Distinct orientation of AlN thin films deposited on sapphire substrates by laser ablation

Crystalline, 50-1200 nm thick aluminum nitride (AlN) films were deposited on sapphire by laser ablation of sintered AlN targets. On c-, r-, m-, and a-cut sapphire the AlN films were (00.1), (11.0), (10.0), and (00.1) oriented, respectively. X-ray diffraction goniometry revealed the inplane orientation of AlN on c-, r-, and m-cut sapphire to be c: AlN[1000]parallel to Al2O3 [2100], r:AlN[2010]parallel to Al2O3[1000], and m: AlN[0100]parallel to Al2O3 [0001]. TEM showed a smooth surface of AlN(11.0) grown on r-cut sapphire in contrast to a rough surface and columnar structure of AlN(00.1) films on c-cut sapphire. AFM revealed atomically flat films at initial stages of film growth independent of the substrate orientation. Thick AlN films on c-cut sapphire were rough whereas those on r-cut sapphire remained smooth up to 1 mu m thickness with a rms-roughness as low as 1 nm.