Epitaxial growth of VO2 by periodic annealing

Abstract

We report the growth of ultrathin VO 2 films on rutile TiO 2 (001) substrates via reactive molecular-beam epitaxy. The films were formed by the cyclical deposition of amorphous vanadium and its subsequent oxidation and transformation to VO 2 via solid-phase epitaxy.Significant metal-insulator transitions were observed in films as thin as 2.3 nm, where a resistance change ΔR/R of 25 was measured. Low angle annular dark field scanning transmission electron microscopy was used in conjunction with electron energy loss spectroscopy to study the film/substrate interface and revealed the vanadium to be tetravalent and the titanium interdiffusion to be limited to 1.6 nm. Page 4 of 25The huge metal-insulator transition (MIT) exhibited by VO 2 in the vicinity of roomtemperature has made it a material of interest for uncooled microbolometer arrays, 1 gas sensing, 2 optical limiting, 3 and most recently MIT transistors. 4,5 In bulk single crystals this MIT occurs at a transition temperature (T c ) of 340 K and is accompanied by a change in structure from a hightemperature tetragonal form, to a low-temperature monoclinic form. 6 The change in resistivity through this transition in bulk VO 2 single crystals has been measured to be five orders of magnitude with a temperature hysteresis 0.5-1 K. 7 The change in resistivity in thick films (>100 nm) can be as high as four orders of magnitude, [8][9][10] but in thin films (<10 nm) is less than three orders of magnitude in all reports to date. 11-13While VO 2 presents an opportunity for emergent switching devices and sensors, its large carrier concentration (~10 In this paper we describe a process for the growth of ultrathin VO 2 films by reactive molecular-beam epitaxy (MBE) and show that they exhibit clear MITs in films as thin as 2.3 nm.We investigate the properties of these films with four-circle x-ray diffraction (XRD), low-angle annular dark field (LAADF) scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), and electronic transport measurements. We also describe a standardized method for calculating the magnitude, hysteresis, and T c of these transitions using Savitsky-Golay smoothing derivatives. 16 All films in this paper were grown by MBE in a Veeco Gen10. X-ray diffraction spectra were collected with a Rigaku Smartlab system utilizing Cu Κ α1 radiation with a 220 Ge twobounce incident-beam monochromator and a 220 Ge two-bounce diffraction side analyzer crystal. STEM images were taken with an FEI Tecnai G 2 F20. VO 2 film thicknesses were calculated with data from Rutherford backscattering spectrometry (RBS) assuming the calibration films had bulk VO 2 density. Electrical transport data was taken using the standard four-contact van der Pauw method in a Quantum Design Physics Property Measurement System (PPMS) with contacts made using gold wire and silver paint. All growth temperatures were measured using a thermocouple in the substrate cavity, but not in contact with the substrate.During growth the film was monitored using reflection ...

Wide bandgap BaSnO3 films with room temperature conductivity exceeding 104 S cm−1

“…The highest known σ 300 K for common transparent conducting oxides (ITO=indium tin oxide, ZnO) and other metallic oxides as conductive electrodes for high frequency applications (VO 2 , LSCO=La 0.5 Sr 0.5 CoO 3 , SrTiO 3 , LSMO=La 0.7 Sr 0.3 MnO 3 , SRO=SrRuO 3 ) are shown for comparison (refs 49, 50, 51, 52, 53, 54, 55). …”

Mott Memory and Neuromorphic Devices

“…Consequently, the growth of these materials often requires careful control of temperature, stoichiometry, and defects to achieve large on/off ratio. Significant progress has been made on both epitaxial [107]- [109] and nonepitaxial growths [110] of high-quality Mott insulators by PLD [111]- [113], sputtering, e-beam evaporation [114], atomic layer deposition [115], [116], chemical vapor deposition [117], [118], molecular beam epitaxy [119], [120], and sol-gel methods [121] in the form of thin films [122]- [125] and nanostructures [126]- [130]. Low-temperature deposition techniques combined with postannealing procedures have also been widely studied [115], [131].…”

Simple and Low-temperature Growth of High Thermal Performance W-Ti Co-Doped VO2 Films on a Scale Wafer

“…[9][10] All these applications, especially microelectronic devices, require manufacturing of VO 2 films with superior quality on a wafer scale with a low cost. [11][12] There have been many fabrication methods of high quality VO 2 films, such as pulsed laser deposition (PLD), 13 magnetron sputtering, 14 molecular beam epitaxy (MBE) 15 and chemical solution deposition technique. 16 However, most of them are not proper for practical industrial applications because high quality films growth needs a wafer scale.…”