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.     

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.     

Epitaxial growth of silicon carbide layers by sublimation “Sandwich method” (II) structural defects and growth mechanism

Structural defects of α-SiC epitaxial layers grown by sublimation “sandwich-method” in vacuum at the temperatures ranging from 1600 to 2100 °C have been investigated by X-ray topography and optical microscopy methods. It was shown, that perfect SiC layers with the homogeneous polytype structure and small dislocation density (≦ 10² cm⁻²) may be obtained on the substrates with any crystallographic orientation at the conditions close to quasi-equilibrium one. The presence of impurities and silicon deficiency in the vapour phase, lead usually to the deterioration of morphological and structural perfection of SiC layers. There are the following structural defects: uncoherent polytype inclusions (mainly β-SiC), pores, dislocations, specific stacking faults. Morphological peculiarities of the SiC epitaxial layers and possible growth mechanisms are discussed.     

Studies on syntactically coalesced Silicon Carbide polytype structures

The structure of two new silicon carbide polytypes, found in syntactic coalescence with a basic structure 6 H, have been worked out. The structure of polytype 21 H is [(33)₂63] and that of polytype 93R is [(33)₄34]₃. The polytype 93R belongs to the structure series [(33)_n34]₃. One of the (00.1) growth face reveals the growth spiral. The splitting of diffraction spots indicates the existence of a high period polytype in the crystal. This high period polytype have been identified as 558H or 1674R. The growth mechanism have been discussed in the light of above observations.     

Factors affecting the formation of misoriented domains in 6H‐SiC single crystals grown by PVT method

Misoriented domains (MDs) are common defects in 6H‐SiC single crystals. We performed an experimental study on the formation of MDs in 2‐inch 6H‐SiC single crystals. Micro‐Raman spectroscopy revealed that the polytype of MDs was mainly 4H‐SiC. By changing growth conditions, it was found that the MDs' formation was closely related to growth rate and the position of highest temperature relative to growth interface. When the growth rate of ingots was relatively high the MDs were more likely to form. Furthermore, the nearer growth interface the position of highest temperature was, the larger the size of the MDs. Based on our experimental findings we suggested that the MDs' formation and the polytype switching from 6H‐ to 4H‐SiC were due to too large axial and/or radial temperature gradients.The results would be helpful to improve the quality of SiC single crystals grown by PVT technique. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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。     

Polymorphic–polytypic transition induced in crystals by interaction of spirals and 2D growth mechanisms

The relationship between crystal polymorphism and polytypism can be revealed by surface patterns through the interlacing of the growth spirals. Simple high-symmetry structures as SiC, ZnS, CdI2 and more complex low-symmetry layered structures as n-paraffins, n-alcohols and micas are concerned with polymorphic–polytypic transition. In this paper, we will show for the first time, through in situ AFM observations and X-ray diffractometry, that a protein polymorph (P2₁2₁2₁α-amylase) locally changes, during growth, to a monoclinic P2₁ polytype, thanks to the screw dislocation activity. The interplay between spiral steps and 2D nuclei of the polytypes coexisting in the same crystalline individual allows to foresee the consequences on the crystal quality. The discussion is extended to other mineral and biological molecules and a new general rule is proposed to explain the interactions between surface patterns and the bulk crystal structure.     

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.     

Microstructures on Zirconium Trisulphide Single Crystals

Physical vapour transport grown single crystals of zirconium trisulphide have been examined by optical microscope, and the study of growth patterns on their finished surfaces reveal the screw dislocation mechanism by which the crystals have grown.     

8英寸导电型4H-SiC单晶衬底制备与表征

使用物理气相传输法(PVT)通过扩径技术制备出直径为209 mm的4H-SiC单晶,并通过多线切割、研磨和抛光等一系列加工工艺制备出标准8英寸SiC单晶衬底。使用拉曼光谱仪、高分辨X射线衍射仪、光学显微镜、电阻仪、偏光应力仪、面型检测仪、位错检测仪等设备,对8英寸衬底的晶型、结晶质量、微管、电阻率、应力、面型、位错等进行了详细表征。拉曼光谱表明8英寸SiC衬底100%比例面积为单一4H晶型;衬底(004)面的5点X射线摇摆曲线半峰全宽分布在10.44″~11.52″;平均微管密度为0.04 cm^-2;平均电阻率为0.020 3 Ω·cm。使用偏光应力仪对8英寸SiC衬底内部应力进行检测表明整片应力分布均匀,且未发现应力集中的区域;翘曲度(Warp)为17.318 μm,弯曲度(Bow)为-3.773 μm。全自动位错密度检测仪对高温熔融KOH刻蚀后的8英寸衬底进行全片扫描,平均总位错密度为3 293 cm^-2,其中螺型位错(TSD)密度为81 cm^-2,刃型位错(TED)密度为3 074 cm^-2,基平面位错(BPD)密度为138 cm^-2。结果表明8英寸导电型4H-SiC衬底质量优良,同比行业标准达到行业先进水平。     

Investigation of the formation and growth processes of silicon carbide monocrystals

The formation and growth processes of SiC monocrystals from vapour phase by sublimation method have been investigated. It is shown that the peaks of the evaporation and growth whiskers are the centres of SiC monocrystals formation. One of the mechanisms of development of the whisker into SiC platelet is established. It is shown that on the naturally mirror like crystal face either the edge of the crystal or the screw dislocation, situated on the crystal edge, is the source of the steps. On the stepped crystal face the root of the crystal is the source of the steps.     

