Silicon Carbide Crystals Research Articles

On the phase transformation of single-crystal 4H–SiC during nanoindentation

Microstructural changes of single-crystal 4H silicon carbide (SiC) induced by nanoindentation under various conditions were investigated. It was found that nanoindentation at different crystal orientations induced different Raman spectroscopic characteristics. Cross-sectional observation by transmission electron microscopy indicated that a very deep subsurface damage region was formed where dislocations occurring along the basal planes, crystal grains rotation and micro-cracks were observed. The microstructures of the damage regions were strongly affected by the nanoindentation conditions. Coupled analysis of lattice fringes and Raman spectra indicated that a phase transformation from 4H–SiC to 3C–SiC occurred during nanoindentation, which has never been reported before. Furthermore, the 4H to 3C phase transformation strongly depended on the indenter orientation with respect to the SiC crystal. These findings are meaningful for low-damage precision machining of SiC substrates.

Reduction of Dislocation Density in Bulk Silicon Carbide Crystals Grown by PVT on Profiled Seeds

The distribution of extended defects in silicon carbide (SiC) crystals grown on profiled seeds by the sublimation (physical vapor transport) method has been studied by optical microscopy in combination with chemical etching. It is established that free lateral growth on protruding relief elements (mesas) is accompanied by a sharp decrease in the density of threading dislocations and micropipes. The decreased density of dislocations is retained after growing a thick layer that involves the overgrowth of grooves that separated individual mesas.

HPHT Diamond Crystallization in the Mg-Si-C System: Effect of Mg/Si Composition

Crystallization of diamond in the Mg-Si-C system has been studied at 7.5 GPa and 1800 °C with the Mg-Si compositions spanning the range from Mg-C to Si-C end-systems. It is found that as Si content of the system increases from 0 to 2 wt %, the degree of the graphite-to-diamond conversion increases from about 50 to 100% and remains at about this level up to 20 wt % Si. A further increase in Si content of the system leads to a decrease in the graphite-to-diamond conversion degree down to complete termination of diamond synthesis at Si content >50 wt %. Depending on the Si content crystallization of diamond, joint crystallization of diamond and silicon carbide and crystallization of silicon carbide only are found to take place. The cubic growth of diamond, typical of the Mg-C system, transforms to the cube-octahedron upon adding 1 wt % Si and then to the octahedron at a Si content of 2 wt % and higher. The crystallized diamonds are studied by a suite of optical spectroscopy techniques and the major characteristics of their defect-and-impurity structure are revealed. The correlations between the Si content of the Mg-Si-C system and the properties of the produced diamond crystals are established.

Two-step SiC solution growth for dislocation reduction

We propose a two-step silicon carbide (SiC) solution growth method for dislocation reduction to produce the high-quality silicon carbide SiC crystals. The two-step growth consists of the growth on a Si face and a C face. Firstly, seed crystal with low threading dislocation density was prepared by the growth on a Si face utilizing the threading dislocation conversion. Secondly, the growth on the C face was conducted on the prepared seed crystal with low threading dislocation density to reduce the density of basal plane dislocations and keep smooth growth surface. We demonstrate that the two-step growth leads to the reduction of the density for all types of dislocations by two orders of magnitude compared to the initial density of the seed crystal.

(Invited) SiC Growth Parameter Evolution Utilizing Infrared Thermal Imaging Towards Realization of Extremely Low Dislocation Bulk SiC

