Stoichiometry enhanced exciton-phonon interactions in ZnO epilayers

Stoichiometry enhanced exciton-phonon couplings have been studied in ZnO epilayers grown on Al₂O₃ substrates by pulsed laser deposition. Rutherford backscattering spectroscopy indicated the stoichiometry, while clear hexagonal morphology revealed the Zn-polar growth of ZnO epilayers. From these high-quality and single-crystalline ZnO epilayers free-exciton emissions were observed at 3.30-3.31 eV with a low resistivity of ≤10 ^-2 Ω cm. Theoretical models were considered for fitting the experimental data to estimate the exciton-phonon interactions in ZnO epilayers. The enhanced exciton-phonon coupling, effective phonon energy and electron-phonon interaction were found to be 680 meV, 65.5 meV and 0.093 meV K^-1, respectively. These larger interactions between excitons and phonons even with the lattice dilation have been attributed to the higher Fröhlich constant due to the strong localization energy in the stoichiometric ZnO epilayers.