Oliver Marquardt

Coaxial multishell (In,Ga)As/GaAs nanowires for near-infrared emission on Si substrates.

Efficient infrared light emitters integrated on the mature Si technology platform could lead to on-chip optical interconnects as deemed necessary for future generations of ultrafast processors as well as to nanoanalytical functionality. Toward this goal, we demonstrate the use of GaAs-based nanowires as building blocks for the emission of light with micrometer wavelength that are monolithically integrated on Si substrates. Free-standing (In,Ga)As/GaAs coaxial multishell nanowires were grown catalyst-free on Si(111) by molecular beam epitaxy. The emission properties of single radial quantum wells were studied by cathodoluminescence spectroscopy and correlated with the growth kinetics. Controlling the surface diffusivity of In adatoms along the NW side-walls, we improved the spatial homogeneity of the chemical composition along the nanowire axis and thus obtained a narrow emission spectrum. Finally, we fabricated a light-emitting diode consisting of approximately 10(5) nanowires contacted in parallel through the Si substrate. Room-temperature electroluminescence at 985 nm was demonstrated, proving the great potential of this technology.

A comparison of atomistic and continuum theoretical approaches to determine electronic properties of GaN/AlN quantum dots

In this work we present a comparison of multiband k.p-models, the effective bond-orbital approach, and an empirical tight-binding model to calculate the electronic structure for the example of a truncated pyramidal GaN/AlN self-assembled quantum dot with a zincblende structure. For the system under consideration, we find a very good agreement between the results of the microscopic models and the 8-band k.p-formalism, in contrast to a 6+2-band k.p-model, where conduction band and valence band are assumed to be decoupled. This indicates a surprisingly strong coupling between conduction and valence band states for the wide band gap materials GaN and AlN. Special attention is paid to the possible influence of the weak spin-orbit coupling on the localized single-particle wave functions of the investigated structure.

Polarization-induced charge carrier separation in polar and nonpolar grown GaN quantum dots

We have performed systematic studies of wurtzite GaN/AlN quantum dots grown on polar and nonpolar surfaces. For this purpose, experimentally observed quantum dot geometries have been employed within an eight-band k⋅p model. The spatial separation of electrons and holes due to polarization potentials is found to be much larger in nonpolar than in polar grown quantum dots. In order to improve the electron-hole overlap and thus the recombination rates, we have varied the shape, size, and the periodic arrangement of nonpolar quantum dots. We observed the strongest improvement of the charge carrier overlap in nonpolar quantum dots that have a reduced dimension. If the size is reduced below 60% of the dimensions reported recently in literature, this increase is clearly more pronounced than for the polar quantum dots, indicating much better recombination rates in smaller nonpolar quantum dots.

Plane-wave implementation of the real-space k⋅p formalism and continuum elasticity theory

Abstract In this work we demonstrate how second-order continuum elasticity theory and an eight-band k ⋅ p model can be implemented in an existing density functional theory (DFT) plane-wave code. The plane-wave formulation of these two formalisms allows for an accurate and efficient description of elastic and electronic properties of semiconductor nanostructures such as quantum dots, wires, and films. Gradient operators that are computationally expensive in a real-space formulation can be calculated much more efficiently in reciprocal space. The accuracy can be directly controlled by the plane-wave cutoff. Furthermore, minimization schemes typically available in plane-wave DFT codes can be applied straightforwardly with only a few modifications to a plane-wave formulation of these continuum models. As an example, the elastic and electronic properties of a III-nitride quantum dot system are calculated.

Strong dipole coupling in nonpolar nitride quantum dots due to Coulomb effects

Optical properties of polar and nonpolar nitride quantum dots (QDs) are determined on the basis of a microscopic theory which combines a continuum elasticity approach to the polarization potential, a tight-binding model for the electronic energies and wavefunctions, and a many-body theory for the optical properties. For nonpolar nitride quantum dots, we find that optical absorption and emission spectra exhibit a weak ground-state oscillator strength in a single-particle calculation whereas the Coulomb configuration interaction strongly enhances the ground-state transitions. This finding sheds new light on existing discrepancies between previous theoretical and experimental results for these systems, as a weak ground state transition was predicted because of the spatial separation of the corresponding electron and hole state due to intrinsic fields whereas experimentally fast optical transitions have been observed.

