Paper No P39: Optimizing the Deposition Rate of Vacuum‐Grown n‐Octylphosphonic acid Monolayer for Low‐Voltage Thin‐Film Transistors

A self‐assembled monolayer of n‐octylphosphonic acid (C 8 PA) is prepared from vapor phase in vacuum. C 8 PA thickness corresponding to several monolayers is deposited on aluminum oxide (AlO x ) and subsequently heated to leave a monolayer of chemisorbed molecules. The effect of C 8 PA deposition rate on a 15‐nm‐thick, bilayer AlO x /C 8 PA dielectric and low‐voltage p‐channel organic thin‐film transistors (OTFTs) is studied. The increase in the deposition rate from 0.1 to 7.0 Å/s leads to increase in the field‐effect mobility from 0.039 to 0.061 cm 2 /Vs, while the threshold voltage remains around −1.55 V. At the same time, the off‐current is reduced from 2.3 × 10 −12 to 1.3 × 10 −12 A, the subthreshold slope is lowered from 100 to 89 mV/decade and the on/off current ratio is increased from ∼10 5 to ∼10 6 . The leakage current density of AlO x is reduced from 1 × 10 −7 to 4 × 10 −8 A/cm 2 at 3 V when C 8 PA monolayer is added on top of it. In addition, pentacene grain size on AlO x /C 8 PA is larger than that on AlO x . The overall performance of AlO x /C 8 PA OTFTs is superior to that of AlO x OTFTs.