Low temperature plasma processing of nc-Si/a-SiNx:H QD thin films with high carrier mobility and preferred (220) crystal orientation: a promising material for third generation solar cells

Abstract

In view of potential applications in third generation nc-Si solar cells, nanocrystalline silicon quantum dots (nc-Si QDs) embedded in amorphous hydrogenated silicon-nitride dielectric matrix (a-SiN_x:H), nc-Si/a-SiN_x:H QDs thin films, are produced by single step low temperature plasma processing of silane (SiH₄) and ammonia (NH₄) diluted in H₂, using planar inductively coupled rf plasma-CVD. By decreasing the deposition temperature over 400–100 °C, the undoped nc-Si/a-SiN_x:H QDs thin films of varying crystallinity (82–37%) are obtained with Si-ncs of average size ∼5.7–1.3 nm and number density ∼10¹¹ to 10¹² cm⁻², providing a significantly wide range of band gap and high optical absorption (>10⁵ cm⁻¹) with associated very high electrical conductivity, σ_d ∼ 5.6 × 10⁻³–2.7 × 10⁻⁷ S cm⁻¹ along with high carrier concentration, n_e ∼ 9 × 10¹³–1.8 × 10¹⁰ cm⁻³, significantly high electron mobility, μ_e ∼ 426–103 cm² V⁻¹ s⁻¹ and photosensitivity varying within ∼1 × 10¹–3 × 10³. At reduced temperature (100 °C), although the crystalline volume fraction decreases, the overall crystallinity is mostly populated by the ultra-nanocrystalline component with dominant (220) crystallographic orientation which has favored electrical transport in stacked layered devices. High-density tiny nc-Si QDs induce a reasonable widening of the band gap even at very low nitrogen content, while sufficiently high σ_d with improved photosensitivity, (σ_Ph/σ_d ∼ 3 × 10³) is a consequence of significantly high electron mobility attained by virtue of efficient defect passivation by the high atomic hydrogen density of the low-pressure ICP plasma that finally assists in obtaining a promising low temperature grown material for suitable applications in third generation tandem structure nc-Si solar cells.