Surface scattering in molecular field-effect devices

• Applicant: Professor Dr. Roland Kersting
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 551242636

Although many applications of organic field-effect transistors (FETs) are proposed, the fundamental properties of charge carrier transport in these devices are far from being understood. Recent works indicate that surface roughness scattering within the channel region limits the carriers' mobility in thin-film FETs of molecular semiconductors. Such scattering events occur on scales comparable to a few unit cells of the crystalline lattice. Investigations of the underlying physics would therefore profit from an experimental technique that exclusively probes local transport properties. We propose to apply THz electromodulation spectroscopy for studying the impact of interface roughness on carrier transport in high-mobility molecular semiconductors. The technique probes the transport of mobile charge carriers, but is insensitive to the impact of grain boundaries, traps, and injection barriers. This will permit an undisturbed view onto the intrinsic transport properties. Temperature-dependent measurements will allow roughness scattering to be distinguished from other processes, such as phonon scattering or dynamic localization. Planarization and chemical modification of the insulator beneath the molecular semiconductor will provide access to the physical origin of surface roughness scattering. The physical understanding of the experimental data will be supported by model calculations of the scattering processes as well as by self-consistent calculations of the carrier distributions within the structures. One important question is how local scattering affects the transport across macroscopic scales as they appear in FETs. This will be studied by comparing the THz data with transport measurements obtained by classical characterization methods such as IV characteristics of FETs and van der Pauw measurements. The expected results advance not only fundamental understanding, but may also stimulate the development of novel device structures, which may lead to a significant leap in mobilities that brings organic FETs closer to application.

• DFG Programme: Research Grants