Passivation of dangling bonds in amorphous Si and Ge by gas adsorption

Amorphous Si and Ge films, prepared by molecular-beam deposition under ultrahigh vacuum at relatively low temperature, absorb gases such as ${\mathrm{H}}_2$, ${\mathrm{F}}_2$, ${\mathrm{O}}_2$, ${\mathrm{SiF}}_4$, ${\mathrm{GeF}}_4$, etc. Because of the high reactivity of the dangling bonds present, these molecules are broken down to their atomic constituents, thereby passivating the dangling bonds and producing chemisorption. It is highly significant that the dangling bonds on internal surfaces of microvoids in amorphous silicon can break down molecular hydrogen while on a cleaved c-Si surface they cannot. Posthydrogenation of amorphous germanium in ultrahigh vacuum with a low-energy differentially pumped sputtering gun resulted in a-Ge:H having a photoconductivity at room temperature, under an Air Mass 1 light, almost double the dark conductivity.