Defect Reduction in AlN Epilayers Grown by MOCVD 
via Intermediate-Temperature Interlayers
In this work, significant reduction of the density of threading dislocations (TDs) in AlN epilayers grown on sapphire substrates via metalorganic chemical vapor deposition has been obtained by insertion of thin intermediate-temperature interlayers (IT-ILs). The growth temperature of the IT-ILs ranged from 750°C to 950°C after the initial growth at high temperature of 1200°C. Detailed characterizations were performed to understand the mechanisms of the reduction in dislocation density. It is found that the relatively low growth temperature of the IT-ILs can modify the originally two-dimensional (2D) growth mode to a three-dimensional (3D) growth process and creates a high density of small islands at the interface. During the subsequent growth of the high-temperature AlN layer, the AlN islands initially coalesce to form larger grains as the growth proceeds, and some TDs present in the previous AlN epilayers are found to bend near the interlayer, leading to dislocation merging and annihilation.