Optical and electrical properties of amorphous zinc tin oxide thin films examined for thin film transistor application

Abstract

Structural, electronic, and optical properties of amorphous and transparent zinc tin oxide films deposited on glass substrates by pulsed laser deposition (PLD) were examined for two chemical compositions of Zn:Sn=1:1 and 2:1 as a function of oxygen partial pressure (PO2) used for the film deposition and annealing temperature. Different from a previous report on sputter-deposited films [Chiang et al., Appl. Phys. Lett. 86, 013503 (2005)], the PLD-deposited films crystallized at a lower temperature <450 °C to give crystalline ZnO and SnO2 phases. The optical band gaps (Tauc gaps) were 2.80−2.85 eV and almost independent of oxygen PO2, which are smaller than those of the corresponding crystals (3.35−3.89 eV). Films deposited at low PO2 showed significant subgap absorptions, which were reduced by postthermal annealing. Hall mobility showed steep increases when carrier concentration exceeded threshold values and the threshold value depended on the film chemical composition. The films deposited at low PO2<2 Pa had low carrier concentrations. It is thought that the low PO2 produced high-density oxygen deficiencies and generated electrons, but these electrons were trapped in localized states, which would be observed as the subgap absorptions. Similar effects were observed for 600 °C crystallized films and their resistivities were increased by formation of subgap states due to the reducing high-temperature condition. High carrier concentrations and large mobilities were obtained in an intermediate PO2 region for the as-deposited films.