Device Applications
The chapter deals with experimental aspects of topological insulators and their device applications in electronics and optoelectronics.
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Device Applications
Device applications of TI materials are based on their semiconductor properties and in this capacity they can be used in typical generic semiconductor electronic devices such as Schottky diodes, photodetectors, field-effect transistors, and thermoelectric units. In addition, the essentially topological spin-momentum locked states, which are absent in normal semiconductors, lead to the use of TI materials as a template for a variety of spintronic devices such as random access memory, information storage, and microwave sources. Since the topological states are pinned to surfaces, the quality of contacts to various metals as well as the Fermi level position at the metal/TI interface affects the accessibility of the states, carrier mobility, and the quality of the Schottky barriers.
8.1
Contacts and Gating
Native defects (mostly chalcogen vacancies) in Bi2(TexSe1 x)3 create n-type doping and pin the Fermi level in the conduction band [1, 2]. The band bending near the surface forms an accumulation layer, thus creating a 2D electron gas and making it difficult to distinguish between 2D electrons and surface topological states. The density of the native defects depends on the growth method, post-growth treatment, as well as the choice of substrate and buffer layer. It was shown that proper postgrowth annealing can reduce the density of chalcogen vacancies that move the Fermi level in the bulk bandgap close to the Dirac point [3]. The carrier density in an accumulation layer can be managed by electrostatic gating with metal contacts which could be either Ohmic or Schottky type, depending on the choice of metal. The active region of most optoelectronic devices consists of single or double heterojunctions characterized by variable energy gaps, high carrier
© Springer Nature Switzerland AG 2020 V. Litvinov, Magnetism in Topological Insulators, https://doi.org/10.1007/978-3-030-12053-5_8
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8 Device Applications
mobility, and the ability to confine nonequilibrium carriers. From this perspective, the various heterojunctions that a TI could form with the normal semiconductors are under intensive study. Several examples of contacts and heterojunctions are given below. First-principle calculations, performed for Bi2Se3/(Au, Pt, Ni, Pd, graphene) contacts predict n-type Ohmic contacts only, regardless of the choice of metals from the group. Graphene and Au were found to have the weakest charge transfer to TI, thus providing the best Ohmic contacts which do not interfere with the topological states and preserve their spin-momentum locked character [4]. To prevent charge transfer to other metals, it was proposed to use a thin large bandgap dielectric layer placed between a metal and a TI. This type of contact is normally used as a gate to achieve surface conduction that is tunable from n- to p-type by gate voltage. Effective gating has been reported for (Bi1xSbx)2Te3 in a geometry with top [5, 6] and back (through the dielectric substrate) gates [7]. To get ambipolar surface conduction, the bulk con
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