Electrons (blue balls) and holes (red balls) show random thermal motion before the terahertz pulse hits the sample. . |
Researchers
at the Max-Born-Institute, Berlin, Germany, have observed the extremely
fast onset of electrical resistance in a semiconductor by following
electron motions in real-time.
When
you first learned about electric currents, you may have asked how the
electrons in a solid material move from the negative to the positive
terminal. In principle, they could move ballistically or ‘fly’ through
the solid, without being affected by the atoms or other charges of the
material. But this actually never happens under normal conditions
because the electrons interact with the vibrating atoms or with
impurities. These collisions typically occur within an extremely short
time, usually about 100 fs (10-13 seconds, or a tenth of a trillionth of
a second). So the electron motion along the material, rather than being
like running down an empty street, is more like trying to walk through a
very dense crowd. Typically, electrons move only with a speed of 1m per
hour, they are slower than snails.
Though
the electrons collide with something very frequently in the material,
these collisions do take a finite time to occur. Just like if you are
walking through a crowd, sometimes there are small empty spaces where
you can walk a little faster for a short distance. If it were possible
to follow the electrons on an extremely fast (femtosecond) time scale,
then you would expect to see that when the battery is first turned on,
for a very short time, the electrons really do fly unperturbed through
the material before they bump into anything.
This
is exactly what scientists at the Max-Born-Institute in Berlin recently
did in a semiconductor material and report in the current issue of the
journal Physical Review Letters.
Extremely short bursts of terahertz light (1 THz = 1012 Hz, 1 trillion
oscillations per second) were used instead of the battery (light has an
electric field, just like a battery) to accelerate optically generated
free electrons in a piece of gallium arsenide. The accelerated electrons
generate another electric field, which, if measured with femtosecond
time resolution, indicates exactly what they are doing. The researchers
saw that the electrons travelled unperturbed in the direction of the
electric field when the battery was first turned on. About 300 fs later,
their velocity slowed down due to collisions.
In
the attached movie, we show a cartoon of what is happening in the
gallium arsenide crystal. Electrons (blue balls) and holes (red balls)
show random thermal motion before the terahertz pulse hits the sample.
The electric field (green arrow) accelerates electrons and holes in
opposite directions. After onset of scattering this motion is slowed
down and results in a heated electron-hole gas, i.e., in faster thermal
motion.
The
present experiments allowed the researchers to determine which type of
collision is mainly responsible for the velocity loss. Interestingly,
they found that the main collision partners were not atomic vibrations
but positively charged particles called holes. A hole is just a missing
electron in the valence band of the semiconductor, which can itself be
viewed as a positively charged particle with a mass 6 times higher than
the electron. Optical excitation of the semiconductor generates both
free electrons and holes which the terahertz bursts, our battery, move
in opposite directions. Because the holes have such a large mass, they
do not move very fast, but they do get in the way of the electrons,
making them slower.
Such
a direct understanding of electric friction will be useful in the
future for designing more efficient and faster electronics, and perhaps
for finding new tricks to reduce electrical resistance.
High-Field Transport in an Electron-Hole Plasma: Transition from Ballistic to Drift Motion
