Si–VO₂ Hybrid Materials with Tunable Networks of Submicrometer Metallic VO₂ Domains Provide Enhanced Diode Functionality

It has been shown that optical devices that use vanadium dioxide (VO 2 ) metastructure allow us to flexibly modulate properties like the phase of the reflected or transmitted electromagnetic waves over a relatively wide range. VO 2 is a strongly correlated material and exhibits an insulator-to-metal transition that can be induced by external stimuli such as heating. During an insulator-to-metal transition, metallic domains emerge and grow, and therefore the metallic domains in a VO 2 layer behave like a tunable electrode. In this study, we found that the use of such a VO 2 layer in a silicon-based diode results in an enhanced response to terahertz light that can be controlled via the metastructure. This device can make use of nonlinear conduction induced by the avalanche effect in silicon (Si) under terahertz pulse irradiation. We fabricated a VO 2 thin-film with submicrometer metallic domains on a Si substrate and evaluated the change in the current through Si as the phase transition in VO 2 progresses via heating. Near the phase-transition temperature, a relatively strong signal was observed even at low terahertz pulse energies. This phenomenon is attributed to the electric-field enhancement resulting from the formation of a network of metallic domains that contain nanogaps and thus enhances impact ionization in Si. The results suggest that devices with such VO 2 layers inherently incorporate a certain amount of tunable electrical capacitance and inductance, which can provide novel device functionalities.