Paper EM2-WeM12
Enhanced Performance Metal/Insulator/Insulator/Metal (MIIM) Tunnel Diodes

Wednesday, November 12, 2014, 11:40 am, Room 314

Thin film metal-insulator-metal (MIM) tunnel devices are gaining interest for applications such as hot electron transistors, diodes for optical rectenna based IR energy harvesting, IR detectors, large area macroelectronics, and selector diodes to avoid the sneak leakage in RRAM crossbar arrays. For many of these applications, figures of merit include high asymmetry and strong nonlinearity of current vs. voltage (I-V) behavior at low turn on voltages (V_ON). The common strategy to achieving rectification in MIM devices relies on Fowler-Nordheim tunneling (FNT) conduction in conjunction with the use of dissimilar work function metal electrodes to produce an asymmetric, polarity dependent electron tunneling barrier. The properties of single layer MIM diodes are dominated by the choice of insulator. Performance is limited by the workfuction difference that can be achieved between the electrodes as well as the metal-insulator band offsets. Wide bandgap oxides are limited by high V_ON. Narrow bandgap dielectrics such as Ta₂O₅ and Nb₂O₅ are attractive because the small barrier heights allow for low turn-on voltages. However, because conduction in these materials is based on emission rather than tunneling, they may not be suitable for high speed rectification. Recently, we showed that a nanolaminate pair of insulators (Al₂O₃/HfO₂) can be used to form MIIM diodes with enhanced performance over single layer MIM diodes and demonstrated that observed enhancements in low voltage asymmetry are due to "step tunneling," a situation in which an electron may tunnel through only the larger bandgap insulator instead of both.¹

In this work, we show that MIIM diodes may require only one of the insulators to be dominated by tunneling and thus allow use of narrow band gap insulators for tunnel devices. Atomic layer deposition (ALD) was used to deposit nanolaminate insulators on smooth amorphous metal bottom electrodes. We demonstrate that Ta₂O₅, a narrow bandgap dielectric dominated by thermal emission, may be combined with Al₂O₃, a wide bandgap dielectric dominated by tunneling, to achieve high asymmetry, low V_ON MIIM diodes whose overall performance is dominated by tunneling. The performance of a variety of other bilayer MIIM diodes (HfO₂/Ta₂O₅, ZrO₂/Ta₂O₅, Al₂O₃/ZrO₂, and HfO₂/ZrO₂) will be discussed as well. These results advance the understanding needed to engineer thin film tunnel devices for microelectronics applications.

1. N. Alimardani and J.F. Conley, Jr., Appl. Phys. Lett. 102, 143501 (2013).