Microstructure, vacancies and voids in hydrogenated amorphous silicon

Rajendra Timilsina

Abstract

This dissertation presents a theoretical and computational study of microstructure, vacancies and voids in hydrogenated amorphous silicon (a-Si:H). The microstructure consists of all possible silicon-hydrogen bonding configurations such as SiH, SiH2 , SiH3 and SiH 4 . However, it is highly dominated (approximately 75% or even more) by monohydride (SiH) configurations. Furthermore, the hydrogen atoms locate in both clustered and diluted phases; as a result, the distribution becomes highly inhomogeneous. Approximately 5% of hydrogen atoms reside in a form of isolated monohydrides at the lower (7 at.%) concentration whereas such configurations do not appear at the higher concentrations (≥14 at.%). The microstructure is further enriched with different types of vacancies such as mono- and divacancies. At the lower hydrogen content, it consists of split divacancies whereas voids appear in the higher concentrations (≥16 at.%). Structures of the voids are highly irregular and their internal surfaces consist of 6-16 hydrogen atoms. The microstructure further shows hydrogen molecules within the voids at the higher (≥16 at.%) concentrations. However, the concentration of the molecules is very low, in a range of 0.9-1.4% of the total hydrogen atoms. In order to investigate the microstructure in further detail, an approximate calculation of the nuclear magnetic resonance (NMR) line spectra is performed. The approximated line spectrum is a superposition of broad (19-50 kHz) and narrow line widths (1.7-6 kHz) depending on the concentration of hydrogen atoms. These observations are in excellent agreement with infrared (IR), nuclear magnetic resonance (NMR), multiple quantum nuclear magnetic resonance (MQ-NMR) and calorimetry experiments as well as ab initio calculations.