Excitonic recombination radiation in undoped and boron-doped chemical-vapor-deposited diamonds

The correlation between free-exciton and bound-exciton recombination has been investigated by cathodoluminescence in high-quality and impurity-controlled chemical-vapor-deposited (CVD) diamond films. The films are formed by [CO(5%)]/[${\mathrm{H}}_2$] using microwave-plasma CVD and are doped with ${\mathrm{B}}_2$${\mathrm{H}}_6$ during deposition. In moderately doped (below 3×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$) semiconducting diamonds, the intensity ratio between bound-exciton and free-exciton recombination reflects the boron concentration in the films. It indicates the possibility for a measure of the acceptor concentration in semiconducting diamond. The temperature dependence of the recombination of the excitons bound to neutral acceptors from 80 to 160 K shows the activation energy of 50–60 meV which is comparable to the binding energy of the excitons bound to acceptors. In the same temperature range, the intensity of free-exciton recombination is almost constant due to the high exciton binding energy of 80 meV. As the boron concentration increases, the effective dissociation of the bound excitons becomes low and the emission is stable above 200 K. In heavily doped samples (above ${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$), the direct recombination without phonon emission from the conduction band to the acceptor level takes place.