Achievement of polarity reversion from Al(Ga)-polar to N-polar for AlGaN film on AlN seeding layer grown by a novel flow-modulation technology

N-polar AlGaN epi-layer was realized on AlN seeding layer grown with a novel flow-modulation method. The polarity reversion from Al(Ga)-polar to N-polar for AlGaN/AlN films was confirmed by KOH etching and subsequent observation with optical microscope as well as high-resolution X-ray diffraction (HR-XRD) measurement. In particular, the dependence of crystalline quality and defect size in the N-polar AlGaN epi-layers on the V/III ratio was investigated with HR-XRD and scanning electron microscopy (SEM). It was found that as the full width at half maximum (FWHM) value of the X-ray rocking curve (XRC) varied with the V/III ratio in a fluctuating mode for the N-polar AlGaN epi-layers, and a FWHM value as small as 450 arcsec was achieved for the sample grown with a V/III ratio of 988. Moreover, it was revealed by the SEM measurement that the maximum diagonal length of hexagonal cone on the surface of the N-polar AlGaN epi-layers decreased sharply when a V/III ratio of 1236 was used although the crystalline quality and the surface morphology of the N-polar AlGaN epi-layers were not improved simultaneously. The peculiar migration of the group-III atoms on the surface of the N-polar AlGaN epi-layer associated with the molar ratio of TMA/(TMA + TMG) was considered to be responsible for this result.