Identification of extended defect and interface related luminescence lines in polycrystalline ZnO thin films grown by sol–gel process
The luminescence lines related to extended defects and interfaces in polycrystalline ZnO thin films grown by sol–gel process are deeply investigated by combining temperature-dependent photoluminescence and cathodoluminescence imaging with high-resolution transmission electron microscopy. A typical broad emission band is shown in the range of 3.316 to 3.333 eV and mainly consists of two distinct contributions. At high energy, a 3.333 eV line is associated with interfaces (i.e., free surfaces and grain boundaries) and predominates for small ZnO nanoparticles owing to their high density. The intensity ratio of the excitonic to interface-related transitions is low in this first configuration and the 3.333 eV line is characterized by an activation energy of 12.0 ± 1.2 meV and a Huang-Rhys factor of 0.54 ± 0.05 at 12 K. At low energy, a 3.316 eV line is attributed to basal plane stacking faults that are mostly of I1-type and prevail for large ZnO nanoparticles. The 3.316 eV line is characterized by an activation energy of 6.7 ± 0.8 meV and a Huang Rhys constant of 0.87 ± 0.03 at 12 K. Basal plane stacking faults are most likely formed as the coalescence process proceeds with the decomposition and crystallization processes during annealing. As shown by low-temperature monochromatic cathodoluminescence imaging, the luminescence corresponding to the 3.316 eV line is, in this second configuration, limited to some specific area (i.e., large nanoparticles), and the relative intensity ratio of the excitonic to interface-related transitions is increased due to the smaller free surface area and density of grain boundaries.