Bandtail States and Carrier Transport in Amorphous Silicon

在这项研究中，我们追求了非平衡载流子动力学的理论，非平衡载流子动力学包含局域带尾态，其中有限的相互作用允许通过隧道。为了定量地解释由调制光电流测量确定的频率相关的光载流子漂移迁移率，深入探讨了傅里叶域解决方案。理论分析与在宽频率和温度范围内对氢化非晶硅进行的实验结果很好地一致，并且揭示了隧道跃迁的包含大大加速了局域带尾态载流子的热化。同时考虑了带载流子直接捕获和带尾载流子隧穿转移到复合中心的广义复合模型，表明即使在室温下，隧穿重组也优先发生。本研究还进行了调制光激发下结光电流的频域测量，称为频率分辨飞行时间，以分别评估电子和空穴的漂移迁移率。测量的p-i-n样品的电子飞行时间谱由于在非色散输运状态下的传输时间效应而呈现振荡结构。谱拍的分析确定了频率相关的漂移迁移率，并对该材料的输运特性进行了定量评估。在强光照射下，飞行时间实验观察到空穴漂移迁移率明显降低，这说明光诱导的非晶硅网络的无序性对价带边缘附近的电子态有很强的影响。

In this study, we have pursued a theory for nonequilibrium carrier kinetics in amorphous semiconductors containing localized bandtail states among which finite interactions are allowed via tunneling. Fourier domain solutions are explored intensively in order to quantitatively interpret the frequency-dependent photocarrier drift mobility determined by modulated photocurrent measurement. The theoretical analysis is in good agreement with experiments carried out for hydrogenated amorphous silicon over a wide range of frequencies and temperatures, and discloses that the inclusion of tunneling transitions considerably accelerates carrier thermalization in localized bandtail states. A generalized recombination model which considers both direct capture of band carriers and tunneling transfer of bandtail carriers into recombination centers is also examined in detail, suggesting that even for room temperature, the tunneling recombination takes place preferentially. Also performed in this study is the frequency-domain measurement of junction photocurrent under a modulated photoexcitation, called the frequency-resolved time of flight, in order to evaluate the drift mobilities of electrons and holes, separately. The electron time of flight spectra measured for a p-i-n sample exhibit an oscillatory structure due to a transit time effect in a non-dispersive transport regime. The analysis of the spectral beat determines the frequency dependent drift mobility, and yields quantitative assessments of the transport properties of this material. Under an intense light exposure, the time of flight experiments observe a sizable reduction of the hole drift mobility, which illustrates that the light induced disordering of amorphous silicon network gives a strong impact on the electronic states near the valence band edge.