Gate Screening on Remote Phonon Scattering for Pentacene Organic TFTs: Holes Versus Electrons

Abstract

Bottom-gated pentacene organic thin-film transistors (OTFTs) were fabricated on p-type and n-type Si wafers with different resistivities, with NdTaON as a high- ${k}$ gate dielectric. Although the pentacene grain size and dielectric surface roughness were nearly the same for all the samples, the carrier mobility improved with decreasing resistivity for both types of OTFTs (p-gate and n-gate), indicating that similar to the electron concentration, the hole concentration in the gate electrode can also affect the carrier mobility. This is attributed to the remote phonon scattering of the high- ${k}$ gate dielectric on the channel carriers, which can be screened by the holes in the gate electrode. The p-gate and n-gate samples with the lowest gate–electrode resistivity of $0.005~\Omega \cdot $ cm achieve the highest mobility of 1.15 and 1.37 cm $^{{2}}/\text{V}\cdot \text{s}$ , respectively. Moreover, for the same gate-doping concentration, the p-gate sample shows lower mobility than the n-gate sample. This is likely because under a negative gate voltage ( ${V}_{g}$ ), the p-gate is depleted to make the actual ${V}_{g}$ smaller than the applied ${V}_{g}$ , thus resulting in a lower extracted carrier mobility. In addition, such a depletion effect leads to a lower hole concentration near the gate–electrode/gate–dielectric interface, thus weakening the gate screening effect on the remote phonon scattering.