TABLE OF CONTENTS Although amorphous covalent insulators are a natural consequence of the theory of amorphous state, an interest in these systems has been generally limited. The term of amorphous insulator has appeared occasionally in literature for a long time, e.g. [167], but it rather expresses materials with a very low conductivity and has no concern with the present conception of their band structure. The amorphous insulator, to our mind, according to the model presented in Sec. 5.2.1, is an amorphous material without the extended states in the conduction and the valence bands. Thus, only localized states exist there. Of course, we will confine our discussion to materials fabricated by plasma polymerization. It should be, however, pointed out at once that most of the plasma deposited films seem to be rather a-insulators than a-semiconductors, though in general, sometimes uncritically indeed, their electronic structure is described just as for an a-semiconductor.
The model for the density of states of amorphous insulator was proposed in 1984 [22]. Since that time, investigations performed on various thin plasma films have given more experimental evidence for the existence of the a-insulator state [9], [19], [52], [166], [168]. Figure 5.22 shows a sketch of the band model for the best understood at present amorphous insulator, namely a-Si xC y:H material plasma polymerized from organosilicon precursors [19], [22]. Main features of this model, however, should be recognized as characteristic for the whole group...
