Morphological variations in Au_xSi_y nanostructures under variable pressure and annealing conditions

Abstract

Well-ordered, substrate symmetry-driven, Au_xSi_y structures of average size ~25 nm were formed under ultra-high vacuum (UHV) conditions using molecular beam epitaxy method. Post-annealing was done at 500 °C in three different vacuum conditions: (1) low vacuum (LV) (10⁻² mbar), (2) high vacuum (HV) (10⁻⁵ mbar) and (3) UHV (10⁻¹⁰ mbar) (MBE chamber). For both HV and LV cases, the Au_xSi_y nanostructures were found to have their corners rounded unlike in UHV case where the structures have sharp edges. In all the above three cases, samples were exposed to air before annealing. In situ annealing inside UHV chamber without exposing to air resulted in well-aligned rectangles with sharp corners, while sharp but irregular island structures were found for air exposed and UHV annealing system. The role of residual gases present in LV and HV annealing environment and inhibition of lateral surface diffusion due to the presence of surface oxide (through air exposure) would be discussed. Annealing at various conditions yielded variation in the coverage and correspondingly, the average area of nanostructures varied from a ~329 nm² (as deposited) to ~2,578 nm² (at high temperature). High-resolution transmission electron microscopy (planar and cross section) has been utilized to study the morphological variations.