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

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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.