The NASA Glenn Research Center (GRC) has employed infrared thermal imaging technology in a hot wall chemical vapor deposition (CVD) system for thermal measurement of silicon carbide (SiC) crystals mounted vertically in the gas stream. The use of infrared thermal imaging technology revealed deficiencies in growth parameters that had previously not been observable utilizing traditional noncontact monitoring techniques. GRC has been investigating the lateral expansion (⊥ to (0001)) of pseudo SiC fiber seed crystals using a custom designed horizontal flow hot wall CVD system in proof of concept research, for a radically new bulk growth process. This bulk growth process would, ideally produce SiC boules with just one single centrally located screw dislocation (SD). Dislocations in general have a negative impact on SiC devices and device fabrication. Current commercially available SiC substrates have a dislocation density of ~1000 cm2. A key requirement of this process is the lateral homoepitaxial growth expansion of a SiC fiber without the introduction of additional defects. This requires maintaining growth temperatures along the length of the SiC fiber. In the absence of purpose grown fiber like seed crystals, pseudo seed crystals were created by mechanically dicing thin slivers from a-plane and m-plane oriented boule slices. The slivers (~17000 x 2000 x 400) µm’s were subsequently subjected to gaseous etching in hydrogen to produce needle like seed crystals with fiber like dimensions with a width of ~100 µm’s. The seed crystals were oriented vertically in a gas stream of the horizontal CVD system. Traditional optical pyrometry coupled with a small CCTV aligned along the horizontal axis was used to be provide real time thermal and optical data of the CVD. Due to the extremely small cross-sectional area of the seed crystals, the thermal radiation was insufficient to be measured with optical pyrometry and was ineffective in resolving the temperature of the vertical slivers. Only the temperature of the carrier holding the seed crystals could be resolved. A commercially available infrared thermal imaging system equipped with an extra-long working distance (530mm) lens replaced the pyrometer and CCTV. The thermal imager requires that the target be only 3 x 3 pixels to obtain accurate temperatures. When coupled with long working distance optics the temperature of the seed crystals could be resolved. A thermal gradient that was previously unobserved was now viewable. Growth parameters were adjusted to compensate for the gradient preventing nucleation of 3C-SiC at the tip of the seed crystal. A growth rate (⊥ to (0001)) of ~80 μm/hr was achieved. Total lateral growth was > 3.5mm. Initial analysis by synchrotron white beam x-ray topography (SWBXT) confirms, that the fiber growth was homoepitaxial, matching the polytype of the respective underlying region of the seed sliver. Although the as grown SiC contained a high density of basal plane dislocations (BPD’s) it appears that the BPD’s originated in the pseudo seed crystal without additional BPD’s introduced during the growth.

Sublimation Growth of 4 and 6 Inch 4H-SiC Low Defect Bulk Crystals in Ta (TaC) Crucibles

Recently, the wide bandgap semiconductors, especially silicon carbide (SiC), have become more important due to the unique electrical and thermophysical properties that make them applicable to a variety of electronic devices (Schottky and PiN diodes, JFETs, MOSFETs, etc.). For these applications, the crystals need to be manufactured with highest possible quality, both structural and chemical, at reduced cost. This requirement places rather extreme constraints on the crystal growth as the simultaneous goals of high quality and low cost are generally incompatible.Refractory metal carbide technology, particularly, tantalum carbide (TaC), was originally developed for application in highly corrosive and reactive environments. Yu. Vodakov [1] demonstrated for the first time advantages of use of refractory metal carbides for PVT growth of SiC and later AlN bulk crystals. In the present paper we discuss the effect of refractory metal on PVT growth of large diameter 4H SiC bulk crystals.

Bulk Growth of Large Area SiC Crystals

The growth of large diameter silicon carbide (SiC) crystals produced by the physical vapor transport (PVT) method is outlined. Methods to increase the crystal diameters, and to turn these large diameter crystals into substrates that are ready for the epitaxial growth of SiC or other non homogeneous epitaxial layers are discussed. We review the present status of 150 mm and 200 mm substrate quality at Cree, Inc. in terms of crystallinity, dislocation density as well as the final substrate surface quality.

Study of Nanoscale Inhomogeneities in Silicon Carbide Crystals via Small-Angle X-Ray Scattering

The structure of 4H-SiC and 6H-SiC monocrystals’ nanoscale defects was studied by the small-angle X-ray scattering (SAXS) method. Analysis of dispersion indicatrices has shown the presence of inhomogeneities in the form of linear fractals in silicon carbide crystals of 6H and 4H polytypes, mass fractals in 6H polytype and inhomogeneities partition’s fractal surface in 4H polytype.

