Sensitizing States in Intrinsic Hydrogenated Amorphous Silicon and Their Effect on Photoconductivity

本質非晶矽氫中的致敏狀態及其對光電導率之效應

李世鴻(Shih-Fong Lee) ； 郭宗仁(Chung-Jen Kuo) ； 林寬輝(Kuan-Huei Lin)

《大葉學報》 2卷1期 (1993/12) Pp. 81-95

https://doi.org/10.7119/JDYU.199312.0081

摘要

致敏狀態是其電子及電洞陷捕截面差異非常大之狀態。具有非常小電子陷捕截面之致敏狀態，電洞可在此狀態長期駐留而不會與自由電子複合。非晶矽氫中在接近價帶邊緣的能帶隙中具有上述的致敏狀態，當電洞之準費米能階掃過這些致敏狀態時，會導致能帶隙中狀態駐留電子的重分配作用，從而提高自由電子之生命週期，並提高非晶矽氫之光電導率。此致敏狀態對於非晶矽氫材料之複合動力及其元件光電特性有深遠影響。本論文經由量測本質非晶矽氫之光電導率特性及其與量測溫度之關係，並使用計算機模擬，以研究此致敏狀態之密度、特性、及其所處之能量位置。尤其敏感度分析更明確顯示，致敏狀態是影響非晶矽氫光電導率之最重要因素，特別是其所處之能量，對光電導率之效應尤為顯著，此效應可以解釋非晶矽氫在室溫及低溫時光電導率特性歧異紛亂現象之成因。

關鍵字

非晶矽氫 ； 致敏狀態 ； 光電導率 ； 複合動力 ； 計算機模擬

並列摘要

Sensitizing states are the energy states whose electron and hole capture cross sections are very different from each other. Sensitizing states, having very small capture cross section for electrons, are the trapping states where holes can reside for a long period of time without recombining with free electrons. These sensitizing states, which are located deep in the valence band tail, can have a large effect on the recombination kinetics in hydrogenated amorphous silicon (a-Si:H). In this study, steady state photoconductivities on intrinsic, high quality a-Si:H films have been measured over an extended temperature range to study the effect of the sensitizing states on the recombination kinetics in a-Si:H. Numerical analysis based on the detailed balance of carrier generation and recombination was used to model the experimental results on both annealed and light-soaked states. In order to accurately model the experimental results, temperature-dependent gap state parameters, such as carrier generation rate and mobility gap, were also taken into account for the first time. This allowed the densities, nature, and energy location of the different gap states species over an extended energy range to be derived. In these films, the photoconductivities at room temperature are dominated by the midgap states having densities in annealed state and light-soaked state of 2×10^(15) cm^(-3) and 4×10^(15) cm^(-3), respectively. The photoconductivities which are found to be relatively insensitive to temperature changes and do not exhibit pronounced peaks such as those have been observed by others, are found to be consistent with the presence of sensitizing states located at 0.3eV to 0.4eV from the valence band edge with the densities similar to those of the dangling bond midgap defect states. These densities are significantly smaller than those reported by McMahon and Xi. It is also found that light soaking increases not only the total density of midgap states but also the total density of the sensitizing states. This is in direct contrast to the safe hole trap model in which a corresponding decrease in the density of the sensitizing states after light soaking is expected. Calculations using the detailed analysis also shows that the photoconductivity and its temperature dependence are very sensitive to the energy location of sensitizing states even when their densities are comparable to those near midgap. This clearly demonstrates that the existence of sensitizing states is a possible explanation for the large differences in photoconductivities of intrinsic, undoped a-Si:H.

並列關鍵字

hydrogenated amorphous silicon ； sensitizing states ； photoconductivity ； recombination kinetics ； numerical modeling