Optical bandgap and width of Urbach tail of lithium doped zinc oxide films deposited by SILAR method

Undoped and Lithium (Li) doped Zinc Oxide (ZnO)films were prepared by Successive Ionic Layer Adsorption and Reaction (SILAR) technique using 0.1M Zinc Sulphate with Ammonia as the cationic solution. Doping is done at different atomic weight percentage (atm. wt%) 5, 10 and 15 by adding Lithium Chloride to the cationic solution. The samples were annealed at 500◦C for one hour after deposition. The UV-Visible Transmittance and Absorbance of the samples were recorded from 250nm to 900nm.The reflectance data of the samples were estimated and the plot of reflectance suggested that the quality of the films is good except for Li:ZnO film at 5atm.wt%.The Optical constants ’n’ and ’k’ were calculated for different incident wavelengths on the samples. The variation of optical constants with incident wavelengths suggested that the Li: ZnO sample at 5atm.wt% has relatively high values when compared to other samples. The Urbach energy (Ee) or width of band tail is calculated from the (lnα-hν) plot and the Optical bandgap energy of the samples were calculated fron the (αhν)2 − hν plot. The bandgap energy was observed to be high for the undoped samples and decreases to normal value for 5atm.wt% sample. The bandgap energy again increases with doping concentration. The Urbach energy is found to be increasing with increase in doping concentration. These variations could be explained using the amorphous-crystalline nature and defect related effects in the samples due to doping.