A-Si:H prepared by ion bombardment activated hydrogen incorporation during Si evaporation

Abstract

a-Si:H is prepared by vacuum evaporation of Si from a crucible kept at high positive potential relative to the substrate. The concomitant bombardment of the growing film surface with ionized vapor atoms (Si+) is found to activate the incorporation of hydrogen atoms cracked off from residual water molecules (pressure=2×10−7 Torr); a-Si:H containing up to 25 at. % of H may thus be obtained. The dark conductivity changes from activated behavior (low conductivity) to hopping (high conductivity) behavior by screening off this bombardment, while the dangling bond content (2×1017<Ns <1018 cm−3), as determined by electron-spin resonance, does not change within an order of magnitude. The hydrogen incorporation into the Si structure is studied by infrared absorption, revealing most of the incorporated hydrogen to prevail near the void surfaces in the Si–H2 configuration. The results point at the existence of an inhomogeneous film structure (microstructure), the specific features of which are controlled by the ion bombardment.