Monocrystalline 6H-SiC Research Articles

Equilibrium shapes and surface selection of nanostructures in 6H-SiC

The equilibrium shape of 6H-SiC nanostructures and their surfaces were studied by analyzing nano-void (∼10 nm) shapes, which were introduced in monocrystalline 6H-SiC by high-temperature neutron irradiation, using transmission electron microscopy. The nano-voids were determined to be irregular icosahedrons truncated with six {1¯100}, twelve {1¯103}, one smaller top-basal, and one larger bottom-basal planes, which suggests that {1¯100} and {1¯103} are the next stable surface class after the basal planes. The relatively frequent absence of the {1¯100} surface in the nano-voids indicated that the (1¯103¯) surface type is energetically rather stable. These non-basal surfaces were found not to be atomically flat due to the creation of nanofacets with half unit-cell height in the c-axis. The {1¯100} and {1¯103} surfaces were classified as two and four face types according to their possible nanofacets and surface termination, respectively. We also discuss the surface energy difference between the (1¯103¯) and (1¯103) face types in relation to the energy balance within the equilibrium, but irregular, polyhedron, in which the (1¯103) surface had double the surface energy of the (1¯103¯) surface (∼3900 erg/cm2).

Experimental study of surface performance of monocrystalline 6H-SiC substrates in plane grinding with a metal-bonded diamond wheel

Monocrystalline SiC has long been recognised as an ideal third-generation semiconductor material for application in situations where there is high frequency, high power and high voltage. Because of the hardness, stiffness and strength of monocrystalline SiC, these materials are extremely difficult to machine in order to produce the extremely flat, smooth and damage-free surfaces required in many industrial applications. This paper presents the results of plane grinding experiments which were carried out using a metal-bonded diamond wheel. The process parameters of these have demonstrated a significant influence on surface roughness. Surface quality improved and roughness decreased as the wheel feed rate decreased. On the substrate surfaces, the particle trajectory density increased closer to the centre, producing better surface quality. At large feed rates, the material removed is mainly brittle fracture, and the plough scratch widths and gully depths on the ground surface are greater. At small feed rates, plastic removal predominates, becoming more significant closer to the centre and giving a smoother surface. Median and transverse cracks occur in the subsurface damaged layer, and the crack depths are consistent with the surface roughness Rz and gully morphology. Then, the critical grit depth of cut and the maximum undeformed substrate thickness values under different grinding conditions were calculated in order to analyse the characteristics of the subsurface damage and the material removal mechanism. Finally, an optimised process was achieved using orthogonal experiments, with processing parameters for rough, half-fine and fine grinding. This process gives flat substrates with surface roughness Ra 0.012 μm, SSD < 4 μm and TTV < 3 μm, which achieves the same level of precision as lapping.

Material removal mechanism of cluster magnetorheological effect in plane polishing

A cluster magnetorheological (MR) plane polishing based on a combination of magnetorheological finishing (MRF) and cluster mechanism (arrange in a regular array) has been developed for large planar optical workpiece. In this paper, the polishing pressure of an individual MR micro-grinding head on workpieces was investigated, and the mathematical model for cluster MR plane polishing was established based on the Preston equation and the properties of the workpiece material. An arc-shaped polishing belt was processed on monocrystalline 6H-SiC and Si wafer by the cluster MR polishing apparatus which polishing disk inlaid with cylindrical flat bottom magnetic pole arrange in the same direction regularly and monocyclically. The profile of arc polishing belt’s cross section and the surface morphology were observed and analysed to verify the obtained model. The abrasives with different size were found to have an equal effect on the processed surface because of the accommodated-sinking effect of the polishing pad constituted by the cluster MR micro-grinding head. It was found that the monocrystalline 6H-SiC and Si wafer were polished in a plastic mode with a smooth, damage-free surface.

