Paper EM+OX-WeA12
Fabrication and Characterization of Metal-Insulator-Insulator-Metal (MIIM) Tunnel Diodes

Wednesday, October 31, 2012, 5:40 pm, Room 009

MIM tunnel diodes have been proposed for high speed applications such as hot electron transistors, IR detectors, and optical rectennas for IR energy harvesting as well as backplanes for LCDs. The majority of these applications require highly asymmetric and non-linear I-V behavior at low applied voltages. The standard approach to achieving asymmetric operation in MIM devices is through the use of metal electrodes with different workfunctions (Φ_M). However, the amount of asymmetry achievable using this method is limited by the Φ_M difference that can be obtained using practical metals. In this work, we use an alternative approach to achieving asymmetric and non-linear operation – engineering of the insulating tunnel barrier using nanolaminate pairs of insulators with different bandgaps and band-offsets to produce asymmetric tunnel barriers. Electrons tunneling from one metal electrode to the other will see a different shape barrier depending on the direction of tunneling and the bias applied.

Recently, we found that atomic scale roughness at the bottom metal-insulator interface can dominate the I-V characteristics of MIM diodes, even overwhelming the influence of Φ_M difference. By using the amorphous metal ZrCuAlNi (ZCAN) as an ultra-smooth bottom electrode in combination with high quality dielectrics deposited via ALD, we were able to fabricate MIM diodes dominated by FN tunneling. In this work, nanolaminate insulator stacks consisting of either HfO₂/Al₂O₃, Ta₂O₅/Al₂O₃,ZrO₂/Al₂O₃, or HfO₂/ZrO₂ are deposited on sputtered ZCAN bottom electrodes via ALD. Al dots form the top gate electrode.

MIIM I-V characteristics were found to be sensitive to the relative thickness as well as the arrangement of the individual dielectric layers. In the ZCAN / Al₂O₃ / HfO₂ / Al orientation, the asymmetric tunnel barrier opposes the effect of the asymmetric Φ_M, a larger current is measured at positive bias, and asymmetry is lower than for a neat Al₂O₃ insulator device. However, in the ZCAN / HfO₂ / Al₂O₃ / Al orientation, the asymmetric tunnel barrier enhances the asymmetric Φ_M, a larger current is measured at negative bias, and asymmetry is greater than for neat Al₂O₃ or HfO₂ insulator devices. High asymmetry is seen when conduction in both dielectric layers is dominated by FN tunneling rather than bulk limited mechanisms. Ta₂O₅/Al₂O₃, e.g., did not show an enhancement in asymmetry.