High sensitivity of Franz-Keldysh oscillations in photoreflectance spectra for probing morphology in Al_xGa _1−xN/GaN heterostructures

¹ R & D Department, High Frequency and Optical Device Works, Mitsubishi Electric Corporation, 4-1 Mizuhara, Itami, Hyogo 664-8641, Japan
² Department of Applied Physics, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan

Corresponding author: Takeuchi.Hideo@db.MitsubishiElectric.co.jp

Received: 6 September 2006
Revised: 16 October 2006
Accepted: 24 October 2006
Published online: 17 January 2007

Abstract

We demonstrate that Franz-Keldysh oscillations (FKOs) observed by photoreflectance (PR) spectroscopy are highly sensitive to the surface morphology of Al_xGa_1−xN layers in Al_xGa_1−x N heterostructures. Three Al_0.2Ga_0.8N/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_0.2Ga_0.8N/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_0.2Ga_0.8N/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.