surface states (rus. поверхностные состояния) — electronic states that are spatially localisised near the surface of solid.

Description

Understanding of surface states naturally emerged from studying the band theory of confined crystals. In a discussion of the nature of surface states, one generally distinguishes between Tamm Surface states arising due to termination of the crystal lattice at the boundary, and Shockley states localisised at impurities or defects on the surface or in the oxide layer covering the surface. Tamm states are formed by allowed energy bands separated by forbidden bands. Surface allowed bands can be located at energies corresponding to both forbidden and allowed bulk bands.

In 1932, Tamm, while considering a simple one-dimensional model of a semi-infinite crystal as a sequence of delta potential barriers limited by a potential wall, came to a fundamental conclusion about the possible existence of states whose wave functions are localisised to the crystal surface.

A fundamentally different approach to dealing with surface states than that proposed by Tamm was proposed by Shockley in 1939. He considered the electron energy levels in a chain of eight atoms. The calculation showed that forbidden bands appear in the spectrum of electron energy, and within each of those there are two states related to the fact that there are two edge atoms at the ends of the linear chain. The electrons in these two atoms are in the same unique position as the electrons of atoms near the surface of a three-dimensional crystal, where, according to Shockley, one can also expect surface states.

In a real crystal, surface electronic states correspond to the coordinatively unsaturated surface atoms. Usually, their emergence results in alteration, surface reconstruction, i.e., the displacement of surface atoms both in a plane tangent to the surface and normal to it, which creates a surface structure with a period equal to several periods of the three-dimensional lattice or incommensurable with them. The character of the reconstruction depends on the crystallographic orientation of the surface, the method of its preparation, as well as the type and concentration of adsorbed impurities or the presence of the oxide layer on it.

Interest in the electronic surface states is associated not only with the desire to understand the physics of surface phenomena, but also with vital technological needs. Surface states act as recombination centres for electrons and holes, thereby reducing the number of carriers and degrading characteristics of diodes, transistors, solar cells and other semiconductor devices.

Illustrations

а — One-dimensional model potential of a semi-infinite lattice. Two types of wave functions in
а — One-dimensional model potential of a semi-infinite lattice. Two types of wave functions in a semi-infinite crystal: b — volume states; c — surface states.

Authors

  • Zotov Andrey V.
  • Saranin Alexander A.
  • Naymushina Daria A.

Sources

  1. Oura K. et al. Surface Science: An Introduction // Springer, 2010 - 452 pp.
  2. Surface states // Physical encyclopedia V. 3 (in Russian) / Ed. by Prokhorov A.M. — Мoscow: Sovetskaja ehnciklopedija, 1988. 651–652 pp.

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