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, and the amount of 02 and/or H&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 0 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 0 and H atoms, and in the relationship between these bonding environments and the source of the oxygen atoms (02 or H20). 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.

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Films of amorphous hydrogenated silicon were deposited from silane and disilane plasmas in a planar diode reactor using rf and dc excitation. Substrate temperature (Ts), total pressure (pT), interelectrode spacing (d), and excitation power (P) were systemically varied. Films were characterized in terms of H content and bonding, optical gap (Eo), ambipolar diffusion length (ld), and performance in p-i-n solar cell structures. The extent of silane polymerization to disilane and higher homologs by homogeneous plasma reactions is indicated as a dominant factor in controlling the composition and properties of a-Si:H deposited form rf- and dc-excited silane plasmas. The optimization of film properties at Ts=230 °C is found to be independently controlled by P, pT, and d inasmuch as they determine the degree of plasma polymerization. Films prepared under conditions promoting plasma polymerization resemble films made from SiH4–Si2H6 mixture feedstocks with nonpolymerizing conditions, exhibiting increased H content, dihydride density, and Eo as well as poorer electronic properties. The similarity of film properties for a variety of rf and dc nonpolymerizing plasmas suggests that the identity of the reactive monomeric film-forming specie(s) is either invariant or noncritical. These films have Eo≂1.7 eV, ld≂0.3 μm, and produce p-i-n solar cells with efficiencies≂7.5%. The roles of homogeneous and heterogeneous processes were investigated. Implications for maximum achievable deposition rates and silane utilization are discussed.

On the correlation among photodegradation, charged dangling bonds and microstructure in hydrogenated amorphous silicon

Interactions between radical growth precursors on plasma-deposited silicon thin-film surfaces Dependence of thinfilm microstructure on deposition rate by means of a computer simulationStep coverage in the vacuum deposition of thin metal films