AVS2015 Session EM+AS+SS-MoA: MIM Diodes, Functional Oxides, and TFTs

Metal Insulator Metal (MIM) diodes that work on fast mechanism of tunneling have been used in a number of very high frequency applications such as (Infra Red) IR detectors and optical Rectennas for energy harvesting. Their ability to operate under zero bias condition as well as the possibility of realizing them through additive techniques makes them attractive for (Radio Frequency) RF applications. However, two major issues namely, high surface roughness at the metal-insulator junction which effects the reliability of the diode, and very high resistance (typically in Mega Ohms) which complicates its matching with RF antenna have prevented its wide spread use in RF rectennas.

In this work, various metal deposition methods such as sputtering and electron beam evaporation are compared in pursuit of achieving low surface roughness. Amorphous metal alloy has also been investigated in terms of its low surface roughness. Zinc oxide has been studied for its suitability as a thin dielectric layer for MIM diodes. Finally, comprehensive RF characterization of MIM diodes has been performed in two ways: 1) by standard S-parameter methods, and 2) by investigating their rectification ability under zero bias operation.

It is concluded from the Atomic Force Microscopy (AFM) imaging that surface roughness as low as sub 1 nm can be achieved reliably from crystalline metals such as copper and platinum. This value is comparable to surface roughness achieved from amorphous alloys, which are non-crystalline structures and have orders of magnitude lower conductivities. Relatively lower resistances of the order of 1 Kilo Ohm with a sensitivity of 1.5 V^-1 have been obtained through DC testing of devices with MIM diode structure of platinum/zinc oxide/ titanium. Finally, RF characterization reveals that input impedances in the range of 300 Ω to 25 Ω can be achieved in the low GHz frequencies (from 0.5-10 GHz). From the rectification measurements at zero bias, a DC voltage of 4.7 mV has been obtained from an incoming RF signal of 0.4 W at 2.45 GHz, which indicates the suitability of these diodes for RF rectenna devices without providing any bias. These preliminary results indicate that with further optimization, MIM diodes are attractive candidates for RF energy harvesting applications.

If the antenna can be designed to absorb wavelengths in the range of a few hundred THz with multi-antenna array design, it results in high conversion efficiency due to power production from various light sources between ultraviolet (UV) and infrared (IR) radiation that is often thought of as heat and exists beyond the visible range for humans. One of the problems in this idea, however, is the nature of visible or IR light to oscillate at ultra-high frequencies. Therefore, a rectifier working at such an ultra-high frequency should be developed with a highly efficient coupling between antenna and light. Because Schottky diode is limited to frequencies less than ~ THz level, nanometer size MIM diode structure has been suggested as alternative design. Two different metals have used normally to make an asymmetric characteristic of current-voltage. However the work function difference between the metals cannot produce a high asymmetry, which causes a poor rectifier performance, even though the structure can be driven in THz range. To solve this issue, we used a structural asymmetric MIM design. The planar asymmetric design using various metals or grapheme showed better asymmetric I-V characteristics than that of simple MIM structure. In addition, for the vertical aligned design, single multi-wall carbon nanotube was formed as one electrode to get high tunneling current caused by the structural effect of sharp tip. The structural asymmetry can make a different field density states to the metals, which induces a high rectify characteristics. The contrast ratio between the forward and the reverse bias is ~10⁴ level. The estimated cut-off frequency is about 4.74THz. The electrical characteristics are stable up to 423K.

The response of a multiwall carbon nanotube to visible light has been reported to be consistent with conventional radio antenna theory. Researchers have proposed that this result might be exploited to realize an optical rectification device – that is, a device that converts free-propagating electromagnetic waves at optical frequencies to localized d.c. electricity. However, an experimental demonstration of this concept requires that the multiwall carbon nanotube antenna be coupled to a diode that operates on the order of 1 petahertz (switching speed on the order of a femtosecond). Ultralow capacitance, on the order of a few attofarads, could allow a diode to operate at these frequencies; and the development of metal-insulator-metal tunnel junctions with nanoscale dimensions has emerged as a potential path to diodes with ultralow capacitance, but these structures remain extremely difficult to fabricate and couple to a nanoscale antenna reliably. Here we demonstrate optical rectification by engineering metal-insulator-metal tunnel diodes at the tips of multiwall carbon nanotubes, which act as the antenna and metallic electron emitter in the diode. This performance is achieved using diode areas based on the diameter of a single carbon nanotube (about 10 nanometers), geometric field enhancement at the carbon nanotube tips, and a low work function semi-transparent top metal contact. Using vertically-aligned arrays of the diodes, we measure d.c. open-circuit voltage and short-circuit current at visible and infrared electromagnetic frequencies that is due to a rectification process, and quantify minor contributions from thermal effects. Our devices show evidence of photon-assisted tunneling, and exhibit zero-bias diode responsivity on the order of 0.1 amps per Watt and zero-bias differential resistance as low as 100 ohms-centimeter squared under illumination. Additionally, power rectification is observed under simulated solar illumination. Numerous current-voltage scans on different devices, and between 5-77 degrees Celsius, show no detectable change in diode performance, indicating a potential for robust operation.

