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 (T_s), total pressure (p_T), interelectrode spacing (d), and excitation power (P) were systemically varied. Films were characterized in terms of H content and bonding, optical gap (E_o), ambipolar diffusion length (l_d), 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 T_s=230 °C is found to be independently controlled by P, p_T, 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 E_o 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 E_o≂1.7 eV, l_d≂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.