Effects of He plasma treatment on zinc oxide thin film transistors

We report on the effects of He plasma treatment for reducing contact resistances of ZnO thin-film transistors (TFTs). A TFT with Ti (20 nm)/Au (40 nm) ohmic contacts was fabricated by exposing the ohmic contact area to He plasma before the formation of the Ti/Au ohmic contacts. Schottky like behavior was turned into ohmic behavior by the He plasma treatment, indicating the effectiveness of the treatment for reducing the contact resistance for ZnO TFTs. Interestingly, the sheet resistance of the ZnO channel layer decreased from 53.8 GΩ/sq to 6.41 GΩ/sq although the channel portion of ZnO was not treated by the He plasma. The transconductance for a 20-μm-gate TFT was also improved from 4.6 × 10^-6 S/mm to 1.2 × 10^-4 S/mm.

I. Intoroduction

Oxide semiconductors have drawn considerable attention as a pixel driver of flat-panel displays. We have been studying zinc oxide (ZnO) TFTs (Thin Film Transistor: TFT) as one of the candidates. For the performance improvements of TFTs, the reduction of the access resistance including contact resistance is indispensable. Recently, it has also been reported that a helium (He) plasma treatment is a promising method as a way of reducing the access (channel) resistance for IGZO TFTs [1]. The mechanism of the reduction in the resistance is reported to be the formation of oxygen vacancies by the He plasma treatment. We employed the He plasma treatment for reducing contact resistance for ZnO TFTs.