Excitonic Emission from High Quality Homoepitaxial Diamond Films

Recently, deep-ultraviolet (deep-UV) light emitting diodes (LEDs) based on diamond have been proposed using a free-exciton emission from diamond. Presently, it is well known that commercially available LEDs and laser diodes (LDs) are based on direct band-gap semiconductors such as GaAs and other III-V compound semiconductors. On the other hand, diamond is an indirect band-gap semiconductor. In order to realize commercially available deep-UV LEDs and LDs based on the free-excitons of the diamond, therefore, the nature of the indirect free-excitons from the diamond should have some advantages that overcome the handicap of the indirect band-gap. The excitons in high quality diamond crystal have a high binding energy (80 meV), excitonic emission can be observed even up to room temperature. We have successfully observed the strong deep-UV emission of 235 nm (ie., 5.27 eV) due to free-exciton at room temperature in cathodoluminescence (CL) spectra from homoepitaxial diamond films prepared by microwave plasma assisted chemical vapor deposition (CVD) using CH4 gas system. More recently, under the condition of the very low CH4 concentrations less than 0.05%, we have successfully grown homoepitaxial diamond films with atomically flat surface. In these films, we found that the free-exciton emission intensity not only becomes stronger than the previous one, but the excitonic emission intensity increases super-linearly with the electron bean current in the high excitation region above a threshold value at room temperature. The nonlinear effects strongly suggest that application of free-exciton emission of diamond to deep-UV LEDs is very promising. In this paper, we report detailed behaviors of the nonlinear effects in free-exciton emission of CVD diamond films studied by the CL measurements.