Synthesis and Optimization of Luminescent Si Nanoparticles by CO_2 Laser Annealing and Si Nanocrystal Light Emission in Microcavities

摘要

We report synthesis and photoluminescence optimization of luminescent Si nanocrystals in silicon rich oxide films using a CO_2 laser beam. Laser annealing allows for a very localized heat deposition. This results in appreciable temperature rise in an area that is equivalent to only a few spot sizes. This could be important in CMOS back-end compatible processing where high temperatures on the entire wafer scale might not be acceptable. Furthermore, temperature optimization studies in furnace annealing are time consuming because the furnaces have to be programmed to each individual temperature and the stabilization takes long times. In CO_2 laser annealing, the entire temperature range for nanocrystal formation is available along the radial and axial directions of the laser spot - thereby allowing temperature optimization in a single short experiment. Presence of crystalline nanoparticles is ascertained using structural analysis techniques like transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). We also report luminescence optimization with respect to laser power and annealing time. It is observed that laser annealing in an air ambient results in two peaks in the luminescence spectrum — on in the visible at 570 nm and one in the near infra read at 800 nm. Origin of luminescence in these two peaks is probed by hydrogen passivation and time resolved measurements. In the second part of the paper, we focus on continuous wave characterization of photoluminescence from Si nanocrystals embedded in microdisk resonators. There have been numerous reports on observation of continuous-wave and transient gain in planar optical waveguides with Si nanocrystal active layer. However, there are relatively very few investigations focusing on photoluminescence emission from Si nanocrystals (quantum dots) embedded in on-chip optical microcavities. Microcavities spectrally filter the luminescence from the quantum dots and, depending on the (Q/V) ratio, can significantly alter the spontaneous photoemission from the quantum confined excitons. In our work, planar microcavities are patterned on the emitter layer by high resolution electron beam lithography and a combination of dry and wet chemical etching. Fabrication procedure is optimized to maximize the ratio of the quality factor and the mode volume. Continuous-wave photoluminescence measurements are performed by top-pumping the resonators with a 488 nm line of an argon ion laser. We study the photoemission from the microdisks for the polarization dependence, and quality factors. Contributions of various mechanisms leading to the observed loss are estimated. We believe that our studies will help gain further insight into photoemission physics of the group-IV nanostructures.