Transport of photocarriers in Cd xHg 1−xTe graded-gap structures

Abstract

A theoretical and experimental study has been made on the transport of light injected excess charge carriers in Cd xHg 1−xTe variable band gap materials. A theoretical analysis is given of the ambipolar motion of the charge carriers in terms of diffusion and drift in the electric fields associated with the gradients of the band gap, lifetime, effective masses and doping level. The photoelectromagnetic effect (P.E.M.) has been calculated for two limiting cases: when the electric field is predominantly associated with a gradient of band gap (ideal case), and when the net electric field is zero (pseudohomogeneous case). It is shown that the two situations can be distinguished by analysing the variation of the P.E.M. effect in a magnetic field. Two different experimental studies have been made: a study of the optical transmission spectra which leads to the determination of the profiles of the structures and the spatial distribution of the absorbed photons, and a study of the P.E.M. effect and the photoconductivity as a function of several parameters: magnetic field, wavelength and temperature. The interpretation of these results shows that the structures studied can be characterized as pseudohomogeneous and an estimation of the variation of the diffusion length (1·5–18·5 μ) and the lifetime (5·10 −11–2·10 −8 sec) of the electrons in the composition range 0·15 < x < 0·25 has been made.