Influence of Bombardment on the Nanostructure and Microstructure in Amorphous Germanium.

In this thesis a study of the morphology of thin films deposited by rf diode, dc magnetron, and ion assisted evaporation is carried out. Particular attention is devoted to the role of bombardment. Amorphous germanium is used as a model system for this investigation. In the rf diode study, thick films are found to exhibit a density increase as sputtering gas pressure is decreased, resulting in some cases to an observed hyperdensity. An explanation based on stress-induced phase transformation is presented. For the dc magnetron studies, films exhibiting a minimum in void fraction were produced. This is attributed to the bombardment by reflected neutrals. Ion assisted evaporation is employed to provide a decoupling between bombardment energy and flux. The energy range from 15 to 600 eV is investigated. It is determined that small energies and fluxes provide substantial modification of the void network. A dosage effect is also observed for the film density, with the energy deposited into the film being a normalization factor. A specific damage threshold is measured at ~18 eV/Ge atom, and attributed to displacement type damage. This is found to be in very good agreement with a predicted value of 17.8 eV/Ge atom as determined by a simple relationship developed in this thesis. Neutron activation analysis experiments show an increase in the amount of incorporated inert gas as the energy and flux is increased. This is suggested to have a possible effect on the short-range and intermediate-range order network structure. The optical band gap is also measured and found to change with bombardment, indicating that the electrical as well as structural properties can be varied. These changes are discussed in terms of two different mechanisms. The fractal properties of pyrolytic graphite are investigated and discussed as a method for obtaining a quantitative description of thin film evolution. The main result of this study can be summarized as follows: small changes in bombardment can give rise to large variations in thin film morphology and these changes, especially at low energies and flux ratios, are measured for the first time in detail in this thesis.