Charge transfer between organic molecules and epitaxial graphene on metals

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química. Fecha de lectura: 04-10-201

General atomistic approach for modeling metal-semiconductor interfaces using density functional theory and nonequilibrium Green's function

Metal-semiconductor contacts are a pillar of modern semiconductor technology. Historically, their microscopic understanding has been hampered by the inability of traditional analytical and numerical methods to fully capture the complex physics governing their operating principles. Here we introduce an atomistic approach based on density functional theory and non-equilibrium Green's function, which includes all the relevant ingredients required to model realistic metal-semiconductor interfaces and allows for a direct comparison between theory and experiments via I-V bias curves simulations. We apply this method to characterize an Ag/Si interface relevant for photovoltaic applications and study the rectifying-to-Ohmic transition as function of the semiconductor doping.We also demonstrate that the standard "Activation Energy" method for the analysis of I-V bias data might be inaccurate for non-ideal interfaces as it neglects electron tunneling, and that finite-size atomistic models have problems in describing these interfaces in the presence of doping, due to a poor representation of space-charge effects. Conversely, the present method deals effectively with both issues, thus representing a valid alternative to conventional procedures for the accurate characterization of metal-semiconductor interfaces

Schottky barrier lowering due to interface states in 2D heterophase devices

The Schottky barrier of a metal-semiconductor junction is one of the key quantities affecting the charge transport in a transistor. The Schottky barrier height depends on several factors, such as work function difference, local atomic configuration in the interface, and impurity doping. We show that also the presence of interface states at 2D metal-semiconductor junctions can give rise to a large renormalization of the effective Schottky barrier determined from the temperature dependence of the current. We investigate the charge transport in n- and p-doped monolayer MoTe$_2$ 1T'-1H junctions using ab-initio quantum transport calculations. The Schottky barriers are extracted both from the projected density of states and the transmission spectrum, and by simulating the IT-characteristic and applying the thermionic emission model. We find interface states originating from the metallic 1T' phase rather than the semiconducting 1H phase in contrast to the phenomenon of Fermi level pinning. Furthermore, we find that these interface states mediate large tunneling currents which dominates the charge transport and can lower the effective barrier to a value of only 55 meV.Comment: 6 figure

Prospective role of multicenter bonding for efficient and selective hydrogen transport

Multicenter bonding is shown to be able to dramatically reduce atomic transport barriers in solids. Theoretical analysis of H atoms in a nanoporous polymorph of ZnO (SOD-ZnO) shows intercage hopping to be aided by four-center bonds which: (i) radically reduce the sterically hindered H-transport barrier to be close to that found in Pd membranes, and (ii) induce p doping. SOD-ZnO is also shown to be thermodynamically favored under triaxial tension and selective for encapsulating weakly perturbed H atoms. Such materials have potential use in atomic transport, control, and purification