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 AIN target. The AiN films deposited at 800 OC and laser energy densities in the range of 2-3 J/cm(2) 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 degrees. 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[(1) over bar 2 (1) over bar 0]\\Al2O3[0 (1) over bar 10] and AlN[10 (1) over bar 0]\\Al2O3[(2) over bar 110]. This is equivalent to 30 degrees 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 5-6x10(13) ohm cm with a breakdown field of 5 x 10(6) V/cm. At higher laser energy densities greater than or equal to 10 J/cm(2) and lower temperatures less than or equal to 650 degrees C, the deposited films were nitrogen deficient and contained free metallic aluminum, both df which degrade the microstructural, electrical, and optical properties of the AlN films. (C) 1995 American Institute of Physics.