Electronic Structures and Optical Properties of 6H- and 3C-SiC Microstructures and Nanostructures from X-ray Absorption Fine Structures, X-ray Excited Optical Luminescence, and Theoretical Studies

Abstract

We report a comparative study of the electronic and optical properties of five silicon carbide (SiC) materials of different crystal structures (6H and 3C polytypes), sizes (micro- and nanocrystals), and morphologies (nanowires of SiC−SiO2 core−shell structures and oxide-free nanowires). X-ray absorption near-edge structures (XANES) at both Si K- and C K-edge have been used to investigate the electronic structures of SiC. Theoretical calculations using density functional theory (DFT), the WIEN2k code are in good accord with the experiment. It is found that both 6H- and 3C-SiC have similar XANES at Si K-edge hence similar local structure at the Si site but slightly and more noticeable difference at the C K-edge, which is due to the difference in band gaps among the polytypes. The spectra of core−shell nanowires are found to have both SiO2 and SiC contributions in which SiO2 is dominant. The SiO2 shell can be almost completely removed by hydrofluoric acid treatment. X-ray excited optical luminescence (XEOL) was used to measure light emission from SiC by tuning the excitation photon energy across both Si K- and C K-edge. Luminescence was observed from all SiC samples upon X-ray excitation, though at different wavelengths depending on the crystal structures and morphologies. By measuring the yield of the total luminescence (300−800 nm) and luminescence at selected wavelengths with excitation photon energy across the absorption edge, we are able to ascribe the luminescence at various wavelengths to surface states and defects, quantum confinement, and SiO2 surface/SiC-SiO2 interface in core−shell nanowires. The implications of these findings are discussed.