Role of disorder and correlations in the metal-insulator transition in ultrathin ${\mathrm{SrVO}}_3$ films

Metallic oxide ${\mathrm{SrVO}}_3$ represents a prototype system for the study of the mechanism behind thickness-induced metal-to-insulator transition (MIT) or crossover in thin films due to its simple cubic symmetry with one electron in the $3d$ state in the bulk. Here we report a deviation of chemical composition and distortion of lattice structure existing in the initial 3 unit cells of ${\mathrm{SrVO}}_3$ films grown on ${\mathrm{SrTiO}}_3$ (001) from its bulk form, which shows a direct correlation to the thickness-dependent MIT. In situ photoemission and scanning tunneling spectroscopy indicate a MIT at the critical thickness of ∼3 unit cells (u.c.), which coincides with the formation of a $(\surd 2\times \surd 2)R{45}^{\circ }$ surface reconstruction. However, atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy show depletion of Sr, and a change of V valence, thus implying the existence of a significant amount of oxygen vacancies in the 3 u.c. of ${\mathrm{SrVO}}_3$ near the interface. Transport and magnetotransport measurements further reveal that disorder, rather than electron correlations, is likely to be the main cause for the MIT in the ${\mathrm{SrVO}}_3$ ultrathin films.

Figure 1

(a) A structural model of ${\mathrm{SrVO}}_3$ film grown on ${\mathrm{SrTiO}}_3$ along the [001] direction. (b) The reflection high-energy electron diffraction (RHEED) intensity oscillation curve and pattern of a 20-u.c. ${\mathrm{SrVO}}_3$ film. (c,d) The scanning tunneling microscopy (STM) images of a 3- and a 50-u.c. ${\mathrm{SrVO}}_3$ film, respectively, with line profiles shown in the insets. The sizes of the images are 300 nm × 300 nm.

Figure 2

(a,b) The ultraviolet photoemission spectra (UPS) of ${\mathrm{SrVO}}_3$ films with thicknesses of 1–4 and 25 u.c. near the Fermi edge ($E_F$), and the corresponding intensity change at the Fermi edge with increasing thickness. (c) Scanning tunneling spectroscopy $I\text{−}V$ curves for a 0.1% Nb-doped ${\mathrm{SrTiO}}_3$ substrate and ${\mathrm{SrVO}}_3$ films with different thicknesses. (d) $dI/dV\text{−}V$ curves for the samples in (c), measured by the lock-in amplifier.

Figure 3

Low-energy electron diffraction (LEED) patterns from the surface of ${\mathrm{SrVO}}_3$ films with thicknesses of 1–4, 50, and 100 u.c. at $E=80\mathrm{eV}$. (d) Intensity profiles along the cut line across the integer spot (–1,1) and the fractional spot (–0.5,0.5) from the patterns displayed in (c). The red dotted line is shown in the 100-u.c. pattern in (c) as the red dotted line. The red circles mark the fractional spots.

Figure 4

(a) Low-magnification HAADF-STEM image taken along the [100] direction with intensity profile (yellow curve) across the interface of a 50-u.c. SVO film on STO(001). The arrows indicate the darker region in the SVO near the interface. (b) Enlarged HAADF-STEM image with intensity profile superimposed and ball model of the structure. The red dotted line marks the position of the interface and the yellow dotted line the SVO layers with lower intensity from the interface. (c) Selected area diffraction pattern taken along the [100] direction from the SVO film and STO substrate. (d) ABF-STEM image of the SVO film with structure model superimposed.

Figure 5

(a) HAADF-STEM image across the interface between a 50-u.c. SVO film and STO substrate taken along the [100] direction. (b) Out-of-plane lattice constant as a function of distance from the interface $\left(x=0\right)$, measured from the HAADF-STEM image by averaging 20 u.c. along the [010] direction. The lattice constants for bulk STO and SVO are indicated by dotted lines. (c) False colored elemental maps for Sr (green), Ti (red), and V (blue), with lateral averaged profiles overlaid. The red dotted line marks the interface and the off-stoichiometry region of the film is highlighted. (d) Oxidation state of Ti and V ions across the interface. (e,f) Background subtracted V-$L_{2,3}$ and Ti-$L_{2,3}$ EELS spectra. (g,h) The energy of V-$L$ and Ti-$L$ edge peaks (the dashed lines are guide to the eyes).

Figure 6

(a) Resistivity vs temperature plot of ${\mathrm{SrVO}}_3$ films with thicknesses of 3–20 u.c. The arrows mark the resistivity minima. (b) The conductivity of the 4–10-u.c. films, fitted to the logarithm of the temperature in the low-temperature region (5–30 K for the 10-u.c. film, and 5–50 K for the 4–8-u.c. films). (c) The resistance of a set of 10-u.c. films as grown and postannealed under 850 °C in high vacuum for different times.

Figure 7

Magnetoresistance (MR) in magnetic field perpendicular to the SVO films at different temperature and with the thickness of (a) 4-, (b) 6-, and (c) 10-u.c. as-grown films, compared with (d) 10-u.c. film postannealed in high vacuum under 850 °C for 40 min.