Scanning tunneling microscopy investigation of III-V compound semiconductors and novel 2D nanolattices

The research activity described in this thesis is mainly devoted to study the fundamental properties of alternate channel materials to bulk silicon. Indeed, there is consensus, in the scientific and industrial community, that silicon is approaching its ultimate scaling limit. Today, increasing the performance of integrated circuits by scaling silicon metal-oxide-semiconductor field effect transistor (MOSFET) is becoming more and more difficult and, therefore, a systematic survey of possible alternate solutions should be considered. This purpose is taken into account by exploring two paradigmatic candidates, namely, a promising III-V semiconductor, In0.53Ga0.47As(001), and a novel two-dimensional (2D) material, silicene, the silicon counterpart of graphene, which does not exist in nature. The former represents a mid-term option fully compatible with silicon-based processing, in a More Moore strategy, while the latter represents a completely different approach that can be envisioned for ultimate device downscaling either in a More Moore perspective or even further for More than Moore prospects. Such a systematic survey has to be carried out with probes which might consider the atomic properties, since the progressive shrinking of devices dimensions is pointing towards atomic-scale. In this framework, the choice of Scanning Tunneling Microscope (STM) represents a suitable probe which can address either the morphological properties or the electronic ones through Scanning Tunneling Spectroscopy (STS), which is the most powerful STM-related capability, since it allows for atomic-scale characterization of the local density of states (LDOS) of the sample. The local electronic properties of In0.53Ga0.47As are investigated at the interface with Al2O3. Two pristine surfaces, the As-rich (2x4) reconstruction and the group-III-rich (In/Ga) (4x2) reconstruction are scrutinized by STM. STS shows that for both n-type doped reconstructions the Fermi level is initially pinned near the valence band. However, upon in situ growth of the Al2O3 thin film by Molecular Beam Epitaxy (MBE), partial unpinning occurs, while post-deposition annealing restores the original pinned condition in a different extent depending on the surface reconstruction. This behavior is rationalized in terms of an interface dipole induced by positive charges in the as-grown oxide, which are suppressed upon annealing. Hence, this comparison shows that the (2x4) reconstruction is more favorable for application-oriented perspectives of In0.53Ga0.47As as an active channel in MOSFET devices. Despite the high expectations, graphene, the nowadays best-known 2D material, has severe limitations for logic electronics due to its gapless nature. These limitations could be possibly overcome by silicene (and germanene). Here, a thoroughly study about the formation by MBE of various 2D phases of silicon on the Ag(111) surface depending on coverage and substrate temperature is reported. The resulting scenario is depicted by several silicene phases showing different periodicities and orientations with respect to the silver substrate. These structural phases stem from the intrinsic flexibility of silicene originated by its buckled structure, in contrast to graphene. The periodically modulated LDOS of silicene superstructures evidenced by STS spectra could be ascribed to the symmetry breaking in the triangular sublattices originated by the presence of buckled honeycomb lattice. The instability of silicene upon air exposure leads to the necessary encapsulation which is provided by an aluminum-based capping layer. This configuration allows the transfer of the samples for ex situ Raman spectroscopy measurements. Both experimental and theoretical Raman spectra show the presence of E2g mode, namely G peak in graphene, which is the fingerprint of honeycomb lattice, and by other vibrational modes activated by the intrinsic disorder related to the buckling. Both theoretical models and experiments proved that silicene phases have different mixture of sp2-sp3 hybridization that means different bond lengths, bond angles, and buckling parameters. Moreover these phases of silicene exhibit different electronic properties, ranging from semiconducting to semi-metal character. The perspective of future high performance logics should probably experience an intermediate step at III-V compound semiconductors, but it is very likely to expect that 2D materials will remain a hot topic in future electronics.