Influence of Deposition Conditions on the optical and Electronic Properties of a-Ge:H

ABSTRACTWe have measured the optical absorption edge spectra for absorption coefficients between 1 and 105 cm-1, the photoluminescence spectra at 77 K, the conductivity-temperature relations, and the photoconductivity magnitude and spectral dependence for a series of r.f. glow discharge films of a-Ge:H. The films were deposited at the powered cathode and unpowered anode of a diode capacitive reactor having very different electric fields and plasma conditions, while substrate temperature, H2 dilution of a GeH4 plasma and applied r.f. power were varied. The structure of the films are radically different, with the anode-deposited films displaying a microstructure of low density material (voids) and the cathode-deposited films displaying homogeneity similar to that of device-quality a-Si:H. A self consistent explanation of the differences in measured optical and electronic properties is given, taking full account of the structural observations. From this analysis the conditions required for the production of a-Ge:H of good photoelectronic quality may be inferred. Preliminary structural, optical and photoelectronic data for a-Si1.x Gex :H of large x, prepared under conditions extrapolated from the a-Ge study, indicate significant improvements from current data in the literature.