Oxidation-Induced Anisotropic Subsurface Segregation in SnTe Nanowires

In this study, we present the results of structural investigations of topological crystalline insulator (TCI) SnTe nanowires (NWs) grown by two different methods: autocatalytic and gold-assisted. The growth was performed using two distinct approaches: molecular beam epitaxy (MBE) and physical vapor deposition (PVD). Cross-sectional studies of oxidized NWs reveal a pronounced, unusual, anisotropic elemental distribution and a complex oxidation behavior. Taking into account the differences in the diffusion coefficients of the constituent elements and the high concentration of intrinsic cation vacancies, a two-step mechanism has been proposed. The initial step involves the formation of a thin surface oxide layer based on the Cabrera-Mott model adapted for semiconductors. This is followed by interdiffusion, described as the Kirkendall effect, in which tin migrates outward, whereas oxygen migrates inward into the NW. The process is not entirely uniform, and throughout the formation of the amorphous oxide skin, crystalline filaments are carved out as a result of ion migration. Concurrently, filaments act as channels, facilitating ion exchange through the associated vacancy diffusion mechanism. For Au-catalyzed NWs, thin Au films in the form of a ferrule extending several tens of nanometers were observed beneath the nanodroplets along the sidewalls, effectively suppressing oxygen adsorption in these regions. Chemical analysis of oxidized NWs coated with a Ti/Au bilayer, commonly employed for electrical contacts, shows that Ti binds oxygen from the oxide. However, this does not alter the previously created chemical segregation. The results clarify and advance the understanding of interface stability and oxidation in IV-VI semiconductor nanostructures, offering guidance for device integration.