Doped amorphous semiconductors

Abstract

The article reviews the advances that have been made during the past two years in the new field of substitutionally doped a-semiconductors, particularly a-Si. After introducing some of the basic concepts used in the treatment of a-materials, the preparation and doping of a-semiconductors from the gas phase and the role of hydrogen are discussed in some detail. Recent results on doping by ion implantation have also been included. The following sections are concerned with the effect of n and p-type impurities on the electronic transport properties of a-Si and a-Ge. The discussion is based on results from conductivity, drift mobility, Hall effect and thermoelectric power measurements and leads to the main conclusion that the donors introduce a new hopping path through the system which begins to dominate over tail-state hopping when their density exceeds about 10¹⁸ cm^-3. Recent photo-conductivity and luminescence results on doped a-Si are then discussed; they give information on recombination and also demonstrate the pronounced effect of doping on these properties. In the final section amorphous p-n junctions are considered as well as their possible applications in photovoltaic solar energy conversion.