Deviation from bulk in the pressure-temperature phase diagram of ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films

We found atypical pressure dependence in the transport measurements of the metal to insulator transition (MIT) in epitaxial thin films of vanadium sesquioxide (${\mathrm{V}}_2{\mathrm{O}}_3$). Three different crystallographic orientations and four thicknesses, ranging from 40 to 500 nm, were examined under hydrostatic pressures $\left({\mathrm{P}}_h\right)$ of up to 1.5 GPa. All of the films at transition exhibited a four order of magnitude resistance change, with transition temperatures ranging from 140 to 165 K, depending on the orientation. This allowed us to build pressure-temperature phase diagrams of several orientations and film thicknesses. Interestingly, for pressures below 500 MPa, all samples deviate from bulk behavior and show a weak transition temperature $\left(T_c\right)$ pressure dependence $(d{T}_c/d{P}_h=1.2\times {10}^{-2}\pm 0.3\times {10}^{-2}\mathrm{K}/\mathrm{MPa}),$ which recovers to bulklike behavior $(3.9\times {10}^{-2}\pm 0.3\times {10}^{-2}\mathrm{K}/\mathrm{MPa})$ at higher pressures. Furthermore, we found that pressurization leads to morphological but not structural changes in the films. This indicates that the difference in the thin film and bulk pressure-temperature phase diagrams is most probably due to pressure-induced grain boundary relaxation, as well as both plastic and elastic deformations in the film microstructure. These results highlight the difference between bulk and thin films behaviors.

Figure 1

The RSM for ${\mathrm{V}}_2{\mathrm{O}}_3$ 100 nm films grown on different orientations of $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$. The data at $q_x=0$ represents the typical out-of-plane measurement. The RSM for film grown on (a) $a$-plane (1 0 0) out-of-plane orientation. Both ${\mathrm{V}}_2{\mathrm{O}}_3$ and $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ peaks are visible. Both in-plane and out-of-plane crystallographic orientations in ${\mathrm{V}}_2{\mathrm{O}}_3$ follow the $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ orientations. Films grown on (b) $r$-plane (0 1 2; sapphire peaks are not visible due to a slight miscut in the substrate) and (c) $m$-plane (1 1 0). For the (2 2 0) plane, only the ${\mathrm{V}}_2{\mathrm{O}}_3$ peak is visible because the $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ peak is outside the scan range. In (b) and (c), faint polycrystalline rings (two to three orders of magnitude more faint than the associated peaks) are visible, suggesting that a small portion of the film is polycrystalline. The measurements were done without a monochromator to increase the overall diffraction intensity, which resulted in the presence of several contamination lines. The primary x-ray wavelength is 1.5406 Å associated with Cu $\mathrm{K}\alpha 1$; however, significant contributions to the detected signal come from Cu $\mathrm{K}\alpha 2$ $(\lambda =1.5444\AA )$ and W L1 $(\lambda =1.0248\AA )$. For example, in Fig. 3, the (300) peak has another peak below the (300)${\mathrm{V}}_2{\mathrm{O}}_3$ associated with the W L1 of (300)$\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ and an even more faint fourth peak from W L1 of (300)${\mathrm{V}}_2{\mathrm{O}}_3$.

Figure 2

Resistance versus temperature dependence at different pressures for (a) $r$ plane, (b) $a$ plane, and (c) $m$ plane. The transition temperature decreases monotonically with increased pressure. Only absolute resistance measurements are reported because the samples were small $<1\mathrm{m}{\mathrm{m}}^2)$ and the variation in the geometry of the indium contact point made accurate evaluation of resistivity across the sample difficult. However, the magnitude and shape of transition is comparable or better to those previously reported in literature (larger change, sharper transition) [18, 23]. The difference in shape of the curves can be attributed to interfacial microstructure [18, 24].

Figure 3

Structural and microstructural characterization of a 100 nm ${\mathrm{V}}_2{\mathrm{O}}_3$ thin film grown on $r$-plane $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$: (a) XRD of the (0 1 2) out-of-plane peak before and after pressurization. Since the pressure cell accepts only very small sample sizes (less than $1\times 1\mathrm{m}{\mathrm{m}}^2$), the pattern after pressurization is noisier. (b) AFM image after pressurization; (c) XRR data and fit of data, before pressurization $({\chi }^2=0.0198)$. Reflectivity data after pressurization not shown since very noisy.

Figure 4

Transition temperature as a function of pressure. In both panels, the dashed line represents the experimental single crystal bulk behavior [28]. (a) Phase diagrams plotted as a function of crystallographic orientation for 100 nm thin films. (b) Phase diagrams plotted as a function of film thickness for $r$-cut sapphire. The uncertainty in pressure was determined from a pressure measurement before and after the temperature sweep. The 3 K uncertainty in transition temperature is a worst case uncertainty in the location of the inflection point in the curve.

Figure 5

Transition temperature as a function of pressure, with a constant $T$ offset subtracted from each of the phase diagrams in Fig. 4. All data collapse onto two master lines (below 600 MPa, slope $-0.012\pm 0.003\mathrm{K}/\mathrm{MPa}$ and above $600\mathrm{MPa},-0.039\pm 0.003\mathrm{K}/\mathrm{MPa}$); a small slope region and a large slope region. Dashed line represents the experimental single crystal bulk behavior with a slope of $0.05\text{K/MPa}$ [28].