High temperature growth of AlN by plasma-enhanced molecular beam epitaxy

AlN(0001) films have been grown by plasma-enhanced molecular beam epitaxy on sapphire(0001) surfaces utilizing monoenergetic activated-nitrogen beams at elevated substrate temperatures (900-1300 degrees C). AlN films synthesized under stoichiometric conditions, with ECR microwave powers of less than 100 W, substrate temperatures of 925-1150 degrees C and a low temperature buffer laver exhibit narrow X-ray diffraction rocking curve widths [6 arcmin, AlN(0002)]. Smooth surface morphologies with typical RMS surface roughness of similar to 15 Angstrom are found for similar to 1 mu m AlN growth when the lower range of substrate temperatures are used. The phi-scan of the AlN(10 (1) over bar 3) X-ray Bragg reflection has six-fold symmetry with peak widths of 1.6 degrees, indicating that the highest quality films are aligned, albeit with small angle grain boundaries in the a-b plane. Lattice constants of 3.079 Angstrom (a) and 5.036 Angstrom (c) are inferred from the X-ray diffraction data, indicating a bi-axial compressive strain of 1.03% in the a-b plane. Even smoother surfaces with an RMS roughness similar to 6 Angstrom can be produced if pre-growth surface nitridation is eliminated. in that case, degradation in the AlN thin-film crystal quality is found, as judged by the 24 arcmin rocking curve widths. The influence of growth conditions (i.e. substrate temperature, ratio of activated-nitrogen to Al flux, ion kinetic energy, pre-growth sapphire nitridation and the properties of the buffer layer) on the resulting crystal quality and surface morphology are directly addressed. (C) 1999 Elsevier Science S.A. All rights reserved.