OPTIMIZATION OF PROCESS PARAMETERS FOR THE DEPOSITION OF IMPROVED A-GEH BY DC MAGNETRON SPUTTERING

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 X 10(22) atoms/cm3 and contains up to 4 X 10(21) H atoms/cm3. 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 alpha (hnu=0.6 eV) < 10 cm-1 and Urbach energy E0 < 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 P(dc) < 100 W, i.e., a growth rate of 1 mum/h or less. The ratio of photo- to dark conductivity reaches a maximum value of sigma(ph)/sigma(D)=0.3 for sigma(D)=10(-4) (OMEGA cm)-1 and correspondingly a normalized photoconductivity of etamutau=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.