Growth and properties of high quality a-(Si,Ge):H alloys using helium and hydrogen dilution

We report on the growth and properties of a-(Si,Ge):H films and devices prepared using dilution with both hydrogen and helium. The films and devices were grown using a remote ECR plasma discharge. Helium dilution allowed for a higher growth rate than achieved using only hydrogen dilution. It was found that hydrogen dilution was always necessary to achieve good electronic properties. The films were characterized using photoconductivity and subgap absorption measurements. Defect density was measured using space-charge limited current in device-like samples. It was found that good quality films, with Urbach energies below 50 meV, and subgap α ∼ 1/cm could be grown down to a Tauc bandgap of 1.33 eV. The defect densities increased with increasing Ge content, but were still in the range of 1-2 × 10¹⁶ /cm³-eV. Substrate-type p-i-n devices were fabricated in these materials, and good fill factors were obtained. The films and devices were tested for stability.