Pulsed Laser Depostion of Silicon Carbide Thin Films: Growth, Bonding and Thermal Characterization

Abstract

Silicon Carbide (SiC), in its crystalline form, in a wide band-gap semiconductor that has attracted much interest as a potential replacement of Si silicon in high power / high temperature devices with a wide range of modern technological fields. Its amorphous counterpart (a-SiC) has also its fair share of applications because of its tuning optical, electronic and mechanical properties. Recently, there has been greater attention directed towards the study of the thermal properties of such materials from both the fundamental and technological aspects. In this thesis, we have achieved the synthesis of silicon carbide thin films on Si substrates by pulsed laser deposition with the aim of investigating their thermal properties as the nano-structure of the films transitions from amorphous to crystalline depending on growth conditions. The effects of the several experimental process parameters (laser repetition rate, growth pressure and temperature) on the nature of the grown films were revealed through several materials characterization techniques. Surface profilometry was used to measure film thickness and growth rate while surface morphology was imaged using Scanning Electron Microscopy (SEM). X-Ray Diffraction (XRD) and Raman Spectroscopy allowed the assessment of crystallinity and the determination of the type of bonding of the films, respectively. We used a photothermal-beam-deflection technique (laser flash) to determine the thermal diffusivity of the grown layers. We were able to develop synthesis process that is reproducible where the growth rate depends almost linearly on laser repetition rate up to 25 Hz. Process controllability becomes problematic at higher repetition rate such as 50 Hz. We have also found that growth rate is significantly reduced at deposition pressures of 10 mTorr and above, due to the scattering of the ablated species by the background Ar gas. Deposition temperature plays a crucial role in determining the bonding configuration in the layers. At 300 oC, the grown layers are amorphous with the onset of crystallites formation detected at 600 oC. Fully crystallized layers are observed at 950 oC. The thermal diffusivity was determined at the onset of crystallization in the SiC layers, and a value of 1.12 cm2/s was reported, showing a significant enhancement compared to the Si substrate (0.88 cm2/s). These results are very encouraging as they pave the way towards a full investigation of the evolution of the thermal properties of unhydrogenated silicon carbide materials as their nanostructure evolves from an amorphous to a fully crystalline state.