On the electronic transport properties of amorphous (Si,Ge) alloys: charged scattering centers and compositional disorder

The electronic transport properties of a-(Si,Ge):H alloys over the entire range of Ge content (0-100%) grown using low pressure, reactive ECR plasma deposition with high H dilution and subtle (sub-ppm) B-doping have been investigated in detail by employing the microwave photomixing technique. Strong evidence for the existence of long-range potential fluctuations in a-(Si,Ge):H alloys has been found from the measurements of electric field dependence of the drift mobility. It was observed for the first time that the film transport properties do not degrade monotonically with increasing Ge content; there exists a valley point near the middle of the composition range, where the strongest potential fluctuations occur as a result of a significant increase in the charged defect density. Beyond this point, the film quality increases again, but still much worse than that for the Si end. The effect of potential fluctuations can be enhanced by adding Ge or Si to the alloy system resulting in deterioration of the transport properties of a-(Si,Ge):H alloys. Our present results demonstrate that the increased charged scattering centers and compositional disorder upon adding Ge or Si to the alloys play an important role in the potential fluctuations.