Thin film metal-insulator-metal (MIM) rectifying devices using double, triple or quadruple insulator layers are currently the focus of attention for the development of next-generation optical nantennas. The interest is driven by their nanoscale footprint, room temperature operation, zero bias voltage requirement, and ease of integration with Complementary Metal Oxide Semiconductor technology. Highly asymmetric and nonlinear current-voltage (IV) behaviour at low applied voltages is critical for these applications. The operation of a MIM device is based on quantum mechanical tunnelling through a thin insulating film positioned between two metal electrodes. Hence, the operation speed of MIM devices depends on the tunneling transmission time, typically in the range of femto seconds or less, and theoretically can reach to a few 100 THz or even into the range of the solar spectrum. Figures of merit for MIM devices include the asymmetry, nonlinearity, and turn-on voltage. Rectification performance in MIM devices is generally limited by the work function difference that can be achieved between the metal electrodes, the barrier heights at either interface, and the mechanism of charge transport through the insulator. The choice of insulator is crucial. Wide band gap insulators provide high turn-on voltage. Narrow band gap insulators, such as Ta₂O₅ or Nb₂O₅, are thus attractive since small metal/insulator barrier heights allow for low turn-on voltage. For stable, temperature insensitive, high-speed operation, conduction through the insulator should be dominated by tunneling.

In this paper, we present comprehensive experimental and theoretical work on tunnel-barrier rectifiers comprising double (Nb₂O₅/Al₂O₃) and triple (Ta₂O₅/Nb₂O₅/Al₂O₃) insulator configurations engineered to enhance low voltage nonlinearity. There are two mechanisms that allow metal-insulator-insulator-metal rectifiers to have a high nonlinearity while keeping the resistance low: (i) resonant tunnelling, and (ii) so-called step tunnelling. Both will be discussed in the paper. A modified multi-layer Tsu-Esaki method has been used for IV calculations from the transmission coefficient by the transmission matrix method. The theoretical work indicates that the onset of resonant tunneling in MIIM and MIIIM rectifiers can be adjusted to close to zero volts by appropriate choice of work function difference of the metal contacts, the thickness of insulator layers, and the depth of quantum well. The double and triple insulator rectifiers were fabricated using atomic layer deposition and rf magnetron sputtering, while the metal contacts including Al and Ag were defined by photolithography or shadow mask and deposited by thermal evaporation. The thickness, band gap, roughness, band offsets, work functions and electron affinities have been extracted from variable angle spectroscopic ellipsometry, atomic force microscopy, x-ray and inverse photoelectron spectroscopy on fabricated devices to ascertain quality of the interfaces and measure barriers. The key rectifier properties, asymmetry, non-linearity and responsivity have been assessed from current voltage measurements performed in the range 293-370 K. A superior low voltage asymmetry (116 at 0.6 V) and responsivity (17 A/W at 0.7 V) for MIIIM rectifiers has been observed in advance of state-of-the-art experimental values. The results demonstrate enhanced rectification by atomically multilayering tunnel barriers in cascaded and non-cascaded MIIIM arrangements, for inclusion in optical nantennas.

 Acknowledgement. The work has been funded by EPSRC, UK, under project EP/K018930/1.