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S. NONOMURA*, T. GOTOH*', M. NISHIO*, T. SAKAMOTO*, M KONDO**, A. MATSUDA** and S.NITTA* *Department of Electrical Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan "**ThinFilm Silicon Solar Cells Superlab, ETL, Ibaraki, Japan

ABSTRACT The structural aspect of photodegradation effect in hydrogenated amorphous silicon has been investigated by the use of the simple and sensitive detection technique, the laser opticallever bending method, for a small expansion or extraction in thin films. The volume change induced by the thermal expansion due to the photothermal effect and the residual expansion was observed in hydrogenated amorphous silicon prepared by PECVD. The latter residual expansion was persistent after light soaking and was recovered by thermal annealing at 200 'C. The time dependence of the volume expansion with light soaking shows the same time dependence of photoinduced defect density. The photoinduced volume changes normalized by the initial volume are the order of 10-s-10o, which values are two orders smaller than chalcogenide glasses such as a-As 2S'. The normalized volume change of a-Si:H with the different sample preparation conditions of PECVD such as the hydrogen dilution ratio r (r = SilH/12) and substrate temperature is shown. Also it is demonstrated that the photoinduced expansion is observed in hydrogenated amorphous silicon prepared by photo CVD and hotwire CVD methods. The spatial extent related to a photoinduced defect creation in a-Si:H is estimated.

INTRODUCTION The light induced decrease of photoconductivity by Steabler-Wronski [1] is a first finding of photoinduced phenomenon in hydrogenated amorphous silicon (a-Si:H). It was elucidated that the phenomenon is caused by the defect creation [2,3]. Many models [4] have tried to interpret the mechanism of the light induced defect creation, but the conclusive consensus is not obtained even in the present stage. It is well known that chalcogenide glasses show the large fractional thickness change due to the band gap light soaking. The estimated thickness change is 3.8x10 3 in a-As 2S3 [5], and the structural change is detectable by XRD. On the other hand, no direct evidence of structural change in a-Si:H has been reported until now. Recently, several groups have reported experimental evidences of macroscopic structural change using the polarized electroabsorption method [6] and the laser ablation time of flight mass spectroscopy [7], after proposing the possibility of structural change [8] by 1.Present address; Laboratory of Photoelectronic Materials, Department of Applied Physics, Hokkaido University, 337 Mat. Res. Soc. Syrup. Proc. Vol. 557 c 1999 Materials Research Society

Fritzsche. These obtained changes are indirect and very subtle as compared to those of chalcogenide glasses. In generally, a-Si:H has a high restricted network and the small number of dangling bond's creation (the order of 1016) by light soaking. These are the reasons being difficult to detect the related volume change in a-Si:H by the current methods. Th