Direct and indirect band gaps | EPFL Graph Search

In semiconductor physics, the band gap of a semiconductor can be of two basic types, a direct band gap or an indirect band gap. The minimal-energy state in the conduction band and the maximal-energy state in the valence band are each characterized by a certain crystal momentum (k-vector) in the Brillouin zone. If the k-vectors are different, the material has an "indirect gap". The band gap is called "direct" if the crystal momentum of electrons and holes is the same in both the conduction band and the valence band; an electron can directly emit a photon. In an "indirect" gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice. Examples of direct bandgap materials include amorphous silicon and some III-V materials such as InAs and GaAs. Indirect bandgap materials include crystalline silicon and Ge. Some III-V materials are indirect bandgap as well, for example AlSb. Implications for radiativ

Thermal scanning probe lithography for 2D material-based gas sensing

Berke Erbas

Recently, two-dimensional (2D) material based gas sensing, especially transition metal dichalcogenide-based sensing, has been widely investigated thanks to its room temperature sensing ability. Unlike metal oxide based sensors, 2D material-based sensing can be carried out without a heater, therefore it reduces the power consumption and makes them promising candidates for smartphones and wearable electronic devices. Besides gas sensing ability, these 2D materials are interesting and promising candidates for electronics and optoelectronics applications. For example, monolayer molybdenum disulfide (MoS2), which is a 2D semiconductor with direct bandgap, is one of the promising candidates for transistor but it has low mobility compared to silicon. To replace silicon-based devices, strain engineering of MoS2 is widely investigated. It is observed that tensile strain tunes the bandgap of MoS2, enhances carrier mobility and improves NO2 gas sensing by piezotronic effect. Different strategies have been used to strain MoS2 such as flexible substrates to stretch the exfoliated material on top of them, silicon/ dielectric substrates with surface roughness or silicon/dielectric substrates patterned with nanocone structure to locally strain transferred MoS2. Herein, field effect transistor with three-dimensionally patterned gate oxide is designed, fabricated and electrically characterized for sensing application. Thermal scanning probe lithography, which is a direct three-dimensional nanofabrication technique, is used for the fabrication of sinusoidal surfaces. By transferring the exfoliated MoS2 flakes on top of these sinusoidal gate oxide, biaxial strain is induced. Unlike counterparts, thermal scanning probe lithography-based patterning provides us design flexibility, which controls the direction of strain and the strain rate. As a result of Raman spectroscopy, the shifts in out-of-plane vibration (A1g) and in-plane vibration (E1 2g) modes are up to 0.87 cm−1 and 2.60 cm−1, respectively. According to the theoretical Raman spectra calculations of strained MoS2 [1], these shifts correspond to an average strain of 0.58% in a circular area with 1 μm diameter. The strain of 0.58% is obtained for a sinusoidal pattern having a 23 nm amplitude and a 440 nm period. Besides, as a result of photoluminescence characterization, it is shown that an average strain of 0.39% leads to a 35 meV redshift for A excitation peak position. Consequently, the strain of MoS2 on the surface patterned by thermal scanning probe lithography tunes the bandgap of MoS2 by 90.11 meV/%.

2021

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

Optoelectronic and magnetic properties induced by atomic defects in two-dimensional semiconductors

Alessio Scavuzzo

Two-dimensional (2D) materials have attracted increasing attention over the last decade owing to their remarkable mechanical, electrical and optical properties. Following the groundbreaking discovery of graphene, a plethora of other atomically-thin materials have emerged. They cover a broad spectrum of electronic character including semiconductors, metals and insulators. The functionalities of 2D materials can be further extended by stacking individual layers into vertical heterostructures. The resulting interlayer electrical coupling enables harnessing complementary properties of the constituent materials and exploring novel fundamental physics phenomena. Their versatility and intriguing optoelectronic properties make 2D materials promising alternatives and compliments to current semiconductor technologies which suffer from scale-down limitations. Among the 2D semiconductors, transition metal dichalcogenides (TMDCs) such as MoS2 and WSe2 stand out due to their direct band gap, strong spin-orbit coupling and valley degree of freedom. This class of materials could thus enable a new generation of 2D optoelectronic devices in which information is processed through the electron spin or valley pseudospin, and accessed by optical means. Another promising 2D material is hexagonal boron nitride (h-BN), which is a transparent wide band gap insulator with excellent mechanical properties. Furthermore, the absence of dangling bonds and its strong dielectric screening capability make h-BN an ideal substrate for 2D materials. In general, crystalline defects such as vacancies, adatoms, and substitutional impurities have a significant impact on the properties of atomically-thin materials. Defects in TMDCs can trap free excitons, thus creating single-photon sources, while in h-BN they can introduce localized electronic states deep within its large band gap, resulting in stable quantum emission at room temperature. Defects can also be exploited to modulate the characteristics of the host 2D material, thus introducing functionalities that are absent in the pristine form. One option is the controlled introduction of substitutional impurities in TMDCs in order to induce tunable magnetic properties. The present thesis aims at exploring the optoelectronic properties of quantum emitters in h-BN and the magnetic properties arising from substitutional V doping in WSe2. Along these lines, a further task is to identify novel approaches to control the functionalities of the two different 2D semiconductors. In the first experimental part, the electrical tuning of single photon emitters in atomically-thin h-BN is investigated by applying a vertical or horizontal electric field. The large and robust Stark shift attained with both configurations testifies the efficiency of the electrical tuning to control the emission and underscores the ability of this method to address fundamental properties of the quantum emitters. The second experimental part describes the results of circular polarization-resolved PL and magnetotransport measurements performed with the aim of probing ferromagnetic order in V-WSe2. Although the transport measurements on few-layer flakes point toward a layered antiferromagnetic phase, the PL data gained on monolayer sheets suggest the presence of small local ferromagnetic domains. Taken together, these findings provide a valuable basis for further investigations on tailoring the properties of 2D materials via the controlled introduction of defects.

EPFL2020