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

本研究项目由三个子课题组成；1)“闪角极化电吸收（GAPEA）技术”的发展；2)鉴定光致结构变化及其对氢化非晶硅（a-Si: H）电子性能光致退化的影响；3)评价光致内应力/体积变化。光反射几何中的GEPEA在理论和技术上都已确立。新的表征方法已经应用于实际的a-Si: H - p-i-n结样品，明确地表明，正如我们之前基于共面几何PEA测量的研究所表明的那样，光暴露会增加网状无序。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，a-Si: H薄膜在光照射下膨胀，密度趋于降低，这与PEA比值因子的时间行为一致，表明非晶态网络结构的无序性。结果使我们得出结论，在光照射下，非晶态网络结构发生了大规模的变化，这可能导致光诱导产生亚稳态悬垂键缺陷。基于简单位点无序紧结合模型的理论考虑给出了一种可能的解决方案；如果光暴露只对价带边缘态产生显著影响，则观察到的极化电吸收信号、电子迁移率和光学乌尔巴赫尾的光诱导变化都可以得到一致的解释。对A - sin: H/ A - si: H异质结构内部光发射测量的初步研究表明，在光照射下，价带尾斜率变宽，这表明我们关于光致无序物理的新思想是有效的。少

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