Effect of lithium doping in the optical and structural properties of zinc oxide films by SILAR method

In this work, undoped and Lithium (Li) doped Zinc oxide (ZnO) films were deposited on glass substrates using SILAR (Successive Ionic Layer adsorption and Reaction) method. Here 0.1M zinc sulphate mixed with ammonia solution is taken as the cationic precursor and distilled water at 70 °C as the anionic precursor. Lithium doping is done by adding anhydrous Lithium chloride (LiCl) in cationic solution at different atomic weight percentage (5%, 10% and 15%) as required.The samples were annealed for 1 hour at a holding temperature of 500 °C.The X ray diffraction analysis showed that the undoped samples are in amorphous phase. The doped (5 wt.%) samples showed peaks corresponding to ZnO hexagonal wurtzite structure and the calculations revealed that the lattice parameters are close to the standard values. But the Li doped samples at atomic weight percentages 10 and 15 showed much variation from ZnO crystal structure and 15 wt.% sample was completely turned to amorphous state. The UV-Visible transmission was 80% for the undoped sample. The transmission reduced to below 1.5% for the Li:ZnO film at 5 wt.% doping. Again the transmission properties improved with increase in doping and was 80% for the film with 15 wt.% doping.The photoluminescence spectra of the samples were drawn. The undoped sample showed a strong broader peak centered near 400 nm, corresponding to near band edge emission and some minor peaks at longer wavelength region, indicating the presence of defects. After doping at atomic weight 5%, this shorter wavelenghth emission had been reduced and the longer crystal defect related emission became dominant, which may be the cause for reduction in transmission of the sample. With increase in doping concentration it has been observed that the shorter wavelength emission became more dominant and is clearly shown by the 15wt.% sample.