Amorphous hydrogenated carbon-germanium films (a-Ge X C Y :H) were fabricated by plasma chemical vapor deposition in an audio frequency (af) three-electrode reactor using tetramethylgermane (TMGe) as a source compound. Two types of the material, namely semiconducting (a-S) and insulating (a-I) films characterized by quite different electronic properties, were produced. For example, electrical conductivity at room temperature, T room , amounts to approximately 10 -18 S/m (with activation energy E A ≈ 0.9 eV) and 10 -4 S/m (E A ≈ 0.3 eV) for a-I and a-S, respectively. This very drastic change in the electronic structure of the a-Ge X C Y :H films can be caused by a very small variation of the plasma deposition conditions and therefore it is termed a-I-a-S transition. The previous attempts to explain the nature of the a-I-a-S transition from the molecular and supermolecular point of view (quantitative chemical microanalysis, XPS and IR spectrscopies, TEM, SEM, electron diffraction, and AFM) have failed. In this paper Raman spectroscopy has been used to this end. It has been found that the carbon structure does not determine the electronic properties of the films and sp 2 sites are not responsible for the a-I-a-S transition. This conclusion is supported by investigations on a-Ge X C Y :H films, in which sp 2 sites are purposely created. On the other hand, a drastic difference in the Ge atoms dispersion in the a-I and a-S has been found. In the a-I films a great amount of Ge atoms is molecularly dispersed, whereas in the a-S films the vast majority of Ge atoms is in the form of a-Ge several-nanometersized clusters. It is suggested that these clusters play a crucial role in the formation of the amorphous semiconductor network and they are responsible for the a-I-a-S transition.

Electronic properties of a-GexCy:H films plasma-deposited in a new type of audiofrequency reactor with three parallel electrodes have been investigated. It has been found that small changes in coupling capacity in the reactor can cause a drastic step change in the electronic structure of the deposited films. This effect is attributed to a transition from amorphous dielectric to amorphous semiconductor.

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