Low-temperature Electrodeposition of Crystalline Semiconductor Materials.

Abstract

Crystalline group IV semiconductor materials, silicon (Si) and germanium (Ge) are essential building blocks in energy conversion, storage and optoelectronic devices. The state-of-art material synthetic methods fail to offer a non-energy-intensive solution for producing crystalline group IV semiconductor materials using easy-to-access apparatuses under ambient conditions. The primary aim of this thesis is to develop a cost-effective synthetic method, namely electrochemical liquid-liquid-solid (ec-LLS) growth, for preparing these materials at low temperatures using simple instruments and chemicals. The key innovation of the ec-LLS approach is the utilization of liquid metal electrodes in an electrodeposition process, during which the liquid metal serves simultaneously as a conductive substrate for current flow and as a solvent phase for semiconductor crystallization. The unique combination of electrodeposition and liquid-phase crystallization in this strategy opens new possibility for low temperature preparation of crystalline group IV semiconductors. This thesis will test a few key hypotheses regarding the fundamental and practical aspects of ec-LLS. Chapter 2 focuses on the Ge electrodeposition on liquid pool electrodes with various compositions to demonstrate the versatility of the ec-LLS approach. The significant role of liquid metal electrodes in the crystal formation process will be highlighted by the X-ray diffraction data. Chapter 3 expands the application of ec-LLS strategy to the controlled electrodeposition of Ge nanowires using nano-sized growth catalyst. As-deposited Ge nanowires will also be tested as Li+ battery anodes without further processing. Chapter 4 details the direct epitaxial growth of single-crystalline Ge nanowires at room temperature by the ec-LLS approach. Discrete Ga nanoparticles will be used as the seeding catalyst for the Ge nanowire growth on a single crystal Ge wafer. Electron microscopy evidence supporting the notion of epitaxial growth will be presented. Chapter 5 demonstrates the application of ec-LLS strategy for electrodeposition of crystalline Si at temperature as low as 80 oC from an organic electrolyte. SiCl4 precursor in propylene carbonate will be electrochemically reduced onto liquid Ga pool electrode to form high-coverage elemental Si. In summary, the collected results from this thesis will endorse ec-LLS as a non-energy-intensive synthetic method for producing crystalline group IV semiconductor materials.