Christopher Prohl

Evolution of the InAs wetting layer on GaAs(001)-c(4×4) on the atomic scale

Scanning tunneling microscopy was used to investigate the development of the InAs wetting layer on the GaAs(001)-c(4×4) surface. At low InAs coverages signatures of indium agglomerations form on the surface, before an abrupt change to a (4×3) reconstructed monolayer of In2/3Ga1/3As occurs at about 2/3 ML of deposited InAs. Further indium deposition leads to a second layer with α2(2×4) and β2(2×4) structural units on the surface.

Growth of In0.25Ga0.75As quantum dots on GaP utilizing a GaAs interlayer

Coherent In0.25Ga0.75As quantum dots (QDs) are realized on GaP(001) substrates by metalorganic vapor phase epitaxy in the Stranski-Krastanow mode utilizing a thin GaAs interlayer prior to In0.25Ga0.75As deposition. Luminescence is observed between 2.0 eV and 1.83 eV, depending on the thickness of the In0.25Ga0.75As layer. The critical thickness for the two-dimensional to three-dimensional transition of the layer is determined to 0.75 to 1.0 monolayers. A mean activation energy of 489 meV for holes captured by In0.25Ga0.75As quantum dots is measured by deep-level transient spectroscopy, yielding a hole storage time of 3 µs at room temperature.

Spatial structure of In0.25Ga0.75As/GaAs/GaP quantum dots on the atomic scale

In0.25Ga0.75As/GaAs quantum dots grown by metalorganic vapor-phase epitaxy in a GaP matrix have been investigated on the atomic scale using cross-sectional scanning tunneling microscopy. The quantum dots have a truncated pyramidal shape with a reversed cone stoichiometry profile. All deposited indium is found within the quantum dots and the occasionally observed quantum rings, while the wetting layer has a GaAsP composition without any indium inside. This indicates an intense lateral material transfer during growth.

Electronic properties of dysprosium silicide nanowires on Si(557)

The electronic properties of self-assembled dysprosium silicide nanowires on Si(557) are studied by angle-resolved photoelectron spectroscopy. Using a toroidal electron energy analyzer, the energy surfaces of the nanostructures are imaged. At dysprosium coverages exceeding one monolayer, metallic nanowires with a two-dimensional electronic structure are formed on [111]-oriented terraces, consisting of hexagonal DySi2 monolayers or Dy3Si5 multilayers with the c-axis in [111] direction of the substrate.

Atomic structure and strain of the InAs wetting layer growing on GaAs(001)-c(4×4)

Using scanning tunneling microscopy, the authors studied the wetting layer evolution of InAs on GaAs(001)-c(4×4) and unraveled the different surface reconstructions during this process. At low coverages the deposited InAs material is first stored at defects and then at the hollow sites of the GaAs(001)-c(4×4) reconstruction. Close to an InAs coverage of 2/3 monolayer (ML), the whole surface abruptly reconstructs into an In2/3Ga1/3As monolayer, showing mainly a (4×3) reconstruction. Further deposited InAs is arranged in three different InAs(001)-(2×4) reconstructions on top of the In2/3Ga1/3As layer. After quantum dot occurrence above about 1.4 ML of InAs, a material transport away from the wetting layer is observed by a partial reappearance of the underlying (4×3) reconstruction. A detailed analysis of the observed reconstructions clearly shows that their specific atomic arrangements lead to a reduction of strain, while increased amounts of strain at the wetting layer start to build up above about 1.4 ML ...

