Amorphous hydrogenated germanium films have been deposited in a conventional glow discharge reactor (diode as well as triode configuration) from GeH4 with and without additional H2 dilution. In this study special emphasis was put on samples deposited at different substrate temperatures, different neutral gas densities and dilution ratios. The deposition rate has been varied from 0.2 to 2.5 Å/s. Optical, structural and electronic properties have been studied by VIS/NIR thin film spectroscopy, PDS, Raman backscattering, FTIR, as well as temperature dependent dark and photoconductivity. Hydrogen content was monitored by quantitative SIMS analysis and FTIR spectroscopy.
By ‘soft’ deposition, i.e. high pressure and H2 dilution ratio, Fermi level positions near midgap (Eact =0.48eV at Eg=1.0ev) are achieved. Nevertheless the optimum ratio σph/σd is restricted to 0.6 at room temperature (λ=632.8nm). To point out the influence of Fermi level position for transport properties, we investigated two samples with different Fermi level positions in detail. σd(T) is strongly dependent on Fermi level positions, as well as the absolute values of σph(T), whereas the temperature dependence of σph(T) does not differ greatly. This is an indication, that Fermi level position and recombination centers of a-Ge:H can be effectively influenced by changing deposition parameters.