Discovery of ferroelectricity (FE) in binary oxides enables the advent of FE memories and a plethora of novel CMOS compatible building blocks spanning from the logic domain to high-density storage and neuromorphic computing. In this paper we develop the first comprehensive model of vertical Ferroelectric Field Effect Transistor, V-FeFET, to identify sources of variability, understand retention problems, and point a path to improving reliability and enabling high-density storage FE memories with extended endurance.

We combine density functional theory and many body perturbation theory to investigate the electronic properties of Si(100) and Ge(100) surfaces terminated with halogen atoms (-I, -Br, -Cl, -F) and other chemical functionalizations (-H, -OH, -CH3) addressing the absolute values of their work function, electronic affinity and ionization potential. Our results point out that electronic properties of functionalized surfaces strongly depend on the chemisorbed species and much less on the surface crystal orientation. The presence of halogens at the surface always leads to an increment of the work function, ionization potential and electronic affinity with respect to fully hydrogenated surfaces. On the contrary, the presence of polar -OH and -CH3 groups at the surface leads to a reduction of the aforementioned quantities with respect to the H-terminated system. Starting from the work functions calculated for the Si and Ge passivated surfaces, we apply a simple model to estimate the properties of functionalized SiGe surfaces. The possibility of modulating the work function by changing the chemisorbed species and composition is predicted. The effects induced by different terminations on the band energy line-up profile of SiGe surfaces are then analyzed. Interestingly, our calculations predict a type-II band offset for the H-terminated systems and a type-I band offset for the other cases.

First-principles calculations of work function tuning induced by different chemical terminations on Si(100) surface are presented and discussed. We find that the presence of halogen atoms (I, Br, Cl, and F) leads to an increase of the work function if compared to the fully hydrogenated surface. This is a quite general effect and is directly linked to the chemisorbed atoms electronegativity as well as to the charge redistribution at the interface. All these results are examined with respect to previous theoretical works and experimental data obtained for the (100) as well as other Si surface orientations. Based on this analysis, we argue that the changes in the electronic properties caused by variations of the interfacial chemistry strongly depend on the chemisorbed species and much less on the surface crystal orientation.

Silicon is today the electronic material par excellence. Nevertheless the increasing demand for new, innovative and more efficient devices has driven scientists to explore new functionalities in Si-based materials. In silicon photonics the introduction of second-order nonlinearity by proper material engineering would be highly desirable. However a bulk second-order dipolar nonlinear optical susceptibility in Si is forbidden due to the bulk crystal centrosymmetry. Different approaches have been used to break this inversion symmetry: interfacing Si with different materials and/or introducing strain. In this paper we theoretically investigate second-harmonic generation, described by the second-order nonlinear susceptibility chi((2)) in Si/Ge heterostructures. The role of symmetry and strain will be carefully analyzed also through a comparison with the computed results for unstrained and strained bulk Si and SiC systems, the first system being initially centrosymmetric, the second from the start non centrosymmetric. Thus we are able to elucidate the type of strain and symmetry breaking necessary to induce, tune and enhance second-harmonic generations in different energy regions for Si-based systems.

A systematic theoretical study of the work function behavior for Si1−xGex heterostructures over the whole composition range, from Si (x = 0) to Ge (x = 1), is presented. Our results, obtained through Density Functional Theory calculations and in good agreement with experimental evidences, show that increasing the Ge content lowers the work function value. We find that in order to exactly reproduce this behaviour in relation to the work function of pure Ge and Si systems and their concentrations, a deviation from the linear Vegard's rule is necessary. However, the calculated bowing parameter is very small, thus making the simple linear interpolation a valid approximation to obtain the work function of complex SiGe alloys.

