Amorphous Barrier Research Articles

Efficient diffusion barrier layers for the catalytic growth of carbon nanotubes on copper substrates

We have investigated the growth of carbon nanotube (CNT) films on copper substrates by the catalytic chemical vapour deposition route. Ferrocene was used as the catalyst precursor and toluene was the carbon feedstock. The copper substrates were coated with nitride and oxide amorphous ceramic barrier coatings in order to prevent diffusion of the iron catalyst during growth. It was found that virtually no CNT grew on pure copper, but long and densely packed mats of CNTs could be grown on TiN-coated copper. Copper substrates coated with SiN x and In 2O 3:Sn (ITO) also showed better results than pure copper, although the CNT density was much lower than that obtained from TiN/Cu. Auger electron spectroscopy (AES) showed that Fe diffusion occurred into SiN x /Cu and ITO/Cu substrates, which partially inhibited its catalyst activity. In contrast, AES did not detect the presence of diffused Fe into the TiN coating. The estimation of the diffusion coefficient by AES depth profiles for Fe in SiN x , was 3 · 10 −3 nm 2 s −1. This value establishes an upper limit for Fe diffusion on substrates for proper nanotube nucleation and growth. Secondary ion mass spectrometry provided complementary information on the composition profiles with depth.

Large tunnel magnetoresistance in magnetic tunnel junctions using a Co2MnSi Heusler alloy electrode and a MgO barrier

A large tunnel magnetoresistance (TMR) ratio of 753% has been observed at 2 K in a magnetic tunnel junction (MTJ) using a Co2MnSi Heusler alloy electrode and a crystalline MgO tunnel barrier. This TMR ratio is the largest reported to date in MTJs using a Heusler alloy electrode. Moreover, we have observed a large TMR ratio of 217% at room temperature (RT). This TMR at RT is much larger than that of MTJs using an amorphous Al-oxide tunnel barrier. However, the temperature dependence of the TMR ratio is still large because of inelastic tunneling in the antiparallel magnetic configuration.

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.

Electrical conductance properties for magnetic tunnel junctions with MgO barriers

We measured inelastic electron tunneling (IET) spectra and conductance for MgO tunneling magnetoresistance (TMR) films to obtain information on the ferromagnetic/barrier layer interface. The IET spectra showed the difference between amorphous and crystalline structures in the barrier. In the magnetic tunnel junction (MTJ) with a crystalline barrier the IET spectra indicated an Mg-O phonon peak at a low bias voltage by measurement with a parallel magnetization configuration. On the other hand, no peak was observed in the MTJ with an amorphous barrier.

Magnetoresistance in Co∕Pt based magnetic tunnel junctions with out-of-plane magnetization

Submicron magnetic tunnel junctions exhibiting perpendicular magnetic anisotropy have been prepared by sputtering. They associate a hard and a soft electrode based on Co∕Pt multilayers, separated by an amorphous alumina barrier. The soft electrode is either free or exchange biased by an antiferromagnetic layer. The magnetoresistance ratio reaches 8% at room temperature after patterning junctions with diameter down to 200nm. The macroscopic magnetic properties were investigated by extraordinary Hall effect and conventional magnetometry measurements. The magnetic moments of both electrodes are out of plane. Two well-separated switching fields allow the realization of well-defined parallel and antiparallel configurations of the magnetizations.

Bias Voltage Dependence of Tunneling Anisotropic Magnetoresistance in Magnetic Tunnel Junctions with MgO andAl2O3Tunnel Barriers

Tunneling anisotropic magnetoresistance (TAMR) is observed in tunnel junctions with transition metal electrodes as the moments are rotated from in-plane to out-of-plane in sufficiently large magnetic fields that the moments are nearly parallel to one another. A complex angular dependence of the tunneling resistance is found with twofold and fourfold components that vary strongly with bias voltage. Distinctly different TAMR behaviors are obtained for devices formed with highly textured crystalline MgO(001) and amorphous Al2O3 tunnel barriers. A tight-binding model shows that a fourfold angular dependence can be explained by the presence of an interface resonant state that affects the transmission of the contributing tunneling states through a spin-orbit interaction.

