K. Pötschke

Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots

A systematic variation of the exciton fine-structure splitting with quantum dot size in single quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from to as much as with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band theory and the configuration interaction method were performed. Different sources for the fine-structure splitting are discussed, and piezoelectricity is pinpointed as the only effect reproducing the observed trend.

106years extrapolated hole storage time in GaSb∕AlAs quantum dots

A thermal activation energy of 710meV for hole emission from InAs∕GaAs quantum dots (QDs) across an Al0.9Ga0.1As barrier is determined by using time-resolved capacitance spectroscopy. A hole storage time of 1.6s at room temperature is directly measured, being three orders of magnitude longer than a typical dynamic random access memory (DRAM) refresh time. The dependence of the hole storage time in different III–V QDs on their localization energy is determined and the localization energies in GaSb-based QDs are calculated using eight-band k⋅p theory. A storage time of about 106years in GaSb∕AlAs QDs is extrapolated, sufficient for a QD-based nonvolatile (flash) memory.

Structure and intermixing of GaSb/GaAs quantum dots

We present cross-sectional scanning tunneling microscopy results of GaSb quantum dots in GaAs, grown by metalorganic chemical vapor deposition. The size of the optically active quantum dots with base lengths of 4–8 nm and heights of about 2 nm is considerably smaller than previously published data obtained by other characterization methods. The local stoichiometry, obtained from atomically resolved images, shows a strong intermixing in the partly discontinuous wetting layer with an average GaSb content below 50%, while the GaSb content of the partly intermixed quantum dots is between 60% and 100%.

Temperature-Dependent Small-Signal Analysis of High-Speed High-Temperature Stable 980-nm VCSELs

980-nm VCSELs based on submonolayer growth show for 20-Gbit/s large-signal modulation clearly open eyes without adjustment of the driving conditions between 25degC and 120degC. To access the limiting mechanism for the modulation bandwidth, a temperature-dependent small-signal analysis is carried out on these devices. Single-mode devices are limited by damping, whereas multimode devices are limited by thermal effects, preventing higher photon densities in the cavity.

Atomic Structure of Buried InAs Sub-Monolayer Depositions in GaAs

The atomic structure of sub-monolayer depositions fabricated by an alternating deposition of 0.5 monolayer InAs and 16 monolayer GaAs is revealed by cross-sectional scanning tunneling microscopy. The resulting InAs-rich structures are about 5 nm wide and laterally separated from each other by about 2 nm, yielding a very high density above 1012 cm-2. The InAs-rich material is not only found within the deposition layer, but remarkably segregated with a segregation coefficient R~0.7 over several monolayers along the growth direction. A similar segregation coefficient is found in the case of only 4 monolayer GaAs spacer thickness, revealing a more general growth mechanism for sub-monolayer depositions.

Atomic structure and optical properties of InAs submonolayer depositions in GaAs

Using cross-sectional scanning tunneling microscopy and photoluminescence spectroscopy, the atomic structure and optical properties of submonolayer depositions of InAs in GaAs are studied. The submonolayer depositions are formed by a cycled deposition of 0.5 monolayers InAs with GaAs spacer layers of different thicknesses between 1.5 and 32 monolayers. The microscopy images exhibit InAs-rich agglomerations with widths around 5 nm and heights of up to 8 monolayers. A lateral agglomeration density in the 1012 cm−2 range is found. During the capping of the InAs depositions a vertical segregation occurs, for which a segregation coefficient of ∼0.73 was determined. In the case of thin GaAs spacer layers, the observed segregation forms vertically connected agglomerations. The photoluminescence spectra exhibit peaks with linewidths below 10 meV and show a considerable dependence of the peak energy on the spacer thickness, even up to 32 monolayers GaAs, indicating a long range electronic coupling.Using cross-sectional scanning tunneling microscopy and photoluminescence spectroscopy, the atomic structure and optical properties of submonolayer depositions of InAs in GaAs are studied. The submonolayer depositions are formed by a cycled deposition of 0.5 monolayers InAs with GaAs spacer layers of different thicknesses between 1.5 and 32 monolayers. The microscopy images exhibit InAs-rich agglomerations with widths around 5 nm and heights of up to 8 monolayers. A lateral agglomeration density in the 1012 cm−2 range is found. During the capping of the InAs depositions a vertical segregation occurs, for which a segregation coefficient of ∼0.73 was determined. In the case of thin GaAs spacer layers, the observed segregation forms vertically connected agglomerations. The photoluminescence spectra exhibit peaks with linewidths below 10 meV and show a considerable dependence of the peak energy on the spacer thickness, even up to 32 monolayers GaAs, indicating a long range electronic coupling.

Size-Tunable Exchange Interaction in InAs/GaAs Quantum Dots

Single epitaxial quantum dots are promising candidates for the realization of quantum information schemes due to their atom-like electronic properties and the ease of integration into optoelectronic devices. Prerequisite for realistic applications is the ability to control the excitonic energies of the dot. A major step in this direction was recently reached by advanced self-organized quantum-dot growth, yielding ensembles of equally shaped InAs/GaAs dots with a multimodal size distribution. The well-defined sizes of spectrally well separated subensembles enable a direct correlation of structural and excitonic properties, representing an ideal model system to unravel the complex interplay of Coulomb interaction and the quantum dot’s confining potential that depends on size, shape, and composition. In this paper we focus on the exciton-biexciton system with emphasis on the excitonic fine-structure splitting. Across the whole range of size variations within our multimodal quantum dot distribution a systematic trend from +520µeV to −80µeV is found for decreasing dot size. To identify the underlying effects calculations of the fine-structure splitting are performed. A systematic variation of the structural and piezoelectric properties of the modeled quantum dots excludes shape anisotropy and tags piezoelectricity as a key parameter controlling the fine-structure splitting in our quantum dots.

Online control of quantum dot laser growth

Reflectance anisotropy spectroscopy (RAS) was used to control in-situ the complex MOCVD growth process of both InGaAs/GaAs quantum dots (QDs) and lasers with such QDs in the active region. RAS spectra of complete device runs including the first vertical cavity surface emitting QD laser (QD VCSEL) grown using MOCVD are presented.