Structural characterization of AlGaN/AlN Bragg reflector grown by metalorganic chemical vapor deposition

A thirty-pair AlGaN/AlN distributed Bragg reflector (DBR) targeted at a center wavelength of 320 nm was grown on a 2-inch sapphire substrate by metalorganic chemical vapor deposition (MOCVD). It is free of cracks in the main area of the wafer except for a ∼4 mm periphery observed under optical microscope. The measured reflectance spectrum shows a peak reflectivity of 93% at 313 nm and a bandwidth of 13 nm. Further investigation into the interfaces was performed using Auger electron spectroscopy (AES), scanning transmission electron microscope (STEM) and energy dispersive X-ray fluorescence spectrometer (EDX). It was found that the constitutional change is abrupt when AlGaN is deposited on an AlN layer but gradual in reverse, which resulted in a stack of quasi-three-layer periods and high Al composition and therefore both the center reflectivity and the bandwidth were depressed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)