Transport characteristics of L-point and Г-point electrons through GaAs-Ga_(1-x)Ai_xAs-GaAs(111} double heterojunctions

We present here a study on the transport characteristics of L‐point and Γ‐point derived electrons through abrupt GaAs–Ga_(1−x)Al_xAs–GaAs(111) double heterojunctions. The use of complex‐k band structures in the tight‐binding approximation and transfer matrices provide a reasonably accurate description of the wave function at the GaAs–Ga_(1−x)Al_xAs interface. A representation of the wave function in terms of bulk complex‐k Bloch states is used in the GaAs regions where the potential is bulklike. A representation of the wave function in terms of planar orbitals is used in the central Ga_(1−x)Al_xAs region where the potential deviates from its bulk value (i.e., interfacial region). Within this theoretical framework, realistic band structure effects are taken into account and no artificial rules regarding the connection of the wave function across the interface are introduced. The ten‐band tight‐binding model includes admixture in the total wave function of states derived from different extrema of the GaAs conduction band. States derived from the same extremum of the conduction band appear to couple strongly to each other, whereas states derived from different extrema are found to couple weakly. Transport characteristics of incoming L‐point and Γ‐point Bloch states are examined as a function of the energy of the incoming state, thickness of the Ga_(1−x)Al_xAs barrier, and alloy composition x. Transmission through the Ga_(1−x)Al_xAs barrier is either tunneling or propagating depending on the nature of the Bloch states available for strong coupling in the alloy. Since Bloch states derived from different extrema of the conduction band appear to couple weakly to each other, it seems possible to reflect the low velocity L‐point component of the current while transmitting the high velocity Γ‐point component

Surface core excitons in III-V semiconductors

Recent experiments have shown that the cation core excitons on the (110) surface of many III-V semiconductors have very large binding energies.(^1) They are sometimes reported to be bound by as much as ≳0.8 eV, tightly bound compared to bulk binding energies of ≾0.1 eV. To explore this phenomenon, we have calculated the binding energies and oscillator strengths of core excitons on the (110) surface of GaAs, GaSb, GaP, and InP

Fermi-level position at a semiconductor-metal interface

We have investigated the phenomenon of Fermi-level pinning by charged defects at the semiconductor-metal interface. Two limiting cases were investigated. In the first case we modeled an infinitely thick metallic coverage. In the second case we modeled a submonolayer coverage by using a free semiconductor surface containing defects. In both cases we assumed that most of the defect-induced interface states are localized inside the semiconductor, not more than a few angstroms away from the metal. Under these conditions we have estimated the difference in Fermi-level position between n- and p-type semiconductors to be less than 0.05 eV in the case of a thick metallic coverage. This difference was shown to be the maximum possible one, and it occurs only when there is no pinning. When there is pinning, this difference is even smaller. No such upper bound on the difference in Fermi-level position exists in the case of submonolayer coverage. We have also found that the defect density required to pin the Fermi level is ∼10^14 cm^-2 in the case of a thick metallic coverage, but only ∼10^12 cm^-2 in the case of a submonolayer coverage

Near-band‐gap photoluminescence of Hg_(1−x)Cd_xTe

The results of photoluminescence studies of Hg_(1−x)Cd_xTe with x=0.32 and 0.48 for temperatures between 5 and 30 K are described. In the x=0.32 and x=0.48 material, band‐to‐band, band‐to‐acceptor, and donor‐to‐acceptor luminescencelines are observed. We report the first observation of bound‐exciton luminescence in HgCdTe, which we observe in the samples with x=0.48

Theoretical studies of electronic properties of semimagnetic superlattices in a magnetic field

We present our first theoretical study of the electronic properties of superlattices formed from semimagnetic semiconductors. Both Cd0.8Mn0.2Te/Cd0.7Mn0.3Te and Hg0.95Mn0.05Te/ Cd0.78Mn0.22Te systems are considered explicitly. Magnetic field splittings are calculated with and without the exchange interaction. We find that the exchange interaction dominates the magnetic effects in the wide-gap Cd0.8Mn0.2Te/Cd0.7Mn0.3Te system while the Landau level shift is also important in the Hg0.95Mn0.05Te/ Cd0.78Mn0.22Te system. We present calculations of the superlattice band-gap variation with temperature and its derivative with magnetic field as a function of the superlattice layer thickness. Variation of the band offset in determining the values of the various quantities is examined

k⋅p theory of semiconductor superlattice electronic structure in an applied magnetic field

We present a k⋅p theory of semiconductor superlattices in an applied magnetic field. We consider superlattices with a [001] growth axis and the magnetic field along the growth axis. A single-basis set for the constituent materials is provided by a zone-center pseudopotential calculation with a reference Hamiltonian. The Γ15 valence and Γ1 conduction states are coupled with a spinor and treated explicitly. Nearby energy states are treated in Löwdin perturbation theory with the k⋅p operator and the difference between the material pseudopotential and the reference pseudopotential as the perturbation. The calculation is carried out consistently to first order in wave functions and second order in energies. Magnetic, exchange (in semimagnetic materials), spin-orbit, and strain (in strained-layer superlattices) interactions are included between the explicitly included states. When inversion-asymmetry and warping terms are dropped in the Hamiltonian, a Landau index becomes a good quantum number. Bloch and evanescent states are computed for a fixed Landau index in each material. Interface matching of the constituent-material bulk eigenfunctions is accomplished with use of results derived for the normal component of the current density operator. The Landau indices are not mixed by the interface matching. Superlattice translational symmetry is used to derive an eigenvalue equation for the superlattice wave vectors and eigenfunctions. The numerical implementation of the formal results is described and used to investigate a nonmagnetic superlattice Ga0.47In0.53As/Al0.48In0.52As and a semimagnetic superlattice Hg0.95Mn0.05Te/Cd0.78Mn0.22Te

Charge order, dynamics, and magneto-structural transition in multiferroic LuFe2_2O4_4

We investigated the series of temperature and field-driven transitions in LuFe$_2$O$_4$ by optical and M\"{o}ssbauer spectroscopies, magnetization, and x-ray scattering in order to understand the interplay between charge, structure, and magnetism in this multiferroic material. We demonstrate that charge fluctuation has an onset well below the charge ordering transition, supporting the "order by fluctuation" mechanism for the development of charge order superstructure. Bragg splitting and large magneto optical contrast suggest a low temperature monoclinic distortion that can be driven by both temperature and magnetic field.Comment: 4 pages, 3 figures, PRL in prin

Pothole Reporting System

The purpose of this project is to create a pothole detection device that can be attached to the underside of a commercial vehicle. Potholes cost motorists around 6.4 billion dollars annually, thus demonstrating the need for a system to aid with the detection and reporting of potholes. The four systems we needed to consider for the implementation of this project were the power system, the sensing system, the data processing system, and the reporting and logging system. Power pulled from the vehicle will enable the sensors and data processing module. The data processing module will analyze the readings from the sensors and output pothole data to the logging and reporting system. The logging and reporting system, located on an android mobile device, will store the pothole locations on a cloud server