ZnAl2O4 decorated Al-doped ZnO tetrapodal 3D Network: Microstructure, Raman and Detailed Temperature Dependent Photoluminescence Analysis
3D networks of Al-doped ZnO tetrapods decorated with ZnAl2O4 particles synthesised by the flame transport method were investigated in detail by optical techniques combined with morphological/structural characterisation. Low temperature photoluminescence (PL) measurements revealed spectra dominated by the near band edge (NBE) recombination in the UV region, together with broad visible bands which peak position shifts depending on the ZnO:Al mixing ratios. A close inspection of the NBE region evidences the effective doping of the ZnO structures with Al, as corroborated by the broadening and shift of its peak position towards the expected energy associated with exciton bound to Al. Both temperature and excitation density-dependent PL results point to an overlap of multiple optical centres contributing to the broad visible band, with peak position dependent on the Al content. While in the reference sample the wavelength of the green band keeps unchanged with temperature, in the composites the deep level emission showed a blue shift with increasing temperature, likely due to the distinct thermal quenching of the overlapped emitting centres. This assumption was further validated by the time-resolved PL data, which clearly exposed the presence of more than one optical centre in this spectral region. PL excitation analysis demonstrated that the luminescence features of the Al-doped ZnO/ZnAl2O4 composites reveal noticeable changes not only in the deep level recombination, but also in the material’s bandgap, when compared with the ZnO reference sample. At room temperature, the ZnO reference sample exhibits free exciton resonance at ~3.29 eV, whereas the peak position in the Al-doped ZnO/ZnAl2O4 samples occurs at ~3.38 eV due to the Burstein-Moss shift, commonly observed in heavily doped semiconductors. Considering the energy shift observed and assuming a parabolic conduction band, a carrier concentration of ~1.82 ×1019 cm-3 was estimated for the Al-doped ZnO/ZnAl2O4 samples.