Josephson Junction Microscope for Low-Frequency Fluctuators

The high-Q harmonic oscillator mode of a Josephson junction can be used as a novel probe of spurious two-level systems (TLSs) inside the amorphous oxide tunnel barrier of the junction. In particular, we show that spectroscopic transmission measurements of the junction resonator mode can reveal how the coupling magnitude between the junction and the TLSs varies with an external magnetic field applied in the plane of the tunnel barrier. The proposed experiments offer the possibility of clearly resolving the underlying coupling mechanism for these spurious TLSs, an important decoherence source limiting the quality of superconducting quantum devices.

Probing momentum distributions in magnetic tunnel junctions via hot-electron decay

The tunnel momentum distribution in a (magnetic) tunnel junction is probed by analyzing the decay of the hot electrons in the Co metal anode after tunneling, using a three-terminal transistor structure in which the hot-electron attenuation is sensitive to the tunnel momentum distribution. Solid state amorphous Al2O3 barriers and the vacuum barrier of a scanning tunneling microscope are compared. For the former the attenuation length in nominally the same Co is strikingly larger (factor of 2), implying a more isotropic tunnel momentum distribution for Al2O3 barriers.

Elimination of two level fluctuators in superconducting quantum bits by an epitaxial tunnel barrier

Quantum computing based on Josephson junction technology is considered promising due to its scalable architecture. However, decoherence is a major obstacle. Here, we report evidence for improved Josephson quantum bits (qubits) using a single-crystal ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ tunnel barrier. We have found an $\ensuremath{\sim}$80% reduction in the density of the spectral splittings that indicate the existence of two-level fluctators (TLFs) in amorphous tunnel barriers. The residual $\ensuremath{\sim}20%$ TLFs can be attributed to interfacial effects that may be further reduced by different electrode materials. These results show that decoherence sources in the tunnel barrier of Josephson qubits can be identified and eliminated.

Recent advances on physico-chemical characterization of passive films by EIS and differential admittance techniques

Thin Nb 2O 5 anodic films (∼20 nm thick) grown in phosphoric acid solution have been characterised by EIS and differential admittance study in a large range of potential and frequency. The overall electrical behaviour has been interpreted by means of the theory of amorphous semiconductor Schottky barrier in presence of a non-constant density of states (DOS). A comparison of DOS for films grown in different electrolytes is reported.

Interface stability between amorphous ferromagnetic layer and Al oxide barrier in tunneling magnetoresistive films at elevated temperatures

The interface stability and microstructure between amorphous and crystalline ferromagnetic (FM) layers Fe56Co24B20 and Co70Fe30 (at. %) and oxide barrier layers (AlO) deposited by physical vapor deposition, in both as-deposited and annealed states, have been studied using hysteresis loops for magnetic measurement, x-ray photoelectron spectroscopy for elemental depth profiling, and transmission electron microscopy for atomic-level microstructure. AlO was found to be amorphous on both amorphous Fe56Co24B20 and polycrystalline Fe30Co70 FM layers. Substantial Fe diffusion towards the AlO layer and Al diffusion towards the FM layer are clearly observed for the Fe56Co24B20∕AlO system when annealed above 360°C and will likely cause magnetic tunneling junction devices made from this system to fail.

Vinyltrimethylsilane (VTMS) as a Probe of Chemical Reactivity of a TiCN Diffusion Barrier-Covered Silicon Surface

This paper presents the first molecular level investigation of chemical reactivity of a surface of an amorphous diffusion barrier film deposited on a Si(100)-2 x 1 single crystal. Vinyltrimethylsilane (VTMS) is chosen as a probe molecule because of its chemical properties and because of its role as a ligand in a common copper deposition precursor, hexafluoroacetylacetonato-copper-vinyltrimethylsilane, (hfac)Cu(VTMS). The surface chemistry of vinyltrimethylsilane on titanium carbonitride-covered Si(100)-2 x 1 has been investigated using multiple internal reflection Fourier transform infrared spectroscopy (MIR-FTIR), Auger electron spectroscopy (AES), thermal desorption mass spectrometry, and computational analysis. On a film with nominal surface stoichiometry TiC(x)N(y) (x approximately y approximately 1) preannealed to 800 K, VTMS adsorbs molecularly at cryogenic temperatures even at submonolayer coverages; the major pathway for its temperature-programmed evolution is desorption. Adsorption at room temperature leads to chemisorption via a double-bond attachment. A set of computational models was designed to investigate the possible adsorption sites for a VTMS molecule on a TiCN-covered Si(100)-2 x 1 surface. A comparison of the computational predictions for a variety of possible adsorption sites with the results of thermal desorption and infrared measurements suggests that approximately 90% of the adsorbed VTMS is chemisorbed along the Ti-C bond while approximately 10% is chemisorbed on a Ti corner atom, the minority site of the surface. The Ti-N bond is not participating in the chemisorption process.

