Anomalous optical processes in photoluminescence from ultrasmall quantum dots of ZnO

Ensembles of alumina capped ZnO quantum dots (ZQDs) were grown using pulsed laser deposition. The ZQDs of mean radii comparable to and smaller than the pertinent excitonic Bohr radius (∼2.34 nm), called ultrasmall quantum dots, show size dependent optical absorption edges, which follow the strong confinement model. In this model the confinement energy and Coulombic interaction energy of the localized electron-hole pairs are significantly higher than their correlation energy and the optical transitions are perceived to be nonexcitonic in nature. In photoluminescence (PL) spectra of such ZQDs of mean radius of ∼2.3 nm at temperatures of 6 K and above, the primary recombinations are found to be due to the surface bound and Al donor bound electron-hole pairs. The band-edge recombination peak of the PL spectra appeared at about 70 K and above, which was found to be about 166 meV Stoke and/or thermally redshifted with respect to the experimentally observed absorption edge. Almost all of the PL spectra at different temperatures conspicuously showed the LO and 2LO phonon replicas of the primary transitions, suggesting strong coupling between the recombining charge carriers and the LO phonon, which is rather unusual for nonexcitonic recombinations. The temperature dependent PL peak positions followed the well known Varshni’s relation with fitting parameters close to that of the bulk ZnO. The peak intensity of the observed PL transitions followed the normal law of thermal quenching which could be fitted with the Arrhenius equation having activation energy of about 10 meV. Temperature dependence of full width at half maximum of the PL peaks when fitted with the Hellmann and O’Neill models did not result in a close match. However, from this fit one could estimate a value of the carrier-LO phonon coupling coefficient of ∼980 meV, which is higher than that reported earlier for the ZQDs. These observations are hitherto unfamiliar and expected to provide further insight into the basic optical processes in alumina capped ultrasmall ZQDs.