HIGH-MOBILITY LOW-THRESHOLD-VOLTAGE PENTACENE THIN-FILM TRANSISTORS PREPARED AT RAPID GROWTH-RATES BY PULSED-LASER DEPOSITION

Organic thin films are of increasing interest as active media in thin- film electronic devices. Pulsed-laser deposition (PLD) constitutes a n ovel and highly promising but at present ill-characterized recent addi tion to available fabrication techniques. In this paper, we report ver y promising measurements on PLD films of pentacene, comparing them wit h the characteristics of their more conventional counterparts obtained by thermal evaporation (TE). It is shown that PLD is capable of achie ving significant improvements in device characteristics, whilst simult aneously allowing films to be deposited at least 100 times faster than TE. Surface morphology analysis by atomic force microscopy and scanni ng electron microscopy suggests that the enhanced properties are assoc iated with appreciably improved molecular ordering. PLD thin-film tran sistors (TFTs) deposited onto room-temperature substrates exhibit a fi eld-effect mobility mu(FE) Of 3 x 10(-2) cm(2) V-1 s(-1), a 0.25 V thr eshold voltage, and an on-to-off current ratio of more than 1400. In c ontrast, TE devices prepared under otherwise identical conditions exhi bit a mu(FE) Of only about 10(-4) cm(2) V-1 s(-1), a 0.8 V threshold v oltage and an on-to-off ratio of 240. The mobility values for the PLD TFTs are already the highest reported for undoped pentacene devices an d are sufficient to make the material viable for prototype active circ uits. Moreover, our most recent experiments have established that rais ing 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 molecul e! This is accompanied by further improvements in electrical conductiv ity, offering exciting possibilities for devices of even higher perfor mance.