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.     

The structure,perfection and annealing behaviour of SiC needles grown by a VLS mechanism

Over 50 different single crystals of SiC, grown by a vapour-liquid-solid mechanism, have been examined by optical and X-ray diffraction methods to determine their structure, perfection and annealing behaviour. All the crystals studied had either the hexagonal close-packed 2H (ABAB…) structure or the cubic close-packed 3C (ABCABC…) structure or a combination of both, and invariably contained a random distribution of stacking faults on their basal planes. The thermal stability of the structures was studied by annealing them between 1300–2200 °C in an inert argon atmosphere, and reexamining the structure of the crystals after each annealing run. Structural transformations were found to commence in different crystals at different temperatures between 1400 and 1800 °C. The transformations were found to be irreversible, not martensitic and appear to involve the nucleation of the new phase by the insertion of stacking faults followed by its growth during the annealing. The mechanism of the solid-state transformations, the influence of faults and impurities, and the thermal stability of different SiC structures are discussed.     

An X-ray investigation of growth and deformation faults in a vapour grown 2H SiC crystal

Single crystals of 2H SiC grown by hydrogen reduction of methyltrichlorosilane at 1400°C frequently contain a high concentration of random stacking faults in their hexagonal closepacked | AB | AB | … structure. This given rise to diffuse streaks along reciprocal lattice rows parallel to the c¹ axis for h ? k ≠ 0 mod 3. To investigate the nature of stacking faults in these crystals, the intensity distribution along the 10. l reciprocal lattice row of a 2H SiC crystal was recorded on a 4-circle computer-controlled single crystal diffractometer. The halfwidths of 10. l reflections with l even and l odd were found to be 0.36 and 0.24 reciprocal units respectively. It is observed (i) that the 10. l reflections with l even are highly broadened and (ii) that the halfwidths of l even and l odd reflections are in the ratio of 3 : 2. This suggests that the stacking faults present are predominantly growth and deformation faults. Since the fault concentration is very high, exact theoretical expressions for the halfwidths of 10. l reflections were used to calculate the growth and deformation fault probabilities (α and β) from the observed half widths, without neglecting the second and higher order terms in α and β. It is found that α = 0.11 and β = 0.20. The deformation fault probability (β) is surprisingly high for hard and brittle material like SiC which does not undergo plastic deformation easily. It is suggested that several deformation fault configurations have resulted from a clustering of growth faults.     

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.     

Solvent effect on the polytype structure of CdI2 crystals

Single crystal platelets of cadmium iodide have been grown from alcohols: n-propyl, isobutyl and isoamyl and from their aqueous solutions. The structures of 880 crystals has been identified and the effect of solvent on the polytype structure of CdI₂ has been discussed regarding: the stability of the basic structures 2H and 4H, the structure of complex polytypes, the effect of the solvent on the structure of faults, the period of polytypes. Some relations indicating the role of the solvent in the formation of polytype structure of CdI₂ have been found.     

Vapour growth and epitaxy of minerals and synthetic crystals

Surface microtopographs of the following crystals, both natural and synthetic, grown from pure vapour phase (PV), by chemical vapour transport (CVT), from high temperature solution (HTS) and hydrothermal solution (HS) are compared according to the criteria of (1) whether spirals are circular or polygonal and (2) how wide is the step separation of the spiral; SiC (PV, CVT, both synthetic), hematite (CVT, HTS, natural and synthetic), corundum (CVT, HTS, synthetic), mica minerals (PV, CVT, HS, natural and synthetic) and beryl (CVT, HTS, HS, natural and synthetic). Clear differences in morphology and step separation were found between crystals grown from vapour phase and solutions, between PV and CVT, as well as between natural and synthetic crystals. The differences are analysed in conjunction with the recent developments of computer simulations on spiral morphology. The results show interaction between solid and fluid plays very important role in determining the spiral morphology. Oriented intergrowth between different crystals well known among minerals, such as epitaxy, topotaxy, co-axial intergrowth, exsolution etc. are briefly summarized. It is also briefly explained how these relations are used in understand the growth or cooling histories of natural minerals.     

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.