In Situ Observation of Epitaxial Li–Si-Nanostructure Formation on Si(111)

The formation kinetics of lithium silicon phases was studied in a molecular beam epitaxy environment under ultrahigh vacuum (UHV) conditions. A heated SKIM substrate was exposed to an incoming lithium flux at a gradually decreasing substrate temperature. The onset of the nucleation and the formation of Li-Si phases were simultaneously observed by in situ synchrotron X-ray diffraction (XRD), azimuthal reflection high-energy electron diffraction (ARHEED), and line-of-sight quadrupole mass spectrometry (QMS) of the desorbing Li. First, Li-induced reconstructions formed, followed by the appearance of nanostructures which grow epitaxially with respect to the Si substrate. Both crystal structures, namely Li2Si7 and Li21Si5, which are thermodynamical stable at elevated temperatures were identified. They form with an epitaxial relationship such that [111](si) parallel to [010](Li12s7,) and [11 (2) over bar](Si) parallel to [100](Li12Si7) as well as [111](Si) parallel to [111](Li21Si5) and [01 (1) over bar](Si) parallel to [01 (1) over bar](Li21Si5). Finally, three-dimensional Li structures appeared on the surface. The in situ measurements were supplemented by ex situ atomic force microscopy and scanning electron microscopy showing the real-space information on three-dimensional Li Si structures. Similarities and differences compared to a Li-ion battery structure containing a Si anode are discussed. A significant mobility of Si and Li to form nm-sized crystal structures is concluded.