Investigation on the formation mechanism, structure and optical properties of ultra-narrow silicon nanowires by chemical vapor etching

Abstract

As one of the most important semiconducting materials in modern industry, silicon has been actively researched for both basic scientific research and technological applications in the past decades. The increasing need for miniaturized devices that can work faster with less energy consumption has been a driving force for silicon-based micro- and nano-electronics. Nanostructured silicon, especially silicon nanowires (SiNWs) exhibit unique electronic, optical, and chemical properties, making them promising building blocks for the functional systems. In addition, the indirect bandgap of bulk Si could be tuned to a wider direct bandgap as the diameter of SiNWs approaches the carrier de Broglie wavelength due to quantum confinement effects. However, it has been a challenging task to fabricate highly-dense and oriented SiNWs with diameters less than 5 nm for practical applications.A variety of approaches have been developed for SiNWs fabrication, including the noble metal nanoparticle-catalyzed vapor-liquid-solid method as a representative bottom-up approach and the metal-assisted chemical etching as a representative top-down approach. Nonetheless, the typical large diameters (10-100 nm) of grown SiNWs limit the quantum confinement effects for tunable electronic and optical properties. Moreover, the low nanowire density and effective surface area per unit volume are unfavored for large-scale applications that require mat-like architectures. In order to fully exploit the superior properties of quantum SiNWs, a catalyst-free fabrication of high-density, well-aligned ultra-narrow SiNWs (2-5 nm) with a completely new formation mechanism has been recently developed, despite that the detailed formation mechanism, crystalline structures and properties of nanowires remain elusive. In this study, to better understand the fundamental knowledge, the equilibrium concentration calculations have been performed to reveal the major etchants (SiCl4 and HCl) and the major products (SiCl2, SiHCl3 and H2). Based on the equilibrium calculations, the H2 supply has been optimized for a more ordered nanowires array. Also, the sources of oxidant gases, which are responsible for the origination of ultra-narrow nanowires, have been investigated. The severely contracted diamond cubic structure and [100] orientation of the SiNWs have been confirmed by high resolution transmission electron microscopy and x-ray diffraction. The Raman spectrum and photoluminescence (3.5 eV) of SiNWs exhibit significant phonon and electronic confinement effects, indicating great potentials for uses in tunable electronics, optics and energy systems where highly controllable electronic structures and large surface area are required.