Chemical bonding of hydrogen and oxygen in glow-discharge–deposited thin films ofa-Ge:H anda-Ge:(H,O)

Abstract

We have grown thin films of a-Ge:H and a-Ge:(H,O) by the glow-discharge process, and have used infrared (ir) absorption spectroscopy to study the local bonding of hydrogen and oxygen as a function of the substrate temperature $T_s$ and the amount of ${\mathrm{O}}_2$ and/or ${\mathrm{H}}_2$O added to the germane gas mixture in the plasma. The temperature dependence of the ir features associated with monohydride and dihydride groups in a-Ge:H alloys is different in these films, grown at relatively high plasma power, from what has been reported in films of a-Si:H grown from silane plasmas at lower power levels. In particular, dihydride incorporation in these a-Ge:H films displays a distinct temperature threshold, rather than a competition with the monohydride bonding as in the a-Si:H films. The bonding of H and O atoms in the ternary alloys a-Ge:(H,O) is also different from what has been reported for the corresponding Si ternary alloys, both in the nature of the local atomic arrangements of the O and H atoms, and in the relationship between these bonding environments and the source of the oxygen atoms (${\mathrm{O}}_2$ or ${\mathrm{H}}_2$O). These differences in bonding in a-Ge:H and a-Ge:(H,O) films with respect to the corresponding a-Si alloy films reflect differences in both the plasma-phase precursor chemistry and temperature-dependent reactions at the growth surface.