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Nanostructures
for Terahertz Frequencies
Many of the devices operating in
the terahertz frequency range, such as the quantum cascade laser and quantum
well infrared photodetectors, rely on intersubband transitions of electrons
in the conduction band. These devices
work very well at liquid helium temperatures where thermal effects are
suppressed, but at room temperature kT is 25 meV, and 1 THz is only 4 meV. Quantum well based devices are
subject to huge amounts of in plane scattering as the temperature increases,
which causes the devices to cease to operate.
It has been shown however that increasing the confinement of the electrons
so that they are no longer free to move in plane increases the efficiency of
these devices. However, current
methods of doing this usually involve applying large magnetic fields perpendicular
to the quantum well plane, which is not practical for creating a device. Quantum posts are the first
nanostructure which is able to provide this confinement with intersubband
transitions that occur in the THz region.
The quantum posts are self-assembled nanostructures based on epitaxial
quantum dots. They are roughly
cylindrical regions of InGaAs embedded in GaAs.
Figure: Cross sectional TEM of two
quantum posts. They are the lighter slightly
conical regions. The posts are grown using
molecular beam epitaxy (MBE), and stacking quantum dots on top of each other
so that they actually touch. This
gives us essentially a large quantum dot with a height adjustable by the
number of cycles we grow. The exciting
thing is that we control the energy of the intersubband transitions in the
posts by engineering the height during growth. Posts of different heights will resonate at
different terahertz frequencies.
Current work on this
project focuses on demonstrating this height dependence of the resonant
frequencies, as well as attempts to implement the posts into cascade
structures. Please refer to the publications
section of the site for more information on the quantum posts. Back to research. |
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