CdSe is used as a prototype to show the implications of valence-band degeneracy for the optical properties of strongly quantum-confined nanocrystals. Absorption spectra and photoluminescence spectra obtained under intermediate and strong pulsed excitation show the presence of new structures. The energy levels for the electron and the hole are calculated with the spherical confinement, the nonparabolicity of the conduction band, and the valence band degeneracy taken into account. The oscillator strengths of the dipole-allowed transitions are also calculated. This model is found to be in good agreement with the experimental observations, which originate mainly from the quantization of the energy spectrum of holes with due account given to valence-band degeneracy.

Using first-principles linear muffin-tin orbital density functional band structure calculations, the ordering of the states in the wurtzite ZnO valence-band maximum, split by crystal-field and spin-orbit coupling effects, is found to be ${\Gamma }_{7\left(5\right)\mathrm{}}>{\Gamma }_{9\left(5\right)\mathrm{}}>{\Gamma }_{7\left(1\right)\mathrm{}},$ in which the number in parentheses indicates the parent state without spin-orbit coupling. This results from the negative spin-orbit splitting, which in turn is due to the participation of the Zn $3d$ band. The result is found to be robust even when effects beyond the local density approximation on the Zn $3d$ band position are included. Using a Kohn-Luttinger model parametrized by our first-principles calculations, it is furthermore shown that the binding energies of the excitons primarily derived from each valence band differ by less than the valence-band splittings even when interband coupling effects are included. The binding energies of $n=2\mathrm{}$ and $n=1\mathrm{}$ excitons, however, are not in a simple $1/4$ ratio. Our results are shown to be in good agreement with the recent magneto-optical experimental data by Reynolds et al. [Phys. Rev. B 60, 2340 (1999)], in spite of the fact that on the basis of these data these authors claimed that the valence-band maximum would have ${\Gamma }_9$ symmetry. The differences between our and Reynolds’ analysis of the data are discussed and arise from the sign of the Landé g factor for holes, which is here found to be negative for the upper ${\Gamma }_7$ band.

We present a comprehensive study of the free-exciton and exciton-polariton photoluminescence in wurtzite GaN. Using polarization-dependent measurements we were able to resolve the fine-structure energy splittings in the $n=1\mathrm{}$ state of the A exciton and to determine the energy separation between the $1S$ and ${2P}_{\pm 1}$ states as $19.7\pm 0.2\hspace{0.5em}\mathrm{meV}.$ For the $n=1\mathrm{}$ state, the evolution of the emission from two transverse polariton branches ${\Gamma }_{5T1}$ and ${\Gamma }_{5T2},$ the longitudinal exciton ${\Gamma }_{5L},$ and the dipole-forbidden exciton ${\Gamma }_6$ in magnetic fields up to 15 T have been studied in Faraday configuration. We have estimated the value of the parallel effective g factor of the hole mixed into the $1S$ state of the A exciton as $g_{A,1S}^{\Vert }=2.25\mathrm{}\pm \mathrm{0.2.}$ To describe these data a theory is developed for the exciton energy structure in hexagonal semiconductors with wurtzite symmetry in zero and in weak external magnetic fields which takes into account the effect of the hexagonal lattice anisotropy and the coupling of all excitonic states belonging to different valence subbands. The effect of the exciton interaction with polar optical phonons is considered. The theory is very successful in describing the free-exciton emission and magnetoluminescence in wurtzite GaN. The effective mass parameters and the magnetic Luttinger constant as well as the effective Rydberg numbers and binding energies of the A, B, and C excitons are determined from a comparison of the theory with experimental data.

ZnO single crystals, epilayers, and nanostructures often exhibit a variety of narrow emission lines in the spectral range between 3.33 and 3.35 eV which are commonly attributed to deeply bound excitons ($Y$ lines). In this work, we present a comprehensive study of the properties of the deeply bound excitons with particular focus on the $Y_0$ transition at 3.333 eV. The electronic and optical properties of these centers are compared to those of the shallow impurity related exciton binding centers ($I$ lines). In contrast to the shallow donors in ZnO, the deeply bound exciton complexes exhibit a large discrepancy between the thermal activation energy and localization energy of the excitons and cannot be described by an effective mass approach. The different properties between the shallow and deeply bound excitons are also reflected by an exceptionally small coupling of the deep centers to the lattice phonons and a small splitting between their two electron satellite transitions. Based on a multitude of different experimental results including magnetophotoluminescence, magnetoabsorption, excitation spectroscopy (PLE), time resolved photoluminescence (TRPL), and uniaxial pressure measurements, a qualitative defect model is developed which explains all $Y$ lines as radiative recombinations of excitons bound to extended structural defect complexes. These defect complexes introduce additional donor states in ZnO. Furthermore, the spatially localized character of the defect centers is visualized in contrast to the homogeneous distribution of shallow impurity centers by monochromatic cathodoluminescence imaging. A possible relation between the defect bound excitons and the green luminescence band in ZnO is discussed. The optical properties of the defect transitions are compared to similar luminescence lines related to defect and dislocation bound excitons in other II–VI and III–V semiconductors.