Study of Carrier Transport Property in Amorphous Semiconductors by Light Excited Hall effect

载波迁移率是影响设备性能的基本物理量之一。在这方面，对载流子迁移率的了解对于改进材料加工和器件设计是必不可少的。然而，对于太阳能电池领域中备受关注的非晶半导体，包括非晶硅合金（a-Si），没有简单的方法可以直接测量“自由”载流子迁移率，而传统的方法是依靠一些特定的模型，从TOF“总电荷-载流子”迁移率的温度依赖性中推导出来的。本研究旨在建立一种新的工具，利用极化电吸收和霍尔测量来测定带边缘附近的自由载流子迁移率，并研究合金、掺杂、制备条件和制备后的各种处理对迁移率的影响。磷掺杂后，电子迁移率下降到未掺杂时的三分之一左右，而空穴迁移率基本保持不变。如果远程势涨落在决定输运性质方面起着关键作用，那么电子和空穴的迁移率应该会因带电杂质和/或缺陷的加入而相等地降低，然而，这与我们的实验观察相矛盾。此外，我们发现在掺杂B的材料上具有定性相同的迁移率行为；电子迁移率下降，但空穴迁移率没有显著降低。这些发现似乎暗示，掺杂引起的迁移率变化不应归因于经典的电荷效应，而杂质原子本身的掺入可能是至关重要的。

Carrier mobility is one of the fundamental physical quantities which largely impact the device performance. In this regard, the knowledge of the carrier mobility is indispensable for the refinements of material processing as well as device design. However, for amorphous semiconductors including amorphous silicon alloys (a-Si) which are of great interest in solar cell field, no simple means have been available for the direct measurement of the "free" carrier mobility, instead it is conventionally deduced from the temperature dependence of the TOF "total charge-carrier" mobility by relying on some specific models. The present work aims to establish a new tool for the determination of free carrier mobility near the band edge by the use of polarized electroaborption and Hall measurements, and to investigate how mobilities are affected by alloying, doping, preparation conditions and/or various treatments after preparation. Upon Phosphorous doping, the electron mobility decreases to about one-third of that in undoped case, while the hole mobility remains almost unchanged. If the long-range potential fluctuation plays a critical role in determining the transport property, then both the electron and hole mobilities should be equally reduced by the incorporation of charged impurities and/or defects, which is, however, in contradiction with our experimental observation. Furthermore, we have found on B doped materials a qualitatively identical behavior of mobilities ; drop in the electron mobility, but no significant reduction in the hole mobility. These findings seem to imply that the doping-induced change in mobilities should not be attributed to classical charge effects, but the incorporation of impurity atoms itself may be of central importance.