Photoconductivity of Si/Ge buffers, superlattices, and multiple quantum wells

Experimental data of photoconductivity (PC) are presented to determine opti cal and electrical properties of Si/Ge buffers, strained layer superlattice s (SLS) and multiple quantum wells (MQW). The measured spectra can be decom posed into three absorption thresholds representing the substrate, the buff er, and the superlattice, respectively. In dependence on photon energy a ph ase 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 addition al voltage between substrate and film. A time-resolved analysis of the PC-s ignal reveals that the negative PC is due to a charge exchange between shal low impurity centers of the Si substrate and the film. (C) 1999 Elsevier Sc ience S.A. All rights reserved.