Heteroepitaxy and Highly Oriented Diamond Deposition

Surface investigations on different nucleation pathways for diamond heteroepitaxial growth on iridium

: Progress continued in 1992 in the two major thrust areas of the diamond program; diamond consolidation, and heteroepitaxial nucleation. We have been developing a consolidation technology as one approach to large area diamond single crystals. During this phase of the program, techniques were developed for (a) bonding of natural diamond seed crystals to non-diamond substrates, (b) overgrowth for consolidation, (c) ion-implantation for lift-off of thin diamond platelets. Experimental and theoretical progress was also made on heteroepitaxial growth of diamond. Theoretical modeling indicated a possible role of subsurface carbon in Ni(111) in preventing diamond nucleation. Experimental studies have provided evidence for several roles for oxygen on diamond surfaces during diamond growth. Progress was also made in developing techniques for microstructural characterization of CVD diamond. TEM sample preparation methods were developed for cross-sectional and plan-view analysis of diamond films and substrates. A simple method for rapidly assessing defect densities in CVD and natural diamond was also developed.

Silicon carbide (SiC) has been considered as one of the alternative power semiconductor materials due to its excellent properties such as wide band gap, high break down field, and high thermal conductivity. Although SiC with 3C (zinc blende) is suitable for high-frequency power devices from its high electron mobility and high electron-saturation-velocity, SiC is well known as having dozens of polytypes including 4H and 6H in addition to 2H (wurtzite) and 3C that consist of different ordered stacking sequences of SiC atomic double layers. The 3C-SiC is believed to be more stable than the hexagonal structure at low temperatures. In contrast, 2H-SiC, which has the simplest stacking sequence, is rarely observed at high temperatures. Moreover, point defects also affect the stable polytype in SiC. Heavily Nitrogen (N)-doped single crystals of 6H-SiC were completely transformed into 3C-SiC by annealing in vacuum at presence of Si vapor at 2080 K and 2180 K (hereafter Case I). The 3C-SiC-like bands was also observed in 4H-SiC epilayers with heavily N-doping after thermal annealing (hereafter Case II). Presuming that vacancy formation is related to temperature, point defects such as vacancy and N in SiC are crucial factor for the structural transformation between hexagonal and cubic structures. In this study, the structural transformation in SiC is simply interpreted by considering point defect formation with the aid of ab initio calculations. Ab initio total energy calculations in a √3×√3×12 unit cell imply that stable stacking for SiC is 6H with Si-vacancy (VSi), 4H with C-vacancy (VC) and N substituting for Si (NSi), and 3C with N substituting for C (NC), whereas 2H-stacking does not appear as a stable stacking in these systems. Furthermore, the formation energy calculations reveal that vacancy formation depends on C chemical potential m C, where VSi and VC formations are favorable at large m C (corresponding to C-rich) and small m C (Si-rich), respectively. On the other hand, NC formation is more favorable than NSi formation over the entire range of m C and independent of conditions because of very small difference in covalent radius between N and C. Comparing these calculated results with the experimental finding in the Case I for transformation from 6H to 3C, thermal treatment with presence of Si vapor at high temperatures suppresses VSi formation favoring 6H-stacking while it enhances VC formation to be occupied by N (NC) in addition to N substitution for C (NC) that transform 6H to 3C. The Case II can be also interpreted by considering NC. Thermal annealing for N-doped 4H-SiC favored by VC easily produces NC with N occupying VC and substituting for C to realize 3C-SiC bands at high N-doped region in 4H-SiC epilayers. These results suggest that NC is crucial for the transformation from hexagonal stacking such as 6H and 4H to cubic stacking 3C. In order to give physical interpretation for the transformation to 3C-SiC with NC, the interaction energies Jn between nth neighbor SiC double layers are estimated in the range of n ≤ 3 using the axial next nearest neighbor Ising (ANNNI) model and calculated total energies for SiC polytypes obtained in this study. The calculated results elucidate that the absolute value of positive J 1 (51.70 meV) with J 3 (9.92 meV) favoring 3C-stacking is much larger than that of negative J 2 (-16.66 meV) inducing hexagonal stacking. It should be noted that the Jn values are one order of magnitude greater than those of perfect SiC such as J 1 (2.33 meV), J 2 (-1.41 meV), and J 3 (0.51 meV). This implies that the layer interaction in SiC becomes strong and long-range due to N-doping. Considering coordinate of (|J 2|/J 1, J 3/J 1) in the ANNNI phase diagram, it is also found that (0.31, 0.19) for SiC with NC is located in the stable region of 3C and is more distant from the multi-phase degeneracy point (0.5, 0) inducing polytype and phase boundary between 3C and 4H than (0.61, 0.21) for perfect SiC. This clarifies that the large J 1 in SiC with NC stabilizes 3C-stacking to dominate the structural transformation to 3C due to N-doping. Furthermore, atomistic interaction between the nearest neighbor layers corresponding to J 1 is also qualitatively discussed in terms of electrostatic interaction between bond charges located at the center of interatomic bonds and that between ionic charges located at the lattice sites on the basis of a simple energy description and charge distribution obtained by ab initio calculations.

