PhD Oral Exam – Bahareh Yaghootkar

Abstract

The large surface area and high aspect ratio of nano-structures make them promising candidates as a fundamental building block for manufacturing various devices. The potential applications of silicon nano-structure array include but are not limited to electron emitters, sensors, solar cells, rechargeable batteries, and hydrogen storage devices.

With advances in nanotechnology, various techniques have been reported for synthesis and fabrication of nano-structures. However, these techniques like chemical vapor deposition and vapor liquid solid suffer from the need of very sophisticated and high cost equipment. Furthermore, the need of high operating temperature, high vacuum, and catalyst material such as gold are major challenges of these techniques. On the other hand some fabrication techniques such as top-down approaches involve complicated fabrication steps that ultimately increase the cost of the device.

Therefore, a rising impetus has been devoted to development of less complicated and low-cost fabrication techniques of silicon nano-structure.

The goal of this thesis was to introduce novel and cost-effective fabrication methods which also maintain the benefits of CMOS compatibility. Two non-lithography top-down approaches were introduced for fabrication of silicon nano-structures array with capability of controlling the structure characteristics.

The first fabrication approach consists of three steps:

1. patterning of silicon surface in TMAH using anisotropic etching technique
2. formation of porous layer on patterned silicon surface using electrochemical anodic etching
3. treatment of porous silicon layer using an alkaline etching to reveal the silicon nano-structure array

The second fabrication approach consisted of two steps, namely: Anisotropic etching followed by electrochemical etching. The main idea behind this approach was that unlike the first approach the electrochemical etching is performed in transition regime not porous silicon formation regime.

These techniques allowed for the controlling the characteristics and morphology of silicon nano-structures.
Completely different morphologies of nanostructures were achieved as a result of transforming the electrochemical process from porous silicon formation to transition regime.

A study on effect of type of dopant, p- and n-type, on over-mentioned fabrication methods was also investigated.