Intrinsic And Boron-doped Silicon Nanocrystals: Synthesis, Properties And Applications In Silicon Photonics

Silicon photonics is a technology to implement various optical functionalities in silicon. It has found a wide variety of applications, including life sciences, medicine, communication, computing, sensing, energy, and so on. The main interest is associated with the possibility to merge electronics and photonics in the same chip. This is very appealing since it allows exploitation of both the high computation capability of electronics and the high communication bandwidth of photonics. However, to achieve monolithically integrated silicon photonics, the main limitation is the lack of any practical Si-based light sources:either efficient light emitting diodes (LEDs) or Si lasers. Taking into account the main limitation that bulk Si is an indirect band-gap semiconductor, strategies have been proposed to improve the light emission and to realize Si-based light source. Among all the strategies, low-dimensional silicon (nano-Si) is quite promising due to quantum-confinement effect and interface effect.In this thesis, various approaches towards a Si-based light source based on nano-Si have been intensively investigated. The optoelectronic properties, the related physical mechanism and the applications of all-Si-based silicon-rich oxide (SRO) films and boron-doped silicon-rich oxide films have been systematically addressed. In the following, the primary achievements in this work are described.(1) High density of silicon nanocrystals (Si-NCs) embedded in silicon oxide film is achieved by plasma enhanced chemical vapor deposition (PECVD) technique and a successive high temperature annealing process. The passivation of nonradiative states and defects at the Si-SiO2interface is realized by performing H passivation of the Si-SiO2interface through standard forming gas annealing in order to increase the radiative yield without affecting the emission mechanism. Taken the optimized film as active layer combined with lithographic patterning and wet/dry etching processes, array of several thousands of5-10μm diameter microdisk resonators are formed. As Si-NCs posses a wide photoluminescence (PL) band in the visible and NIR region; when they are embedded in a dielectric microcavity, it is sufficient to observe whispering-gallery modes (WGMs) generated by optical injection via spontaneous emission of the Si-NCs. We report on subnanometer WGM resonances corresponding to quality factors (Q-factors) as high as3000around the wavelength of800nm, which to our knowledge are the highest among the previously reported values in Si-NC-based microdisk systems. We stress that the SRO material optimization (low-loss, positive material gain) should play a key role for further enhancement of the observed Q-factors of few thousands. Even with low while inhomogeneously broadened gain spectrum of Si-NC, microdisk resonators with similar Q-factors should be potential candidates to allow for a low-threshold laser action.(2) We report on a study of the recombination dynamics of Si-NCs embedded in a planar WGM resonator. Fundamental properties of exciton dynamics in Si-NCs, in particular their absorption cross-section and excited carrier-related losses, can be extracted from continuous-wave spectroscopy by analyzing the resonance linewidths at different excitation powers. Observation of nonlinear drifts of mode peak positions in the same experiment allows us to model the nonlinear refractive index of the nanocrystalline material. The theoretical results confirm that the observed sublinear blue and linear redshifts of resonance peak positions are induced by excited carrier effects and thermal heating at low and high pump powers, respectively. The extracted thermo-optic coefficient, kT=1.46×104K-1, and excited carrier refraction, kEC=-1.07×10-23cm3in Si-NCs, are of relevance since they may induce important modulation of the fine modal structure of an optically active cavity.(3) Boron-doped silicon-rich oxide films with a series value of Si excess and boron concentration are prepared by co-sputtering technique followed with the annealing treatment. The effect of boron-doping on the chemical composition and microstructure of Si-NCs embedded in silicon-oxide matrix are well studied. With low value of Si excess (Si/O atomic ratio～0.52), there are sub-nanometer scale Si or even atomic scale Si aggregates embedded in SiO2matrix. While for moderate value of Si excess (Si/O atomic ratio～0.67), there are Si-NCs with diameter of2-5nm embedded in SiO2matrix. And for high value of Si excess (Si/O atomic ratio～1.1), there are larger size Si-NCs embedded in SiO2matrix, which become elliptical-shaped and are overlapped. The investigation of X-ray photoelectron spectroscopy of Si2p and B1s core level spectra suggests that B atoms exist in Si substitutional sites of atomic scale or sub-nanometer scale or nanometer-scale Si as well as in silicon oxide matrix and/or in interface between matrix and Si aggregates.(4) For high values of Si excess, an almost4-5orders of magnitude decrease of the sheet resistance is achieved due to the significant increase of the carrier density by the electrically activated doping. For moderate and low values of Si excess, intense white photoluminescence from boron-doped silicon-rich oxide films is observed. The PL efficiency is of several percent, which is a promising result in Si-base solid state white light emitting. The influence of Si excess, boron concentrations, and annealing temperatures is examined with the aim of optimizing the PL efficiency and clarifying the PL mechanism. The interface region between sub-nanometer scale/nanometer-scale Si and SiO2matrix, or the atomic scale Si aggregates in SiO2matrix are most likely housing the luminescence centers which are formed by the promotion of boron and are active in white light emission properties. In particular, for moderate values of Si excess, our boron-doped silicon-rich oxide films possess both the intense white luminescence property and enhanced electroconductivity. In summary, this system has the following advantages:since all elements of the film (B, O, Si) are "CMOS periodic table" elements, the production is completely compatible with existing microelectronics technology and is of low cost; by adjusting the Si excess, boron content and annealing temperature, one can tune the white light emitting, which is a considerably simple fabrication process; the electrically activated boron doping can improve the electrical transport properties of the film, which makes it more suitable for Si-based optoelectronic integration.