Influence of excitation wavelength on photoluminescence spectra of Al doped ZnO films

Dependence of photoluminescence (PL) spectra of Al doped zinc oxide (AZO) films in near band edge (NBE) and green regions on the excitation wavelength (λex) has been investigated. A redshift has been observed in both the regions with increase of λex. The redshift of free exciton emission (FX) peaks with increase of λex may be due to different contributions of excitonic emissions and their phonon replicas. The intensity of these peaks decreases exponentially with increase of λex. The peaks related to first order transverse optical longitudinal optical replicas of the FX (FX-TO-1LO) phonon replica disappeared beyond λex=220 nm. The peaks assigned to FXAn=1 and FXAn=2 become visible for λex≥340 nm and become stronger for λex=360 nm. The separation between the two consecutive phonon replicas decreases with decrease of excitation energy due to more activity of lower energy phonon replicas. In case of undoped ZnO films, three emission peaks in NBE, blue and green regions have been observed. The redshifting with increase of λex in NBE, blue and green regions are found to be 7, 2 and 13 meV, respectively.