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Title: Organic field-effect transistors
Author: Chua, L.-L.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2007
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In this thesis, we demonstrated that divinyltetramethyldisiloxane-benzocyclobutene (BCB), which has previously been used as an isolation dielectric in III-IV semiconductor devices, in fact makes an excellent gate dielectric material in OFETs after suitable purification. Robust ultra-thin films with high glass transition temperature and high dielectric breakdown strength can be obtained by simple spin-coating followed by rapid-thermal-anneal to above 250°C. With this material, we were able to demonstrate remarkable performance in polymer OFETs and explore several aspects of their physics. In Chapter 2, we introduce the use of BCB as a good candidate for solution-processable organic gate dielectric. Pinhole-free ultra-thin gate dielectric film as thin as 50nm can be made from this material. With this gate dielectric, robust continual cyclic operation of poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(phenylene-(N-(p-2-butylphenyl-imino-phenylene)) (TFB) FETs at 120°C was achieved. Previously, the thinnest practical solution-processable gate dielectric thickness was >300 nm-thick. In Chapter 3, we demonstrated self-organised polymer semiconductor/dielectric FETs fabricated using a spontaneous and an unusual vertical phase separation of the TFB polymer semiconductor and the BCB dielectric materials during film spinning. This method enables the formation of semiconductor and dielectric layers at the same time without exposing their interface to air. Using these devices, we established that a critical root-mean-square interface roughness of 0.7 nm (measured on the 100 nm length scale) could be tolerated without loss of mobility of the devices, probably related to the hopping of the carries at the interface. In Chapter 4, we demonstrated using this non-trapping BCB dielectric the generality of n-type field-effect conduction across a wide range of polymer organic semiconductors. We showed that this was previously suppressed by interface trapping of the accumulated electrons by the –OH group in the gate dielectrics that have often been used. We found electron mobilities very similar to, if not larger than, hole mobilities across a range of organic semiconductors. Therefore, many (though not all) π-conjugated materials are by their nature ambipolar and can support both electron and hole conduction nearly equally well. Their previous classification into “n-type” and “p-type” materials is thus somewhat arbitrary. Finally, in Chapter 5, we used BCB as the top gate dielectric and fabricated fully functional double-gate OFETs over a bottom gate dielectric. We showed that such devices exhibit electrostatic coupling of the two gates occurs to produce an “AND” logic gate.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available