Defect Engineering in VO₂ Thin Films via He⁺ Irradiation

Vanadium dioxide is a desirable material for neuromorphic computing applications due to its nonlinear electrical transport properties. These are the result of a metal-insulator transition (MIT) that can be accompanied by up to a four orders of magnitude change in electrical conductivity. At ~68°C, the MIT is coupled with a structural phase transition (SPT) as VO₂ transforms from an insulting monoclinic phase to a metallic rutile phase. In order to utilize VO₂ as a dynamically controllable material, it is critical to understand how to control 1) the electrical conductivities of two phases, and 2) aspects of the structural transition itself, including the extent of phase coexistence. Previous research has attempted to alter the MIT by use of chemical dopants that either decrease the MIT transition temperature, such as W and Mo, or increase the transition temperature, such as Ge [1-2]. Irradiation has also been used to modify MIT behavior, primarily by creating vacancy and interstitial defects that in turn alter the phase transition behavior [3-4]. Finally, the use of different substrates to grow highly oriented VO₂, notably Al₂O₃ and TiO₂ single crystal substrates, has been shown to shift the transition temperature as low as 35°C and as high as 80°C due to the resulting anisotropic compression/tension on the c-axis of VO₂ [5-6]. However, less research has focused on the intersection of strain and defects caused by irradiation.<br/>The goal of this research is to establish the effect of defects caused by irradiation on the MIT behavior and electrical transport in VO₂ thin films. Polycrystalline VO₂ thin films were grown on SiO₂(100nm)/Si, while epitaxial VO₂ was grown on Al₂O₃ (0001). Photolithography was used to make both 4-terminal and 2-terminal devices at unique crystallographic directions [10-10] for sapphire, due to the six-fold symmetry. To study device switching performance, a range of device widths and lengths were used ranging from 1 to 60 um and 1 to 25 um. These films were irradiated with He+ ions under two distinct energy regimes(10 keV, and 2 MeV), and fluences ranging from 5E13 to 5E16 cm^-2. We will describe the conjunction of coupled strain and irradiation defects on the electrical conductivities of the two phases of VO₂, as well as the hysteresis, width of phase transition, and temperature of transition. In this study, VO₂is shown to be fairly resistant to irradiation and can also be used for rad-hard material applications. This research furthers the exploration of the limits of the metal-insulator transition in VO₂ via nanoscale material manipulation for neuromorphic applications.<br/><br/><b>1. P. Jin, & S.Tanemura, “V1− xMoxO2 thermochromic films deposited by reactive magnetron sputtering,” Thin Solid Films, 281, 239-242, 1996.</b><br/><b>2. K. Shibuya, M. Kawasaki, & Y. Tokura, “Metal-insulator transition in epitaxial V 1− x W x O 2 (0≤ x≤ 0.33) thin films,” Applied Physics Letters, 96(2), 022102, 2010.</b><br/><b>3. E. M. Heckman, L. P. Gonzalez, S. Guha, O.J. Barnes, & A. Carpenter, “Electrical and optical switching properties of ion implanted VO2 thin films,” Thin Solid Films, 518(1), 265-268, 2009.</b><br/><b>4. A. Gupta, R. Singhal, J. Narayan, and D. K. Avasthi, “Electronic excitation induced controlled modifications of semiconductor-to-metal transition in epitaxial VO2 thin films,” Journal of Materials Research, vol. 26, no. 23, pp. 2901–2906, 2011.</b><br/><b>5. K. Nagashima, T. Yanagida, H. Tanaka, and T. Kawai, "Stress relaxation effect on transport properties of strained vanadium dioxide epitaxial thin films," Physical Review B, vol. 74, no. 17, 2006-11-17 2006, doi: 10.1103/physrevb.74.172106.</b><br/><b>6. Y. Zhao et al., "Structural, electrical, and terahertz transmission properties of VO2 thin films grown on c-, r-, and m-plane sapphire substrates," Journal of Applied Physics, vol. 111, no. 5, p. 053533, 2012-03-01 2012, doi: 10.1063/1.3692391.</b>