High-mobility low-threshold-voltage pentacene thin-film transistors prepared at rapid growth rates by pulsed-laser deposition

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Abstract
Organic thin films are of increasing interest as active media in thin-film electronic devices. Pulsed-laser deposition (PLD) constitutes a novel and highly promising but at present ill-characterized recent addition to available fabrication techniques. In this paper, we report very promising measurements on PLD films of pentacene, comparing them with the characteristics of their more conventional counterparts obtained by thermal evaporation (TE). It is shown that PLD is capable of achieving significant improvements in device characteristics, whilst simultaneously allowing films to be deposited at least 100 times faster than TE. Surface morphology analysis by atomic force microscopy and scanning electron microscopy suggests that the enhanced properties are associated with appreciably improved molecular ordering. PLD thin-film transistors (TFTs) deposited onto room-temperature substrates - 2 2- 1- 1 exhibit a field-effect mobility of 3 10 cm V s, a 0.25 V threshold FE voltage, and an on-to-off current ratio of more than 1400. In contrast, TE devices prepared under otherwise identical conditions exhibit a of only FE - 4 2- 1- 1 about 10 cm V s, a 0.8 V threshold voltage and an on-to-off ratio of 240. The mobility values for the PLD TFTs are already the highest reported for undoped pentacene devices and are sufficient to make the material viable for prototype active circuits. Moreover, our most recent experiments have established that raising the substrate temperature during PLD deposition to 473 K yields a dramatic reduction in the surface roughness of films, to a value of 0.4 nm, which is comparable with the dimensions of the pentacene molecule! This is accompanied by further improvements in electrical conductivity, offering exciting possibilities for devices of even higher performance.

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