A study on low-voltage pentacene organic thin-film transistors with Nd-based and La-based oxides as gate dielectrics

Abstract

Recently, organic thin-film transistors (OTFTs) are widely applied in flexible electronic products due to their low cost, compatible fabrication and high flexibility. However, OTFTs still face some challenges. The first is the high operating voltage due to the large threshold voltages of OTFTs. As compared to other thin-film transistors (TFTs), another challenge is the low carrier mobility of OTFTs, leading to lower speed in electronic applications. Therefore, through the investigations on new gate dielectrics and dielectric-surface modification, the main objective of this research is to reduce the threshold voltage and improve the carrier mobility of pentacene OTFTs. Firstly, the effects of gate electron concentration on the performance of pentacene OTFT with neodymium-tantalum oxide (NdTaON) as gate dielectric is studied. By using n-type Si-gate with a high doping concentration of 1.3 x 1019/cm3, the carrier mobility of the transistor can be up to 1.47 cm2/V·s, which is much higher than 0.69 cm2/V·s for a lower doping concentration of 1.5 x 1015/cm3. The reason for the mobility improvement should be due to the gate screening on the remote phonon scattering of the high-k dielectric layer. More plasmons generated in the Si-gate with higher doping concentration can enhance the coupling with the dipoles in the gate dielectric to reduce the remote phonon scattering on the charge carriers in the pentacene layer. Then, the screening effect of p-type Si gate is compared with that of n-type Si for pentacene OTFT’s with NdTaON gate dielectric. Experimental results indicate that p-Si gate with higher doping concentration can also provide stronger screening effect, thus improving the carrier mobility. Moreover, for the same doping concentration, the carrier mobility of a p-Si gate device is lower than that of its n-Si gate counterpart, which should be due to that the formation of a depletion region in the p-Si gate under a negative operating voltage. Moreover, pentacene OTFTs with NdxNb(1-x)O, NdxNb(1-x)ON and NdxHf(1-x)ON as gate dielectrics are investigated. The OTFTs with Nd0.95Nb0.05O, Nd0.95Nb0.05ON and Nd0.85Hf0.15ON can achieve carrier mobility of 2.00, 2.24 and 2.16 cm2/V·s respectively, which are all over 20 times higher than that of a control device with pure Nd oxide as gate dielectric. Meanwhile, the threshold voltages of the devices are as low as -1.57 V, -0.79 V and -0.91 V, respectively. Besides, by using glass as the substrate and ITO as the gate electrode, a transparent pentacene OTFT with Nd0.76Ta0.24ON gate dielectric is fabricated at a low temperature of 200 ℃, and a low threshold voltage of -0.2 V as well as a high mobility of 2.32 cm2/V·s are obtained. Lastly, a pentacene OTFT with La0.87Ti0.13ON gate dielectric fabricated at a low temperature of 200℃ achieves a high mobility of 2.6 cm2/V·s, low threshold of -1.5 V and small sub-threshold swing of 0.07 V/dec. Besides, with LaTiON as the gate dielectric, the effects of the gate-electrode/gate-dielectric interlayer on the carrier mobility are further explored, indicating that thicker interlayer can reduce the gate screening on the remote phonon scattering to degrade the carrier mobility.