Structural and Optoelectronic Properties of Thin Film LaWN3

Nitride perovskites are an emerging class of materials that have been predicted to display a range of interesting physics and functional properties, but they are highly underexplored due to the difficulty of synthesizing oxygen-free nitrides. LaWN3, recently reported as the first oxygen-free nitride perovskite, exhibited polar symmetry and a large piezoelectric coefficient. However, the predicted ferroelectric switching was hindered by a large leakage current, which motivates better understanding of the electronic structure and optical properties of this material. In this article, we study the structure and optoelectronic properties of thin film LaWN3 in greater detail, employing combinatorial techniques to correlate these properties with cation stoichiometry. We report a two-step synthesis method that utilizes a more common radio frequency substrate bias instead of a nitrogen plasma source, yielding nanocrystalline films that are subsequently crystallized by ex situ annealing. We investigate the crystal structure and composition of the combinatorial films produced by this method, finding polycrystalline La-rich films and highly textured W-rich films. The measured optical absorption onset (2.5-3.5 eV) and temperature- and magnetic field-dependent resistivity (0.0055-2 ω cm for W-rich and La-rich LaWN3, respectively) are consistent with semiconducting behavior and are highly sensitive to cation stoichiometry, which may be related to amorphous impurities: metallic W or WNx in W-rich samples and insulating La2O3 in La-rich samples. The fractional magnetoresistance is linear and small, consistent with defect scattering, and a W-rich sample has n-type carriers with high densities and low mobilities. We demonstrate a photoresponse in LaWN3: The resistivity of a La-rich sample is enhanced by ∼28% under illumination and at low temperature, likely due to a defect trapping mechanism. The physical properties of LaWN3 are highly sensitive to cation stoichiometry, like many oxide perovskites, which therefore calls for precise composition control to utilize the interesting properties observed in this nitride perovskite.