Interface-dominated growth of a metastable novel alloy phase
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Biswarup Satpati Institute of Physics, Sachivalaya Marg, Bhubaneswar 751 005, India (Received 10 March 2005; accepted 19 April 2005)
A new D023 metastable phase of Cu3Au was found to grow at the interfaces of Au/Cu multilayers deposited by magnetron sputtering. The extent of formation of this novel alloy phase depends upon an optimal range of interfacial width primarily governed by the deposition wattage of the direct current magnetron used. Such interfacially confined growth is utilized to grow a ∼300-nm-thick Au/Cu multilayer with thickness of each layer nearly equal to the optimal interfacial width which was obtained from secondary-ion mass spectrometry (SIMS) data. This growth technique is observed to enhance the formation of the novel alloy phase to a considerable extent. The SIMS depth profile also indicates that the mass fragment corresponding to Cu3Au occupies the whole film while x-ray diffraction (XRD) shows almost all the strong peaks belonging to the D023 structure. High-resolution cross-sectional transmission electron microscopy shows the near-perfect growth of the individual layers and also the lattice image of the alloy phase in the interfacial region. Vacuum annealing of the alloy film and XRD studies indicate stabilization of the D023 phase at ∼150 °C. The role of interfacial confinement, the interplay between interfacial strain and free energy, and the hyperthermal species generated during the sputtering process are discussed.
I. INTRODUCTION
In metallic multilayer systems, the large free energies available at the interfaces can help to form interfacial alloys when these free energies are comparable to or larger than the enthalpy of formation of the probable alloy phases.1,2 In many such systems the alloy formed is of a non-bulk, metastable nature.3–5 For multilayers of immiscible metals, there exists a positive mixing enthalpy that in effect acts as an energy barrier to mixing.6,7 On the other hand, for miscible metallic systems, the formation enthalpy for different metastable crystalline or ordered phases are mostly negative and very close to each other.8 Interfacial alloy formation in multilayers have been reported to take place via solid-state reaction,9,10 mechanical alloying,11–13 ion beam irradiation,14,15 etc. Especially in case of the immiscible systems, alloying is achieved via the ion beam irradiation technique, which can supply adequate energy to overcome the energy barrier. Almost all of the metastable alloys reported for immiscible systems are amorphous in nature, the basic idea being the persistence of a random arrangement of atoms formed by forced mixing and a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0355 J. Mater. Res., Vol. 20, No. 10, Oct 2005
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subsequent rapid quenching of the supplied energy.16 The probability of obtaining ordered alloy phases increases as we go over to the miscible systems. Localdensity approximation (LDA) calculations for Au/Cu syst
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