Single-crystal organic field effect transistors with the hole mobility ∼8 cm2/V s

Abstract

We report on the fabrication and characterization of single-crystal organic p-type field-effect transistors (OFETs) with the fieldeffect hole mobility µ ~ 8 cm 2 /V⋅s, substantially higher than that observed in thin-film OFETs. The single-crystal devices compare favorably with thin-film OFETs not only in this respect: the mobility for the single-crystal devices is nearly independent of the gate voltage and the field effect onset is very sharp. Subthreshold slope as small as S = 0.85 V/decade has been observed for a gate insulator capacitance C i = 2 ± 0.2 nF/cm 2 . This corresponds to the intrinsic subthreshold slope S i ≡ SC i at least one order of magnitude smaller than that for the best thin-film OFETs and amorphous hydrogenated silicon (α-Si:H) devices. a) Electronic mail: podzorov@physics.rutgers.edu b) Also at P. N. Lebedev Physics Institute, 119991 Moscow, RussiaThe quest for high-performance organic field-effect transistors (OFETs) has resulted in a significant increase of the charge carrier mobility µ. 1 In the best devices based on thin organic films, values of µ up to ~ 1.5 cm 2 /V⋅s have been reported. 2 This performance is already comparable with that of amorphous-silicon FETs. 3 However, there are still several important issues to be resolved, most of them being associated with grain boundaries and interfacial disorder in organic thin films. Indeed, currently these structural defects are the major factor which limits the mobility, 2, 4 causes the dependence of the mobility on the gate voltage, 5,6 and results in the broadening of the on/off transition. 7 Grain boundaries can be eliminated in devices fabricated on single crystals of organic semiconductors, which enables to explore the role of other factors.Recently, we developed a technique for the fabrication of single-crystal OFETs, 8 which allowed us to completely eliminate the inter-crystalline boundaries. In this Letter, we report on the optimization of this technique, which has resulted in a dramatic increase of the field effect hole mobility up to µ ~ 8 cm 2 /V⋅s. The large magnitude of µ is not the only advantage of the single-crystal devices: their mobility is nearly gate-voltage independent, and the onset of conductivity is very sharp. Comparison between single-crystal and thin-film OFETs helps to identify the characteristics of the latter devices, which are associated with structural defects.High-quality rubrene crystals have been grown from the vapor phase in a stream of ultra-high-purity hydrogen in a horizontal reactor. 9 Several key factors affect the crystal quality. One of the important parameters is the difference in temperature between the sublimation zone and the growth zone, ∆T = T sblm -T growth , which is an analog of the supersaturation at thermal equilibrium. The regime of small supersaturation, when T sblm is set close to the sublimation threshold of rubrene, is crucial for the mobility improvement. The crystal growth in this regime proceeds by the flow of steps at a very low rate (≤ 5×10 -7 cm/s in a direction perpendicula...

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