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°C). AlN films synthesized under stoichiometric conditions, with ECR microwave powers of less than 100 W, substrate temperatures of 925–1150°C and a low temperature buffer layer exhibit narrow X-ray diffraction rocking curve widths [6 arcmin, AlN(0002)]. Smooth surface morphologies with typical RMS surface roughness of ∼l5 Å are found for ∼1 μm AlN growth when the lower range of substrate temperatures are used. The φ-scan of the AlN 101̄3 X-ray Bragg reflection has six-fold symmetry with peak widths of 1.6°, indicating that the highest quality films are aligned, albeit with small angle grain boundaries in the a-b plane. Lattice constants of 3.079 Å (a) and 5.036 Å (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 ∼6 Å 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.