Valence and conduction bands | EPFL Graph Search

In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature, while the conduction band is the lowest range of vacant electronic states. On a graph of the electronic band structure of a semiconducting material, the valence band is located below the Fermi level, while the conduction band is located above it. The distinction between the valence and conduction bands is meaningless in metals, because conduction occurs in one or more partially filled bands that take on the properties of both the valence and conduction bands. Band gap Band gap In semiconductors and insulators the two bands are separated by a band gap, while in conductors the bands overlap. A band gap is an energy range in a solid where no electron states can exist due

Ab initio study of interface states at metal contacts to III-IV semiconductors

Thomas Maxisch

We present a theoretical study of the physical characteristics of metal/semiconductor junctions. Using first principle pseudopotential calculations, we have investigated the nature of electronic states with energies within the semiconductor band gap of representative abrupt, defect-free, anion-terminated metal/III-V interfaces. Namely, we focused on Al contacts to GaAs(001), AlAs(001) and cubic GaN(001) as well as on Al, Au and Cu junctions to cubic GaN(001). Recent advances in Schottky barrier concepts emphasize the possible relationship between interface states and the formation of the Schottky barrier. We aim at understanding the atomic-scale mechanisms responsible for interface states as well as their role in the Schottky barrier formation process. At As-terminated Al/GaAs(001) and Al/AlAs(001) junctions, resonant and localized interface states occur at the J point of the interface 2D Brillouin zone near the Fermi energy in the semiconductor midgap region. They correspond to intermetallic bonds between the outermost cation atoms of the semiconductor and the interfacial Al atoms of the metal. These interface states derive from an interaction between localized states of the Al(001) surface and semiconductor conduction band states, mediated by localized states of the unreconstructed, As-terminated semiconductor (001) surface. Our results indicate that interface states of the intermetallic, bonding-like kind could play an important role in the transport properties of metal/AlxGa1-xAs junctions. We have also investigated the electronic structure of Al, Au and Cu junctions to cubic, N-terminated GaN(001). The localized interface state reported for As-terminated Al/GaAs(001) and Al/AlAs(001) junctions occurs also at metal/GaN interfaces under the condition that atoms on the outermost atomic plane of the metal are placed in front of the outermost semiconductor cation. This indicates that the formation mechanism of this state is a very general one. In contrast to Al/AlxGa1-xAs junctions, these states occur at energy much larger than EF for the contacts to GaN. Thus, they are not expected to contribute significantly to the electronic transport of the latter interfaces. However, a large number of interface states attributed to d-type orbitals occur over a wide energy range including EF at contacts of noble metals to GaN.

EPFL2003

From hidden order to antiferromagnetism: Electronic structure changes in Fe-doped URu2Si2

Ji Dai, Emmanouil Frantzeskakis, Kevin Huang

In matter, any spontaneous symmetry breaking induces a phase transition characterized by an order parameter, such as the magnetization vector in ferromagnets, or a macroscopic many electron wave function in superconductors. Phase transitions with unknown order parameter are rare but extremely appealing, as they may lead to novel physics. An emblematic and still unsolved example is the transition of the heavy fermion compound URu2Si2 (URS) into the so-called hidden-order (HO) phase when the temperature drops below T-0 = 17.5 K. Here, we show that the interaction between the heavy fermion and the conduction band states near the Fermi level has a key role in the emergence of the HO phase. Using angle-resolved photoemission spectroscopy, we find that while the Fermi surfaces of the HO and of a neighboring antiferromagnetic (AFM) phase of well-defined order parameter have the same topography, they differ in the size of some, but not all, of their electron pockets. Such a nonrigid change of the electronic structure indicates that a change in the interaction strength between states near the Fermi level is a crucial ingredient for the HO to AFM phase transition.

NATL ACAD SCIENCES2021

Charge-carrier Dynamics in Metal Oxides and Hybrid Lead Perovskites investigated by Time-Resolved UV and X-ray Spectroscopy

Thomas Charles Henry Rossi

In modern ultrafast optoelectronic technologies based on wide band gap insulators, the non-equilibrium dynamics of photogenerated charges plays a major role. Unravelling the mechanisms of interaction between these charge carriers and their environment is crucial to support the design of more efficient devices. Especially, the absorption of photons with more energy than the optical band gap generates electrons-hole pairs (EHP) whose transport and dissociation as excitons or free charge carriers is of pivotal importance for the charge separa- tion at photovoltaic interfaces. The advent of ultrafast pump-probe optical spectroscopy in the deep-UV provides access to the dynamics of EHP through a variety of transient modifications of the optical spectrum. In this Thesis, the cooling of electrons in ZnO and methylammonium lead bromide perovskite (MAPbBr3), two broadly used direct band gap semiconductors, is tracked as they end up and accumulate at the bottom of the conduction band. ZnO has a fast cooling time of the order of 100 fs which efficiently converts photon excess energy into lattice heat while MAPbBr3 has a rather slow electron cooling time of the order of 400 fs, favorable for charge separation at interfaces as it drives the injection of hot carriers. Additionally, the high energy carried by the UV probe photons provides access to the photodynamics at different high symmetry points in the Brillouin zone. In a second step, these two semiconductors have been associated to other materials to form prototypical photovoltaic assemblies such as dye-sensitized (ZnO/N719) and solid-state lead perovskite sensitized (TiO2/MAPbBr3) interfaces. Upon electron injection into the transition metal oxide (ZnO or TiO2), dramatic changes are observed in the absorption close to the optical band gap from which the timescale of electron injection is obtained. It highlights how slight screening length and chemical potential changes can generate large modifications in the optical properties through many-body effects. At both interfaces, a delayed appearance of the electron at the bottom of the conduction band is observed which is due the formation of a bound state. This Thesis extends beyond the band insulators to the photodynamics of NiO, a strongly correlated charge-transfer insulator. Excitation above the optical band gap generates a large photoinduced absorption below the optical band gap which is characteristic of the interplay between electronic and low energy bosonic excitations. The dressing of electronic excitations with bosonic modes generates low energy excitation modes on ultrafast timescales which generate a competition between itinerant and localized resonances with a high degree of tunability. The progress made in resonant X-ray based time-resolved spectroscopies allows the investigation of the degree of charge localization around specific atoms in a system. Photogenerated electron localization in NiO is studied by time-resolved X-ray absorption spectroscopy, causing bond elongations and the formation of an electron-polaron in less than 100 ps. In a last part, the linear dichroism of the steady-state X-ray absorption spectrum of anatase TiO2 at the K-edge is studied which provides an unambiguous assignment of the pre-edge transitions as their orbital composition is strongly anisotropic and sensitive to the crystal orientation.

EPFL2018