Sample-dependent excitation behavior of the structured green luminescence band in ZnO

Highlights: • 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. 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.