  Self-assembled monolayers (SAMs) have been used to improve both the positive
and negative bias-stress stability of amorphous indium gallium zinc oxide
(IGZO) bottom gate thin film transistors (TFTs). N-hexylphosphonic acid (HPA)
and fluorinated hexylphosphonic acid (FPA) SAMs adsorbed on IGZO back channel
surfaces were shown to significantly reduce bias stress turn-on voltage shifts
compared to IGZO back channel surfaces with no SAMs. FPA was found to have a
lower surface energy and lower packing density than HPA, as well as lower bias
stress turn-on voltage shifts. The improved stability of IGZO TFTs with SAMs
can be primarily attributed to a reduction in molecular adsorption of
contaminants on the IGZO back channel surface and minimal trapping states
present with phosphonic acid binding to the IGZO surface.
