Unser Artikel mit dem Titel "Angular dependence of light trapping in In0.3Ga0.7As/GaAs quantum-well solar cells" von X. H. Li, P. C. Li, D. Z. Hu, D M. Schaadt, und E. T. Yu wurde in J. Appl. Phys., 2014 115, 044303 veröffentlicht.
Abstract:
The dependence of light trapping effects in In0.3Ga0.7As/GaAs quantum-well solar cells on wavelength and incident angle is experimentally characterized and analyzed. Separation of active device layers from their epitaxial growth substrate enables integration of thin-film semiconductor device layers with nanostructured metal/dielectric rear contacts to increase optical absorption via coupling to both Fabry-Perot resonances and guided lateral propagation modes in the semiconductor.
The roles of Fabry-Perot resonances and coupling to guided modes are analyzed via photocurrent response measurements and numerical modeling for light incident at angles of 0 degrees (normal incidence) to 30 degrees off normal. Light trapping enables external quantum efficiency at long wavelengths as high as 2.9% per quantum well to be achieved experimentally, substantially exceeding the similar to 1% per quantum well level typically observed.
Increased long wavelength quantum efficiency is shown in experimental measurements to persist with increasing angle of incidence and is explained as a consequence of the large number of guided modes available in the device structure.
Unser Artikel mit dem Titel "Influence of hole shape/size on the growth of site-selective quantum dots" von Ch. J. Mayer, M. Helfrich, und D M. Schaadt wurde in Nanoscale Res. Lett., 2013 8, 504 veröffentlicht.
Abstract:
The number of quantum dots which nucleate at a certain place has to be controllable for device integration. It was shown that the number of quantum dots per nucleation site depends on the size of the hole in the substrate, but other dimensions of the nucleation site are vague.
We report on the influence of hole shape on site-selectively grown InAs quantum dots (QDs) by molecular beam epitaxy. Dry etching of the GaAs wafers was used because of its high anisotropic etching characteristic. Therefore, it was possible to verify the influence of several hole shape parameters on the subsequent QD growth independently.
We show that the nucleation of these QDs depends on several properties of the hole, namely its surface area, aspect ratio of the surface area, and depth. Especially, the aspect ratio shows a big influence on the number of nucleating QDs per site. With knowledge of these dependencies, it is possible to influence the number of QDs per site and also its distribution.
Unser Artikel mit dem Titel "Controlling structural properties of positioned quantum dots" von M. Helfrich, B. Terhalle, Y. Ekinci, und D M. Schaadt wurde in J. Crystal Growth, 2013 371, 39 veröffentlicht.
Abstract:
The effects of in situ annealing on positioned InAs quantum dots grown on pre-structured GaAs substrates were investigated. Atomic force microscopy and scanning electron microscopy data were used to analyze the evolution of quantum dots during post growth annealing.
Two different annealing regimes are found which allow for increasing or decreasing the quantum dot size. The shape of the initially elongated QDs becomes more symmetric during annealing. Furthermore, the number of quantum dots per site can be controlled by annealing time.
Additionally, it is found that the width of the occupation probability distribution decreases during annealing. The results are discussed and compared to annealing experiments with self-assembled quantum dots.
Die Zusammenarbeit mit unseren Partnern aus Taiwan wird weiter intensiviert durch den Besuch von Austauschdoktorand Herrn Jerry Li.
Unser Artikel mit dem Titel "A portable molecular beam epitaxy system for in situ x-ray investigations at synchrotron beamlines" von T. Slobodskyy, P. Schroth, D. Grigoriev, A. A. Minkevich, D. Z. Hu, D. M. Schaadt, und T. Baumbach wurde in Rev. Sci. Instrum., 2012 83, 105112 veröffentlicht.
Abstract:
A portable synchrotron molecular beam epitaxy (MBE) system is designed and applied for in situ investigations.
The growth chamber is equipped with all the standard MBE components such as effusion cells with shutters, main shutter, cooling shroud, manipulator, reflection high energy electron diffraction setup, and pressure gauges. The characteristic feature of the system is the beryllium windows which are used for in situ x-ray measurements.
An UHV sample transfer case allows in vacuo transfer of samples prepared elsewhere. We describe the system design and demonstrate its performance by investigating the annealing process of buried InGaAs self-organized quantum dots.
Unser Artikel mit dem Titel "Improvement of GaN epilayer by gradient layer method with molecular-beam epitaxy" von Y.-L. Chen, I. Lo, M.-H. Gau, C.-H. Hsieh, M.-W. Sham, W.-Y. Pang, Y.-C. Hsu, J.-K. Tsai, R. Schuber, und D. Schaadt wurde in Thin Solid Films, 2012 520, 6134 veröffentlicht.
