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Elemental Semiconductors
Published in Lev I. Berger, Semiconductor Materials, 2020
Among the elements of Group V, only phosphorus in its orthorhombic and monoclinic modifications has semiconductor properties. The other pnictogens — arsenic, antimony, and bismuth — are semimetals (see, e.g., Dresselhaus3.390). Moss,3.1 in the process of measurements of electrical, optical, and photoelectrical properties of thin films, found, notwithstanding, that gray arsenic behaves as a semiconductor with the energy gap of 1.14 eV (from temperature dependence of electrical conductivity). The photoconductivity of gray arsenic is characterized by the long-wave limit between 1.19 eV and 1.23 eV in agreement with the data on the conductivity vs. temperature measurements. Moss reported also on photoconductivity in thin layers of antimony with activation energy close to 0.1 eV. It is possible that these results prove the existence of the quantum dimensional effect mentioned before.3.386
Heusler Compound: A Novel Material for Optoelectronic, Thermoelectric, and Spintronic Applications
Published in Niladri Pratap Maity, Reshmi Maity, Srimanta Baishya, High-K Gate Dielectric Materials, 2020
As semi-metals have lower charge carriers than metals, they typically have low electrical and thermal conductivities. In addition, they usually show high diamagnetic susceptibilities and lattice dielectric constants. Some of the examples of recently discovered semi-metallic compounds among the classes of half-Heusler and full-Heusler alloys are NdPtBi, YPtBi, ScPtBi, GdPtB, LiBaBi, HoPdBi, YAuPb, LuPdSb, CrTiVAl, Ru2TaAl, Ru2NbGa, Ru2CrIn, Fe2VGa, and more to explore. Ru2TAl is a type of semi-metallic Heusler alloy whose energy band and density of states are presented in Figure 9.15. First principle calculations revealed an indirect overlap between electron and hole pockets that gives a pseudogap in the neighborhood of EF (Angell et al., 1983; Tseng et al., 2017). Such a state of affairs provides a practical interpretation for the semi-metallic behavior in Ru2TaAl, as shown in Figure 9.15.
Semimetal Electronics
Published in Klaus D. Sattler, st Century Nanoscience – A Handbook, 2020
Alfonso Sanchez-Soares, Christian König, Conor O’Donnell, Jean-Pierre Colinge, James C. Greer
Semimetals are materials with an electronic structure somewhat in between that of metals and semiconductors. In metals, the electronic structure is characterized by a partially filled CB; that is, the Fermi level lies inside at least one band, thus providing electrons in the vicinity with a large number of states for electrical conduction. In contrast to metals, semiconductors and insulators have well-defined VB and CBs separated by an energy bandgap. Electrons occupying states at the top of the VB thus have no available states for conduction, unless they are promoted to the bottom of the CB, requiring relatively large energies on the order of electron volts. Therefore, semiconductors allow control of current flow in gated devices by shifting band alignments between device regions through application of electrostatic potentials with magnitudes on the order of volts. The electronic structure of semimetals closely resembles that of semiconductors in that VB and CB can be clearly identified, but exhibit a small overlap between them instead of being separated by a bandgap. Although the list of elemental semimetals is limited and includes arsenic, antimony, bismuth, and some allotropes of tin and carbon, the number of reported alloys and low-dimensional materials with SM properties is ever increasing. Some examples include mercury telluride, tin-germanium alloys, and some transition metal dichalcogenides (TMDs) such as tungsten ditelluride [10]. Figure 10.3 schematically illustrates the characteristic band structure of metals, semiconductors/insulators, and semimetals.
Thermoelectric transport in graphene and 2D layered materials
Published in Nanoscale and Microscale Thermophysical Engineering, 2019
Another advantage of 2D materials is the possibility to tune their band gap. This could be achieved by changing the number of layers, applying strain or electric field and changing the structural composition of the material (by hydrogenation or oxidation). For example, arsenic is a typical group V semimetal in its bulk form. A puckered monolayer honeycomb structures of arsenic, called arsenene, is a semiconductor with indirect band gap of 0.831 eV [17]. Varying the number of layers, one can observe a smooth transition from semi-metallic to semiconducting state. It was shown that arsenene can be a direct gap semiconductor, a metal or a semimetal with Dirac cone similar to graphene depending on the strength and direction of applied strain [17]. Experimentally, strain could be applied by growing 2D materials on substrates that are having lattice mismatch with the deposited 2D material. Applying a perpendicular electric field to a buckled honeycomb structure, another 2D form of arsenic, is shown to reduce and eventually close the band gap around 6 V/nm. In the next section of this article, we will summarize some of the latest advances in a handful classes of 2D materials.