An investigation of terahertz response in monocrystalline 6H-SiC for electro-optic sampling

We theoretically investigate the feasibility of terahertz detection via electro-optic (EO) sampling using 6H-SiC single crystal. The frequency response is simulated based on the principle of phase-matching condition. The optical dispersion of 6H-SiC was calculated by Sellmeier equation. In collinear incidence approach, the THz detectable bandwidths are simulated by a frequency response function at different excitation wavelengths. The cut-off frequency as a function of crystal thickness is revealed. In non-collinear incidence approach, the phase-matching condition can be achieved by using a silicon prism to couple the THz radiation into 6H-SiC single crystal. The crossing angle between THz radiation and incident optical beam is subject to the THz dispersion of Si prism and group index of 6H-SiC. The relation between THz coherence length and crossing angle is discussed. Both approaches display that 6H-SiC performs a broadband THz response for EO sampling at 515 nm. The sensitivity of EO sampling of 6H-SiC is triple times higher than GaP. In combination of the high critical breakdown field, 6H-SiC is consider to be a promising candidate for detecting high field THz radiation.

Structural evolution of SiC nanostructured and conventional ceramics under irradiation

Nanostructured ceramics are envisaged for next generation nuclear reactors. In this study, we performed irradiation experiments on nanostructured and microstructured 3C-SiC samples, with 95MeV Xe ions at room temperature. This energy permits the observation of the combined electronic and nuclear interaction with matter. The structural evolution of the sample under irradiation was followed by grazing incidence X-ray diffraction and transmission electron microscopy. The results do not reveal a complete amorphization, despite value of displacement per atom overcoming the total amorphization threshold determined for the monocrystalline 6H-SiC at room temperature with low energy ions. This may be attributed to competing effects between nuclear and electronic energy loss in this material.

Effect of Test Temperature and Strain Rate on the Yield Stress of Monocrystalline 6H-SiC

Effect of Test Temperature and Strain Rate on the Yield Stress of Monocrystalline 6H-SiC

Effect of Test Temperature and Strain Rate on the Yield Stress of Monocrystalline 6H-SiC

The critical resolved shear stress for activating the (0001) 〈21-1-0〉 slip system of monocrystalline 6H-SiC has been determined as a function of test temperature and strain rate via constant-displacement compression tests. Tests were conducted at temperatures between 550 and 1300 °C at strain rates of 1.3×10—4, 6.3×10ü—5 and 3.1×10ü—5 s—1. The current study shows that 6H-SiC crystals can be plastically deformed via relatively modest resolved shear stresses on the basal plane at temperatures as low as ≈︂550 °C. For temperatures below ≈︂1300 °C for the fast and intermediate strain rates, and for temperatures below ≈︂1100 °C for the slow strain rate, the stress exponent n, and the activation enthalpy H(2.1 ± 0.7) eV, respectively. At higher temperatures at the slowest strain rate, the activation enthalpy was determined to be (4.5 ± 1.2) eV. Subsequent to the deformation tests, transmission electron microscopy (TEM) was used to rationalize some of the results.

Experimental and Theoretical Analysis of the High Temperature Thermal Conductivity of Monocrystalline SiC

The thermal properties of monocrystalline 6H-SiC have been studied within the temperature range 300-2300 K. The thermal diffusivity a was measured with the laserflash method up to 2300 K. The specific heat capacity c{sub p} was determined from room temperature up to 1650 K. From thermal diffusivity data, the c{sub p} data fitted by the Debye-model and the density {rho} taken partly from measurements and partly from literature the thermal conductivity {kappa} was evaluated from {kappa} = a(T) x {rho}(T) x c{sub p}(T) to give {kappa}/[Wm{sup -1}K{sup -1}]{approx_equal}451700 x (T/[K]){sup -1.29}. The theoretical analysis of the given high temperature thermal conductivity data and of low temperature data taken from literature shows, that the phonon contribution is dominant within the whole temperature range (8-2300 K). Within a Callaway analysis a Grueneisen-parameter {gamma} = 1.5 was found. Unipolar and bipolar electronic contributions to the thermal conductivity seem to be negligible. At temperatures {>=}1500 K additionally heat transport by radiation becomes relevant. (orig.) 15 refs.