Abstract:
We demonstrated a molecular beam epitaxy method to resolve the dilemma between structural and morphological quality in growth of the GaN epilayer. A gradient buffer layer was grown in such a way that the N/Ga ratio was gradually changed from nitrogen-rich to gallium-rich.
The GaN epitaxial layer was then grown on the gradient buffer layer. In the X-ray diffraction analysis of GaN(002) rocking curves, we found that the full width at half-maximum was improved from 531.69″ to 59.43″ for the sample with a gradient buffer layer as compared to a purely gallium-rich grown sample. Atomic force microscopy analysis showed that the root-mean-square roughness of the surface was improved from 18.28 nm to 1.62 nm over an area of 5 × 5 μm2 with respect to a purely nitrogen-rich grown sample.
Raman scattering showed the presence of a slightly tilted plane in the gradient layer. Furthermore we showed that the gradient layer can also slash the strain force caused by either Ga-rich GaN epitaxial layer or AlN buffer layer.
Mathieu Helfrich hat seine Doktorarbeit erfolgreich beendet.
Unser Artikel mit dem Titel "Microstructure of PAMBE-grown InN layers on Si(111)" von R. Walther, D. Litvinov, M. Fotouhi, R. Schneider, D. Gerthsen, R. Vöhringer, D. Z. Hu, und D. M. Schaadt wurde in J. Crystal Growth, 2012 340, 34 veröffentlicht.
Abstract:
AlN layers with a thickness of 250 nm were grown by plasma-assisted gas source molecular-beam epitaxy on Si(111) at substrate temperatures between 600 °C and 900 °C. The surface morphology and microstructure of the AlN layers were analyzed by scanning and transmission electron microscopy.
Different defect types are observed in the AlN layers and at the AlN/Si(111) interfaces as a function of the temperature: inclusions of pure Al in the Si-substrate, crystallites of the cubic AlN phase, dislocations, stacking faults and inversion domain boundaries. The formation and concentration of the defects depends strongly on the substrate temperature during the growth.
X-ray diffraction rocking curves for the (0002) reflection yield minimum full width at half maximum values for the sample grown at the 900 °C under Al-rich conditions indicating optimum structural quality. However, the discussion of the entity of defects will show that a more differentiated view is required to assess the overall quality of the AlN layers.
Unser Artikel mit dem Titel "Semiconductor heterostructures and optimization of light-trapping structures for efficient thin-film solar cells" von C. O. McPheeters, D. Hu, D. M. Schaadt, und E. T. Yu wurde in J. Opt., 2012 14, 024007 veröffentlicht.
Abstract:
Sub-wavelength photonic structures and nanoscale materials have the potential to greatly improve the efficiencies of solar cells by enabling maximum absorption of sunlight. Semiconductor heterostructures provide versatile opportunities for improving absorption of infrared radiation in photovoltaic devices, which accounts for half of the power in the solar spectrum. These ideas can be combined in quantum-well solar cells and related structures in which sub-wavelength metal and dielectric scattering elements are integrated for light trapping.
Measurements and simulations of GaAs solar cells with less than one micron of active material demonstrate the benefits of incorporating In(Ga)As quantum-wells and quantum-dots to improve their performance.
Simulations that incorporate a realistic model of absorption in quantum-wells show that the use of broadband photonic structures with such devices can substantially improve the benefit of incorporating heterostructures, enabling meaningful improvements in their performance.
Unser Artikel mit dem Titel "Efficient single-photon extraction from quantum-dots embedded in GaAs micro-pyramids" von D. Rülke, D. M. Schaadt, H. Kalt, und M. Hetterich wurde in Appl. Phys. Lett., 2012 100, 251101 veröffentlicht.
Abstract:
We demonstrate an easy method to fabricate efficient single-photon sources based on In(Ga)As quantum-dots embedded in reversed GaAs micro-pyramids. It relies on a single wet-chemical etching step utilizing an AlAs sacrificial layer.
Due to the pyramidal shape of the cavities, we have been able to separate a small number of quantum-dots from the self-assembled ensemble and improve the extraction efficiency for single photons. The latter is predicted by finite difference time domain and finite elements method simulations to be about 80%–90% over a broad spectral range of 40 nm.
Single-photon emission has been proven experimentally by means of auto-correlation measurements.
