Investigations on the reasons for degradation of zinc tin oxide thin film transistor on exposure to air

Amorphous zinc tin oxide thin film transistor (ZTO TFT) was fabricated in the inverted-staggered (top contact) structure on Si/SiO2 substrate. The on-off ratio and sub-threshold swing were 2.55 × 106 and 2.3 V/dec respectively. The comparison of transfer characteristics of as prepared and 20 days air exposed ZTO TFT indicates that the threshold voltage and sub-threshold slope are highly influenced by ambient conditions. After exposure to air the device parameters such as µsat, VT and SS changed from 0.44 cm2/V s to 0.49 cm2/V s, 15 V to 18.5V and 2.3 V/dec to 9 V/dec respectively. The positive shift in threshold voltage can be due to the adsorption of oxygen molecules on the backchannel. The increase in sub-threshold swing after air exposure suggests acceptor like trap creation in the channel layer caused by adsorption of water molecules. The deposition of passivation layer on top of the device reduces the aging effect hence stability of device under ambient condition was improved. Density of states (DOS) of as prepared and 20 days air exposed TFT has been investigated for both passivated and unpassivated devices. Comparison of DOS for the TFTs suggested the formation of large number of trap levels in the ZTO channel layer by water adsorption and it was reduced by the passivation of channel layer of ZTO device.