Carrier lifetime in amorphous semiconductors

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Abstract
In amorphous semiconductors, because of the band-tail and -gap states, the excess carrier lifetime becomes sensitive to many experimental details. An attempt is made to clarify the relationship between carrier lifetime, density of states, and measurement details. The results show that in a steady-state photoconductivity measurement, the loss of carriers by recombination is determined by the density of deep states and the position of the quasi-Fermi level. In a transient measurement, the limited observation time and the contact also play important roles, besides the density of states. The shallow states in the energy band control the drift mobility and thus affect the lifetime indirectly. In an extreme case, the lifetime can become longer as the defect density increases. These points are illustrated with the electron lifetime derived from photoconductivity, time of flight, and delayed-field charge collection for hydrogenated amorphous silicon and silicon-germanium alloys.