Tailoring the preferable orientation relationship and shape of alpha-FeSi(2)nanocrystals on Si(001): the impact of gold and the Si/Fe flux ratio, and the origin of alpha/Si boundaries

The growth of α-FeSi₂ nanocrystal ensembles on gold-activated and gold-free Si(001) surfaces at different Si/Fe flux ratios via molecular beam epitaxy is reported. The study reveals that the utilisation of gold as a catalyst regulates the preferable orientation relationship (OR) of the nanocrystals to silicon and their morphology at a given Si/Fe flux ratio. α-FeSi₂ free-standing crystals with continuously tuned sizes from 30 nm up to several micrometres can be grown with an α(001)//Si(001) basic OR under gold-assisted conditions and an α(111)//Si(001) OR under gold-free growth conditions on a Si(001) surface. The preferred morphology of nanocrystals with a particular OR can be altered through changes to the Si/Fe flux ratio. Herein, the microstructure and basic OR between the silicide nanocrystals and the silicon substrate, and the formation of nanocrystal facets were analysed in detail with the help of microscopic techniques and simulation methods based on the analysis of near coincidence site (NCS) distributions at silicide/silicon interfaces. On the basis of the simulations used, we managed to reveal the nature of the interfaces observed for the main types of α-FeSi₂ nanocrystals grown. Three types of interfaces typical for nanoplates with an α(001)//Si(001) basic OR, which are (i) stepped, (ii) stressed, and (iii) flat, are explained based on the tendency for the NCS density to increase at the interface. The results presented reveal the potential for the bottom-up fabrication of α-FeSi₂ nanocrystals with tuned physical properties as potentially important contact materials and as building blocks for future nanoelectronic devices.