Time-of-flight measurement of hole transport in lightly boron-doped a-Si:H films

Abstract

Effects of slight boron-doping on the mobility and the mobility-lifetime ( μτ ) product of holes in glow-discharged hydrogenated amorphous silicon films have been investigated by means of time-of-flight measurement. When a small amount of boron in the range of the flow rate ratio B 2 H 6 /SiH 4 = 17–50 ppm is doped, hole transport at room temperature becomes non-dispersive, in contrast with a dispersive nature in undoped films. The hole mobility and the μτ product at room temperature have large values at these flow rate ratios. However, when the flow rate ratio B 2 H 6 /SiH 4 is further increased, holes show again dispersive transport accompanying a reduction in mobility and μτ product. The activation energy of the hole mobility is nearly 0.4 eV, independent of boron-doping level. These results are tentatively interpreted in terms of an introduction of localized states associated with boron or on the basis of a two-phase model in which a-Si:H films are considered to be composed of an entangled network of paramicrocrystalline zones and highly disordered Si:H alloy regions.