Al2O3 Tunnel Barrier Research Articles

Robust Low Voltage Program-Erasable Cobalt-Nanocrystal Memory Capacitors with Multistacked Al2O3/HfO2/Al2O3 Tunnel Barrier

An atomic-layer-deposited Al2O3/HfO2/Al2O3 (A/H/A) tunnel barrier is investigated for Co nanocrystal memory capacitors. Compared to a single Al2O3 tunnel barrier, the A/H/A barrier can significantly increase the hysteresis window, i.e., an increase by 9 V for ±12 V sweep range. This is attributed to a marked decrease in the energy barriers of charge injections for the A/H/A tunnel barrier. Further, the Co-nanocrystal memory capacitor with the A/H/A tunnel barrier exhibits a memory window as large as 4.1 V for 100 μs program/erase at a low voltage of ±7 V, which is due to fast charge injection rates, i.e., about 2.4 × 1016 cm−2s−1 for electrons and 1.9 × 1016 cm−2s−1 for holes.

Role of the electronic structure on the relationship between the crystallinity of CoFe and its tunneling magnetoresistance

The influence of the crystallinity of CoFe on tunneling magnetoresistance is investigated in magnetic tunnel junctions with an amorphous Al2O3 tunnel barrier. An enhancement in the tunneling magnetoresistance is found when the CoFe is made amorphous compared to when it is crystalline. Ab initio electronic structure calculations show substantial differences in the band structures of crystalline and amorphous forms of bulk CoFe alloys but a decreased spin polarization at the Fermi energy in the amorphous phase. We speculate that the increased tunneling magnetoresistance is rather due to changes in bonding at the interface between Al2O3 and CoFe.

Sign of tunnel spin polarization of low-work-function Gd/Co nanolayers in a magnetic tunnel junction

Magnetic tunnel junctions having a low-work-function Gd/Co nanolayer at the interface with an Al2O3 tunnel barrier are shown to exhibit both positive and negative values of the tunnel magnetoresistance. The sign of the tunnel spin polarization of the Gd/Co nanolayer electrode depends on the thickness of the Gd and Co layers, temperature, and applied voltage. This reflects the nature of the interaction between the conduction electrons of the rare-earth and transition metals.

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.

Electrical injection and detection of spin-polarized carriers in silicon in a lateral transport geometry

We present the electrical injection, detection, and magnetic field modulation of lateral diffusive spin transport through silicon using surface contacts. Fe∕Al2O3 tunnel barrier contacts are used to create and analyze the flow of pure spin current in a silicon transport channel. Nonlocal detection techniques show that the spin current detected after transport through the silicon is sensitive to the relative orientation of the magnetization of the injecting and detecting contacts. Hanle effect measurements demonstrate that the spin current can be modulated by a perpendicular magnetic field, which causes the spin to precess and dephase in the transport channel.

Electrical spin-injection into silicon from a ferromagnetic metal/tunnel barrier contact

The electron’s spin angular momentum is one of several alternative state variables under consideration on the International Technology Roadmap for Semiconductors (ITRS) for processing information in the fundamentally new ways that will be required beyond the ultimate scaling limits of silicon-based complementary metal–oxide–semiconductor technology1. Electrical injection/transport of spin-polarized carriers is prerequisite for developing such an approach2,3. Although significant progress has been realized in GaAs (ref. 4), little progress has been made in Si, despite its overwhelming dominance of the semiconductor industry. Here, we report successful injection of spin-polarized electrons from an iron film through an Al2O3 tunnel barrier into Si(001). The circular polarization of the electroluminescence resulting from radiative recombination in Si and in GaAs (in Si/AlGaAs/GaAs structures) tracks the Fe magnetization, confirming that these spin-polarized electrons originate from the Fe contact. The polarization reflects Fe majority spin. We determine a lower bound for the Si electron spin polarization of 10%, and obtain an estimate of ∼30% at 5 K, with significant polarization extending to at least 125 K. We further demonstrate spin transport across the Si/AlGaAs interface.

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.

