CATHODOLUMINESCENCE SPECTROSCOPY OF FREE AND BOUND EXCITONS IN CHEMICAL-VAPOR-DEPOSITED DIAMOND

Cathodoluminescence (CL) spectroscopy was used to study exciton recomb ination at temperatures from 80 to 280 K in undoped and boron-doped di amond particles grown by hot-filament chemical vapor deposition. Spect ral lines due to free and bound excitons were observed in the near-ban d-gap region, 4.6-5.5 eV. (The band gap of diamond is at 5.49 eV.) In the undoped particles, free-exciton lines were observed at 5.27 and 5. 12 eV. Another set of lines, not previously reported, was observed bet ween 4.7 and 5.0 eV, with the most intense lines at 4.757, 4.832, and 4.950 eV. The latter set of lines is attributed to excitons bound to l attice defects, possibly dislocations. In the boron-doped particles, e xcitons bound to boron acceptors were found to dominate the near-band- gap CL spectrum at low temperature. The most intense acceptor-bound-ex citon line is at 5.20 eV. In both boron-doped and undoped particles, t he exciton lines were much less intense in {111} crystal-growth sector s than in {100} sectors. Quenching of the exciton luminescence due to nonradiative recombination is believed to be the cause of the reduced intensity in the {111} sectors. The temperature dependence of the inte nsities, peak positions, and peak widths of the exciton lines was exam ined. The free and acceptor-bound excitons in the boron-doped particle s were found to be in thermal equilibrium with each other. In the undo ped particles, the free and defect-bound excitons were not in thermal equilibrium; the higher-energy bound-exciton lines (at 4.832 and 4.950 eV) decreased more rapidly with increasing temperature than either th e lowest-energy bound-exciton line (at 4.757 eV) or the free-exciton l ines. This behavior is attributed to thermally activated transitions f rom the higher-energy bound-exciton states to the lowest-energy state. The full width at half maximum (FWHM) of the free-exciton line, W(FE) , increased with temperature at about the rate predicted for excitatio ns thermalized near the bottom of a parabolic energy band: W(FE)(T) = W(FE)(0) + 1.795kT. The FWHM's of the defect-bound- and acceptor-bound -exciton lines also increased with temperature, but at slower rates th an for the free exciton. The peak positions of the exciton lines incre ased slightly with increasing temperature.