Strain splitting of the Γ5 and Γ6 free excitons in ZnO

High-quality ZnO crystals have been grown by vapor-phase techniques and by the hydrothermal method. Depending on the surface preparation technique, some hydrothermally grown crystals contain strain. These strains result in energy shifts of the free excitons as well as relaxation of the selection rules. The ${\Gamma }_6$ unallowed exciton is observed in these samples without the application of a magnetic field. The ${\Gamma }_6$ exciton is also observed to split in a strain field, consistent with the ${\Gamma }_9$ symmetry for the top valence band in ZnO. The ${\Gamma }_5$ and ${\Gamma }_6$ excitons have been observed to split in the strain field. The splitting is believed due to combined strain and electron–hole spin exchange.