Search Results - (Author, Cooperation:R. A. Buhrman)

2018-03-06

American Physical Society (APS)

0031-9007

1079-7114

Physics

Condensed Matter: Electronic Properties, etc.

2018-10-04

American Physical Society (APS)

1098-0121

1095-3795

Physics

Magnetism

2012-05-05

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1089-7550

AIP Digital Archive

Physics

We have used ballistic electron magnetic microscopy to image, with nanometer resolution, the magnetization behavior of Co/Cu/Co trilayer films in the presence of a magnetic field. Films prepared both by thermal evaporation and by magnetron sputtering have been studied. In each case we have observed both large, ∼500 nm, domain structures, and much smaller, apparently randomly dispersed, regions of magnetic misalignment between the Co layers that persist to fields 〉100 Oe. We find the details of the ballistic electron transport through the films to be different on small length scales, ∼50 nm, for the two types of growth methods. © 2000 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Three applications of ballistic electron microscopy are used to study, with nanometer-scale resolution, the magnetic and electronic properties of magnetic multilayer thin films and tunnel junctions. First, the capabilities of ballistic electron magnetic microscopy are demonstrated through an investigation of the switching behavior of continuous Ni80Fe20/Cu/Co trilayer films in the presence of an applied magnetic field. Next, the ballistic, hot-electron transport properties of Co films and multilayers formed by thermal evaporation and magnetron sputtering are compared, a comparison which reveals significant differences in the ballistic transmissivity of thin film multilayers formed by the two techniques. Finally, the electronic properties of thin aluminum oxide tunnel junctions formed by thermal evaporation and sputter deposition are investigated. Here the ballistic electron microscopy studies yield a direct measurement of the barrier height of the aluminum oxide barriers, a result that is invariant over a wide range of oxidation conditions. © 2001 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

It is demonstrated that polycrystalline thin films of high Tc superconducting YBa2Cu3O7−x can be grown with clean grain boundaries, i.e., without a boundary layer of segregation or different phase. In clean stoichiometric samples, angular misorientations of grains may be the origin of weak link behavior. High-resolution scanning transmission electron microscope images of films grown on ZrO2 and MgO by reactive evaporation, reactive sputtering, and laser ablation show atomic lattice images of clean grain boundaries. X-ray microanalysis with a 10 A(ring) spatial resolution also indicates no composition deviation at the grain boundaries. Grain sizes and epitaxial relations of samples prepared by different methods are characterized.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We discuss the results of a study on the growth by laser ablation of YBa2Cu3O7 thin films on polycrystalline and annealed vicinal (001) MgO substrates. In both instances the films were found to grow predominantly with the c axis normal to the plane of the substrate, regardless of the orientation of the MgO surface. In the case of the vicinal substrates the films were found to have superconducting properties comparable to those obtained with films grown on (001) oriented, annealed single-crystal substrates.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

By patterning ∼1-μm-wide microbridges in laser ablated YBa2Cu3O7 films containing high-angle tilt boundaries, weak links have been isolated with critical currents low enough to avoid self-screening effects. The current-voltage characteristics of these high-angle tilt-boundary weak links are well described by the resistively shunted junction model, if noise rounding is included. The response of the supercurrent to magnetic field and temperature indicates that the weak links are spatially nonuniform, consisting of relatively small areas of high critical current density, Jc, separated by areas with very low or zero Jc. The response to rf power suggests that the current-phase relation is nonsinusoidal.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Spin filtering of ballistic electrons by ultrathin cobalt films of thicknesses ranging from 0.2 to 3.5 nm has been studied experimentally using nonmagnetic metal–ferromagnet–superconductor nanocontacts. In such systems the flow of electrons with energies below the superconducting gap is very sensitive to any net spin polarization of the electron current. This effect was used to quantitatively measure the transmission rates of up and down spin electrons passing through an individual ferromagnetic layer of nanometer thickness. © 1999 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

