Growth of Si:Ge Alloy Nanowires

Abstract

In this thesis, I will present the research on gas-phase catalytic growth of Si1-xGex alloy nanowires for photonic application. These group-IV semiconductor alloys offer a continuously variable system with a wide range of crystal lattices and energy band-gaps, leading to various electrical and optical properties. Indeed, in microelectronics, Si1-xGex thin films offer a lattice engineered platform for strained Si carrier channels with enhanced carrier mobility in Si/Si1-xGex heterostructures. More interestingly, the continuous variation in the energy band gap in Si1-xGex provides challenging opportunities for Si optoelectronics because it potentially allows emission and detection of the photons in the wavelength range of optical fiber communication. Light emission from these group-IV semiconductors is, however, only poorly achieved because of the relatively slow radiative recombination of electron hole (e-h) pairs across the indirect energy band-gap. One promising strategy to enhance luminescence efficiency, at least in Si photonics, is to spatially confine e-h pairs at the nanometer scale, that is light emission from Si nanocrystals. Hence, one can speculate that efficient light emission for optical communication can be achieved from Si1-xGex nanocrystals whose size is smaller than their characteristic length scale for light-emission processes. Here I report the growth of single-crystalline Si1-xGex nanowires, whose relative composition is controllably tuned over the entire composition range. I then present experimental demonstration of the optical band-edge shift from near-infrared to visible regions with alloying of Si and Ge, and their spatial confinement at the nanometer scale, and we interpret our observations as energy band-gap modulation in Si1-xGex nanowires. Specifically, we show that the optical band-edge of thick Si1-xGex nanowires can be adjusted by the intrinsic bulk alloying effects, and can be tuned further with the control of the diameters of the thin Si1-xGex nanowires by the extrinsic size effects. We note that previous reports demonstrated the growth of single-crystalline Si1-xGex nanowires by various catalyst-assisted syntheses, such as laser ablations, vapor transports, and chemical vapor syntheses. Nevertheless, no report on the modulation of the band gaps in Si1-xGex nanowires, which is our primary concern of this Letter, is available to the best of our knowledge to date. As a follow-up study, I report the axially graded heteroepitaxy and a confocal Raman scattering imaging of an individual one-dimensional Si1-xGex alloy crystal, where the relative composition of Si and Ge is continuously modulated in a wide range along the individual nanowires, sharing the same crystal structures with the continuously varying lattices. The optical properties of Si1-xGex nanowires are particularly interesting, because they can provide unique opportunities to investigate both the alloying and size effects at the nanometer scale on their energy-band gaps at the same time. On-wire, composition- and lattice modulated Si1-xGex alloy nanowires in this study thus can represent as a model system of continuously energy-band gaps within a well defined size and dimension for the size dependent optical properties. By employing a confocal Raman scattering imaging technique done on individual Si1-xGex nanowires, we also demonstrate that the local variations in compositions can be spatially and spectrally resolved along the individual nanowires. The spatially resolved spectra of the Raman phonon bands (νSi–Si, νSi–Ge, or νGe–Ge) allow us to optically estimate the local composition along the wire axis within the 500 nm spatial resolution.My research about many interesting physics observed in Si1-xGex nanowires provide us a good chance to study interesting size effect and band modulation in nanometer scale and suggest the general implication for group IV photonics.