Berlin researchers use 2D terahertz spectroscopy to better understand semiconductors

Aug. 5, 2011
Researchers from several Berlin institutions have banded together to try to better understand the nature of electrons in semiconductors, using two-dimensional (2D) terahertz spectroscopy to visualize the link between an electron cloud's size and the strength of its interaction with the semiconductor.

Berlin, Germany--Researchers from several Berlin institutions have banded together to try to better understand the nature of electrons in semiconductors, using two-dimensional (2D) terahertz spectroscopy to visualize the link between an electron cloud's size and the strength of its interaction with the semiconductor.1 The researchers hail from the Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, the Paul-Drude-Institut für Festkörperelektronik, and the Helmholtz-Zentrum Berlin für Materialien und Energie.

They have shown that electrons in a semiconductor are best described as a cloud with a size of a few nanometers. The cloud size is determined by the interaction of the electron with vibrations in the crystal lattice.

The carriers of an electric current through a semiconductor are mobile electrons that move with high velocities through the crystal lattice; in the process, they lose part of their kinetic energy to the lattice. In semiconductors like gallium arsenide, the positively and negatively charged ions of the crystal lattice vibrate with a period of about 100 fs; the vibrations are quantized in units of phonons. When an electron interacts with the crystal lattice (the so-called electron-phonon interaction), energy is transferred from the electron to the lattice in the form of phonons.

Smaller electron cloud, more interaction

The researchers found that the strength of the electron-phonon interaction depends sensitively on the spatial extent of the electron's charge cloud (or, more colloquially, the electron's "size"); reducing the electron size leads to an increase of the interaction by up to a factor of 50. This results in a strong coupling of the movements of electrons and ions. Electron and phonon together form a new quasiparticle called a polaron.

To visualize this phenomenon, the researchers created double quantum wells from gallium arsenide and gallium aluminum arsenide (GaAs/AlGaAs), in which the energies of the movements of electrons and ions were tuned to each other. The electron-phonon interactions were studied with 2D terahertz spectroscopy (the terahertz pulses were created by difference-frequency mixing of two IR pulses). The many peaks in the 2D spectrum revealed the presence of polarons and helped determine the electron-phonon coupling strengthleading to a measurement of the electron-cloud size, which was 3 to 4 nm.

The new method shows the importance of electron-phonon coupling in the optical spectra of semiconductors, which will be of interest for the development of optoelectronic devices with custom-tailored optical and electric properties.

REFERENCE:

1. W. Kuehn et al., Phys. Rev. Lett. 107, 067401 (2011); J. Phys. Chem. B 115, 5448 (2011).

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

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