Atomistic Modelling of III-V Semiconductors: from a single tetrahedron to millions of atoms

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

Role of segregation in InAs/GaAs quantum dot structures capped with a GaAsSb strain-reduction layer

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 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. © 2009 The American Physical Society

Role of segregation in InAs/GaAs quantum dot structures capped with a GaAsSb strain-reduction layer

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 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. © 2009 The American Physical Society

Empirical bond order potential calculations of the elastic properties of epitaxial InGaSbAs layers

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

The synthesis and high-level expression of a β2-adrenergic receptor gene in a tetracycline-inducible stable mammalian cell line

High-level expression of G-protein-coupled receptors (GPCRs) in functional form is required for structure–function studies. The main goal of the present work was to improve expression levels of β2-adrenergic receptor (β2-AR) so that biophysical studies involving EPR, NMR, and crystallography can be pursued. Toward this objective, the total synthesis of a codon-optimized hamster β2-AR gene suitable for high-level expression in mammalian systems has been accomplished. Transient expression of the gene in COS-1 cells resulted in 18 ± 3 pmol β2-AR/mg of membrane protein, as measured by saturation binding assay using the β2-AR antagonist [3H] dihydroalprenolol. Previously, we reported the development of an HEK293S tetracycline-inducible system for high-level expression of rhodopsin. Here, we describe construction of β2-AR stable cell lines using the HEK293S-TetR-inducible system, which, after induction, express wild-type β2-AR at levels of 220 ± 40 pmol/mg of membrane protein corresponding to 50 ± 8 μg/15-cm plate. This level of expression is the highest reported so far for any wild-type GPCR, other than rhodopsin. The yield of functional receptor using the single-step affinity purification is 12 ± 3 μg/15-cm plate. This level of expression now makes it feasible to pursue structure–function studies using EPR. Furthermore, scale-up of β2-AR expression using suspension cultures in a bioreactor should now allow production of enough β2-AR for the application of biophysical techniques such as NMR spectroscopy and crystallography

Under pressure: control of strain, phonons and bandgap opening in rippled graphene

Two-dimensional (2D) layers like graphene are subject to long-wavelength fluctuations that manifest themselves as strong height fluctuations (ripples). In order to control the ripples, their relationship with external strain needs to be established. We therefore perform molecular dynamics (MD) of suspended graphene, by the use of a newly developed force field model (MMP) that we prove to be extremely accurate for both C Diamond and Graphene. The MMP potential successfully reproduces the energy of the r-bonds in both sp3 and sp2 configuration. Our MD simulations and experimental electron microscopy analysis reveal that ordered and static ripples form spontaneously as a direct response to external pressure. Furthermore the morphology of graphene and strain response of the crystal bonds differ depending on the particular directions where external pressure is present. Different regions of the strained graphene sheet are then investigated by tight-binding. Localised bandgap opening is reported for specific strain combinations, which also results in particular signatures in the phonon spectrum. Such controllable morphological changes can therefore provide a means to practically control and tune the electronic and transport properties of graphene for applications as optoelectronic and nanoelectromechanical devices