Unser Artikel mit dem Titel "Investigation of buried quantum dots using grazing incidence X-ray diffraction" von P. Schroth, T. Slobodskyy, D. Grigoriev, A. Minkevich, M. Riotte, S. Lazarev, E. Fohtung, D. Z. Hu, D. M. Schaadt, und T. Baumbach wurde in Mater. Sci. and Eng. B, 2012 177, 721 veröffentlicht.
Abstract:
Self-organized, buried InAs quantum dots covered by an AlAs diffusion barrier were investigated under UHV conditions using grazing incidence X-ray diffraction. The experimental data is compared to the simulated results obtained by Finite Element Method and Distorted Wave Born Approximation.
We have found that the simulated data could be compared to the experimental one only after convolution by the resolution element which can be estimated from the experiment.
By adjusting the simulation parameters we were able to find good agreement between the simulated and the measured data.
Unser Artikel mit dem Titel "Nanoparticles and Efficiency Enhancement in Plasmonic Solar Cells" von R. Wang, M. Pitzer, Lj. Fruk, D. Z. Hu, und D. M. Schaadt wurde in J. Nanoelectron. and Optoe., 2012 7, 3 veröffentlicht.
Abstract:
In this study, the effect of various nanoparticles with different sizes, shapes and densities on the photocurrent of plasmonic solar cells was investigated. Nanospheres and nanocubes of Ag and Au were synthesized by polyol and seed-mediated method to serve as plasmonic scattering centers. Special immobilization treatments of the surface of GaAs based solar cells were performed to achieve the control over the densities of the particles.
The photocurrents of the plasmonic solar cells with varying densities and shapes of Ag and Au nanoparticles were measured, respectively. While the immobilization of the Ag particles on GaAs surface can be well controlled, it proved difficult for Au particles.
The highest efficiency enhancement was achieved for Ag nanospheres with a surface coverage of about 11%. If the density of Ag nanoparticles is too large, clusters are formed which lead to additional enhancement at longer wavelengths, however the overall photocurrent enhancement is lowered due to absorption of light in the particles.
Unser Artikel mit dem Titel "Magnetic properties of Cu-doped GaN grown by molecular beam epitaxy" von P. R. Ganz, G. Fischer, Ch. Sürgers, und D. M. Schaadt wurde in Phys. Rev. B, 2012 85, 165204 veröffentlicht.
Abstract:
Cu-doped GaN films were epitaxially grown by molecular beam epitaxy on C-plane sapphire substrates with an AlN buffer layer. Growth under metal-rich and nitrogen-rich conditions was investigated for different Cu-to-Ga beam-equivalent-pressure ratios xBEP. The samples were characterized by scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction.
Films within a narrow range of xBEP around 1% exhibit ferromagnetic behavior with a Curie temperature higher than 400 K. For higher xBEP>1%, islands of a Cu-Ga compound are predominantly formed at the surface as nonferromagnetic precipitates.
Our detailed study shows that the saturation magnetization of ferromagnetic films with xBEP≈1% decreases with increasing film thickness. This suggests that the ferromagnetism arises from defects, such as threading dislocations, created by the incorporation of Cu into the GaN.
Herr Christian Mayer hat seine Diplomarbeit erfolgreich beendet und mit seiner Doktorarbeit angefangen.
Unser Artikel mit dem Titel "Transmission electron microscopy investigation of AlN growth on Si(111)" von D. Litvinov, D. Gerthsen, R. Vöhringer, D.Z. Hu, und D. M. Schaadt wurde in J. Crystal Growth, 2012 338, 283 veröffentlicht.
Abstract:
AlN layers with a thickness of 250 nm were grown by plasma-assisted gas source molecular-beam epitaxy on Si(111) at substrate temperatures between 600 °C and 900 °C. The surface morphology and microstructure of the AlN layers were analyzed by scanning and transmission electron microscopy.
Different defect types are observed in the AlN layers and at the AlN/Si(111) interfaces as a function of the temperature: inclusions of pure Al in the Si-substrate, crystallites of the cubic AlN phase, dislocations, stacking faults and inversion domain boundaries. The formation and concentration of the defects depends strongly on the substrate temperature during the growth.
X-ray diffraction rocking curves for the (0002) reflection yield minimum full width at half maximum values for the sample grown at the 900 °C under Al-rich conditions indicating optimum structural quality. However, the discussion of the entity of defects will show that a more differentiated view is required to assess the overall quality of the AlN layers.
Herr Yunhua Wu und Frau Ruoli Wang haben mit ihrer Doktorarbeit angefangen.
