Effect of hydrogen dilution on the optical properties of hydrogenated amorphous silicon prepared by plasma deposition

Abstract

A series of experimental studies has been made on the effect of the dilution of silane with hydrogen on optical properties of hydrogenated amorphous silicon films (a-Si:H) prepared by plasma deposition as functions of the gas-volume ratio γ (=[SiH₄]/([SiH₄] + [H₂])) and the substrate temperature T _s. The effect of hydrogen dilution has been discussed in terms of the obtained values of the deposition rate R _d, the optical gap E _g, the Urbach energy E _u, the defect density N _d, the hydrogen content C _H and the refractive index n ₀ and their correlations between them and the hydrogen-bonding configuration estimated from infrared absorption spectra. Its effect strongly depends on T _s and decreases N _d and E _u from γ = 100% to about 20%. The effect for reducing them is discussed in terms of the surface-limited optimal growth model containing the hydrogen dilution effect. On decreasing γ from about 10%, both N _d and E _u increase in spite of the decrease in R _d. This result is due to the onset of the formation of microcrystalline phase and is discussed in terms of the ‘selective etching’ model. An a-Si: H film with E _u less than 50 meV is prepared at our optimal growth conditions: γ ≈ 20% and T _s, ≈ 200°C.