Temperature dependence of the photoconductivity of gallium-doped hydrogenated amorphous germanium films

This work reports a study of the photoconductivity (PC) of Ga-doped a-Ge:H films as a function of temperature and photon flux. The experimental data d iscussed here refer to: (a) the photocurrent, Tpc, at fixed photon energy ( 1.3 eV) and fixed photon flux (F approximate to 2.4 x 10(16) cm(-2) s(-1)) in the 14-380 K temperature range and (b) the variation of the exponent, ga mma(I-PC F-7) with temperature (55-380 K) and doping level for Fin the 8 x 10(14)-3 x 10(16) cm(-2) s(-1) range. In all cases, I-PC is approximately i ndependent of temperature, T, in the low temperature region (T < 40 K) and increases for T > 40 K. The I-PC Of the undoped and the most lightly doped samples increases up to a maximum at T similar to 230 K, and decreases for higher temperatures (thermal quenching, TQ). The maximum peak of I-PC becom es smaller and appears as a shoulder as the doping level increases. However , this shoulder can still be interpreted as an evidence of TQ. The onset te mperature of the TQ becomes smaller as the Ga concentration increases. A ga mma(min), corresponding to a T-min is measured. Both gamma(min) and T-min v ary with doping. The gamma(min) and T-min are maxima for compensated sample s and decrease as the Fermi energy shifts from midgap in either direction. (C) 2000 Elsevier Science B.V. All rights reserved.