In last decade the organic field effect transistors (OFETs) have drawn our attention in electronics. Recent intensive studies have revealed that carrier injection process dominantly rules the performance of OFETs. However it is still not sufficient to discuss the details of the carrier injection. Here we employ a novel time-resolved microscopic second harmonic generation (TRM-SHG) measurement that enables us to distinguish carrier injection process from carrier transport process, by measuring the migration of electric field caused by injected carrier motion. [1,2]. We show that SHG signal enhanced from the region near the source electrode decays after applying a pulse gate voltage Vgs at Vgd =0 V, but it strongly depends on the metal work-function of electrodes (Au, Ag). Further the enhanced SHG decay is also dependent on the amplitude of Vgs, suggesting that a space charge field formed by injected carriers dominates the decay of the SHG intensity. In this report, with defining the interfacial resistance based on a Schottky injection process, we discuss the details of the decay of SHG observed near the source electrode in terms of carrier injection and contact resistance, and conclude that contact resistance could be changeable depending on the behavior of injected carriers in OFETs.
Sample used in the experiment was top-contact pentacene FET with Ag source-drain electrodes. Laser pulse (wavelength: 1120nm) was focused onto the channel region (Fig.1). The SHG signal was mainly enhanced near the edge of the source electrode. According to the SHG signal of pentacene film, the enhanced SH intensity was in proportion to the square of the local electric field formed in pentacene layer, and it decayed as shown in Fig.2. This result suggests that injected carriers form a space charge field, which gradually cancels out the Laplace field generated in the OFET. Interestingly, the enhanced SH intensity decay was dependent on the amplitude of Vgs with different relaxation times (Fig.2). Based on these results, we argue that an increase of space charge field at the organic-metal interface leads to decrease of interface resistance.
各種検索
サポート
ヘルプ