Evolution of Metallicity in Vanadium Dioxide by Creation of Oxygen Vacancies

Tuning of the electronic state of correlated materials is key to their eventual use in advanced electronics and photonics. The prototypical correlated oxide (${\mathrm{VO}}_2$) is insulating at room temperature and transforms to a metallic state when heated to $67°\mathrm{C}$ (340 K). We report the emergence of a metallic state that is preserved down to 1.8 K by annealing thin films of ${\mathrm{VO}}_2$ at an ultralow oxygen partial pressure ($P_{{\mathrm{O}}_2}\sim {10}^{-24}\mathrm{atm}$). The films can be reverted back to their original state by annealing in oxygen, and this process can be iterated multiple times. The metallic phase created by oxygen deficiency has a tetragonal rutile structure and contains a large number of oxygen vacancies far beyond the solubility at equilibrium (greater than approximately 50 times). The oxygen starvation reduces the oxidation state of vanadium from ${\mathrm{V}}^{4+}$ to ${\mathrm{V}}^{3+}$ and leads to the metallization. The extent of resistance reduction (concurrent with tuning of optical properties) can be controlled by the time-temperature envelope of the annealing conditions since the process is diffusionally driven. This experimental platform, which can extensively tune oxygen vacancies in correlated oxides, provides an approach to study emergent phases and defect-mediated adaptive electronic and structural phase boundary crossovers.

Figure 1

Metallization of vanadium dioxide at extremely low oxygen partial pressure. (a) Experimental setup for annealing ${\mathrm{VO}}_2$ at low oxygen partial pressure. (b) Real-time oxygen partial pressure monitored by a zirconia-based oxygen sensor, where pristine ${\mathrm{VO}}_2$ is annealed at $P_{{\mathrm{O}}_2}\sim {10}^{-24}\mathrm{atm}$. The inset shows the $P_{{\mathrm{O}}_2}$ in a narrow time window. (c) At low oxygen partial pressure, oxygen vacancies are created in ${\mathrm{VO}}_2$, and the oxidization state of vanadium is reduced from ${\mathrm{V}}^{4+}$ to ${\mathrm{V}}^{3+}$. (d) By introducing a high concentration of oxygen vacancies, the metal-insulator transition is suppressed and a metallic nonvolatile ${\mathrm{VO}}_{2\text{−}\delta }$ state is observed down to very low temperatures. The process can be reversed by oxygen annealing.

Figure 2

Manipulation of electrical properties of ${\mathrm{VO}}_2$ thin films after annealing at $P_{{\mathrm{O}}_2}\sim {10}^{-24}\mathrm{atm}$. (a) Resistivity-temperature curves of 40-nm PVD-grown ${\mathrm{VO}}_2$ annealed at $300°\mathrm{C}$ and $400°\mathrm{C}$, respectively, for 2 h. After annealing, the thin films show metallic behavior down to room temperature. (b) Corresponding derivative of the resistivity-temperature curve for various annealing conditions. (c) Resistivity-temperature curves of 40-nm ALD-grown ${\mathrm{VO}}_2$ annealed at $400°\mathrm{C}$ for 2 h. The metallization behavior of ${\mathrm{VO}}_2$ under low oxygen partial pressure is reproducible with ALD-grown films. (d) After metallization of 40-nm PVD-grown ${\mathrm{VO}}_2$ at low $P_{{\mathrm{O}}_2}$ for $400°\mathrm{C}$ 2 h in (a), the sample is further annealed in pure oxygen at $300°\mathrm{C}$ for 2, 4, and 6 h. The resistivity-temperature curves show that the metal-insulator transition is recovered after annealing in oxygen.

Figure 3

Mid-IR reflectance of ${\mathrm{VO}}_2$ before and after annealing at $P_{{\mathrm{O}}_2}\sim {10}^{-24}\mathrm{atm}$. (a) Temperature-dependent reflectance spectrum of pristine ${\mathrm{VO}}_2$ deposited on a sapphire substrate. After annealing at $P_{{\mathrm{O}}_2}\sim {10}^{-24}\mathrm{atm}$ at $400°\mathrm{C}$ for 2 h, the reflectance spectrum of the sample is shown in (b), where weak temperature-dependent behavior is observed. The sample maintains a high reflectance when cooled, demonstrating the vanishing of metal-insulator transition after annealing. (c) Single-wavelength ($\lambda =8\mu \mathrm{m}$) temperature-dependent reflectance of ${\mathrm{VO}}_2$ after annealing at low oxygen partial pressure at $400°\mathrm{C}$ for 2 h. The reflectance of the pristine sample increases sharply when heated past the metal-insulator-transition temperature. After annealing at low oxygen partial pressure, the sample shows high reflectance, similar to that of the thermally induced metallic state of the pristine sample. Moreover, the reflectance of the annealed sample decreases slightly with heating, indicative of metallic behavior.

