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 NiO_x of ∼2.1–2.5 eV, which correlates with the high zero-bias dynamic resistance (R_D0) of ∼500 kΩ of an associated MIM diode. These values render NiO_x to be unsuitable for use in a rectenna device. Better performance has been observed from diodes based on TiO₂ and ZnO_x films. The best rectification performance was achieved for a Au/2.6 nm ZnO_x/Cr diode, scaled down to 1 μm² device area, showing a zero-bias dynamic resistance of R_D0 = 71 kΩ, zero-bias responsivity β₀ = 0.28 A/W, and a coupling efficiency of η_c = 2.4 × 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. [ABSTRACT FROM AUTHOR]