Excitonic recombination radiation in phosphorus-doped CVD diamonds

Free-exciton and bound-exciton recombination radiations are observed reproducibly at different doping levels using cathodoluminescence in phosphorus-doped chemical-vapor-deposited (CVD) diamond thin films. The films are grown by microwave-plasma-assisted CVD, and are doped with phosphine during deposition. From the energy difference between free- and bound-exciton recombination radiations, the binding energy of free excitons to neutral donors is found to be 90 meV. The intensity of bound-exciton recombination radiation decreases as the temperature increases. On the other hand, the intensity of free-exciton recombination radiation increases apparently from 80 to 150 K in samples with phosphorus-carbon concentration ratios of 200, 500, and 1000 ppm. Using the rate equation for transfer processes among free and bound excitons, the radiative process with increasing temperature is explained. The dissociated bound excitons are directly transferred to free excitons with increasing temperature.