D. Litvinov

Investigations on the Stranski-Krastanow growth of CdSe quantum dots

We have investigated the growth kinetics of the self-assembled formation of coherently strained CdSe islands. We have found that two distinctly different types of islands are formed in succession. Analyzing the density distribution function of the two dominating size classes of islands, we show that islands of an average diameter of about 16 nm (type B islands) are correlated with a phase transition via a Stranski–Krastanow growth process. The other islands with a diameter of less than 10 nm (type A islands) is formed during the growth of the first 2 ML. At a coverage of about 3.1 ML CdSe stacking faults appear, indicating the beginning of the plastic relaxation of the quantum dot structure.

Coexistence of planar and three-dimensional quantum dots in CdSe/ZnSe structures

Two well distinguishable classes of nanoscale islands were identified in CdSe/ZnSe quantum dot structures by optical spectroscopy and transmission electron microscopy. For 2.1 to 3.1 monolayer CdSe deposition, coherent three-dimensional (3D) islands, formed in the Stranski–Krastanow (SK) mode, are found with typical diameters of ∼16 nm and a coverage-dependent density of up to 3×1010 cm−2. Simultaneously, small islands with lateral extensions below 10 nm and a density of ∼5×1011 cm−2 are formed by strain-modified island growth. Whereas the 3D SK islands dominate the emission properties at room temperature, the latter smaller islands determine the optical properties at temperatures below 120 K.

Quantum dot formation by segregation enhanced CdSe reorganization

The influence of the growth conditions during capping of CdSe/ZnSe quantum structures grown on GaAs(001) by molecular-beam epitaxy (MBE) were systematically investigated by high-resolution x-ray diffraction, transmission electron microscopy, and temperature dependent, partly time-resolved photoluminescence spectroscopy. The results clearly indicate formation of quantum wells with potential fluctuations if conventional MBE is used for capping the CdSe by ZnSe. In contrast, quantum dot formation occurs using migration enhanced epitaxy for this growth step. In the latter case, quantum dots can be obtained without formation of stacking faults.

Electrical spin injection from ZnMnSe into InGaAs quantum wells and quantum dots

We report on efficient injection of electron spins into InGaAs-based nanostructures. The spin light-emitting diodes incorporate an InGaAs quantum well or quantum dots, respectively, as well as a semimagnetic ZnMnSe spin-aligner layer. We show a circular polarization degree of up to 35% for the electroluminescence from InGaAs quantum wells and up to 21% for InGaAs quantum dots. We can clearly attribute the polarization of the emitted photons to the spin alignment in the semimagnetic layer by comparison to results from reference devices (where the ZnMnSe is replaced by ZnSe) and from all-optical measurements.

Atomic structure of MBE-grown GaAs nanowhiskers

The structural properties of MBE-grown GaAs and Al0.3Ga0.7 As nanowhiskers were studied. The formation of wurtzite and 4H-polytype hexagonal structures with characteristic sizes of 100 nm or larger in these materials was demonstrated. It is concluded that the Au-Ga activation alloy symmetry influences the formation of the hexagonal structure.

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.

Ordered arrays of vertically correlated GaAs and AlAs quantum wires grown on a GaAs(311)A surface

We study GaAs–AlAs short-period superlattices (SPSLs) grown on a GaAs(311)A surface using plan-view transmission electron microscopy (TEM). A strong in-plane compositional modulation with a period of 3.2 nm along the [011] direction is revealed by TEM under chemically sensitive imaging conditions and in high-resolution TEM. Our results confirm the formation of highly ordered vertically aligned arrays of GaAs and AlAs quantum wires formed via self-organized growth. Bright photoluminescence (PL) at room temperature in the green and yellow spectral range is observed.

Systematic investigation into the influence of growth conditions on InAs/GaAs quantum dot properties

The influence of the conditions during growth of InAs/GaAs quantum-dot structures on GaAs(001) by molecular-beam epitaxy was investigated systematically with respect to achieving quantum-dot photoluminescence in the 1 eV range. The growth temperature, As flux, growth rate, InAs deposit, and growth interruption time before cap layer growth were varied. Photoluminescence spectroscopy and transmission electron microscopy were used to study the optical and structural properties. Large InAs quantum dots with photoluminescence in the 1 eV range were obtained at a low growth rate of 0.0056 ML/s. Analyzing in particular the low-growth-rate regime, we found that an InAs deposition of at least 2.4 ML and a growth temperature of 500−510 °C were crucial to obtain large quantum dots with a high size uniformity. Composition analyses by transmission electron microscopy revealed a significantly higher In concentration in the quantum dots grown at low growth rate compared to high-growth-rate samples.

Transmission electron microscopy investigation of segregation and critical floating-layer content of indium for island formation in In x Ga 1 − x As

We have investigated

Room-temperature lasing of strain-compensated CdSe/ZnSSe quantum island laser structures

{\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}