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 AlN0001∥Al2O30001 and in-plain alignment of AlN1̄21̄0∥Al2O301̄10 and AlN101̄0∥Al2O32̄110. 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.