Plasma polymerization and deposition of amorphous hydrogenated silicon from rf and dc silane plasmas

Abstract

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.