The atomic arrangement in amorphous silicon, both evaporated and PE-CVD material, is not completely random because silicon shows strong preference for four fold configuration and bond lengths close to the value in a crystal. Because of the non-ideal arrangement of the atoms, there is a certain number of atoms where not all four valence electrons undergo bonding. Such dangling bonds can be measured by electon spin resonance because they contain a single unpaired electron. For sputtered and evaporated materials, Brodsky reported dangling bond densities in the order of 1020 cm-3 [Brodsky-1969prl]. The single electron can either be stripped easily, or it can attract another electron for pairing. Thus, dangling bonds act as amphoteric defect in charge transport, and their high density prevented the use of evaporated or sputtered films in electronic devices.
PE-CVD material is generally grown from silicon containing gases like silane (SiH4). Before its application in PE-CVD, silane was used widely in the fabrication of crystalline silicon because it can easily be purified to high levels by distillation. After purification, ingots of crystalline silicon are grown by pyrolisis at high temperature, but at temperatures below 500°C, silane molecules are quite stable as long as no moisture or oxygen is around. If we would like to deposit silicon from silane gas at moderate temperatures, we have to aid the dissociation, e.g. with a glow discharge [Sterling-1965sse]. The birth of amorphous silicon as electronic material is is generally traced to a later report of the same group where they present conductivity measurements on thin films deposited by the glow discharge method [Chittick-1969jecs].
Brodsky reported defect densities in the order of 1016 cm-3 in PE-CVD material [Brodsky-1979jncs]. This may still seem high, but it is already much better than material deposited by sputtering or evaporation. It was suspected that the differences are due to incorporation of hydrogen during the plasma process [Brodsky-1970prb], but it took a few years to produce clear experimental evidence. The presence of hydrogen in PE-CVD material was first shown by annealing experiments; starting from about 300°C, hydrogen bonds start breaking and hydrogen comes out of amorphous silicon, a process called evolution or effusion [Knights-1976aip, Brodsky-1977apl]. After some confusion and unclear interpretations of IR absorption data, the type of bonding in the network could be clarified by analyzing bending and stretching modes of silicon-hydrogen bonds which appear at specific frequencies [Brodsky-1977prb].
Eventually it became clear that that the incorporation of hydrogen into the material is inherent to PE-CVD growth as long as the deposition is carried out at moderate temperatures, and it emerged that hydrogen can attach to open silicon bonds, thereby effectively passivating the dangling bonds. Because there is a significant amount of hydrogen in PE-CVD material and because it plays such a fundamental role in defect passivation, we should rightfully call this material hydrogenated amporphous silicon (a-Si:H).