Structural Properties of AlN Grown on Sapphire at Plasma 
Self-Heating Conditions Using Reactive Magnetron Sputter Deposition
Aluminum nitride (AlN) films were grown on sapphire by reactive magnetron sputter deposition in N2 discharges at plasma self-heating conditions. The growth temperature was as low as 94°C. The structural properties resulting from different substrate biases and growth pressures were investigated by atomic force microscopy, x-ray diffraction (XRD) measurements, and transmission electron microscopy (TEM). At 20 mTorr of N2 with most sputtered species thermalized, films exhibited both AlN (0002) and $$ (10\overline{1} 1) $$ XRD peaks, with the AlN (0002) intensity initially increasing with ion energy above 15 eV, showing enhanced film quality with an optimum of 25 eV. At a lower growth pressure of 5 mTorr with energetic sputtered species, the AlN $$ (10\overline{1} 1) $$ peak disappeared and the crystallinity of AlN improved, exhibiting relaxed epitaxial AlN. The measured lattice parameter was 0.4975 nm, which was 0.10% smaller than that of bulk. The epitaxial relationship of a single-crystal AlN film was confirmed by pole figure and cross-sectional TEM. These results demonstrate that control of ion energy and energy of the sputter-deposited species is critical for film deposition at low temperature.