Photoluminescence Processes in ZnO Thin Films and Quantum Structures

Abstract

ZnO, a well-known direct and wide bandgap semiconductor is found to show intricate photoluminescence (PL) spectra in thin films and quantum structures such as quantum wells and quantum dots (QDs). In ZnO, thin films grown on sapphire substrates using pulsed laser deposition (PLD) an intense PL in the UV region at about 3.35 eV was observed, which corresponded to near band-edge emission due to the excitonic recombinations. The deep level emission in the visible spectral region of 2–3 eV, which is found to be due to off stoichiometry of the ZnO films, i.e., oxygen vacancies, zinc interstitial, and other structural defects, was almost negligible compared to the near band-edge emission. The strong near band-edge emission in UV spectral region was found to have fine structures consisting of various peaks mainly due to donor and acceptor bound excitons and their phonon replicas, which changed their position and intensity with temperature. In ZnO/Mg _x Zn_1−x O multi-quantum wells (MQWs) with well layer thickness in the range of ~4 to 1 nm on (0001) sapphire substrates also grown using PLD under the optimized conditions, we observed size-dependent blue shift in ZnO bandgap due to the quantum confinement effect. The PL spectra of these ZnO MQWs recorded at 10 K showed that line width of the PL peaks increased with decreasing well layer thickness, which was attributed to fluctuations in the well layer thickness. The temperature-dependent PL peak positions for the MQWs were found to shift gradually toward red end of the spectrum with increase in temperature up to 300 K due to the temperature-dependent thermal expansion/dilation of the lattice and carrier-phonon scattering. This dependance was found to be consistent with the well-known Varshni’s empirical relation. Ensembles of alumina capped ZnO quantum dots (ZQDs) also grown using pulsed laser deposition with mean radii comparable to and smaller than the pertinent excitonic Bohr radius (~2.34 nm), called ultra-small QDs showed size-dependent optical absorption edges. These absorption spectra were found to be consistent with the strong confinement model, in which the confinement energy and Coulombic interaction energy of the localized electron-hole pairs are taken to be significantly higher than their correlation energy and the optical transitions are perceived to be non-excitonic in nature. In PL spectra of such ZQDs of mean radius of ~2.3 nm at temperatures of ~6 K and above the primary recombinations were found to be due to the surface bound and Al donor bound electron-hole pairs. The near band-edge recombination peaks of the PL spectra appeared at the sample temperature of ~70 K and beyond. These peaks were found to be ~166 meV Stoke and/or thermally red shifted with respect to the experimentally observed absorption edge. Almost all the PL spectra of the ZQDs at different temperatures showed the LO and 2 LO phonon replicas of the primary transitions, which suggests strong coupling between the recombining charge carriers and the LO phonons. The temperature-dependent spectral positions of the PL peaks for the ZQDs also followed the above stated Varshni’s relation with fitting parameters close to that of the bulk ZnO. The intensity of the PL peaks was found to follow the normal mechanism of thermal quenching which could be fitted with the Arrhenius type of equation having activation energy of ~10 meV. Temperature dependence of FWHM of the PL peaks when fitted with the Hellmann and O’Neill models did not result in a close match. Although one could estimate a value of the carrier-LO phonon coupling coefficient of ~980 meV from this fit, but this was found to be much higher than that reported earlier for the ZQDs. These studies are expected to provide deeper insight into the basic optical processes in ZnO thin films, quantum wells, and QDs.