Tunnel Barrier Roughness Dependence of Magnetic Tunnel Junction with Synthetic Antiferromagnetic Pinned Layer

MTJs of structure Si/SiO2/Ta/Ru/IrMn/CoFe/Ru/CoFe/Al-O/CoFe/NiFe/Ru with different surface roughness of bottom electrode were prepared by sputtering, and it was investigated that the dependence of TMR ratio and resistance-area product (RA) on plasma oxidation time, thickness and surface roughness of tunnel barrier, and the tunneling characteristics of junction devices through I-V curves. To get resistance of below RA 10 kΩμm2, oxidation time of 10 s for 8 Å thick Al layer was required. In this case, thickness of Al2O3 barrier layer was 12.5 ~ 14 Å. For the 13 Å thick Al2O3 tunnel barrier, TMR ratio of optimized MTJ with uniform tunnel barrier was about 45% at bias voltage of 100 mV. Also the barrier height and the barrier width fitted to Simmon's relation were 12.3 Å and 3.07 eV, respectively, and these values agreed with that of MTJ within error range. The I-V curve and TMR ratio versus bias voltage curve of MTJ with rough tunnel barrier were linear and asymmetric, respectively, but in case of MTJ with uniform tunnel barrier, these curves were non-linear and symmetric, respectively. It was confirmed that the smooth surface of bottom electrode was a basic requirement for MTJ.

High quality ferromagnetic 0 and π Josephson tunnel junctions

The authors fabricated high quality Nb∕Al2O3∕Ni0.6Cu0.4∕Nb superconductor-insulatorferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic Ni0.6Cu0.4 layer and on the ambient temperature, the junctions were in the 0 or π ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The Al2O3 tunnel barrier allows one to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter βc increases exponentially with decreasing temperature and reaches βc≈700 at T=2.11K. The critical current density in the π state was up to 5A∕cm2 at T=2.11K, resulting in a Josephson penetration depth λJ as low as 160μm. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the Ni0.6Cu0.4 layer and different transparencies of the interfaces.

Preparation of arrays of metallic nanoparticles exhibiting coulomb blockade at room temperature: an approach based on the self‐organization of metal‐loaded diblock‐copolymers

AbstractThe self‐organization of HAuCl4 loaded poly (styrene)‐block‐poly (2‐vinylpyridine) (PS‐b‐P2VP) diblock‐copolymers resulting in hexagonally ordered arrays of Au nanoparticles is exploited to prepare a system exhibiting single electron charging effects at room temperature. For this purpose, the metallic nanoparticles are deposited on top of an Al2O3 tunnel barrier separating them from a Niobium thin film backelectrode grown on a c‐cut Sapphire substrate. The single electron system is completed by the metal tip of a scanning tunneling microscope (STM), which is positioned right above the nanoparticle and serves as the second electrode. The STM additionally allowed taking current‐voltage I (V) characteristics revealing a large gap around zero bias typical of a Coulomb blockade. This interpretation is supported by the good agreement between experimental I (V) curves and their theoretical description. Copyright © 2006 John Wiley & Sons, Ltd.

Cobalt-Al2O3-silicon tunnel contacts for electrical spin injection into silicon

The resistance of Co–Al2O3–Si tunnel contacts for electrical spin injection from a ferromagnet into silicon is investigated. The contacts form a substantial Schottky barrier, 0.7eV, which plays a dominant role in the electronic transport. On Si with a low doping concentration (∼1015cm−3), the contact resistance is affected by the Al2O3 tunnel barrier only in the forward bias. In the reverse bias (the spin injection condition), the Schottky barrier results in a very high contact resistance, ∼102Ωm2. While the contact resistance is improved to ∼10−2Ωm2 using Si with a high doping concentration (∼5×1019cm−3), it is still about five to six orders of magnitude higher than the value needed for resistance matching to silicon.

Fabrication of high quality ferromagnetic Josephson junctions

We present ferromagnetic Nb/Al2O3/Ni60Cu40/Nb Josephson junctions (SIFS) with an ultrathin Al2O3 tunnel barrier. The junction fabrication was optimized regarding junction insulation and homogeneity of current transport. Using ion-beam-etching and anodic oxidation we defined and insulated the junction mesas. The additional 2nm thin Cu-layer below the ferromagnetic NiCu (SINFS) lowered interface roughness and ensured very homogeneous current transport. A high yield of junctional devices with jc spreads less than 2% was obtained.

Spin polarization in ferromagnet/insulator/superconductor structures with the superconductor on top of the barrier

The tunneling spin polarization of CoFe, NiFe, and pure Ni was measured using superconducting tunneling spectroscopy for Al2O3 tunnel barriers with the ferromagnetic material both above and below the barrier. The spin polarization was found to be very similar for Co–Fe in both cases but for Ni and Ni–Fe alloys the spin polarization was significantly lower when the Al2O3 barrier was deposited on top of the ferromagnetic material. We attribute this to oxidation of the ferromagnet at the ferromagnet/Al2O3 interface resulting from an imperfect formation of the barrier.

