Investigations on pulsed laser deposited Li-doped Zno thin films

Abstract

Li-doped ZnO (Zn1−xLixO2, with x= 0.000, 0.005, 0.010 and 0.015) thin films were deposited by pulsed laser deposition (PLD) method and the effect of variation of various parameters like Li-doping percentage, substrate, thickness variation and effect of UV radiation on the resistivity and other properties of thin films was observed. The Zn1−xLixO2 films with different Li content were grown epitaxially on c-axis oriented sapphire -Al2O3 (001) substrate with dominant (002) peak of ZnO hexagonal wurtzite structure. We observed that the resistivity increased on increasing Li content due to decrease in carrier concentration. The next series was made on Quartz substrate and the optical band was observed to be decreasing which is a contradiction to the results observed in case of Sapphire Substrate. Further, for the study of thickness variation, Zn1−xLixO2 (x=0.015) thin films with thickness 50nm, 75nm and 100nm were deposited. We observed that there is only slight change in the FWHM (Full Width Half Maximum) showing the highly oriented nature despite of different thicknesses. The resistivity gradually increased on increasing the thickness of films due to the increased scatterinf centres for carriers. Since the band gap of ZnO lies in the near UV spectral region, an unusual feature of metal-to-semiconductor transition (MST) was observed in the temperature range of 200-250 K on illuminating UV radiation (λ= 365nm) on undoped and Li-doped ZnO thin films while the same films in dark condition manifested purely semiconductor characteristics throughout the entire range of temperature (300-10K). This unexpected behavior can be explained on the phenomenon of trapping and de-trapping of carriers in the defect band of ZnO.