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

Dry method for monolayer deposition of n-octylphosphonic acid (C₈PA) on the surface of aluminium oxide (AlO_x) 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. AlO_x functionalized with such C₈PA monolayer exhibits leakage current density of ∼10⁻⁷ A/cm² at 3 V, electric breakdown field of ∼6 MV/cm, and a root-mean-square surface roughness of 0.36 nm. The performance of low-voltage pentacene thin-film transistors that implement this dry AlO_x/C₈PA gate dielectric depends on C₈PA desorption time. When the desorption time rises from 25 to 210 min, the field-effect mobility increases from ∼0.02 to ∼0.04 cm²/V s, threshold voltage rises from ∼−1.2 to ∼−1.4 V, sub-threshold slope
decreases from ∼120 to ∼80 mV/decade, off-current decreases from ∼5 × 10⁻¹² to ∼1 × 10⁻¹² A, on/off current ratio rises from ∼3.8 × 10⁴ to ∼2.5 × 10⁵, and the transistor hysteresis decreases from 61 to 26 mV. 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.