GROWTH-MECHANISM OF GAN GROWN ON SAPPHIRE WITH ALN BUFFER LAYER BY MOVPE

TEM (transmission electron microscopy) as well as SEM was carried out to clarify the growth mechanism of GaN grown by MOVPE on a sapphire (alpha-Al2O3) substrate with an AlN buffer layer. Cross-sectional TEM has revealed that GaN has many defects near the GaN/AlN interface, composed of columnar fine crystals and trapezoid crystals, but the defect density decreases abruptly for the layer of GaN thicker than about 300 nm. From the fine structure of the GaN/AlN interface, a new growth mechanism using the AlN buffer layer which is important to obtain a uniform and high-quality GaN layer has been found. It consists of the following stages: (1) high-density nucleation of GaN, (2) geometric selection arranging the crystallographic direction of the GaN columnar crystals and (3) highly lateral growth velocity of the trapezoid islands.