Effect of annealing on room temperature photoluminescence of polymeric precursor derived ZnO thin films on sapphire substrates

The study of the effect of annealing on the room temperature photoluminescence (PL) of polycrystalline, textured zinc oxide (ZnO) thin films on sapphire substrates, synthesized via aqueous polymeric precursors, is presented. ZnO thin films exhibit strong, broad, and asymmetric near band edge ultra-violet (UV) PL peak without deep-level emissions. There is an increase of grain size and texture coefficient along the c-axis for the ZnO thin films with annealing temperature. XRD data reveals the presence of strain in the ZnO annealed films. The near band edge PL peak was deconvoluted using Gaussian approximations into contributions from free exciton recombination (FX) and its longitudinal optical (LO) phonon replicas. Deconvolution of the PL peaks reveals that the PL peak position does not shift with annealing. It is observed that with an increase in texturing, grain growth, and strain in the ZnO films, a first order LO phonon replica (FX-1LO) is present, which is forbidden in bulk ZnO crystals. Annealing of preferentially oriented ZnO films at high temperatures leads to a strain-induced exciton-phonon coupling due to an increase in grain sizes and texturing of films.