Transparent In/SeO₂ Thin Film Transistors Designed for Gigahertz/Terahertz Technologies

Herein, thin films of selenium oxide are coated onto transparent indium substrates with thickness of 150 nm under vacuum pressure of 10^–5 mbar. In/SeO₂ optical receivers are structurally, optically and electrically characterized. Induced crystallization of tetragonal SeO₂ showing homogeneous composition and continuous film formation is achieved via indium substrates. Indium thin films enhance the light absorbability and optical conductivity without altering the energy band gap of SeO₂. Strong interaction between In and Se at the ultrathin interface of In/SeO₂ led to the formation of a new second band gap of 0.92 eV relating to direct allowed transitions in InSe. Indium substrates increased the dielectric constant of SeO₂ by more than four times, making SeO₂ suitable for nonlinear optical applications. The terahertz cutoff frequency changed in the range of 0.9–14.0 THz. In-depth analysis of the optical conduction in In/SeO₂ films showed that the films display drift mobility, plasmon frequency and free carrier density values that render In/SeO₂ a suitable candidate for fabrication of thin film transistors. The transistors displayed microwave resonator features presented by double band stop filters. The filters showed return loss values larger than 20 dB and voltage standing wave ratios of 1.0 at 1.16 GHz. A negative capacitance effect is also observed for the transistors under study.