Abstract
Visible light emission from silicon nanostructures formed by Si+ ion implantation into a SiO2 matrix and subsequently annealed at high temperatures (mainly 1300°C and 900°C) in various annealing atmospheres has been investigated. Various analyses techniques, such as Rutherford Backscattering Spectroscopy (RBS), Secondary Ion Mass Spectroscopy (SIMS), Photoluminescence (PL), and Transmission Electron Microscopy (TEM) were employed to characterize the structures in terms of their composition and optical properties. RBS and SIMS analyses revealed nitrogen and carbon impurities in the samples which we conclude originate from contamination, such as N2 and CO+ in the ion beam. PL analysis with a 488 nm Ar laser at 300 K showed that there was no visible PL from the samples before Si+ implantation or from the samples after Si+ implantation but before annealing. Also, N+ implantation gave rise to no PL. Si+ implanted samples with 2 x 1017 Si+ cm-2 and 6 x 10 17 Si+ cm-2 exhibited, after annealing at 1300°C for 30 minutes in a nitrogen ambient, strong visible PL, with a broad spectrum at peak wavelengths of 580 nm and 760 nm, respectively. There was a weak dependence of the PL peaks at 580 nm and 760 nm on annealing tune and annealing temperature. However, there was no PL from N+ implanted and annealed samples. Annealing the Si+ implanted samples in forming gas (FG) at lower temperatures (up to 1000°C) increased the PL peak intensity up to a factor of two, however, the PL peak wavelengths were the same. It is concluded that hydrogen annihilates the non-radiative recombination pathways. This effect provides evidence for surface states playing an important role in light emission. From PL analysis, using a short wavelength laser (325 nm), it was found that silica samples showed one broad PL spectrum at a peak wavelength of 440 nm, whereas samples consisting of a 1 mum thick SiO2 film exhibited several peaks which we found to be due to optical interference. Detailed observations of the fine structure in the PL spectra at low temperature (18 K), from the silica samples which were annealed in FG at 900°C revealed strong evidence for interaction between excitons and Si-O vibrations localized in a very small region. TEM analysis showed that there were precipitates after annealing at 1300°C in N2 in a sample implanted at a dose of 2 x 10 17 Si+ cm2 with an energy of 400 keV whilst HTEM analysis showed that the microcrystallites varied in size from 2.5 nm to 7.5 nm. However, TEM failed to show any precipitates in samples which were implanted with the same dose at an energy of 200 keV and have strong PL, and this is also another strong indicator that the PL emission is not simply due to quantum confinement. In this thesis, we propose a luminescent model to describe the mechanism for light emission. Physically the emitting structures are envisaged to have three regions, namely, a Si core (Si precipitate), an interfacial transition region whose composition varies from Si to SiO2, and the surrounding SiO2 matrix. Light emission occurs by a two step process involving generation and confinement of excitons in the Si core, whose band structure is modified from that of bulk crystalline silicon, and radiative recombination through the interaction of excitons and Si-O vibrations within the interfacial transition region. The justification for this model is discussed in this thesis.