Defects in hydrogenated amorphous silicon-germanium alloys studied by photomodulation spectroscopy

The sub-band-gap photomodulation (PM) spectra of a series of amorphous hydrogenated silicon-germanium alloys (a-Si1-xGex:H) were measured. For dilute alloys (x<0.1) two PM bands were observed. One band had previously been associated with the D center in unalloyed a-Si:H. The relative strength of this band decreased as the alloy parameter x increased. We associated the second band with a germanium-related D center (Ge-D). For x>0.1 only this second (Ge-D) band was observed. The Ge-D bands peak was shifted by 0.4 eV from the band gap for the entire range of alloys. We analyzed this band using the assumption that the D0 and the D- levels of the Ge-D are symmetrical about the midgap energy. We estimated that the thermal levels of the D0 and the D- are 0.5 eV above the valence-band edge and 0.5 eV below the conduction-band edge, respectively. The result that the levels shift with alloying so as to track the band edges differs with some previous work which assumed constant level positions. The relaxation energy was 0.4 eV for all alloys while the effective correlation energy decreased linearly with x from 0.7 (for x=0.1) to 0.0 eV (for x=1). A model for the Ge-D which may explain its levels tracking of the band edges is tentatively proposed.