Raman spectroscopic investigation of ZnO and doped ZnO films, nanoparticles and bulk material at ambient and high pressures

Multichannel micro-Raman spectroscopy has been used for systematic investigation of pure and doped-ZnO in the form of thin films, nanosize powders and sintered bulk material under ambient conditions. High-pressure Raman spectra of the sintered ZnO:Al bulk material have been measured to 11.7 GPa in order to evaluate residual stresses in deposited films as well as to characterize the wurtzite to rocksalt phase transition at and above 9.3 GPa. High temperature Raman spectra of Al:ZnO are reported to 973 K that show pre-resonance enhancement of the intensities of several Raman bands above 200 C with blue-green laser excitation. All films studied were of the wurtzite phase, fine-grained, and exhibited varying degrees of c-axis orientation with respect to the substrate normal depending upon the deposition conditions. It has been found that the ratio of intensities of the high-frequency E-1/E-2 and 2LO/E-2 Raman modes increase with increasing defect content of the films and as-prepared powders, The E-2 mode frequency shift of the ZnO films relative to the single crystal frequency indicates varying degrees of residual tensile stresses in these films depending on preparation method and processing. The changes in measured Raman spectra on post deposition annealing of films and nanoscale powders are correlated with defect content of these samples.