Impact of Electrode Roughness on Metal-Insulator-Metal (MIM) Diodes and Step Tunneling in Nanolaminate Tunnel Barrier Metal-Insulator-Insulator-Metal (MIIM) Diodes
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In this chapter, the impact of electrode roughness and bilayer insulator tunnel barriers on the performance of metal-insulator-metal (MIM) diodes are discussed. The effect of bottom electrode roughness on the current versus voltage (I–V) characteristics of asymmetric electrode M1IM2 tunnel diodes is discussed first. Atomic layer deposition (ALD) is used to deposit high quality insulators independent of bottom metal electrode. It is shown that bottom electrode roughness can strongly influence the I–V characteristics of M1IM2 diodes, overwhelming even the metal work function difference induced asymmetry. Devices with smoother bottom electrodes are shown to produce I–V behavior with better agreement with Fowler–Nordheim tunneling theory as well as yield a higher percentage of well-functioning devices. By combining high quality uniform tunnel barriers deposited by ALD with atomically smooth (~0.3 nm RMS roughness) bottom electrodes, highly nonlinear and asymmetric MIM tunnel diodes with good reproducibility and stable I–V behavior are produced. Next, the impact of nanolaminate bilayer insulator tunnel barriers on asymmetric metal work function metal-insulator-insulator-metal (M1I1I2M2 & M1I2I1M2) devices is discussed. It is demonstrated that bilayer tunnel barriers can be arranged to either enhance, oppose, or even reverse the asymmetry induced by the asymmetric work function electrodes. These results represent experimental demonstration that step tunneling (a step change in the tunneling distance through a bilayer tunnel barrier) can dominate the I–V asymmetry of M1IIM2 diodes with asymmetric work function electrodes. By combining bilayer tunnel barriers with asymmetric metal electrodes, devices are made with voltage asymmetry and nonlinearity that exceed that of standard single layer asymmetric electrode M1IM2 devices as well as that of symmetric electrode M1I1I2M1 devices.
KeywordsAtomic Layer Deposition Bottom Electrode Band Diagram Resonant Tunneling Tunnel Barrier
This work was supported in part by grants from the National Science Foundation (through DMR-0805372 and an REU supplement), the U.S. Army Research Laboratory (through W911NF-07-2-0083), and the Oregon Nanoscience and Microtechnologies Institute. The authors thank Matt Chin, Madan Dubey, and Steve Kilpatrick of the U.S. Army Research Lab for sputtered Pt films and support, Prof. John Wager, Bill Cowell, and John McGlone of the Oregon State University School of Electrical Engineering and Computer Science for the ZrCuAlNi films used in this study, Prof. Douglas Keszler of the Oregon State University Dept. of Chemistry, Wei Wang for assistance with AFM, Chris Tasker for equipment support, Dr. P. Eschbach for assistance with TEM imaging, Cheng Tan and Ben Lambert for assistance with data collection, and Dr. David Evans of Sharp Labs of America for evaporated Ir and Pt films.
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