Electron microscopy investigations of purity of AlN interlayer in AlxGa1−xN/GaN heterostructures grown by plasma assisted molecular beam epitaxy

The electron microscopy was used to characterize the AlN interlayer in AlxGa1−xN/AlN/GaN heterostructures grown by plasma assisted molecular beam epitaxy (PAMBE). We show that the AlN interlayer grown by PAMBE is without gallium and oxygen incorporation and the interfaces are coherent. The AlN interlayer has the ABAB stacking of lattice planes as expected for the wurtzite phase. High purity of AlN interlayer with the ABAB stacking leads to larger conduction band offset along with stronger polarization effects. Our studies show that the origin of lower sheet resistance obtained by PAMBE is the purity of AlN interlayer.