It is crucial to understand the compositional dependence of the bandgap of the Ni1–xMgxO alloy as it is a promising bandgap-tunable material for ultraviolet (UV) and deep-UV photodetectors. However, the dependence of the bandgap of this material on the Mg content, x, is still a subject of debate, as the experimentally measured optical bandgap of Ni1–xMgxO is 2–3 eV lower than that of pure MgO (7.77 eV), in contrast to that predicted by conventional bandgap bowing theories. Here, we demonstrate that the Ni1−xMgxO alloy has two bandgaps: (i) the Ni-3d bandgap Eg3d, i.e., the bandgap between the valence band (O 2p) and Ni-3d(eg) bands, where the charge-transfer (CT) transition energy weakly depends on the Mg content and (ii) the alloy bandgap EgNi1xMgxO, i.e., the bandgap between the valence band and the conduction band of the Ni1−xMgxO alloy, which is related to the Ni 4s/Mg 3s states; the alloy bandgap obeys the conventional bandgap bowing model. The Ni1−xMgxO absorption spectra at low Mg contents are difficult to deconvolute because the electronic bands originating from the Ni-3d bands and the alloy conduction band overlap. The band structure described above elucidates the anomalous characteristics of the bandgap of Ni1–xMgxO, i.e., most of the optical bandgap values reported so far are smaller than the expected value corresponding to the transition from the valence band to the alloy conduction band because they were evaluated based on the absorption due to the O 2p-Ni 3d(eg) CT transition.

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