Deposition and photoconductivity of hydrogenated amorphous silicon films by the pyrolysis of disilane

The thermal decomposition of disilane (Si2H6) in a hydrogen or helium flow has been used for the deposition of hydrogenated amorphous-silicon (a-Si:H) films on the surface of Corning 7059 glass substrates at 450–500 °C. The reaction product consists of monosilane and trisilane in addition to the unreacted disilane and ethylsilane (the major impurity in commercial disilane). The concentration of Si2H6 in the reaction mixture has been found to strongly affect the deposition rate and the photoconductivity of a-Si:H films. At a given Si2H6 concentration, the deposition rate of a-Si:H films increases exponentially with temperature. At a given substrate temperature, the AM1 conductivity of a-Si:H films increases with increasing Si2H6 concentration and approaches 10−5 Ω cm−1 at Si2H6 concentrations higher than about 4%, and the conductivity ratio in better films is about 105. The conductivities of CVD a-Si:H films have been found to show negligible change under illumination over a period of several days. The optical band gap of a-Si:H films has been determined to be 1.65–1.68 eV. The doping of a-Si:H films with arsenic and boron has been carried out, and their activation energies determined.