Metal oxide thin-film transistors (TFTs) are becoming the mainstream for display backplanes. These TFTs are fabricated with traditional photolithographic techniques, typically on rigid substrates. In our lab, we explore approaches that are more “print-compatible”, with broad alignment tolerance and no small-gap mask features. We deposit zinc oxide (ZnO) semiconductors, aluminum oxide (Al₂O₃) dielectrics, and aluminum-doped zinc oxide conductors by the fast, atmospheric pressure, large-area-compatible, spatial atomic layer deposition (SALD) process. In addition to depositing good-quality thin-film transistor layers at temperatures at and below 200 °C, this process can work with a wide variety of rough and deformable substrates.

Here we describe vertical TFT and circuit architectures that unite process simplicity with high performance. The liberal design rules result from vertical transistors with self-aligned source and drain contacts that define the sub-micron channel length. Using 10-micron design rules for both the minimum line/space dimensions and for alignment tolerances, we have fabricated 9-stage ring oscillators with greater than 1 MHz oscillation frequency, at supply voltage below 6 V. Starting with a gate layer with a re-entrant profile on the edge, these devices use spatial ALD to conformally coat the Al₂O₃ gate dielectric and ZnO semiconductor, and a line-of-sight deposition process such as evaporation for the aluminum electrodes. Individual device characteristics as well as circuit performance will be discussed.

Geometrically asymmetric tunneling nanostructures are of interest to make ultra-high frequency diodes for applications in detection and solar energy harvesting. Atomic layer deposition (ALD) is one of the most promising techniques for fabrication of tunneling nanostructures. In previous work, it has been demonstrated that individual metal-vacuum-metal (MVM) tunnel junctions with a gap distance of 1-2 nm can be fabricated by selective-area ALD of Cu onto Pd templates. However, optimizing nonlinearity and scaling up to large arrays of tunneling devices both introduce new challenges that include achieving precise control of nucleation and good quality conformal growth on sharply defined asymmetric nanostructures.

​In this study, the fabrication of large arrays of MVM tunnel junctions is investigated using selective-area ALD. Nano-patterned Pd nanostructures with sharp asymmetric features are prepared as seed layers for planar, geometrically-asymmetric junctions on SiO₂ / silicon substrates by high-resolution electron beam lithography. Selective-area ALD applied to patterned Pd nanostructures allows tuning the size of junctions to nanometer dimensions. Microscopy and chemical analysis are used to evaluate nanostructure morphology, tunnel junction uniformity, and selective area growth characteristics. In-situ electrical measurements are used to measure DC current-voltage curves and nonlinearity. It was found that film nucleation and growth selectivity can be greatly affected by different pre-deposition sample treatments. UV/Ozone (UVO) cleaning and hydrogen annealing before ALD both enhance the nucleation of Cu thin films on Pd seed layers. In addition, UVO treatment promotes selective growth on Pd vs. SiO₂ areas while boiling samples in water to hydroxylate SiO₂ surface area contributes to a loss of selectivity. In-situ measured electrical data during ALD growth demonstrate a gradual convergence to tunneling with sub-nm control provided by the ALD method. However, control of tunneling non-linearity and geometric asymmetry is complicated by an incomplete understanding of the growth mechanism and the morphology evolution of nanostructures. There is a compromise between conditions that promote good ALD growth and those that maintain geometric asymmetry. We conclude with suggestions to promote growth, maintain sharp asymmetric features, and achieve non-linear tunneling characteristics.