Structural properties of Al_xGa_1-xN grown on sapphire by molecular beam epitaxy

Structural properties of Al_xGa_1-xN epilayers grown on (0 0 0 1) sapphire substrate by molecular beam epitaxy are investigated in the range of AlN molar fraction from 0.16 to 0.76. The AlN molar fraction estimated by X-ray diffraction agrees well with that of Rutherford backscattering spectroscopy, showing a good linear relationship. The uniform Auger electron spectroscopy depth profile and linear dependence of average atomic concentration of all the constituents of Al_xGa_1-xN epilayers on AlN molar fraction imply that the growth of Al_xGa_1-xN epilayers with variation of AlN molar fraction is well controlled without the compositional fluctuation in depth of the epilayer. It is observed by atomic force microscopy that the surface grain shape of Al_xGa_1-xN epilayer changes from roundish to a coalesced one with increasing AlN molar fraction.