53 research outputs found
Superlattices and NiPi structures in new forms of cascade solar cells
The activity in the field of photovoltaic semiconductor superstructures is described. Progress was accomplished in the two principal directions previously defined in our initial proposal, i.e.: (1) Theoretical investigation of the optical properties of superlattices; and (2) New solar cell concepts and device modeling. Although important information concerning the optical constants of superlattices and multiple quantum well structures was obtained from our computer model, most of the theoretical efforts have progressively shifted from the former to the latter aspect of the project because of the discovery of a new kind of photovoltaic device which may exhibit improved performances with respect to conventional solar cells
Joule heating induced negative differential resistance in free standing metallic carbon nanotubes
The features of the characteristics of metallic carbon nanotubes (m-NTs)
in different experimental setups are studied using semi-classical Boltzmann
transport equation together with the heat dissipation equation to account for
significant thermal effects at high electric bias. Our model predicts that the
shape of the m-NT characteristics is basically controlled by heat removal
mechanisms. In particular we show that the onset of negative differential
resistance in free standing nanotubes finds its origins in strong transport
nonlinearities associated with poor heat removal unlike in substrate-supported
nanotubes.Comment: 3 pages, 3 figure
Optical constants of GaAs-AlGaAs superlattices and multiple quantum wells
The optical properties of GaAs-Al sub x Ga sub 1-xAs superlattices are calculated as a function of the frequency and superlattice structure. The comutations are performed using a partition method which combines the vectors k.p method with the pseudopotential technique. The influence of the super-structure on the electronic properties of the systems is accounted for by appropriate quantization conditions. The anisotropy and structure dependence of the dielectric constant result mainly from the contribution of the gamma region while the contributions of the other regions of the Brillouin zone are rather insensitive to the superlattice structure. The superlattice index of refraction values are shown to attain maxima at the various quantized transition energies, where for certain structures, the difference between the refractive indices of the superlattices and its corresponding Al sub x Ga sub 1-xAs alloy can be as large as 2%. In general results are in good agreement with the experimental data
Soft Carrier Multiplications by Hot Electrons in Graphene
By using Boltzmann formalism, we show that carrier multiplication by impact
ionization can take place at relatively low electric fields during electronic
transport in graphene. Because of the absence of energy gap, this effect is not
characterized by a field threshold unlike in conventional semiconductors, but
is a quadratic function of the electric field. We also show that the resulting
current is an increasing function of the electronic temperature, but decreases
with increasing carrier concentration
Engineering Exchange Coupling in Double Elliptic Quantum Dots
Coupled elliptic quantum dots with different aspect ratios containing up to
two electrons are studied using a model confinement potential in the presence
of magnetic fields. Single and two particle Schroedinger equations are solved
using numerical exact diagonolization to obtain the exchange energy and
chemical potentials. As the ratio between the confinement strengths in
directions perpendicular and parallel to the coupling direction of the double
dots increases, the exchange energy at zero magnetic field increases, while the
magnetic field of the singlet-triplet transition decreases. By investigating
the charge stability diagram, we find that as inter-dot detuning increases, the
absolute value of the exchange energy increases superlinearly followed by
saturation. This behavior is attributed to the electron density differences
between the singlet and triplet states in the assymetric quantum dot systems.Comment: 5 pages, 5 figure
Superstructure high efficiency photovoltaics
A novel class of photovoltaic cascade structures is introduced which features multijunction upper subcells. These superstructure high efficiency photovoltaics (SHEP's) exhibit enhanced upper subcell spectral response because of the additional junctions which serve to reduce bulk recombination losses by decreasing the mean collection distance for photogenerated minority carriers. Two possible electrical configurations were studied and compared: a three-terminal scheme that allows both subcells to be operated at their individual maximum power points and a two-terminal configuration with an intercell ohmic contact for series interconnection. The three-terminal devices were found to be superior both in terms of beginning-of-life expectancy and radiation tolerance. Realistic simulations of three-terminal AlGaAs/GaAs SHEP's show that one sun AMO efficiencies in excess of 26 percent are possible
Semiconductor superlattice photodetectors
Superlattice photodetectors were investigated. A few major physical processes in the quantum-well heterostructures related to the photon detection and electron conduction mechanisms, the field effect on the wave functions and the energy levels of the electrons, and the optical absorption with and without the photon assistance were studied
Semiconductor superlattice photodetectors
A superlattice photomultiplier and a photodetector based on the real space transfer mechanism were studied. The wavelength for the first device is of the order of a micron or flexible corresponding to the bandgap absorption in a semiconductor. The wavelength for the second device is in the micron range (about 2 to 12 microns) corresponding to the energy of the conduction band edge discontinuity between an Al/(sub x)Ga(sub 1-x)As and GaAs interface. Both devices are described
Semiconductor superlattice photodetectors
Two novel types of superlattice photodetectors were studied. The first was a superlattice photomultiplier and the second a photodetector based on the real space transfer mechanism. A summary of the results is presented
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