La electrónica de semiconductores
genera, controla y amplifica la corriente eléctrica en dispositivos como los
transistores. Los portadores de corriente eléctrica son los electrones , lo
cuales se mueven a altas velocidades (10000 a algunos millones de metros por
segundo) en las estructuras cristalinas de los semiconductores. Sin embargo al
hacer esto, pierden una parte de su energía cinética al chocar con los átomos
en dichas estructuras, y como la energía no se pierde, se transmite en forma de
vibración a dichos átomos. En el arsenuro de galio por ejemplo dichas
vibraciones tienen un periodo de 100fs (1 fs = 10-15 s = 1 billionth part of one millionth of a
second).
In the microcosm of electrons and ions such vibrations are quantized.
This means that the vibrational energy can only be an integer multiple of a
vibrational quantum, also known as a phonon. 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 such vibrational
quanta.
Berlin researchers report in the
latest issue of the scientific journal Physical Review Letters that
the strength of the electron-phonon interaction depends sensitively on the
electron size, i.e., on the spatial extent of its charge cloud. Experiments in
the time range of the lattice vibration show that 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 quasi particle, a polaron.
To visualize this phenomenon, the researchers used a nanostructure made
from gallium arsenide and gallium aluminum arsenide, in which the energies of
the movements of electrons and ions were tuned to each other. The coupling of
both movements was shown by a new optical technique. Several ultrashort light
pulses in the infrared excite the system under study. The subsequent emission
of light by the moving charge carriers is measured in real time. In this way
two-dimensional nonlinear spectra (see Fig.) are generated, which allow the
detailed investigation of coupled transitions and the determination of the
electron-phonon coupling strength. From the coupling strength one finds the
size of the electron cloud, which is just 3-4 nanometers. Furthermore, this new
method shows for the first time the importance of electron-phonon coupling for
optical spectra of semiconductors. This is of interest for the development of
optoelectronic devices with custom-tailored optical and electric properties.
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