Response-Time Measurements of Exciton and Pair Radiative Recombination Associated with the Zn-O Isoelectronic Complex in GaP, 4 to 100 °K

Experiments are described in this paper which increase our knowledge of radiative recombination in GaP(Zn, O) and which clarify several points of controversy that have recently arisen in the literature concerning the kinetics of GaP(Zn, O) luminescence. In particular, impulse-excitation experiments are described which yield an exciton-hole capture cross section of ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ at 50 °K. Time-resolved spectral measurements verified this interpretation of the impulse experiments. This value is over three orders of magnitude greater than that deduced by previous workers. This result confirms that, at room temperature, exciton holes may reasonably be assumed to be in thermal equilibrium with valence-band holes, thus simplifying the description of excitonic recombination kinetics. Other experiments described here include a study of the nearly coincident exciton and pair red bands over the temperature range 4-100 °K. These bands were separately observed using variable-intensity photoexcitation and spectral discrimination. The temperature-dependent time-decay behavior normally associated with the $A$ and $B$ transitions of an exciton bound to an isoelectronic center was observed for the first time in the GaP(Zn, O) system and the values ${\tau }_A=35\mathrm{}$ nsec and ${\tau }_B=400\mathrm{}$ nsec assigned. An upper bound on the pair-band strength was placed at $1.5\times {10}^{-13\mathrm{}}(N_A-N_D)$ ${\sec }^{-1\mathrm{}}$ on the basis of low-excitation-intensity time-decay measurements. Structure was observed on the usually featureless low-energy side of the red band in one crystal over a wide temperature range. A peculiar behavior observed in the time decay of the red band in the temperature range 60-80 °K is attributed to the thermalization of electrons from shallow donor levels at their subsequent recapture by the Zn-O complexes. This interpretation was substantiated by spectral and time-decay observations on the green pair band associated with the shallow donors. Finally, an explanation of the negative photoconductivity effect reported by Nelson and Rodgers at 20 °K is suggested, in which impurity conductivity, the dominant conductivity mechanism at this temperature, is decreased by the filling of ionized acceptor sites following photoexcitation.