Ground-state Exciton Binding Energy Research Articles

Near-field magnetoabsorption of quantum dots

We investigate the effect of an external magnetic field of variable orientation and magnitude (up to $20\phantom{\rule{0.3em}{0ex}}\mathrm{T}$) on the linear near-field optical absorption spectra of single and coupled III-V semiconductor quantum dots. We focus on the spatial as well as on the magnetic confinement, varying the dimensions of the quantum dots and the magnetic field. We show that the ground-state exciton binding energy can be manipulated utilizing the spatial and magnetic confinement. The effect of the magnetic field on the absorption spectra, increasing the near-field illumination spot, is also investigated. The zero-magnetic-field ``structural'' symmetry can be destroyed varying the magnetic field orientation and this affects the near-field spectra. The asymmetry induced (except for specific orientations along symmetry axes) by the magnetic field can be revealed in the near-field but not in the far-field spectra. We predict that near-field magnetoabsorption experiments, of realistic spatial resolution, will be in the position to bring to light the quantum dot symmetry. This exceptional symmetry-resolving power of the near-field magnetoabsorption is lost in the far field. The influence of the Coulomb interactions on the absorption spectra is also discussed. Finally, we show that certain modifications of the magnetoexcitonic structure can be uncovered using a realistically acute near-field probe of $\ensuremath{\approx}20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.

The influence of interdiffusion on the binding energy of excitons in In xGa 1− xN yAs 1− y /GaAs quantum wells

The influence of a technologically important effect of interdiffusion of atoms between well and barrier layers on the ground state exciton binding energy has been investigated as a function of the well width, for different quaternary material compositions. The migration of the group III atoms through the quantum well interfaces has been assumed to be dominant. Significant changes of the exciton binding energy depending on the diffusion length have been observed.

Exciton and donor binding energies in quantum-well wires and quantum dots a fractional-dimensional space approach

A simple method for calculating the free-exciton binding energies in the fractional-dimensional-space model for single-quantum-well structure has been extended to quantum-well wires and quantum dots, in which the real anisotropic system is modelled through an effective isotropic environment with a fractional dimension. In this scheme, the fractional-dimensional parameter is chosen via an analytical procedure and involves no ansatz. We calculated the ground-state binding energies of excitons and donors in quantum-well wires with rectangular cross sections. Our results are found to be in good agreement with previous variational calculations and available experimental measurements. We also discussed the ground-state exciton binding energy changing with different shapes of quantum-well wires.

Photoreflectance evidence of the N-induced increase of the exciton binding energy in an InxGa1−xAs1−yNy alloy

The binding energy of the heavy-hole ground-state exciton in In0.25Ga0.75As1−yNy/GaAs single quantum wells (y=0, 0.011) was experimentally derived by photoreflectance measurements. We measured a binding energy of 6.6 and 8.5 meV for the N-free and the N-containing sample, respectively. The observed increase of the exciton binding energy can be accounted for by an increase of the exciton reduced mass of about 30% upon N introduction into the InxGa1−xAs lattice, consistently with recent experimental results and in agreement with earlier theoretical predictions.

Determination of the orbital lineup at reactive organic semiconductor interfaces using photoemission spectroscopy

We determined the orbital lineup of the tris (8-hydroxyquinolinato) gallium (Gaq3)/Mg interface using combined x-ray and ultraviolet photoemission spectroscopy (XPS and UPS) measurements. The Gaq3/Mg system is a prototypical model structure for organic electron/low work function electrode transporting materials interfaces found in organic light emitting diodes (OLED). A Gaq3 thin film was grown in 15 steps on a previously sputter-cleaned Mg substrate starting at a 1 Å nominal thickness up to a final thickness of 512 Å. Before, and in between the growth steps, the sample surface was characterized by XPS and UPS. The results indicate the formation of a reaction layer of about 12 Å thickness at the Mg interface, which resulted in a 0.96 V interface dipole potential. At Gaq3 coverages higher than 256 Å, a strong charging shift occurred in the overlayer related UPS-emission lines, which was identified by measuring the high binding energy cutoff (secondary edge) of both the XP and UP spectra. The several magnitudes different x-ray and ultraviolet source photon intensities allow pinpointing charging shifts with high sensitivity. Due to the low work function of the reacted interface layer, the Gaq3 electronic states are aligned at a binding energy below the substrate Fermi edge that exceeds the magnitude of the optical gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). This allowed the conclusion that the ground state exciton binding energy of Gaq3 needs to be larger than 0.43 eV. Based on these considerations, the lowest possible electron injection barrier matching the experimental data was estimated to be 0.15 eV.

Excitonic structure in a quantum well under the tilted magnetic field

By using an appropriate coordinate transform we have calculated variationally the ground state exciton binding energy in the single square well under the tilted magnetic field. The dependence of the binding energy to the magnetic field strength, well width, and the direction of the field is discussed. We conclude that for small well widths the exciton binding energy is very sensitive to the tilt angle.

