Metal nanoparticle (NP) surface coatings, supporting Localized Surface Plasmons (LSPs), have been widely reported to enhance the excitonic luminescence in ZnO by increasing the spontaneous emission rate. In this work, the enhanced UV-emission of Al-coated a-plane ZnO single crystals with varying carrier densities (ne = 1013 - 1017 cm-3) were systematically studied, using photoluminescence (PL), cathodoluminescence (CL) and valence band photoemission spectroscopy.
The low temperature PL and CL of the uncoated ZnO exhibited a sharp UV emission at 3.37 eV originating from excitonic recombination and a broad visible defect-related deep level (DL) emission. Adding a 1 nm-thin Al surface coating resulted in an enhanced UV, accompanied by a changed DL emission. The UV enhancement was attributed to an additional, faster relaxation pathway through the Al-LSPs, revealed by depth-resolved CL, showing the greatest UV enhancement close to the surface. A strong UV enhancement dependence on the intrinsic carrier density in ZnO was observed with a maximum of 17 times for the highest carrier density. The increasing ne reduces the surface depletion layer width from 3,095 nm to 44 nm, resulting in higher surface band bending allowing for deeper competitive donor recombination ionization which consequently increases the UV emission. Power-dependent PL data of the uncoated ZnO exhibited a clear trend of decreased UV emission with increasing excitation power, indicating the formation of additional competitive exciton recombination channels. Conversely, the UV emission of the Al-coated ZnO was not affected by the excitation power increase, suggesting the passivation of surface recombination channels by the Al coating. This effect was found to be more pronounced in ZnO single crystals with higher intrinsic carrier densities.
An up to 17-fold UV enhancement of Al-coated ZnO for the highest carrier density (~1017 cm-3) was observed in PL which was attributed to an additional, faster recombination channel via LSPs in the Al NPs. However other processes induced by the Al surface coating also contribute to this UV enhancement, including quenching of surface recombination channels in the uncoated ZnO and the formation of a surface depletion region by adding the thin Al surface coating.