Up Next

History

First of all, I may not be the right person to recount the history of amorphous silicon. What is usually called by this name today, refers to a material which was first described in the year I was born, back in 1969. Amorphous silicon, i.e. material without near range order was known before, it can easily be obtained by evaporation or sputtering. However, this material is not a useful semiconductor. In 1969, Chittick deposited amorphous silicon by plasma excitation of silane, and it turned out that this process yields a material with two typical semiconducting properties; it showed thermally activated conductivity, and it could be doped by adding phosphorous [Chittick-1969jecs]. When talking of amorphous silicon these days, we refer almost always to a material fabricated with plasma enhanced chemical vapour deposition (PE-CVD).

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 500C, 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 300C, 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).


Up Next