Study of Light Induced Effects on the Carrier Transport Property in Amorphous Silicon

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The frequency dependent (ac) conductivity being characteristic to disordered solids has been long a subject of great interest, and argued from both the microscopic and macroscopic pictures of various conduction systems. Recent experimental studies have revealed an existence of "universal" frequency-scaling of the normalized ac conductivity, that is found to be well reproduced theoretically for extremely disordered system in which hopping charge conduction via strongly localized/bound sites is predominant. What is excluded in these discussions is the ac electronic conductivity near the mobility edge in amorphous semiconductors which is likely to exhibit similar scaling feature. The aim of this work is, then, to pursue the universal ac scaling for the mobility edge electronic conductivity as well as to infer to the information to be extracted from the measurements, especially the light induced effects on the band-edge carrier transport. Frequency and temperature dependent electronic conduction near the mobility edge is investigated on the basis of the renormalization group treatment for weak localization problems. The ac conductivity normalized by its dc counterpart is found to follow a simple universality equation being quite similar to that obeyed in common for various different hopping conduction systems. The theoretical results have been applied to the experimental data sets acquired on the ac conductivity of hydrogenated amorphous silicon in the well-annealed and light-soaked states, indicating that the the energy width of the conduction band edge localized states is not changed by light-soaing, and stays at aroud 100 meV.