Determination of valence and conduction band offsets in Zn0.98Fe0.02O/ZnO hetero-junction thin films grown in oxygen environment by pulsed laser deposition technique: A study of efficient UV photodetectors

Abstract

Here we present ZnO based thin films deposited in oxygen environment by pulsed laser deposition technique for UV photodetection applications. The valence and conduction band alignments are the key parameter required in understanding the type of charge carrier participating in conduction process. The valence and conduction band offset measurements have been carried out using the X-ray photoelectron spectroscopy (XPS) on the pristine ZnO, Fe (2 wt%) -doped ZnO (Zn0.98Fe0.02O) and the hetero-junction of Zn0.98Fe0.02O with ZnO (Zn0.98Fe0.02O/ZnO). XPS studies revealed that the valence band is upward shifted with 0.37 eV, whereas, there is a downward shifting of −0.393 eV in the conduction band. Structural analysis of the deposited films has been done using X-ray diffraction studies, which revealed the polycrystalline nature of all the films having wurtzite crystal phase. AFM studies suggest that the size of the grains decreases with Fe-doping in ZnO network due to different ionic radii of Zn and Fe. UV–Vis studies suggest the decrease in optical band gap with Fe-doping and hetero-junction formation. The optical band gap energy has also been obtained from the fitting of experimental data of absorption spectra with Mott -Davis's model and Hydrogenic excitonic model. Both the models are well corroborated with each other. The photoconduction behavior of the deposited films has been analyzed under UV light. The pristine ZnO film has a photoresponse of about 25% which is found to increase in Zn0.98Fe0.02O (∼36%) film. The responsivity values are found to be 1.56, 2.17 and 1.86 mA/W for the pristine ZnO, Zn0.98Fe0.02O and Zn0.98Fe0.02O/ZnO thin films, respectively.