The quantum-confined Stark effect in a single self-assembled CdSe/ZnSe quantum dot was studied by means of highly spatially resolved photoluminescence spectroscopy. A nanotechnological approach making use of a capacitor-like geometry enabled us to apply a well-defined lateral electric field on the quantum dots. Stark shifts of up to 1.1 meV were obtained, which can be well fitted by a purely quadratic dependence on an electric field. In quite good agreement with theoretical calculations, an exciton polarizability of 4.9×10−3 meV/(kV/cm)2 can be extracted, while the permanent dipole moment in the lateral direction is found to be negligible.

Stark effect and polarizability in a single CdSe/ZnSe quantum dot

An empirical ${s}_{c}{p}_{a}^{3}$ tight-binding (TB) model is applied to the investigation of electronic states in semiconductor quantum dots. A basis set of three $p$ orbitals at the anions and one $s$ orbital at the cations is chosen. Matrix elements up to the second nearest neighbors and the spin-orbit coupling are included in our TB model. The parametrization is chosen so that the effective masses, the spin-orbit splitting and the gap energy of the bulk CdSe and ZnSe are reproduced. Within this reduced ${s}_{c}{p}_{a}^{3}$ TB basis the valence $(p)$ bands are excellently reproduced and the conduction $(s)$ band is well reproduced close to the $\ensuremath{\Gamma}$ point, i.e., near to the band gap. In terms of this model much larger systems can be described than within a (more realistic) $s{p}^{3}{s}^{*}$ basis. The quantum dot is modeled by using the (bulk) TB parameters for the particular material at those sites occupied by atoms of this material. Within this TB model we study pyramidal-shaped CdSe quantum dots embedded in a ZnSe matrix and free spherical CdSe quantum dots (nanocrystals). Strain effects are included by using an appropriate model strain field. Within the TB model, the strain effects can be artificially switched off to investigate the influence of strain on the bound electronic states and, in particular, their spatial orientation. The theoretical results for spherical nanocrystals are compared with data from tunneling spectroscopy and optical experiments. Furthermore the influence of the spin-orbit coupling is investigated.

Tight-binding model for semiconductor nanostructures

Publisher Summary This chapter describes the program package digital analysis of lattice images (DALI), which was developed to quantify high-resolution transmission electron microscopy (HRTEM) image information. These programs are applied to semiconductor heteroepitaxial layers where the strain state and the composition on an atomic scale are of particular interest. The basis of one possible approach to solve the task of determination of strain and composition on an atomic scale is measurement of local lattice parameters—that is, measurement of the distance between adjacent atomic columns. This simply requires the detection of the intensity maxima positions in a high-resolution image that can be considered a fingerprint of the local lattice parameter. It is not necessary to know the actual position of the atomic columns with respect to the intensity maxima position if the transmission electron microscope (TEM) specimen thickness does not change significantly and composition-insensitive imaging conditions are chosen to avoid chemical shifts of the contrast pattern. Local composition can be extracted directly if the relationship between composition and lattice parameter is known.

Atomic Scale Strain and Composition Evaluation from High-Resolution Transmission Electron Microscopy Images

We report on the temperature dependence of the band-edge photoluminescence decay of organically capped colloidal ZnSe quantum dots (QDs) in the size range from 4.0 to 7.5 nm. A similar trend is observed for all investigated sizes: the decay time is short (∼5 ns) above 20 K and increases sharply below 20 K, eventually reaching a constant value (270–400 ns) at sufficiently low temperatures (<4 K). The temperature regime in which the decrease of lifetime occurs depends on the QD size and is lower for larger QDs. This behavior can be modeled by a Boltzmann distribution between a lower long-lived and a higher short-lived exciton states, with an energy separation ranging from 3.3 ± 0.2 to 1.5 ± 0.1 meV in the 4.0 ± 0.3 to 7.5 ± 0.5 nm size range. We show that this energy separation is consistent with coupling of the lowest exciton state to a confined acoustic phonon.

Unravelling the Size and Temperature Dependence of Exciton Lifetimes in Colloidal ZnSe Quantum Dots

Electric-field-dependent absorption of znse-based quantum wells : the transition from two-dimensional to three-dimensional behavior

Interference pattern due to standing polariton waves have been observed in thin planparallel ZnSe layers. The samples consist of pseudomorphic ZnSe/ZnCdSe quantum well structures grown by molecular beam epitaxy with cap layers of various thicknesses. The interference pattern appearing in reflectance and photoluminescence excitation spectra were fitted to calculated spectra applying a simple theoretical model. This fit results in the determination of the deadlayer thickness and the evaluation of the Luttinger parameter γ1 and γ2.

M. Meier

Papers

Hole mass determination in znse by observation of standing polariton waves

Composition dependent determination of band offsets in {ZnCdSe}/{ZnSe} and {ZnSe}/{ZnSSe} SQW by optical means

Growth, structural and optical characterization of MBE {ZnCdSe}/{ZnSe} quantum wells