Dr. Daniel M. Schaadt ist zum Professor für Energiewandlung am Institut für Energieforschung und Physikalische Technologien an der Technischen Universität Clausthal ernannt worden. Sitz der Arbeitsgruppe ist hauptsächlich am EnergieCampus der TU Clausthal in Goslar. Die Arbeitsgruppe wird in den nächsten Monaten inklusive Ausstattung nach Goslar wechseln.
Herr Yunhua Wu und Frau Ruoli Wang haben ihre Diplomarbeiten erfolgreich beendet.
Ralf Schuber hat seine Doktorarbeit erfolgreich beendet.
Omar Tanirah hat seine Masterarbeit erfolgreich beendet.
Martin Pitzer hat seine Diplomarbeit erfolgreich beendet.
Unser Artikel mit dem Titel "Improvement of performance of InAs quantum dot solar cell by inserting thin AlAs layers" von D. Z. Hu, C. O. McPheeters, E. T. Yu und D. M. Schaadt wurde in Nanoscale Res. Lett., 2011 6, 83 veröffentlicht.
Abstract:
A new measure to enhance the performance of InAs quantum dot solar cell is proposed and measured. One monolayer AlAs is deposited on top of InAs quantum dots (QDs) in multistack solar cells. The devices were fabricated by molecular beam epitaxy. In situ annealing was intended to tune the QD density.
A set of four samples were compared: InAs QDs without in situ annealing with and without AlAs cap layer and InAs QDs in situ annealed with and without AlAs cap layer. Atomic force microscopy measurements show that when in situ annealing of QDs without AlAs capping layers is investigated, holes and dashes are present on the device surface, while capping with one monolayer AlAs improves the device surface.
On unannealed samples, capping the QDs with one monolayer of AlAs improves the spectral response, the open-circuit voltage and the fill factor. On annealed samples, capping has little effect on the spectral response but reduces the short-circuit current, while increasing the open-circuit voltage, the fill factor and power conversion efficiency.
Herr Chrisitan Mayer hat als Diplomand angefangen. Er wird sich mit der Optimierung der Lithographie für gezielt-gewachsene InAs-Quantenpunkte beschäftigen.
Herr Michael Maschek hat als gemeinsamer Diplomand mit der Arbeitsgruppe H. v. Löhneysen angefangen. Er wird sich mit anomalem Hall-Effekt an Kupfer-dotiertem GaN beschäftigen.
Unser Artikel mit dem Titel "Undamped collective surface plasmon oscillations along metallic nanosphere chains" von W. Jacak, J. Krasnyj, J. Jacak, A. Chepok, L. Jacak, W. Donderowicz, D. Z. Hu und D. M. Schaadt wurde in J. Appl. Phys., 107, 084304 veröffentlicht.
Abstract:
The random-phase-approximation semiclassical scheme for describing plasmon excitations in large metallic nanospheres (with radius similar to 10-60 nm) is developed for the case when a dynamical electric field is present. The spectrum of plasmons in metallic nanospheres is determined including both surface and volume-type excitations and their mutual connections.
It is demonstrated that only dipole-type surface plasmons can be excited by a homogeneous dynamical electric field. The Lorentz friction due to irradiation of electromagnetic energy by plasmon oscillations is analyzed with respect to sphere dimension. The resulting shift in resonance frequency due to plasmon damping is compared with experimental data for various sphere radii.
Wave-type collective oscillations of surface plasmons in long chains of metallic nanospheres are described. The undamped region for collective plasmon propagation along the metallic chain is found in agreement with previous numerical simulations.
Im Wintersemester 2010/11 wird Dr. Daniel Schaadt zusammen mit PD Dr. Michael Hetterich die Vorlesung "Solid State Optics" halten.
Des Weiteren wird Dr. Daniel Schaadt auch das Kapitel "Halbleiternanostrukturen" in der CFN-Ringvorlesung "Grundlagen der Nanotechnologie I" unterrichten.
Als Teil der Zusammenarbeit mit Prof. Ikai Lo, Prof. Liuwen Chang, Prof. Chien-Chen Kuo und Prof. Mitch Chou, besuchen Dr. Daniel Schaadt als Visiting Professor und Dr. Dongzhi Hu als Visiting Postdoc die National Sun Yat-Sen Universität in Kaohsiung, Taiwan. Wärend seines Aufenthalts wird Dr. Schaadt die Vorlesung "solid state optics" halten.
Unser Artikel mit dem Titel "Quantification of Sample Thickness and In-Concentration of InGaAs Quantum Wells by Transmission Measurements in a Scanning Electron Microscope" von T. Volkenandt, E. Müller, D. Z. Hu, D. M. Schaadt, und D. Gerthsen wurde in Microscopy and Microanalysis, 16, 604 veröffentlicht.
