Diamond LED Utilizing Free Exciton Recombination Emission

Diamond free exciton (FE) recombination emission located around 235 nanometers in wavelength is expected to be highly efficient and a good candidate for deep UV LED. We previously reported that an external efficiency (eta (sub ext)) of 8 x 10-5 was obtained at room temperature from a pn junction sample composed of a S-doped CVD layer and a B-doped synthesized substrate. But a peak intensity ratio of band-A to that of FE emission (rA/FE) was about 100. This undesired behavior seemed to be caused by lattice defects on the mechanically polished p-type substrate. In order to improve LED characteristics, a B-doped layer was grown on a B-doped substrate by MW-assisted CVD method at the following conditions; C/H atomic ratio 0.05%, B/C atomic ratio 50 ppm, temperature 875 C and thickness 0.8 microns. Atomic force microscope analysis showed that the averaged roughness (Ra) of the surface was reduced from 0.48 nm to 0.32 nm. Then S-doped CVD layer growth and device separation by reactive ion etching were done the same way as previous sample. A typical r(sub A/FE) of thus fabricated LED's was greatly suppressed to 0.5, which suggested a defect concentration reduction at the junction. Maximum eta (sub ext) of 2.7 x 10-4 and maximum output power of FE emission of 17 micro Watts was obtained.