Photoemission study of $\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$ interfaces

We have measured photoemission spectra of two kinds of $\mathrm{Ti}{\mathrm{O}}_2$-capped $\mathrm{V}{\mathrm{O}}_2$ thin films, namely, that with a rutile-type $\mathrm{Ti}{\mathrm{O}}_2$ capping layer $(r\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2)$ and that with an amorphous $\mathrm{Ti}{\mathrm{O}}_2$ capping layer $(a\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2)$. Below the metal-insulator transition temperature of the $\mathrm{V}{\mathrm{O}}_2$ thin films, $\sim 300\mathrm{K}$, $\mathrm{Ti}{\mathrm{O}}_2$ and $\mathrm{V}{\mathrm{O}}_2$ form an interface between a band insulator with the $d^0$ electronic configuration and a Mott (or Mott-Peierls) insulator with the $d^1$ electronic configuration. Metallic states were, however, not observed at the interfaces, in contrast to the interfaces between the $d^0$ band insulator $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ and the $d^1$ Mott insulator $\mathrm{La}\mathrm{Ti}{\mathrm{O}}_3$. We discuss possible origins of the difference between $\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$ and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3∕\mathrm{La}\mathrm{Ti}{\mathrm{O}}_3$, and suggest the importance of the polarity discontinuity in the latter interface. The stronger incoherent part was observed in $r\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$ than in $a\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$, suggesting Ti-V atomic diffusion due to the higher deposition temperature for $r\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$.

Figure 1

(Color online) Electrical resistivity of the $\mathrm{V}{\mathrm{O}}_2$, $a\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$, and $r\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$ thin film sample.

Figure 2

(Color online) $\mathrm{Ti}2p$ XAS spectra of $r\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$ and $a\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$ compared with spectra reported by Kucheyev et al. (Ref. 27).

Figure 3

(Color online) Valence-band $\mathrm{V}2p\to 3d$ resonant photoemission spectra of $a\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$ measured at room temperature. The $\mathrm{V}3d$ band was strongly enhanced at $518\mathrm{eV}$. Vertical lines indicate the LMM Auger peak position.

Figure 4

(Color online) $\mathrm{V}2p\to 3d$ resonant photoemission spectra of $\mathrm{V}{\mathrm{O}}_2$, $a\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$, and $r\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$ films in the $\mathrm{V}3d$ band region. The photon energy corresponds to the $\mathrm{V}2p\to 3d$ absorption edge and, therefore, these spectra were free from contributions from the $\mathrm{Ti}{\mathrm{O}}_2$ capping layers.

Figure 5

(Color online) Angle dependence of the photoemission spectra of $r\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$ in the $\mathrm{V}3d$ band region. The result indicates that the strong incoherent part of $r\text{‐}\mathrm{Ti}{\mathrm{O}}_2$ arose from the interface region.

Figure 6

(Color online) Photoemission spectra of $r\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$ and $a\text{‐}\mathrm{Ti}{\mathrm{O}}_2∕\mathrm{V}{\mathrm{O}}_2$ in the metallic and insulating phases of $\mathrm{V}{\mathrm{O}}_2$.