|Probing Organic Transistors with Infrared Beams|
Silicon-based transistors are well-understood, basic components of contemporary electronic technology. In contrast, there is growing need for the development of electronic devices based on organic polymer materials. Organic field-effect transistors (FETs) are ideal for special applications that require large areas, light weight, and structural flexibility. They also have the advantage of being easy to mass-produce at very low cost. However, even though this class of devices is finding a growing number of applications, electronic processes in organic materials are still not well understood. A group of researchers from the University of California and the ALS has succeeded in probing the intrinsic electronic properties of the charge carriers in organic FETs using infrared spectromicroscopy. The results of their study could help in the future development of sensors, large-area displays, and other plastic electronic components.
Arguably, one of the main challenges in achieving these goals is finding a means of investigating the intrinsic electronic properties of the charge carriers in organic FETs without the need for metallic contacts that could interfere with the properties under study. In FETs, the charge carriers are confined to a nanometer-thick layer at the semiconductor–insulator interface, buried under several layers of the device. This makes it difficult to experimentally study injected charge carriers using some of the most informative experimental techniques in the arsenal of physicists and chemsists, including scanning tunneling microscopy, photoemission spectroscopy, and inelastic x-ray and neutron scattering.
In this research, the scientists instead employed infrared light to study the electronic processes in organic FETs that are based on poly(3-hexylthiophene), a semiconducting polymer featuring exceptionally high charge-carrier mobility. In such materials, charge carriers can induce infrared vibrations of the polymer chain. In addition, when these charges are displaced under the influence of an electric field, they drag the local polarization cloud of the molecular chains with them, forming so-called polarons. The scientists have been able to employ infrared spectroscopy to directly probe the vibrational modes and polarons in a functional organic FET device. This information is difficult or impossible to obtain using other experimental techniques.
Furthermore, infrared spectromicroscopy can also be used to explore the distribution of charges in the conducting channel of the FETs with high spatial resolution, made possible by the exceptionally high brightness and small focal-spot size of the infrared beams at the ALS. Using the infrared Beamline 1.4.3, the researchers were able to acquire infrared spectra from individual spots less than 10 microns in diameter. By scanning the beam over the conducting channel of the device and measuring the infrared spectra in different areas, they were able to map out the density of the charge carriers in different regions and examine its evolution as the applied voltage is increased. Comparisons were made between FETs having insulating layers of either SiO2 or TiO2. The latter, having a high dielectric constant, are much desired in FETs because they allow for a much higher density of charge carriers in their channels than SiO2-based FETs.
However, the measurements revealed severe restrictions of the charge-carrier channel length for TiO2-based devices, a significant departure from the behavior expected for an "ideal" FET, in which the charge density increases linearly with voltage and is uniform in the channel. This is particularly important if one wants to use these insulators in chemical or biological sensors. In general, these experiments indicate that infrared spectroscopy and spectromicroscopy offer researchers unique tools to explore physical phenomena at the nanoscale occurring at the semiconductor–insulator interface in organic FETs. The team hopes to employ the same techniques in the study of other materials in FET devices, including polymers, organic molecular crystals, and transition-metal oxides.
Research conducted by Z.Q. Li, N. Sai, M. Di Ventra, and D.N. Basov (University of California, San Diego); G.M. Wang, D. Moses, and A.J. Heeger (University of California, Sant Barbara); and M.C. Martin (ALS).
Research funding: National Science Foundation, Petroleum Research Fund, and U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.
Publication about this research: Z.Q. Li, G.M. Wang, N. Sai, D. Moses, M.C. Martin, M. Di Ventra, A.J. Heeger, and D.N. Basov, "Infrared imaging of the nanometer-thick accumulation layer in organic field-effect transistors," Nano Letters 6, 224 (2006).