Study of Light-induced Structural Change in Hydrogenated Amorphous Silicon by the Glancing-angle Polarized Electroabsorption Technique

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This research program is comprised of three sub-subjects ; 1) development of "Glancing-angle Polarized Electroabsorption (GAPEA) Technique", 2) identification of light-induced structural change and its consequence for light-induced degradation of electronic properties in hydrogenated amorphous silicon (a-Si : H), and 3) evaluation of light-induced internal stress/volume change. The GEPEA in the light-reflection geometry has been established both in the theoretical and technological aspects. The new characterization method has been applied to practical a-Si : H p-i-n junction samples, definitely indicating that the net work disorder is increased by light exposure as had been suggested by our earlier study based on the coplanar-geometry PEA measurements. The internal stress as well as mass density have been measured to provide alternative evidences for the light-induced structural change by the use of the bending-beam method and floatation method, respectively. It is found from these res … More ults that a-Si : H films expand and the density tends to decrease upon light-exposure, the temporal behaviors of which coincide with that of the PEA ratio factor indicating disorderness of the amorphous network structure. The results permit us to conclude that a large scaled change in the amorphous network structure occurs under light-exposure, which might proceed the light-induced creation of metastable dangling bond defects. Theoretical consideration based on the simple site-disordered tight-binding model gives a possible solution ; observed light-induced changes in the polarized electroabsorption signal, electron mobility and optical Urbach tail could be all consistently explained if light exposure would give a striking impact only on the valence band edge states. A preliminary examination involving internal photo emission measurements on a-SiN : H/a-Si : H heterostructures demonstrates the valence band tail slope is broadened upon light exposure, implying the validity of our new idea on the physics of the light-induced disorder. Less