Nasir Alimardani

Advancing MIM Electronics: Amorphous Metal Electrodes

Effectively controlling quantum mechanical tunneling through an ultrathin dielectric represents a fundamental materials challenge in the quest for high-performance metal-insulatormetal (MIM) diodes. Such diodes are the basis for alternative approaches to conventional thin-fi lm transistor technologies for large-area information displays, [ 1 , 2 ] various types of hot electron transistors, [ 2–6 ] ultrahigh speed discrete or antennacoupled detectors, [ 7–14 ] and optical rectennas. [ 15 ] MIM diodes have been fabricated by anodization, [ 1 ] thermal oxidation, [ 8–11 , 14 ]

Impact of electrode roughness on metal-insulator-metal tunnel diodes with atomic layer deposited Al2O3 tunnel barriers

Metal-insulator-metal (MIM) tunnel diodes on a variety of high and low work function metals with various levels of root-mean-square roughness are fabricated using high quality atomic layer deposited Al2O3 as the insulating tunnel barrier. It is found that electrode surface roughness can dominate the current versus voltage characteristics of MIM diodes, even overwhelming the impact of metal work function. Devices with smoother bottom electrodes are found to produce current versus voltage behavior with higher asymmetry and better agreement with Fowler-Nordheim tunneling theory, as well as a greater percentage of functioning devices.

Step tunneling enhanced asymmetry in asymmetric electrode metal-insulator-insulator-metal tunnel diodes

The impact of nanolaminate insulator tunnel barriers on asymmetric metal workfunction metal-insulator-insulator-metal (MIIM) devices is investigated. We demonstrate experimentally that bilayer insulators introduce additional asymmetry and can be arranged to either enhance or oppose the asymmetry induced by the asymmetric workfunction electrodes. It is also shown that step tunneling can dominate the I-V asymmetry of M1IIM2 diodes. By combining bilayer tunnel barriers with the standard approach of asymmetric metal electrodes, we are able to achieve low voltage asymmetry and non-linearity exceeding both that of standard single layer asymmetric electrode metal-insulator-metal devices as well as symmetric electrode M1I1I2M1 devices.

Investigation of the impact of insulator material on the performance of dissimilar electrode metal-insulator-metal diodes

The performance of thin film metal-insulator-metal (MIM) diodes is investigated for a variety of large and small electron affinity insulators using ultrasmooth amorphous metal as the bottom electrode. Nb2O5, Ta2O5, ZrO2, HfO2, Al2O3, and SiO2 amorphous insulators are deposited via atomic layer deposition (ALD). Reflection electron energy loss spectroscopy (REELS) is utilized to measure the band-gap energy (EG) and energy position of intrinsic sub-gap defect states for each insulator. EG of as-deposited ALD insulators are found to be Nb2O5 = 3.8 eV, Ta2O5 = 4.4 eV, ZrO2 = 5.4 eV, HfO2 = 5.6 eV, Al2O3 = 6.4 eV, and SiO2 = 8.8 eV with uncertainty of ±0.2 eV. Current vs. voltage asymmetry, non-linearity, turn-on voltage, and dominant conduction mechanisms are compared. Al2O3 and SiO2 are found to operate based on Fowler-Nordheim tunneling. Al2O3 shows the highest asymmetry. ZrO2, Nb2O5, and Ta2O5 based diodes are found to be dominated by Frenkel-Poole emission at large biases and exhibit lower asymmetry. The ...

Conduction processes in metal–insulator–metal diodes with Ta2O5 and Nb2O5 insulators deposited by atomic layer deposition

Metal–insulator–metal diodes with Nb2O5 and Ta2O5 insulators deposited via atomic layer deposition are investigated. For both Nb2O5 and Ta2O5, the dominant conduction process is established as Schottky emission at small biases and Frenkel–Poole emission at large biases. Fowler–Nordheim tunneling is not found to play a role in determining current versus voltage asymmetry. The dynamic dielectric constants are extracted from conduction plots and found to be in agreement with measured optical dielectric constants. Trap energy levels at ϕT ≈ 0.62 and 0.53 eV below the conduction band minimum are estimated for Nb2O5 and Ta2O5, respectively.

Enhancing metal-insulator-insulator-metal tunnel diodes via defect enhanced direct tunneling

Metal-insulator-insulator-metal tunnel diodes with dissimilar work function electrodes and nanolaminate Al2O3-Ta2O5 bilayer tunnel barriers deposited by atomic layer deposition are investigated. This combination of high and low electron affinity insulators, each with different dominant conduction mechanisms (tunneling and Frenkel-Poole emission), results in improved low voltage asymmetry and non-linearity of current versus voltage behavior. These improvements are due to defect enhanced direct tunneling in which electrons transport across the Ta2O5 via defect based conduction before tunneling directly through the Al2O3, effectively narrowing the tunnel barrier. Conduction through the device is dominated by tunneling, and operation is relatively insensitive to temperature.

Stability and bias stressing of metal/insulator/metal diodes

The performance and stability of metal/insulator/metal tunnel diodes was investigated as a function of interfacial roughness using Al, Ir, Pt, and ultra-smooth amorphous multi-metal (ZrCuAlNi) bottom electrodes with uniform Al2O3 tunnel dielectrics deposited via atomic layer deposition. Current density versus field behavior and device yield were found to be a function of interfacial roughness with smoother electrodes exhibiting more ideal behavior and higher percentages of working devices. A preliminary investigation of DC bias stressed devices suggests that interfacial roughness plays a large role in stability and reliability as well.

Impact of Electrode Roughness on Metal-Insulator-Metal (MIM) Diodes and Step Tunneling in Nanolaminate Tunnel Barrier Metal-Insulator-Insulator-Metal (MIIM) Diodes

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.

Electrical stressing of bilayer insulator HfO 2 /Al 2 O 3 metal-insulator-insulator-metal (MIIM) diodes

Bilayer metal/insulator/insulator/metal (MIIM) diodes of HfO2 and Al2O3 using asymmetric metal gates are investigated for their susceptibility to trap charge. Comparing the effects of constant current electrical stressing for varying thicknesses of insulators, the authors have formulated an adaptation of the Fowler-Nordheim derivative method such that an estimate of the charge centroid location can be obtained for bilayer insulators. In this work it has been found the addition of an HfO2 layer into an Al2O3 MIM device leads to increased charge trapping and greater shifts in the I-V characteristic.

Step tunneling enhanced asymmetry in metal-insulator-insulator-metal (MIIM) diodes for rectenna applications

We combine nanolaminate bilayer insulator tunnel barriers (Al2O3/HfO2, HfO2/Al2O3, Al2O3/ZrO2) deposited via atomic layer deposition (ALD) with asymmetric work function metal electrodes to produce MIIM diodes with enhanced I-V asymmetry and non-linearity. We show that the improvements in MIIM devices are due to step tunneling rather than resonant tunneling. We also investigate conduction processes as a function of temperature in MIM devices with Nb2O5 and Ta2O5 high electron affinity insulators. For both Nb2O5 and Ta2O5 insulators, the dominant conduction process is established as Schottky emission at small biases and Frenkel-Poole emission at large biases. The energy depth of the traps that dominate Frenkel-Poole emission in each material are estimated.