The kinetics of donor-acceptor pair transitions with strong phonon coupling in GaP

Abstract

Donor-acceptor pair luminescence in GaP was studied by time-resolved spectroscopy, by measuring and analysing the integral band decay over many decades of intensity and time and by measuring and analysing the temperature dependence of the luminescence intensity. We report here the results obtained with some or all of these techniques for pairs involving deep as well as shallow centres. Those with a deep centre are: S P-Si P and Si Ga-Si P, in which the acceptor is the deep centre, and O P-C P and O P-Zn Ga, in which the donor is the deep centre. These pairs all have a broad, phonon-dominated luminescence band. The pairs involving shallow centres include Si Ga-C P, S P-C P and Te P-C P. Due to the different way of momentum conservation in these pairs, those involving a donor on a Ga-site have strong phonon co-operation and weak zero-phonon (ZP) transitions, whereas those with a donor on a P-site have strong ZP transitions. With time-resolved spectroscopy, well-resolved structure, due to a ZP pair band and its phonon replicas, is obtained for nearly all pairs. A comparison is made with the structure obtained by measuring in the stationary state at very low excitation densities. Some trends in the strength of phonon co-operation are noted. In the case of Si Ga-C P pairs, sharp replicas of ZP pair lines with a momentum-conserving phonon are also resolved. Using the integral band decay technique and results from time-resolved spectroscopy we have determined the transition probabilities for the total luminescence band as well as for the ZP band for nearly all pairs mentioned. The influence of incomplete saturation of the pairs during excitation with a focussed beam of an argon-ion laser is discussed and approximately corrected for. Extrapolated to zero pair separation, the total transition probability ranges from ∽ 10 5s -1 for Si Ga-C P pairs to ∽ 15 × 10 5s -1 for O P-C P and O P-Zn Ga pairs. The ZP probability ranges from ∽ 4 × 10 3s -1 for Si Ga-C P and Si Ga-Si P pairs to ∽ 4 × 10 5s -1 for S P-C P pairs. The transition probability for pairs in GaP is discussed in relation to the site occupied by the donor and the depth of the centres involved. The relatively high dope concentrations used in some cases, ∽ 1 to 2 × 10 18cm -3, are discussed briefly in relation to “concentration quenching”. The temperature dependence of the luminescence intensity of S P-Si P pairs is satisfactorily analysed with a simple linear model. The same model is applied to similar literature data on the S P-C P pair luminescence. Using in addition data on the average pair transition probability, trapping rate constants for hole capture by the Si P and C P acceptors were calculated to be ∽ 10 -9 cm 3s -1. Evidence is presented that the trapping rate constant for holes by the Zn Ga-O P complex is about 2 × 10 -9 cm 3s -1.