SiC Epitaxial Growth on Carbon

AbstractThe possibility of single crystal SiC expitaxial growth on freestanding amorphous carbon films (500–1000 Å) as well as thin amorphous carbon layers deposited on mono-crystalline SiC seeds, by conventional physical vapor transport (PVT) technique, is demonstrated. Preliminary experiments indicate that under certain specific growth conditions, 3D SiC single crystals (100 – 600 Å) of different polytypes can be grown on freestanding amorphous carbon layers, with more or less equal probability of formation for each polytype. On the other hand, under low axial temperature gradients (&lt; 30°C/cm), the SiC epitaxial growth on carbon is amorphous in nature. Also, experimental results that demonstrate two-dimensional single crystal SiC epitaxial growth on an amorphous carbon film deposited on mono-crystalline 6H-SiC wafer, is presented. Experiments performed in our laboratory indicate that monocrystalline SiC growth is possible on amorphous carbon layers upto 0.1 μm thickness.

Experimental Study of Ternary Pd-Si-C Phase Equilibria and Pd/SiC Interface Reactions

The phase diagram of Pd-Si-C was investigated by experimental methods (XRD, SEM/EDX) and approximate thermodynamic calculations. Based on these results several isothermal sections between 700 and 1000°C are developed. Bulk diffusion couples between Pd and monocrystalline SiC were studied in the same temperature range, 700 to 900°C. A comparison of reaction behaviour between monocrystalline 6H-SiC and ceramic HIP-SiC with Pd is given. The formation of periodic bands consisting of Pd 2 Si + C and Pd 2 Si is clearly observed in the Pd/6H-SiC diffusion zone. These periodic bands are obscured in the Pd/HIP-SiC diffusion couples. In addition, a fine periodic substructure in the two-phase layer (Pd 2 Si + C) is detected. Furthermore, at least a modest joining pressure may be required to produce the reaction zone and the periodic bands. The sequence of reaction layers as observed in the diffusion couples, SiC/C+Pd 2 Si/Pd 2 Si/-/C+Pd 2 Si/Pd 2 Si/Pd 3 Si/Pd, could be related to the isothermal section at 800°C. At 850 °C a liquid phase is formed in the diffusion zone. Interpretations pertaining to the observed diffusion paths are discussed based on the ternary solid state equilibria.

Solid State Phase Equilibria and Reactive Diffusionin the Cr-Si-C System

The phase diagram of Cr-Si-C was investigated by experimental methods (XRD, SEM/EDX) and approximate thermodynamic calculations. Based on these results the stability of the ternary phase Cr 5 Si 3 C (τ) up to 1400°C is confirmed and a tie line between Cr 3 C 2 and SiC is proposed. Bulk diffusion couples of Cr on monocrystalline 6H-SiC were prepared by a special deposition scheme and the interface reactions were studied in the temperature range between 1000 and 1200 °C. The sequence of reaction layers as observed in the diffusion couples, Cr/Cr 23 C 6 /Cr 7 C 3 /Cr 7 C 3 + Cr 3 Si/Cr 3 Si/Cr 5 Si 3 C/SiC, could be related to the isothermal section at 1000°C. Three stability diagrams were constructed for the Cr-Si-C system at 1000 °C. Interpretations pertaining to the observed diffusion paths are discussed based on the ternary solid state equilibria, thermodynamic calculations and the stability maps.

