Prepare with care: Low contact resistance of pentacene Field-Effect transistors with clean and oxidized gold electrodes

Abstract In recent years, a number of researchers have voiced concerns regarding the increasing amount of publications which report record performance characteristics of organic fieldeffect transistors (OFETs). These devices often make use of elaborate architectures, ranging from surface treatment by selfassembled monolayers to contactlimited doping, injection layer introduction etc. While these techniques are claimed to improve the device performance, they often result in poorly defined device interfaces, such as that of the organic semiconductor with the dielectric and electrode surfaces, which render a rigorous theoretical description of the interface effects close to impossible. Furthermore, the effects of ambient exposure of prepared devices are often neglected during the analysis. In this work we introduce a completely highvacuumbased manufacturing and electrical characterization of OFETs using commercially available gold bottomcontact bottomgate FET structures, which enables a rigorous exclusion of any air contact of the devices. Using the example of the prototypical and widely studied organic semiconductor pentacene, we compare various initial cleaning procedures of the FETstructures prior to the vacuum deposition of pentacene films on the device characteristics and complement the electrical in situ analysis by ex situ AFM measurements of the pentacene film morphology. We demonstrate that O 2 plasmacleaning with subsequent thermal annealing under vacuum conditions results in welldefined interfaces of the pentacene film with clean surface of gold electrodes, which exhibit a remarkably low width normalized contact resistance of 1.19  kΩ cm. Additionally, we show that the formation of gold oxide on top of the electrodes as a result of O 2 plasmacleaning without subsequent heating is beneficial for the charge carrier injection, resulting in a contact resistance of only 0.16  kΩ cm – to the best of our knowledge, one of the lowest yet reported value for pentacenegold systems. In addition, the comparative rigorous vacuumbased processing and analysis allows in particular to study also the effects of controlled exposure to ambient air – even for a short time – as well as aging under different storage conditions on the device performance.