Electron and spin transport studies of gated lateral organic devices

Abstract

In view of the many, often contradictory, reports of magneto-resistance (MR) in spin valve stacks containing a layer of organic semiconductor, mostly of the small molecule variety, we have investigated interdigitated lateral structures with an organic layer deposited in the narrow gap between two ferromagnetic electrodes, which are well-suited for studying charge and spin transport in novel (high resistivity) semiconducting materials. For the channel material we used three different organic semiconductors, the small molecule tris-(8-hydroxyquinoline) aluminum (Alq3), single crystals of pentacene, and the conductive polymer poly(3-hexylthiophene) (P3HT). The channel length was 80 nm. Temperature-dependent current-voltage characteristics reveal that in all instances the current is limited by field-assisted thermionic injection over an energy barrier at the metal/organic interface. No measurable magneto-resistance was observed down to 7 K. The interface energy barrier, together with the vastly different electronic structure of metals and organics close to the Fermi level, preclude spin injection. Nonetheless, unlike the case of inorganic semiconductors, the insertion of an artificial tunnel barrier at the contact did not improve spin injection. Gate-dependent measurements exhibited short-channel effects and transistor operation with on/off ratios of 103, but no magneto-resistance. We suggest the observations are a consequence of the formation of bipolaron-states at increasing carrier concentration.