Abstract:
High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images of electron-transparent samples show dominant atomic number (Z-) contrast with a high lateral resolution. HAADF STEM at low electron energies <30 keV is applied in this work for quantitative composition analyses of InGaAs quantum wells.
To determine the local composition, normalized experimental image intensities are compared with results of Monte Carlo simulations. For verification of the technique, InGaAs/GaAs quantum wellstructures with known In concentration are used. Transmission electron microscopy samples with known thickness are prepared by the focused-ion-beam technique.
The method can be extended to other material systems and is particularly promising for the analysis of materials that are sensitive toward knock-on damage.
Unser Artikel mit dem Titel "Growth of M-plane GaN on (100) LiGaO2 by plasma-assisted molecular beam epitaxy" von R. Schuber, M. M. C. Chou und D. M. Schaadt wurde in Thin Solid Films, 518, 6773 veröffentlicht.
Abstract:
Growth of M-plane GaN on (100) LiGaO2 was achieved using plasma-assisted molecular beam epitaxy. Thermal annealing of the LiGaO2 wafer was found to lead to a substrate surface suitable for growth. Structural and morphological analysis was performed using x-ray and reflective high energy electron diffraction, scanning electron and atomic force microscopy.
X-ray diffraction results show very high phase purity and a relaxation state of the GaN film close to 80%. The surface morphology, showing characteristic M-plane streaks, is flat and smooth.
Unser Artikel mit dem Titel "Radius dependent shift of surface plasmon frequency in large metallic nanospheres: theory and experiment" von W. Jacak, J. Krasnyj, J. Jacak, A. Chepok, L. Jacak, W. Donderowicz, D. Z. Hu und D. M. Schaadt wurde in J. Appl. Phys., 107, 124317 veröffentlicht.
Abstract:
Theoretical description of oscillations of electron liquid in large metallic nanospheres with radius of few tens of nanometer is formulated within random-phase-approximation semiclassical scheme in jellium model with retardation included via Lorentz friction. Spectrum of plasmons is determined including both surface and volume type excitations.
It is demonstrated that only surface plasmons of dipole type can be excited by homogeneous dynamical electric field. The Lorentz friction due to irradiation of electromagnetic wave by plasmon oscillations is analyzed with respect to the sphere dimension. The resulting shift in resonance frequency turns out to be strongly sensitive to the sphere radius.
The form of electromagnetic e-m response of the system of metallic nanospheres embedded in the dielectric medium is found. The theoretical predictions are verified by a measurement of extinction of light due to plasmon excitations in nanosphere colloidal water solutions, for Au and Ag metallic components with radius from 10 to 75 nm.
Theoretical predictions and experiments clearly agree in the positions of surface plasmon resonances and in an emergence of the first volume plasmon resonance in the e-m response of the system for limiting big nanosphere radii, when dipole approximation is not exact.
Unser Artikel mit dem Titel "Growth of A-plane GaN growth on LiGaO2(010) by plasma-assisted molecular beam epitaxy" von R. Schuber, M. M. C. Chou, P. Vincze, Th. Schimmel, und D. M. Schaadt wurde in in J. Crystal Growth, 312, 1665 veröffentlicht.
Abstract:
The (010) surface of LiGaO2 is closely lattice matched to A-plane (11-20) GaN making it an interesting candidate as a substrate for heteroepitaxy of non-polar GaN. We demonstrate successful, first-time growth of A-plane GaN on (010) LiGaO2 using plasma-assisted molecular beam epitaxy.
Structural and morphological analysis is performed using X-ray and reflective high energy electron diffraction, scanning electron and atomic force microscopy. Very high phase purity of A-plane GaN is shown. Apart from defects of the epitaxial film originating from substrate scratches, the film is smooth and shows an rms roughness of 10 nm over an area of 8x8 µm2.
Herr Omar Tanirah hat als Masterstudent angefangen. Er wird sich mit Photolumineszenz von Nitridschichten beschäftigen.
Im folgenden Sommersemester 2010 wird Dr. Daniel Schaadt zusammen mit PD Dr. Michael Hetterich die Vorlesung "Halbleiterphysik" halten.
Des Weiteren wird Dr. Daniel Schaadt auch als Betreuer im Hauptseminar "Halbleiter-Nanostrukturen" mitarbeiten.
Raimund Vöhringer hat seine Diplomarbeit erfolgreich beendet.
