Light-Excited Structural Instability of a-Si:H
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• State Laboratory for Surface Physics, Institute of Semiconductors & Center for Condensed Matter Physics, Chinese Academy of Sciences, P. 0. Box 912, Beijing, 100083, China, ams@mlnmi, cnc. ac. cn
•**Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, P. 0. Box 603, Beijing, 100080, China, zhangdl~aphy01.iphy.ac.cn
ABSTRACT With the accumulation of experimental data, it has been recognized by many that the lightinduced metastable change of a-Si:H, Staebler-Wronski effect (SWE), may be related to a structural instability of the whole a-Si:H network. However, direct evidence of such a structural change is still lacking. In the present paper, the efforts of our laboratory in this direction will be reviewed, including the light-induced changes of Si-H bond absorption, low frequency dielectric response, and an apparent photo-dilation effect.
INTRODUCTION The main obstacle toward a fruitfuil device application of a-Si:H is the light-induced degradation of the material discovered by Staebler and Wronski in 1977 [1]. In the last two decades much progress has been made in the understanding of SWE and the improvement of the stability of a-Si:H films. However, the physical mechanism of the effect still remains a controversial issue. At the center of the debate are two important problems: The first is the role hydrogen plays during the metastable change. And the second one is whether the SWE reflects some statistical redistribution of the whole a-Si:H network because of a light-induced instability of the structure, or the effect involves only some special bond configurations. In this paper, we will present a review of some results in these respects of our lab during the last few years.
SI-H BOND CHANGE AND STRUCTURAL INSTABILITY From the very beginning of the discovery of SWE, the role of hydrogen has always been the center of attention. Since hydrogen in a-Si:H is used to passivate the enormous amount of dangling bonds, it is natural to think that the light-induced increase of defects should be accompanied by some change of Si-H bonds. There have been continuous efforts trying to detect such a change, but, unfortunately, the meaning of these results has been quite
controversial [2, 3, 4]. 697 Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society
We have developed a differential method for detecting subtle changes in Si-H bonds of a-Si:H. The essence of the method is to make the strong background absorption signal become a common mode and to pick up only the light-induced change by a phase-lock technique [5]. Denoting the detection limit of the usual IR spectrometer by (;u which depends on the noises and drifts of both light source and detector, the detection limit of the present method is 6D=8TC;u, where 6 T represents the sample's inhomogeneity. The differential method has a great advantage over the conventional approach when the sample is homogeneous, i.e., 6 T is small. The method, therefore, can easily be used to check the homogeneity of a sample.*It is usuall
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