Isolated Voids in Amorphous Silicon and Related Materials Measured by Effusion of Implanted Helium

Effusion measurements of hydrogen and implanted helium are reported for (undoped) amorphous and crystalline Si:H and related materials. Effusion of helium observed at temperatures > 600°C is attributed to isolated voids present in the material from the preparation process. While rather high void densities are detected for amorphous silicon films prepared by such deposition techniques like vacuum evaporation or sputtering, much smaller densities are found for plasma grown hydrogenated amorphous silicon (a-Si:H). For device-grade a-Si:H, the density of cavities which can trap helium is estimated to be about 2x1018/cm3at most, suggesting that crystalline silicon type divacancies are not the major hydrogen incorporation site.