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Optical Properties of Defects

  • Karl W. Böer
  • Udo W. PohlEmail author
Living reference work entry

Later version available View entry history

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Abstract

The optical absorption spectrum of lattice defects provides the most direct information about their electronic properties; general characteristics are obtained from such spectra even without detailed knowledge about the defect structure. Two substantially different types of absorption spectra occur: line spectra near the fundamental band-to-band absorption originating from shallow-level defects and usually broad spectra from deep-level, tight-bonding defects. Shallow defects are described by a hydrogen-like model using a Rydberg energy modified by the effective mass and dielectric constant. Lattice coupling and relaxation of these centers is only weak. Deep centers show strong electron-lattice coupling described by the Huang-Rhys factor, which expresses the mean number of emitted phonons during lattice relaxation after electron capture. Electronic and vibronic properties of deep centers are described in a configuration-coordinate diagram, with optical excitation and emission processes represented by vertical transitions according to the Franck-Condon principle. Photoionization refers to an excitation from a defect center into a band and leads to edge-shaped absorption.

At high defect density, the localized wavefunctions overlap and form an impurity band, which at densities exceeding the Anderson-Mott limit merges with band states. Such extended density of states leads to an Urbach tail in the optical absoption near the band edge. In addition, the absorption edge of band-to-band transitions in heavily doped semiconductors is blueshifted due to the Burstein-Moss effect resulting from band filling.

Keywords

Band tailing Band-to-band absorption Burstein-Moss effect Configuration-coordinates Deep-level defects Defect-center relaxation Effective-mass defect Electron-lattice coupling Franck-Condon principle Huang-Rhys factor Hydrogen-like model Inhomogeneous broadening Lattice coupling Line shape Oscillator strength Shallow-level defects Urbach tail 

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Copyright information

© Springer International Publishing AG 2017

Authors and Affiliations

  1. 1.NaplesUSA
  2. 2.Institut für Festkörperphysik, EW5-1Technische Universität BerlinBerlinGermany

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