Nano scale phase separation in Au-Ge system on ultra clean Si(100) surfaces

Abstract

We report on the phase separation in Au–Ge system leading to the formation of lobe-lobe (bi-lobed) Au-Ge nanostructures under ultra high vacuum (UHV) conditions (≈3 × 10−10 mbar) on clean Si(100) surfaces. For this study, ≈2.0 nm thick Au samples were grown on the substrate surface by molecular beam epitaxy. Thermal annealing was carried out inside the UHV chamber at temperature ≈500 °C and following this, nearly square shaped AuxSi1-x nano structures of average length ≈48 nm were formed. A ≈2 nm Ge film was further deposited on the above surface while the substrate was kept at a temperature of ≈500 °C. Well ordered Au-Ge nanostructures where Au and Ge residing side by side (lobe-lobe structures) were formed. In our systematic studies, we show that, gold-silicide nanoalloy formation at the substrate (Si) surface is necessary for forming phase separated Au-Ge bilobed nanostructures. These results show that the Au–Ge bonding is unstable in nature. Electron microscopy (TEM, STEM–EDS, SEM) studies were carried out to determine the structure of Au–Ge nano systems. Rutherford backscattering spectrometry measurements show gold inter-diffusion into substrate while it is absent for Ge.