Study of structural and electrical properties of ZnO thin film for Thin Film Transistor (TFT) applications

This work reports the room temperature deposition of undoped ZnO thin film using RF magnetron sputtering technique for thin film transistor (TFT) applications. RF sputtered ZnO thin film of thicknesses 100 and 200 nm were deposited over n-type silicon (Si) substrate using 99.9% pure ZnO target. For the deposition of ZnO thin films, a deposition pressure of 28 mTorr and RF power of 50 W was maintained. The gas flow rate was 5 and 20 sccm for oxygen and argon respectively. Structural and surface morphological characterization was done using X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) techniques. XRD analysis of the thin film gives a dominant X-ray diffraction peak corresponding to strong c-axis oriented phase of hexagonal wurtzite structure of ZnO. The XRD results confirm the polycrystalline nature of the thin film and shows crystallinity improvement with increase in film thickness. Further, FESEM results confirm that the grain size increases with the film thickness. The surface roughness is studied through AFM while surface elemental survey is done using XPS. To measure the resistivity of deposited films, four point probe method was used. The resistivity of undoped 100 nm thick ZnO film is found to be 4.47 kΩ cm, while the resistivity of undoped 200 nm thick ZnO film reduces to 724.13 Ω cm because of improved crystallinity.