Optimization of process parameters for the deposition of improved a-Ge:H by dc magnetron sputtering

Abstract

A detailed study of the influence of hydrogen and argon partial pressures, substrate temperature, and dc power on the composition and the optoelectronic properties of hydrogenated amorphous germanium (a-Ge:H) is presented. The sputtered a-Ge:H has a typical Ge density of 4×1022 atoms/cm3 and contains up to 4×1021 H atoms/cm 3. It is chemically stable and shows no sign of postoxidation in the IR spectra over a period of one year. A low midgap absorption according to α(hν=0.6 eV)<10 cm-1 and Urbach energy E 0<50 meV is observed for the best films which are prepared at the lowest argon pressure p(Ar)=0.5 mTorr and a dc power of Pdc<100 W, i.e., a growth rate of 1 μm/h or less. The ratio of photo- to dark conductivity reaches a maximum value of σph/σ D=0.3 for σD=10-4 (Ω cm) -1 and correspondingly a normalized photoconductivity of ημτ=10-5 cm2/V is achieved. The increased dark conductivity of the a-Ge:H films is explained in terms of an unintentional n-type doping effect.