Tungsten-oxide thin films for a high-temperature semiconductor hydrogen detector based on a 6H-SiC crystal

We demonstrate the possibility of fabricating high-efficiency semiconductor hydrogen detectors based on tangsten-oxide thin films deposited onto silicon-carbide crystals by reactive pulsed-laser deposition. The obtained WO3/SiC structures ensure a noticeable voltage shift ΔU on the reverse branch of the I−V characteristics without commonly used catalyst layers of platinum-group metals. The ΔU value reached 2.1 V at a detected hydrogen concentration of 0.2% in air at 350°C. The response times to hydrogen inlet and recovery of the WO3/SiC structure after hydrogen outlet are found to be much shorter than those for the Pt/WO3/SiC structure. The high performances of the fabricated WO3/SiC sensor are due to the layered structure of the orthorhombic phase of tungsten oxide, which consists of loosely packed microcrystalline plates containing nanocrystals smaller than 100 nm.

Investigation of SiC Polytypes for PVT Process Control Using Ultraviolet Fluorescence Technique

Silicon carbide (SiC) is a wide-bandgap semiconductor with high saturation velocity and thermal conductivity. So, It is well-known as a potential substrate for high-power, high-temperature and high-frequency devices. Commercial 4H- and 6H-SiC wafers are commonly grown by physical vapor transport (PVT) method and some emerging techniques such as high temperature chemical vapor deposition (HTCVD) and top seeded solution growth (TSSG) is still under developing. Because SiC has large number of polytypes determined by their stacking order during crystal growth, the polytype inclusion has been a common issue during SiC growth, especially at the early stage of development of new process or new equipments. When photon is incident on the surface of SiC crystal and the energy of photon is larger than the band gap of SiC (2.9 ~ 3.3eV in the UV range), the photon excites the electron in the valence band to the conduction band. Excited electron goes to the lower states emitting heat and relaxes to the valence band emitting photon that has longer wavelength that the incident light. The phenomena that wavelength is shifted to the longer wavelength by material is called fluorescence. Thus, polytypes with different energy band gap can be identified with the fluorescence color if UV ray is incident ray with sufficient energy for excite electron in the valence band. In this study, we observed the fluorescent color of various SiC polytypes with a lab-developed UV fluorescence equipment. The UV fluorescence is non-destructive technique to characterize the grown sample without any further sample preparations, so that this technique is applied to develop a new PVT process in the early stages. SIMS, Photoluminescence and Raman spectroscopy were used for identify the fluorescence color considering polytypes, dopants and defects. Stress mapping using high resolution X-ray diffraction and Raman spectroscopy were used for analyzing the effect of polytype formation on residual stress distribution on the grown crystal. Glazing-incidence X-ray topography using white beam were conducted for evaluating the grown SiC crystals at Pohang accelerator laboratory.

Polytype inclusions and polytype stability in silicon-carbide crystals

On the basis of both published data and our own experimental data we consider the main aspects of the problem related to ensuring polytype stability for ingots of grown 4H and 6H silicon-carbide compounds.

SiC nanowires synthesized from graphene and silicon vapors

The preparation of silicon carbide (SiC) nanowires is basically important for its potential applications in nanodevices, nanocomposites, etc. In the present work, a simple route was reported to synthesize SiC nanowires by heating commercial graphene with silicon vapors and no catalyst. Characterization by scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, electron energy scattering, X-ray diffraction, and Raman dispersive spectrum demonstrates the products are composed of β-SiC crystal. The SiC nanowires have the average diameter of about 50 nm and length of tens of micrometers. The vapor–solid mechanism was employed to interpret the SiC nanowires growth. Gaseous SiO which was produced by the reaction of Si powders with its surface oxidation reacted with the solid graphene to form SiC crystal nuclei. And SiC crystal nuclei would act as active sites for further growing into nanowires.

