Effect of depth of traps in ZnO polycrystalline thin films on ZnO-TFTs performance

Abstract

ZnO thin films were processed by radio frequency magnetron sputtering at room temperature on p-Si/SiO2 substrates under pure argon (Ar:O2=100:0vol.%) and argon–oxygen mixture (Ar:O2=99:1vol.%) gas environment. Morphological, optical and electrical characteristics of the ZnO films are reported, and they show a clear relationship with the gas mixture employed for the sputtering process. Scanning Electron Microscopy revealed the formation of grains of 15.3 and 19.9nm average sizes and thicknesses of 59nm and 82nm for films growth in pure argon and argon–oxygen, respectively. Photoluminescence measurements at room temperature showed the violet emission band (centered at 3eV) which was only detected in the ZnO film grown under pure argon. From thermally stimulated conductivity measurements two traps with 0.27 and 0.14eV activation energies were identified for films grown in pure argon and argon–oxygen mixture, respectively. The trap at 0.27eV is associated with a level located below the conduction band edge and it is supported by the PL band centered at 3eV. Both types of ZnO films were used as the active channel layer in thin film transistors with thermal SiO2 as gate dielectric. Field effect mobility, threshold voltage and current ratio were improved in the devices with ZnO channel deposited with the argon–oxygen mixture (99% Ar/1% O2 vol.). Threshold voltage decreased from 25V to 15V, field effect mobility and current ratio increased from 0.8 to 2.4cm2/Vs and from 102 to 106, in that order.