Comparative Study on MOCVD Growth of a-Plane GaN Films on r-Plane Sapphire Substrates Using GaN, AlGaN, and AlN Buffer Layers

In this work, we have comparatively investigated the effects of the GaN, AlGaN, and AlN low-temperature buffer layers (BL) on the crystal quality of alpha-plane GaN thin films grown on r-plane sapphire substrates. Scanning electron microscopy images of the alpha-plane GaN epilayers show that using an AlGaN BL can significantly reduce the density of surface pits. The full-width at half-maximum values of the (11 (2) over bar0) x-ray rocking curve (XRC) are 0.19 degrees, 0.36 degrees, and 0.48 degrees for the films grown using Al(0.15)Ga(0.85)N, GaN, and AlN BLs, respectively, indicating that an AlGaN BL can effectively reduce the mosaicity of the films. Room-temperature photoluminescence shows that the AlGaN BL results in lower impurity incorporation in the subsequent alpha-plane GaN films, as compared with the case of GaN and AlN BLs. The higher crystal quality of alpha-plane GaN films produced by the Al(0.15)Ga(0.85)N BL could be due to improvement of BL quality by reducing the lattice mismatch between the BL and r-sapphire substrates, while still keeping the lattice mismatch between the BL and epitaxial alpha-plane GaN films relatively small.