Domain nucleation across the metal-insulator transition of self-strained ${\mathrm{V}}_2{\mathrm{O}}_3$ films

Bulk ${\mathrm{V}}_2{\mathrm{O}}_3$ features concomitant metal-insulator (MIT) and structural (SPT) phase transitions at $T_{\mathrm{C}}\sim 160$ K. In thin films, where the substrate clamping can impose geometrical restrictions on the SPT, the epitaxial relation between the ${\mathrm{V}}_2{\mathrm{O}}_3$ film and substrate can have a profound effect on the MIT. Here, we present a detailed characterization of domain nucleation and growth across the MIT in (001)-oriented ${\mathrm{V}}_2{\mathrm{O}}_3$ films grown on sapphire. By combining scanning electron transmission microscopy and photoelectron emission microscopy (PEEM), we imaged the MIT with planar and vertical resolution. We observed that upon cooling, insulating domains nucleate at the top of the film, where strain is lowest, and expand downwards and laterally. This growth is arrested at a critical thickness of 50 nm from the substrate interface, leaving a persistent bottom metallic layer. As a result, the MIT cannot take place in the interior of films below this critical thickness. However, PEEM measurements revealed that insulating domains can still form on a very thin superficial layer at the top interface. Our results demonstrate the intricate spatial complexity of the MIT in clamped ${\mathrm{V}}_2{\mathrm{O}}_3$, especially the strain-induced large variations along the $c$ axis. Engineering the thickness-dependent MIT can provide an unconventional way to build out-of-plane geometry devices by using the persistent bottom metal layer as a native electrode.

Figure 1

(a) Schematic representation of the ${\mathrm{V}}_2{\mathrm{O}}_3$ unit cell in the corundum and monoclinic phases. (b) X-ray reflectivity of four (001)-oriented ${\mathrm{V}}_2{\mathrm{O}}_3$ films grown on (001)-oriented sapphire, with thicknesses ranging between 18 and 300 nm. Measurements were done at room temperature. (c) θ-2θ scans around the 006 peaks of the ${\mathrm{V}}_2{\mathrm{O}}_3$ films and the sapphire substrates. The vertical dashed line shows the position of the (006) peak for relaxed bulk ${\mathrm{V}}_2{\mathrm{O}}_3$. Measurements were done at room temperature. (d) Two-point resistance vs temperature for the four ${\mathrm{V}}_2{\mathrm{O}}_3$ films. (e) Resistance normalized by the film's resistance at 300 K, to better compare the MIT suppression.

Figure 2

Reciprocal-space maps around the 119 direction for (a) a 300-nm film and (b) a 50-nm film. Several temperatures are shown. Several twins are possible and visible for the monoclinic phase. No twinning is observed in the corundum phase.

Figure 3

Cryogenic STEM LAADF electron micrographs of the 300-nm-thick ${\mathrm{V}}_2{\mathrm{O}}_3$ film from 180 to 99 K. Because LAADF imaging in STEM is very sensitive to lattice distortion (largely diffraction contrast), it is used here to reveal crystalline defects, grain boundaries, and thickness variation as well as to dissociate the corundum and the monoclinic crystal structure of ${\mathrm{V}}_2{\mathrm{O}}_3$. The extended bright and dark areas correspond to the monoclinic and the corundum phases of the ${\mathrm{V}}_2{\mathrm{O}}_3$ material, respectively.

Figure 4

Four-dimensional STEM characterization of the ${\mathrm{V}}_2{\mathrm{O}}_3$ films at cryogenic temperature. (a) Locally averaged electron diffraction pattern in log scale recorded at 125 K from a monoclinic ${\mathrm{V}}_2{\mathrm{O}}_3$ region in the 300-nm-thick film (indexed using the corundum phase notation). (b) Locally averaged electron diffraction pattern in log scale recorded at 125 K from the 50-nm-thick ${\mathrm{V}}_2{\mathrm{O}}_3$ film. (c) Intensity map from the circled areas in (b) capturing the singularity from the monoclinic phase in the 50-nm ${\mathrm{V}}_2{\mathrm{O}}_3$ film, from 180 to 99 K.

Figure 5

Strain characterization of the 300-nm-thick ${\mathrm{V}}_2{\mathrm{O}}_3$ film at (a) 293 K and (b) 125 K. The different panels correspond to uniaxial relative deformation maps ($E_{xx}$, $E_{zz}$) of the ${\mathrm{V}}_2{\mathrm{O}}_3$ film, using the sapphire substrate as the reference. (c) Vertical $E_{xx}$ and $E_{zz}$ profile lines, integrated along the $x$ axis. Both 293 and 125 K are shown. The vertical black dashed line shows where the interface between substrate and film is. (d) Enlarged plot of the $E_{xx}$ components from (c). Compressive strain increases as the interface is approached. The green dotted line in (c) and (d) marks the expected $E_{xx}$ for the unstrained bulk ${\mathrm{V}}_2{\mathrm{O}}_3$ relative to the sapphire, as mentioned in the main text.

Figure 6

X-ray absorption near the vanadium L- and oxygen K edges for the (a) 300-nm film at 190 K, (b) 300-nm film at 44 K, (c) 50-nm film at 190 K, and (d) 50-nm film at 44 K. For each panel, scans with light polarized in plane (black) and out of plane (green) are shown.

Figure 7

(a) XLD (V/H) images of the 50-nm film with an incoming beam energy of 519.2 eV. Several temperatures are shown. Temperature scanning was done by increasing temperature. Cooldown measurements are shown in Fig. S4. (b) X-ray absorption for the 50-nm film at 44 K with light polarized in plane. The three scans correspond to three regions of interest (contrast domains) in the PEEM map. These were chosen in accordance with the three domain intensities observed. (c) X-ray absorption for the 50-nm film at 44 K with light polarized out of plane. The three scans correspond to three regions of interest (contrast domains) observed in the PEEM map. These were chosen in accordance with the three intensity domains observed. Red arrows in (b) and (c) indicate the energy where the scans in (a) where measured.