Continuous Liquid Metal Printing of High Mobility 2D Oxide Heterostructure Transistors

We introduce a new roller-based Continuous Liquid Metal Printing (CLMP) process for low-temperature, in-air deposition of 2D metal oxides, including InO_x and GaO_x. Our R2R compatible printing method uses Van der Waals forces to transfer high-quality, large area (> 30 cm²) 3.5 nm conductive oxide films generated by a self-limiting Cabrera-Mott oxidization mechanism in < 3s. We apply the CLMP method to fabricate InO_x / GaO_x heterostructure thin film transistors (TFTs) at a maximum process temperature of 200 °C. The CLMP 2D GaO_x layers were designed to provide a modulation doping effect at the back-channel of the 2D InO_x, enhancing the free carrier concentration as well as the electronic mobility. The nature of this InO_x / GaO_x heterointerface was further investigated by AFM, Kelvin probe measurements, photoelectronic spectroscopy in air (PESA), and Raman spectroscopy. XRD and TEM characterization reveal that the 2D GaO_x layer is amorphous as deposited, while the 2D InO_x exhibits large, plate-like grains, an electronically advantageous unique property of CLMP InO_x. An average linear mobility improvement from µ₀ = 12.5 cm²/Vs to µ₀ = 17.5 cm²/Vs is observed in comparing pure InO_x channels with InO_x / GaO_x heterostructure TFTs. These heterostructure devices exhibit accumulation mode operation and an average I_on/I_off of 10⁷. In summary, our work utilizes a breakthrough high-speed Van der Waals printing to fabricate semiconducting 2D oxide heterostructures for applications to displays, photodetectors, and flexible inorganic electronics.