Publications de Klaus Leifer | EPFL Graph Search

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Negatively and positively charged excitons are identified in the spatially resolved photoluminescence spectra of quantum wires. We demonstrate that charged excitons are weakly localized in disordered quantum wires. As a consequence, the enhancement of the ...

The evolution of GaN luminescence under electron beam injection has been studied by means of in situ cathodoluminescence experiments on various epitaxial lateral overgrown samples. It is shown that the ultraviolet (UV) peak of undoped materials experiences a decrease of its intensity as well as a noticeable redshift, while the other extrinsic peaks only experience an intensity decrease. However, in Mg doped materials the UV peak intensity decrease is followed by an increase of its intensity which can even r

The formation mechanisms, structure and optoelectronic properties of Ga(In)As/(Al)GaAs quantum dot (QD) heterostructures grown by organometallic chemical vapor deposition on patterned (111)B GaAs substrates are reviewed. With this approach, it is possible to prepare high-quality semiconductor QDs whose position on a substrate is precisely controlled via a pre-growth photolithography step. The dots are formed at the center of an inverted, tetrahedral pyramid and are surrounded by distinct, low-dimensional ba

We report on the direct absorption measurement of the polarization selection rules in a V-groove QWR waveguide. Attenuation measurements in a 1 mm long waveguide were performed from 10 K up to room temperature. These measurements yielded the QWR DOS selection rules for lateral (TE) and vertical (TM) confinement directions. 1D heavy-hole like e(n)h(n) subband features were observed in the TE absorption spectra. Effective mass and 4 x 4 k . p models have been used to evaluate the band to band absorption. For

Thin film multilayers play a crucial role in the efficient transport of neutrons from the source to the experiments at the Swiss Neutron Spallation source SINQ. In order to develop and optimize the neutron optical components, a magnetron sputtering facility has been installed at PSI aimed at the deposition of thin films covering large substrate area. Initial uniformity tests are very encouraging showing thickness variation of only 1% for 5 nm layers over an area of 500 X 400 mm2. A detailed study of the str

A novel technique for tuning the strength of quantum confinement in site-controlled semiconductor quantum dots (QDs) is introduced and investigated theoretically and experimentally. The method makes use of controlled local growth rates during metalorganic ...

In situ cathodoluminescence experiments have been performed to follow the time dependence of the UV luminescence in epitaxial lateral overgrowth GaN specimens. The decrease of the observed intensity and red-shift of the UV peak are interpreted in terms of non-radiative defect introduction and diffusion. This leads to strain relaxation of the GaN epilayer, which is initially under compressive strain. Monochromatic UV and yellow CL images show that dislocations act as efficient non-radiative recombination cen

We report on the growth and optical properties of dense arrays of single GaAs/AlGaAs quantum dot (QD) heterostructures with pitches as small as 300 nm. The samples were grown by organometallic chemical vapor deposition in dense inverted pyramids on {1 1 1}B GaAs substrate pre-patterned using electron beam lithography and wet chemical etching. The growth conditions such as deoxidation and growth temperatures, growth rates, and V/III ratio, had to be chosen quite differently from those employed with micron-si

We describe the first stage of the evolution of CL spectra, intensity and dislocation contrast under low keV electron beam for ELO-GaN with a low dislocation density. The UV and yellow intensities are decreased by beam irradiation. We have observed a broadening of the UV peak towards low energies followed by a red shift. This is explained in terms of an electron beam activation of non-radiative centers which relax partially the compressive strain, The dislocation contrast is lowered, but the dislocations be

Hybrid lead halide perovskites have reached comparable efficiencies to state-of-the-art silicon solar cell technologies. However, a remaining key challenge toward commercialization is the resolution of the perovskite device instability. In this work, we id ...

We have studied systems of asymmetric coupled GaAs V-groove quantum wires p-i-n diodes, in which the electrons and holes are injected into different wires. Despite the use of a 7 nm thick AlGaAs tunnel barrier, and although we demonstrate that holes tunnel more slowly than electrons, we observe a very efficient and fast tunneling between the quantum wires (QWR's). We attribute this to the achievement of a resonance between hole levels in the two QWR's. Photoluminescence excitation experiments are compared w

Codeposition of hydrocarbons is a severe problem during focused electron beam writing of pure metal nanostructures. When using organometallic precursors, a low metal content carbonaceous matrix embedding and separating numerous nanosized metal clusters is formed. In this work, we present a new and easy approach to obtain high purity gold lines: the use of inorganic PF3AuCl as a precursor. Electrical resistivities as low as 22 µOhms cm at 295 K (ten times the bulk Au value) were obtained. This is to our know

We present a model for the interplay between the fundamental phenomena responsible for the formation of nanostructures by metalorganic vapor phase epitaxy on patterned (001)/(111)B GaAs substrates. Experiments have demonstrated that V-groove quantum wires ...

