M. Rabe

Self-assembled CdSe quantum dots Formation by thermally activated surface reorganization

Abstract The formation of self-assembled CdSe quantum dots on ZnSe by a reorganization process occurring during growth interruption and thermal activation is demonstrated. Optical data yield a dot size in the 3–5 nm range and a density of 10 10 –10 11 cm −2 . Our measurements reveal a complex coupling with lattice vibrations.

Transmission electron microscopy investigation of structural properties of self-assembled CdSe/ZnSe quantum dots

CdSe quantum dots on ZnSe, grown by molecular beam epitaxy and formed during reorganization of an initially uniform film by thermal activation, are microstructurally elucidated in cross section and plan view, using transmission electron microscopy. In diffraction contrast, an almost uniform wetting layer is clearly visible. Dark contrast features with a distinctly larger extension into growth direction mark the location of quantum dots. Individual quantum dots can be identified in high-resolution imaging both by lattice expansion and contrasts arising from their strain fields. Plan-view images show the coexistence of two classes of quantum dots with an average lateral size of ⩽10 nm (area density 100 μm−2) and 10–50 nm (20 μm−2), respectively. The shape of the larger entities is pyramidlike.

Electronic excitations and longitudinal optical phonon modes of self-assembled CdSe quantum dots revealed by microprobe studies

The electronic states of self-assembled CdSe quantum dots and the longitudinal optical phonon mediated energy relaxation among these states are investigated. Measurements with microprobe resolution demonstrate that each quantum dot develops specific phonon modes. There is no evidence for a well-defined phonon bottleneck in the intra-quantum-dot relaxation. We observe single quantum-dot bi-exciton emission and bi-exciton binding energies exceeding 20 meV.

Influence of the growth procedure on the Cd distribution in CdSe/ZnSe heterostructures: Stranski–Krastanov versus two-dimensional islands

Molecular beam epitaxy is used to grow different types of ZnSe/CdSe/ZnSe heterostructures. The topography of the bare CdSe surface studied with in situ atomic force microscopy is compared with high-resolution transmission electron microscopy data on overgrown structures. The growth procedure critically influences morphology and Cd distribution. Only use of thermal activation after low-temperature CdSe deposition enables the accomplishment of a distinct Stranski–Krastanov (SK) morphology with three-dimensional islands with a core of pure CdSe. Interdiffusion effects during activation of the SK transition as well as overgrowth are of minor importance.

Growth and magneto-optical properties of sub 10 nm (Cd, Mn)Se quantum dots

We report on the formation of self-assembled semimagnetic (Cd, Mn)Se quantum dots via thermally activated reorganization of an initially two-dimensional film. Incorporation of Mn causes a marked decrease of the dot density, while the size is only slightly affected. Magneto-optical data are consistent with a homogeneous distribution of the magnetic ions. We observe g factors as large as 200, enabling control of individual carrier spins on a sub 10 nm length scale.

Formation and stability of II–VI self-assembled quantum dots revealed by in situ atomic force microscopy

Abstract A study on self-assembled CdSe quantum dots (QDs) on ZnSe is reported. The use of an in situ ultra-high vacuum atomic force microscope (UHV-AFM) provides the morphological evidence for the thermally activated reorganization of an initially two-dimensional CdSe film into an array of QDs. The QDs have heights between 1.5 and 3.5 nm and lateral diameters smaller than 10 nm. UHV-AFM images do not yield any evidence for Ostwald ripening. We assign the ultra-high stability to the existence of a kinetic barrier prohibiting the heterosystem to pass from the metastable QD state into the thermodynamic equilibrium.

ZnSe-based electro-optic waveguide modulators for the blue-green spectral range

The Pockels effect is used to modulate and switch blue-green light by means of MBE-grown ZnSe waveguides with cladding layers of (Zn,Mn)Se on n-GaAs substrates. In a retarder geometry, the strained birefringent planar waveguide exhibits a contrast ratio larger than 80:1 at 2.485 eV. When the electric field is applied via stripe contacts on the top cladding layer, the waveguiding for TE-polarized light can be confined in the lateral direction. In the centre of the lateral waveguide channel, the contrast ratio is better than 16:1 at 2.48 eV. Switching between adjacent channels is demonstrated with a contrast ratio better than 5:1.

Developments in the use of RHEED for interpreting growth processes in the MBE of wide gap II–VI semiconductors

The reflection high-energy electron diffraction (RHEED) during molecular beam epitaxial growth enables a unique insight of how growth proceeds. The subject as a whole is reviewed and some new results including a Monte-Carlo simulation are added in order to establish a comprehensive assessment of the details of the processes that take place during MBE growth of wide-gap II–VI semiconductors. Processes that take place during growth interruptions, the occurrence of a phenomenon denoted as “material contrast”, which has so far only been reported for II–VI materials, and a detailed analysis of the phase angle of RHEED oscillations are covered.

RHEED reflex profile analysis and phase locked epitaxy of ZnSe on (001) -oriented GaAs

Abstract ZnSe layers have been grown by MBE in both the conventional and the phase-locked epitaxy (PLE) mode. RHEED patterns have been taken at different stages during growth interruptions. The intensity profile along the 00 rod has been analyzed with respect to the surface morphology. During growth interruptions, the surface is smoothed by two processes with different time constants. In the PLE mode, surprisingly, the present results indicate that the number of RHEED oscillation periods observed does not coincide with the number of monolayers obtained.

Dark exciton states and magneto-excitons in wide-gap II-VI quantum wells

Abstract This paper summarizes magneto-optical studies on the fine structure of the exciton ground state as well as magneto-exciton states in ZnSe-related quantum wells. Relevant parameters like binding energies, in-plane masses and the ground-state splitting due to the electron—hole exchange interaction are precisely determined.