Dissertation

Spectroscopic studies of amorphous thin films

Petrich, Mark Anton

1987

Overview

Amorphous semiconductors are important in applications such as solar photovoltaic devices, xerography, and thin film transistors. However, the relationships between the structure and properties of amorphous semiconductors are not well understood. Despite a lack of fundamental understanding, it has been shown that amorphous semiconductor properties can be tailored to a specific application by the addition of alloying elements, or by variation in the production process. Characterizing the structures that lead to these desired properties, and determining the effects of processing chemistry on the resulting materials, will lead to an optimization of amorphous semiconductor technology. The amorphous hydrogenated semiconductors that I have studied include amorphous silicon, silicon carbide, silicon nitride, and amorphous carbon. All of these materials were prepared by plasma enhanced chemical vapor deposition, a commonly used deposition technique in the solid-state materials industry. By using Fourier transform infrared spectroscopy, we were able to obtain qualitative information about chemical bonding environments in newly deposited materials. Information about the electronic structure was provided by optical absorption spectroscopy in the near IR, visible, and ultra-violet regions. The atomic compositions of our materials are known from Rutherford backscattering (for the non-hydrogen elements) and proton nuclear magnetic resonance (for the hydrogen). We used solid-state nuclear magnetic resonance (NMR) to provide quantitative and selective information about local bonding configurations. Carbon NMR studies of amorphous carbon have identified the relative amounts of planar and tetrahedral bonding in this material. Studies of amorphous silicon carbide have revealed that doubly bonded carbons exist in this material, even if the ratio of carbon to silicon is less than one. A variation of the basic experiment showed that these doubly bonded carbons are not bound to hydrogen. The microstructure of fluorinated silicon nitride was inferred from the results of silicon, hydrogen, and fluorine NMR experiments. A relatively new technique, multiple quantum NMR, was used to observe the detailed hydrogen microstructure of amorphous silicon and amorphous silicon carbide. In device quality materials, the dominant hydrogen microstructural unit is a group of five to seven hydrogen atoms, all covalently bonded to silicon or carbon.

• Publisher: ProQuest Dissertations & Theses
• Subject: Chemical engineering
• ISBN: 9798206207750