Growth of 2D silicon structures by molecular beam epitaxy

Abstract

The continuously expanding portfolio of 2D materials represents a precious set of building blocks for engineering next generation electronic and optical devices. A recent addition to this family is represented by silicene, the 2D allotrope of silicon. This material is characterised by a structure similar to the one of graphene, showing a hexagonal arrangement of Si atoms, albeit slightly buckled out of plane. This new, synthetic material combines an ultrahigh carrier mobility with the unique opportunity to tune its band gap by applying an external electric field. For these reasons it could be exploited to realise field effect transistors with a two-dimensional channel. Silicene still remains a vastly unexplored material: to fully unleash its potential, it is crucial to acquire additional knowledge regarding its chemical and physical properties as well as optimised growth processes and substrates. Furthermore, it is of pivotal importance to develop and investigate processes that allow to integrate this 2D material in functional devices. In this thesis, the well-known growth of silicene on Ag(111) is firstly replicated. The grown layer is thoroughly analysed by means of low energy electron diffraction, X-ray photoemission spectroscopy, angle resolved UV photoemission spectroscopy and low energy electron microscopy. Additionally, a novel method for encapsulating silicene and protecting it from oxidation is developed: the 2D layer is capped, directly in-UHV, by exfoliated few-layer graphene or hBN flakes, enabling the utilisation of ex situ analysis tools, such as Raman spectroscopy and spectroscopic ellipsometry. The vibrational properties of silicene are studied, with particular focus on the polarisability properties, which strictly reflect the two-dimensional nature of the grown layers. In the second part, the growth of silicene on Au(111) is explored. By using a combination of LEED, LEEM, ARUPS and Raman analysis, as well as first principle calculations, the grown layer is fully characterised, showing that it comprises a highly biaxially strained silicene phase. This offers a powerful new platform for future investigations of the effects of strain in 2D silicon-based materials. In the final part of the thesis, CaF2 is epitaxially grown on silicene/Ag(111), to study the possibility of engineering a gate insulation layer to efficiently control the flow of carriers through the two-dimensional material. The presented data show that CaF2 grows well on silicene, forming an ordered structure and maintaining its insulating properties. At the same time, silicon atoms in the silicene layer do not change their chemical state due to the presence of CaF2. Raman spectroscopy data reveal that the buried silicene layer undergoes a structural modification, due to the presence of the insulating layer, nevertheless maintaining a 2D configuration. It is shown that the modified silicene layer possesses an enhanced optical absorption in the visible range, if compared to crystalline-diamond or amorphous silicon thin layers. The results pave the way for combining CaF2 and silicene in next-generation electronic devices.