Asymmetric ferroelectricity and conduction of anomalous leakage current were observed in heteroepitaxial BaTiO3 thin films grown by rf magnetron sputtering at 600° C on three different electrode/substrate combinations: SrRuO3/SrTiO3, Pt/MgO and Nb doped SrTiO3. The voltage shift of hysteresis loops of the capacitance was a linear function of the thickness of the BaTiO3 films and became as large as 10 V in the film with a thickness of 410 nm. The asymmetry did not disappear even after a heat treatment carried out at 800° C in air. On the other hand, a steep increase in the leakage current was observed in the heteroepitaxial films when a positive voltage was applied. The origin of both the asymmetric hysteresis and the anomalous conduction is discussed in terms of asymmetric crystal structure caused by misfit dislocations introduced in heteroepitaxial growth.

Silicon carbide (SiC) nanomaterials are being used in a large range of fields because of their excellent mechanical strength, high thermal resistance and conductivity, high chemical stability and being semiconductors. In the present review, we evaluate SiC nanomaterials as membranes and adsorbents to separate different species. In addition, applications of SiC nanomaterials as membranes or in a membrane structure are described. In this regard, experimental studies which used porous SiC in membrane structure confirmed that SiC can enhance the performance of any membrane because of its large surface area, anti-fouling nature, high porosity, and high-water permeability. Next, molecular dynamics simulation studies examined the ability of SiC nanosheets and SiCNTs in water desalination, gas and dissolved molecule separations. Ultimately, based on the reviewed articles and their findings, areas requiring further attention by researchers are proposed.

Silicon carbide (SiC) material has many outstanding physical and mechanical properties such as high strength, high hardness, low density, high thermal conductivity, low thermal expansion coefficient, large band-gap, and excellent oxidation and corrosion resistances [1–3]. It is a leading material for components and devices operating at high temperature, high power and under harsh environments [4–5]. Micro-sized SiC particles and whiskers are commonly used as reinforcement materials for ceramics, metals and alloys in various structural and tribological applications [6–7].

Silicon carbide (SiC) is a promising semiconductor for high-power devices due to its superior material properties; high breakdown field, high electron saturation velocity, and high thermal conductivity. To implement SiC power devices, pn junction must be formed in the SiC. However, ion implantation for impurity doping has several issues for the SiC. For example, while a high-temperature (~1700 °C) post-implantation annealing is required to electrically activate implanted species [, it induces generation of crystallographic defects in the SiC, such as segregation of carbon atoms at the surface from the SiC bulk [. Therefore, development of new technology for local doping of SiC is highly demanded.