Interplay between Coulomb interaction and quantum-confined Stark-effect in polar and nonpolar wurtzite InN/GaN quantum dots

In this paper we systematically analyze the electronic structures of polar and nonpolar wurtziteInN/GaN quantum dots and their modification due to the quantum-confined Stark effect caused by intrinsic fields. This is achieved by combining continuum elasticity theory with an empirical tight binding model to describe the elastic and single-particle electronic properties in these nitride systems. Based on these results, a many-body treatment is used to determine optical absorption spectra. The efficiency of optical transitions depends on the interplay between the Coulomb interaction and the quantum-confined Stark effect. We introduce an effective confinement potential which represents the electronic structure under the influence of the intrinsic polarization fields and calculate the needed strength of Coulomb interaction to diminish the separation of electrons and holes.

Room temperature magnetoelectric properties of type-II InAsSbP quantum dots and nanorings

Quaternary InAsSbP quantum dots (QDs) and quantum rings (QRs) are grown on InAs (100) substrates by liquid phase epitaxy. High resolution scanning electron and atomic force microscopes are used for the characterization. The room temperature optoelectronic and magnetoelectric properties of the InAsSbP type-II QDs and QRs are investigated. For the QD-based structures, specific dips on the capacitance-voltage characteristic are revealed and measured, which are qualitatively explained by the holes thermal and tunnel emissions from the QDs. Specific fractures at room temperature are experimentally found in the magnetic field dependence of an electric sheet resistance for the InAsSbP QRs-based sample.

Growth process, characterization, and modeling of electronic properties of coupled InAsSbP nanostructures

Quaternary III-V InAsSbP quantum dots (QDs) have been grown in the form of cooperative InAsSb/InAsP structures using a modified version of the liquid phase epitaxy. High resolution scanning electron microscopy, atomic force microscopy, and Fourier-transform infrared spectrometry were used to investigate these so-called nano-camomiles, mainly consisting of a central InAsSb QD surrounded by six InAsP-QDs, that shall be referred to as leaves in the following. The observed QDs average density ranges from 0.8 to 2 × 109 cm-2, with heights and widths dimensions from 2 to 20 nm and 5 to 45 nm, respectively. The average density of the leaves is equal to (6-10) × 109 cm-2 with dimensions of approx. 5 to 40 nm in width and depth. To achieve a first basic understanding of the electronic properties, we have modeled these novel nanostructures using second-order continuum elasticity theory and an eight-band k·p model to calculate the electronic structure. Our calculations found a clear localization of hole states in th...

Electronic properties of site-controlled (111)-oriented zinc-blende InGaAs/GaAs quantum dots calculated using a symmetry-adapted k⋅p Hamiltonian

In this work, we present and evaluate a (111)-rotated eight-band k ⋅p Hamiltonian for the zinc-blende crystal lattice to investigate the electronic properties of site-controlled InGaAs/GaAs quantum dots grown along the [111] direction. We derive the rotated Hamiltonian including strain and piezoelectric potentials. In combination with our previously formulated (111)-oriented continuum elasticity model, we employ this approach to investigate the electronic properties of a realistic site-controlled (111)-grown InGaAs quantum dot. We combine these studies with an evaluation of single-band effective mass and eight-band k ⋅p models, to investigate the capabilities of these models for the description of electronic properties of (111)-grown zinc-blende quantum dots. Moreover, the influence of second-order piezoelectric contributions on the polarization potential in such systems is studied. The description of the electronic structure of nanostructures grown on (111)-oriented surfaces can now be achieved with significantly reduced computational costs in comparison to calculations performed using the conventional (001)-oriented models.

Polarization effects due to thickness fluctuations in nonpolar InGaN/GaN quantum wells

We have employed continuum elasticity theory and an eight band k·p model to study the influence of thickness fluctuations in In0.2Ga0.8N quantum wells grown along the [112¯0] direction in GaN. Such fluctuations are the origin of polarization potentials that may spatially separate electrons and holes in the vicinity of a thickness fluctuation and therefore reduce the efficiency of light emitters. Our calculations reveal that even shallow fluctuations of one or two monolayers can induce a significant spatial separation of electrons and holes, in particular, if the lateral extent of such a fluctuation is large.