| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 96 === This study uses atomic layer deposition (ALD) to develop ultra-thin high-dielectric-constant (high-k) gate dielectrics for poly-3-hexthylthiophene (P3HT) organic thin film transistors (OTFTs), aiming to minimize the operation voltage of the OTFTs without compromising the field-effect mobility and the on/off ratio. By optimizing the precursor exposure condition to improve the nucleation and coverage of ALD films, we demonstrate that Al2O3 films, HfO2 films and Al2O3/HfO2 alternating laminates can all achieve adequate film quality to function as the gate dielectric at thickness < 50 nm, which is significantly below the minimum thickness of high-k dielectrics previously reported for OTFTs. With high capacitance and low leakage current, all of the three types of ALD high-k dielectrics reduce the threshold voltage (Vth) of OTFT to < 3 V, down from 26.7 V of the control which uses SiO2 as the gate dielectric; the lowest Vth of 1.2 V is achieved with a 10-nm ALD Al2O3 dielectric layer. Although the ALD high-k dielectrics effectively lower the Vth, they also decrease the field-effect mobility of the resultant OTFTs due to the charge trapping effect of their high-polarity end groups. This trade-off is eliminated by modifying the surface of the high-k dielectrics with a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS), whose low-polarity end groups prevent charge-trapping interactions with the P3HT layer. The SAM layer also reduces current leakage through the gate, thereby further lowering the Vth of all OTFTs with ALD high-k dielectrics to < 2 V.
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