In-plane structural anisotropy and polarized Raman-active mode studies of nonpolar AlN grown on 6H-SiC by low-pressure hydride vapor phase epitaxy

Nonpolar a-plane and m-plane AlN layers were grown on a-plane and m-plane 6H-SiC substrates by low-pressure hydride vapor phase epitaxy (LP-HVPE), respectively. The effects of growth temperature were investigated. Results showed that surface roughness was reduced by increasing the temperature for both a-plane and m-plane AlN layers. In-plane morphological anisotropy was revealed by scanning electron microscopy and atomic force microscopy, which was used to image the morphological and structural transitions with temperature. Anisotropy in on-axis X-ray rocking curves was also detected by high-resolution X-ray diffraction. However, compared with the a-plane AlN layer, a smooth surface was easily obtained for the m-plane AlN layer with good crystalline quality. The optimal temperature was lower for the m-plane AlN layer than that for the a-plane AlN layer. The stress characteristics of nonpolar AlN layers were studied using polarized Raman spectra. Results showed the presence of anisotropic in-plane stresses within the epitaxial nonpolar AlN layers.