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

Abstract

A self‐assembled monolayer of n‐octylphosphonic acid (C8PA) is prepared from vapor phase in vacuum. C8PA thickness corresponding to several monolayers is deposited on aluminum oxide (AlOx) and subsequently heated to leave a monolayer of chemisorbed molecules. The effect of C8PA deposition rate on a 15‐nm‐thick, bilayer AlOx/C8PA 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 cm2/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 × 10−12A, the subthreshold slope is lowered from 100 to 89 mV/decade and the on/off current ratio is increased from ∼105 to ∼106. The leakage current density of AlOx is reduced from 1 × 10−7 to 4 × 10−8 A/cm2 at 3 V when C8PA monolayer is added on top of it. In addition, pentacene grain size on AlOx/C8PA is larger than that on AlOx. The overall performance of AlOx/C8PA OTFTs is superior to that of AlOx OTFTs.

Ozone oxidation methods for aluminum oxide formation: Application to low-voltage organic transistors

“…The presence of monolayer on top of aluminum oxide reduces the leakage current and changes the hydrophilic surface into hydrophobic [8]. The vacuum vapor growth of C 8 PA monolayer was optimized with respect to the post-deposition annealing time [19,20], deposition temperature [21], and deposition rate [22]. All capacitors were completed by evaporating a 50-nm-thick gold layer [19].…”