Approaching the Trap-Free Limit in Organic Single-Crystal Field-Effect Transistors

We present measurements of rubrene single-crystal field-effect transistors with textbooklike transfer characteristics, as one would expect for intrinsically trap-free semiconductor devices. Particularly, the high purity of the crystals and the defect-free interface to the gate dielectric are reflected in an unprecedentedly low subthreshold swing of 65 mV/decade, remarkably close to the fundamental limit of 58.5 mV/decade. From these measurements, we quantify the residual density of traps by a detailed analysis of the subthreshold regime, including a full numerical simulation. An exceedingly low trap density of D-bulk = 1 x 10(13) cm(-3) eV(-1) at an energy of approximately 0.62 eV is found. This result corresponds to one trap per eV in 10(8) rubrene molecules. The equivalent density of traps located at the interface (D-it = 3 x 10(9) cm(-2) eV(-1)) is as low as in the best crystalline Si/Si field-effect transistors. These results highlight the benefit of having van derWaals bonded semiconducting crystals without electronically active states due to broken bonds at the surface.