Observation of Deep Defect Relaxation Processes in Hydrogenated Amorphous Silicon-Germanium Alloy

Abstract

Modulated photocurrent (MPC) spectroscopy has been used to investigate the energy distribution of deep defect states in photo-CVD grown a-SilxGex:H alloys. We observe a distinct electron trapping feature at a thermal energy of 0.55eV below Ec for higher Ge concentration (x = 0.62) alloys. For those samples with Ge concentration between 30-50 at.%, an anomalously large phase shift was observed within a temperature window between 150K to 240K. The modulated photocurrent exhibits strong quenching at the edges of this window. We suggest that these MPC spectra are controlled by deep defect relaxation processes in this temperature region, which causes electron thermal emission processes to be greatly suppressed. This effect can be reduced by increasing the reverse bias. Under such conditions, the energy position of these defect states can be roughly estimated to lie near Ec -0.33eV. This result supports the recent result obtained from transient photocapacitance and photocurrent measurement.