Electronic states in the gap of amorphous silicon-germanium alloys

The electron spin resonance spectra of a-Si_0.7Ge_0.3:H alloys are investigated as a function of doping with PH₃ and B₂H₆. Undoped and lightly-doped samples have Si and Ge dangling bonds as the main deep paramagnetic defects. From the doping dependence of the corresponding spin signals a higher average energy for the Ge dangling bond levels is deduced. The conduction band tail resonance (g = 2.0083) observed for strong P-doping is interpreted as due to fairly delocalized wavefunctions averaging over Si and Ge antibonding states. In contrast, the valence band tail states adjacent to the dangling bond levels are dominated by localized GeGe bonding orbitals.