Optimization of the Properties of Undoped a-Ge:H

Abstract

ABSTRACTHigh density, non-porous, highly photoconductive films of undoped a-Ge:H showing minimal microstructure have been prepared out of an r.f. glow discharge of GeH4 on substrates mounted on the powered electrode of a diode reactor. These films, prepared under the systematic variation of substrate temperature, discharge power and dilution of the plasma by H2 are markedly different from those produced on the unpowered electrode. An optimum level of applied power is found which does not differ significantly from that used to prepare state-of-the-art a-Si:H. For the reactor geometry used, dilution of the plasma with H2 is found to be essential to the preparation of high quality a-Ge:H. These films contain similar amounts of bound hydrogen to that found in good a-Si:H, and also relatively large quantities of unbound hydrogen. Transport is activated with a Fermi level near mid-gap, the dangling bond spin density is 5 × 1016 spins/cm3, and the samples exhibit photoluminescence. The electron μÄ has been measured using the TOF technique. The conditions for preparation of optimized material are quite different for those found to optimize the preparation of high quality a-Si:H in this reactor. This is asserted to be a principal cause for our finding, along with other laboratories, of inferior properties for a-SiGe:H alloys when compared with those of a-Si:H.