Thin Solid Films, 164 (1988) 217 220 217 AN INTERESTING UNCONVENTIONAL PHOTOINDUCED CHANGE IN THE DARK CONDUCTIVITY OF a-Si:F:H* S. C. DE, GAUTAM GANGULY, SWATI RAY AND A. K. BARUA Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032 (India) The results of light exposure on dark conductivity, its activation energy, and the photoconductivity of a-Si:F:H samples are presented. A light-induced decrease in photoconductivity by a factor of about 3 is observed. The changes in dark conductivity are rather unusual. The direction of the change is found to depend on the position of the Fermi level. At a particular activation energy (about 0.85 eV) no light-induced change in the dark conductivity occurs. This behaviour has been explained in terms of a model. The temperature dependence of the defect creation and annealing has also been reported. 1. INTRODUCTION Light-induced changes in the opto-electronic properties 1 and the consequent degradation of the performance of amorphous silicon thin-film photovoltaic devices 2 has been an active area of research. One of the reasons for the interest in fluorinated hydrogenated amorphous silicon thin films, first prepared by Ovshinsky and Madan 3, has been the reduced light-induced metastable changes in the properties of the material 4. Extensive study of the properties of a-Si:F:H as a function of the deposition parameters 4' 5 has established that device quality material can be prepared by the glow discharge deposition technique. We have undertaken an intensive study of the effects of light soaking on the dark conductivity, activation energy and photoconductivity of a-Si:F:H thin films. Here, we report mainly the unusual changes in the dark conductivity and activation energy observed in a-Si:F:H films deposited by the r.f. glow discharge deposition technique. 2. EXPERIMENTAL DETAILS The a-Si:F:H samples were prepared from mixtures of silicon tetrafluoride (SiF4) and hydrogen, in a capacitively-coupled plasma CVD system. The samples used in this study were deposited on glass (Corning 7059), and conductivity measurements were performed with aluminium electrodes in a coplanar geometry *Paper presentedat the 7th International Conference on Thin Films, New Delhi, India, December 7 11, 1987. 0040-6090/88/$3.50 © ElsevierSequoia/Printed in The Netherlands