Al2O3 Barrier Research Articles

Sub-nm equivalent oxide thickness on Si-passivated GaAs capacitors with low Dit

A thin amorphous silicon interlayer, inserted between the III-V semiconductor and the gate dielectric is expected to prevent III-V oxidation, as required for high-mobility channel transistors. We demonstrate that the addition of a thin Al2O3 barrier layer between the a-Si and the high-k HfO2, together with optimized post-metallization annealing, is the key to reduce the a-Si consumption and to achieve a highly scaled gate stack with equivalent oxide thickness of ∼0.8 nm. The evolution of the interfaces during growth and the quality of the stack are investigated by in-situ X-ray photoelectron spectroscopy and electrical measurements on metal-oxide-semiconductors capacitors.

Effect of heat‐treatment on the performance of gas barrier layers applied by atomic layer deposition onto polymer‐coated paperboard

AbstractThe effect of heat treatment on the gas barrier of the polymer‐coated board further coated with an Al2O3 layer by atomic layer deposition (ALD) was studied. Heat treatment below the melting point of the polymer followed by quenching at room temperature was used for the polylactide‐coated board [B(PLA)], while over‐the‐melting‐point treatment was utilized for the low‐density polyethylene‐coated board [B(PE)] followed by quenching at room temperature or in liquid nitrogen. Heat treatment of B(PLA) and B(PE) followed by quenching at room temperature improved the water vapor barrier. However, because of the changes in the polymer morphology, quenching of B(PE) with liquid nitrogen impaired the same barrier. No improvement in oxygen barrier was observed explained by, e.g., the spherulitic structure of PLA and the discontinuities and possible short‐chain amorphous material around the spherulites forming passages for oxygen molecules. This work emphasizes the importance of a homogeneous surface prior to the ALD growth Al2O3 barrier layer. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

P‐65: Flexible Gas‐Barrier Technologies with Inorganic Nanolaminate, Surface Modifications and Its Measurements

AbstractThe paper present transparent gas barrier technologies such as RF‐sputtered inorganic nanolaminate, hydrophobic/hydrophilic surface modifications, and its measurements. Highly conformal and uniform deposition of Al2O3 and SiO2 barriers and hydrophilic dots on nano‐imprinted hydrophobic surface structures successfully reduced water permeation rates according to the Ca degradation measurements.

The role of the spray pyrolysed Al2O3 barrier layer in achieving high efficiency solar cells on flexible steel substrates

Thin film chalcopyrite solar cells grown on light-weight, flexible steel substrates are poised to enter the photovoltaic market. To guarantee good solar cell performance, the diffusion of iron from the steel into the CIGSe absorber material must be hindered during layer deposition. A barrier layer is thus required to isolate the solar module from the metal substrate, both electronically and chemically. Ideally the barrier layer would be deposited by a cheap roll-to-roll process suitable to coat flexible steel substrates. Aluminium oxide deposited by spray pyrolysis matches the criteria. The coating is homogeneous over rough substrates allowing comparatively thin barrier layers to be utilized. In this article, solar cell results are presented contrasting the device performance made with a barrier layer to that without a barrier layer. Secondary Ion Mass spectrometry (SIMS) measurements show that the spray pyrolysed barrier layer diminishes iron diffusion to the chalcopyrite absorber layer. The role of sodium, imperative for the growth of high efficiency chalcopyrite solar cells, and how it interacts with Al2O3 is discussed.

Inelastic electron tunneling spectrum from surface magnon and magnetic impurity scatterings in magnetic tunnel junctions

Analytic expressions for contributions to the inelastic electron tunneling spectrum (IETS) from surface magnon scattering and magnetic impurity scattering are obtained. It is shown that surface magnon scattering alone does not lead to peaks in the IETS. The peaks at small bias often observed in the IETS of magnetic junctions are due to magnetic impurity scattering, in agreement with the traditional model for zero-bias anomaly. These impurity resonance peaks can sometimes split due to the impurities' magnetic coupling to the electrodes. Measurements of AlO and MgO barrier junctions yield excellent agreement to the theory. The experiment further shows that the magnetic impurities in MgO barriers are strongly coupled to the electrodes but those in AlO barriers are not magnetically coupled to the electrodes.

