Materials Science Forum Vol. 711

Abstract: Recent research efforts in growth of 3C-SiC are reviewed. Sublimation growth is addressed with an emphasis on the enhanced understanding of polytype stability in relation to growth conditions, such as supersaturation and Si/C ratio. It is shown that at low temperature/supersaturation spiral 6H-SiC growth is favored, which prepares the surface for 3C-SiC nucleation. Provided the supersaturation is high enough, 3C-SiC nucleates as two-dimensional islands on terraces of the homoepitaxial 6H-SiC. Effect of different substrate surface preparations is considered. Typical extended defects and their electrical activity is discussed. Finally, possible novel applications are outlined.

Abstract: In this work we report on the study of twin boundary (TB) evolution during heteroepitaxial growth of 3C-SiC on patterned 4H-SiC(0001) substrate by vapour-liquid-solid (VLS) mechanism. Ge₅₀Si₅₀ melt was used at a temperature of 1450°C. 3C-SiC deposit was obtained on top and outside the mesas. Some lateral enlargement of these mesas was observed but it was systematically homoepitaxial. Elimination of TBs inside the 3C-SiC deposit on top of the mesas was observed for specific mesa shape and/or orientation of the sidewalls. Though three–fold or six-fold symmetry mesas are recommended for TB elimination, originally circular mesas lead also to the same result due to initial faceting toward hexagonal shape.

Abstract: The hetero epitaxial growth of 3C-SiC on nominally on-axis 4H-SiC is reported. A horizontal hot-wall CVD reactor working at low pressure is used to perform the growth experiments in a temperature range of 1200-1500 °C with the standard chemistry using silane and propane as precursors carried by a mix of hydrogen and argon. The optimal temperature for single-domain growth is found to be about 1350 °C. The ramp up-conditions and the gas-ambient atmosphere when the temperature increases are key factors for the quality of the obtained 3C layers. The best pre-growth ambient found is carbon rich environment; however time of this pre-treatment is crucial. A too high C/Si ratio during growth led to polycrystalline material whereas for too low C/Si ratios Si cluster formation is observed on the surface. The addition of nitrogen gas is also explored.

Abstract: In this work we report on the growth of cubic silicon carbide using CBr₄ and silane as precursors at different C₃H₈/CBr₄ flow ratios. The layers were deposited on 2’’ (001) Si wafers by means of the VPE technique in a horizontal cold-wall reactor with induction heating. The growths were performed at atmospheric pressure, in H₂ atmosphere and involved several steps: Si thermal etching; carburisation; epitaxial growth. Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were performed to observe the film morphology and defects and correlate them with the gas phase composition. Results show that the addition of CBr₄ to the standard SiH₄ and C₃H₈ precursor can change the crystalline nature and the morphology of the grown SiC.

Abstract: In this work, we focus our attention on the characterization of 3C-SiC films, grown within a CVD reactor, on Si substrates. It will be shown how the growth procedures influence the SiC film structure and quality with the growth rate used during the growth used as example. Evaluation of crystal structure has been conducted by X-Ray Diffraction (XRD), Raman microscopy and Transmission Electron Microscopy (TEM). Overall film quality increases if films are grown under low growth rate conditions, thanks also to an important reduction in the density of micro-twins. The trend of the full widths at half maximum (FWHMs) of SiC rocking curves, considered good ‘quality indicator’ as their broadenings are affected by crystallographic defects, as a function of 3C-SiC thickness shows a saturated regime for very thick films, due to the saturation of stacking fault density after 50 μm of growth. This work wants to suggest a reasonable path for the characterization of the material structure that can be useful, anywhere and in any time, to assess if the morphology and microstructure of our films are satisfactory and to drive towards the desired improvement.

Abstract: 3C-SiC lattice parameters, both in-plane and out-of-plane, have been studied as a function of the temperature (up to 773 K) by performing X-Ray Diffraction (XRD) measurements in coplanar and non-coplanar geometry during the thermal treatments. A tetragonal distortion of the 3C-SiC cell has been observed, with a=b≠c, resulting from a tensile stress status induced by the presence of Si substrate. A linear expansion coefficient of about 4.404 × 10^-6 K^-1 at 773 K has been obtained for a 15 μm thick 3C-SiC film grown on (100) Si substrate. The discrepancy with the value reported in literature of 5.05 × 10^-6 K^-1 at 800 K [Slack et al., Journal of Applied Physics 46, 89 (1975)] may be related to the different nature of samples used.

Abstract: The crystal growth of 3C-SiC onto silicon substrate by Vapour-Liquid-Solid (VLS) transport has been investigated. In the studied growth configuration, propane feeds a SiGe liquid phase contained in 10 µm-deep etched trenches on the Si substrate. Before SiGe deposition, the substrate surface and the trench walls were coated with a thin (100 - 200 nm) CVD-grown 3C-SiC seeding layer. For the VLS growth, the temperature was increased up to 1280°C, above the SiGe alloy melting point, at which point propane was added to start VLS growth. X-ray diffraction shows that some SiC is grown epitaxially onto the CVD seeding layer. However, cross-section SEM observations have evidenced that SiC has grown as trapezoidal islands and not as an uniform layer. Backscattered electron images also clearly show a deep penetration of germanium into the substrate through the SiC seeding layer. This penetration was found to be strongly reduced when increasing the seeding layer thickness from 100 to 200 nm.

Abstract: The design, fabrication and characterisation of flexural-mode 3C-SiC resonators are presented. Single-clamped, double-clamped beams and circular membranes, with aluminium and platinum electrodes patterned on top, have been implemented. The structures’ resonant frequency has been investigated optically when the devices are actuated mechanically and electro-thermally. Electro-thermal mixing of two input electrical signals with two different frequencies has been demonstrated. The temperature stability of the 3C-SiC resonators has been characterised.

Abstract: SiC is a candidate material for micro- and nano-electromechanical systems (MEMS and NEMS). The hetero-epitaxial growth of 3C-SiC on silicon substrates allows one to overcome the traditional limitations of SiC micro-fabrication, but the high residual stress created during the film grow limits the development of the material for these applications. In this work, in order to evaluate the amount of residual stress released from the epi-film, different micro-machined structures were developed. Through the measurement of natural resonant frequencies and Raman shift analysis, a strong relationship between the mechanical proprieties of the material (Young’s modulus) and the film crystal quality (defect density) was observed.