Herr Yunhua Wu und Frau Ruoli Wang haben als Diplomanden angefangen. Herr Wu wird sich mit Nitridschichten zur Photoelektrolyse von Wasser beschäftigen. Frau Wang wird an der Herstellungen von Nanopartikeln arbeiten. Sie wird gemeinsam mit Frau Dr. Fruk betreut.
Unser Artikel mit dem Titel "Cu-doped GaN grown by molecular beam epitaxy" von P. R. Ganz, C. Sürgers, G. Fischer und D. M. Schaadt wurde in in J. Phys.: Conf. Series, 200, 062006 veröffentlicht.
Abstract:
Cu-doped GaN is a promising candidate for a nitride-based diluted magnetic semiconductor. Theoretical predictions show the possibility of ferromagnetism and high spin-polarization for certain arrangements of Cu atoms in the GaN lattice. Initial experimental results have already indicated ferromagnetism.
However, the influence of structural defects on the ferromagnetic order in Cu-doped nitrides is not clear. Hence, the origin of the ferromagnetism is still under debate. We have used density functional theory (DFT) to verify previous theoretical predictions and to investigate the effects of the position of Cu atoms on the ferromagnetic properties.
Our DFT calculations show high degrees of spin-polarization, independent of the arrangement of Cu atoms. Additionally, we have investigated the growth of Cu-doped GaN by molecular beam epitaxy. The influence of parameters, such as Cu to Ga ratio and growth temperature, on the structural and magnetic properties will be discussed.
Martin Pitzer hat als gemeinsamer Diplomand der Arbeitsgruppen Fruk und Schaadt angefangen.
Er wird sich hauptsächlich mit der optischen Charakterisierung von Nanopartikeln in photovoltaischen Bauelementen beschäftigen.
Unser Artikel mit dem Titel "Lateral ordering, strain and morphology evolution of InGaAs/GaAs(001) Quantum Dots (QDs) due to high temperature post growth annealing" von M. Riotte, E. Fohtung, D. Grigoriev, A.A. Minkevich, T. Slobodskyy, M. Schmidbauer, T. H. Metzger, D. Z. Hu, D. M. Schaadt, und T. Baumbach wurde in in Appl. Phys. Lett., 96, 083102 veröffentlicht.
Abstract:
The effect of postgrowth annealing on shape and ordering of a single layer of InGaAs/GaAs(001) quantum dots is investigated by three dimensional grazing incidence small angle x-ray scattering. A transition from disordered dots to two-dimensional lateral ordering is found. This transition is accompanying a quantum dot shape transformation. Grazing incidence diffraction measurements relate the observed ordering type to strain driven self organization.
The role of different growth conditions leading to lateral correlation is discussed by comparing the results to recent experimental achievements in the field.
Unser Artikel mit dem Titel "Morphology and stress evolution of InAs QD grown and annealed in-situ at high temperature" von D. Z. Hu, A. Trampert, und D. M. Schaadt wurde in in J. Crystal Growth, 312, 447 veröffentlicht.
Abstract:
The post growth annealing of InAs quantum dots (QDs) at relatively high temperature was investigated by an in-situ stress cantilever beam setup. For samples annealed at 500 °C, stress accumulated during QD formation relaxes below the value which was built-up during wetting-layer growth.
AFM images taken at different annealing stages reveal that QDs ripen first and then dissolve within 10 min of annealing. These observations are explained by a combination of In desorption, especially at the beginning of annealing, and interdiffusion between Ga and In.
Unser Artikel mit dem Titel "Pulsed Electrical Spin Injection into InGaAs Quantum Dots: Studies of the Electroluminescence Polarization Dynamics" von P. Aßhoff, W. Löffler, H. Flügge, J. Zimmer, J. Müller, B. Westenfelder, D. Z. Hu, D. M. Schaadt, H. Kalt, und M. Hetterich wurde in in AIP Conf. Proc., 1199, 369 veröffentlicht.
Abstract:
We present time-resolved studies of the spin polarization dynamics during and after initialization through pulsed electrical spin injection into InGaAs quantum dots embedded in a p-i-n-type spin-injection light-emitting diode. Experiments are performed with pulse widths in the nanosecond range and a time-resolved single photon counting setup is used to detect the subsequent electroluminescence.
We find evidence that the achieved spin polarization shows an unexpected temporal behavior, attributed mainly to many-carrier and non-equilibrium effects in the device.
Unser Artikel mit dem Titel "GaAs micro-pyramids serving as optical micro-cavities" von M. Karl, T. Beck, S. Li, D. Z. Hu, D. M. Schaadt, H. Kalt, und M. Hetterich wurde in in AIP Conf. Proc., 1199, 369 veröffentlicht.