(Invited) ALD Moisture Barrier for Cu(InGa)Se2 Solar Cells

We report that gas diffusion barriers grown by atomic layer deposition (ALD) can protect moisture-sensitive, thin-film solar cells based on a Cu(In, Ga)Se2 absorber. The solar conversion efficiency for cells with ALD Al2O3 directly deposited, or laminated with a flexible polymer front sheet coated with an ALD barrier, was measured after stressing for more than 1000 hr in "damp heat" at 85{degree sign} C/85% RH with simulated solar illumination. For cells with ALD Al2O3 barriers, either directly deposited or laminated, the conversion efficiency decreased by only 3% after 1000 hr "damp heat" stressing. This supports ALD as an effective moisture-barrier strategy for extending the lifetime of flexible, thin-film CIGS solar cells, which are attractive for future building integrated applications.

Are Al2O3 and MgO tunnel barriers suitable for spin injection in graphene?

We report on the structural impact on graphene and multi-layers graphene of the growth by sputtering of tunnel barriers. Sputtered Al2O3 and MgO barriers were chosen for their well-known efficiency as spin injectors in spintronics devices. The impact of the growth on the structure of graphene and up to 4-layer flakes was analyzed by Raman spectroscopy. This study reveals that for Al2O3 growth, the impact is moderate for a monolayer and decreases sharply for bilayers and above. In the case of MgO all the flakes underwent a strong amorphization. Moreover, this reveals that while single layer graphene is believed to offer the best spin transport properties, the better robustness of multilayer graphene may ultimately make it a better choice for spintronics devices.

Energy level alignment and electrical properties of (Ba,Sr)TiO3/Al2O3 interfaces for tunable capacitors

The interface formation between Ba0.6Sr0.4TiO3 and Al2O3 has been studied using photoelectron spectroscopy with in situ sample preparation. A negligible valence band discontinuity, corresponding to a ∼5.6 eV barrier for electron transport at the BST/Al2O3 interface is determined. Current-voltage measurements show that the leakage current can be significantly reduced by inserting the Al2O3 barrier layer between barium strontium titanate (BST) and Pt electrode. Different charge injection behavior depending on Al2O3 thickness is observed, which correspond well with the experimentally determined energy band diagrams. Direct tunneling from the metal electrode into the BST conduction band through the Al2O3 barrier layer is observed.

Studies of Al2O3 barriers for use in tunnel junctions for nonlocal spin detection experiments

Aluminum oxide films were grown on Si under ultrahigh vacuum conditions for use as tunnel barriers in spin injection studies. X-ray photoelectron spectroscopy was performed to characterize the film stoichiometry. It was observed that all the aluminum was bonded to the oxygen for the films grown in 1 nm steps. Whereas the 2 nm sample grown in one 2 nm step left a partially unoxidized aluminum film. Current-voltage measurements were performed and fitted to a tunnel model. The resistance area products fall within the range needed for spin injection and nonlocal readout.

Hot electron transport in magnetic tunnel transistors with an epitaxial MgO tunnel barrier

We fabricated fully epitaxial magnetic tunnel transistors (MTTs) consisting of a Fe(001)/MgO(001) emitter and a Fe(001) based on a n-GaAs(001) collector. We observed much higher transfer ratio (α) and magnetocurrent (MC) ratio than those of the previously reported MTTs that had an amorphous AlO barrier. These improvements are due to the coherent tunneling and single-crystalline base layer. In addition, a pronounced peak structure appeared in the emitter bias dependence of α, which might be attributed to the interface resonance states at the Fe(001)/GaAs(001) interface.

