Photoelectrochemical (PEC) solar light-induced water splitting has received significant attention for CO2-free hydrogen production. Recently, oxynitride semiconductors, such as LaTiO2N and ATaO2N (A = Ca, Sr, and Ba) are extensively studied as promising candidates for PEC water splitting under the visible light due to their chemical stability and appropriate band structure. Among these oxynitrides, BaTaO2N has a smaller absorption edge (~1.9 eV) and thus high-efficiency is expected. In order to suppress the grain boundaries and defects deactivating photocarriers, single crystalline electrode is better than ceramic- or particle-based electrode. However, there are only few reports on the synthesis of BaTaO2N thin films due to lack of commercially available substrate with good lattice matching. In this study, we fabricated BaTaO2N epitaxial thin films by nitrogen plasma assisted pulsed laser deposition (NPA-PLD) on SrTiO3 (STO) substrates (–5.4% mismatch), and improved their crystallinity by inserting a lattice-matched BaSnO3 (+0.1% mismatch) as a buffer layer.
BaTaO2N epitaxial thin films were grown on STO (100) and (110) substrates with atomically flat surfaces at various substrate temperature (Ts). A BaTaOx target was ablated under supply of nitrogen gas activated by an RF plasma source. XRD measurements revealed that the films grown on STO (110) at Ts ≥550 °C showed diffraction peaks from (110)-oriented perovskite BaTaO2N. However, the films grown at Ts ≥650 °C included impurity phases, whereas the films grown at Ts <600 °C showed weaker diffraction intensity as Ts decreased. The BaTaO2N films grown on STO (100) showed lower crystallinity than those on STO (110) substrates. The BaTaO2N film fabricated on STO (110) at 600 °C showed clear yellow color indicating almost stoichiometric chemical composition. On the other hand, its surface roughness was relatively large; root mean square (RMS) of ~5 nm.
To improve the crystallinity and surface roughness, double buffer layers of BaSnO3 (~90 nm)/Sr0.5Ba0.5SnO3 (~10 nm) were epitaxially grown on STO (110) before the deposition of BaTaO2N. XRD pattern of the BaTaO2N thin film fabricated on the buffer layers indicated epitaxial growth of a (110)-oriented perovskite BaTaO2N thin film with much better crystallinity than the films directly grown on STO (110) substrate. The surface roughness of the BaTaO2N film was also improved to RMS of ~1 nm. The band gap of the film was determined as 2.01 eV from a Tauc plot of (αhν)1/2 vs hν, which shows a good agreement with the previous report on thin films (2 eV).