Characterisation of AlxGa1-xN films prepared by plasma induced molecular beam epitaxy on C-plane sapphire

AlxGa1-xN films were grown on c-plane sapphire by plasma induced molecular beam epitaxy with 0 less than or equal to x less than or equal to 1. The composition and purity of the AlxGa1-xN layers was determined by elastic recoil detection analysis with a relative error of 5% for the Al content. Both X-ray diffraction and atomic force microscopy indicate only a slight decrease in epitaxial quality of the AlxGa1-xN films with increasing Al content up to x = 0.65. X-ray diffraction is used to separate the effects of thermally induced biaxial compressive stress and the alloy composition on the shift of interplanar spacings by measuring both lattice constants. The deviation of the c/a ratio from that of fully relaxed films is a quantitative measure of the biaxial compressive stress leading to a distortion of the unit cell. Values up to 0.5 GPa were observed. By the method proposed, the determination of alloy composition can be corrected for this effect. The results obtained by this method are in very good agreement with the elastic recoil detection measurements substantiating the validity of Vegard's law. These results, compared with optical measurements, indicate that the bowing parameter of the optical bandgap is 1.3 eV within the experimental error.