Properties of polymorphous silicon-germanium alloys deposited under high hydrogen dilution and at high pressure

Hydrogenated polymorphous silicon-germanium samples with Ge content x up to 0.35 have been deposited in a plasma-enhanced chemical vapor deposition system from silane and germane precursors highly diluted in hydrogen and in a high pressure range. The optical, defect-related, and transport properties of both types of carriers have been studied using a set of complementary techniques before light soaking, after light soaking, and after subsequent annealing. Before light soaking, we observe a small deterioration of the electron transport properties with increasing Ge content. This is related to the widening of the conduction band tail and to the increase of a deep defect band with emission energies at about 0.4 eV below the conduction transport path, which is attributed to Ge dangling bonds. The hole transport properties are much less sensitive to the incorporation of Ge, which we attribute to the unchanged Urbach tail parameter. Light soaking results in a deterioration of the transport properties due to an increase of the density of states, the relative effect being less pronounced for the highest Ge contents due to the higher native defect concentrations. Again, the electron transport properties are more affected than the hole transport properties, which appear quite stable against light soaking. Annealing at 460 K following light soaking is found to restore the initial state more easily in the low Ge content samples. Compared to samples deposited under more conventional conditions (no or less hydrogen dilution and low pressure) the striking features of our alloys are that (i) they exhibit an enhanced stability of the transport properties against light soaking and (ii) the characteristic time of the light-soaking kinetics is shorter. These features reveal the polymorphous character of our alloys, related to a specific hydrogen microstructure observed by infrared spectroscopy and hydrogen effusion, which indicate that the amorphous matrix exhibits an improved order like in hydrogenated polymorphous silicon.