(Digital Presentation) Optimization of MIM Rectifiers for Terahertz Rectennas
Authors: Tekin Serdar B. , Almalki Saeed , Vezzoli Andrea , O’Brien Liam , Hall Steve , Chalker Paul R. , Mitrovic Ivona Z. 
Published: July 2022

There is a significant demand for harvesting renewable infrared (IR) energy from unused heat sources. The rectifying antenna (rectenna) device has the ability to capture alternating current (AC) IR radiation and rectify it into usable direct current (DC) electricity. Metal-Insulator-Metal (MIM) diodes have shown to be the most prominent contenders for rectenna applications. This is due to their ultra-fast current transport mechanism in the femtosecond range by means of quantum mechanical tunnelling. Optical rectification at 28.3 THz has recently been demonstrated by rectenna devices based on MInM diodes using Au/Al2O3/Ti [1] and Ti/TiO2/ZnO/Al [2] configurations. Although the results are promising, the overall conversion efficiency is quite low, 2.05 × 10-14 [1], mainly due to the poor rectification properties of the diodes. Other recent research [3-5] has focused on the combination of stoichiometric and non-stoichiometric oxides with the aim of engineering the barrier heights to achieve low dynamic resistance (R0
), high responsivity (β0
) and asymmetry (η0
) at zero-bias where the results are very promising for self-biased rectennas. The most recent experimental breakthrough was achieved by Ni/NiO/AlOx/CrAu bowtie rectennas that feature a device area of 0.035 µm2, low R0
of 13 kΩ and high β0
of 0.5 A/W. The results show that 5.1% coupling efficiency and 1.7 × 10−8% power conversion efficiency can be achieved with correct optimization of oxide stack. Furthermore, the most recent theoretical study [6] shows that the β0
of the MI2M diodes can be further improved to ~ 5 A/W by keeping the impedance match between the diode and the antenna at around 100 Ω. The proposed Ti/1 nm TiO2/1 nm Nb2O5/Ti rectenna design can achieve diode cut-off frequency (fc
) of 17 THz and resistance × capacitance (RC) time constant of 9 fs assuming the diode area of 0.01 µm2. Liverpool group has demonstrated recently [7,8] the effect of resonant tunnelling in non-cascaded (Al/Ta2O5/Nb2O5/Al2O3/Al) and cascaded (Al/Nb2O5/Al2O3/Ta2O5/Al) triple insulator diode structures with an oxide thickness ratio of 1:3:1 (in nm) deposited by atomic layer deposition (ALD). The diodes exhibit superior β = 5 A/W at 0.2 V and η = 12 at 0.1 V, with a drawback of high R0
due to high barrier heights between the metal/oxide layers.

In this paper, we further optimize the MInM diode configurations so that the metal/oxide barrier is significantly lowered to ~ 0.1 eV. This is achieved by using the combinations of rectenna contender oxides such as Al2O3, NiO and ZnO that have high electron affinity and low dynamic permittivity [9]. Ultra-thin (≤ 5 nm) insulating layers were fabricated using radio frequency (RF) magnetron sputtering and ALD. Metal electrodes were deposited by thermal evaporation and RF sputtering using shadow mask, photolithography and nanolithography processes. The device areas range from 100 µm × 100 µm, 1 µm × 1 µm and 100 nm × 100 nm depending on the patterning process to observe the effect of device scaling on the conduction mechanisms and direct current (DC) rectification properties. The deposited oxide layers were measured by variable angle spectroscopic ellipsometry (VASE) to ascertain their thickness, uniformity and optical constants. DC current voltage measurements were performed on fabricated diodes to evaluate key rectification parameters such as R0
, β0
, η and non-linearity (fNL
) around zero-bias. Complementary theoretical calculations were performed to substantiate the experimental results and allow comparison of different MInM diode configurations such as NiO/Al2O3, ZnO/Al2O3, NiO/ZnO and NiO/ZnO/Al2O3. This work shows that the coupling efficiency at IR cut-off frequencies can be improved by optimizing the barriers in MInM rectifiers.

References. [1] Jayaswal et al.,Materials Today Energy, 7, 1-9 (2018); [2] Elsharabasy et al., IEEE J. Photovoltaics, 9, 1232, (2019); [3] Matsuura et al., Sci. Rep. 9, 1-7 (2019); [4] Weerakkody et al., ACS Appl. Nano Mater. 4, 2470–2475 (2021); [5] Belkadi et al., Nat. Commun., 12, 1–6 (2021); [6] Elsharabasy et al., Results Mater. 11, 100204 (2021); [7] Mitrovic et al., ECS Trans. 72, 287 (2016); [8] Tekin et al., Solid State Electron. 185, 108096 (2021); [9] Mitrovic et al., Materials, 14, 5218 (2021).