Lifetimes of bound excitons in CdS

Abstract

Weakly bound excitons in CdS have giant oscillator strengths, which lead to exceedingly fast radiative lifetimes. We have been able to measure the lifetime of the I2 line, an exciton bound to a neutral donor, and the lifetime of the I1 line, an exciton bound to a neutral acceptor. We find τI2 = 0.5 ± 0.1 nsec and τI1 = 1.03 ± 0.1 nsec. The measurements were made at 1.6°K. The lifetimes are measured by exciting the luminescence with an argon laser, modulated at 100 mc, and measuring the time delay of the luminescence with a 100 mc phase sensitive detector. Previous calculations of Rashba and Gurgenishvili predicted radiative lifetimes which were an order of magnitude shorter than our measured values. They used an incorrect value for the exciton mass. When corrected, their theory gives τI2 = 0.56 nsec and τI1 = 1.85 nsec in reasonably good agreement with our measurements. Thomas and Hopfield measured the absorption oscillator strength of the I2 line. Their measurements predict a radiative lifetime for the I2 line of 0.4 ± 0.1 nsec. This is very close to our measured value and shows that the I2 line decays radiatively. We conclude that the nonradiative Auger effect is negligible for the I2 line and either negligible or small for the I1 line.