Effect of annealing on the electrical, optical and structural properties of hydrogenated amorphous silicon films deposited in an asymmetric R.F. plasma CVD system at room temperature

Abstract

This paper reports the effect of annealing on hydrogenated amorphous silicon films (a-Si : H) deposited by r.f. self-bias technique on cathode in an asymmetric r.f. plasma CVD system at room temperature. Detailed study of the variation of the dark and photoconductivity ( σ D and σ ph) as a function of temperature and light intensity, surface morphology, hydrogen evolution, optical absorption, subgap absorption and related parameters, thermal and structural disorder on the optical-absorption edge, IR vibrational modes and bonded hydrogen content have been carried out on unannealed and annealed samples at different temperatures ( T a) from 100°C to 550°C. It is found that the values of σ ph increase and that of Urbach energy ( E o), subgap defect density ( N d) and the polyhydride to monohydride ratio decrease upto T a=250°C and beyond 250°C the values of σ ph decrease and that of E o, N d and the polyhydride to monohydride ratio increase. The best opto-electronic properties with much improved σ ph and σ ph/ σ D and dominant monohydride bonding are obtained after annealing the room temperature deposited film at 250°C for 1 h. The σ D data obeys a Meyer Neldel rule in annealed a-Si : H films. The value of optical band gap is found to be related to the E o and the hydrogen content. The Urbach energy ( E o) which is a measure of the disorder is the sum of structural and thermal disorder. The structural disorder part decreases with the annealing temperature upto ∼300°C and thereafter it increases. The curves of optical absorption coefficient versus photon energy at different T a converge to a common point.