Large tunnel magnetoresistance in magnetic tunnel junctions using Co2MnX (X = Al, Si) Heusler alloys

We fabricated B2-ordered Co2MnAl and L21-ordered Co2MnSi Heusler alloy films by optimizing various fabrication conditions (substrate, composition of sputtering target, substrate and post-annealing temperature, etc) and applied these films to bottom electrodes of magnetic tunnel junctions (MTJs). We used Al-oxide insulating tunnel barriers for our MTJs and varied oxidation times of Al films to control qualities of the Al-oxide insulating layer and Heusler-alloy/Al-oxide interface. Observed tunnel magnetoresistance (TMR) ratios were extremely sensitive to the structure and surface morphology of the prepared Heusler alloy films. Epitaxially grown Heusler alloy films showed good structural quality, very flat surfaces and enhanced TMR ratios. The behaviour of the TMR ratios towards oxidation time for the preparation of the Al-oxide barriers and the measurement temperature dependence of the TMR ratios were quite different between the MTJs with Co2MnAl and Co2MnSi electrodes. The obtained TMR ratio of 83% at 2 K in the MTJ with epitaxially grown B2-ordered Co2MnAl was large among the MTJs with an amorphous Al-oxide tunnel barrier. This result suggests that B2-ordered Co2MnAl is a highly spin-polarized material, as predicted by our theoretical calculation. Moreover, we observed a very large TMR ratio of 159% at 2 K in the MTJ with a high-quality epitaxially grown L21-ordered Co2MnSi electrode. This TMR ratio is the highest value to date in MTJs using an amorphous Al-oxide tunnel barrier. Spin-polarization of the Co2MnSi bottom electrode obtained from Julliere's formula was about 0.89. This value is also the largest achieved to date for a Heusler material and is much larger than those of conventional ferromagnetic materials such as Co–Fe. This large spin-polarization is attributed to a half-metallic band structure, as predicted by theoretical calculations.

Epitaxial Growth of Cu Nanodot Arrays Using an AAO Template on a Si Substrate

We established a method to clean the Si surface that exists at the bottom of anodic aluminum oxide (AAO) nanoholes after removal of the amorphous barrier layer, and we succeeded in the preparation of epitaxial Cu dot arrays on the Si surface in the nanoholes. The Si surfaces at the AAO nanohole bottoms were cleaned with dilute hydrofluoric acid after annealing at 900°C in Ar ambient, and we sputtered Cu on the AAO template to form Cu dot arrays. This method for preparing nanohole arrays on a single crystalline substrate enables growth of a variety of highly regular epitaxial nanodot/wire arrays.

Selective epitaxial growth of submicron complex oxide structures by amorphous SrTiO3

A chemical-free technique for fabricating submicron complex oxide structures has been developed based on selective epitaxial growth. The crystallinity and hence the conductivity of the complex oxide is inhibited by amorphous SrTiO3 (STO). Using a combination of pulsed laser deposition and electron-beam lithography, amorphous STO barriers are first deposited on a single-crystal substrate. A thin film is then deposited on the patterned substrate with the amorphous STO barriers acting to electrically and physically isolate different regions of the film. Since no chemical or physical etchants come in contact with the deposited film, its integrity and stability are preserved. This technique has produced submicron YBa2Cu3O7−δ and La2∕3Ca1∕3MnO3 structures.