Abstract:
An efficient light-matter coupling requires high-quality (Q) micro-cavities with small mode volume. We suggest GaAs micro-pyramids placed on top of AlAs/GaAs distributed Bragg reflectors to be promising candidates. The pyramids were fabricated by molecular-beam epitaxy, electron-beam lithography and a subsequent wet-chemical etching process using a sacrificial AlAs layer.
Measured Q-factors of optical modes in single pyramids reach values up to 650. A finite-difference time-domain simulation assuming a simplified cone-shaped geometry suggests possible Q-factors up to 3600. To enhance the light confinement in the micro-pyramids we intend to overgrow the pyramidal facets with a Bragg mirror—results of preliminary tests are given.
Unser Artikel mit dem Titel "Reversed pyramids as novel optical micro-cavities" von M. Karl, D. Rülke, T. Beck, D.Z. Hu, D. M. Schaadt, H. Kalt, und M. Hetterich wurde in in Superlattices and Microstruct., 47, 83 veröffentlicht.
Abstract:
Pyramidal micro-cavities represent a novel promising class of semiconductor optical cavities. In contrast to our previous approach based on pyramids sitting on distributed Bragg reflectors, we investigate reversed freestanding GaAs pyramids. The latter are achievable by a wet-chemical etching process where an AlAs sacrificial layer in the epitaxially grown layer structure is used.
In freestanding GaAs pyramids, light is simply confined by total internal reflection at the interface of the high refractive index material GaAs to the surrounding. Due to strong optical confinement within the pyramidal shape, small mode volumes are expected. Quality factors up to 3000 were measured in first structures. However, simulations suggest the possibility of much higher values.
Therefore, these freestanding pyramids are promising for an optimized ratio between quality factor and mode volume, which is crucial for quantum-optical applications.
Unser Artikel mit dem Titel "GaAs photonic crystal slab nanocavities: growth, fabrication, and quality factor" von J. Sweet, B. C. Richards, J. D. Olitzky, J. Hendrickson, G. Khitrova, H. M. Gibbs, D. Litvinov, D. Gerthsen, D. Z. Hu, D. M. Schaadt, M. Wegener, U. Khankhoje, und A. Scherer wurde in in Photonics and Nanostructures - Fundamentals and Applications, 8, 1 veröffentlicht.
Abstract:
In an effort to understand why short wavelength (~1000 nm) GaAs-based photonic crystal slab nanocavities have much lower quality factors (Q) than predicted (and observed in Si), many samples were grown, fabricated into nanocavities, and studied by atomic force, transmission electron, and scanning electron microscopy as well as optical spectroscopy.
The top surface of the AlGaAs sacrificial layer can be rough even when the top of the slab is smooth; growth conditions are reported that reduce the AlGaAs roughness by an order of magnitude, but this had little effect on Q. The removal of the sacrificial layer by hydrogen fluoride can leave behind a residue; potassium hydroxide completely removes the residue, resulting in higher Qs.
Wir haben eine Zusammenarbeit mit der CFN-Arbeitsgruppe von Frau Dr. Ljiljana Fruk bzgl. der Ausnutzung von Nanopartikeln in photovoltaischen Bauelementen gestartet. Dazu wurden zwei Diplomarbeiten ausgeschrieben.
Yu-Chi Hsu hat ihre Messungen in unserer Arbeitsgruppe abgeschlossen und ist wieder nach Kaohsiung zurückgekehrt.
Sie wird dort ihre Studien in in der Arbeitsgruppe von Prof. I. Lo in enger Zusammenarbeit mit uns weiterführen.
Im Rahmen unserer Kollaboration mit der National Sun Yat-Sen University, besucht Ralf Schuber Taiwan für drei Monate.
Im folgenden Wintersemester 2009/10 wird Dr. Daniel Schaadt zusammen mit PD Dr. Michael Hetterich die Vorlesung "Solid State Optics" halten.
Des Weiteren wird Dr. Daniel Schaadt auch das Kapitel "Halbleiternanostrukturen" in der CFN-Ringvorlesung "Grundlagen der Nanotechnologie I" unterrichten.
Prof. Galina Khitrova und Prof. Hyatt Gibbs besuchen uns wieder, um unsere gemeinsamen Wachstumsstudien zu Cavity-Strukturen fortzusetzen.
Als Teil unserer Kollaboration mit der National Sun Yat-Sen University in Kaohsiung in Taiwan, besucht uns Prof. Mitch Chou für mehrere Wochen, um das Wachstum von GaN auf verschiedenen Substraten zu studieren.
