Kirkham, C; (2014) Bi and Mn Nanostructures on the Si(001) Surface. Doctoral thesis , UCL (University College London).

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Abstract

With the increasing miniaturisation of electronics, it is becoming important to study nanoscale systems, down to the control and manipulation of individual atoms. This work focuses on several different structures, all on the technologically important Si(001) surface, including individual spin active Bi adatoms, the Bi nanoline and the Mn nanowire. Research in this area is guided by both experimental results and theoretical simulations. Here I explore the latter, via Density Functional Theory, with a particular focus on simulated STM images, demonstrating both the successes and limitations of these techniques. This work aims to both explain experimental results and suggest new experimental avenues. Adsorption of individual Bi atoms on Si(001) shows promise for quantum computing applications, due to the existence of spin active adsorption sites. However, rapid diffusion makes them unsuitable for real world applications. Selective depassivation of the H:Si(001) surface is shown to be a viable technique for trapping spin active Bi atoms, and offers the possibility of targeted Bi incorporation. Nanolines of Bi, which spontaneously form on Si(001) have been extensively studied, both experimentally and theoretically. Recent experimental STM results have shown a strong bias dependence to the appearance of the nanolines, and here I present simulations which successfully explain these results. I also present further studies into defects on the nanoline. I also studied nanowires that form when Mn is adsorbed on Si(001), which offer the possibility of magnetic nanowires. However, at present their physical structure is still unknown, despite prior efforts to address this. Here I present a thorough investigation into potential models for the Mn nanowire, encompassing prior models, their extensions and other surface or subsurface Mn arrangements. This remains an open problem, although identification of specific features in the experimental images, and deficiencies in previous models, has furthered our understanding of the problem.

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