Photo-Induced Structural Change in Hydrogenated Amorphous Silicon

采用极化电吸收（PEA）方法研究了氢化非晶硅的光致结构变化。场调制吸收由两部分组成，其中一部分是真正的极化依赖的电吸收，作为结构无序的指示，另一部分是由于焦耳加热引起的温度调制引起的热吸收。热吸收成分必须从观察到的场调制吸收信号中去除，以便根据电吸收信号的极化依赖性对结构紊乱进行准确可靠的评估，因为由于光诱导降解引起的样品电阻增加应该对热吸收信号的大小产生很大的影响。采用相分离方法将观测到的场调制吸收信号反卷积为两个分量，结果表明，约有15-25%的观测信号是由Tauc隙区的热吸收效应引起的。然而，对于我们之前的PEA结果，即在光照下结构紊乱增加，不需要任何必要的改变。利用弯曲梁法和浮法分别测量了材料的内应力和质量密度，为光致结构变化提供了替代证据。结果表明，在光照射下，非晶硅薄膜膨胀，密度减小。为了解决它们之间的本质联系，系统地研究了PEA无序、内应力、质量密度和亚稳缺陷密度在光照射下的动态变化。内应力和质量密度的变化似乎强烈依赖于光照射脉冲的持续时间，即使累积照射时间是固定的，而这种特殊的行为不会发生在PEA障碍中。实验结果表明，光诱导的网络紊乱的增加会导致局部网络应力的积累，从而产生亚稳缺陷。因果关系是在简单的物理基础上形成的，这对观察到的光致结构变化和亚稳缺陷产生的动力学以半定性的方式给出了明确的解释。如果我们的新想法是正确的，那么光诱导降解的消除应该通过中断链接来建立；新工作障碍的增加，局部网络应力的积累，亚稳态缺陷的产生和稳定。少

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