Photoinduced Structural Changes in Hydrogenated Amorphous Silicon
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induced dangling bond defect with an unpaired spin at g = 2.0055 that is believed to be a dominant non-radiative recombination center [2]. Recently, several reports have implied that the photoinduced structural change occurs in addition to photo-induced defect creation [3]. It is proposed as a reasonable explanation for the result of a reversible change of the 1/f noise spectrum [4] or a reversible shift of the Si 2p X-ray photoelectron spectroscopy peak [5]. If the photo-induced structural change is identified in physical quantities directly reflecting the overall structural disorder and correlated with the photo-induced defect creation, then a new possibility would emerge for understanding the mechanism of the Staebler-Wronski effect as well as finding a way to eliminate the photo-induced degradation. We have approached the problem by means of the polarized electroabsorption method [6,7], and observed a reversible photo-induced structural change taking place besides the metastable change in the defect system, both of which exhibit markedly different time-evolution characteristics under light exposure and thermal annealing processes [8,9,10]. Polarized electroabsorption (PEA) method is one of the most useful tools for probing structural disorder in amorphous semiconductors, however, there exists a problem to be solved, that is, the field-modulated absorption contains the true polarization-dependent electroabsorption serving as an indicator of the structural disorder as well as the thermoabsorption resulted from the temperature modulation due to Joule heating. The thermoabsorption component has to be removed to make an accurate and reliable evaluation of structural disorder. We evaluate the pure electroabsorption with using the phase-separation procedure which is applied to deconvolute the observed field-modulated absorption signal into the above two components. In order to give another evidence to the photo-induced structural change, change in internal stress and density have been measured by the use of the bending-beam method and floatation method, respectively. It is found that amorphous silicon films expand and the density tends to decrease upon light-exposure, the temporal behaviors of which coincide with that of the PEA ratio 735 Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society
factor indicating disorderness of the amorphous network structure. The purpose of this paper is to present the results of refined PEA measurements as well as to discuss to the photo-induced PEA change in association with macroscopic structural changes elucidated by internal stress and density measurement. EXPERIMENT Undoped a-Si:H samples of 1 to 2 micron meter thick were deposited by the plasma enhanced chemical vapor deposition method from hydrogen diluted 10% silane on a 50 ltm-thick fused silica substrate. To prepare device grade a-Si:H sample, substrate 2temperature and rf power were set at 200 °C and 35 W, respectively. For light soaking, 50 mW/cm Air Mass 1 (AM 1) illumination was used. For the purpose of e
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