Evidence of defect band mechanism responsible for band gap evolution in (ZnO)1−x(GaN)x alloys

It is known that ${(\mathrm{ZnO})}_{1\ensuremath{-}x}{(\mathrm{GaN})}_x$ alloys demonstrate remarkable energy band bowing, making the material absorb in the visible range, in spite of the binary components being classical wide band gap semiconductors. However, the origin of this bowing is not settled; two major mechanisms are under debate: Influence of the orbital repulsion and/or formation of a defect band. In the present work, we applied a combination of the absorption and emission measurements on the samples exhibiting an outstanding nanoscale level of ${(\mathrm{ZnO})}_{1\ensuremath{-}x}{(\mathrm{GaN})}_x$ homogeneity as monitored by the high resolution electron microscopy equipped with the energy dispersive x-ray analysis and the electron energy loss spectroscopy; moreover the experimental data were set in the context of the computational analysis of the alloys employing density functional theory and quasiparticle $GW$ approximation. A prominent discrepancy in the band gap values as deduced from the absorption and emission experiments was observed systematically for the alloys with different compositions and interpreted as evidence for the absorption gap shrinking due to the defect band formation. Computational data support the argument, revealing only minor variations in the bulk of the conduction and valence band structures of the alloys, except for a characteristic ``tail'' in the vicinity of the valence band maximum. As such, we conclude that the energy gap bowing in ${(\mathrm{ZnO})}_{1\ensuremath{-}x}{(\mathrm{GaN})}_x$ alloys is due to the defect band formation, presumably at the top of the valence band maximum.