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By solving the continuity equation of excitons under steady excitation, a theoretical model for the excitonic luminescence of semiconductors was developed taking into account the exciton diffusion and surface recombination. The theoretical model was used to analyze the photoluminescence (PL) spectra of ZnO obtained from the bulk single-crystal samples with and without surface passivation, showing that the nonradiative recombination on the surface is an important channel of losing excitons, thus substantially reducing the PL quantum efficiency of excitons at room temperature. In addition, the surface recombination was found to have impacts on the fine structure of excitonic luminescence at low temperature. Using the theoretical model, the diffusion length of excitons at room temperature was estimated and found to be different from sample to sample, strongly depending on the sample processing. The theoretical model was demonstrated to be capable of accurately fitting the temperature-dependent PL intensity of passivated samples and showed that the exciton diffusion has significant impacts on the dynamics of excitonic luminescence at high temperature. [ABSTRACT FROM AUTHOR]

The effects of surface conditions on photo-induced-carrier (PIC) diffusion are studied by solving one-dimensional continuity equation. Using various forms of ZnO samples with different surface conditions, the PICs lost at sample surface are substantiated to have important contributions to the visible luminescence. For unpassivated ZnO nanorods and single-crystal samples that undergo a high-temperature annealing in air, the visible luminescence originating from the PICs diffused to the surface is estimated more than 99.8 % and 70 % at room temperature, respectively, and more than 99.99 % and 97 % at low temperature. The structureless luminescence caused by PICs, such as the green, yellow, and orange luminescence bands are well fitted under the framework of tail-state luminescence, while the structured green band is suggested to be in association with a specific type of surface states formed during high-temperature annealing in oxidizing atmosphere and can be fitted with Huang-Rhys model by taking into account tail-state luminescence. [ABSTRACT FROM AUTHOR]

Multiphonon assisted optical emission processes of ZnO in the vicinity of excitonic luminescence were investigated using the excitation-emission maps and the photoluminescence (PL) excitation spectra recorded at different emission photon energies. Multiphonon resonant Raman scattering (RRS) was found to have the temperature dependence similar to the excitonic luminescence, but not to be influenced by the surface conditions and annealing process. The multiphonon RRS and the longitudinal optical (LO) phonon replicas of PL lines were substantiated to originate from the cascade scattering of exciton polaritons with successively emitting LO phonons. The second-order Raman processes were observed to take part in the capture of exciton polaritons by the defects, forming the neutral donor bound excitons. [ABSTRACT FROM AUTHOR]

A new fine structure of red luminescence (RL) was observed in unintentionally doped ZnO at low temperature. The new RL was demonstrated to be different from the one assigned to Fe3+ center by Heitz et al and visible only under the excitation of light capable of generating excitons. The RL can be extremely enhanced at the incident frequency in resonance with the donor exciton and the intensities of some adjacent lines separated by ∼4 meV are thermally populated with the increase in temperature. The new structured RL was studied in the terms of photoluminescence (PL) and PL excitation spectra and the origin was discussed taking into account the internal transitions of transition metal elements, structure defects, and native point defects. There is a great possibility that the new structured RL arises from the excited states converted from bound excitons, for example, the excitons or the electron and hole pairs bounded by the donor and acceptor pairs (DAPs) of O and Zn vacancies, because the binding energy determined by the equation of DAP fluorescence is highly in accordance with the theoretic values reported in the literature. [ABSTRACT FROM AUTHOR]

The photoluminescence spectra excited by different wavelength light from ZnO samples with and without SiO x passivation demonstrated that the excitation behavior of structured green luminescence (GL-S) band is determined by the surface conditions or the morphological forms. The lowest energy of photons able to excite the GL-S band varied from 3.380 eV down to a value far below the zero phonon line of GL-S band, depending on the defect density in the region near to the sample surface. The imperfections like cracks, scratches, and voids etc. were found decisive for the effectiveness of SiO x passivation on the GL-S band, which is removable only for the samples with good surface conditions. In addition, the phonon energy in GL-S band was determined to be ~3 meV lower than that of longitudinal optical phonons and thus was associated with the surface optical mode. The evidence of a link between the GL-S band and the optical emissions on sample surface revealed that there is no necessity to connect the GL-S band with any specific defects in the inside of samples. This optical emission mechanism is not only helpful to settle the relevant issues of visible luminescence in the research of ZnO, but also important to the study of other phenomena related to photoexcitation. • GL-S band of ZnO is substantiated to originate from the optical emissions on surface. • Excitation behavior of GL-S band depends the density of defects in the near surface region. • Surface imperfections are decisive for the effectiveness of SiO x passivation on GL-S band. • Phonons in the GL-S band is assigned to the surface optical mode. [ABSTRACT FROM AUTHOR]

The optical emissions of O-annealed ZnO nanorods (ZnO NRs) before and after surface passivation are studied at temperatures of 295 to 17 K in terms of photoluminescence (PL) spectra, PL excitation spectra, and PL quantum efficiency. Because the ZnO NR samples maintain the surface free of mechanical damages during the sample preparation, unambiguous evidence is obtained to substantiate that the structured green luminescence (GL-S) bands come from the optical emissions on the sample surfaces. We suggest that the GL-S bands are excited by the excitons diffused to the sample surface and thus substantially reduced in the PL spectra of ZnO NRs after surface passivation. PL fitting shows that many kinds of surface states may have contributions to the optical emissions in the GL-S bands, leading to the variation of GL-S bands with annealing temperature and ambient environment. [Display omitted] • Flower-like ZnO NRs are successfully passivated by depositing a layer of SiO x films. • PL quantum efficiency of ZnO NRs before and after passivation is determined. • GL-S bands of ZnO NRs are substantiated to come from the sample surface. • GL-S bands is suggested to be excited by the excitons diffused to the surface of ZnO NRs. [ABSTRACT FROM AUTHOR]