Metal-insulator transitions in VO2 thin films

VO2 is a prototypical metal-insulator with a transition temperature slightly above room temperature. We are developing phase-field models to capture both the structural and electronic orders and their interplay in VO2 and explored a number of applications in conjunction with device-like in-operando experiments.

  • Simulation of voltage oscillation – Using an established phase-field model of VO2 by incorporating the heat equation, we are studying the voltage oscillation in VO2. The findings in this work are also expected to shed light on the long debate on whether the electron correlation effect is responsible for the voltage-driven insulator-metal transition in VO2.
Figure 3

 

Voltage oscillation in VO2 subject to direct voltages. (a) Experimentally measured electric field–current curve. (b) Simulated electric field–current curve. The inset is a schematic of the device geometry. (c) Experimentally measured voltage drop across the series resistor as a function of time. (d) Simulated voltage drop across the series resistor as a function of time.

 

  • Transient monoclinic metallic phase – Working in collaboration with Lindenberg group at Stanford, we studied transient monoclinic metallic phase during the insulator-metal transition in polycrystalline VO2 thin films. Upon applying a voltage, the rutile metallic phase first nucleates at regions with lower transition temperature and grows into the surrounding monoclinic insulating phase, turning it into monoclinic metallic phase.
Figure 4

 

Phase-field simulation of transient monoclinic metallic phase upon electrical excitation (B) and validation by experiments from Lindenberg group at Stanford (C).

 

  • Bending of VO2 films under voltages – We expereimentally probe VO2 thin film 2-terminal devices in-operando with an electrical pulse and an X-ray pulse probe on ns-μs time scales. It is shown that VO2 switches from an insulating phase to a metallic tetragonal phase with an incubation period followed by a rapid switch. We are using  phase-field simulations to undeerstand the incubation period arising from joule heating and the experimentally observed potato chip stress effect on the substrate.
Figure 5 Probing the electronically driven insulator to metal transition. (a) Custom in-operando setup developed for Beamline 2ID-D to study the structural evaluation accompanying the electronically driven insulator-to-metal (IMT) transition in VO2 ultrathin films fabricated into a 2-terminal configuration. 

 

  • Growth of VO2 thin films on sapphire and rutile – Evolution of a phase pattern may be affected by a number of factors including the inhomogeneities of vanadium valence states, twin domain walls, local oxygen deficiency or surplus in a film. To validate the computational predictions, we are growing VO2 thin films on 3-inch sapphire substrates to fabricate 2- and 3-terminal devices for observing the metal/insulating phase pattern formation under DC and AC currents, turn-on and turn-off characteristics, and ultrafast pump-probe dynamics. These will facilitate the validation of the phase-field model to capture the complex order parameters and their interplays.