Epitaxial ScGaN films were grown on GaN-on-sapphire substrates by NH 3 -molecular beam epitaxy (MBE). The Sc content remained below 2% for all films, as determined by the simulation and fitting of 0002 v-2u X-ray diffractograms, while no deviation from the wurtzite structure was detected using high resolution X-ray diffraction. ScGaN growth rates decreased significantly as the Sc flux increased and remained low compared to the growth rates of GaN grown under similar conditions, while Sc incorporation rates remained low even at Sc effusion cell temperatures as high as 1400 8C. Atomic force microscopy (AFM) height images indicate that higher Sc fluxes are associated with improved film coalescence. These data indicate that a surfactant layer of Sc may be present at the film surface during growth. The dislocation density of the lower Sc content films was similar to that of the underlying GaN, though the bending and introduction of additional dislocations with an a-component was observed for higher Sc contents. Basal plane stacking faults connected by prismatic stacking faults were also observed, in higher Sc-content ScGaN films, along with a broad luminescence band near 580 nm.
The characteristic defect microstructure of epitaxial wurtzite ScxGa1−xN films on GaN was investigated. Threading dislocations in ScxGa1−xN are inclined with respect to [0001] to relieve in-plane compressive strains. However, increased densities of I1-type basal plane stacking faults and nanoscale lamellar zinc-blende inclusions in the (0001) plane occur with increasing x, in contrast to ScxAl1−xN. This behaviour occurs because the formation energy of the zinc-blende phase is close to that of the wurtzite phase for low Sc-content ScxGa1−xN but not for ScxAl1−xN.
High energy-resolution fluorescence-detected X-ray absorption spectroscopy and density functional theory calculations were used to investigate the local bonding and electronic structure of Sc in epitaxial wurtzite-structure Sc x Ga 1-x N films with x ≤ 0.059. Sc atoms are found to substitute for Ga atoms, accompanied by a local distortion involving an increase in the internal lattice parameter u around the Sc atoms. The local bonding and electronic structure at Sc are not affected strongly by the strain state or the defect microstructure of the films. These data are consistent with theoretical predictions regarding the electro nic structure of dilute Sc x Ga 1-x N alloys.
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