Fabrication of high-quality Al-polar and N-polar AlN templates through self-forming tiny-pit layers and polarity inversion

A fabrication technique for high-quality AlN templates using only metalorganic vapor phase epitaxy, which involves a tiny-pit AlN layer optimized by changing the growth temperature and V/III ratio, is proposed. Combining the tiny-pit AlN layer with the main AlN layer can be beneficial for achieving epitaxial lateral overgrowth and reducing threading dislocation density (TDD). According to transmission electron microscopy observations, the TDD in the AlN layer on the tiny-pit layer was 7.3 × 109 cm–2, and the TDD was further reduced to 2.6 × 108 cm–2 by adopting multiple tiny-pit layers. Tiny voids were observed at the interface between the tiny-pit AlN and main AlN layers, and the radius of curvature of the AlN layer also increased compared with that for a conventional AlN template by reducing stress in the AlN layer. In addition, the N-polar AlN layer was grown using Al-polar tiny-pit AlN layers through polarity inversion on a sapphire substrate with an offcut angle of 2.0°. Consequently, it was found that the Al-polar tiny-pit AlN is highly effective in improving the crystalline quality of the N-polar AlN layer. The surface flatness of the N-polar AlN layer with the tiny-pit layer and polarity inversion retained the conventional N-polar AlN template.