Sol–gel MgO thin films for insulation on SiC

Components for space telescopes using high quality silicon carbide (SiC) produced via the chemical vapor composite (CVC) process are currently under development. This CVC process is a modification of chemical vapor deposition (CVD) and results in a dramatic reduction in residual stress of the SiC deposit. The resultant CVC SiC material has high modulus, high thermal conductivity and can be polished to better than 1nm RMS surface roughness, making it ideal for space telescopes requiring lightweight, stiff and thermally stable components. Moreover, due to its lower intrinsic stress, CVC SiC is much more readily scaled to large sizes and manufactured into the complex geometries needed for the telescope assemblies. Results are presented on the optical figure for a lightweight 15cm CVC SiC mirror demonstrating low wavefront error (<30nm peak-to-valley and <5.1nm rms). Theoretical and experimental modal analysis measured the first four resonant modes of the mirror and found a first modal frequency in the vicinity of 2100 Hz, representing a highly stiff mirror.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Limitations of silicon (Si) based power electronic devices can be overcome with Silicon Carbide (SiC) because of its remarkable material properties. SiC is a wide bandgap semiconductor material with larger bandgap, lower leakage currents, higher breakdown electric field, and higher thermal conductivity, which promotes higher switching frequencies for high power applications, higher temperature operation, and results in higher power density devices relative to Si [1]. The proposed work is focused on design of a SiC gate driver to drive a SiC power MOSFET, on a Cree SiC process, with rise/fall times (less than 100 ns) suitable for 500 kHz to 1 MHz switching frequency applications. A process optimized gate driver topology design which is significantly different from generic Si circuit design is proposed. The ultimate goal of the project is to integrate this gate driver into a Toyota Prius plug-in hybrid electric vehicle (PHEV) charger module. The application of this high frequency charger will result in lighter, smaller, cheaper, and a more efficient power electronics system.

Silicon carbide samples synthesized from silicon by topochemical substitution of atoms are studied by the ion channeling method. The results of the analysis unambiguously demonstrate the occurrence of structural heteroepitaxy. The lattice of synthesized silicon carbide of hexagonal polytype 6H is epitaxially matched in the 〈0001〉 direction with the lattice grating grid array network of an initial substrate silicon in the 〈111〉 direction. The main features of structural self-coupling matching in this epitaxial heterocomposite are revealed. Despite the very large silicon carbide and silicon lattice parameter mismatch, the misfit dislocation density at the interface is low, which is a feature of the topochemical substitution method leading to comparable structures.

Silicon carbide (SiC) is a wide bandgap semiconductor and it exhibits excellent electronic and chemical properties. Fabrication of SiC devices on Si wafers of large diameter is desired to reduce their production cost. For the fabrication of electronic devices in the SiC layer, however, an electronic isolation between SiC and Si substrate is required because of the leakage current between the SiC and the substrate. Therefore the application of the SOI (Si on insulator) technique to the SiC on insulator (SiCOI) structure has been eagerly investigated [1, 2]. SiCOI structure has been investigated for the applications of piezo-resistive sensors and micro electromechanical systems (MEMS) operating at physically and chemically harsh environments [3]. SiC growth on SOI substrates, however, is very difficult owing to thermal instability of the thin top-Si layer. During the thermal annealing of SOI substrates at the substrate temperature lower than 1000°C depending on the top Si layer thickness, Si atoms agglomerate and the Si islands and voids would be formed [4, 5]. Because outdiffusion of the Si atoms into SiC layer is induced in the case of the SiC growth on Si layer, void formation takes place at lower temperatures than that in the case of the thermal annealing. The SiC growth at much lower temperature than 1000°C, therefore, is required. In our previous study, 3C-SiC epitaxial films were grown at 750°C by hot-mesh CVD, a kind of hot-wire CVD which utilizes the catalytic decomposition of source gases by heated tungsten (W) wires with a mesh structure [6], using monomethylsilane (MMS) as a source gas. In this paper, the epitaxial growth of 3C-SiC films on SOI substrates was investigated by the hot-mesh CVD method. And their piezoresistive property was measured for the application of a pressure sensor.

