Nitrogen substitution for oxygen is an anion-engineering strategy for tailoring the rich physical properties of oxides. In this work, we investigate the effect of nitrogen doping in titanium monoxide (TiO) and the underlying mechanism, utilizing epitaxial titanium oxynitride Ti(O,N) films grown using nitric oxide (NO)-assisted molecular beam epitaxy (MBE). The resulting film with the ${\mathrm{TiO}}_{0.6}{\mathrm{N}}_{0.4}$ stoichiometry exhibits a superconducting transition temperature ($T_c$) of about 2.5 K, higher than the $T_c$ of pure TiO ($\sim 0.5\mathrm{K}$) yet lower than that of TiN ($\sim 6\mathrm{K}$). However, the $T_c$ variation as a function of anion ratio shows a large discrepancy with the theoretical $T_c$ trend computed based on the Eliashberg equation and density functional theory (DFT). Interestingly, we find that the series of Ti(O,N) films follows the Mooij correlation in the normal-state resistivity versus temperature—typically associated with amorphous metals—manifesting the critical importance of disorder. Our analysis reveals that strong disorder in TiO originates from stoichiometric Ti and O vacancies ($V_{\mathrm{O}}$ and $V_{\mathrm{Ti}}$), while the N substitution for O above 10% effectively mitigates the vacancy formation but introduces moderate chemical disorder at the anion sublattice. Our DFT calculations demonstrate that disorder suppresses superconductivity through the resultant strong momentum-dependent decoherence in the low-energy electronic and high-frequency phonon structures. N substituting for O weakens the lattice instability inherent to TiO, thereby enhancing the $T_c$ primarily due to reduced disorder. Our work suggests that substituting N for O could serve as a viable experimental method for tuning disorder and superconductivity in the series of Ti(O,N) materials, opening new avenues for their applications in quantum devices.

Close

PrevNext

View figure in article

• Electronic structure
• Superconductivity
• First-principles calculations
• Molecular beam epitaxy