Hydrogen in hydrogenated amorphous silicon thick film and its relation to the photoresponse of the film in contact with molybdenum

Hydrogenated amorphous silicon films of thickness 0.5-7 μm on molybenum substrates were deposited from silane by dc glow discharge and studied by mass spectrometric observation of the evolution of hydrogen upon heating and correlating this information with the photoresponse. The films were found to contain two types of hydrogen, namely weakly bonded hydrogen, which evolved at 365°C and was the minority, and strongly bonded hydrogen, which evolved at 460-670°C and was the majority. The proportion of strongly bonded hydrogen increased with increasing film thickness and with increasing substrate temperature during deposition. The total amount of hydrogen increased when the substrate temperature was decreased from 350 to 275°C. The strongly bonded hydrogen resided throughout the thickness of the film, whereas the weakly bonded hydrogen resided near the film surface. The evolution of the strongly bonded hydrogen was diffusion controlled, with an activation energy of 1.6 eV. The strongly bonded hydrogen enhanced the photoresponse, whereas the weakly bonded hydrogen degraded the photoresponse.