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Browsing by Author "Cha, Limei"

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    A metastable HCP intermetallic phase in Cu-Al bilayer films
    (2006) Cha, Limei; Rühle, Manfred (Prof. Dr. Dr. h.c.)
    For the present study, three kinds of layered Cu/Al films have been fabricated. The first kind of samples were multilayered Cu/Al films deposited by sputtering on (001) Si. The individual layer thicknesses were 100 nm, 200 nm and 400 nm, while the total film thickness of 800 nm was kept constant, thus leading to multilayer systems with 8, 4 and 2 layers, respectively. The second type of samples were Cu/Al bilayer films grown on (0001) sapphire by sputtering, with individual layer thicknesses of 400 nm. The third type of samples were bilayer films (100 nm Cu and 100 nm Al) deposited on (0001) sapphire by MBE at room temperature. Applying conventional transmission electron microscopy (CTEM) and X-ray diffraction (XRD) ( -2 scans and pole figure measurements), different epitaxial growth behaviors were found in these films. All multilayer films from the first type were polycrystalline. The second type of films show a (111) FCC texture and possess intermetallic phases at the interfaces. The orientation relationship (OR) between the bilayer film and the substrate was: <110> (111)Cu // <110> (111)Al // <2-1-10> (0001)sapphire. The films of the third kind are mainly epitaxial, with the following OR between films and the substrate: [-110] (111)Cu //[-110] (111)Al // [10-10] (0001)sapphire. The films from the third group show the best epitaxial behavior and thus the major work in this thesis was performed on these samples. It was found that an interlayer with a thickness of 8 nm is located between the pure Cu and the pure Al film as observed by CTEM, using a JEOL JEM-2000FX. HRTEM investigations performed with a JEM ARM-1250 operated at 1250kV displayed that along [111]FCC, the atomic structure of the interlayer has an ABAB stacking sequence, which is identical with a hexagonal close-packed (HCP) structure in [0001] direction, but not with the ABCABC stacking sequence of Cu and Al in [111]FCC. The changing intensities of atomic columns suggested that different layers are occupied by different atoms. To understand the structure of the interlayer in detail, a HCP atomic model was constructed and used for image simulations which were performed with the soft-package named electron microscopy simulation (EMS). The lattice parameters of the HCP structure at the interlayer were determined from a model which gave the best agreement between the experimental and simulated images. The parameters are: a = b = 0.256 nm, c = 0.419 nm, ? = 120°, with the space group of P-6m2. For the quantitative HRTEM the program of iterative digital image matching (IDIM) was used. The agreements between the simulated images and the experimental images are 93.1 % and 93.4 % for <110>FCC and <112>FCC directions, respectively. The orientation relationship between the two FCC metal layers and the HCP interlayer is: <110> (111)FCC // <11-20> (0001)HCP. Furthermore, lattice distortion analysis (LADIA) revealed that the lattice parameters of the HCP phase are increasing from the near-Cu-side to the near-Al-side, although the symmetry of the HCP structure remains. The chemical composition of the interlayer was investigated by energy dispersive X-ray spectroscopy (EDS) in a VG HB501UX dedicated scanning transmission electron microscope (STEM). EDS linescans were performed from pure Al to pure Cu layers. To obtain the absolute chemical composition of the interlayer, quantitative X-ray microanalysis was necessary. Thus, a standard CuAl2 alloy sample was employed to obtain the experimental k-factor of the Cu-Al system. Using the determined k-factor, the results indicated that there is 27 ~ 58 at % of Al in the interlayer and that the Al concentration is gradually increasing from the near-Cu-side to the near-Al-side. This result is in agreement with Hume-Rothery laws for alloy phases. However, beam broadening, which is influenced by the thickness of the specimen and the nature of the investigated material, leads to a smeared diffusion-like concentration profile. Nevertheless, the EDS results together with image simulations, performed using different chemical concentrations of the HCP phase, revealed a compositional gradient across the interlayer. The formation mechanism of the intermetallic phase, which locates at the Cu/Al interface and possesses a broad composition range and a HCP structure, can be explained by Hume-Rothery laws and Shockley partial dislocations, cooperating with diffusions. In order to examine the stability of this HCP phase, in-situ heating experiments were performed in the HRTEM at ~ 600°C. After cooling the sample to RT, the HCP phase disappears and three equilibrium intermetallic phases i.e. CuAl, CuAl2, and Cu9Al4 form. This suggests that the HCP intermetallic phase is metastable. Ex-situ heating experiments were performed at different temperatures to obtain the temperature range in which the HCP metastable phase will be stable. According to the XRD measurements, it is found that the metastable HCP phase exists below 120°C.
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