STUDIES OF SUBGAP ABSORPTION AND RELATED PARAMETERS BY THE CONSTANT PHOTOCURRENT METHOD OF HIGH-RATE DEPOSITED HYDROGENATED AMORPHOUS-SILICON FILMS

The constant photocurrent method (CPM) has been used to measure subgap-related parameters such as absorption coefficient-alpha, characteristic energy E0 of tail states and density of subgap defect states together with an estimate of the band gap of hydrogenated amorphous silicon (a-Si:H) films prepared at various deposition rates. A higher deposition rate in the home-made plasma chemical vapour deposition system was obtained by an improved design of powered electrode and an earthed shield coupled with optimization of process parameters. The results of our measurement show that with increase in deposition rate of a-Si:H films from 6.7 to 18 angstrom s-1, the values of the characteristic energy E0, defect density N(s) and band gap E(g) increases from 55.9 to 65.2meV, from 3.3 x 10(16) to 9.8 x 10(16) cm-3 and from 1.73 to 1.80eV respectively. CPM measurements were further extended on light-soaked and annealed samples and it was found that E0 and the defect density increase after light soaking and the effect is reversed after annealing the sample at 160-degrees-C for 1 h.