UC Riverside

Recently, semiconductor nanostructures have generated a continuously growing interest owing to their intriguing physical properties and potential for future technology development. Quasi-one-dimensional nanowires (NWs), usually defined as round or polyhedral shaped cross-sectional structures with a high aspect ratio of 1000 or more, is one of the most active research areas for nanoelectronics, optoelectronics and sensors. Precise controlled growth of NWs, including dimensions, growth rate, morphology, growth direction, and composition, is essential from the viewpoints of fundamental materials fabrication and their performance in future applications. However, there are still many challenges in the fabrication of NWs with controlled dimensions and properties. This dissertation focuses on investigating the fundamental synthesis aspects of epitaxially grown III-V NWs by chemical vapor deposition (CVD).

First, gold (Au) catalyst - assisted growth of InSb NWs directly on lattice mismatched (~7%) InAs (100) substrates have been studied. The influences of NW diameter and growth temperature on the NW growth rate and morphology have been investigated. The results indicate that NW growth is limited by the growth species direct impingement and the diffusion of surface adatoms. Next, the dependence of morphology of epitaxial InSb NWs on CVD growth parameters over InSb (100) substrates has been investigated. NW length and tapering factors correlated to morphological variations are determined as a function of growth parameters including growth temperature (300°C-480°C), powder source, and duration of growth. Results showed that NW morphology is influenced by axial and radial growth modes, reflecting the competition of various growth mechanisms under different growth conditions. We discovered that Indium droplets may also promote the growth of NWs with a similar role to that of catalytic Au nanoparticles. A potential growth model is accordingly described and discussed.

An optimum growth condition for straight and minimally tapered InSb NWs has been established. The as-grown NWs are investigated using Transmission Electron Microscopy (TEM), indicating a dominant <110> growth direction with the zincblende crystal structure. Current-voltage (I-V) measurements of single nanowire field effect transistors (NW-FET), suggest that NWs are of n-type.

A study of the effect of growth temperature on the growth direction of InSb NWs has been studied. A uniform growth direction of InSb NWs is achieved by optimizing the growth temperature profile. Results demonstrate that growth temperature plays an important role when controlling the NW growth direction similar to research on other types of III-V semiconductor NWs. We also investigated the large assembly of NW arrays aligned on the substrate using both in-situ controlled growth method and ex-situ contact printing method.

Finally, accurate electrical characterization of NWs and their ultimate implementation in electronics devices fabrication of metal contacts to semiconductor NWs with good ohmic behavior and low contact resistance has been studied. NW-FETs have been fabricated and I-V characteristic was measured to study the NW electrical properties and performance of devices. The improvements of the contact quality and other properties are discussed.

Our findings provide an in-depth understanding of semiconductor NW growth via chemical vapor deposition, and the first basis for further investigations on how to achieve controlled growth of individual and arrayed NWs, and novel device applications for switching and computation based on future development of nanoscale specific integrated circuits - NASICs.