Determination of the Al mole fraction and the band gap bowing of epitaxial AlxGa1−xN films

Abstract

${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ alloys were grown on $c$ -plane sapphire by plasma-induced molecular beam epitaxy. The Al content $x$ was varied over the whole composition range $\text{(0⩽x⩽1).}$ The molar Al fraction was deduced from x-ray diffraction and for comparison by elastic recoil detection analysis. The composition of the alloys calculated from the lattice parameter $c$ underestimates $x$ . This is due to a deformation of the unit cell. The exact Al mole fraction and the biaxial strain of the alloys can be calculated by an additional determination of $a$ , using asymmetric reflections. The results obtained by x-ray diffraction and elastic recoil detection provide evidence for the validity of Vegard’s law in the AlGaN system. In addition, the deviation of the band gap from a linear dependence on $x$ was investigated. We found a downward bowing with a bowing parameter $\text{b=1.3\hspace{0.17em}}\mathrm{eV}.$