L. Ivanova

Quantum ring formation and antimony segregation in GaSb∕GaAs nanostructures

GaSb quantum rings in GaAs were studied by cross-sectional scanning tunneling microscopy. The quantum rings have an outer shape of a truncated pyramid with typical lateral extensions between 10 and 30nm and heights between 1 and 3nm, depending on the molecular beam epitaxy growth conditions. A clear central opening of varying diameter and more or less conical shape, filled with GaAs, is characteristic for the GaSb rings. The self-organized formation of quantum rings during the growth and subsequent fast overgrowth of GaSb quantum dots is attributed to a combination of large strain with strong Sb segregation. The latter is enabled by extensive group-V atomic exchange reactions at the GaSb∕GaAs interfaces, which are quantitatively evaluated from the atomically resolved microscopy data.

Confined states of individual type-II GaSb/GaAs quantum rings studied by cross-sectional scanning tunneling spectroscopy.

Combined cross-sectional scanning tunneling microscopy and spectroscopy results reveal the interplay between the atomic structure of ring-shaped GaSb quantum dots in GaAs and the corresponding electronic properties. Hole confinement energies between 0.2 and 0.3 eV and a type-II conduction band offset of 0.1 eV are directly obtained from the data. Additionally, the hole occupancy of quantum dot states and spatially separated Coulomb-bound electron states are observed in the tunneling spectra.

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.

Nitrogen-induced intermixing of InAsN quantum dots with the GaAs matrix

We investigated the influence of nitrogen incorporation on the growth of InAsN∕GaAs quantum dots (QDs) using cross-sectional scanning tunneling microscopy. Nitrogen exposure during InAs growth leads to a rather strong dissolution and the formation of extended almost spherical InGaAs QDs with a very low nitrogen content. Nitrogen atoms are instead observed in the surrounding GaAs matrix, and indium atoms are even found underneath the nominal base plane of the QDs. These effects are related to a rather low solubility of nitrogen within InAs, leading to high strain between indium-rich QDs and the surrounding nitrogen-rich matrix.

Formation of InAs/InGaAsP quantum-dashes on InP(001)

Self-assembled InAs/InGaAsP/InP(001) nanostructures are investigated using cross-sectional scanning tunneling microscopy. Atomically resolved images at both the (110) and the (1¯10) cleavage surface show InAs quantum dashes with almost binary composition and a truncated pyramidal shape. The quaternary matrix material directly above the InP substrate already shows a tendency toward decomposition, which gradually increases along the [001] growth direction, in particular above quantum dash layers. This decomposition, in turn, leads to an enhanced vertical correlation in the nucleation of further quantum dash layers.

Contrast mechanisms in cross-sectional scanning tunneling microscopy of GaSb/GaAs type-II nanostructures

Cross-sectional scanning tunneling microscopy results on GaSb quantum wells and dots in GaAs are found to exhibit a narrow, sharply defined contrast of the nanostructure at negative sample bias, but a smoothly broadened contrast at positive sample bias. This contrast is related to the specific type-II band alignment of GaSb/GaAs heterostructures in combination with tip-induced band bending. The corresponding model is quantitatively verified by numerical simulations of band bending and tunnel current profiles combined with calculations of cleavage-induced strain relaxation.

InAs nanostructures on InGaAsP/InP(001): Interaction of InAs quantum-dash formation with InGaAsP decomposition

Cross-sectional scanning tunneling microscopy is used to study the spatial structure and composition of self-assembled InAs nanostructures grown on InGaAsP lattice matched to the InP substrate. Images of the (110) and (1¯10) cleavage surfaces reveal InAs quantum dashes of different lateral extensions. They are found to be about 60 nm long, about 15 nm wide, about 2 nm high, and to consist of pure InAs. Furthermore, the quaternary InGaAsP matrix material below, in between, and above the quantum-dash layers shows a strong lateral contrast variation, which is related to a partial decomposition into columns of more InAs-rich and more GaP-rich regions. The effect is particularly pronounced along the [110] direction. A quantitative analysis of this strain-induced contrast yields a decomposition characterized by variations of the group-III and/or group-V concentrations in the order of ±10%. The data strongly indicate that the strain at the growth surface induced by the decomposition of the underlying matrix mate...

Formation Of InAs/InGaAsP Quantum Dashes

Self‐assembled InAs/InGaAsP/InP(001) nanostructures are investigated using cross‐sectional scanning tunneling microscopy. Atomically resolved images show elongated nanostructures with binary composition and a truncated pyramidal shape. The investigation of the InGaAsP/InP interface shows a tendency of the quaternary matrix material towards decomposition and indicates InAs quantum‐dash formation by nucleation on initially slightly decomposed InAs‐rich regions of the InGaAsP.