We discuss strain simulations of quantum dot structures covered with a GaSbAs strain reducing capping layer in the presence of Sb segregation. Cross Sectional Scanning Tunneling Microscopy shows strong Sb and In segregation in the material surrounding the quantum dot. Using the three layer model originally proposed for the SiGe system by D. J. Godbey, M. G. Ancona, J. Vac. Sci. Technol. A 15, 976 (1997) we accurately calculate the segregation profile and include a non uniform composition to our models. Using atomistic modeling, we present strain maps of the quantum dot structures that show the propagation of the strain into the GaAs region is strongly affected by the shape and composition of the strain reduction layer.

In this paper, we show the use of an optimally parameterized empirical potential of the Abell-Tersoff type and demonstrate that we can obtain a deep level of insight into the properties of the epitaxially grown quaternary alloy InGaAsSb. We find that the strain energy as a function of composition does not follow intuitive averages between the binary constituents and that the theoretical behaviour appears to be substantiated by experimental evidence of growth of InAs self-assembled quantum dots capped by GaSbAs.

Nuclear spin polarization dynamics are measured in optically pumped individual $\text{GaAs}/{\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}$ interface quantum dots by detecting the time dependence of the Overhauser shift in photoluminescence spectra. Long nuclear polarization decay times of $\ensuremath{\approx}1\text{ }\text{min}$ have been found indicating inefficient nuclear spin diffusion from the GaAs dot into the surrounding AlGaAs matrix in externally applied magnetic field. A spin-diffusion coefficient two orders lower than that previously found in bulk GaAs is deduced.

We report a combined experimental and theoretical analysis of Sb and In segregation during the epitaxial growth of InAs self-assembled quantum dot structures covered with a GaSbAs strain-reducing capping layer. Cross-sectional scanning tunneling microscopy shows strong Sb and In segregation which extends through the GaAsSb and into the GaAs matrix. We compare various existing models used to describe the exchange of group III and V atoms in semiconductors and conclude that commonly used methods that only consider segregation between two adjacent monolayers are insufficient to describe the experimental observations. We show that a three-layer model originally proposed for the SiGe system [D. J. Godbey and M. G. Ancona, J. Vac. Sci. Technol. A 15, 976 (1997)] is instead capable of correctly describing the extended diffusion of both In and Sb atoms. Using atomistic modeling, we present strain maps of the quantum dot structures that show the propagation of the strain into the GaAs region is strongly affected by the shape and composition of the strain-reduction layer.

In this paper we show the use of an optimally parameterized empirical potential of the Abell–Tersoff type to study the strain energy of the quaternary alloy InGaAsSb. We use our results to compute modified segregation energies in an improved kinetic model of segregation for the combined effects of group III and V exchange processes during epitaxial growth and compare with experimental data from Scanning Tunnelling Microscopy.

Sizable nuclear spin polarization is pumped in individual electron-charged $\mathrm{In}\mathrm{P}∕\mathrm{Ga}\mathrm{In}\mathrm{P}$ dots in a wide range of external magnetic fields ${B}_{Z}=0--5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ by circularly polarized optical excitation. We observe nuclear polarization of up to $\ensuremath{\approx}40%$ at ${B}_{Z}=1.5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ corresponding to an Overhauser field of $\ensuremath{\approx}1.2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. We find a strong feedback of the nuclear spin on the spin pumping efficiency. This feedback, which is produced by the Overhauser field, leads to nuclear spin bi-stability at low magnetic fields of ${B}_{Z}\ensuremath{\approx}0.3\ensuremath{-}1\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. We find that the splitting in magnetic field between the trion radiative recombination peaks markedly increases, when the Overhauser field in the dot cancels the external field. This counterintuitive result is shown to arise from the opposite contribution of the electron and hole Zeeman splittings to the optical transition energies.

Modelling of III-V semiconductor materials and nanostructures has been a very active field in the last 15 years. The rapid development in the material synthesis of low dimensional structures for optical applications has triggered a world wide interest for modelling methods capable of accurately describing systems comprising millions of atoms. With the development of empirical or semiempirical methods, together with the ever increasing computational power available to scientists, it is now possible to model e.g. quantum dots inside simulation boxes comprising 3 million atoms. In this talk we will review the most recent developments in the field of empirical atomistic methods, particularly the bond order potentials, and discuss its links and reliance on ab initio calculations. The links between these methods and modeling of segregation effect will also be discussed.

Nuclear polarization dynamics are measured in the nuclear spin bistability regime in a single optically pumped InGaAs/GaAs quantum dot. The controlling role of nuclear spin diffusion from the dot into the surrounding material is revealed in pump-probe measurements of the nonlinear nuclear spin dynamics. We measure nuclear spin polarization decay times in the range of 0.2-5 s, strongly dependent on the optical pumping time. The long nuclear spin decay arises from polarization of the material surrounding the dot by spin diffusion for long (>5s) pumping times. The time-resolved methods allow the detection of the unstable nuclear polarization state in the bistability regime otherwise undetectable in cw experiments.

Sizable nuclear spin polarization is pumped in individual InP/GaInP dots in a wide range of external magnetic fields B_ext=0-5T by circularly polarized optical excitation. We observe nuclear polarization of up to 40 and corresponding to an Overhauser field of 1.2T. We find a strong feedback of the nuclear spin on the spin pumping efficiency. This feedback, produced by the Overhauser field, leads to nuclear spin bi-stability at low magnetic fields of Bext=0.5-1.5T. We find that the exciton Zeeman energy increases markedly, when the Overhauser field cancels the external field. This counter-intuitive result is shown to arise from the opposite contribution of the electron and hole Zeeman splittings to the total exciton Zeeman energy.

The impact of the capping material on the structural properties of self-assembled InAs quantum dots (QDs) was studied at the atomic scale by cross-sectional scanning tunneling microscopy. Capping with lattice matched layers and with strained layers was analyzed. When the different capping materials are lattice matched to the substrate, the differences in the QD properties can be dominated by chemical effects: InAs/InP QDs capped with InP have a 2 ML smaller height than those capped with InGaAs or InGaAsP due to As/P exchange induced decomposition. The height of the dots is found to be much more strongly affected when strained capping layers are used. InAs/GaAs, QDs capped with InGaAs are considerably taller than typical GaAs-capped dots. When GaAsSb is used as the capping layer, the dots are almost full pyramids with a height of 9.5 nm, indicating that dot decomposition is almost completely suppressed. This indicates that the dot/capping layer strain plays a major role in inducing dot decomposition during capping.

The influence of a GaAsSb capping layer on the structural properties of self-assembled InAs∕GaAs quantum dots (QDs) is studied on the atomic scale by cross-sectional scanning tunneling microscopy. QDs capped with GaAs0.75Sb0.25 exhibit a full pyramidal shape and a height more than twice that of the typical GaAs-capped QDs, indicating that capping with GaAsSb suppresses dot decomposition. This behavior is most likely related to the reduced lattice mismatch between the dot and the capping layer.

We show that by illuminating an InGaAs/GaAs self-assembled quantum dot with circularly polarized light, the nuclei of atoms constituting the dot can be driven into a bistable regime, in which either a thresholdlike enhancement or reduction of the local nuclear field by up to 3 T can be generated by varying the pumping intensity. The excitation power threshold for such a nuclear spin "switch" is found to depend on both the external magnetic and electric fields. The switch is shown to arise from the strong feedback of the nuclear spin polarization on the dynamics of the spin transfer from electrons to the nuclei of the dot.