G.J. Van Der Kolk

On the composition range of amorphous binary transition metal alloys

Abstract A comparison is made between various models which predict the ability to form an amorphous phase with a relatively high crystallization temperature by rapid quenching or vapour deposition. These models are compared with recent experimental information. It is demonstrated that the effect of atomic size alone is not sufficient to describe the range of formation of amorphous phases. The crystal structure of the constituent elements must also be taken into account. The structural stability is correlated with the average number of valence electrons per atom Z . Easy amorphous phase formation is expected for those values of Z where structural stability is at a minimum. The comparison with experimental results suggests that the relative weight of the structural stability is different for 3d, 4d and 5d transition metal series.

BINDING OF HELIUM TO METALLIC IMPURITIES IN TUNGSTEN - EXPERIMENTS AND COMPUTER-SIMULATIONS

A W(100) single crystal was implanted with low doses Ag, Cu, Mn, Cr, Al or In. Subsequent heating to 1600 K removed all vacancies and left the implants in substitutional positions. Low energy He was injected, and binding of He to the substitutional impurities was observed. Binding energies were found as high as 1.25 eV for one He atom. Pair potential calculations were performed; the calculated binding energies correspond reasonably with the measured ones.

Microstructural studies of the growth of aluminum films with water contamination

Al films were grown at three different water partial pressures. The structure of the films at various thicknesses was observed with transmission electron microscopy. It was found that all three cases grew initially by island formation. (i) At greater thicknesses the film grown at the lowest pressure has large grains and a uniform thickness. (ii) The medium contaminated film has relatively clean regions with large grains and heavily contaminated regions with small grains. Furthermore, the film is not uniform in thickness; the clean regions are much thicker than the contaminated ones. Contamination of Al nucleates at discontinuous regions. Starting at the boundary Al substrate vacuum the contamination extends up to the Al surface. The contaminant layer prevents coalescence and recrystallization of Al, explaining the small grain size in certain areas. (iii) The heavily contaminated film has small grains and a uniform thickness. It is observed that at relatively small thicknesses (50 nm) the Al surface is alr...

Buffer layers for superconducting YBaCuO thin films on Si and SiO2.

By direct deposition onto hot substrates, using laser ablation, crystalline YBa2Cu3O7?? (123) was obtained at 650°C on SiO2, but not on (100) Si substrates. The 123 film did not show a superconducting transition due to interfacial reactions. The failure temperature of insulating buffer layers, such as tantalum oxide and hafnium oxide, is around 500 °C. Although MgO and BaZrO3 show a high stability in contact with 123 at 900 °C, they fail as a diffusion barrier at much lower temperatures. Below 400 °C barium diffuses through MgO, which itself remains unaffected. Using BaZrO3 the same happens around 700 °C. BaF2 fails as a diffusion barrier below 400 °C. Using laser ablation, high quality 123 films were grown on ZrO2 buffer layers above 650 °C. For the first time we report superconducting transitions of 123 deposited at 650 °C onto an amorphous metal alloy, Ir45Ta55. The problems encountered using conducting buffer layers are either a low reaction temperature with 123 (HfB2 and HfN) or oxidation of the metal alloy (Ir45Ta55) around 400 °C. Intermediate noble metal layers silver and Ag/Au/Ag could not prevent oxygen diffusion towards the underlying buffer layer.

Thermodynamics of the stability of amorphous alloys of two transition metals

A scheme, recently proposed by Miedema and Niessen (MN), by which the concentration range is predicted in which amorphous binary transition metal alloys have high crystallization temperatures, has been tested for several alloy systems. The predictions are based on calculated values of the enthalpies of the amorphous phase and the f.c.c., h.c.p. and b.c.c. solid solutions. Thin films of W-Os, W-Ir, Ta-Re, Ta-Os, Ta-Ir, V-Pd, Nb-Pd and Ta-Pd were deposited at room temperature. The crystallization behaviour of films that were amorphous as deposited was investigated. For the 5d-5d systems, crystallization temperatures from 750 °C to 950 °C were observed. Using transmission electron microscopy and X-ray diffraction, some complex metastable phases were found as crystallization products. In agreement with the MN scheme plots of the crystallization temperature vs. average valence z generally exhibit a maximum around -z = 6.7. However, the range of alloy compositions over which amorphous alloys are formed disagrees, in most cases, with the predicted one. We propose improvements for the calculation of the enthalpy curves.

Range of slow positrons in metal overlayers on Al

Polycrystalline Pd and amorphous PdTa films on Al substrates were studied by a variable energy positron beam and by Rutherford backscattering. Since positron diffusion in the overlayers is limited, the range follows directly from the Doppler broadening as a function of incident positron energy. To observe possible effects of positron backscattering, a sandwich of Al/Pd/Al was studied as well. It was found that the mean penetration depth is not described well by z(E)=A(μg/cm2)×En(E), if A and n are assumed to be material and energy independent.

