Effect of Deposition Temperature in RF Sputtered ZnO Thin Films on ZnO TFT Performance

Abstract

ZnO thin films are deposited using RF magnetron sputtering by varying argon: oxygen gas flow rates and substrate temperatures. The structural and optical characterization of ZnO thin films are systematically carried out using X-ray diffraction (XRD), SEM, UV-visible spectroscopy and X-ray photoelectron spectroscopy (XPS). Dominant (002) Grazing incidence (GI) XRD peak on samples deposited at 300°C with Ar:O₂ (16:4) ratio suggest c-axis orientation both on the bulk and surface of ZnO thin film. Increase in the crystallite grain size were observed as the deposition temperature is increased from Room temperature (RT) to 300°C, leading to the reduction in grain boundaries. Absorption analyses show the reduction in band-tail states within the bandgap, supporting annihilation of defects, on the samples deposited at 250°C and 300°C. XPS spectra confirm the improved O₂ incorporation and reduction in oxygen vacancies in sample deposited at 300°C. Highest hall mobility of 46.09 cm²/V-sec has been observed on sample deposited at RT, and is dominated by defects. Whereas, films deposited at 250°C and 300°C exhibit Hall bulk mobilities of 20.43 cm²/V-sec and 31.63 cm²/V-sec, respectively. Further, bottom-gate ZnO thin film transistors (TFTs) are also fabricated on SiO₂/p-Si substrate. Variation in substrate temperature showed performance enhancement in terms of leakage current, threshold voltage, sub-threshold swing and I_ON/I_OFF ratio. Devices deposited at 300°C resulted in O₂-rich surface through chemisorption, which led to the reduction in leakage current of upto 10^-12A and 10-fold reduction in sub-threshold swing from 30V to 2.8V. Highest field-effect mobility of 1.1 cm²/V-sec has been achieved when the ZnO thickness in the TFT is reduced to 50 nm.