# Progress in THz Rectifier Technology: Research and Perspectives
### THz Diode Technology: State of the Art of Metal Insulator Metal (MIM) Diode
Tantalum nitride (TaN) was deposited and patterned to form the top electrodes. The electrodes of all samples were deposited using 500 nm thick Al for good ohmic contacts. The results indicate a very high capacitance density of 19.13 fF/\(\upmu\)m\({}^{2}\) due to the perovskite SrTiO\({}_{3}\) (STO) with a very high dielectric constant of 145. This MIM capacitor also displays a quadratic voltage coefficient of 610 ppm/V\({}^{2}\) and a good leakage current of 5 \(\times\) 10\({}^{-9}\) A/cm\({}^{2}\) at 2 V, which is attributed to the inserted Al\({}_{2}\)O\({}_{3}\). This combination of materials makes this diode a promising candidate for higher frequencies. In [51], the authors present a fabrication and characterization of high-sensitivity copper-copper oxide-copper (Cu-CuO-Cu) metal-insulator-metal tunnel junctions. The MIM diode was fabricated on 300 nm of silicon dioxide (SiO\({}_{2}\)) placed on silicon (Si) substrates. Electrodes patterning was conducted by electron beam lithography. Cu electrodes were shown as 100 nm thick and 2 nm of CuO was deposited using RF sputtering. In this study, there were Cu-CuO-Cu symmetrical MIM tunnel junctions with 2 \(\times\) 2 \(\upmu\)m\({}^{2}\) of contact area; the results indicate maximum responsivity of 4.497 V\({}^{-1}\) at a bias voltage of 153 mV and zero-bias dynamic resistance \(R_{d}\) of 180 K\(\Omega\). In [52], the authors present quantum-tunneling metal-insulator-metal diodes made by rapid atmospheric pressure chemical vapor deposition. The authors realized and characterized an asymmetric diode Pt/Al\({}_{2}\)O\({}_{3}\)/Al printed on 500 nm of SiO\({}_{2}\) deposited on Si. The diode consists of Al\({}_{2}\)O\({}_{3}\) with thickness of 6 nm sandwiched between Pt/Al with thickness of 100 nm. Materials and deposition are indicated in [52]. The results are obtained by considering two different methods, atmospheric pressure chemical vapor deposition (AP-CVD) and plasma-enhanced atomic layer deposition (PEALD) techniques. With the first method, Pt-Al\({}_{2}\)O\({}_{3}\)-Al shows an _Asym_ of 110 for 1.5 V, an _NL_ of 6 for 1.4 V, an \(S\) of 9 V\({}^{-1}\) for 1.2 V, and current voltage (I-V) responses of 1 \(\times\) 10\({}^{-10}\) A with a voltage bias of 2 V, TOV of 1.4 V, and zero-bias dynamic resistance of \(R_{d}\) = 5 \(\times\) 10\({}^{12}\)\(\Omega\). With the second method, Pt-Al\({}_{2}\)O\({}_{3}\)-Al shows an _Asym_ of 30 for 2 V, an _NL_ of 30 for 1.8 V, an S of 22 V\({}^{-1}\) for 1.8 V, and current voltage (I-V) responses of 1 \(\times\) 10\({}^{-11}\) A with a voltage bias of 2 V, TOV of 1.75 V, with zero-bias dynamic resistance \(R_{d}\) of 7 \(\times\) 10\({}^{12}\)\(\Omega\). In [53], the authors present a model, design, and fabrication of antenna (bowtie)-coupled metal-insulator-metal diodes for IR sensing. In detail, an asymmetric diode Al-Al\({}_{2}\)O\({}_{3}\)-Au designed to rectify up to 60 THz is given. Metal-insulator-metal diodes were fabricated on 100 nm SiO\({}_{2}\) placed on Si substrates. Three layers of device were patterned by using the electron beam lithography (EBL) system. The bottom layer was 495 K polymethyl methacrylate (PMMA) and the top layer was 950 K PMMA. Bilayer resist was used in lithography. The first electrode made of aluminum is 65 nm thick whereas different thicknesses of Al\({}_{2}\)O\({}_{3}\) were deposited in order to investigate the thickness dependence of the characteristics of the devices. In the second electrode, 5 nm of chromium (Cr) followed by 65 nm thick gold (Au) were deposited. The results obtained indicate an \(S\) of 14.46 V\({}^{-1}\) for 600 mV, a density current voltage (J-V) response of 4.0 \(\upmu\)A/cm\({}^{2}\) for a voltage bias of 800 mV, a current of 12 nA, and zero-bias dynamic resistance \(R_{d}\) of 100 k\(\Omega\) for 0.6 V voltage bias. In [19], the authors present a model, design, fabrication, and characterization of an asymmetric Al-Al\({}_{2}\)O\({}_{3}\)-Cr MIM diode able to rectify up to 28.3 THz.