Photoluminescence in sputtered amorphous silicon-hydrogen alloys

Abstract

Results of photoluminescence (PL) experiments performed on amorphous silicon-hydrogen alloys prepared by sputtering in a plasma containing hydrogen are presented. A single, featureless peak centred near 1.3 eV and of 0.3 eV f.w.h.m. is observed upon excitation with 2.4 eV light in alloys of hydrogen concentration c_H≥5 at.% held at temperatures below 200 K. The PL quantum efficiency and peak energy are increasing functions of c_h. A high-temperature decrease in PL is observed, and electric fields of 10⁵ V/cm are found to be sufficient to quench the PL. It is contended that the original argument in the literature concerning the necessity for Stokes shifts of the order of 0.5 eV is wrongly based. From the present data we conclude that radiative recombination in our films may best be viewed in the framework of a rigid-band model with only small perturbations caused by distortional effects of the host matrix. The high-temperature thermal quenching of the PL is analysed in terms of two quenching mechanisms: one of high activation energy which dominates the quenching at high temperatures, and a second of much lower activation energy which dominates the quenching at low temperatures. The former (latter) involves the thermal ionization of a thermalized (thermalizing) electron-hole pair. We conclude that it is reasonable that the removal of gap states by the hydrogen permits radiative recombination by some route, either through a specific recombination centre or as a result of dribbling down through a high density of states. Results presented on PL in oxygenated a-Si : H suggest strongly that the hydrogen creates radiative recombination centres by providing states in the gap.