Near-field optical microscopy and scanning Kelvin microscopy studies of V-defects on $AlGaN∕GaN$ films

$AlxGa1−xN$ thin film was grown on undoped $GaN$∕sapphire (0001) substrate by metalorganic chemical vapor deposition. V-defects were directly observed by atomic force microscopy (AFM) with various size of $0.5–2μm$ in diameter. In a previous study, the microphotoluminescence spectra showed an extra peak $(Iv=350nm)$ inside the V-defect besides the near-band-edge emission $(Inbe=335nm)$⁠. To achieve better spatial resolution, we used near-field scanning optical microscopy (NSOM) and scanning Kelvin-force microscopy (SKM) to probe the V-defect in detail. The NSOM spectra showed that the intensity of the $Iv$ band increased gradually from V-defect edges to its center, while $Inbe$ remained unchanged. Besides, the SKM measurements revealed that the Fermi level decreased from the flat region to V-defect center by about $0.2eV$⁠. These results suggest that the $Iv$ band could be related to shallow acceptor levels, likely resulting from $VGa$ defects.