The excellent electrical and thermal transport properties of copper (Cu) make it an attractive material for electronic applications. However, its poor intrinsic mechanical properties and oxidation resistance are strongly limiting the field of application. Alloying Cu with chromium (Cr) is an effective way of increasing the oxidation resistance of such alloys. However, under equilibrium conditions the solubility limit of Cr in Cu is far below 1 at.%. Supersaturated Cu-Cr alloys can be synthesized by non-equilibrium thin film deposition techniques. The high cooling rates bring another positive effect of grain refinement that leads to a strong increase of the mechanical properties. In a thin film with nominal composition of Cu₆₇Cr₃₃ (at.%) nanoscale phase separation is characterized by energy dispersive X-ray spectroscopy (EDS) in an aberration-corrected scanning transmission electron microscope (STEM). The electronic structure of Cu in body centered cubic (bcc) crystal structure is analyzed by localized electron energy loss spectroscopy (EELS) and experimental spectra are contrasted to spectra calculated by density functional theory (DFT).

Co-evaporation using molecular beam epitaxy (MBE) obtains metastable Cu-Cr alloy thin films with nominal thickness of 300 nm and composition of Cu₆₇Cr₃₃ (at.%). Selected area electron diffraction confirms the bcc crystal structure of the thin films with columnar grains of ~50 nm in diameter. Aberration-corrected STEM in combination with EDS establishes compositional fluctuations within the grains on the order of 1 – 5 nm. The domains adopt the bcc crystal structure shown in the HAADF(High Angle Annular Dark-Field)-STEM image of Fig. 1. The chemical phase separation in Cu- and Cr-rich domains with composition of Cu₈₅Cr₁₅ (at.%) and Cu₄₂Cr₅₈ (at.%) is illustrated in Fig. 2. The alignment of the interface between the Cu- and Cr-rich domains shows a preference for {110}-type habit plane. The electronic structure of the Cu-Cr thin films is investigated by EELS and is contrasted to an fcc-Cu reference sample given in Fig. 3. The main differences between bcc- and fcc-Cu are related to differences in van Hove singularities in the electron density of states. In Cu-Cr solid solutions with bcc crystal structure a single peak after the L₃-edge, corresponding to a van Hove singularity at the N-point of the first Brillouin zone is observed. Spectra computed for pure bcc-Cu and random Cu-Cr solid solutions with 10 at.% Cr confirm the experimental observations. Changes in electronic structure of supersaturated solid solutions of Cu by alloying with Cr are discussed in detail.

Figures:

Fig. 1: HAADF-STEM image of Cu- and Cr-rich domains within a grain. The inset shows the diffractogram of the <111> zone axis.

Fig. 2: STEM-EDS elemental map of the region shown in Fig. 1. The chemical phase separation into Cu- and Cr-rich regions (red and blue) is confirmed.

Fig. 3: a) HAADF-STEM image indicating regions of STEM-EELS measurement. b) Corresponding Cu-L23 edge of Cu- and Cr-rich regions in comparison to an fcc-Cu reference sample. Cu in bcc crystal structure shows one peak D after the L3-edge (A), in fcc crystal structure two peaks B and C are observed.

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EMC Abstracts - https://emc-proceedings.com/abstract/nanoscale-phase-separation-and-electronic-structure-of-metastable-bcc-cu-cr-alloy-thin-films/