Sn-assisted heteroepitaxy improves ZnTiN2 photoabsorbers

Abstract

Sustainable production of liquid fuels from abundant resources, such as carbon dioxide and water, may be possible through photoelectrochemical processes. Zinc titanium nitride (ZnTiN₂) has been recently demonstrated as a potential photoelectrode semiconductor for photoelectrochemical fuel generation due to its ideal bandgap induced by cation disorder, shared crystal structure with established semiconductors, and self-passivating surface oxides under carbon dioxide reduction operating conditions. However, substantial improvements in crystalline quality and optoelectronic properties of ZnTiN₂ are needed to enable such applications. In this work, we investigate the heteroepitaxial growth of ZnTiN₂ on c-plane (001) sapphire substrates. Growth on sapphire improves crystal quality, while growth on sapphire at elevated temperatures (300 °C) yields highly-oriented, single-crystal-like ZnTiN₂ films. When Sn is incorporated during these epitaxial growth conditions, notable improvements in ZnTiN₂ film surface roughness and optoelectronic properties are observed. These improvements are attributed to Sn acting as a surfactant during growth and mitigating unintentional impurities such as O and C. The single-crystal-like, 12% Sn-containing ZnTiN₂ films exhibit a steep optical absorption onset at the band gap energy around 2 eV, electrical resistivity of 0.7 Ω cm, and a carrier mobility of 0.046 cm² V⁻¹ s⁻¹ with n-type carrier concentration of 2 × 10²⁰ cm⁻³. Density functional theory calculations reveal that moderate substitution of Sn (12.5% of the cation sites) on energetically-preferred cation sites has negligible impact on the optoelectronic properties of cation-disordered ZnTiN₂. These results are important steps toward achieving high performance PEC devices based on ZnTiN₂ photoelectrodes with efficient photon absorption and photoexcited carrier extraction.