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IR Spectroscopic Techniques Probe Organic Field Effect Transistors

Photonics Spectra
Mar 2006
Daniel S. Burgess

Researchers at the University of California, San Diego, in La Jolla, the University of California, Santa Barbara, and Lawrence Berkeley National Laboratory in Berkeley, Calif., have employed Fourier transform infrared (FTIR) spectroscopy and synchrotron FTIR spectromicroscopy to study organic field effect transistors. Their findings indicate that the techniques are well suited for exploring the intrinsic properties of charge carriers in the devices.

The organic field effect transistors were grid-electrode devices with dimensions of 10 × 14 mm.

Despite interest in the devices for applications from chemical and biological sensors to flexible displays, the electronic processes in organic field effect transistors are not well understood, said Zhiqiang Li, a graduate student in the physics department at the San Diego campus. A particular challenge is finding non-contact means of investigating the charges in the channel region.

Li noted that the structure of field effect transistors makes it difficult to experimentally study injected charge carriers using some of the most informative techniques in the toolbox of physicists and chemists, such as scanning tunneling microscopy, photoemission spectroscopy, and inelastic x-ray and neutron scat-tering. The carriers are confined to a nanometer-thick layer at the semiconductor/insulator interface that is buried under several layers of the device. Experimental investigations thus have tended to involve DC transport probes.

Fourier transform infrared (FTIR) spectroscopy and synchrotron FTIR spectromicroscopy were employed to investigate charge injection in bottom-contact field effect transistors based on poly (3-hexylithiophene), illustrated here in cross section. Two types of gate insulators were used: a 200-nm-thick layer of SiO2 and a bilayer of 6 nm of SiO2 and 180 nm of TiO2. Images courtesy of Zhiqiang Li.

In contrast, he said, FTIR spectroscopy is a noncontact, nondestructive method that is well suited for investigating the mechanism of the electronic transport and the nature of voltage-induced electronic states in the field effect transistors. When mobile electrons or holes in organic materials are displaced under the influence of a DC electric field, he explained, they drag the local “polarization cloud” of the molecular chains with them, forming polarons that can be probed spectroscopically.

Spectromicroscopy further enables the interrogation of charges in the conducting channel with high spatial resolution. The technique relies on the intrinsic brightness of the radiation produced by a synchrotron — in the case of Lawrence Berkeley’s Advanced Light Source, used in the investigators’ work, it is on the order of 200 times brighter at 10 µm than conventional IR sources. The high brightness enables the beam to be focused with little loss to diffraction-limited spot sizes on the order of 2 to 10 µm.

In their research, the scientists studied 10 × 14-mm bottom-contact field effect transistors based on the semiconducting polymer poly(3-hexylthiophene). The devices featured V-shaped electrodes and a gate insulator layer of either 200 nm of SiO2 or 6 nm of SiO2 and 180 nm of TiO2. They employed an FTIR spectrometer from Bruker Optics Inc. of Billerica, Mass., in the spectroscopic studies and a Nicolet microscope and FTIR spectrometer from Thermo Electron Corp. of Waltham, Mass., for spectromicroscopy.

The decay of the charge carrier density with distance from the electrodes were imaged using infrared beam lines from the Advanced Light Source sy nchrotron at Lawrence Berkeley Natinal Laboratory.

They found that the field effect transistors with the SiO2 insulator displayed no change in the injected charge carrier density in the entire conducting channel with lengths of several millimeters, consistent with the behavior of an ideal device. Those with the high dielectric constant SiO2/TiO2 bilayer, in contrast, displayed a rapidly decaying carrier density.

The experiments, Li said, indicate that FTIR spectroscopy and spectromicroscopy offer researchers unique tools to explore organic field effect transistors. IR spectromicroscopy’s ability to evaluate the quality of gate insulators in the devices may play a role in the search for high dielectric constant insulators to replace SiO2 in metal-oxide semiconductor field effect transistor architectures.

The research team hopes to employ the techniques in the study of other materials in the devices, he said, including polymers, organic molecular crystals and transition-metal oxides.

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