First Principles Phase Diagram Calculation and Theoretical Investigation of Electronic Structure Properties of ZnO_1-xSe_x for Photoanode Applications

Terminal solid solutions in the ZnO_1-xSe_xsystem (0 ≤ x ≤ 0.15, 0.95 ≤ x ≤ 1), exhibit extreme bandgap reduction attributable to band anti-crossing (BAC). In this work, we perform a theoretical investigation of extreme alloying in this system (0 ≤ x ≤ 1). The temperature-composition phase diagram of ZnO_1-xSe_xis obtained via cluster expansion method and Monte Carlo simulations. For 0 ≤ x ≤ 0.15, a solid solution in the wurtzite structure and for 0.8 ≤ x ≤ 1, a solid solution in the sphalerite structure is obtained. The alloy system exhibits a miscibility gap in the range 0.15 ≤ x ≤ 0.8. Only the solid solutions are seen to obey bandgap reduction predicted by BAC. The band gap of the alloys, calculated using the delta-sol method shows a bowing behaviour as predicted by the BAC model. Difference in the electronegativities of O and Se atoms in the lattice leads to hybridization of O-2p and Se-4p electronic states. Interaction between these electronic states also leads to a split in the valence band edge at O-rich end and a split in the conduction band edge at the Se-rich end. The effective mass, estimated from the density of states, of holes at the O-rich end and that of electrons at the Se-rich end, increases with alloying. These fundamental insights will help in employing suitable alloy compositions for optimal photocurrent density when these materials are used as photoanodes.