High quality epitaxial aluminum nitride layers on sapphire by pulsed laser deposition

We have grown high quality epitaxial AlN layers on sapphire substrates by pulsed laser ablation of a stoichiometric AlN target. The AlN films deposited at 800 °C and laser energy densities in the range of 2–3 J/cm2 were found to be epitaxial with the c axis normal to the Al2O3(0001) surface. The x-ray rocking curve of epitaxial AlN films yielded a full width at half maximum of 0.21°. The selected area electron diffraction patterns and high resolution transmission electron microscopy also revealed that the films were epitaxial with an orientational relationship of AlN[0001]∥Al2O3[0001] and in-plain alignment of AlN[1̄21̄0]∥Al2O3[01̄10] and AlN[101̄0]∥Al2O3[2̄110]. This is equivalent to 30° rotation in the basal plane of the AlN film with respect to the sapphire substrate. The absorption edge measured by ultraviolet-visible spectroscopy for the epitaxial AlN film was sharp and the band gap was found to be 6.1 eV. The electrical resistivity of the films was about 5–6×1013 ohm cm with a breakdown field of 5×106 V/cm. At higher laser energy densities ≥10 J/cm2 and lower temperatures ≤650 °C, the deposited films were nitrogen deficient and contained free metallic aluminum, both of which degrade the microstructural, electrical, and optical properties of the AlN films.