In this work we explore the microscopic mechanisms responsible for Random Telegraph Noise (RTN) current fluctuations in HfOx Resistive Random Access Memory (RRAM) devices. The statistical properties of the RTN current fluctuations are analyzed in a variety of reading conditions by exploiting the Factorial Hidden Markov Model (FHMM) to decompose the complex RTN traces in a superimposition of two-level fluctuations. We investigate the physical mechanisms that could be responsible for the RTN current fluctuations by considering two options that are the Coulomb blockade effect and the metastable-to-stable transition of defect assisting the Trap-Assisted-Tunneling (TAT) charge transport. Physics-based simulations show that both options allow reproducing the RTN current fluctuations. The electron TAT via oxygen vacancy defects, responsible for the current in High Resistive State (HRS), is significantly altered by the electric field caused by electron trapping at defects (i.e. neutral interstitial oxygen), not directly involved in charge transport. Similarly, the transition of oxygen vacancies into a stable-slow defect configuration (still unidentified in HfOx) can temporarily switch off the current, thus explaining the RTN.

Work function in Si1-xGex heterostructures with Ge content in the 6% to 49% range was studied with high energy resolution by combining Kelvin force microscopy and X-ray photoelectron emission microscopy. Although the two methods are based on distinct physical mechanisms, we show that both techniques give the same work function differences between each Si1-xGex layer, as small as 20 meV. To detect such small work function differences, we put in evidence the necessity of preparing the Si1-xGex sample surface with polishing, HF etching and Ar+ sputtering. Such surface preparation allows, in principle, to reduce the deleterious influence of surface states, coming for instance from carbon atoms or native oxide, on quantitative work function extraction. We show in this paper that even after such a sample surface preparation, a strong band bending can be present, which causes a contrast inversion on the surface of the material and yields an artificially lower surface work function with respect to theoretical values. By using density functional theory simulations, we demonstrate that such inversion is likely due to residual carbon present on the surface.

Second-harmonic generation is described by the second-order nonlinear susceptibility chi((2)) which, in the electric-dipole approximation, requires a noncentrosymmetric medium. It is very challenging and of high technological interest to search whether it is possible to find away to break inversion symmetry in centrosymmetric crystals in order to induce second-order nonlinearities. A new intriguing way to observe second-order nonlinear phenomena is strain. Here, we present a detailed analysis of the correlation between the strain and the chi((2)) in both centrosymmetric and noncentrosymmetric materials. We considered Si and SiC as test materials and we studied different types of strain (tensile/compressive), in different directions (uniaxial/biaxial) and for different light-polarization directions. We found which is the type of strain necessary in order to induce, tune, and enhance second-harmonic generation in different energy regions for centrosymmetric and noncentrosymmetric materials.

Starting from experimental findings and interface growth problems in Si/Ge superlattices, we have investigated through ab initiomethods the concurrent and competitive behavior of strain and defects in the second-harmonic generation process. Interpreting the second-harmonic intensities as a function of the different nature and percentage of defects together with the strain induced at the interface between Si and Ge, we found a way to tune and enhance the second-harmonic generation response of these systems.

