Experimental data of photoconductivity (PC) are presented to determine optical and electrical properties of Si/Ge buffers, strained layer superlattices (SLS) and multiple quantum wells (MQW). The measured spectra can be decomposed into three absorption thresholds representing the substrate, the buffer, and the superlattice, respectively. In dependence on photon energy a phase change of 180 degrees between the illuminating reference signal and the PC response is observed at the fundamental band gap of the substrate. The phase shift represents a negative PC which can be suppressed by an additional voltage between substrate and film. A time-resolved analysis of the PC-signal reveals that the negative PC is due to a charge exchange between shallow impurity centers of the Si substrate and the film.