Surface Smoothing Mechanism of AlN Film by Initially Alternating Supply of Ammonia

A buffer technique that initially alternates supply of ammonia (IASA) is employed for AlN film growth on sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Atomic force microscopy (AFM) study reveals that step-flow-like growth morphology is achieved for the AlN film with the root-mean-square (rms) roughness as small as 0.336 nm. In contrast, the surface morphology of thick AlN film grown directly on sapphire substrate shows rough grainy feature with a large rms value of 28.9 nm. The mechanism leading to superior morphology by introducing IASA process is investigated using transmission electron microscopy (TEM), hot wet chemical etching and scanning electron microscopy (SEM) techniques. Evidence is presented that their morphological differences are attributed to strain reduction and polarity inversion. The present work provides insight into the AlN epitaxial growth and indicates that IASA is an effective method to realize atomically smooth AlN film.