Amorphous and microcrystalline silicon films grown at low temperatures by radio-frequency and hot-wire chemical vapor deposition

Abstract

The effect of hydrogen dilution on the optical, transport, and structural properties of amorphous and microcrystalline silicon thin films deposited by hot-wire (HW) chemical vapor deposition and radio-frequency (rf) plasma-enhanced chemical vapor deposition using substrate temperatures (T-sub) of 100 and 25 degrees C is reported. Microcrystalline silicon (mu c-Si:H) is obtained using HW with a large crystalline fraction and a crystallite size of similar to 30 nm for hydrogen dilutions above 85% independently of T-sub. The deposition of mu c-Si:H by rf, with a crystallite size of similar to 8 nm, requires increasing the hydrogen dilution and shows decreasing crystalline fraction as T-sub is decreased. The photoconductivity, defect density, and structure factor of the amorphous silicon films (a-Si:H) are strongly improved by the use of hydrogen dilution in the T-sub range studied. a-Si:H films with a photoconductivity-to-dark conductivity ratio above 10(5), a deep defect density below 10(17) cm(-3), an Urbach energy below 60 meV and a structure factor below 0.1 were obtained for rf films down to 25 degrees C (at growth rates similar to 0.1-0.4 Angstrom/s) and for HW films down to 100 degrees C (at growth rates similar to 10 A/s), using the appropriate hydrogen dilution. In the low T-sub range studied, the growth mechanism, film properties, and the amorphous to microcrystalline silicon transition depend on the flux of atomic hydrogen available. The properties of the films are compared to those of samples produced at 175 and 250 degrees C in the same reactors.