Effect of the oblique excitation and detection on the cathodoluminescence spectra

We have elucidated the effect of oblique excitation and detection on cathodoluminescence (CL) spectra. A new CL system was developed, in which the light collector (detector) is set normal to the electron beam direction (excitation). The direction of excitation and detection was varied by tilting the specimen. CL spectra from free-standing GaN platelet were recorded with various tilt angles. The band edge emission has suffered serious suppression when the detector is set at the horizontal direction. We attributed it to the reabsorption of band edge emission. However, this suppression is not serious for the shallow level and deep level emissions because these lights have not suffered reabsorption effect. The gradual decrease at higher tilt angle may be explained with the geometrical effect of reducing the excitation volume due to oblique specimen surface. These findings suggest that the geometrical factor may give serious modification on the CL spectra when the specimen surface normal is tilted more than 60° from the direction of detector.