Statistical model for prebreakdown current jumps and breakdown caused by single traps in magnetic tunnel junctions

To obtain reliable magnetic tunnel junctions (MTJs) for sensor and memory applications, the quality of the Al2O3 tunnel barrier is extremely important. Here, we studied the reliability of MTJs with a 1.6 nm Al2O3 tunnel barrier formed by ultraviolet light assisted oxidation. In the stress measurements, prebreakdown current jumps and, finally, breakdown are observed. We show, by using statistics, that both the current jumps and the final breakdown can be attributed to single trap generation. Moreover, we can relate the current jump height to the trap location. In this way, we reveal the breakdown mechanism in MTJs and illustrate the importance of reliability studies.

Reduced temperature and bias-voltage dependence of the magnetoresistance in magnetic tunnel junctions with Hf-inserted Al2O3 barrier

A modified tunnel barrier structure for the magnetic tunnel junction (MTJ) was fabricated by inserting a Hf layer in the middle of the Al2O3 tunnel barrier. MTJs with the Hf-inserted barrier show a higher tunnel mangnetoresistance (TMR) ratio and weaker temperature and bias-voltage dependence of TMR compared to the MTJs with a conventional Al2O3 barrier. The enhancement of the TMR ratio and the reduction of the temperature and bias-voltage dependence were attributed to the reduction of defects in the barrier.

Room temperature operation of a high output current magnetic tunnel transistor

The structure and properties of a magnetic tunnel transistor with high current output at room temperature are presented. The transistor marries a two-terminal magnetic tunnel junction with an Al2O3 tunnel barrier and a GaAs collector. The output current depends on the spin-dependent transport of hot electrons in the base layer of the transistor, which is formed from a single ultrathin ferromagnetic film. At a bias voltage of 1.4 V across the tunnel barrier, output currents larger than 1 μA and magnetocurrent changes of 64% are obtained at room temperature.

Tunnel barrier properties of stressed ferromagnetic tunnel junctions

The oxide breakdown properties of ultra-thin (~1 nm), naturally oxidised Al2O3 tunnel barriers in magnetic tunnel junctions were studied using ramped and constant stress experiments. During stress measurements at 1.35 V, a fast breakdown of the junction was observed. The time-to-breakdown is evaluated using Weibull statistics, as commonly utilised in SiO2 reliability studies.

Resistance and tunneling spectra of aligned multiwalled carbon nanotube arrays

The resistance and tunneling spectra of samples formed by depositing silver electrodes at the two ends of aligned, template-grown, carbon nanotube arrays were measured in the temperature range 0.67–440 K. Two types of samples were fabricated, one with small oxide tunnel junctions separating the carbon nanotubes from the metal electrodes, the other with a significant Al2O3 tunnel barrier. The measurements indicate the presence of three regimes for dI/dV(V). For T>220 K, dI/dV(V) and the zero-bias conductivity show a broad minimum and an activation temperature dependence suggesting semiconductor behavior. In the temperature range 10<T<140 K, the zero-bias conductivity shows a square-root temperature dependence. For T<2 K, a very steep rise in the zero-bias tunneling resistance is observed with a strong simultaneous suppression of the tunneling conductivity near the Fermi energy. Coulomb blockade is suggested as a plausible explanation of the observed behavior.

Electrical noise in hysteretic ferromagnet–insulator–ferromagnet tunnel junctions

Low frequency noise has been measured in magnetic tunnel junctions that have Al2O3 tunnel barriers and magnetoresistance values up to 35% at 295 K. Fluctuations in voltage were found to cross over from Johnson noise to shot noise at low bias voltages, in quantitative agreement with theories of noise in quantum ballistic systems. 1/f resistance noise, where f is frequency, predominates at larger biases and is proportional to the mean current squared. This noise is attributed to trapping processes and it depends sensitively on the relative position of the oxide edge and the ferromagnet–Al interface.

Spin dependent tunneling in 80NiFe/LB film with ferrocene and tris(bipyridine)ruthenium derivatives/Co junctions

80NiFe/LB-film/Co tunnel junctions with poly-N-dodecyl-acrylamide and its copolymer with the ferrocene derivative (Fc) or the tris(bipyridine)ruthenium complex (Ru) have been fabricated. FTIR spectra and small angle X-ray diffraction patterns for Fc, Ru and their mixture LB films indicated the formation of highly ordered molecular structure. The Bohr magneton number per one ferrocene moiety showed no difference between the film of Fc and the mixed film with Ru in the dark. However, UV-visible absorption and fluorescence spectra indicated that the electron transfer process in the mixed LB films occurred by an irradiation of light. The tunnel junctions using these LB films showed tunnel magnetoresistive effect. Its magnetoresistive ratio was 0.4% at room temperature being very small in comparison with the tunnel junction with Al2O3 tunnel barrier. The reason for this was considered to be residual water in the LB film causing a structural change of Co or the oxidation of the Co surface, so that the spin polarization could have become low.