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/).