Abstract :
Calculations are presented to show that the primary limitation on the room-temperature quantum efficiency of the red luminescence in zinc-and oxygen-doped GaP is due to nonradiative Auger recombination of electrons and excitons trapped at Zn-O nearest-neighbor complexes. Significant improvements in the low-level quantum efficiency can be achieved (up to values of 100%) through reduction of the free-hole concentration by intentional compensation of highly zinc-doped material, the latter being necessary to provide a large concentration of Zn-O recombination centers. Because the recombination time of the trapped carriers increases inversely with hole concentration, large low-level quantum efficiencies are found to be incompatible with fast speed of response and with maximum brightness. This happens because at low hole concentrations the long recombination time leads to a rapid saturation of the Zn-O complexes simultaneously with a lengthened radiative lifetime. Detailed model calculations are presented characterizing the best state-of-the-art material (7% efficiency) as well as the theoretical optimum. The 7% LPE diodes are best characterized by 1017/cm3Zn-O pairs and a minority-carrier lifetime of 2 ns, corresponding to a maximum brightness of 15 lm/ cm2at a current density of 10 A/cm2. A reduction in background impurity concentration by a factor of 2 is predicted to raise the brightness by 25%. In order to achieve the theoretical maximum brightness of 27 lm/cm2at 10 A/cm2an order of magnitude reduction in residual impurities is required. For current densities of the order of 1 A/cm2, a factor of 3 improvement in brightness is predicted over the presently attainable value of 2 lm/cm2.
