Spectroscopic evidence of the formation of (V,Ti)O2 solid solution in VO2 thinner films grown on TiO2(001) substrates

Abstract

We have prepared VO 2 thin films epitaxially grown on TiO 2 (001) substrates with thickness systematically varied from 2.5 to 13 nm using a pulsed laser deposition method, and studied the transport property and electronic states of the films by means of resistivity and in situ synchrotron photoemission spectroscopy (SRPES). In resistivity measurements, the 13-nm-thick film exhibits a metal-insulator transition at around 290 K on cooling with change of three orders of magnitudes in resistivity. As the film thickness decreases, the metal-insulator transition broadens and the transition temperature increases. Below 4 nm, the films do not show the transition and become insulators. In situ SRPES measurements of near the Fermi level valence band find that the electronic state of the 2.5-nm-thick film is different than that of the temperature-induced insulator phase of VO 2 itself although these two states are insulating. Ti 2p core-level photoemission measurements reveal that Ti ions exist near the interface between the films and TiO 2 substrates, with a chemical state similar to that in (V,Ti)O 2 solid solution. These results indicate that insulating (V,Ti)O 2 solid solution is formed in the thinner films. We propose a simple growth model of a VO 2 thin film on a TiO 2 (001) substrate. Near the interface, insulating (V,Ti)O 2 solid solution is formed due to the diffusion of Ti ions from the TiO 2 substrate into the VO 2 film. The concentration of Ti in (V,Ti)O 2 is relatively high near the interface and decreases toward the surface of the film. Beyond a certain film thickness (about 7 nm in the case of the present 13-nm-thick film), the VO 2 thin film without any Ti ions starts to grow. Our work suggests that developing a technique for preparing the sharp interface between the VO 2 thin films and TiO 2 substrates is a key issue to study the physical property of an ultrathin film of "pure" VO 2 , especially to examine the presence of the novel electronic state called a semi-Dirac point phase predicted by calculations.

High temperature coefficient of resistance of low-temperature-grown VO2 films on TiO2-buffered SiO2/Si (100) substrates

“…A similar increase in resistivity and suppression of the MIT have been reported for VO 2 films grown on single-crystal TiO 2 substrates at high growth temperatures, and they have been attributed to interdiffusion of Ti and V atoms at the interface. 20,21 In fact, we confirmed by means of secondary-ion mass spectroscopy that Ti atoms diffused into a VO 2 film grown at 770 K (data not shown).…”

Thickness-dependent metal-to-insulator transition in epitaxial VO2 films

“…It is reported that nanoscale line cracks due to thermal stress can induce strain relaxation in VO 2 films with thicknesses ranging from 10 to 30 nm [16]. In addition, if the thickness is only within several nanometers, the VO 2 films will show exotic properties [21,22]. Moreover, the thickness also affects the performance in VO 2 -based nanoscale devices [23,24].…”

Epitaxial growth of VO2 by periodic annealing

“…The limited interdiffusion and consistent tetravalent oxidation state of vanadium throughout the film make it possible for substantially thinner VO 2 films to exhibit MITs. 4,27,28 The growth process described has yielded the thinnest films yet to show an MIT and the only VO 2 thin films grown by MBE to show an MIT.…”