Evidence for positive spin polarization in Co with SrTiO3 barriers

Recent negative tunneling magnetoresistance results with epitaxial SrTiO3 and TiO2 tunnel barriers were attributed to tunneling that favor d-like states from Co, resulting in negatively polarized electron tunneling. In order to confirm this, we carried out direct measurements of spin polarization of Co with SrTiO3 barriers using a superconductor as a spin detector. The polarization was observed to be positive (+31%), in contrast to published results. Furthermore, studies conducted on magnetic tunnel junctionswith SrTiO3 and TiO2 tunnel barriers are consistent with the direct measurements. The implications of our results, for the case of amorphous barriers, in comparison to crystalline barriers is discussed.

Void formation in the Cu layer during thermal treatment of SiNx/Cu/Ta73Si27/SiO2/Si systems

AbstractThe thermal stability of a SiNx passivation layer and its influence on the annealing behavior of an amorphous Ta73Si27 diffusion barrier deposited between copper and SiO2 were analyzed by X‐ray diffraction, glow discharge optical emission spectroscopy, Auger electron spectroscopy, scanning electron microscopy, and transmission electron microscopy. During heat treatment at a temperature Tan = 500 °C, diffusion of Cu atoms out of the Cu metallization into the SiNx passivation occurs. The Cu diffusion intensifies with increasing annealing temperature and annealing time and seems to be a necessary precondition for a defect formation process observed within the Cu metallization. Depending on the chemical composition of the SiNx/Cu interface, voids in the μm‐range can be formed within the Cu film. Compared to an unpassivated sample, heat treatment leads to a reduced diffusion of Ta atoms from the barrier through the copper into the SiNx/Cu interface. The barrier crystallization process into Ta5Si3 occurring during annealing at Tan = 600 °C is principally not affected by the presence of a SiNx passivation. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystallization and failure behavior of Ta-Ni nanostructured/amorphous diffusion barriers for copper metallization

This work examines the thin-film properties and diffusion barrier behavior of sputtered Ta-Ni films, aiming at depositing highly crystallization-resistant and conductive diffusion barriers for Cu metallization. Structural analysis indicates that the as-deposited Ta-Ni films indeed have a glassy structure and are free from highly resistive intermetallic compounds. Examining Si/Ta-Ni/Cu stacked samples reveals that thermally induced failure of amorphous Ta-Ni barriers is triggered by the barrier’s reaction with the silicon substrate at temperatures around 700°C. The effectiveness of the amorphous Ta-Ni thin film thus can be substantially enhanced by effectively blocking diffusion of copper toward the underlying silicon.

Radial Distribution Function Analysis of the Amorphous Barrier Layer in Magnetic Spin Tunnel Junctions

Spin tunnel junctions consist of two ferrromagnetic layers separated by an amorphous insulating barrier layer which is a few nanometers thick. The barrier layer is the most critical layer in terms of magnetic transport properties and yet, of all the layers, the structure of this layer has been the least investigated, mainly due to difficulties in carrying out structural investigations of nanovolumes of amorphous materials. In this paper we demonstrate how the technique of radial distribution function analysis using electrons can be used to investigate such small volumes, by applying it to the aluminium oxide amorphous layer in a junction. The analysis results in a radial distribution function which matches those obtained by neutron and X-ray diffraction from bulk material.

Nanostructured TaN(O)/TaN Barrier Film Formed by Oxygen Plasma Treatment for Copper Interconnect

Oxygen plasma was used to treat an ultrathin TaN barrier layer (10 nm) to improve barrier performance. X-ray diffraction, transmission electron microscopy, and measurements of electrical properties, were used to evaluate the barrier performance against Cu diffusion. Nanocrystallization and oxidation occurred after treatment with oxygen plasma. A nanostructured amorphous TaN(O) layer with a grain size of was formed on the surface of the TaN layer. The junction diodes resulted in large leakage currents after annealing at 525°C for 1 h, while the junction diodes retained their electrical integrity after annealing at 650°C. Nanocrystallization effects of plasma treatments are believed to be able to suppress Cu penetration into the Si substrate and, hence, improve the barrier performance. Nanostructured amorphous barrier layers can lengthen grain structures to alleviate effectively Cu diffusion, thereby acting as much more effective barriers than conventional TaN films. © 2004 The Electrochemical Society. All rights reserved.