Wurtzitic gallium nitride (GaN) films of high crystalline quality can be deposited on γ-LiAlO2(100) substrates by low-energy ion-beam-assisted molecular-beam epitaxy (IBA-MBE) [1]. Such GaN films exhibit m-plane orientation, i.e. the GaN c-axis and a-axis lie parallel to the substrate surface. In contrast to typically c-plane oriented GaN films the growth direction of m-plane GaN is non-polar. Therefore, the build-up of strain-induced electric fields perpendicular to the film is avoided. Additionally, the lattice mismatch between m-plane GaN and γ-LiAlO2 is anisotropic with values of 0.3 % and 1.7 % along the c-axis and a-axis of GaN, respectively.
Aim of this study is to present and examine the observation that the crystalline, as well as the optical quality of the GaN films and the mechanical stress of the films are strongly correlated. The films were deposited on heated γ-LiAlO2(100) substrates by evaporation of gallium using a conventional effusion cell and by simultaneous irradiation by nitrogen ions possessing a maximum kinetic energy of 25 eV. The substrate temperature was varied between 550 and 750°C. The crystalline quality of the films was characterized using highly resolved x-ray diffraction and high resolution transmission electron microscopy. The optical quality was estimated by photoluminescence spectroscopy. The strain of the films was also obtained by x-ray diffraction.
The results show that at optimal substrate temperatures between 650 and 750°C the nitrogen ion to gallium atom flux ratio is a crucial parameter influencing the crystalline quality of the GaN films. In dependence of the growth regime, Ga-rich or N-rich, the GaN films are of high crystalline and optical quality, but compressively stressed, or of minor crystalline and optical quality with low mechanical stress, respectively. The presented results are discussed taking into account the influence of the preparation parameters on the GaN film properties, the anisotropic difference in thermal expansion coefficients of film and substrate, as well as the anisotropic lattice mismatch.
[1] J.W. Gerlach, A. Hofmann, T. Höche, F. Frost, B. Rauschenbach, G. Benndorf, Appl. Phys. Lett. 88 (2006) 011902
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