Cu₃Si is used as a catalyst for the production of technologically highly important chlorosilanes, an intermediate compound in the production of ultrapure silicon for the semiconductor industry [1]. Copper silicides and copper germanides have also been studied as materials for applications as contacts and interconnects in Si and Ge-Si electronic devices [2]. Different structures of Cu₃Si and Cu₃(SiGe) have been reported over the years [3,4,5], however, new aspects have been revealed by this study.

Nanoobjects of various shapes were prepared by the CVD method using organometallic precursors (SiH­­₄, EtSiH₄, BuSiH₃, and their mixtures with H₂) and copper substrates at temperature of about 500 °C. For comparison and more variability in composition, bulk samples of various compositions (Cu₇₈Si₂₂, Cu₇₇Si₂₃, Cu₇₆Si₂₄, Cu₇₅Si₂₅, Cu₇₄Si₂₆) were prepared by arc melting.

Samples were screened by SEM/EDX/EBSD and powder XRD. Selected samples were studied by single-crystal XRD and TEM. TEM was performed on a Philips CM 120 (LaB₆, 120kV) equipped with a NanoMEGAS precession unit DigiStar, an Olympus SIS CCD camera Veleta (2048×2048), and an EDAX windowless EDX detector Apollo XLTW. Precession-assisted electron diffraction tomography (EDT) in microdiffraction setup was used to acquire data for structure characterization of nanoobjects.

In the Cu₃Si solid solution (Fig. 1), two variants were identified sharing the same average structure (P6₃/mmc, a=4.06Å, c =14.66Å). The structures of the two variants (diagonal (D) and off-diagonal (O)) differ in the placement of satellite reflections, which are caused by strong modulation of the honeycomb copper layers. The D and O variants are most likely stabilized by composition. The D-variant was observed in bulk sample with composition of Cu₇₇Si₂₃ and also in the nanoplatelets prepared by CVD on Cu-substrates, whereas the O-variant was present in the samples richer in silicon. Moreover, additional periodicity along c-axis was detected in Cu₃Si compared to Cu₃(SiGe) with c=7.33Å (Fig. 2). Temperature experiments are currently under progress, and will be also presented.

[1] Bernard, F.; Souha, H.; Gaffet, E. Mater. Sci. Eng., A 2000, 284, 301–306.

[2] An, Z.; Ohi, A.; Hirai, M.; Kusaka, M.; Iwami, M. Surf. Sci. 2001, 493, 182–187.

[3] Solberg, K. J. Acta Crystallogr. 1978, A34, 684–698.

[4] Wen, Y. C.; Spaepen, F. Philos. Mag. 2007, 87, 5581–5599.

[5] Palatinus, L.; Klementová, M.; Dřínek, V.; Jarošová, M.; Petříček, V., Inorg. Chem. 2011, 50, 3743–3751.

[6] The study was supported by the Czech Science Foundation under project No. 15-08842J.

Figures:

Fig. 1: (a) Cu3Si structure viewed along [100] - Cu - black, Si - light grey. (b) Strongly modulated honeycomb layer filled with copper atoms results in diagonal and off-diagonal variants of the structure.

Fig. 2: Sections through reciprocal space acquired from EDT data of (a) Cu3Si nanowire prepared by CVD, (b) Cu¬3(SiGe) nanoplatelet prepared by CVD.

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