Exciton Structure and Zeeman Effects in Cadmium Selenide

Abstract

The exciton spectra of CdSe has been observed and identified by optical reflection, absorption and Zeeman structure at 1.8°K. The reflection and absorption spectra indicate the presence of two nonoverlapping exciton series. From observed optical selection rules, the conduction band is identified as having ${\Gamma }_7$ symmetry. The two series correspond to the ${\Gamma }_9$ and ${\Gamma }_7$ valence bands split by the crystal field by approximately 200 ${\mathrm{cm}}^{-1\mathrm{}}$. A third series, at higher energies, has been observed in absorption corresponding to a ${\Gamma }_7$ valence band state split by spin-orbit effects from the other two states by approximately 3490 ${\mathrm{cm}}^{-1\mathrm{}}$. The $n_1=1, 2, 3, \mathrm{and} 4$ states of the first (${\Gamma }_9$) series, the $n_2=1\mathrm{} \mathrm{and} 2$ states of the second (first ${\Gamma }_7$) series, and the $n_3=1\mathrm{}$ state of the third (second ${\Gamma }_7$) series have been observed and identified in absorption. The series limit of the first series, corresponding to the band gap, has been measured to be 14 850 ${\mathrm{cm}}^{-1\mathrm{}}$. A semi-empirical theory of exciton structure in the presence of an external magnetic field developed in the effective mass approximation has been used to obtain the band parameters at K=0 of CdSe from observed exciton spectra. The effect of the finite photon momentum has been observed through changes in the Zeeman structure of the $n_1=2\mathrm{}$, $P$ states upon 180° rotation of the magnetic field in the plane perpendicular to the crystal $C$ axis. Some deviations between experiment and the theoretical predictions of the ellipsoidal effective mass theory have been observed. The relation between these deviations and the possibility of toroidal energy surfaces in CdSe is discussed briefly.