The Study of Conditions for the Silicon Carbide Crystals Formation in the Complex Composition Metal Melt

The results of thermodynamic modeling and experimental studies are presented in this article. The aim of this work is to determine the conditions of silicon carbide crystals synthesis in the complex composition metal melts. The "FactSage" software was used for thermodynamic modeling. The phase diagrams, in the form of liquidus surfaces, allow determining the metal composition with minimum melting temperature and presenting the ranges of concentrations and temperatures for which the equilibrium product of the interaction between the components of the metal melt is silicon carbide. The results of experimental research confirmed the possibility of growing silicon carbide crystals in the complex metal melts at low temperatures. The results of the obtained sample of complex alloy examination (conducted with the scanning electron microscope JEOL JSM–6460LV with a energy dispersion spectrometer by "Oxford Instruments" used for performing qualitative and quantitative microprobe analysis) helped to reveal the crystals corresponding to SiC composition.

Influence of calcination temperature on CuO–CeO2/SiC catalysts for SO3 decomposition in the sulfur–iodine cycle for hydrogen production

The copper-cerium composite oxides supported on silicon carbide (SiC) catalysts (CuO–CeO2/SiC) for sulfuric acid decomposition were studied in sulfur-iodine (SI or IS) cycle. The influence of calcination temperatures (800 °C–1000 °C) of CuO–CeO2/SiC catalysts on their activities and morphology were investigated in this study. The activity of CuO–CeO2/SiC was prominently higher than that of CuO–CeO2 composite oxides catalyst. The SO3 conversion ratio of CuO–CeO2/SiC catalyst increased with calcination temperature from 800 °C to 900 °C, while decreased with calcination temperature at 900 °C–1000 °C. XRD pattern, TEM (HRTEM) images and Energy Dispersive X-ray (EDX) spectra revealed that copper–cerium composite oxides were well dispersed and immobilized on the surface of SiC grains. XRD pattern showed that the accelerated transformation of SiC crystals to amorphous SiO2 and the rapid expansion of CeO2 sub-grains at the calcination temperature of 900 °C–1000 °C, as well as the accelerated amorphization of CuO crystals at 800 °C–900 °C. The agglomeration of catalysts with sintering temperature was also proven by the BET test. At a higher calcination temperature, X-ray photoelectron spectroscopy (XPS) of Cu2p spectra revealed the reduction of more Cu2+ in the process of SiC oxidation. According to TEM (HRTEM), XPS, and infrared radiation, the amorphous SiO2 was produced by the oxidation of superficial SiC particles, and the thickness of the SiO2 layers on SiC grains increased with the calcination temperature. A simple mechanism depicted the oxidation of CuO–CeO2/SiC was created.

A first step toward bridging silicon carbide crystal properties and physical chemistry of crystal growth

The links between the occurrence of a given silicon carbide (SiC) polytype and crystal growth conditions have been mainly empirically described. Although studies are a little more advanced, a similar description can be applied to point defects and especially dopants in the crystals. We propose a method to determine such links, based on a coupled thermodynamic and mass transport model, where SiC is treated as a solid solution. We implemented such an approach to the case of the seeded sublimation growth process, which is currently the industrial bulk growth process for SiC single crystalline ingots. The computation of both Si and C activities in the SiC crystal, between the SiC–C and SiC–Si two-phase equilibria, allowed first the assessment of the SiC crystal chemistry and second the linking of SiC–C and SiC–Si chemistry with the growth process parameters. We demonstrate that depending on the temperature, pressure and a temperature gradient, the activities of Si and C in the crystals can be described and tuned. The case of cubic polytype (3C-SiC) formation is specifically discussed. Its stabilization requires a high Si activity and a low C activity in the SiC crystal, meaning that the synthesis conditions must be close to the limit at the SiC + Si phase boundary.

