Amorphous Hydrogenated Silicon, a-Si:H

Abstract

The research in the field of amorphous semiconductors, from the very beginning, has been driven by both the scientific interest in basic aspects of disorder in the properties of solids and technological applications. In the early years chalcogenide glasses were at the center of the interest owing to thin-film applications in imaging, xerography, memory and switching devices. At that time amorphous silicon and amorphous germanium, a-Si and a-Ge, were of more academic scientific interest. As simple elemental tetrahedrally bonded amorphous semiconductors, they served as model systems in which the disorder was less complicated, being defined not by chemical composition but by the structural disorder only. These amorphous semiconductors were prepared as thin films, about 0.1–1 μm thick, on glass or quartz substrates by a variety of methods such as thermal or e-gun evaporation, sputtering, ion bombardment and electrolysis. The simplest model for the structure of tetrahedrally bonded semiconductors is the continuous random network (CRN) structure in which the average coordination number is close to 4. Fluctuations in the bond angles and nearest-neighbor distances lead to a loss of long-range order even in the second-neighbor shell. This loss of long-range order is the characteristic structural feature of amorphous semiconductors. As a result, important theoretical concepts which are based on periodicity (Bloch’s principle) fail, such as band structure, k-vector, Bloch states, effective masses and optical selection rules. The optical spectra of amorphous semiconductors appear to be more or less broadened versions of their crystalline counterparts, which shows that the density-of-states distribution is the decisive quantity; this is largely determined by the nature and structure of the chemical bonding. Perhaps the most obvious effect of disorder is the localization of electronic states, in particular near the band edges, which strongly affects the transport properties.