Kinetics of green and red-orange pair luminescence in GaP

Decays of the donor-acceptor pair luminescence have been measured as a function of the pair distance R at liquid He temperatures using a pulsed laser beam excitation and applying gated integration techniques to display the decays. The decay time constants were derived from the exponential decays below R ≈ 20 A (on the resolved pair lines) and from the major components of the non-exponential decays at larger R. The decay time depends exponentially on R, the dependence increasing in the “green” pair sequence (S-C), (S-Zn) and (S-Cd) with the characteristic lengths 9.1 A, 7.7 A and 7.0 A. For the “red-orange” pairs (S-Si) no such length could be derived owning to the strong phonon replication. The radiative transition probabilities extrapolated to zero R are ≈ 5 × 105 sec-1 for the “green” pairs and ≈ 105 sec-1 (rough estimate) for the “red-orange” pairs. The zero-phonon transition probabilities were found to be ≈ 4 × 105 sec-1 and ≈ 3 × 103 sec-1 respectively. Possible causes of decay distortion above R ≈ 20 A are discussed as well as thermal effects.