5.5 Amorphous Insulator-Amorphous Semiconductor Transition

The critical strength of disorder, which determines the type of electronic structure, is closely connected with the molecular structure of the material. Going along a theoretical way, for instance, from frozen mixture of silane (SiH ₄) and methane (CH ₄) (which is a molecularly amorphous material) via a mixture of polymer-like chains ([-C-C-], [-Si-Si-] and [-C-Si-]), then more and more cross-linked chains (covalent amorphous material), hydrogenated carbon-silicon glasses (a-Si _xC _y:H), up to a-Si _xC _y:H alloys, one should agree to the fact that completely different models describe the electronic structure of the materials lying at both ends of the way. Thus, we have to expect that at some point of the way a rapid transition in the electronic structure from one to another type must occur.

In fact, it has been found that contrary to a-Si _xC _Y:H alloys, which are generally known as typical semiconducting materials ( Sec. 5.3.3), a-Si _xC _Y:H glasses (pp-organosilicon glasses) rank rather among a-insulators (Sec. 5.4). If these considerations are correct, the monotonic change of the molecular structure from glass to alloy, which results in a change of the strength of disorder, should cause a transition from a-insulator to a-semiconductor, i.e. the transition from a material with no extended states to a material with extended states. In fact, such an effect has been observed in films plasma-deposited from methylosilanes with varying methyl contents (from 0 to 4...