The role of Ca traces in the passivation of silicon dioxide dielectrics for electron transport in pentacene organic field effect transistors

Recently, n-type transport in organic field effect transistors (OFETs) incorporating pentacene on a silicon dioxide (SiO(2)) dielectric has been demonstrated by Ahles et al. [Appl. Phys. Lett. 85, 4499 (2004)]. The electron transport was made possible by modifying the dielectric/semiconductor interface using traces of Ca. While the facilitation of electron current in pentacene remained unclear at that point, an interface near filling of electron trap states in the transistor channel or on the SiO(2) dielectric could be suggested as a possible explanation. In the following the influence of the Ca interlayer on the n-type transport in pentacene based OFETs will be correlated with an x-ray photoelectron spectroscopy analysis of the SiO(2)/Ca interface, in dependence of the Ca layer thickness. It is demonstrated that for low thicknesses an oxidized Ca insulator is formed on the SiO(2) dielectric, allowing for the observed pentacene electron transport. The formation of the oxide is suggested to compensate available electron traps, in the form of hydroxyl groups on the SiO(2) surface, thus reducing the trap density and isolating remaining interface traps from the transistor channel. Furthermore, it is substantiated for an increased Ca layer thickness, that metallic Ca remains in the oxidized Ca interlayer. This leads to a substantial degradation of the OFET charge carrier transport properties. It is found that up to a Ca layer thickness of similar to 12 angstrom, the metallic Ca fraction in the oxidized Ca layer can be effectively reduced by thermal or cyclic electrical stress, leading to a permanent improvement in the n-type OFET transport properties. (c) 2008 American Institute of Physics.