The structural and optical characteristics studies of quantum nanostructures for single-electron transistors materials

Abstract

Silicon (Si) nanocrystals embedded in Si dioxide (SiO2) matrix has attracted a strong interest due to its ability to emit light in visible spectral region, simplicity of producing, and compatibility with present technology. This thesis discusses the structural and optical properties of Si nanocrystals formed by rapid thermal annealing of sub-stoichiometric Si oxide films (SiOx, x<2) with large excess of Si. The SiOx thin films were deposited with co-sputtering of SiO2 and Si target using radio frequency magnetron sputtering. The Si-to-SiO2 ratio in SiOx was controlled by varying the deposition parameters, including the deposition time, substrate temperature, deposition power, and number of Si targets being placed on the SiO2 target. Subsequently, films were rapid-thermal-annealed in nitrogen gas for two minutes at various temperatures. High temperature annealing induced the decomposition of SiOx into Si nanocrystals and SiO2. Surface morphology analysis of as-deposited films with atomic force microscope shows that optimum surface roughness and grain size were obtained at 180 W, 100 oC and 120 minutes deposition. As estimated from the infra-red (IR) absorption peak, x-value in SiOx decreased by increasing the number of Si targets, deposition power and substrate temperature, but increased with increasing deposition time. X-ray diffraction study revealed that Si nanocrystals with 22 nm to 7 nm in diameter and preferred (111) orientation were formed in SiOx films with x<1.85 after annealing at 1000 oC and above. The average size of nanocrystals increased as the excess Si concentration and the annealing temperature increased. Films consisted of nanocrystals exhibit red-to-IR emission, which corresponded to band energy of about 1.5 eV, indicating the bandgap enlargement of nanocrystals compared to bulk Si. The presence of Stokes shift between absorption and emission suggested that emission was originated from radiative recombination via radiative centres associated with the surface states of nanocrystals. The bandgap of Si nanocrystals is tunable by controlling its size.