Plasma-deposited hydrogenated carbon–germanium semiconducting films doped with acceptor and donor centers

Hydrogenated amorphous carbon–germanium (a-GeXCY:H) films were doped with acceptors (boron, aluminum and gallium) and donors (phosphorous). The films were prepared by plasma enhanced chemical vapor deposition (PECVD), using an r.f. (13.56 MHz) reactor with a coplanar electrode geometry. Investigations on electrical conductivity and optical absorption were carried out. All used dopants caused a maximal increase in the conductivity of approximately two orders of magnitude. Based on more thorough examinations performed on the boron and phosphorous-doped films, it has been suggested that both acceptor and donor dopands modify only p-type conductivity of the undoped films and any transition to n-type is not observed. Boron creates an additional band of acceptors whereas phosphorous eliminates the oxygen donor centers reducing thereby the compensation of the native acceptors.