Electronic and optical properties of glow-discharge amorphous silicon-carbon alloys

Abstract

Measurements are reported on the optical gap, dark conductivity and photoconductivity of hydrogenated amorphous silicon-carbon films (a-SiC : H) prepared from the glow-discharge decomposition of silane-methane mixtures in He for gas compositions g= (SiH₄)/(SiH₄ + CH₄) between 1 and 0. The optical gap E_g goes through a maximum at g ∼ 0·35 and the slope of the absorption edge decreases continuously as g is decreased from 1 to 0. The dark conductivity of the alloys, and also of a-C : H has been measured in the temperature range 300–470 K. Two conduction regimes are observed: extended-state conduction with an activation energy close to E_g /2 at high temperatures and hopping conduction in the conduction band tail at low temperatures. In the latter regime a well-defined activation energy is observed and from the change in the activation energy at the temperature of the transition, T _B ∼ 350 K, we conclude that the conduction band tail width ΔE increases continuously when the carbon concentration is increased up to g ∼ 0·25–0·30. The conductivity data in both regimes are found to follow the Meyer-Neldel rule. Finally, it is shown that the photoconductivity efficiency is decreased by about eight orders of magnitude when g decreases from 1 to 0, and that the temperature dependence of the photocurrent is thermally activated with an activation energy of 0·34 ± 0·2 eV between 293 and 400 K. The behaviour of carbon-rich samples is discussed in terms of known structural properties.