1 - Defect States Spectroscopy in Amorphous Semiconductors

Abstract

Amorphous semiconductors are characterized by properties that are absent in their crystalline counterparts. On the one hand, they are “unsuitable” objects both for experimentalists and from the theoretical point of view; on the other hand, they have widespread technical applications. The principle of detailed balance is a result of the microscopic reversibility of electron kinetics. A prerequisite for the establishment of thermal equilibrium requires that the forward and reverse rates are identical. For isothermal reactions, the equilibrium constant remains unchanged. The principle of detailed balance is of fundamental importance to establish helpful relations between reaction and equilibrium constants because both are at the initial thermal equilibrium; in addition, at the new equilibrium after the relaxation of the perturbation, the net forward and reverse reaction rates are zero. Defect states in the forbidden (mobility) gap may act as electron or hole traps depending on their states of occupancy. The introduction of quasi-Fermi levels for trapped electron and holes allows the classification of trapping states as shallow or deep traps. The involvement of electron states in the reaction process decreases with increasing energy from the quasi-Fermi level for trapped electrons up to the edge of the conduction band, or with decreasing energy from the quasi-Fermi level for trapped holes down to the top of valence band. Free carriers falling into one of these trapping states are reemitted with high probability back into the band from which they came from.