&lt;sec&gt;Silicon carbide (SiC) is widely used due to the lower coefficient of thermal expansion (CTE), high thermal conductivity and excellent mechanical properties. However, the self-diffusion coefficient of SiC relative to that of oxide ceramics is very low, it is difficult to sinter at lower temperature. The &lt;i&gt;β&lt;/i&gt;-spodumene has ultra low or even negative thermal expansion coefficient combined with good thermal and chemical durability, which melts at 1423 ℃. Accordingly, the present study focuses on the use of &lt;i&gt;β&lt;/i&gt;-spodumene as a flux at lower sintering temperature and the preparation of lower CET composite ceramics. The effects of spodumene on the sintering behavior, phase relations, thermal expansion and mechanical properties of spodumene/silicon carbide composites are discussed.&lt;/sec&gt;&lt;sec&gt;A high pure &lt;i&gt;β&lt;/i&gt;-spodumene LiAlSi&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;6&lt;/sub&gt; compound with nearly zero thermal expansion coefficient is synthesized via solid phase sintering. Spodumene/silicon carbide composites are fabricated by the adding 25, 30, 35 and 40 mass% synthesized &lt;i&gt;β&lt;/i&gt;-spodumene powder to 75, 70, 65 and 60 mass% &lt;i&gt;α&lt;/i&gt;-SiC matrix, respectively. Both &lt;i&gt;β&lt;/i&gt;-spodumene and SiC are fabricated by conventional pressureless liquid sintering technique, and the batches are uniaxially pressed into discs and rectangular bars, then sintered at 1550 ℃ for 2 h in an Ar atmosphere.&lt;/sec&gt;&lt;sec&gt;The results show that the SiC and &lt;i&gt;β&lt;/i&gt;-spodumene do not react during sintering, and the &lt;i&gt;β&lt;/i&gt;-spodumene changes from tetragonal phase into hexagonal phase, the cell volume has a considerable shrinkage. A certain amount of liquid phase can help to enhance the density, improve Young’s modulus and promote the sintering behavior of SiC. When the feedstock contains 35% &lt;i&gt;β&lt;/i&gt;-spodumene, the Young’s modulus reaches to (204.2 ± 0.5) GPa. Excess porosity is formed when liquid phase is too much during sintering, The Young's modulus of the sample 40SP is (119.6 ± 0.5) GPa. It is determined that the Young’s modulus of these materials are affected by porosity and internal microcracks. This study indicates that the content of &lt;i&gt;β&lt;/i&gt;-spodumene and porosity are the dominant factors to control the CET of composites, but the porosity has a stronger effect. Besides, the microcracks, which are formed by the interaction of various internal stresses, are also an impotant factor. The materials with nearly zero thermal expansion are developed in a lower temperature range from –150 ℃ to 25 ℃, the spodumene content in the most stable composite reaches 40 mass%, and the CET of composite is close to that of Si (α&lt;sub&gt;25 ℃&lt;/sub&gt; = 2.59 × 10&lt;sup&gt;–6&lt;/sup&gt; ℃&lt;sup&gt;–1&lt;/sup&gt;) in a temperature range of 25–480 ℃.&lt;/sec&gt;

Abstract. In the current work, the effect of copper (Cu), manganese (Mn), and silicon carbide (SiC) nanoparticles on the thermal stability and conductivity of glass fiber-epoxy resin composites was discussed. Thermogravimetric Analysis (TGA) was done as per ASTM E 1131 to study the thermal degradation of composites from 30°C to 800°C. Thermal conductivity increases with the percentage of fillers of SiC, Mn, and Cu nanoparticles in the GFER polymer composites. The combination of epoxy, Glass Fibre and SiC nanoparticle-loaded composites was found highest thermal conductivity. TGA discloses that for SiC, Mn and Cu NPs glass fibre epoxy resin composites thermal stability increased with the percentage weight fraction of nanoparticles. The addition of nanoparticles to glass fibre epoxy resin polymer composites causes an increase in thermal properties due to the nanoparticles acting as catalysts for decomposition and less mass loss observed in TGA. The Thermal Conductivity of 20% SiC nanoparticles were found to be 0.37 W/mK. In Overall comparison, SiC nanoparticle-loaded composites exhibit significantly improved thermal stability and are more suited where the heat transfer plays the predominant role in high thermal conductivity and more thermal stability application fields.

The advantage of Silicon Carbide (SiC) based devices are less thermal management requirements and smaller passive components which result in higher power density. SiC devices have higher blocking voltages, lower on-state resistance and switching losses and higher thermal conductivity and operating temperatures. SiC devices can operate at higher voltages, higher frequencies and higher junction temperatures than comparable Si devices, which results in significant reduction in weight and size of the power converter and increase in system efficiency.