Optoelectronic and structural properties of Ge-rich narrow band gap nc-SixGe1-x absorber layer for tandem structure nc-Si solar cells

Abstract

Germane (GeH4) and silane (SiH4) have grossly different dissociation behaviors. In the conventional preparation process, the inclusion of GeH4 in the gas mixture under nc-Si compatible growth conditions results in the spontaneous dissociation of GeH4 in the plasma into various lower hydrides of Ge with lower mobility than Si-hydrides. Ge-radicals exhibit a higher viscosity co-efficient and smaller surface diffusion length when compared to Si-radicals, which prompt the arbitrary and excess residence of Ge atoms at the growth sites, eventually destroying the development of nanocrystallinity in the film network. In the present work, the controlled inclusion of Si atoms during the growth of the network at ~220 °C under plasma conditions compatible for growing high-quality nc-Ge film has been pursued to produce nc-SiGe alloy material through which the dissociation of SiH4 becomes admissible for sustained nanocrystallinity. Systematic changes in the optoelectronic and structural characteristics of the intrinsic nc-SixGe1–x films from H2-diluted (SiH4 + GeH4)-plasma have been studied and correlated. The fabricated nc-SixGe1–x (x > 0) films belong to the Ge-nanocrystal dominated mixed-phase materials bearing a-SiGe matrix. The nc-SixGe1–x film sustaining significant crystallinity with an adequately widened optical band gap to Eg ~1.23 eV due to the incorporation of Si and its corresponding high dark conductivity (σD~4.72×10−4 S cm−1) appears appropriate for the absorber layer in the bottom sub-cell of nc-Si tandem solar cells.