John F. Conley

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.

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 Nb2O5u2009=u20093.8u2009eV, Ta2O5u2009=u20094.4u2009eV, ZrO2u2009=u20095.4u2009eV, HfO2u2009=u20095.6u2009eV, Al2O3u2009=u20096.4u2009eV, and SiO2u2009=u20098.8u2009eV with uncertainty of ±0.2u2009eV. 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 ϕTu2009≈u20090.62 and 0.53u2009eV below the conduction band minimum are estimated for Nb2O5 and Ta2O5, respectively.

Solution based prompt inorganic condensation and atomic layer deposition of Al2O3 films: A side-by-side comparison

A comparison was made of Al2O3 films deposited on Si via prompt inorganic condensation (PIC) and atomic layer deposition (ALD). Current–voltage measurements as a function of annealing temperature indicate that the solution-processed PIC films, annealed at 500u2009°C, exhibit lower leakage and roughly equivalent breakdown strength in comparison to ALD films. PIC films are less dense than as-deposited ALD films and capacitance–voltage measurements indicate a lower relative dielectric constant. On the basis of x-ray photoelectron spectroscopy, transmission electron microscopy, and energy dispersive x-ray spectroscopy, it is found that the 500u2009°C anneal results in the formation of a ∼6u2009nm thick interfacial SiO2 layer at the Si interface. This SiO2 interfacial layer significantly affects the electrical performance of PIC Al2O3 films deposited on Si.

Atomic layer deposition of two dimensional MoS2 on 150 mm substrates

Low temperature atomic layer deposition (ALD) of monolayer to few layer MoS2 uniformly across 150u2009mm diameter SiO2/Si and quartz substrates is demonstrated. Purge separated cycles of MoCl5 and H2S precursors are used at reactor temperatures of up to 475u2009°C. Raman scattering studies show clearly the in-plane (E12g) and out-of-plane (A1g) modes of MoS2. The separation of the E12g and A1g peaks is a function of the number of ALD cycles, shifting closer together with fewer layers. X-ray photoelectron spectroscopy indicates that stoichiometry is improved by postdeposition annealing in a sulfur ambient. High resolution transmission electron microscopy confirms the atomic spacing of monolayer MoS2 thin films.

Bipolar resistive switching in an amorphous zinc tin oxide memristive device

The integration of amorphous zinc tin oxide (ZTO) into crossbar memristor device structures has been investigated where asymmetric devices were fabricated with Al (top) and Pt (bottom) electrodes. The authors found that these devices had reproducible bipolar resistive switching with high switching ratios >104 and long retention times of >104u2009s. Electrical characterization of the devices suggests that both filamentary and interfacial mechanisms are important for device switching. The authors have used secondary ion mass spectrometry to characterize the devices and found that significant interfacial reactions occur at the Al/ZTO interface.

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.

Improved oxidation resistance of organic/inorganic composite atomic layer deposition coated cellulose nanocrystal aerogels

Cellulose nanocrystal (CNC) aerogels are coated with thin conformal layers of Al2O3 using atomic layer deposition to form hybrid organic/inorganic nanocomposites. Electron probe microanalysis and scanning electron microscopy analysis indicated the Al2O3 penetrated more than 1500u2009μm into the aerogel for extended precursor pulse and exposure/purge times. The measured profile of coated fiber radius versus depth from the aerogel surface agrees well with simulations of precursor penetration depth in modeled aerogel structures. Thermogravimetric analysis shows that Al2O3 coated CNC aerogel nanocomposites do not show significant thermal degradation below 295u2009°C as compared with 175u2009°C for uncoated CNC aerogels, an improvement of over 100u2009°C.

Atomic layer deposition of bismuth oxide using Bi(OCMe2iPr)3 and H2O

This is the publisher’s final pdf. The article is copyrighted by the American Vacuum Society and published by the American Institute of Physics Publishing. It can be found at: http://scitation.aip.org/content/avs/journal/jvsta.

Investigation of growth parameter influence on hydrothermally grown ZnO nanowires using a research grade microwave

Hydrothermal growth methods allow for low-temperature synthesis of ZnO nanowires (NWs) directly on a variety of novel substrates required for proposed flexible substrate applications such as power-harvesting fabrics [1]. Although hydrothermal growth opens up a new realm of application possibilities, a major drawback of this method is growth time — up to 20 hours or longer have been reported as necessary to yield desired NW morphologies [2]. The use of microwave heating of the nutrient solution has recently been reported to speed the growth process [3]. Although hydrothermal growth temperature has been reported to be a critical process variable [4], the conventional microwave used previously [3] did not offer a direct method of controlling process temperature. Instead, indirect control of growth temperature was attempted by microwave power level adjustment. Other typical microwaves change power level through variations in duty cycle. In this work, a research grade microwave oven, equipped with integrated temperature control, was used to systematically investigate the impact of growth temperature, growth time, and solution concentration on ZnO NW length, diameter, aspect ratio, growth orientation, density, and wire morphology.