Irreversible changes in doping efficiency and hydrogen bonding in the equilibrium state ofa-Si:H

We report unexpected irreversible changes in the conductivity ({σ}), dangling-bond density (DB), infrared-vibrational-absorption spectrum, and other properties of hydrogenated amorphous silicon (a-Si:H) by annealing samples above the glass transition temperature but below the deposition temperature. After annealing, both {σ} and the DB density decrease for P-doped samples, suggesting a decrease in donor-doping efficiency. For B-doped samples, both {σ} and the DB density increase after annealing, suggesting an increase in acceptor-doping efficiency. In the infrared-absorption spectrum, an increase at wave number 2085 cm^-1, together with a decrease at 1940 cm^-1 with the same magnitude, and an increase at 620 cm^-1 are observed after annealing. Possible explanations are discussed. No H effuses out of the sample during this irreversible annealing. Only a few percent of H rearranges its bonding. Our results show that a-Si:H is not in equilibrium after deposition but that it relaxes slowly to a more stable state above the equilibrium temperature.