Photo-Induced Structural Change in Hydrogenated Amorphous Silicon

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Polarized electroabsorption (PEA) method has been used to study photo-induced structural changes in hydrogenated amorphous silicon.The field-modulated absorption consists of two components, one of which is the true polarization-dependent electroabsorption serving as an indicator of the structural disorder, and the other is the thermoabsorption originated in the temperature modulation due to Joule heating.The thermoabsorption component has to be removed from the observed field-modulated absorption signal to make an accurate and reliable evaluation of structural disorder in terms of the polarization dependence of the electroabsorption signals, since an increase in the sample resistance owing to the light-induced degradation should give a large impact on the magnitude of the thermoabsorption signal.The phase-separation procedure has been applied to deconvolute the observed field-modulated absorption signal into two components, leading to a result that about 15-25% of the observed signal a … More rises from the thermoabsorption effect for the Tauc gap region.Nevertheless, any essential alternation is not needed for our previous PEA result that the structural disorder is increased upon light-exposure.The internal stress as well as mass density have been measured to provide alternative evidences for the photo-induced structural change by the use of the bending-beam method and floatation method, respectively.It is found from these results that amorphous silicon films expand and the density tends to decrease upon light-exposure.The dynamics of the change in the PEA disorder, internal stress, mass density and metastable defect density under light exposure have been systematically investigated in order to resolve the essential link among them.The change in internal stress as well as mass density appears to be strongly dependent on the duration of light exposure pulse even through the accumulated exposure time is fixed, whereas such a peculiar behavior does not occur for the PEA disorder.The experimental findings lead us to an idea that the photo-induced increase in the net-work disorder shall result in an accumulation of the local net-work stress, and then a creation of metastable defects.The causality relation is formulated in the simplistic physical basis, which gives a clear-cut interpretation for the observed dynamics of the photo-induced structural change and creation of metastable defects in a semi-qualitative manner.If our new idea were true, then the removal of the photo-induced degradation should be established by interrupting the link ; increases in the new-work disorder, accumulation of local net-work stress, and creation & stabilization of metastable defects. Less