Current induced polycrystalline-to-crystalline transformation in vanadium dioxide nanowires

Abstract

Vanadium dioxide (VO₂) exhibits a reversible insulator-metal phase transition that is of significant interest in energy-efficient nanoelectronic and nanophotonic devices. In these applications, crystalline materials are usually preferred for their superior electrical transport characteristics as well as spatial homogeneity and low surface roughness over the device area for reduced scattering. Here, we show applied electrical currents can induce a permanent reconfiguration of polycrystalline VO₂ nanowires into crystalline nanowires, resulting in a dramatically reduced hysteresis across the phase transition and reduced resistivity. Low currents below 3 mA were sufficient to cause the local temperature in the VO₂ to reach about 1780 K to activate the irreversible polycrystalline-to-crystalline transformation. The crystallinity was confirmed by electron microscopy and diffraction analyses. This simple yet localized post-processing of insulator-metal phase transition materials may enable new methods of studying and fabricating nanoscale structures and devices formed from these materials.

Figure 1. The fabricated device and the as-sputtered VO₂ film.

The (a) top and (b) cross-section view schematics of a VO₂ device with Pd contacts. (c) SEM images of a device at two magnifications. (d) X-ray diffraction pattern of the starting VO₂ film before crystallization.

Figure 2. VI and resistivity measurements before and after crystallization.

VI measurements for (a) L = 0.77 μm and W = 2.4 μm and (b) L = 0.99 μm and W = 0.93 μm. (c) The resistivity vs. current corresponding to (a) and the inset is the magnified region in dashed lines. (d) The resistivity vs. temperature for the “before” and “after” states showing the reduced hysteresis and resistivity in the “after” state.

Figure 3. Electron microscopy images and diffraction patterns before and after the crystallization.

Top view SEM images of the VO₂ nanowire device with L = 7.5 μm and W = 0.91 μm (a) before and (b) after the crystallization. XTEM images of a VO₂ nanowire (c) before and (d) after crystallization showing the absence of grains in the “after” state. (e) is the high-resolution TEM and (f) the SAED pattern corresponding to the “before” state in (c). The polycrystallinity is evident in the non-uniform plane orientations in (e) and randomness of the diffraction pattern in (f). (g) is the high-resolution TEM and (h) the SAED pattern corresponding to the “after” state in (d) taken in the VO₂ metallic (rutile) phase. The atomic arrangement is more periodic and the diffraction pattern exhibits clear symmetries that can be matched to the (i) computed diffraction pattern of VO₂ with a rutile crystal structure at view direction [210] using the lattice parameters from ref. .

Figure 4. Thermal modeling of the crystallization.

(a) The measured dissipated power at the critical current compared to the power required to heat the VO₂ nanowires by ΔT = 1470 K calculated using (a) 1D Fourier model for devices with varying W/L. The result of a thermal transport simulation (COMSOL Multiphysics) showing (b) the current density and (c) temperature distribution for VO₂ wire of dimensions L = 0.78 μm and W = 0.41 μm at the critical current, I_C.