Strong carrier localization in 3d transition metal oxynitride LaVO3−xNx epitaxial thin films

Abstract

Perovskite LaVO₃ is a typical Mott insulator that exhibits insulator to metal transition by hole doping via substitution of an alkali earth element for La (La_1–xSr_xVO₃ or La_1−xCa_xVO₃). In this study, we investigated the electrical transport properties of its anion-substituted counterpart, LaVO_3−xN_x (0 ≤ x ≤ 0.71). LaVO_3−xN_x epitaxial thin films with high crystallinities and smooth surfaces were grown by plasma-assisted pulsed laser deposition. The electrical resistivity (ρ) of the thin films was remarkably lower than previously reported values for bulk polycrystals of LaVO_3−xN_x, indicating a suppressed contribution of the resistive grain boundaries to ρ in the present films. Plots of ρ against temperature for the LaVO_3−xN_x thin films showed that they were insulating (dρ/dT < 0) even at the highest doping level (x = 0.71), which is much larger than the threshold values for insulator to metal transition in La_1−xSr_xVO₃ (x = 0.18) and La_1−xCa_xVO₃ (x = 0.3). The curves of ρ against temperature were well described using the Efros–Shklovskii-like variable range hopping model, suggesting that random potential introduced by N substitution in the VX₆ conduction path induces strong carrier localization in LaVO_3−xN_x.