Role of thermal strain in the metal-insulator and structural phase transition of epitaxialVO2films

Abstract

The metal-insulator switching characteristics of ${\mathrm{VO}}_{2}$ play a crucial role in the performances of ${\mathrm{VO}}_{2}$-based devices. In this paper we study high-quality (010)-oriented epitaxial films grown on (001) sapphire substrates by means of electron-beam evaporation and investigate the role of interface defects and thermal strain on the parallel evolution of the metal-insulator transition (MIT) and structural phase transition (SPT) between the monoclinic (insulator) and rutile (metal) phases. It is demonstrated that the highly-mismatched ${\mathrm{VO}}_{2}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ interface promotes a domain-matching epitaxial growth process where the film grows in a strain-relaxed state and the lattice distortions are confined at the interface in regions with limited spatial extent. Upon cooling down from the growth temperature, tensile strain is stored in the films as a consequence of the thermal expansion mismatch between ${\mathrm{VO}}_{2}$ and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$. The thinnest films exhibit the highest level of tensile strain in the interfacial plane resulting in a shift of both the MIT and the SPT temperatures towards higher values, pointing to a stabilization of the monoclinic/insulating phase. Concomitantly, the electrical switching characteristics are altered (lower resistivity ratio and broader transition) as a result of the presence of structural defects located at the interface. The SPT exhibits a similar evolution with, additionally, a broader hysteresis due to the formation of an intermediate, strain-stabilized phase in the M1-R transition. Films with thickness ranging between 100--300 nm undergo a partial strain relaxation and exhibit the best performances, with a sharp ($10{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ temperature range) and narrow (hysteresis $l4{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$) MIT extending over more than four orders of magnitude in resistivity $(6\ifmmode\times\else\texttimes\fi{}{10}^{4})$.