Electronic processes in amorphous semiconductors

Abstract

Applying the effective-mass approach, electronic processes in amorphous semiconductors (a-semiconductors) are reviewed. Expressions of the effective masses of electrons and holes are derived in the real coordinate space. It is shown that a charge carrier has different effective masses in the extended and tail states and not only the magnitude but also the sign of the effective mass changes as the charge carrier crosses its mobility edge. This has been applied to explain the anomalous behavior of the Hall coefficient observed in a-Si:H and other amorphous solids (a-solids). The optical properties of a-solids are also studied. The effective masses of charge carriers are used to calculate Tauc's coefficient in the absorption spectra theoretically, which has not been done before. It is found that while Tauc's plot is correctly obtained using the constant transition matrix element, deviations from Tauc's plot observed in some a-solids may be attributed to a photon-energy-dependent matrix element. Other aspects of the deviation from Tauc's plot are also extensively reviewed. The effective-mass approximation is applied to derive the energies of singlet and triplet excitonic states, which are then used to study the photoluminescence properties of a-semiconductors. It is shown that the double-peak structure (fast and slow) observed in the photoluminescence using the quadrature-frequency-resolved spectra technique (QFRS), originates from the excitonic states. The fast peak arises from singlet and the slow from triplet excitonic states. Finally, the effective-mass approach is applied to study the transport properties of a-solids.