Giant oscillator strength is inherent in excitons that are weakly bound to impurities or defects in crystals.
The spectrum of fundamental absorption of direct-gap semiconductors such as gallium arsenide (GaAs) and cadmium sulfide (CdS) is continuous and corresponds to band-to-band transitions. It begins with transitions at the center of the Brillouin zone, . In a perfect crystal, this spectrum is preceded by a hydrogen-like series of the transitions to s-states of Wannier-Mott excitons.[1] In addition to the exciton lines, there are surprisingly strong additional absorption lines in the same spectral region.[2] They belong to excitons weakly bound to impurities and defects and are termed 'impurity excitons'. Anomalously high intensity of the impurity-exciton lines indicate their giant oscillator strength of about per impurity center while the oscillator strength of free excitons is only of about per unit cell. Shallow impurity-exciton states are working as antennas borrowing their giant oscillator strength from vast areas of the crystal around them. They were predicted by Emmanuel Rashba first for molecular excitons[3] and afterwards for excitons in semiconductors.[4] Giant oscillator strengths of impurity excitons endow them with ultra-short radiational life-times ns.
^Elliott, R. J. (1957). "Intensity of optical absorption by excitons". Phys. Rev. 108 (6): 1384–1389. Bibcode:1957PhRv..108.1384E. doi:10.1103/physrev.108.1384.
^Broude, V. L.; Eremenko, V. V.; Rashba, É. I. (1957). "The Absorption of Light by CdS Crystals". Soviet Physics Doklady. 2: 239. Bibcode:1957SPhD....2..239B.
^Rashba, E. I. (1957). "Theory of the impurity absorption of light in molecular crystals". Opt. Spektrosk. 2: 568–577.
^Rashba, E. I.; Gurgenishvili, G. E. (1962). "To the theory of the edge absorption in semiconductors". Sov. Phys. - Solid State. 4: 759–760.
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