Vibrational Properties of Defective Oxides and 2D Nanolattices

The nanoelectronic evolution, which was driven for many years by the ‘‘aggressive scaling’’ of the complementary metal-oxide-semiconductor (CMOS) devices, needs new approaches in order to face the demands for smaller, more performing, and less power-consuming integrated circuits. A few years ago, high-mobility semiconductors, e.g., germanium and III–V semiconductors, started to be investigated as possible substitutes of silicon as materials for the CMOS channel. On the other hand, dielectric materials with a higher dielectric constant (j) than the native silicon dioxide, such as HfO2, were introduced into CMOS devices a couple of years ago, in order to obtain a larger oxide capacitance, improving the performance of the devices while keeping their power consumption as low as possible. To take effective advantage of the introduction of high-mobility semiconductors and high-j dielectrics in the next generations of CMOS devices, high quality interfaces are required. In the first part of this thesis, we investigate the vibrational properties of defective HfO2 by first-principles simulations, and we compare them with experimental results from inelastic electron tunneling spectroscopy (IETS). This spectroscopic technique is very powerful for the investigation of nanoscale junctions. We also model amorphous defective GeO2, likely present at the interface of Ge/HfO2 gate stacks. Different defects, including three-folded oxygen atoms and divalent germanium centers are investigated. We show how the calculated vibrational spectra of the defective oxides, correlated to IETS measurements, can be successfully used for the investigation of high-mobility/high-j gate stacks interfaces. Recently, the interest of the physics and electronic engineering community in 2D materials, such as graphene, increased exponentially. These materials, made up of one single atomic layer, can be used to exploit quantum confinement effects, resulting in unique electronic and magnetic properties. The linear electronic dispersion observed in graphene, linked to the presence of massless Dirac fermions, was recently predicted also for its silicon and germanium counterparts, the so-called silicene and germane. This is very appealing for nanoelectronic and energy applications, in which materials with an extremely high conductivity are highly demanded. Recent experiments showed that silicene grown on metallic substrates has different structural configurations and presents a characteristic puckering of the silicon atoms, which are in contrast to graphene.In the second part of this thesis, the structural and vibrational properties of silicene on Ag(111) surfaces are calculated. Their comparison with experimental measurements, such as scanning tunneling microscopy and Raman spectroscopy, allows us to investigate the structural but also the electronic properties of different silicene reconstructions on Ag(111). Finally, the possible growth of silicene on nonmetallic templates is theoretically investigated. We show that different layered chalcogenide compounds (i.e., MoX2 and GaX, X=S, Se, Te) can be used as templates for the silicene layer. The van der Waals interaction between the silicene layer and the templates is important for avoiding strong interactions (hybridization) between the silicon atoms and the substrates. The different in-plane lattice parameters of the chalcogenide compounds can be exploited to tune the electronic properties of the silicene layer, preserving in some cases its massless Dirac fermions.