The template effects on AlN/Al0.3Ga0.7N distributed Bragg reflectors grown by MOCVD

AlN/Al0.3Ga0.7N distributed Bragg reflectors (DBRs) are grown on sapphire (0 0 0 1) by metal-organic chemical vapor deposition (MOCVD). The number of stacks of the DBRs varies between 6 and 30. The thickness of the quarter-wave layers is designed such that the target peak reflectance locating at the wavelength of 330 nm. The GaN or AlGaN templates are employed for growing the AlN/AlGaN DBRs. It is found that the templates intensively influence on the surface roughness and the interfacial properties of AlN/AlGaN DBRs. The result of atomic force microscopy indicates that the AlN/AlGaN DBRs grow on GaN template under quasi-two-dimensional mode. And the good interfaces of AlN/AlGaN not only clearly observed by cross-sectional scanning electron microscopy, but also concluded from narrow width and high orders of satellite peaks detected by high-resolution X-ray diffraction. The peak reflectance of 20 stacks DBRs on GaN template reaches up to 82% located at 326 nm.While those DBRs grown on AlGaN template do not show such structural and optical properties due to the rough surface of AlGaN template and the poor interfaces of AlN/AlGaN.