Electronic States in Zinc Magnesium Oxide Alloy Semiconductors: Hard X-ray Photoemission Spectroscopy and Density Functional Theory Calculations

The electronic states of zinc magnesium oxide (Zn1-xMgx)O thin films were determined exactly by hard X-ray photoemission spectroscopy (HX-PES) using synchrotron radiation. The Zn 2p core level was shifted to a higher binding energy along with widening of the bandgap by alloying with MgO, whereas the shift of the 0 Is peak was less than that of the Zn 2p peak. Density functional theory (DFT) calculations revealed that the electronic state of an 0 ion bonding with an adjacent Mg ion is remarkably different from that not bonding with a Mg ion. As a result, the energy shift observed in the 0 Is peak results from a combination of the expansion of the bandgap energy and the chemical shift due to a change in the ionicity. The Fermi level is always situated just below the conduction band; this suggests that a shallow donor can be added, even in the alloy film with very high MgO fraction, for example, x = 0.47.