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

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 gamma (= [SiH(4)]/([SiH(4)] + [H(2)])) 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 gamma = 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 gamma 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: gamma approximate to 20% and T(s) x 200 degrees C.