Photoluminescence and the exciton-phonon coupling in hydrothermally grown ZnO

Near band-edge photoluminescence (PL) from hydrothermally grown bulk ZnO was studied as a function of temperature along with the effects of simultaneous excitation with below-gap photons, allowing for accurate assignment of the emission features not possible from low-temperature data alone. Free exciton emission was clearly observed at low temperatures and dominated the PL spectrum above 100 K. Emission from $A$ excitons bound to three neutral donors dominated the low-temperature PL spectrum. Recombination of $B$ excitons bound to these same neutral donors were also identified along with $A$ excitons bound to the donors in their ionized state. A clear difference in the redshift of free and bound excitons with increasing temperature was observed and attributed to reduced exciton-phonon coupling for the bound excitons. Additionally, Fano resonance of the 1-LO replica of the dominant bound $A$ exciton was observed to reduce its PL intensity which can lead to the misidentification of the 2-LO replica as a donor-acceptor-pair transition.