Low-voltage pentacene organic thin-film transistor by using high-k gate dielectric

Abstract

Organic thin-film transistor (OTFT) has wide application in flexible and wearable electronic products due to its flexibility, easy fabrication and low cost. However, applications of OTFT are facing two major challenges. The first is low speed caused by the low carrier mobility of OTFT as compared with other types of TFT. The second is high operating voltage due to the large threshold voltage of OTFT. Therefore, the main purpose of this research is to investigate new gate dielectrics as well as surface modifications for increasing the carrier mobility and reducing the threshold voltage of OTFT with pentacene as the organic semiconductor.

Firstly, the influence of fluorine plasma treatment on the performance of pentacene OTFT with LaZrO as gate dielectric is investigated. After treating the dielectric in a fluorine plasma, the carrier mobility of the transistor can be greatly improved to 0.72 〖cm〗^2/V·s, which is more than 40 tim es that of one without the plasma treatment. The major reasons should be fewer interface traps and thus larger pentacene grains induced by the fluorine plasma. Moreover, after exposure to the atmosphere without encapsulation for 6 months, all the devices still display good transistor characteristics.

Then, the effects of La incorporation in three transition-metal (TM = Y, Zr and Nb) oxides as gate dielectric of pentacene OTFT have been studied. La incorporated in Zr oxide and Nb oxide greatly decreases their trap density by passivating their oxygen vacancies, resulting in larger pentacene grains grown on them and thus higher carrier mobility for the OTFT due to less grain-boundary scattering. The carrier mobility of the ZrLaO- and NbLaO-OTFTs is about 70 times and 300 times higher than that of their counterparts based on ZrO2 and Nb2O5, respectively. However, La incorporated in Y2O3 increases its trap density by roughening its surface, causing smaller pentacene grains grown and thus lower carrier mobility.

Moreover, the effects of La content (x) on pentacene OTFTs with LaxTa(1-x)Oy or LaxNb(1-x)Oy as gate dielectric are investigated. The OTFT with La0.764Ta0.236Oy achieves a high carrier mobility of 1.21 〖cm〗^2/V·s, which is about 40 times and two times higher than those of the devices using Ta oxide and La oxide, respectively. On the other hand, the OTFT with La0.648Nb0.352Oy as gate dielectric has a small threshold voltage of -1.35 V, despite a slightly lower carrier mobility of 1.14 〖cm〗^2 V^(-1) s^(-1).

By using flexible vacuum tape as the substrate, pentacene OTFT with La0.850Nb0.150Oy as gate dielectric is fabricated at a low temperature of 200 ºC and obtains a very high carrier mobility of 5.32 〖cm〗^2 V^(-1) s^(-1)with negligible hysteresis of -0.032 V and small sub-threshold swing of 0.174 V/dec.

Lastly, by including the generation-recombination process of charge carriers in conduction channel, a model for the low-frequency noise of pentacene OTFT is proposed. In this model, the slope and magnitude of power spectral density for low-frequency noise are related to the traps in the gate dielectric and accumulation layer of the OTFT.