Pt/TiO 2 /Ti Metal-Insulator-Metal Tunnel Diodes for Rectification in an Energy Harvesting System
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Pt/TiO2/Ti Metal-Insulator-Metal Tunnel Diodes for Rectification in an Energy Harvesting System Matthew Chin1, Barbara Nichols1, Richard Osgood III2, Stephen Kilpatrick1, Madan Dubey1, and Nibir Dhar1,3 1 RF & Electronics Division, U.S. Army Research Laboratory, Adelphi, MD 20783, USA 2 Fibers and Materials Physics Division, U.S. Army Natick Soldier Research, Development, and Engineering Center, Natick, MA 01760, USA 3 Microsystems Technology Office, Defense Advanced Research Projects Agency, Arlington, VA 22203, USA ABSTRACT Methods for extracting or harvesting energy from the surrounding battlefield environment are of great importance to the United States Army. Scavenging energy from local environments reduces the required energy and weight transported to the theater. Micro- and nano-scale metalinsulator-metal (MIM) tunnel diodes are being developed to provide half-wave rectification as part of a “rectenna” energy harvesting system, which includes a radiation-collecting antenna, a rectifying MIM tunnel diode, and a storage capacitor. In this work, high-frequency MIM tunnel diodes for power rectification were designed, fabricated and characterized. Planar Pt/TiO2/Ti stacks are being fabricated to create a diode with highly asymmetric I-V characteristics that has a very low threshold voltage. The metals were chosen for their high work function difference, and the insulator was chosen for its barrier height, its compatibility with Ti, and its availability. The energy band diagram and the I-V characteristics were modeled to determine the feasibility of the Pt/TiO2/Ti material system for use as a rectifier diode in a rectenna system. Metals and insulator thin films were deposited onto silicon dioxide/silicon substrates. Pillars with lateral dimensions ranging from 20 μm x 20 μm up to 100 μm x 100 μm were fabricated. The dielectric thickness of the MIM diode was varied from 5 nm up to 50 nm to determine the optimal thickness for quantum tunneling. I-V measurements were taken using an electrical characterization system to confirm a non-linear, asymmetric response on a survey of devices fabricated with varying areas. Preliminary results exhibit asymmetric I-V characteristics with threshold voltages of less than 700 mV. INTRODUCTION The ability to harvest energy from infrared and optical sources is very attractive to the U.S. Army for powering unmanned autonomous systems, micro-robotic platforms, and portable electronics. Scavenging radiation with wavelengths between 0.5 and 15 μm would allow electronic equipment and systems to remain deployed for longer periods of time without the need to recharge or refuel. To meet this energy harvesting need, a “rectenna” system, composed of an antenna and a rectifying diode, is currently being developed for operation approaching or at the optical and infrared (IR) wavelengths. A rectenna converts electromagnetic (EM) radiation into an AC voltage via an antenna, which is then rectified into a DC voltage via a rectifying diode. For rectification at THz frequencies (EM wavelengths between
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