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