Optimal H2-dilution playing key role in accomplishing significant nanocrystallinity with both Si and Ge moieties in SiGe nanocomposite thin film network

Abstract

Accomplishing significant nanocrystallinity in SiGe alloy network during low-temperature growth in RF-PECVD is a challenging task because it involves grossly different decomposition kinetics of source gases GeH4 and SiH4. The major limitation lies in its rapid deterioration of optoelectronic qualities at increased Ge-inclusion, via the overpopulation of dangling bonds, two-phase heterostructure contributing two transport paths, and non-uniform distribution of H-bonding within its two competent counterparts that lead towards complete amorphization of the film network. In the present work, SiGe alloy films are grown in 13.56 MHz RF-PECVD at a low TS ∼220 °C, by varying H2-dilution to the (SiH4 + GeH4) plasma. At very low H2-dilution, Ge-dominated low bandgap SiGe material with a dominant amorphous network and subsequent low dark-conductivity is obtained. At an increased H2-dilution, a highly nanocrystalline SiGe network with significant nc-Ge component is produced; however, a reduced Ge-content with H-bonding in mostly poly-hydride configuration opposes bandgap narrowing. The nc-SiGe film possesses optimally narrow optical bandgap (Eg ∼1.5 eV) by substantial Ge present in the network, and significant dark-conductivity (σD ∼2.38 × 10–3 S.cm−1) arising from considerable crystallinity in both Ge-Ge and Si-Si moieties, prepared at optimum H2-dilution [H2 / (SiH4 + GeH4)] = 25. It appears appropriate for an ideal absorber-layer in the bottom sub-cell of tandem-structured nc-Si solar cells.