Nonpolar and semipolar GaN heteroepitaxy on sapphire for LED application

摘要

There has been increasing research interest in nonpolar and semipolar GaN for high brightness light-emitting application. Due to the very limited supply of GaN bulk substrates, the feasible way of obtaining large-area nonpolar and semipolar GaN material is still through heteroepitaxy on foreign substrates at present. This paper highlighted the major challenges in the heteroepitaxy of nonpolar and semipolar GaN on sapphire and presented the progress in reducing the defect density by a two-step growth technique according to the in situ optical reflectance and the ex situ x-ray analyses and transmission electron microscopy measurements. A defect reduction model was proposed based on the correlation between the morphological evolution and the microstructural development during the two-step growth of nonpolar a-plane GaN. The material research status of nonpolar and semipolar GaN was summarized. A promising approach (orientation controlled epitaxy) was pointed out for a further improvement of nonpolar and semipolar GaN material quality.