Effect of annealing on the electrical, optical and structural properties of hydrogenated amorphous silicon films deposited in an asymmetric RF plasmaCVD system at room temperature

This paper reports the effect of annealing on hydrogenated amorphous silico n films (a-Si : H) deposited by r.f self-bias technique on cathode in an as ymmetric r.f. plasma CVD system at room temperature. Detailed study of the variation of the dark and photoconductivity (sigma(D) and sigma(ph)) as a f unction of temperature and light intensity, surface morphology, hydrogen ev olution, optical absorption, subgap absorption and related parameters, ther mal and structural disorder on the optical-absorption edge, IR vibrational modes and bonded hydrogen content have been carried out on unannealed and a nnealed samples at different temperatures (T-a) from 100 degrees C to 550 d egrees C. It is found that the values of sigma(ph) increase and that of Urb ach energy (E-o), subgap defect density (N-d) and the polyhydride to monohy dride ratio decrease upto T-a = 250 degrees C and beyond 250 degrees C the values of sigma(ph) decrease and that of E-o, N-d and the polyhydride to mo nohydride ratio increase. The best opto-electronic properties with much imp roved sigma(ph) and sigma(ph)/sigma(D) and dominant monohydride bonding are obtained after annealing the room temperature deposited film at 250 degree s C for 1 h. The sigma(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 a nd 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 diso rder part decreases with the annealing temperature upto similar to 300 degr ees C and thereafter it increases. The curves of optical absorption coeffic ient versus photon energy at different T-a converge to a common point. (C) 1999 Elsevier Science B.V. All rights reserved.