A-Si:H Deposition from SiH4 and Si2H6 rf-Discharges: Pressure and Temperature Dependence of Film Growth in Relation to α-γ Discharge Transition

Abstract

We present an interpretation of the pressure and temperature dependence of the growth kinetics of hydrogenated amorphous silicon (a-Si:H) in SiH4 and Si2H6 rf-glow discharges. At constant rf power, the a-Si:H deposition rate increases drastically at a given pressure depending on the nature of the gas and on the wall temperature. The threshold nature of this transition is attributed to the onset of an electron avalanche due to ion-induced secondary electron emission and ionization in the plasma sheaths close to the electrodes. We analyze the effect of various plasma parameters governing this α-γ discharge transition in terms of equivalent circuit of the discharge and power dissipation mechanisms. The ratio of a-Si:H deposition rates from SiH4 and Si2H6 rf discharges at the same rf power and flow rate is strongly dependent on the pressure because the α-γ transition occurs at a lower pressure for Si2H6 than for SiH4. The transition is shifted to higher pressures as the temperature increases primarily because of the reduction of gas density, which explains contradicting results in the literature on the influence of temperature on a-Si:H deposition rate. At a given rf power and substrate temperature, the optical, structural and electrical film properties are correlated with the variation of deposition rate as a function of SiH4 or Si2H6 pressure.