| dc.description.abstract | In this thesis, we present a novel laser-assisted chemical vapor deposition (LACVD) system for growing Si nanoclusters embedded in a Si oxide matrix at a room temperature. This LACVD system was constructed by combining a conventional plasma enhanced chemical vapor deposition (PECVD) system and a CO2-excited laser. A gaseous mixture of SiH4 and N2O was used for the reactants.
In view of the point of a high absorption coefficient of a wavelength of 10.6 μm by SiH4, an external CO2 laser beam was guided into a conventional PECVD system. A CO2 laser with various powers was used in a conventional PECVD system to assist the decomposition of SiH4 molecules and also to separate the Si clusters from Si oxide matrix. The phase separation model was proposed to discuss the inconsistency between the calculated and actual composition of O/Si atomic ratio. The dependencies of the photoluminescence (PL) photon energy and intensity on the CO2 laser power were observed. The mixture model of both pure Si and pure oxide phases were also used for calculating the refractive indices of the matrix. These results have demonstrated the existence of laser-induced phase-separated Si nanoclusters and the relation between the sizes of Si nanoclusters and the CO2 laser power were also elucidated.
Varying N2O gas ratio (defined as N2O/total gases) from 0.6 to 0.75 caused the observed PL light emission to vary from 1.65eV to 1.92eV without post-annealing. A phase-separated model and PL measurements were used to identify Si nanoclusters embedded in Si suboxides. Furthermore, the average dot sizes of the films grown by the LACVD system with XN2O=0.6, 0.67, 0.71, and 0.75 are identified by images of transmission electron microscope (TEM) as 3.7, 3.2, 3.0, and 2.7 nm, respectively.
Further, the optical characteristics and annealing behaviors were also studied. The PL emission intensity increased with the annealing temperature until some temperature, then decreased as the annealing temperature increased. Since defect centers in Si suboxides would be gradually annihilated during thermal annealing process, the PL emission intensity originated from defect centers would be reduced with the annealing temperature. The quantum confinement effect is found as the dominant PL emission mechanism in Si nanoclusters embedded in Si suboxides.
Si nanoclusters would be separated from the Si suboxide matrices due to the decomposition assistance of SiH4 by CO2 laser. Raman spectroscopy and photoluminescence measurements of the as-deposited and annealed films were used to identify the Si nanoclusters embedded in the Si suboxide matrices were identified. By studying the light emission behaviors, we deduce that the quantum confinement effect is the dominant mechanism.
A degradation of PL intensity by irradiating the sample with helium-cadmium (He-Cd) laser was also observed. The dependence of PL degradation on long-term irradiation of He-Cd laser was investigated. We found that He-Cd laser induced breakage of Si-H related bonds resulted in Si dangling bonds such as D centers and Pb centers, which are known to reduce the PL intensity. The PL intensity of the He-Cd laser irradiated samples can be improved to the level of as-deposited samples after subjecting the samples to a H2 ambient at 400 °C for 5min. Post-annealing in H2 could also help to increase the PL intensity plays by passivating the defect centers in as-deposited samples. | en_US |