Inverse and normal tunneling magnetoresistance effects in FeCoGd/FeCo/AlO/FeCo multilayers

FeCoGd/FeCo(t)/AlO/FeCo magnetic tunnel junctions were fabricated with a very thin FeCo layer inserted between the FeCoGd layer and the AlO barrier. At room temperature, inverse tunneling magnetoresistance (TMR) effect and normal TMR effect were observed simultaneously in the low and high magnetic field regions, respectively. The absolute values of both inverse and normal TMR ratios, the saturation magnetic field corresponding to the normal TMR effect have similar dependence on t. Transmission electron microscopy studies indicate that a thin granular film is likely formed between the FeCoGd layer and the AlO barrier, which may result in the normal TMR effect at high magnetic fields.

Suppression of Forward Electron Injection from Ru(dcbpy)2(NCS)2 to Nanocrystalline TiO2 Film As a Result of an Interfacial Al2O3 Barrier Layer Prepared with Atomic Layer Deposition

Subnanometer-thick Al2O3 barrier layers on nanocrystalline TiO2 film were prepared with atomic layer deposition (ALD). The method allowed variation of barrier thicknesses at atomic resolution also deep in nanoporous structures, which makes it a superior method as compared to, e.g., sol−gel techniques. In this letter we present results on the effect of Al2O3 barriers of various thicknesses on forward electron injection in dye-sensitized solar cells. A decrease in the amplitude of the oxidized Ru(dcbpy)2(NCS)2 dye absorption signal due to singlet injection was observed already after one deposition cycle that produces a discontinuous layer with nominal thickness of 1 Å. More than two layer coatings also slowed down the triplet injection. The findings indicate suppression of total electron injection, which is probably due to the Al2O3-induced weakening of electronic coupling between the dye and TiO2 as well as modification of the TiO2 electronic structure.

Nano-Engineering by Implanting Al2O3 Nano Particle as Sandwiched Scattering Centers in Between the La0.5Pr0.2Sr0.3MnO3 Thin Film Layers

We report the use of non-magnetic Al2O3 nano particles deposited between two ferromagnetic La0.5Pr0.2Sr0.3MnO3 (LPSMO) manganite layers with an aim to improve the electronic and magnetotransport properties of the layered supper lattice grown on single crystal STO(100) substrate using Pulsed Laser Deposition (PLD) technique. We studied the electronic-transport and magnetotransport properties of this system wherein Al2O3 particles are expected to act as insulating scattering centers between two ferromagnetic LPSMO layers. The scattering due to additional scattering centers (insulating Al2O3 nano particles) could be controlled by application of external field, resulting in high magnetoresistance (MR) approximately 72% as compared to pristine LPSMO film (MR approximately 51%) at temperature close to their T(M) values. In addition, incorporation of nanostructured Al2O3 barrier between the two ferromagnetic LPSMO layers results in a 2-3 fold increase in the values of temperature coefficient of resistance (TCR) and the field coefficient of resistance (FCR) as compared to pristine LPSMO film, suggesting the use of such nanoengineered manganite layered structure for better device application.

Temperature dependent dynamic and static magnetic response in magnetic tunnel junctions with Permalloy layers

Ferromagnetic resonance and static magnetic properties of CoFe∕Al2O3∕CoFe∕Py and CoFe∕Al2O3∕CoFeB∕Py magnetic tunnel junctions and of 25nm thick single-layer Permalloy (Py) films have been studied as a function of temperature down to 2K. The temperature dependence of the ferromagnetic resonance excited in the Py layers in magnetic tunnel junctions shows “kneelike” enhancement of the resonance frequency accompanied by an anomaly in the magnetization near 60K. We attribute the anomalous static and dynamic magnetic response at low temperatures to interface stress induced magnetic reorientation transition at the Py interface which could be influenced by dipolar soft-hard layer coupling through the Al2O3 barrier.

Spin Manipulation in Co-Doped ZnO

We report the clearly observed tunneling magnetoresistance at 5 K in magnetic tunnel junctions with Co-doped ZnO as a bottom ferromagnetic electrode and Co as a top ferromagnetic electrode prepared by pulsed laser deposition. Spin-polarized electrons were injected from Co-doped ZnO to the crystallized Al2O3 and tunnelled through the amorphous Al2O3 barrier. Our studies demonstrate the spin polarization in Co-doped ZnO and its possible application in future ZnO-based spintronics devices.

