Determination of the density of localized electronic states in some amorphous semiconductors from the transient photocurrent

Abstract

The occupancy function of localized states in amorphous semiconductors is calculated from a transient photocurrent of the form n(t) = Ct ^−(1-α) that is observed in undoped a-Si: H and a-As₂Se₃ materials. The form of the time-dependent trap-occupancy function with energy can be described by considering four energy regions. The corresponding distributions are as follows: Boltzmann-type, Fermi-type, energy-independent but time-dependent, and fully occupied. Using this form for the occupancy function, we find that the number of excess free carriers n(t) is linked to the density of localized states g(E) (DOS) via a Volterra integral equation of the second kind. If n(t) = Ct ^−(1-α), this equation can then be solved analytically. The solution shows that the DOS is reasonably well expressed by a single exponential function with a steepness factor T ₀= T/α over a relatively wide energy region below E_c, which is the mobility edge of the conduction band, with the exception of a narrow region very close to E_c. This result agrees with those suggested by most authors.