We have investigated the crystalline quality of CVD diamond films from the viewpoint of the carrier diffusion characteristics by means of cathodoluminescence (CL) method. On one hand, free exciton related luminescences were observed from the whole area of grown CVD films even at room temperature, indicating high quality of the grown films. On the other hand, visible luminescences concluded to come mainly from substrate. The visible CL intensity was found to decrease as an exponential function of the film thickness. The damping length was estimated to be about 10 μm, corresponding to a one-dimensional diffusion length projected to the depth direction.
Furthermore, in order to investigate the effect of surface defects on the luminescence properties, defects were introduced in subsurface region by focused ion beam. A remarkable decrease in CL intensity of the visible luminescence was observed in spite of a large separation between the carrier excited area and the defect-created area. This indicates that substantial amount of excited carriers diffuse to the surface region before diffusing into the substrate region. Thus, the carrier diffusion length should be correctly considered in the evaluation of reasonably high quality diamond films using luminescence methods.