Reflection measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on GaN single crystals are reported. At low temperature (4.2 K) three free exciton lines have been found with energies; E_A=3.4776 eV, E_B=3.4827 eV, and E_C=3.502 eV. The spin‐orbit parameter Δ_so=19.7±1.5 meV and the crystal field parameter Δ_cr=9.3±0.3 meV have been obtained. From temperature dependence of exciton spectra the energy gap dependence has been found: E(T)=E(0)−λ/[exp(β/T)−1] (λ=0.121 eV, β=316 K).

Reflectance and photoluminescence spectra of exciton polaritons in GaN homoepitaxial layers, grown by metalorganic chemical-vapor deposition on GaN single crystals, are reported. Polariton modes involving excitons $A,$ $B,$ and $C,$ which correspond to split valence bands of ${\Gamma }_9,$ ${\Gamma }_7,$ and ${\Gamma }_7$ symmetries, are resolved. Energies of the transverse excitons are found to be, respectively: $E_{\mathrm{TA}}=3.4767\mathrm{}\mathrm{eV},$ $E_{\mathrm{TB}}=3.4815\mathrm{}\mathrm{eV}$ and $E_{\mathrm{TC}}=3.4986\mathrm{}\mathrm{eV},$ at the temperature $T=1.8\mathrm{}\mathrm{K}.$ The shape of the measured emission spectra depends upon donor concentration. This is explained in terms of interbranch polariton scattering.

Systematic measurements of auto- and cross-correlations of photons emitted from individual $\mathrm{Cd}\mathrm{Te}∕\mathrm{Zn}\mathrm{Te}$ quantum dots under pulsed excitation were used to elucidate nonresonant excitation mechanisms in this self-assembled system. Memory effects extending over a few excitation pulses have been detected in agreement with previous reports and quantitatively described by a rate equation model, fitting a complete set of correlation and PL intensity results. The important role of single carrier trapping in the quantum dot was established. An explanation was suggested for the unusually wide antibunching dip observed previously in $X\text{−}X$ autocorrelation experiments on quantum dots under cw excitation.

The neutral donor bound exciton recombination processes in freestanding GaN have been studied. The photoluminescence spectrum shows emission lines related to silicon and oxygen donors. Time-resolved luminescence allows us to correlate the principal donor bound exciton lines with their two-electron satellites. The magnetic field splitting of the two-electron lines is well described by the theory of the hydrogen atom in a magnetic field. For the oxygen donor a 1.5 meV chemical shift and a 30.8 meV effective Rydberg have been evaluated. Two-electron satellites involving excitations to the $2p$ and $2s$ donor states are separated by an energy of 1.0 and 1.3 meV for O and Si impurity, respectively. The temperature dependence of the two-electron emission clearly shows that this separation arises from a splitting of the ground state of the neutral donor bound exciton complex. The nature of this splitting is discussed and it is suggested that it is due to rotational states of donor bound excitons.