A study of Pd–Ta on Si(100) using Auger electron spectroscopy, Rutherford backscattering spectrometry, and variable energy positron annihilation

The applicability of PdxTa1−x as a diffusion barrier on Si has been investigated. For this purpose PdxTa1−x films of 200‐nm thickness (x ranges from 0 to 1) were deposited on Si(100), and the reaction between overlayer and substrate was studied as a function of temperature. Interaction was found to occur at temperatures increasing with the Ta content. The as‐deposited PdxTa1−x films with 0.2≤x≤0.6 were found to be amorphous. The amorphous phase had a higher reaction temperature than the crystalline one, causing a discontinuous step in the reaction temperature. Rutherford backscattering spectrometry spectra revealed that for the Pd‐rich compositions, first a stoichiometric Pd2Si layer formed underneath a pure Ta layer. At higher temperatures TaSi2 formed at the surface. For Ta‐rich compositions Pd2Si formed first as well; however, the reaction temperature was so high that Pd2Si grains formed in a Si matrix. The defect density of the Ta layer, which remained after outdiffusion of Pd, was investigated using ...

Thermal stability of amorphous phases in vapour-deposited binary alloy thin films of palladium with vanadium, niobium and tantalum

Binary alloys of the systems V-Pd, Nb-Pd and Ta-Pd were vapour deposited and investigated by transmission electron microscopy. The atomic fraction,x, was varied in steps of 0.1 from one pure element to the other. The range over which an amorphous phase is observed is found to increase in width going from 3d to 5d alloying element in palladium: the compositions where amorphous phases are found are V1−xPdx (x = 0.5), Nb1−xPdx (x = 0.4 to 0.6) and Ta1−xPdx (x = 0.2 to 0.6). The composition range over which a crystalline phase is found correlates well with the single-phase solid solution region close to the melting temperature in the phase diagram. Crystallization of the V0.5Pd0.5 alloy takes place at 550 K. The amorphous Nb1−xPdx (x = 0.4, 0.5) films crystallize at 850 K, whereas the amorphous Ta-Pd films crystallize between 850 and 1050K, depending on the composition. For Nb1−xPdx (x = 0.4 to 0.5) and Ta1−txPdx (x = 0.2 to 0.6) primary crystallization takes place into an f c c phase. The second crystallization step leads to a phase with a complex structure. The result is a two-phase system. V0.5Pd0.5 and Nb0.4Pd0.6 crystallize polymorphically to an f c c solid solution. The crystallization temperatures for the compositions which display primary crystallization are higher than for the compositions which crystallize by a polymorphic reaction.

Desorption of argon from amorphous Cu60Zr40 films during crystallization

Abstract When films are grown by sputter deposition, usually a fraction of the sputter gas is built into the films. In this study Cu60Zr40 was deposited by argon sputtering. As-deposited Cu60Zr40 films are amorphous. The amorphous films are heated in situ, resulting in the thermally activated release of argon atoms. The argon desorption rate is monitored during heating. For films with a thickness of less than 20 nm the argon desorption rate is at a maximum at the crystallization temperature. For thicker films a second desorption peak is observed at the melting temperature. The results indicate that argon atoms located closer than 20 nm to the surface can escape at the crystallization temperature. Argon atoms buried at larger depths can only desorb at temperatures very close to the melting temperature. The observed crystallization temperature was 740 K for a heating rate of 2 K s−1.

Crystallization of sputtered Cu60Zr40 studied by thermal desorption

Abstract A novel method is used for studying the crystallization of amorphous Cu-Zr film. When film are grown by sputter deposition they usually contain a friction of the sputter gas. In this investigation a Cu60Zr40 target was sputtered in an Ar plasma and an amorphous Cu-Zr film was grown containing 0.13 at. % of Ar. The as-deposited film was heated in situ linearly with time resulting in the release of trapped Ar atoms. The Ar desorption rate is at maximum at the crystallization temperature (740 K at a heating rate of 2 K s−1. A very narrow (20K) desorption peak is observed, which is comparable in width with the crystallization peak observed in differential scanning calorimetry experiments. By varying the film thickness it was found that only those Ar atoms located closer than about 15 nm to the film surface were released at the crystallization temperature. Ar atoms located at greater depths could only desorb when the film temperature was close to the melting temperature of the film material. By variation of the heating rate during the desorption step an effective activation energy was derived of 1.7 ± 0.2 eV. The large discrepancy between the value found and those reported in the literature is discussed. The substrate temperature during deposition was varied and at temperature higher than 450 the trapped fraction of Ar was drastically reduced.