Figure 4

XRD curves of pristine and annealed ${\mathrm{VO}}_2$ thin films (a) over a wide range of angles and (b) around the ${(020)}_M$ peak. The shifting of the ${(020)}_M$ peak to the ${(200)}_T$ peak demonstrates that the high-temperature tetragonal (rutile) structure is maintained down to room temperature after annealing. (c) The resistivity of annealed thin films is measured down to 1.8 K. The metal-insulator-transition temperatures of various Magnéli phases are depicted. The flat resistivity-temperature curve over the entire temperature range indicates the absence of Magnéli phases in the oxygen-deficient ${\mathrm{VO}}_{2\text{−}\delta }$ thin films. (d) XPS analysis of pristine and annealed thin films. The shifting of $\mathrm{V}2p_{1/2}$ to lower binding energy and the appearance of ${\mathrm{V}}^{3+}2p_{3/2}$ photoemission indicate the strong reduction of the oxidization state of the vanadium ion under oxygen deficiency. The estimated oxygen-vacancy level of the annealed sample ${\mathrm{VO}}_{2\text{−}\delta }$ is $\delta =0.08$ at $300°\mathrm{C}$ and $\delta =0.2$ at $400°\mathrm{C}$.

Figure 5

Room-temperature Raman spectra of ${\mathrm{VO}}_2$ after annealing at $P_{{\mathrm{O}}_2}\sim {10}^{-24}\mathrm{atm}$. (a) Raman spectra of PVD-grown 40-nm ${\mathrm{VO}}_2$ annealed at $300°\mathrm{C}$, $350°\mathrm{C}$, and $400°\mathrm{C}$ for 2 h. For the pristine sample, Raman peaks corresponding to the monoclinic structure appear at 129, 196, 224, 308, 386, 498, and $614{\mathrm{cm}}^{-1}$ for $A_g$ symmetry, and 268, 340, and $445{\mathrm{cm}}^{-1}$ for $B_g$ symmetry. For the sample annealed at $400°\mathrm{C}$, the heavily damped Raman peak at $530{\mathrm{cm}}^{-1}$ corresponds to the $A_{1g}$ mode of the rutile structure. Peaks at 421, 575, and $751{\mathrm{cm}}^{-1}$ result from Raman scattering of the ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrate. (b) Relative intensity of the Raman peaks at 196, 224, and $614{\mathrm{cm}}^{-1}$ as a function of the annealing temperature. (c) The $A_g$ mode at $614{\mathrm{cm}}^{-1}$ as a function of the annealing temperature. This Raman mode is related to $\mathrm{V}─\mathrm{O}$ bonding of ${\mathrm{VO}}_2$. (d) The $A_g$ modes at 196 and $224{\mathrm{cm}}^{-1}$ as a function of the annealing temperature. These Raman modes are related to $\mathrm{V}─\mathrm{V}$ bonding and tilting of ${\mathrm{VO}}_2$. Softening of the Raman modes, $\mathrm{V}─\mathrm{O}$ bonding (c), and $\mathrm{V}─\mathrm{V}$ bonding (d) occurs with the increase of the annealing temperature, demonstrating the instability of the monoclinic structure upon the introduction of oxygen vacancies.

Figure 6

Optical properties of ${\mathrm{VO}}_2$ before and after annealing at $P_{{\mathrm{O}}_2}\sim {10}^{-24}\mathrm{atm}$. (a) Real ($n$) and imaginary ($\kappa$) parts of the complex refractive index of pristine ${\mathrm{VO}}_2$ at $25°\mathrm{C}$ and $90°\mathrm{C}$, below and above its metal-insulator-transition temperature. After annealing at $P_{{\mathrm{O}}_2}\sim {10}^{-24}\mathrm{atm}$, the complex refractive index of oxygen-deficient ${\mathrm{VO}}_{2\text{−}\delta }$ at $25°\mathrm{C}$ and $90°\mathrm{C}$ is shown in (b), where Drude-like metallic behavior is observed and no phase transition occurs. (c) Comparison of the optical conductivity between the thermally induced metallic state of pristine ${\mathrm{VO}}_2$ and the oxygen-deficiency-induced metallic state of ${\mathrm{VO}}_{2\text{−}\delta }$ at $90°\mathrm{C}$. The optical conductivity of pristine ${\mathrm{VO}}_2$ across the thermally induced metal-insulator transition is shown in the lower panel. (d)–(f) Schematic illustration of the band structure of pristine ${\mathrm{VO}}_2$ in the insulating state (d) and metallic state (e), while the band structure of oxygen-deficient ${\mathrm{VO}}_{2\text{−}\delta }$ is shown in (f), where strong charge filling in the $\pi *$ and $d_{//}$ orbitals occurs.

Figure 7

Schematic of the Mg-based oxygen trap for achieving ultralow oxygen partial pressure.

Figure 8

Schematic of the zirconia-based oxygen sensor.

Figure 9

Ion, electron, and hole conductivity of zirconia at $550°\mathrm{C}$ calculated based on Eqs. (b3)–(b5).

Figure 10

The experimental Nernst potential measured during low oxygen partial pressure annealing.