Directional ionic transport across the oxide interface enables low-temperature epitaxy of rutile TiO2

Abstract

Heterogeneous interfaces exhibit the unique phenomena by the redistribution of charged species to equilibrate the chemical potentials. Despite recent studies on the electronic charge accumulation across chemically inert interfaces, the systematic research to investigate massive reconfiguration of charged ions has been limited in heterostructures with chemically reacting interfaces so far. Here, we demonstrate that a chemical potential mismatch controls oxygen ionic transport across TiO 2 /VO 2 interfaces, and that this directional transport unprecedentedly stabilizes high-quality rutile TiO 2 epitaxial films at the lowest temperature (≤ 150°C) ever reported, at which rutile phase is difficult to be crystallized. Comprehensive characterizations reveal that this unconventional low-temperature epitaxy of rutile TiO 2 phase is achieved by lowering the activation barrier by increasing the "effective" oxygen pressure through a facile ionic pathway from VO 2-δ sacrificial templates. This discovery shows a robust control of defect-induced properties at oxide interfaces by the mismatch of thermodynamic driving force, and also suggests a strategy to overcome a kinetic barrier to phase stabilization at exceptionally low temperature.

Metal–insulator transition tuned by oxygen vacancy migration across TiO2/VO2 interface

“…The observed suppression of the MIT is consistent with a previous report on oxygen deficient VO 2-δ thin films 17 . We also note that our finding is consistent with a recent repot on VO 2 /TiO 2 heterostructures 43 . By utilizing the proposed “oxygen diode effect” we could tune the MIT and thereby change the ON/OFF ratio by three orders of magnitudes.…”

Heterogeneous integration of single-crystalline rutile nanomembranes with steep phase transition on silicon substrates

“…Indeed, an RSM near the (112) reflection of the TiO 2 NM confirms that the peaks from the VO 2 films and TiO 2 NM showed identical H (i.e., in-plane reciprocal space unit) (Fig. 4c), which indicates that a 10-nm-thick VO 2 film remains coherently strained to the TiO 2 NM along the in-plane direction 27 , 28 .

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“…The sharpened phase transition is attributed to the single-crystalline nature of the VO 2 films perfectly aligned with the underlying single-crystal TiO 2 NM templates. Moreover, the transferred single-crystal TiO 2 NM forms coherently tensile-strained VO 2 films; this epitaxial strain leads to a T MI shift close to room temperature in VO 2 films on TiO 2 NM/SiO 2 /Si compared to relaxed VO 2 films on SiO 2 /Si substrates 27 , 28 , 32 . Along with steep MIT under temperature, these single-crystalline VO 2 films on TiO 2 NM/Si show high endurance during thermal and electrical cycling.…”

“…Symmetrical 2θ-ω scan using synchrotron x-ray scattering on TiO 2 /VO 2 /TiO 2 detected two (002) R Bragg reflections, one from the rutile TiO 2 (2θ = 49.54°) film and substrate, and one from VO 2 film (2θ = 51.75°) (black line in Fig. 2a ) 27 . Due to the close resemblance in crystal structure and small in-plane lattice mismatch of (001) R -VO 2 and (001)-TiO 2 ( 0.86% along a- and b-axis), 17-nm-thick (001) R -VO 2 films are fully strained by biaxial tensile strain; all heterostructure maintained identical in-plane lattice constant (left figure of Fig.…”

“…2b ). Thus, the thickness of VO 2 (~17 nm) was determined to create unstrained and defect-free TiO 2 epitaxial films regardless of film thickness 27 , 28 .

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