Optical and electrical correlation effects in ZnO nanostructures: Role of pulsed laser annealing

This paper reports the preparation and characterization of some high quality polycrystalline ZnO nanofilms (ZNFs) deposited on silicon (Si) substrates via sol gel allied spin coating process. The properties of such deposited ZNFs were modified by Nd:YAG pulsed laser annealing at varied wavelengths. The impact of pulse laser wavelengths on the structural, morphological, optical and electrical characteristics of these nanofilms was evaluated. Annealing at third harmonic generation wavelength of 355 nm was found to produce ZNFs with much better polycrystallinity and strong lattice orientation than the one annealed at the fundamental wavelength of 1064 nm. Furthermore, annealing at the second harmonic generation wavelength of 532 nm could produce a typical ZnO crystal with apparent orientation. The observed modifications in the films structure and morphology were attributed to the laser treatment-mediated thermal effect. FESEM analysis of the films revealed the existence of varied nanostructures depending on the laser wavelength. Photoluminescence (PL) emission spectra of ZNFs exhibited a broad UV excitonic band and a narrow visible band related to defects. The laser irradiation induced generation of the oxygen vacancies were observed to reduce the electrical resistivity of the films and altered the optical band gap energies. Sample annealed at 355 nm disclosed the lowest resistivity of 23.532 x 10-3 omega cm. Raman spectra of ZNFs confirmed the existence of hexagonal ZnO wurtzite structure. The narrowing of the E2high (437 cm-1) Raman phonon mode due to further annealing indicated an improvement in the films crystallinity and reduction in the local atomic defects related to the oxygen vacancy (VO+2). XPS analyses showed an improvement crystallinity and chemisorption of oxygen into the grain boundaries of the films due to the laser treatment in air at lower wavelength. In short, the pulsed laser annealing of ZNFs were established to be far more efficient process for the oxidation of Zn to ZnO than conventional annealing procedures.