I. Luminescence and Free Carrier Decay Times in Semiconductors Containing Isoelectronic Traps

Abstract

This paper describes the measurement of free carrier decay profiles over the temperature range 7°–300°K for GaP:N, GaP:Bi, CdS:Te, and ZnTe:O following their excitation by pulses of electrons from a Van de Graaff accelerator. Contactless microwave transmission and reflectivity techniques were employed to measure the transient conductivity changes. The studies were made principally to obtain a more complete understanding of the recombination mechanisms previously proposed to be operative in CdS:Te and ZnTe:O on the basis of luminescence decay time data. The gross features of the temperature dependence of the free carrier lifetimes correlate well with the luminescence decay time data. Differential equations, formulated to describe the temperature-dependent kinetics, were solved by a combination of numerical and analytical methods to obtain the transient luminescence and free carrier decay profiles. The solutions depend very weakly upon certain capture coefficients. The major variable parameter is in both cases the binding energy ε of the majority carrier in the Coulomb field of the charged isoelectronic trap. For CdS:Te, with εh=20 meV, good agreement was obtained between the calculated and observed free carrier and luminescence decay profiles, thereby providing good support for the model of the recombination mechanism. For ZnTe:O with εe=20 meV, agreement is adequate for the luminescence profiles, but only semiquantitative for the free carrier decay profiles. Reasons for the discrepancies are discussed. For all the materials, at the lowest temperatures, the results indicate very rapid and stable trapping of the carriers with no evidence of a post-excitation conductivity. This is in keeping with the occurrence of highly radiative transitions at these temperatures.