Thermal equilibrium, the Staebler-Wronski effect and potential fluctuations in lithium-doped hydrogenated amorphous silicon

We report the effect of fast thermal quenching (FQ) and light soaking (LS) on electronic transport in lithium-doped hydrogenated amorphous silicon (a- Si:H(Li)), and compare the two instabilities in the same specimen. We find that the metastable changes are qualitatively similar to those in phosphoru s-doped hydrogenated amorphous silicon. The films are in thermal equilibriu m above the temperature T-E, which is lower for heavier doping. However, fo r a given sample, the T-E values are about the same for FQ and LS. Isotherm al relaxation of conductivity a of the metastable states follows a stretche d exponential, but the relaxation parameters for FQ and LS are different. W e also find that the function Q = ln sigma - eS/k shows a large change afte r LS but hardly changes after FQ. The results are explained by assuming tha t the recombination of carriers at the local minima of the potential fluctu ations present on a-Si:H(Li) gives rise to structural changes. The observed increase in the slope of Q versus 1/T after LS is interpreted as an increa se in potential fluctuations. The very small change in Q observed after FQ is explained in terms of the higher mobility of hydrogen at the high temper ature from which a-Si:H is quenched.