Strain splitting of the Gamma(5) and Gamma(6) 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, so me hydrothermally grown crystals contain strain. These strains result in en ergy shifts of the free excitons as well as relaxation of the selection rul es. 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 str ain and electron-hole spin exchange. (C) 1999 American Institute of Physics . [S0021-8979(99)08022-6].