Exciton structure and absorption edge in hexagonal zinc selenide

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Abstract
Optical transitions in hexagonal single crystals of zinc selenide have been investigated by transmission spectroscopy at 15°K. Three exciton series (with n = 1, 2, 3) have been found, and the ground state energies are 2.8520 ± 0.0003, 2.9039 ± 0.0003 and 3.3199 ± 0.0004 eV; the binding energies of these excitons are 0.022 ± 0.001, 0.035 ± 0.002 and 0.019 ± 0.002 eV respectively. With the aid of existing band structure calculations these transitions have been interpreted as being direct and allowed transitions over the minimum band gap associated with split valence bands at the centre of the Brillouin zone ( k = 0). Thus the minimum band gap in hexagonal zinc selenide is 2.874 ± 0.002 eV at 15°K. The spin-orbit and crystal field splitting energies are 0.423 ± 0.002 eV and 0.107 ± 0.002 eV respectively, estimated from the quasi-cubic model. The line width of the excitons has been found to be affected by the thickness of the crystals as well as by the degree of strain, impurities and temperature. From the shape of the absorption curve at energies just below the lowest energy exciton at 78°K, the energy of the longitudinal optical phonon has been estimated as 0.030 ± 0.002 eV. Higher energy transitions at 4.74 and 5.03 eV have been observed and these correspond to transitions between a deeper split valence band and a higher conduction band (∆5 → ∆3) near the centre of the Brillouin zone. The energy for the spin-orbit splitting in this case is 0.29 eV.