Quantum-confinement-effect-driven type-I–type-II transition in inhomogeneous quantum dot structures

We investigate the electronic structures of the inhomogeneous quantum dots within the framework of the effective mass theory. The results show that the energies of electron and hole states depend sensitively on the relative magnitude 77 of the core radius to the capped quantum dot radius. The spatial distribution of the electrons and holes vary significantly when the ratio eta changes. A quantum-confinement-driven type-II-type-I transition is found in GaAs/AlxGa1-xAs-capped quantum dot structures. The phase diagram is obtained for different capped quantum dot radii. The ground-state exciton binding energy shows a highly nonlinear dependence on the innner structures of inhomogeneous quantum dots, which originates from the redistribution of the electron and hole wave functions.

Direct and indirect excitons in coupled GaAs/Al0.30Ga0.70As double quantum wells separated by AlAs barriers.

Heavy-hole and light-hole excitons in symmetric, coupled GaAs/${\mathrm{Al}}_{0.30}$${\mathrm{Ga}}_{0.70}$As double quantum wells have been investigated by photoluminescence and photoluminescence excitation spectroscopy in magnetic fields B\ensuremath{\leqslant}13 T. The excitonic states in these structures can be controlled by varying the width of the AlAs barrier, which separates the GaAs quantum wells. The ground-state exciton binding energy shows a highly nonlinear dependence on the AlAs barrier width which originates from the redistribution of the electron and hole wave functions by the barrier. The barrier width also controls the spatial character of the ground-state excitons. In the case of relatively wide barriers (five monolayers of AlAs) the Coulomb interaction mixes symmetric and antisymmetric single-particle states and generates exciton states, which have predominantly direct or indirect character. With increasing magnetic field the enhancement of the exciton binding energy is stronger for the direct than for the indirect exciton, which leads to an enhanced splitting of indirect and direct excitons as well as to a strong redistribution of oscillator strength from the indirect to the direct exciton. For narrow barriers (one monolayer), on the other hand, spatially direct and indirect character are strongly mixed in all exciton states, leading to a very similar magnetic-field dependence of the energies and oscillator strengths of equivalent exciton states. \textcopyright{} 1996 The American Physical Society.

Excitons in a quantum wire subjected to a magnetic field.

We present variational calculations of the ground-state exciton binding energy and exciton radius in a quantum wire subjected to an external magnetic field. The magnetic field squeezes the exciton wave function, causing the binding energy to become very large and increase superlinearly with the field; at the same time, the exciton radius shrinks. The squeezing is more effective in wider wires, where the exciton wave function is ``softer.'' These results are consistent with recently reported experimental observations.

Exciton-longitudinal-optical-phonon problem in quantum wells

A theory of excitons in quantum wells based on the Bethe-Salpeter equation is presented. The energy gap shift, polaronic effective masses of electrons and holes as well as the ground-state exciton binding energy have been calculated as functions of the layer thickness. In the limit of thin layers the interaction of excitons with LO phonons is found to cause a decrease in the exciton binding energy of the order of 18%.

Interface characteristics and excitonic emissions in ZnSe-ZnSSe superlattices fabricated by low-pressure MOCVD

ZnSe-ZnSSe and ZnSe-ZnS strained-layer superlattices (SLS), exhibiting excellent optical properties, have for the first time been prepared on (100) GaAs substrates by a conventional low-pressure metal-organic chemical-vapor-deposition (MOCVD) technique. The experimental X-ray diffraction patterns are analyzed in terms of the kinematic approximation from which the strains being included in each layer can be quantitatively evaluated. The excitonic line and its spectral shape in the SLS, depending upon the composition of S in the ZnSSe barrier layer, are investigated at 4.2 K; with decreasing the barrier height the ground-state exciton binding energy is considerably lowered and its linewidth becomes narrow. We have found light-hole and heavy-hole exciton transitions in the absorption spectrum and in particular the existence of the light-hole exciton is confirmed by the excitation spectrum.

Oscillatory Spectral Distribution of the Photoelectromagnetic Effect Associated with Direct Interband Transitions in Germanium

A rather simple and straightforward experimental method of studying direct interband optical transitions between Landau levels in semiconductors is achieved by observing the oscillatory spectral distribution of the photoelectromagnetic effect resulting from a quantizing magnetic field. The method is applied here to germanium at a temperature of 6 K, over the energy range 0.9–1.1 eV, using linearly polarized light parallel and perpendicular to magnetic fields of up to 70 kG applied along the [111] direction of the crystals. The spectral oscillations are analyzed in terms of current magneto-optical theories to give a value for the direct band-gap energy (Eg = 0.888 ± 0.001 eV) for the ground state exciton binding energy (Eex = 1.5 ± 0.3 meV) and an estimate of nonparabolicity and excitonic effects on higher-order transitions. A possible relationship between the oscillation amplitude and the bulk mean free path of the carriers is also examined.