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We perform a theoretical study of the optical transitions for different excitonic complexes in highly symmetric strain-free GaAs quantum dots (QDs) fabricated by epitaxially filling nanoholes (NHs) in an AlGaAs surface. NHs are formed by local droplet etching. As a first step, we propose a QD shape modeling consistent with atomic force microscopy (AFM) profiles and an experimental growth procedure. We investigate the QD height dependence of s- and p- shell exciton recombination energies in the framework of the effective mass approximation with an exact numerical diagonalization method. A comparison between theoretical results and available spectroscopic data is carried out. Systematic evolution of the binding energies of neutral (X), charged excitons (X-, X+) and the biexciton (XX), with QD height, is interpreted in terms of a balance between the Coulomb interactions and charge carrier correlation effects. Our calculations demonstrate the important role of the correlation energies in elucidating the bound character of all few-particle states especially the biexciton. [ABSTRACT FROM AUTHOR]

We report photoluminescence (PL) characterization and model simulation of AlInAs/AlGaAs type-II quantum dots (QDs). A thorough and precise determination of the band parameters for QD and matrix materials is given, focusing on the effects of alloy composition and strain state on the electronic properties. Origins of experimentally observed PL emission peaks are identified through a comparison with the band lineup theoretically determined in this work. We interpret the QD emission as originating from indirect type-II transitions involving electrons in the barrier X valley and heavy holes with S and P symmetry. [ABSTRACT FROM AUTHOR]

We have studied the coherent spin dynamics of excitons bound to acceptors, A0X, immersed in a CdTe quantum well by using time resolved photo-induced Faraday rotation. We have also measured the time-resolved differential transmission in order to determine a A0X lifetime of 220 ps, which is independent of the applied magnetic field. We show that at low magnetic field, the spin of A0X is completely frozen during a time, ≅ 4.5 ns, at least twenty times longer than its lifetime. We compare the spin properties of A0X with the spin properties of other charged excitons systems, and we conclude that the hyperfine interaction of the photo-created electron spin with nuclear spins is very likely to be at the origin of the observed spin dephasing times. [ABSTRACT FROM AUTHOR]

The magnetic domain structure in a ferromagnetic (Ga,Mn)As epilayer with perpendicular magnetic easy-axis is investigated using high-resolution Kerr microscopy (KM). We show that, despite strong hysteresis, domain theory for thermodynamical equilibrium can be used to determine the ratio of the domain wall energy and the magnetic energy. Complementary magnetometry and ferromagnetic resonance (FMR) measurements are used to determine the domain wall energy and thickness and the exchange stiffness constant. Index Terms--Magnetic domains, magnetic semiconductors, magnetization reversal, magneto-optic Kerr effect.

The magnetic anisotropy properties of the ferromagnetic semiconductor (Ga,Mn)(As,P) are studied in a material-specific microscopic k.p approach. We calculate the band diagram and the density of states of such quaternary ferromagnetic alloys using a 40 band k.p model as well as antiferro- magnetic s,p-d exchange interaction. Our simulations clearly show that the uniaxial and cubic magnetic anisotropy parameters reveal a pronounced dependence on the vertical strain. We also show that the microscopic calculations are in good agreement with the theoretical and experimental results especially in the description of the fourth-order in-plane and out-of-plane components of the magnetic anisotropy. The anisotropy constants obtained from the microscopic calculations allow us to construct a three-dimensional magnetic free energy surface, which clearly indicates that the easy axis of magnetization depends on the strain conditions. [ABSTRACT FROM AUTHOR]

We present a systematic study on the influence of strain, size, and magnetic field on the electronic properties of InAs/GaAs quantum dots. Using a 40-band k.p model, we have calculated the band diagram of strained InAs, and extract the band parameters which are useful for the electronic properties of InAs/GaAs quantum dots. Then, using an exact numerical diagonalization method on Fourier-Bessel function basis over a large cylinder domain, we calculated numerically the electron and hole eigenenergies and associated wave functions. We considered thereafter the effect of an external applied magnetic field, strain and quantum dot size variation on the charge carrier energy levels. It is clearly found that the strain strongly modifies the quantum dot potential profile, leading to a different electron and hole energy distribution. Our results revealed also that the electron and hole energy spectra change significantly when varying the quantum dot size as well as the magnetic field. Given this striking nanostructure size and magnetic field energy dependent property, these systems provide the opportunity to control and tune their optical and electronic properties through these parameters. [ABSTRACT FROM AUTHOR]

We investigate the effect of postgrowth annealing on the magnetic domain structure and magnetization reversal process of (Ga,Mn)As epilayers grown with tensile strain on a (Ga,In)As buffer. In the case of perpendicular magnetic easy axis, annealing drastically changes the domain structure observed at magnetization reversal. In as-grown samples, strongly anisotropic domain growth is observed. Dendriticlike domain expansion with guided branching along the <110> directions results in a gridlike pattern. This is tentatively attributed to spatial fluctuations of the uniaxial anisotropy constant, correlated with the crosshatch pattern. In annealed samples, domain wall motion is much more isotropic, which likely results from a decrease of the relative amplitude of the uniaxial anisotropy fluctuations with increasing carrier density. However, domain wall motion is impeded by linear or slightly curved defects, hundreds of micrometers long, and pointlike pinning centers. The density of nucleation centers for magnetization reversal strongly decreases upon annealing. [ABSTRACT FROM AUTHOR]

A study was conducted to examine the magnetic domain structure of (Ga,Mn)As epilayers grown with tensile strain on a (Ga,In)As buffer. The results show that the density of nucleation centers of magnetization reversal strongly decreases upon annealing.

The quantum dot size dependence of spin lifetimes and fine structure are studied by polarization-resolved photoluminescence experiments on InAlAs self-assembled quantum dots. Hanle measurements reveal that the dots dimensions influence the scaled spin lifetime ( $\mathrm {g}_{\mathrm {x}}^{\mathrm {e}}\mathrm {T}_{\mathrm {s}}$) and the transverse Landé-g factor ( $\mathrm {g}_{\mathrm {x}}^{\mathrm {e}})$ increases with decreasing dots dimensions. Under magnetic field applied in Faraday geometry, we study the anisotropic exchange splitting of bright exciton states as a function of energy emission (QDs size).The fine structure splitting (FSS) $\thinspace \hslash \omega _{exc}$ increases from 12 to as much as 15 μeV with quantum dots energy emission. [ABSTRACT FROM AUTHOR]