Atomic structure and stoichiometry of In(Ga)As/GaAs quantum dots grown on an exact-oriented GaP/Si(001) substrate

The atomic structure and stoichiometry of InAs/InGaAs quantum-dot-in-a-well structures grown on exactly oriented GaP/Si(001) are revealed by cross-sectional scanning tunneling microscopy. An averaged lateral size of 20 nm, heights up to 8 nm, and an In concentration of up to 100% are determined, being quite similar compared with the well-known quantum dots grown on GaAs substrates. Photoluminescence spectra taken from nanostructures of side-by-side grown samples on GaP/Si(001) and GaAs(001) show slightly blue shifted ground-state emission wavelength for growth on GaP/Si(001) with an even higher peak intensity compared with those on GaAs(001). This demonstrates the high potential of GaP/Si(001) templates for integration of III-V optoelectronic components into silicon-based technology.

Growth and electronic properties of Tb silicide layers on Si(111)

The formation, atomic structure, and electronic properties of Tb silicide layers on the Si(111) surface were studied using scanning tunneling microscopy as well as core-level and angle-resolved photoelectron spectroscopy. For Tb exposures around one monolayer, the formation of a hexagonal TbSi2 monolayer was found, while higher coverages led to the formation of a hexagonal Tb3Si5 multilayer with a 3×3R30° superstructure in the bulk layers. For the monolayer silicide, Si-2p core level spectra show a Fermi level position very close to the conduction band minimum of the silicon substrate, while the Fermi level shifts toward midgap in the multilayer case. The electronic structure of the monolayer is characterized by a Fermi surface consisting of electronlike ellipses around the M¯ points and a holelike state around the Γ¯ point. The effective masses of the band around the M¯ points are strongly anisotropic, with values around 1.45 m 0 in the long direction and 0.16 m 0 in the short direction of the ellipses. In the case of the multilayer, the ellipses around the M¯ points are less eccentric, and there are indications for Umklapp processes due to the 3×3R30° superstructure in the silicide bulk layers. The overall behavior of Tb is found to be similar to that of other trivalent rare earths on Si(111).

Cross-sectional scanning tunneling microscopy of antiphase boundaries in epitaxially grown GaP layers on Si(001)

In a fundamental cross-sectional scanning tunneling microscopy investigation on epitaxially grown GaP layers on a Si(001) substrate, differently oriented antiphase boundaries are studied. They can be identified by a specific contrast and by surface step edges starting/ending at the position of an antiphase boundary. Moreover, a change in the atomic position of P and Ga atoms along the direction of growth is observed in agreement with the structure model of antiphase boundaries in the GaP lattice. This investigation opens the perspective to reveal the orientation and position of the antiphase boundaries at the atomic scale due to the excellent surface sensitivity of this method.

Characterization of anti-phase boundaries at a GaP/Si(001) cross-sectional surface on the atomic scale

We report on a cross-sectional scanning tunneling microscopy (XSTM) investigation of anti-phase boundaries on epitaxial grown GaP layers on a Si(001) substrate. The growth conditions of the GaP layer was chosen to produce a significant anti-phase disorder, to favor the XSTM experiment. A different image contrast for the main phase and the anti-phase domain within the GaP layer could be observed. The image contrast mechanisms of anti-phase boundaries in the XSTM experiment are studied on the atomic scale. This investigation shows the feasibility of such an analysis and the perspective to reveal the atomic arrangement of the anti-phase boundaries as well as the evolution along growth direction due to the excellent surface sensitivity of this method.

Atomic structure and electronic states of InAs(Sb)/GaAs submonolayer quantum dots

Summary form only given. Submonolayer-grown semiconductor nanostructures are promising for high power and high speed laser devices. They are formed by a cycled deposition of the active material with a thickness well below the critical thickness for Stranski-Krastanov transition and well below one monolayer (ML), alternating with several ML thick matrix material. They were successfully implemented in high speed (>25 Gbit/s) vertical-cavity surface emitting lasers operating up to 120°C. In this contribution, the structural changes upon additional supply of Sb are studied on the atomic scale using XSTM. The InAsSb agglomerations show slightly smaller sizes than equivalent submonolayer structures grown without Sb. The structural findings are in close correlation with the different band alignments of the electronic states, showing different behavior for electrons and holes.