The Ni-NiO-Cu Metal-Insulator-Metal (MIM) tunnel diodes were fabricated through electrochemical deposition and thermal oxidation in the confined nanochannels of anodic aluminum oxide templates. Scanning electron microscopy (SEM) investigation reveals the diodes have a contact area of about 0.008 μm 2 , and transmission electron microscopy (TEM) shows that the thickness of NiO insulator layers ranges from 2 nm to 12 nm. The current-voltage (I-V) characteristics of the MIM diodes as prepared exhibit the nonlineara behavior with values strongly depending on the thermal oxidation time and the best zero bias sensitivity is 7.3 V −1 at bias voltage (V bias ) of 0.1 V.MIM tunnel diodes are attracting extensive attention due to their applications in advanced electronic devices such as hot electron transistors, 1 infrared detection, 2 switching memories 3 and rectennas. 4 In these applications, particularly interest lies in the devices operating at terahertz or higher frequencies range which are suitable for imaging, 5 sensing 6 and energy harvesting. 7 For such high frequencies, the MIM diode with a tunneling time in the order of 10 −15 s 8 for electrons is considered as an optimum candidate, since the electrons can flow between the metal electrodes via an ultra-thin insulator layer by the electric tunnel effect in this sandwich structure. Meanwhile, the electrodes with enough high work function difference exhibit non-linear current-voltage behavior that is preferred in rectification. 9 Up to now, diodes of various material combinations have been fabricated, such as Ni-NiO-Cr, 6 Nb-NbOxAg, 10 Al-AlOx-Pt. 11 However, as the MIM diode's equivalent-circuit is the parallel combination of a capacitor and a resistor, 11 the presence of parasitic capacitance restricts the cutoff frequency. 12 Hence, the low capacitance is required for high frequency rectification, that is to say the contact area between the metal layers should be minimized. Thereby the small contact area is the key factor to be considered in the fabrication of the high frequency MIM diodes, which also makes the fabrication of the MIM diode extremely challenging. 13 Currently, the main manufacturing methods to achieve the small contact area are involved in expensive equipments and complicated processing procedures such as photo or E-beam lithographic techniques. 14 Herein, we report a readily accessible approach to fabricate the NiNiO-Cu MIM diodes with a nanoscale contact area for high frequency application. This method is based on that the anodic aluminum oxide (AAO) template's nanochannel size ensures the small contact area of the diodes and that the NiO ultra-thin insulator layer can be prepared by naturally oxidation process, 15 overcoming some drawbacks involved in conventional fabricating procedures. The compositional, micro-structural, electrical properties of the MIM diode as prepared have also been studied.
ExperimentalAn AAO template has a highly ordered nanosize pores and uniform pore depth structure, which is widely used for the fabrication of vari...