Electron affinity of metal oxide thin films of TiO₂, ZnO, and NiO and their applicability in 28.3 THz rectenna devices

The holy grail of achieving efficient operation of infrared (IR) rectennas continues to be the realization of a high performance rectifier. In this paper, we have fabricated metal-insulator-metal (MIM) diodes based on TiO2, ZnO, and NiO thin films using shadow mask evaporation, photolithography, and sputtering. The electron affinities of oxides have been measured by a combination of variable angle spectroscopic ellipsometry and x-ray photoelectron spectroscopy, as well as deduction from the extraction of metal/oxide barrier heights of Fowler-Nordheim tunneling plots. Our results confirm a low value for the electron affinity of NiOx of similar to 2.1-2.5 eV, which correlates with the high zero-bias dynamic resistance (R-D0) of similar to 500 k Omega of an associated MIM diode. These values render NiOx to be unsuitable for use in a rectenna device. Better performance has been observed from diodes based on TiO2 and ZnOx films. The best rectification performance was achieved for a Au/2.6 nm ZnOx/Cr diode, scaled down to 1 mu m(2) device area, showing a zero-bias dynamic resistance of R-D0 = 71 k Omega, zero-bias responsivity beta(0) = 0.28 A/W, and a coupling efficiency of eta(c) = 2.4 x 10(-5)% for rectification at 28.3 THz. The main significance of this study is that it employs a methodology whereby key parameters of the MIM stack are derived from physical measurements, which are then used to assist in the fitting of electrical current-voltage data to produce a reliable appraisal of diode performance in an IR rectenna. (c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).