Intrinsic exciton transitions in high-quality ZnO thin films grown by plasma-enhanced molecular-beam epitaxy on sapphire substrates

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High-quality ZnO thin films have been grown by plasma-enhanced molecular-beam epitaxy onsapphire substrates. Free-exciton absorption and exciton-LO phonon absorption peaks are observedin the films at room temperature, indicating that the exciton states are stable even at roomtemperature. Three excitonic transitions associated with valence bands A, B, and C are clearlyrevealed in the reflectance spectrum measured at low temperatures. This result indicates that theZnO thin films have a perfect wurtzite crystal structure. Biexciton emission is observed in thephotoluminescence spectra at low temperatures, from which the biexciton binding energy isestimated to be 14.5 meV, in good agreement with previous results. Exciton-LO (Ex-LO) andexciton-2LO (Ex-2LO) photon emission peaks are observed at low temperature. The energydifference between the Ex-LO and Ex-2LO bands is about 72.5 meV, which coincides withpreviously reported values of the LO phonon energy for ZnO thin films.
American Institute of Physicsの論文