Design of semiconductor nanowires for high performance photocatalytic and electronic applications

Abstract

Semiconductor nanowires (NWs), such as silicon (Si) and III-V materials, have recently stimulated a great interest in the scientific research community due to their unique electronic and optical properties. The III-V NWs are the fundamental elements in pursuing next-generation nanoelectronics; therefore, it is important to understand the growth mechanism for their large-scale and uniform NWs preparation, as well as to estimate their electronic transport properties for technological applications. In the meanwhile, Si NWs are the promising photocatalysts because of their large specific surface area, small band gaps and reactive Si-H surfaces formed by hydrofluoric acid (HF) treatment; therefore they would have prospective photocatalytic activity in the visible light range, as compared with the ultraviolet photocatalysts. In this regard, it is significant to investigate the physical properties of these NWs for the corresponding photocatalytic applications. In this dissertation, utilizing a solid source chemical vapor deposition (SSCVD) method, excellent carrier mobility indium phosphide (InP) NWs have been synthesized, by utilizing gold (Au) thin film catalyst, and the crystal structure and intrinsic electron transport properties are carefully studied. Notably, the NWs are found to grow via the vapor-liquid-solid (VLS) mechanism with a narrow distribution of diameter uniformly along the entire NW length. Although the grown NWs possess a substantial amount of twin defects, the fabricated NW field-effect transistors (FETs) still exhibit impressive electrical performance with high carrier mobility (~350 cm2/Vs) and ION/IOFF ratio (~106). All these have demonstrated the promising potential of such NWs grown on amorphous substrates for practical applications, as compared to the conventional metalorganic chemical vapor deposition (MOCVD) or molecular bean epitaxy (MBE) grown InP NWs. As Au is well known to be incompatible with the conventional Si-based complementary metal oxide semiconductor (CMOS) technology as the resultant deep level traps in Si greatly degrade the electrical properties, highly crystalline, stoichiometric and dense gallium arsenide (GaAs) NWs are synthesized on amorphous SiO2 substrates using Ni nano-clusters (NCs) as catalysts. The grown NWs have low defect densities and found to grow epitaxially via the vapor-solid-solid (VSS) mechanism with non-spherical NixGay catalytic seeds. The NWs are then configured into field-effect transistors (FETs) showing impressive electrical characteristics with ION/IOFF > 103. Using the same SSCVD method, diameter tailorable GaAs NWs have been prepared in the range of 10-200 nm. Importantly, there are abundant acceptor-like defect states located between the intrinsic nanowire and its amorphous native oxide shell, which can be used to alter electronic transport properties of GaAs nanowires as a function of diameter. Using a NW field-effect transistor (NWFET) device structure, p- to n-channel switching behaviors have been achieved with the increasing NW diameters as the thin NWs (< 40 nm) are fully depleted and the thick NWs (> 70 nm) are non-depleted and the medium diameter NWs (40-70 nm) are semi-depleted by the interface trapping effects. In this case, careful device design considerations are required for achieving the optimal NW device performances. The corresponding NW photovoltaic (PV) cells are then fabricated by using the Schottky contact of the NW with the Au-Ga catalyst tip, with the best performance of Voc ~0.6 V, Jsc ~11 mA/cm2, FF ~0.42 and efficiency ~2.8 % for a NW with diameter of 70 nm. As compared with metal contacts directly deposited on top of the nanowire, this nanoscale contact is found to allevia5te the well-known Fermi-level (EF) pinning to achieve effective formation of Schottky barrier responsible for the superior PV response. It should be noted that this simple structure resulted in comparable efficiency with the complex intentionally prepared p-n junction structured solar cells, showing the potential of these versatile nanoscale contact configurations for future technological device applications. Besides, large-scale uniform nanoporous and nonporous three-dimensional silicon nanowire arrays (SiNWAs) prepared with metal-assisted chemical etching method from different types of Si wafers were explored as photocatalysts for the dye photodegradation. The photocatalytic mechanism is that the dye molecules were decomposed by the ●OH radicals produced by the Si NWs after the absorption of photons with energy equal to or larger than its band gap. Importantly, after the hydrofluoric acid (HF) treatment, the photocatalytic activity of all kinds of SiNWAs increased significantly, which are caused by two reasons: some of the methyl red (MR) molecules can be reduced by hydrogen transfer firstly and then decomposed by ●OH radicals, which plays an important role in decomposing MR; Si NWs can produce ●OH radicals more easily by hydrogen terminated treatment. Besides, the nanoporous SiNWAs showed excellent stability, which can be recovered by HF treatment. The excellent photocatalytic activity and stability of porous Si nanowire arrays may have significant potential for organic waste treatment and environmental remediation.

Date of Award	3 Oct 2012
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Hon Wah Michael LAM (Supervisor) & Ning Bew WONG (Supervisor)