During furnace N2O-based silicon oxynitride growth, the total concentration of NOx species varies strongly with the flow rate of N2O. At low flow rates the N2O decomposes at least partially in the cooler region of the furnace near the gas inlet. This results in lower than expected NOx (x=1,2) concentrations in the oxidizing ambient. At high flow rates, the exothermic decomposition of N2O can heat the inlet region of the furnace, resulting in decomposition at a temperature above the nominal furnace temperature and higher than expected NOx concentrations. These effects can lead to a substantial variation in the concentration of N in the oxynitride as a function of N2O flow. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We discuss the results of a study of the effect of substrate preparation on the microstructure and superconductive properties of YBa2Cu3O7 thin films formed by laser ablation on (001) MgO substrates. Thermal annealing of the substrates is found to be highly effective in producing at fairly low growth temperatures (670 °C), epitaxial, c-axis normal films with good superconductive properties. Alternative surface treatments result in the formation of large angle tilt boundaries and inferior superconductive properties.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The Au/Si(111) interface has been investigated with ballistic electron emission microscopy. The Schottky barrier (SB) height and ballistic transmittance have been measured on interfaces which have been prepared with different types of monolayer-level dopants. Transmission rates but not the SB are found to depend strongly on the resulting degree of interdiffusion of the Au and Si at the interface. An irreversible modification in the transport properties of the buried interface can occur when the system is stressed with electrons injected at several volts above the Schottky barrier.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A high-pressure reactive evaporation process has been used to produce smooth YBa2Cu3O7−y high Tc superconducting films without the necessity of a post-deposition oven anneal cycle. Augmented in some cases by rapid thermal annealing, the process has yielded films with zero resistance transition temperatures above 80 K, and critical current densities above 106 A/cm2 at 4.2 K on both cubic zirconia and SrTiO3 substrates.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Thin-film fabrication techniques for forming three-dimensional "point contacts'' are presented. As-fabricated nanobridges can be modified using electromigration to make the constriction region smaller or dirtier. Scientific applications to quantum transport studies, 1/f noise, and electromigration are discussed.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The superconductive weak link properties of microbridges formed in c-axis normal YBa2Cu3O7−δ polycrystalline thin films containing a variable amount of large angle tilt boundaries have been studied. In the low critical current density limit these weak links have current-voltage (I-V) characteristics that are accurately modeled by the resistively shunted junction model. The I-V's are found to accurately follow a simple scaling law with the product of the critical current and weak link resistance Rn varying linearly with the weak link conductance.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Rugged, high current density NbN1−xCx/MgO/Nb(overdot)N1−xCx tunnel junctions have been fabricated and tested as voltage tunable Josephson junction terahertz oscillators. The emitted radiation from these junctions is detected on chip by a second junction which is capacitively coupled to the first. For oscillator junctions with a critical current density of Jc∼3.5×104 A/cm2 we find that the junction oscillates with a voltage amplitude of ∼1.5 mV. The detected rf voltage level remains essentially constant from 300 GHz to above 1 THz. The oscillator junction produces 0.5 μW of terahertz radiation of which, due to impedance mismatch, 0.01 μW is coupled into the detector junction.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We have studied the behavior of very thin oxide (∼20 A(ring)) metal-oxide-semiconductor tunnel diodes under high electrical field bias. These devices do not usually experience catastrophic breakdown, but can be worn out at high fields through the creation of a low barrier tunneling path. The effective area of the path increases during stress, while the barrier height remains essentially constant at ∼1 eV. The formation of the path is correlated to the presence of multilevel switching fluctuations in the diode current. The same complex fluctuations and excess currents are seen in oxides up to 70 A(ring) where the fluctuations show up as noisy precursors to catastrophic breakdown.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We have used capacitance-voltage (C-V) techniques and x-ray photoelectron spectroscopy (XPS) to study for the first time the electrical and structural properties of thin SiO2 films grown on silicon by plasma anodization and rapid thermal processes (RTO) and then compared them to furnace oxides. We have compared the SiO4 tetrahedral ring structure and the suboxide content of the ∼3-nm-thick interfacial region of these oxides and have found significant structural differences. By correlating these differences with measured electrical differences, we have identified the structural causes of some of the electrical characteristics of the plasma and RTO oxides. In plasma oxides we see larger amounts of silicon dangling bonds, Pb centers, at the Si-SiO2 interface and have identified these dangling bonds as the source of a localized peak of interface states found at 0.3 eV above the silicon valence band. Low-temperature rapid thermal annealing of the plasma oxides relieves localized compressive interfacial strain, apparently by allowing the completion of oxidation at the interface, and reduces the amount of dangling bonds. However, this strain relief simultaneously increases the average SiO4 ring structure at the interface. A larger interfacial SiO4 ring structure is also seen in rapid thermal oxides and has been attributed to the very rapid cooling which takes place at the end of the rapid thermal process. Post-growth thermal processing has been shown to reduce the average ring structure by relieving localized tensile interfacial stress, but this stress relief is accompanied by the appearance of a peak of interface states at about 0.8 eV above the valence band which is attributed to Si–O bonds broken during the anneal. Long furnace anneals of rapid thermal oxides remove these states and give interface state densities comparable to those of furnace oxides.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We describe the results of micro-Raman spectroscopy and optical microscopy studies of basal-plane chain-oxygen vacancy motion in YBa2Cu3O7−δ thin films under the influence of a high current bias near 300 K. Above a threshold level this bias causes vacancy aggregation and then long-range displacement, leading to an enhancement of oxygen order in the region of highest current density and the complex accumulation of oxygen vacancies in the region where the electromigration force is near the threshold level.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Atomic oxygen, which can be liberated as an intermediate product in the decomposition of N2, is shown to be effective in removing N previously incorporated in SiO2 layers grown in N2O. This removal results in a N distribution that is sharply peaked at the Si–SiO2 interface for oxides grown in N2O by rapid thermal oxidation, but in a flat N distribution for N2O oxides grown in a furnace where the concentration of atomic oxygen is generally not substantial at the wafer position. This effect provides a means of tuning N profiles in a manner that may be useful for optimizing oxide quality. © 1995 American Institute of Physics.

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