Dans cette these, je me suis interesse a la description ab initio du processus de generation de seconde harmonique (SHG), qui est une propriete optique non-lineaire des materiaux, et je me suis concentre sur les systemes quantiques confines, a base de silicium. Ces dernieres annees, les etudes ab initio ont suscite un grand interet pour l'interpretation et la prevision des proprietes des materiaux. Il est indispensable d'ameliorer la connaissance des processus non-lineaires et de proposer une description de SHG, a partir des premiers principes. En raison de difficultes importantes, la description de l'optique non lineaire n'a pas encore atteint la precision des phenomenes lineaires. L'etat de l'art des calculs ab initio SHG est represente par l'inclusion des effets a plusieurs corps comme les champs locaux (LF) et l'interaction electron-trou, mais aujourd'hui, l'approche la plus utilisee est l'approximation de particules independantes (IPA), la seule en mesure d'aborder les calculs de structures complexes, tels que des surfaces et des interfaces. Alors que IPA peut etre une bonne approximation pour les systemes massifs, dans des materiaux discontinus d'autres effets peuvent etre predominants. L'objectif de ma these est de donner une analyse du processus de SHG dans des systemes complexes comme les interfaces et les systemes confines a base de silicium, d'inferer de nouvelles connaissances sur le mecanisme physique mis en jeu et son lien avec la nature du systeme. J'utilise un formalisme fonde sur la theorie de la fonctionnelle de la densite dependant du temps (TDDFT) ou les effets a plusieurs corps sont inclus par un choix approprie des noyaux de la TDDFT. Le formalisme et le code ont ete developpes au cours de mon travail, permettant l'etude de materiaux complexes. Mes recherches ont porte sur l'etude de l'interface Si (111)/CaF2 (de type B,T4). Des etudes de convergence montrent l'importance du materiau semi-conducteur par rapport a l'isolant. La reponse est caracteristique d'une region profonde au-dela de l'interface Si, alors que CaF2 converge rapidement juste apres l'interface. La reponse montre une sensibilite aux modifications electroniques, induites dans des etats bien en-dessous de l'interface, et non a la structure ionique du silicium, qui retrouve rapidement la configuration du materiau massif. Une procedure de normalisation pour comparer avec l'experience a ete proposee. Les spectres de SHG ont ete calcules en IPA, et en introduisant les interactions de champs locaux et excitoniques. De nouveaux comportements ont ete observes par rapport aux processus SHG dans GaAs ou SiC, montrant l'importance des effets de champ locaux cristallins. Alors que IPA decrit la position des pics principaux de SHG et que les effets excitoniques modifient legerement l'intensite totale, seuls les champs locaux reproduisent la forme spectrale et les intensites relatives des pics. Cela souligne combien les effets des differents acteurs dans le processus dependent de la nature des materiaux. De nouvelles methodes d'analyse de la reponse ont ete proposees: en effet, le lien direct entre la position des pics et les energies de transition est perdu dans les calculs de SHG : le signal provient d'une equation de Dyson du second ordre ou les fonctions de reponse lineaires et non-lineaire pour des frequences differentes sont melangees. En outre, la complexite du materiau m'a permis d'obtenir des informations sur une grande variete de systemes comme les multicouches et les couches de silicium confinees. Les resultats montrent un bon accord avec l'experience, confirmant la structure de l'interface proposee. Cela souligne la precision du formalisme, la possibilite d'ameliorer nos connaissances sur ces materiaux complexes. Les simulations ab-initio de SHG peuvent etre utilisees comme une technique predictive, pour soutenir et guider les experiences et les developpements technologiques. Les resultats preliminaires sur les structures Si/Ge sont presentes.

In this work we present the ab initio study of crystal local-field effects in second-harmonic generation spectroscopy for an interface material such as Si/CaF${}_{2}$. Starting from an independent particle picture, we demonstrate the fundamental importance of the polarization effects at the interface discontinuity. The estimation of the magnitude of crystal local-field effects for second-order nonlinear response in Si/CaF${}_{2}$ interface was done by a comparative study with the absorption spectroscopy in the linear response. In both cases, we observe that the microscopic fluctuations due to the inhomogeneities of the system cause a decrease of the intensities of the spectra. However, for second-harmonic generation the decrease is selective and completely inhomogeneous while for absorption it is almost rigid. We also compare our theoretical study with experimental data showing unambiguously that only when crystal local fields are included, it is possible to correctly interpret experimental results.

In this work we present the ab initio study of crystal local-field effects in second-harmonic generation spectroscopy for an interface material such as Si/CaF2. Starting from an independent particle picture, we demonstrate the fundamental importance of the polarization effects at the interface discontinuity. The estimation of the magnitude of crystal local-field effects for second-order nonlinear response in Si/CaF2 interface was done by a comparative study with the absorption spectroscopy in the linear response. In both cases, we observe that the microscopic fluctuations due to the inhomogeneities of the system cause a decrease of the intensities of the spectra. However, for second-harmonic generation the decrease is selective and completely inhomogeneous while for absorption it is almost rigid. We also compare our theoretical study with experimental data showing unambiguously that only when crystal local fields are included, it is possible to correctly interpret experimental results.