R. Beyers

Exchange-biased magnetic tunnel junctions and application to nonvolatile magnetic random access memory (invited)

Exchange biased magnetic tunnel junction (MTJ) structures are shown to have useful properties for forming magnetic memory storage elements in a novel cross-point architecture. MTJ elements have been developed which exhibit very large magnetoresistive (MR) values exceeding 40% at room temperature, with specific resistance values ranging down to as little as ∼60 Ω(μm)2, and with MR values enhanced by moderate thermal treatments. Large MR values are observed in magnetic elements with areas as small as 0.17 (μm)2. The magnetic field dependent current–voltage characteristics of an MTJ element integrated with a silicon diode are analyzed to extract the MR properties of the MTJ element itself.

Metastable phase formation in titanium‐silicon thin films

The formation of TiSi2 thin films on silicon substrates has been investigated with several transmission electron microscope techniques. For films formed either by reacting titanium with a silicon substrate or by sintering a codeposited (Ti+Si) mixture, electron diffraction patterns show that a metastable phase—TiSi2 (C49 or ZrSi2 structure)—forms prior to the equilibrium phase—TiSi2 (C54 structure). High‐resolution images indicate that the metastable TiSi2‐silicon interface is atomically sharp, with no ‘‘glassy membrane’’ layer present. The annealing temperature required to transform the metastable TiSi2 to the low resistivity, equilibrium TiSi2 increases as the thin‐film impurity content increases. Previous studies of TiSi2 formation are discussed in light of these results.

Thermodynamic considerations in refractory metal‐silicon‐oxygen systems

Thermodynamic considerations in thin‐film reactions involving refractory metals, refractory metal silicides, silicon, and silicon dioxide are described using ternary phase diagrams. Calculated metal‐silicon‐oxygen phase diagrams for Mo, W, Ta, and Ti are used to explain the reactivity of the metal with silicon dioxide, the effectiveness of native oxide in preventing metal‐silicon interdiffusion, and the formation of silicon dioxide in preference to metal oxides during silicide oxidation. Distinctions are drawn between experimental results which can be explained solely on thermodynamic grounds and those requiring consideration of both thermodynamic and kinetic factors.

Modeling of agglomeration in polycrystalline thin films: Application to TiSi2 on a silicon substrate

An equilibrium model for agglomeration in polycrystalline thin films which considers the energy balance between the grain boundary energy and both surface and substrate interface energies is presented. It predicts that small grain size, low grain boundary energy, and high film surface and interface energies should promote resistance to agglomeration, and shows that the substrate‐film interface can play a significant role in the process. It also predicts a critical grain size limiting formation of a discontinuous island structure. This easily calculable value is significantly smaller than that found in previous modeling. The critical grain size, the importance of the substrate interface, and some of the assumptions are shown to be consistent with transmission microscope observations of TiSi2 thin films deposited on Si substrates.

Phase equilibria in thin‐film metallizations

The determination of stable tie lines in a ternary phase diagram through a limited number of thin‐film reactions is demonstrated. Ternary phase diagrams are then used to explain the stability of refractory metals, silicides, and nitrides during various integrated circuit processing steps. The W–Si–O, Ti–Si–O, Ti–Si–N, and Ti–Al–N systems serve as examples.

Crystallography and microstructure of Y1Ba2Cu3O9−x, a perovskite‐based superconducting oxide

We have investigated the crystallography and microstructure of Y1Ba2Cu3O9−x with transmission electron microscopy and x‐ray diffraction. Y1Ba2Cu3O9−x is a distorted, oxygen‐defect perovskite with ordering of the yttrium and barium ions. Its unit cell is orthorhombic with space group Pmm2 and lattice parameters a=3.893 A, b=11.688 A, and c=3.820 A. The structure is heavily twinned on {101} type planes, possibly due to a tetragonal‐to‐orthorhombic transition above room temperature.

Carbon nanotubes with single-layer walls

Macroscopic quantities of single-layer carbon nanotubes have recently been synthesized by co-condensing atomic carbon and iron group or lanthanide metal vapors in an inert gas atmosphere. The nanotubes consist solely of carbon, sp 2 -bonded as in graphene strips rolled to form closed cylinders. The structure of the nanotubes has been studied using high-resolution transmission electron microscopy. Iron group catalysts, such as Co, Fe, and Ni, produce single-layer nanotubes with diameters typically between 1 and 2 nm and lengths on the order of micrometers. Groups of shorter nanotubes with similar diameters can grow radially from the surfaces of lanthanide carbide nanoparticles that condense from the gas phase. If the elements S, Bi, or Pb (which by themselves do not catalyze nanotube production) are used together with Co, the yield of nanotubes is greatly increased and tubules with diameters as large as 6 nm are produced. Single-layer nanotubes are anticipated to have novel mechanical and electrical properties, including very high tensile strength and one-dimensional conductivity. Theoretical calculations indicate that the properties of single-layer tubes will depend sensitively on their detailed structure. Other novel structures, including metallic crystallites encapsulated in graphitic polyhedra, are produced under the conditions that lead to nanotube growth.

Phase equilibria in metal-gallium-arsenic systems: thermodynamic considerations for metallization materials

We propose a classification scheme for phase equilibria in elemental metal‐gallium‐arsenic systems. Using available data we assign as many metals as possible to seven generic types of ternary phase diagrams. We describe how the phase diagrams can provide a framework for interpreting previous studies of metal reactions with GaAs substrates and for identifying stable materials for GaAs metallizations.

Titanium disilicide formation on heavily doped silicon substrates

Titanium disilicide formation on heavily doped silicon substrates was investigated with sheet resistance measurements, elemental depth profiling, and transmission electron microscopy. As found in a previous study [H.K. Park, J. Sachitano, M. McPherson, T. Yamaguchi, and G. Lehman, J. Vac. Sci. Technol. A 2, 264 (1984)], the TiSi2 growth rate depended on the dopant concentration. The growth rate was highest on undoped substrates, intermediate on heavily phosphorus‐doped substrates, and lowest on heavily arsenic‐doped substrates. However, the critical dopant concentration effect reported by Park et al. was not observed. The uniformity of the titanium‐silicon reaction was not seriously affected by heavy substrate doping. For heavily arsenic‐doped substrates (3.0×1021 As/cm3), TiAs precipitates formed at C49 TiSi2 grain boundaries, and the C49‐to‐C54 transformation temperature increased to 850 °C. For heavily phosphorus‐doped substrates (1.0×1021 P/cm3), no phosphides were unambiguously detected, and the C49‐...

Quaternary phase relations near YBa2Cu3O6+x at 850°C in reduced oxygen pressures

Abstract The quatenary phase relations near YBa 2 Cu 3 O 6+ x at 850°C were determined by oxygen coulometric titration and microanalysis. The phase relations change markedly as the oxygen pressure decreases, due primarily to the formation of a liquid phase and a reduced oxide, BaCu 2 O 2 . Nine invariant reactions were discovered, including decomposition of YBa 2 Cu 3 O 6+ x into Y 2 BaCuO 5 , BaCu 2 O 2 and YBa 3 Cu 2 O 6+ y . The lower limit of oxygen partial pressure at which pure YBa 2 Cu 3 O 6+ x is stable is 4.0×10 −4 atm O 2 at 850°C; the lower limit increases in the presence of BaCuO 2 and Cu 2 O impurities. These results indicate that the temperatures used for single crystal growth of YBa 2 Cu 3 O 6+ x could be lowered by using reduced oxygen pressures or, alternatively, that constant temperature crystal growth could be induced by increasing the oxygen partial pressure.