Epitaxy of (Ga N)1-x (Zn O)x Solid-Solution Thin Films with Widely Tunable Chemical Composition and Strong Visible Absorption

The alloying of wide-band-gap ZnO and GaN causes band-gap reduction, enabling visible-light-driven photocatalysis for high-efficiency water splitting. However, challenges in single-crystal (GaN)1-x(ZnO)x solid-solution synthesis prevent a better understanding of the optical properties and electronic structures. Here, low-temperature epitaxial growth of (GaN)1-x(ZnO)x thin films with a wide tunability of chemical composition is demonstrated by using a multitarget pulsed-laser-deposition (PLD) system. The phase pure (GaN)1-x(ZnO)x solid solution is obtained by alternately depositing GaN and ZnO with the thickness of each GaN/ZnO pair set within one or two unit cells. The band gap of the solid-solution thin films as a function of systematically controlled chemical composition shows asymmetric bowing with a minimum at approximately 2.0 eV for x = 0.65. Furthermore, a large absorption coefficient (>104cm-1) in the visible-light region is observed. The shape of the absorption edge is not consistent with that of a direct-transition semiconductor. First-principles calculation suggests that this inconsistency originates from localization of the valence-band maximum on N atoms bonded with Zn. A technique for fabricating high-quality epitaxial (GaN)1-x(ZnO)x solid solutions is essential for acquiring a deep understanding of the fundamental properties of this system.