Annealing dependent far field of GaAs-(Ga,Al)As LEDs

The far field intensity distribution of spontaneously light-emitting diodes with FABRY-PEROT structure and its dependence on different annealing processes during the diode preparation is studied. The electromagnetic field distribution in the far field is considered for a four-layer-stepdiscontinuity of the dielectric constant in a GaAs-(Ga,Al)As heterostructure diode. Even for a symmetrical dielectric constant profile the maximum luminescence intensity is not emitted necessarily in a plane normal to the light emitting surface of the diode. This cross-eyedness can be influenced by annealing. The changed far-field pattern can be evaluated if one knows the doping profile in the vicinity of the p-n junction and its changes during the annealing. Theoretical statements are in qualitative agreement with experimental results.The authors are grateful to Dr. K.Unger for valuable discussions and to Miss B.Russ for assistance with the measurements. The radioanalytical Zn determination by Mrs. G.Hänold from the VEB Halbleiterkombinat Frankfurt/Oder, Werk Stahnsdorf is gratefully acknowledged. This paper incorporates work carried out in the Arbeitsgemeinschaft AIII-BV-Halbleiter.     

Valence band mixing in GaAs-(AlGa)As heterostructures

In optical emission from electrons confined in quantum wells, the component with polarization normal to the planes was observed to be unexpectedly strong. This suggests a breakdown of the point group symmetry of the highest valence state beyond the reduction from cubic to laminar symmetry. Effective mass theories that include excitonic interactions and conventional symmetry-breaking mechanisms do not account for this phenomenon.     

Interface recombination in P-type GaAs-(AlGa)As quantum well heterostructures

The process of non-radiative interface recombination is considered for quantum well heterostructures. Simple model calculations indicate that, contrary to previous suggestions, the quantum effects do not cause a large reduction in the effective interface recombination velocity, S_eff. In fact, for the same interface quality, quantum effects cause an increase in S_eff.     

Dependence of the lowest impurity state on alloy composition in GaAs-(Al,Ga)As quantum dots

The ground state donor binding energy is estimated using the simple first order perturbation method for a GaAs-Al _x Ga_1−x As spherical quantum dot. The calculated energy is computed as a function of Al-concentration. Donor binding energy is found to be quite sensitive to Al-concentration (x), specifically for smallx. Furthermore, the binding energy is found to be highest for the smallest and the center-doped dot indicating the strongest confinement in those cases.     

The high hydrostatic pressure effect on shallow donor binding energies in GaAs-(Ga, Al)As cylindrical quantum well wires at selected temperatures

We have presented the behavior of a shallow donor impurity with binding energy in cylindrical-shaped GaAs/Ga_0.7Al_0.3As quantum well wires under high hydrostatic pressure values. Our results are obtained in the effective mass approximation using the variational procedures. In our calculations, we have not considered the pressure related Γ−X crossover effects. The hydrostatic pressure dependence on the expectation value of ground state binding energy is calculated as a function of wire radius at selected temperatures. We have also discussed the effects of high hydrostatic pressure and temperature on some physical parameters such as effective mass, dielectric constant, and barrier height. A detailed analysis of these calculations has proved that the effective mass is the most important parameter, which explains the dependency of donor impurity binding energies on the high hydrostatic pressure values.     

Real-Code Genetic Algorithm for Ground State Energies of Hydrogenic Donors in GaAs-(Ga,Al)As Quantum Dots

We present a global optimization method, called the real-code genetic algorithm (RGA), to the ground state energies. The proposed method does not require partial derivatives with respect to each variational parameter or solving an eigenequation, so the present method overcomes the major difficulties of the variational method. RGAs also do not require coding and encoding procedures, so the computation time and complexity are reduced. The ground state energies of hydrogenic donors in GaAs-(Ga,Al)As quantum dots have been calculated for a range of the radius of the quantum dot radii of practical interest. They are compared with those obtained by the variational method. The results obtained demonstrate the proposed method is simple, accurate, and easy implement.     

Giant current-driven domain wall mobility in (Ga,Mn)As

We study theoretically hole current-driven domain wall dynamics in (Ga,Mn)As. We show that the spin-orbit coupling causes significant hole reflection at the domain wall, even in the adiabatic limit when the wall is much thicker than the Fermi wavelength, resulting in spin accumulation and mistracking between current-carrying spins and the domain wall magnetization. This increases the out-of-plane nonadiabatic spin-transfer torque and consequently the current-driven domain wall mobility by 3 to 4 orders of magnitude. Trends and magnitude of the calculated domain wall current mobilities agree with experimental findings.     

Negative and persistent photoconductivity in p-type Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As heterostructures under uniaxial stress

Abstract

Illumination of a double p-Al_0.5Ga_0.5As/GaAs/Al_0.5Ga_0.5As heterostructure by a red light emitting diode results in a negative photoconductivity that, after the diode is switched off, slowly relaxes to a positive persistent photoconductivity, characterised by an approximately 1.5-fold increase of the two-dimensional hole concentration. This metastable state may be explained with a model in which deep electron traps are supposed to be located above the Fermi level on the inverted heterointerface. Under uni-axial compression the hole concentration in the persistent photoconductivity state, as well as in the dark state, demonstrates the same linear decrease.