Dry growth of n-octylphosphonic acid monolayer for low-voltage organic thin-film transistors

Abstract

Dry method for monolayer deposition of n-octylphosphonic acid (C8PA) on the surface of aluminium oxide (AlOx) is presented. Vacuum thermal evaporation is employed to deposit initial thickness corresponding to several C8PA monolayers, followed by a thermal desorption of the physisorbed C8PA molecules. AlOx functionalized with such C8PA monolayer exhibits leakage current density of ∼10−7A/cm2 at 3V, electric breakdown field of ∼6MV/cm, and a root-mean-square surface roughness of 0.36nm. The performance of low-voltage pentacene thin-film transistors that implement this dry AlOx/C8PA gate dielectric depends on C8PA desorption time. When the desorption time rises from 25 to 210min, the field-effect mobility increases from ∼0.02 to ∼0.04cm2/Vs, threshold voltage rises from ∼−1.2 to ∼−1.4V, sub-threshold slope decreases from ∼120 to ∼80mV/decade, off-current decreases from ∼5×10−12 to ∼1×10−12A, on/off current ratio rises from ∼3.8×104 to ∼2.5×105, and the transistor hysteresis decreases from 61 to 26mV. These results collectively support a two stage model of the desorption process where the removal of the physisorbed C8PA molecules is followed by the annealing of the defect sites in the remaining C8PA monolayer.