When AxGa1-xAs layers are grown on V-grooved GaAs substrates by metal organic chemical vapour deposition (MOCVD), a Ga rich vertical quantum well (VQW) appears in the V-groove centre. The width and Ga enrichment of this structure depend on the nominal Al concentration and growth temperature. From EELS linescans and spectrum images, concentration profiles and maps of VQWs are obtained, and a subsequent bandstructure calculation shows up to three confined sublevels in the VQW. Carrier confinement in segments

We have evaluated the structure of high-reflectivity InP/InGaAsP distributed Bragg reflectors grown by chemical-beam epitaxy at a growth rate of 3 mu m/h. Transmission electron microscopy combined with electron energy loss spectroscopy indicate lateral fluctuations in the GaInAsP alloy composition, clearly visible in the 490-510 degrees C growth temperature range, but less perceptible between 460 and 480 degrees C. In the higher temperature range, the modulation in composition shows a higher frequency along

We have studied systems of asymmetric coupled GaAs V-groove quantum wires, in which the electrons and holes are injected into different wires. Our experimental results indicate efficient electron and hole tunneling, despite a 7 nm thick AlGaAs tunnel barrier. Temperature dependent electroluminescence exhibit clear effects of tunneling up to room temperature but cannot distinguish electron/hole tunneling from exciton tunneling.

High resolution and analytical transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize short-period strained-layer Sim-Gen superlattices ( m monolayers Si, n monolayers Ge, total number of periods N≤ 145, total thickness ≃ 200 nm). The superlattices were grown by low-temperature molecular beam epitaxy (T = 300–400°C) on different SiGe alloy buffer layers on Si (100)substrates. The combination of these two methods shows that detailed informations can be obtained about sup

We report on the electric field effects in photoluminescence (PL) and PL excitation (PLE) measurements of reverse-biased GaAs V-groove quantum wires. We observe large redshifts (9 meV at 265 kV/cm) of PL and PLE peaks as well as field dependent intensity and polarization anisotropy variations, which are analyzed by a two-dimensional quantum confined Stark effect model. (C) 1999 American Institute of Physics. [S0003-6951(99)00416-7].

Klaus Leifer

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Enhancement of the binding energy of charged excitons in disordered quantum wires

Cathodoluminescence of epitaxial GaN laterally overgrown on (0001) sapphire substrate. Time evolution with low energy electron beam

Site- and energy-controlled pyramidal quantum dot heterostructures

Direct observation of new transitions in the absorption spectra of a V-groove quantum wire waveguide

Thin Films for Neutron Optics

Mechanisms of Quantum Dot Energy Engineering by Metalorganic Vapor Phase Epitaxy on Patterned Nonplanar Substrates

Defect diffusion and strain relaxation in epitaxial GaN laterally overgrown on (0001) sapphire under low energy electron beam irradiation

Growth and Optical Characterization of Dense Arrays of Site-Controlled Quantum Dots Grown in Inverted Pyramids

Cathodoluminescence intensity and dislocation contrast evolutions under electron beam excitation in epitaxial GaN laterally overgrown on (0001) sapphire

Mechanistic Insights into the Role of the Bis(trifluoromethanesulfonyl) imide Ion in Coevaporated p-i-n Perovskite Solar Cells

Electroluminescence and photoluminescence excitation study of asymmetric coupled GaAs/AlxGa1-xAs V-groove quantum wires

Focused electron beam induced deposition of gold

Decomposition, diffusion, and growth rate anisotropies in self-limited profiles during metalorganic vapor-phase epitaxy of seeded nanostructures

Use of quantitative EELS and cathodoluminescence for study of carrier confinement and diffusion in self organized vertical quantum wells

Evaluation of InP/InGaAsP distributed Bragg reflectors grown by chemical-beam epitaxy

Electrolummescence of asymmetric coupled GaAs/AlGaAs V-groove quantum wires

Structural Characterization of Short-Period SimGEn Superlattices by Transmission Electron Microscopy and X-Ray Diffraction

Two-dimensional quantum-confined Stark effect in V-groove quantum wires: Excited state spectroscopy and theory

Narrow (~4meV) inhomogeneous broadening and its correlation with confinement potential of pyramidal quantum dot arrays

Strongly reduced exciton transfer between quantum wires