High sensitivity of Franz-Keldysh oscillations in photoreflectance spectra for probing morphology in AlxGa 1−xN/GaN heterostructures

We demonstrate that Franz-Keldysh oscillations (FKOs) observed by photoreflectance (PR) spectroscopy are highly sensitive to the surface morphology of Al x Ga 1− x N layers in Al x Ga 1− x N heterostructures. Three Al 0.2 Ga 0.8 N/GaN heterostructures with different surface-morphology profiles, which are confirmed with atomic force microscopy, have been investigated. The X-ray-diffraction patterns are hardly affected by the Al 0.2 Ga 0.8 N/GaN-layer morphology. In contrast, it is revealed that cracks and pits dominating the morphology remarkably reduce the amplitude of the FKOs from the Al 0.2 Ga 0.8 N/GaN layer, which is attributed to the following two mechanisms related to the cracks and pits. One is lifetime broadening due to carrier scattering, and the other is the suppression of the modulation magnitude for the built-in electric field, which is caused by the trapping and recombination of photogenerated carriers at the surface.