638 research outputs found
Optimal design and quantum limit for second harmonic generation in semiconductor heterostructures
The optimal design for infrared second harmonic generation (SHG) is
determined for a GaAs-based quantum device using a recently developed genetic
approach. Both compositional parameters and electric field are simultaneously
optimized, and the quantum limit for SHG, set by the trade-off between large
dipole moments (favouring electron delocalization) and large overlaps
(favouring electron localization), is determined. Optimal devices are generally
obtained with an asymmetric double quantum well shape with narrow barriers and
a graded region sideways to the largest well. An electric field is not found to
lead to improved SHG if compositional parameters are optimized.Comment: 5 pages, 2 figures embedded. To apper in J. App. Phys. (Jan 2nd,
2001
Quantum transport in weakly coupled superlattices at low temperature
We report on the study of the electrical current flowing in weakly coupled
superlattice (SL) structures under an applied electric field at very low
temperature, i.e. in the tunneling regime. This low temperature transport is
characterized by an extremely low tunneling probability between adjacent wells.
Experimentally, I(V) curves at low temperature display a striking feature, i.e
a plateau or null differential conductance. A theoretical model based on the
evaluation of scattering rates is developed in order to understand this
behaviour, exploring the different scattering mechanisms in AlGaAs alloys. The
dominant interaction in usual experimental conditions such as ours is found to
be the electron-ionized donors scattering. The existence of the plateau in the
I(V) characteristics is physically explained by a competition between the
electric field localization of the Wannier-Stark electron states in the weakly
coupled quantum wells and the electric field assisted tunneling between
adjacent wells. The influence of the doping concentration and profile as well
as the presence of impurities inside the barrier are discussed
Electronic transport in quantum cascade structures
The transport in complex multiple quantum well heterostructures is
theoretically described. The model is focused on quantum cascade detectors,
which represent an exciting challenge due to the complexity of the structure
containing 7 or 8 quantum wells of different widths. Electronic transport can
be fully described without any adjustable parameter. Diffusion from one subband
to another is calculated with a standard electron-optical phonon hamiltonian,
and the electronic transport results from a parallel flow of electrons using
all the possible paths through the different subbands. Finally, the resistance
of such a complex device is given by a simple expression, with an excellent
agreement with experimental results. This relation involves the sum of
transitions rates between subbands, from one period of the device to the next
one. This relation appears as an Einstein relation adapted to the case of
complex multiple quantum structures.Comment: 6 pages, 5 figures, 1 tabl
Quantum wells, wires and dots with finite barrier: analytical expressions for the bound states
From a careful study of the transcendental equations fulfilled by the bound
state energies of a free particle in a quantum well, cylindrical wire or
spherical dot with finite potential barrier, we have derived analytical
expressions of these energies which reproduce impressively well the numerical
solutions of the corresponding transcendental equations for all confinement
sizes and potential barriers, without any adjustable parameter. These
expressions depend on a unique dimensionless parameter which contains the
barrier height and the sphere, wire or well radius.Comment: 4 pages, 3 figure
Nonlinear optical properties in a quantum well with the hyperbolic confinement potential
We have performed theoretical calculation of the nonlinear optical properties
in a quantum well (QW) with the hyperbolic confinement potential. Calculation
results reveal that the transition energy, oscillator strength, second-order
nonlinear optical rectification (OR), geometric factor and nonlinear optical
absorption (OA) are strongly affected by the parameters () of
the hyperbolic confinement potential. And an increment of the parameter
reduces all these physical quantities, while an increment of the
parameter enhances them, but not for geometric factor. In addition, it
is found that one can control the optical properties of QW by tuning these
parameters.Comment: 16pages,6 figures,Accepted for publication in Physica
Ultimate performance of Quantum Well Infrared Photodetectors in the tunneling regime
Thanks to their wavelength diversity and to their excellent uniformity,
Quantum Well Infrared Photodetectors (QWIP) emerge as potential candidates for
astronomical or defense applications in the very long wavelength infrared
(VLWIR) spectral domain. However, these applications deal with very low
backgrounds and are very stringent on dark current requirements. In this paper,
we present the full electro-optical characterization of a 15 micrometer QWIP,
with emphasis on the dark current measurements. Data exhibit striking features,
such as a plateau regime in the IV curves at low temperature (4 to 25 K). We
show that present theories fail to describe this phenomenon and establish the
need for a fully microscopic approach
Determination of bound state energies for a one-dimensional potential field
A method for determination of bound state energies for an asymmetric quantum
well with an arbitrary shape of the bottom is suggested. It is shown that how
the equation determining the energy levels can be easily derived if one knows
the electron transmission and reflection amplitudes corresponding to the part
of potential inside the well. The results are applied to three difference test
problems.Comment: a file with three figure
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