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Structure, optical spectra and biaxial stress of (0002) AlN epilayers grown on c-sapphire by high-temperature chemical vapor deposition



(0002) AlN epilayers were grown on c-sapphire at temperatures of 1100, 1200, 1300, 1400, and 1500 °C, respectively, by a homebuilt high-temperature chemical vapor deposition system. The structure, optical properties and biaxial stress of these epilayers were characterized by scanning electronic microscope, XRD, Raman scattering, and UV absorption. The results revealed that smooth layers were obtained below 1400 °C, while stripe-shaped and hexagon-aligned islands grew on the surface of the layer at 1500 °C due to inhomogeneous nucleation in decreased supersaturation. Moreover, the narrowing FWHM of (0002) reflection and phonon modes indicated that the crystal quality improved with increasing temperature. The optical bandgap enlarged with the increasing growth temperature, owing to decreasing dislocations and domain boundaries as well as increasing compressive stress, except that the optical bandgap shrank at 1500 °C because light scattering at the rough surface of the epilayer flattened its optical absorption edge. Furthermore, the results of XRD and Raman studies demonstrated that the biaxial stress in the epilayers turned from tensile stress to compressive stress linearly below 1.8 GPa, resulting in a biaxial stress coefficient of −1.9 cm−1 GPa−1. Therefore, the improving structure together with the increasing biaxial compressive stress of the epilayer enlarged its optical bandgap with a large coefficient of −426 meV GPa−1.