Influence of hydrogen on optical properties of <i>a</i>-Si : H

Abstract

We report on the optical properties of a-Si : H prepared by cathodic sputtering of a Si target in an 80% argon/20% hydrogen reactive gas at ∼9×10−3 Torr. The deposition rate φ (by means of the rf voltage) and the substrate temperature were varied. The total hydrogen content [H] and the concentration of hydrogen involved in SiH bonds [H1] or in SiH2 bonds [H2] were measured. We find [H]≳[H1]+[H2], two times higher in some preparation ranges (ν?15 Å/min or ν=100 Å/min), which implies the occurrence of hydrogen atoms H′ not bonded in the usual SiH or SiH2 forms. The optical absorption coefficient, the optical energy gap E0, and the refractive index ns were measured. E0 decreases as v increases, is independent of Ts for φ≳30 Å/min, and decreases as Ts increases for v≳30 Å/min. ns increases toward the crystalline value as φ increases and Ts decreases. The similarity of the properties of the layers obtained by sputtering and glow discharge is emphasized. We suggest equilibrium between hydrogen bonds as SiH and SiH2 and the other form(s) and that the optical gap is controlled by the deformation of the silicon matrix by the various hydrogen species.