High Temperature Corrosion of Al3Ti-Cr Intermetallics in an Ar-1 %SO2 Gas Atmosphere

Ll2-type Al3Ti−Cr alloys of compositions 67Al−25Ti−8Cr, 66Al−24Ti−10Cr, and 59Al−26Ti−15Cr (in atomic percent) were corrosion tested between 800°C and 1100°C in an Ar−1% SO2 gas atmosphere for up to 150h. Corrosion proceeded mainly via oxidation reactions. The scale consisted primarily of a thin Al2O3 barrier layer. The alloys tested had much better corrosion resistance than TiAl-base alloys or Hastelloy X. Virtually no sulfides were detected in the scale, because of the strong oxidizing tendency of the Al3Ti−Cr alloys having high Al concentrations.

Noise in MgO barrier magnetic tunnel junctions with CoFeB electrodes: Influence of annealing temperature

Low frequency noise has been measured in magnetic tunnel junctions with MgO barriers and magnetoresistance values up to 235%. The authors investigated the noise for different degrees of crystallization and CoFeB∕MgO interface quality depending on the annealing temperature. The authors report an extremely low 1∕f noise, compared to magnetic junctions with Al2O3 barriers. The origin of the low frequency noise is discussed and it is attributed to localized charge traps with the MgO barriers.

Chemical and Magnetic Interface Properties of Tunnel Junctions With Co$_2$MnSi/Co$_2$FeSi Multilayer Electrode Showing Large Tunneling Magnetoresistance

Transport, as well as chemical and magnetic interface properties of two kinds of magnetic tunnel junctions (MTJs) with Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi electrode, Al-O barrier, and Co-Fe counter electrode, are investigated. For junctions with Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi single-layer electrodes, a tunnel magnetoresistance of up to 52% is found after optimal annealing for an optimal Al thickness of 1.5 nm, whereas the room temperature bulk magnetization of the Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi film reaches only 75% of the expected value. By using a [Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> MnSi/Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">times10</sub> multilayer electrode, the magnetoresistance can be increased to 114%, corresponding to a large spin polarization of 0.74, and the full bulk magnetization is reached. For Al thickness smaller than 1 nm, the TMR of both kinds of MTJs decreases rapidly to zero. On the other hand, for 2- to 3-nm-thick Al, the TMR decreases only slowly. The Al thickness dependence of the TMR is directly correlated to the element-specific magnetic moments of Fe and Co at the Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi/Al-O interface for all Al thickness. Especially, for optimal Al thickness and annealing, the interfacial Fe moment of the single-layer electrode is about 20% smaller than for the multilayer electrode, indicating smaller atomic disorder at the barrier interface for the latter MTJ.

All MgB2 tunnel junctions with Al2O3 or MgO tunnel barriers

All MgB2 thin film tunnel junctions with Al2O3 or MgO tunnel barriers were fabricated in situ on Si substrates in a molecular beam epitaxy system and their tunneling characteristics were investigated. In the quasiparticle tunneling spectra of the junction with Al2O3 tunnel barrier, we observed both superconducting gaps of MgB2, while only a small gap was seen with MgO tunnel barrier. Using a microscopic structural analysis, we found that the difference in the spectra is due to the crystal orientation difference of the MgB2 films: the film grown on Al2O3 was polycrystalline whereas the film grown on MgO was c-axis oriented.

Effect of Cr on the High Temperature Corrosion of Al&lt;sub&gt;3&lt;/sub&gt;Ti-Cr Alloys in SO&lt;sub&gt;2&lt;/sub&gt; Atmosphere

Three types of L12-type Al3Ti-Cr alloys, of compositions 67Al-25Ti-8Cr, 66Al-24Ti- 10Cr, and 59Al-26Ti-15Cr in atomic percent, were corrosion tested at 900, 1000, and 1100oC in an Ar-1% SO2 gas atmosphere for up to 150 h. The alloys were resistant to corrosion because a thin Al2O3 barrier layer was formed on the alloy surface due to the high Al content in the alloys. Due to the very strong Al affinity for oxygen, virtually no sulfur was detected inside the oxide scale.