Coulomb-interaction-induced incomplete shell filling in the hole system of InAs quantum dots

The Coulomb-interaction-induced breaking of the Aufbau principle for the hole charging of InGaAs/GaAs quantum dots

The so called 'incomplete hole shell filling' phenomenon, that is the breaking of the Aufbau principle was reported by Reuter et al (Phys. Rev. Lett. 94 026808) in the hole charging spectra of a quantum dot when the results were interpreted in the context of the s/p/d shell system-typical for electrons. We report an example of an inter-particle-interaction induced Aufbau principle violation even if it is applied to the one-particle Kohn-Luttinger eigenstates. We present a [Formula: see text]/configuration-interaction study that concerns multiple holes confined in an InGaAs/GaAs self-assembled cylindrical quantum dot. The eigenenergies and eigenvectors of up to six hole ground states were obtained-along with the corresponding one-hole orbital occupations-and discussed in the context of the Aufbau principle.

Infrared transmission spectroscopy of charge carriers in self-assembled InAs quantum dots under surface electric fields

We present a study on the intersublevel spacings of electrons and holes in a single layer of InAs self-assembled quantum dots. We use Fourier transform infrared transmission spectroscopy via a density chopping scheme for direct experimental observation of the intersublevel spacings of electrons without any external magnetic field. Epitaxial, complementary-doped and semi-transparent electrostatic gates are grown within the ultra high vacuum conditions of molecular beam epitaxy to voltage-tune the device, while a two dimensional electron gas (2DEG) serves as a back contact. Spacings of the hole sublevels are indirectly calculated from the photoluminescence spectrum by using a simple model given by Warburton et al [1]. Additionally, we observe that the intersubb and resonances of the 2DEG are enhanced due to the quantum dot layer on top of the device.

Direct quantitative electrical measurement of many-body interactions in exciton complexes in InAs quantum dots

We present capacitance-voltage spectra for the conduction band states of InAs quantum dots obtained under continuous illumination. The illumination leads to the appearance of additional charging peaks that we attribute to the charging of electrons into quantum dots containing a variable number of illumination-induced holes. By this we demonstrate an electrical measurement of excitonic states in quantum dots. Magnetocapacitance-voltage spectroscopy reveals that the electron always tunnels into the lowest electronic state. This allows us to directly extract, from the highly correlated many-body states, the correlation energy. The results are compared quantitatively to state of the art atomistic configuration interaction calculations, showing very good agreement for a lower level of excitations and also limitations of the approach for an increasing number of particles. Our experiments offer a rare benchmark to many-body theoretical calculations.

Optical imaging of electrical carrier injection into individual InAs quantum dots

We image the micro-electroluminescence (EL) spectra of self-assembled InAs quantum dots (QDs) embedded in the intrinsic region of a GaAs p-i-n diode and demonstrate optical detection of carrier injection into a single QD. Tunneling of electrons and holes into the QDs at bias voltages below the flat-band condition leads to a spectrum of sharp EL lines from a small number of bright spots on the diode surface, characteristic of emission from individual QDs. We explain this behavior in terms of Coulomb interaction effects and the selective excitation of a small number of QDs within the ensemble due to preferential tunneling paths for carriers.

"Artificial atoms" in magnetic fields: wave-function shaping and phase-sensitive tunneling

We demonstrate the possibility to influence the shape of the wave functions in semiconductor quantum dots by the application of an external magnetic field B(z). The states of the so-called p shell, which show distinct orientations along the crystal axes for B(z) = 0, can be modified to become more and more circularly symmetric with an increasing field. Their changing probability density can be monitored using magnetotunneling wave function mapping. Calculations of the magnetotunneling signals are in good agreement with the experimental data and explain the different tunneling maps of the p(+) and p⁻ states as a consequence of the different sign of their respective phase factors.

Enhanced sequential carrier capture into individual quantum dots and quantum posts controlled by surface acoustic waves

Individual self-assembled quantum dots and quantum posts are studied under the influence of a surface acoustic wave. In optical experiments we observe an acoustically induced switching of the occupancy of the nanostructures along with an overall increase of the emission intensity. For quantum posts, switching occurs continuously from predominantly charged excitons (dissimilar number of electrons and holes) to neutral excitons (same number of electrons and holes) and is independent of whether the surface acoustic wave amplitude is increased or decreased. For quantum dots, switching is nonmonotonic and shows a pronounced hysteresis on the amplitude sweep direction. Moreover, emission of positively charged and neutral excitons is observed at high surface acoustic wave amplitudes. These findings are explained by carrier trapping and localization in the thin and disordered two-dimensional wetting layer on top of which quantum dots nucleate. This limitation can be overcome for quantum posts where acoustically induced charge transport is highly efficient in a wide lateral matrix-quantum well.

Electronic excitations in nanostructures: an empirical pseudopotential based approach

Physics at the nanoscale has emerged as a field where discoveries of fundamental physical effects lead to a greater understanding of the solid state. Additionally, the field is believed to have a large potential for technological applications, which has driven a high pace of experimental achievements in fabrication and characterization. From the side of theoretical modeling-so successful in solid state physics in general, since the emergence of density functional theory-we must acknowledge a weak connection to state of the art experimental achievements in the realm of nanostructures. The cause for this partial disconnect resides in the difficulty of the matter, nanostructures being small in size but large in the number of atoms constituting them, and the relevant observables being accessible only through proper treatment of excitations. The large number of atoms and the need for excited state properties makes this a challenging task for theory and modeling. In this contribution we will outline the framework, based on empirical pseudopotentials and configuration interaction, to obtain quantitative predictions of the excited state properties of semiconductor nanostructures using their experimental sizes, compositions and shapes. The methodology can be used to describe colloidal nanostructures of a few hundred atoms all the way to epitaxial structures requiring millions of atoms. The aim is to fill the gap existing between ab initio approaches and continuum descriptions. Based on the pseudopotential idea and the developments of empirical pseudopotentials for bulk materials in the early 1960s, the method has evolved into a powerful tool where the pseudopotential construction has lost some of its empirical character and is now based on modern density functional theory. We will present the construction of these potentials and the way the ensuing wavefunctions are used in a subsequent configuration interaction treatment of the excitation. We will illustrate the available capabilities by recent applications of the methodology to unveil new effects in the optics of nanostructures, quantum entanglement and wavefunction imaging.

Electronic phase diagrams of carriers in self-assembled quantum dots: violation of Hund's rule and the Aufbau principle for holes

We study the ground-state orbital and spin configurations of up to six electrons or holes loaded into self-assembled quantum dots. We use a general phase-diagram approach constructed from single-particle pseudopotential and many-particle configuration interaction methods. The predicted hole charging energies agree with recent charging experiments, but offer a different interpretation: we find that while the charging of electrons follows both Hund's rule and the Aufbau principle, the charging of holes follows a nontrivial charging pattern which violates both the Aufbau principle and Hund's rule.