Inverse Design of Neuronal Action Potentials in Ge Implanted VO₂ Thin Films

Vanadium dioxide (VO₂) is a strongly correlated electronic material that exhibits a metal insulator transition (MIT) at 68°C. The MIT can result in up to four orders of magnitude change in the electrical conductivity, which is coupled with a structural phase transition (SPT) where VO₂ transforms from insulating monoclinic to metallic rutile. Its nonlinear transformation enables non-linear dynamics that make it a desirable material for neuromorphic computing applications. Yet, the use of VO₂ to emulate neuronal behavior requires control of the on/off ratio and aspects of the transition, such as the hysteresis and sharpness of transition. All these factors play a crucial role in extending the lifetime of a VO₂ -based memristor. Previous research has focused on 1) chemical doping of VO₂, [1] 2) applying strain to the material via lattice mismatch [2] and/or 3) creation of defects in order to tune aspects of the transformation [3]. It is poorly understood how modification of intrinsic properties of the film or the MIT translate to desired neuromorphic behavior (e.g., the neuronal action potential of a computing primitive).<br/>The goal of this research is to first establish the feasibility of chemically doping VO₂ thin films with Ge ion implantation at moderate ion energies and various doses. Second, to investigate the changes in the electronic transport behavior and the MIT caused by the addition of various types of defects introduced into the system. Previous work using He⁺ ions at low fluences showed the feasibility of modifying the resistivity of the insulating and metallic phases (ρ_low and ρ_high, respectively) without greatly altering the T_MIT or the on/off ratio [4]. Ideally, by controlling the ρ_low and ρ_high and by decreasing the sharpness of the transition we can maximize the interval at which negative differential resistance (NDR) occurs. This would result in a larger time frame in which device oscillations can occur within the NDR region. Finally, we aim to harness ion implantation as a technique to induce desired current oscillations or action potential-like behavior in simple neuromorphic computational primitives.<br/>Polycrystalline VO₂ thin films were grown on SiO₂(100 nm)/Si substrate. These films were implanted with Ge+ ions at 110 keV, and at fluences ranging from 1x10¹⁴ to 1x10¹⁶ ions/cm². Photolithography was used to make a variety of 2-terminal and 4-terminal devices on the films to study device switching performance. Implanted devices were then subjected to rapid thermal annealing (RTA) at temperatures ranging from 300°C to 900°C to restore the VO₂ crystallinity. This allowed us to identify the RTA temperatures and fluence values that best modified the MIT without amorphizing the film. The present study describes changes to the electrical transport upon Ge⁺ implantation as well as explores different oscillating behaviors observed within the NDR regions for devices subjected to different implantation conditions. Further exploration on the limits of controlling different aspects of the electrical transport behavior will 1) enable a larger class of materials that can emulate the neuronal functions and diverse neuronal dynamics, and 2) introduce a strategy to engineer biomimetic functionality into computational primitives.<br/><br/>References:<br/>1. P. Jin, & S.Tanemura, “V_1-xMo_xO₂ thermochromic films deposited by reactive magnetron sputtering,” Thin Solid Films, 281, 239-242, 1996.<br/>2. E. M. Heckman et. al., “Electrical and optical switching properties of ion implanted VO₂ thin films,” Thin Solid Films, 518(1), 265-268, 2009.<br/>3. K. Nagashima et. al., "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.<br/>4. R. M. Gurrola et. al., “Modulation of electronic transport in VO₂ induced by 10 keV helium ion irradiation”. In progress