Self-assembled ultra-nanocrystalline silicon films with preferred ⟨2 2 0⟩ crystallographic orientation for solar cell applications

Using low-pressure planar inductively coupled plasma CVD at 87% H-2-dilution to the Sift' plasma, nc-Si:H films are prepared that possess preferential growth along < 2 2 0 > crystallographic orientation with I-220/I111 >1.2, bonded H-content of similar to 5.5 at.%, a low microstructure factor of similar to 0.56, along with a reasonably high sigma(D) similar to 5.2 x 10(-4) S cm(-1), Delta E similar to 143 meV and sigma(Ph) similar to 1.4 x 10(-3) S cm(-1). The growth of the nc-Si:H network has been optimized to a moderately high nanocrystallinity (similar to 68%), with an average grain size of similar to 8 nm. The overall network comprises a significant fraction of ultra-nanocrystalline component X-unc/X-nc similar to 0.47, which are dominantly inhabited by the thermodynamically preferred < 2 2 0 > crystallographic orientation that provides convenient electrical transport perpendicular to the film surface and subsequently could facilitate photovoltaic performance. The cross-sectional view of the fracture surface demonstrates columnar structures, closely correlated to the favored growth of the nanocrystallites along < 2 2 0 > crystallographic orientation that retains direction perpendicular to the substrate surface. The underlying phenomena could be demonstrated as a consequence of preferential growth induced by high atomic H density present in the planar inductively coupled SiH4 plasma obtained via much lower H2-dilution compared to that realized in conventional capacitively coupled plasma-CVD. The nc-Si:H films with precise material properties as well as the allied low-pressure ICP-CVD growth process could be of significant use in further progress of nc-Si solar cells. (C) 2015 Elsevier BAT. All rights reserved.