Control of Polarity of AlN Grown on Sapphire Substrate and Growth with Both Al- and N-Polarities

AlN with both Al- and N-polarities in the c-axis direction has attracted attention as a second-harmonic generation device with a quasi-phase-matching structure in the ultraviolet wavelength region. Herein, N-polar AlN on patterned AlN is regrown by metal-organic vapor-phase epitaxy, and AlN lateral polar structures (LPSs) are fabricated without the simultaneous growth of Al- and N-polar AlN layers. A 1 mu m thick Al-polar AlN layer is grown on sapphire, followed by the fabrication of a striped pattern. Subsequently, N-polar AlN is regrown on an exposed sapphire substrate. In the first experiment, the sidewalls after reactive ion etching are not vertical, and voids are formed when N-polar AlN is regrown. KOH wet etching is performed to overcome this problem and improve the verticality of sidewalls. Consequently, a perfect AlN LPS with a minimum period of 2 mu m is fabricated. The verticality of the sidewalls in the LPS and the regrowth conditions of N-polar AlN with a very low V/III ratio are crucial to the fabrication of AlN LPSs with highly vertical interfaces.