals are presented. Doped ZnS single crystals were grown from the melt by Bridgman's high pressure method. Cr dopant converts the structure of ZnS crystals from 3C with individual stacking faults (for pure ZnS) to the complex polytypical structures disordered along one dimension and including specific orderings of cp layers that form polytype cells LH (L = 2l, l = 2, 3, 4 and 5). The structures are characterised by Farkas-Jahnkes statistical parameters π(m, p) and by parameters P_LH of the polytype unit cell formation probability (Kozielski, Tomaszewicz 1986; Palosz, Przedmojski).     

Effect of purification on the polytypism in solution-grown cadmium iodide crystals

The effect of purification on the polytypism in cadmium iodide crystals has been studied by X-ray diffraction in three hundred and sixty polytypes. Formation of small period polytype 2H is governed by both temperature and impurities contained in the starting material. 4H is the most stable polytype and higher occurrence of unidentified polytypes in crystals of purified material has been attributed to free movement of edge dislocations during growth. The results have been examined against empirical conclusions of earlier investigations.     

Zn1−xCdx polytype single crystals with various degree of order

The results of X-ray investigations of the polytype structures formed in solid solutions of Zn_1–xCd_xSe are presented. For compositions of 0.22 < x < 0.35 polytypes 8H, 6H, 4H, and DS (disordered structure) are present. The Zn_1–xCd_xS single crystals of 0 ≦ x ≦ 0 were obtained by Bridgman's method under high protective argon pressure from the melt. The structures are characterised by FARKASZ -JAHNKE statistical parameters π(m,p), and by parameters P_LH of the polytype unit cell formation probability.     

Bulk and epitaxial growth of silicon carbide

Silicon carbide (SiC) is a wide bandgap semiconductor having high critical electric field strength, making it especially attractive for high-power and high-temperature devices. Recent development of SiC devices relies on rapid progress in bulk and epitaxial growth technology of high-quality SiC crystals. At present, the standard technique for SiC bulk growth is the seeded sublimation method. In spite of difficulties in the growth at very high temperature above 2300 °C, 150-mm-diameter SiC wafers are currently produced. Through extensive growth simulation studies and minimizing thermal stress during sublimation growth, the dislocation density of SiC wafers has been reduced to 3000–5000 cm⁻² or lower. Homoepitaxial growth of SiC by chemical vapor deposition has shown remarkable progress, with polytype replication and wide range control of doping densities (10¹⁴–10¹⁹ cm⁻³) in both n- and p-type materials, which was achieved using step-flow growth and controlling the C/Si ratio, respectively. Types and structures of major extended and point defects in SiC epitaxial layers have been investigated, and basic phenomena of defect generation and reduction during SiC epitaxy have been clarified. In this paper, the fundamental aspects and technological developments involved in SiC bulk and homoepitaxial growth are reviewed.     

Mechanism of the 2H → 6H solid state transformation in SiC

Single crystals of 2H SiC transform directly into the 6H (ABCACB) structure when the transformation nucleates at temperatures above 2000°C. The 2H close-packed structure may be transformed to the 6H structure by displacing every third layer. The theory of X-ray diffraction from one-dimensionally disordered crystals undergoing the 2H to 6H structural transformation by such a layer displacement mechanism has been developed. The fact that all the observed solid state transformations in SiC crystals commence with the random insertion of stacking faults and then proceed further to create a statistically ordered structure, permits such a theory to be developed. Exact expressions for the diffracted intensity from such crystals have been obtained and the different diffraction effects observable on single crystal X-ray photographs predicted. A comparison of the theoretically predicted diffraction effects with those visible on the X-ray photographs of SiC crystals undergoing the 2H to 6H transformation shows that the structural transformations in SiC occur by the layer displacement mechanism.     

Numerical simulation of a new SiC growth system by the dual-directional sublimation method

A new SiC growth system using the dual-directional sublimation method was investigated in this study. Induction heating and thermal conditions were computed and analyzed by using a global simulation model, and then the values of growth rate and shear stress in a growing crystal were calculated and compared with those in a conventional system. The results showed that the growth rate of SiC single crystals can be increased by twofold by using the dual-directional sublimation method with little increase in electrical power consumption and that thermal stresses can be reduced due to no constraint of the crucible lid and low temperature gradient in crystals.     

Epitaxial growth of α-SiC layers by chemical vapor deposition technique

Single crystals of α-SiC were grown on α-SiC substrates at a temperature between 1570 and 1630°C with the standard gas flow rate: H₂ ～ 1 liter/min, SiCl₄ ～ 1.7 ml/min and C₃H₈ ～ 0.1 ml/min. The grown layers were transparent greenish-blue, and surfaces were mirror-like. By an X-ray back-reflection Laue pattern and a reflection electron diffraction method, the grown layer was identified as 6H-SiC, one polytype of α-SiC. Crystal growth was influenced by substrate temperature, flow rates of reaction gases and the surface polarity of the substrate. The growth rate decreased with increase of the substrate temperature in the above temperature region. A lamellar structure was observed on the (0001) Si surface and a mosaic structure was observed on the (000$\text{1}$)C surface. The mole ratios of both SiCl₄ and C₃H₈ to H₂ and that of Si to C had some influence on crystal growth. Undoped layers were n-type due to nitrogen. P-type SiC was grown by doping Al during crystal growth. Doping effects were studied by photoluminescence and electrical measurements.     

Experimental Study of Interface Inversion of Tb3ScxAl5‐xO12 Single Crystals Grown by the Czochralski Method

Thermal conditions and rotation rate were examined experimentally for obtaining a flat interface growth of high melting‐point oxide (Tb₃Sc_xAl_5‐xO₁₂ ‐ TSAG) by the Czochralski method. The critical crystal rotation rate can be significantly reduced, of about twice at low and very low temperature gradients comparing to medium temperature gradients in the melt and surroundings of the crystal. The interface shape of TSAG crystals is not very sensitive on crystal rotation rate at small rotations and becomes very sensitive at higher rotations, when the interface transition takes place. The range of crystal rotation rates during the interface transition from convex to concave decreases with a decrease of temperature gradients. At low temperature gradients interface inversion crystals takes place in very narrow range of rotation rates, which does not allow one to growth such crystals with the flat interface. Even changing crystal rotation rate during the growth process in a suitable manner did not prevent the interface inversion from convex to concave and thus did not allow to obtain and maintain the flat interface.