Lifetimes of Bound Excitons in CdS

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 τ_I₂=0.5±0.1 nsec and τ_I₁=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/sec, and measuring the time delay of the luminescence with a 100-Mc/sec phase-sensitive detector. Previous calculations by Rashba and Guygenishvili 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 τ_I₂=0.56 nsec and τ_I₁=1.86 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. We also calculate the lifetime for donor-acceptor pair recombination to be 2.2 nsec as the pair separation goes to zero. This agrees with Colbow's experimental value of 2.5±{}1 nsec. Using the same method, we calculate the lifetime of the Te isoelectronic trap to be 27 nsec. This agrees poorly with Cuthbert and Thomas's measured value of 300 nsec.