| In spite of tremendous challenges, Si is still believed to be of significant importance in the field of optoelectronics. Great interest in light emitting Si materials and Si-based optoelectronic devices has been aroused by the optical properties, especially the characteristics of light emission exhibited by various nanostructured Si materials, which are very different from those of bulk Si. Among these nanostructured Si materials, Si nanocrystals embedded in SiO2 (Si-NCs/SiO2) is one of the hopeful light-emitting Si materials which can most likely get a breakthrough in optoelectronics in the near future.In this dissertation for master degree, the transition from amorphous silicon oxide (a-SiOx) to Si-NCs/SiO2 is studied with Si-NCs/SiO2 as the object material. Si-NCs/SiO2 is prepared by depositing a-SiOx and post-annealing the deposited a-SiOx. With the emphasis of the processes occurring in and the mechanisms responsible for the phase separation in a-SiO2 and the formation of Si nanocrystals (Si-NCs), the transition from a-SiOx to Si-NCs/SiO2 is studied through the characterization of the structure of the as-deposited and annealed samples and the examination of the properties of the samples before and after annealing.Using pure Si as the target material, a-SiOx with amorphous structure and O content x of about 1.2 is deposited by pulsed laser deposition in O2 ambient. With its refractive index of～1.7 and extinction coefficient near zero (～10-4) in the near infrared (NIR) region, the as-deposited a-SiOx shows transparency with optical transmittance over 80% in the visible-NIR region and strong absorption in the ultraviolet (UV) region with the absorption coefficient increasing sharply as the wavelength decreases. At UV light excitation, the as-deposited a-SiOx emits very weak luminescence originating from the defects in the sample. After annealing at 1100℃in N2 atmosphere for 60 min, Si-NCs/SiO2 is formed from a-SiOx, with the mean crystal radius of 4.5 nm and the matrix of SiO2. The annealing process gives rise to a decrease in refractive index (1.65) and an increase in extinction coefficient by one order of magnitude, while the optical transmittance and absorption coefficient remain almost unchanged. The prepared Si-NCs/SiO2 shows excellent characteristics of light emission. At UV light excitation, the prepared Si-NCs/SiO2 emits strong Si-NCs featured luminescence. Our result also show that a-SiOx with x of about 1.2 deposited at 0.7 Pa O2 is optimum in emitting luminescence after annealing.An investigation is performed for the transition from a-SiOx to Si-NCs/SiO2 during thermal annealing by examining the influences of annealing temperature and annealing time on the structure and the properties of the annealed samples. It is found that the transition process is not a simple process of phase separation as is commonly referred to. It includes the precipitation of the excess Si in a-SiOx and the crystallization of the precipitated Si, becoming Si-NCs embedded in the remaining SiO2 matrix. The analysis of Fourier transform infrared spectroscopy evidences that the precipitation of the excess Si and the crystallization of the precipitated Si occur under different conditions and with different rates. The Si precipitation occurs at about 500℃, with the precipitation extent increasing with the annealing temperature, and nearly completes upon annealing at about 1000℃. The results of Raman scattering spectroscopy show that the temperatures for the precipitated Si to crystallize are much higher. At a temperature～1000℃, only partial crystallization starts but is negligible compared to Si precipitation.1100℃is the most appropriate temperature for an efficient Si crystallization and Si-NCs formation, while at higher temperatures the crystals grow too large to emit luminescence. The results of the structure characterization also show that even at the most appropriate temperature, the crystallization rate of the precipitated Si is much lower than the precipitation rate of the excess Si. After annealing for 10 min, there are some Si-NCs in the sample because of Si crystallization. The crystallization of the precipitated Si continues after annealing for 120 min. The precipitated Si atoms exist first as amorphous Si clusters before crystallizing to be Si-NCs.The processes and mechanisms described above on the transition from a-SiOx to Si-NCs/SiO2 can well explain the photoluminescence features and the dependence of the luminescence on annealing conditions. After annealing at 1000℃, the sample emits light featuring Si-NCs related luminescence. The luminescence increases with an increase of the annealing temperature. The strongest luminescence is observed from the sample after annealing at 1100℃. With further increasing the annealing temperature, the luminescence decreases because of the increase in average size of the formed crystals and the decrease in the concentration of the light emitting Si-NCs. In addition, stronger luminescence can be obtained by extending the annealing time at 1100℃, which improves the luminescent performance of the prepared Si-NCs/SiO2... |
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