Degree Type

Dissertation

Date of Award

2015

Degree Name

Doctor of Philosophy

Department

Electrical and Computer Engineering

First Advisor

Vikram Dalal

Abstract

Nanocrystalline Silicon-Germanium (nc-SiGe:H) is a useful material for photovoltaic devices and photo-detectors. Its bandgap can be tuned between Si (~1.1 eV) and Ge (~0.7 eV) by changing the alloy composition during growth. The material exhibits a good absorption extending to the infrared region even with thin layers. However previous work has shown that devices with higher Ge content have poor device performance. Also, very little work has been done previously to measure and understand the defect spectrum of nanocrystalline (Si,Ge). Defects control recombination, and hence, the performance of solar cell devices.

This work deals with studying the fundamental device physics of nc-SiGe:H including defect density, lifetime and mobility and their relationship with impurities, grain size and Hydrogen bonding. Capacitance-Frequency measurements at different temperatures are used to estimate the trap density profile within the bandgap of nc-SiGe:H.

We also study device performance and how to maintain uniform crystallinity in intrinsic layers of devices so as to obtain the best device performance. We show that one can use hydrogen grading or power grading to produce films with uniform crystallinity.

We will report on a systematic study of the varying Germanium content in nc-SiGe:H the relationship between Ge content and transport properties. It is found that upon adding Ge to Si during growth, the intrinsic layer changes from n-type to p-type. This can be reversed back by using ppm levels of phosphorus doping, and devices of reasonable quality can then be obtained. Measurement of defect densities showed that adding ppm levels of phosphine reduced the midgap defect densities

DOI

https://doi.org/10.31274/etd-180810-3943

Copyright Owner

Siva Konduri

Language

en

File Format

application/pdf

File Size

140 pages

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