External impacts on SiC nanostructures in pure and lightly doped silicon carbide crystals

External impacts on SiC nanostructures in pure and lightly doped silicon carbide crystals

(Invited) Structural Characterization of SiC Crystals Grown By Solvent Laser Heated Floating Zone Method and Hot Wall Chemical Vapor Deposition for the Development of a Low Defect Density Bulk Growth Technique for SiC

Structural perfection of silicon carbide (SiC) single crystal is critical in achieving high-performance power devices. This requirement is an ongoing challenge that is being addressed because of the high density of structural defects, particularly threading screw dislocations (TSDs) and basal plane dislocations (BPDs). While the deleterious impact of these defects has been reported, effort in reducing their densities is still ongoing. Researchers at NASA Glenn Research Center (GRC) have recently proposed a new bulk growth process based on axial fiber growth followed by lateral expansion for producing SiC boules with potentially as few as one TSD per wafer [1]. In this techniques, a long continuous single crystal SiC fiber containing a single TSD in the center is grown in the c-direction [0001] and then laterally enlarged (along <1-100> and <11-20> directions) into a boule by chemical vapor deposition (CVD) growth. To implement this novel growth technique, the growth process has been split into two parts that will be optimized separately: (1) Solvent laser heated floating zone (Solvent-LHFZ) method [2] to grow SiC at the end of a 4H-SiC ‘pseudo fiber’ seed using Fe as solvent. This technique combines the fiber growth ability of laser heated floating zone method with single crystal growth ability by traveling solvent method. (2) Lateral expansion of slivers or ‘pseudo-fibers’ of 6H-SiC and mixed 4H/6H polytype SiC by hot wall chemical vapor deposition (HWCVD) [3]. The seed slivers for both growth processes were cut from m-oriented SiC boule slices. Structural characterization of the grown crystals were carried out by synchrotron white beam x-ray topography (SWBXT), optical microscopy, SEM, HRTEM and Raman spectroscopy to map defect types, surface features and distribution [4]. The solvent-LHFZ grown region was single crystal and epitaxial with the seed but however characterized by inhomogeneous strains. The lateral grown regions were also found to be single crystal and epitaxial. The seed used contained stacking disorders and these are reproduced in the lateral growth sections. The Raman spectra confirms the existence of stacking faults with the density of faults higher in the seed than in the grown crystal. Bundles of dislocations are also observed propagating from the seed in m-axis lateral directions. Polytype phase transition and stacking faults were observed by high-resolution TEM (HRTEM), in agreement with SWBXT and Raman scattering. The implication of these results for implementing the novel growth process will be addressed.

Surface-layer composition of titanium alloy after dry grinding by a silicon-carbide wheel

The concentration of ten chemical elements in the surface of a titanium alloy after grinding by a silicon-carbide wheel is determined by means of a Versa 3D twin-beam electron microscope. The content of C, N, O, and Si in a surface layer of up to 200 nm much exceeds the values permitted by the corresponding GOST State Standard. Silicon-carbide crystals are found in the ground surface.

Effect of Porous Graphite for High Quality SiC Crystal Growth by PVT Method

The effect of the porous graphite plate above the source material on properties of silicon carbide (SiC) crystals grown by Physical Vapor Transport method has been investigated. The porous graphite plate was inserted on source powder to produce a more C-rich for the polytype stability of 4H-SiC crystal and a uniform radial temperature gradient. The dendrite structure obtained from SiC source powder in the crucible with porous graphite plate was more densely formed than that in the conventional crucible. The crystal quality of 4H-SiC single crystals grown in porous graphite inserted crucible was revealed to be better than that of crystal grown SiC crystals in the conventional crucible.

Structural Perfection of Silicon Carbide Crystals Grown on Profiled Seeds by Sublimation Method

— The distribution of extended defects in silicon carbide (SiC) crystals grown on profiled seeds by the sublimation (physical vapor transport) method has been studied by optical microscopy in combination with chemical etching and AFM. It is established that free lateral growth on protruding relief elements (mesas) is accompanied by a sharp decrease in the density of threading dislocations and micropipes. The decreased density of dislocations is retained after growing a thick layer that involves the overgrowth of grooves that separated individual mesas.