Temperature-dependent interface reactions and electrical contact properties of titanium on 6H-SiC

In this study the system Ti-Si-C was investigated in terms of two aspects, metallurgical and electrical, in order to understand the formation and the properties of Ti electrical contacts on n-type 6H-SiC. For the metallurgical investigation bulk diffusion couples were prepared from monocrystalline 6H-SiC and Ti and annealed between 700 and 1200 degrees C for different lengths of time. The reaction zones were investigated using a SEM (secondary electron and backscattered electron images as well as energy-dispersive X-ray analysis). For the investigation of the electrical properties Ti contacts were sputter-deposited onto 6H-SiC wafer stripes and annealed at similar temperatures. The contact properties were measured in terms of current-voltage characteristics. We discovered that, over the whole temperature range investigated, the reaction layer growth follows a parabolic growth law which is thermally activated. Above 1200 degrees C the diffusion path from SiC to Ti is SiC/Ti3SiC2/Ti5Si3/two-phase Ti5Si3+TiC1-y/Ti5Si3/Ti. The contacts show ohmic behaviour. Between 1000 and 800 degrees C the diffusion path is: SiC/Ti3SiC2/Ti5Si3/two-phase Ti5Si3+TiC1-y/Ti3Si/Ti. The contacts are also ohmic. Below 700 degrees C the diffusion path is SiC/TiC1-y/two-phase Ti5Si3+TiC1-y/Ti3Si/Ti. The contacts are of Schottky type. It is concluded that the phase in contact with the SiC determines the electrical properties of the junction and the possibility of manufacturing a complete Schottky diode using n-type 6H-SiC and Ti only is demonstrated.

Reactive diffusion between monocrystalline 6H-SiC and chromium

Interface reactions between chromium and monocrystalline 6H-SiC were studied in the temperature range of 1000–1200°C for 24–192 h duration. The reaction layers were characterised by XRD and SEM/EDX techniques. The sequence of reaction layers as observed in the diffusion couples, Cr/Cr23C6/Cr7C3/Cr7C3 + Cr3Si/Cr3Si/Cr5Si3C/SiC, could be related to the isothermal section at 1000°C. A stability diagram was constructed for the Cr-Si-C system at 1000°C, based on the available thermodynamic data by plotting the logarithm of activities of C as a function of relative mole fractions of X(Si) and X(Cr). Based on the thermodynamic calculations and the stability map, interpretations pertaining to the observed diffusion paths are discussed.

Growth rate and surface microstructure in α(6H)–SiC thin films grown by chemical vapor deposition

Monocrystalline 6H-SiC thin films have been epitaxially grown on off-axis 6H-SiC {0001} substrates in the temperature range of 1623–1873 K via chemical vapor deposition. The growth rate was a strong function of the growth temperature and the reactant gas concentration. The activation energies for growth were 64 kJ/mole and 55 kJ/mole for the (0001) Si face and the (0001) C face, respectively. The concentration of growth pits in the films increased as a function of decreasing deposition temperature, increasing concentration of reactant gases and increasing off-axis orientation. Beta-SiC islands were also observed in the epilayers when the (SiH4 + C2H4)/H2 ratio was ≥2.5:3000.

Chemical vapor deposition and characterization of 6H-SiC thin films on off-axis 6H-SiC substrates

High-quality, monocrystalline 6H-SiC thin films have been epitaxially grown on 6H-SiC {0001} substrates which were prepared 3° off-axis from 〈0001〉 towards 〈112̄0〉 at 1773 K via chemical vapor deposition (CVD). Essentially, no defects were generated from the epilayer/substrate interface as determined by cross-sectional transmission electron microscopy (XTEM). Double positioning boundaries which were observed in β-SiC grown on 6H-SiC substrates were eliminated as confirmed by plan-view TEM. A strong dependence of the surface morphology of the as-grown thin films on the tilting orientation of the substrates was observed and reasons for this phenomenon are discussed. The unintentionally doped 6H-SiC thin films always exhibit n-type conduction with a carrier concentration on the order of 1016 cm−3. Au-6H-SiC Schottky barrier diodes were fabricated on the CVD 6H-SiC thin films and it was found that the leakage current at a reverse bias of 55 V was only 3.2×10−5 A/cm2. This is compared to SiC films grown on other substrates.