This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

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The electronic and optical interband transitions are categorized by direct and indirect transitions because the conduction band bottom and the valance band top contain more than one extrema. The electronic and optical band gaps can be calculated by the predominant mechanism of band to band transitions and are normally less than several volts. In the allowed direct transition, the electrons in the valence band transit vertically to the conduction band under the excitation of incident electrons or photons. The absorption coefficient as a function of photon energy in the allowed direct transition can be given by [1],

        ------------------------------ [3951]

where,
       A -- A constant
       v -- The frequency
       h -- The Planck constant
       E_gd -- The allowed direct energy gap.

[1] Tauc JC (1972) Optical properties of solids. North-Holland, Amsterdam.