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 discussed here refer to: (a) the photocurrent, I _PC , at fixed photon energy (1.3 eV) and fixed photon flux ( F≈2.4×10 16 cm −2 s −1 ) in the 14–380 K temperature range and (b) the variation of the exponent, γ( I _PC∝ F ^γ) with temperature (55–380 K) and doping level for F in the 8×10 14 –3×10 16 cm −2 s −1 range. In all cases, I _PC is approximately independent 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∼230 K, and decreases for higher temperatures (thermal quenching, TQ). The maximum peak of I _PC becomes 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 temperature of the TQ becomes smaller as the Ga concentration increases. A γ _min corresponding to a T _min is measured. Both γ _min and T _min vary with doping. The γ _min and T _min are maxima for compensated samples and decrease as the Fermi energy shifts from midgap in either direction.