High Pressure Chemical Deposition of Hydrogenated Amorphous Silicon in Extreme Geometries

Restricted (Penn State Only)
Author:
Day, Todd Douglas
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 31, 2014
Committee Members:
  • John V Badding, Dissertation Advisor
  • Harry R Allcock, Committee Member
  • John B Asbury, Committee Member
  • Ali Borhan, Committee Member
Keywords:
  • hydrogenated amorphous silicon
  • high pressure
  • chemical vapor deposition
  • optoelectronics
  • optical fiber
Abstract:
Hydrogenated amorphous silicon (a-Si:H) is the second most technologically relevant semiconductor, which is being researched for applications such as optoelectronic devices and solar cells. However, the hydride precursor, silane has a significant kinetic barrier to pyrolysis at low enough temperatures for hydrogen passivation to enhance the electronic and photonic properties. Consequently, external activation sources such as radio frequency plasma and heated filaments are used for the synthesis of thin film a-Si:H in a chemical vapor deposition technique at reduced temperatures. However, these complex deposition apparatus are extremely expensive and are limited to planar substrates. The focus of this dissertation is to utilize a high pressure chemical vapor deposition technique for the fabrication of a-Si:H in confined one-, two-, and three-dimensional geometries. The orders of magnitude increase in the precursor pressures utilized in this high pressure approach allows for a-Si:H deposition within meters-long, high aspect ratio templates such as silica optical fiber capillaries. The optical fiber capillaries serve as both the high pressure reactor and substrate in which the high pressure reactant mixture is able to deposit high quality, void-free, atomically smooth a-Si:H wires with micrometer to nanometer diameters. The high pressure decomposition of silane within optical fiber capillaries is intensively studied in order to optimize the material quality by controlling the hydrogen content with appropriate hydride precursor, deposition temperature, and precursor mixture. These a-Si:H micro-wires exhibit exceptional nonlinear optical properties which are useful in telecommunications optoelectronics devices. This high pressure deposition technique can also be extended to planar substrates without the need for external activation sources present in traditional methods. The reaction vessel geometry is a key aspect for the production of heterogeneous films rather than homogeneously nucleated particle formation which has been observed in previous high pressure processes. This high pressure deposition approach can therefore produce inexpensive thin film a-Si:H photovoltaic devices due to the simplicity of the process over that of conventional deposition methods. Furthermore, this high pressure process can be scaled for fabrication of a-Si:H films on large-area, flexible substrates in a roll process. Additionally, three-dimensional nanostructured templates can be infiltrated using this high pressure deposition technique. High-purity, amorphous and crystalline Si can conformally coat nanoscale pores and channels due to the high pressures employed in this process. Fabrication of well-ordered, nanoscale geometries with semiconductor materials can lead to the discovery and study of unparalleled physics related to electronic, photonic, magnetic, and vibrational processes. Electronic grade semiconductor infiltration of nanoscale templates, with voids as small as one nanometer, has not been possible with previous techniques.