Low Filament Temperature Deposition of a-Si:H by Catalytic Chemical Vapor Deposition

Abstract

Hydrogenated amorphous silicon, a-Si:H, is deposited from silane (SiH4) and hydrogen (H2) using a tungsten wire at low filament temperatures (Tfil = 1200°C) by catalytic chemical vapor deposition.The deposition rate increases monotonically with the depositions pressures and shows a maximum at an H2 : SiH4 flow ratio of unity. Vanishingly small deposition rates were observed for silane-only depositions and for H2-to-SiH4 flow ratios of 2.5 and above. The optoelectronic properties show complex dependence on substrate temperature (Tsub). Three intervals of Tsub with distinct optoelectronic were observed: as Tsub increases from 180 to 220°C, the optical bandgap, EgTauc increases from 1.9 to 2.4eV, the dark conductivity,σd, decreases from 10-10 to 10-15 Ω-1cm-1and the photoconductivity, σph, decreases from 10-5 to 10-10Ω-1cm-1 (region (i)). As Tsub increases from 220 to 250 °C, EgTauc decreases to 1.8eV and the photosensitivity, σph/σd decreases to ~ 1 due to an increase of both σd and σph (region (ii)). Throughout these two regions, the photoconductivity γ factor remains between 0.6 and 0.9 and the activation energy of the dark conductivity, Ea,σd, remains between 0.7 and 0.9eV. Above 250°C, the σph and σd remain approximately constant at 10-4 Ω-1cm-1 and γ decreases to below 0.5 and Ea,σd ~ 0.3eV.