Philipp Ganz hat seine Diplomarbeit erfolgreich beendet.
Er wird im November mit der Promotion in der Gruppe beginnen.
Christian Barth hat seine Diplomarbeit erfolgreich beendet.
Dr. Daniel Schaadt wurde von Prof. L. Jacak an die Technische Hochschule in Breslau, Polen eingeladen.
Mathieu Helfrich hat seine Diplomarbeit erfolgreich beendet.
Er wird ab Juli als Doktorand in der Arbeitsgruppe weiterarbeiten.
Im folgenden Sommersemester 2009 wird Dr. Daniel Schaadt zusammen mit PD Dr. Michael Hetterich die Vorlesung "Halbleiterphysik" halten.
Des Weiteren wird Dr. Daniel Schaadt auch als Betreuer im Hauptseminar "Halbleiter-Nanostrukturen" mitarbeiten.
Als Teil der Zusammenarbeit mit Prof. Ikai Lo, Prof. Liuwen Chang, Prof. Chien-Chen Kuo und Prof. Mitch Chou, besucht Dr. Daniel Schaadt als Visiting Professor die National Sun Yat-Sen Universitä,t in Kaohsiung, Taiwan. Wärend seines Aufenthalts wird Dr. Schaadt die Vorlesung "solid state optics" halten.
Wir haben Besuch von Prof. Galina Khitrova und Prof. Hyatt Gibbs von der University of Arizona in Tuscon. Sie werden mit Dr. Dongzhi Hu am Wachstum von Cavity-Strukturen arbeiten.
Raimund Vöhringer hat mit seiner Diplomarbeit in unserer Gruppe begonnen.
Er wird die Arbeit von Frau Trunk teilweise fortsetzen.
Unser Artikel mit dem Titel "Electrical spin injection into InGaAs quantum dots: single dot devices and time-resolved studies" von M. Hetterich, W. Löffler, P. Aßhoff, H. Flügge, J. Müller, B. Westenfelder, D. Z. Hu, D. M. Schaadt, und H. Kalt wurde in in Phys. Stat. Sol. C, 6, 2, 432 veröffentlicht.
Abstract:
In the context of a potential future quantum information processing we investigate the concurrent initialization of electronic spin states in InGaAs quantum dots (QDs) via electrical injection from ZnMn(S)Se spin aligners.
Single dots can be read out optically through metallic apertures on top of our spin-injection light-emitting diodes (spin-LEDs). A reproducible spin polarization degree close to 100% is observed for a subset of the QD ensemble.
However, the average polarization degree is lower and drops with increasing QD emission wavelength. Our measurements suggest that spin relaxation processes outside the QDs, related to the energetic position of the electron quasi-Fermi level, as well as defectrelated spin scattering at the III–V/II–VI interface should be responsible for this effect, leading us to an improved device design.
Finally, we present first time-resolved electroluminescence measurements of the polarization dynamics using nanosecond-pulsed electrical excitation. The latter should enable us to gain a more detailed understanding of the spin relaxation processes in our devices. They are also the first step towards future time-resolved spin manipulation experiments.
Mareike Trunk hat ihre Diplomarbeit erfolgreich beendet.
Sie ist nun Doktorandin an der Universität Oslo in Norwegen.
Dr. Daniel Schaadt wurde zu einem Vortrag an der Technischen Universit&auuml;t Bergakademie Freiberg in Freiberg, Sachsen eingeladen.
Herr Yen-Liang Chen hat uns wieder in Richtung Taiwan verlassen. Wir haben die gemeinsame Zeit genossen und wünschen ihm das Beste für seine Zukunft.
Im folgenden Wintersemester 2008/09 wird Dr. Daniel Schaadt zusammen mit PD Dr. Michael Hetterich die Vorlesung "Solid State Optics" halten. Des Weiteren wird Dr. Daniel Schaadt auch das Kapitel "Halbleiternanostrukturen" in der CFN-Ringvorlesung "Grundlagen der Nanotechnologie I" unterrichten.
Die Zusammenarbeit mit den Gruppen von Prof. Ikai Lo und Prof. Mitch Chou aus Taiwan wird weiter intensiviert durch Frau Yu-Chi Hsu, die uns für etwas mehr als ein Jahr als Austauschdoktorand besucht.
Unser Artikel mit dem Titel "Long-time evolution of the photoluminescence in C- and M-plane GaN/AlN quantum dots upon intense ultraviolet irradiation" von O. Brandt, T. Flissikowski, D. M. Schaadt, U. Jahn, A. Trampert, und H. T. Grahn wurde in in Appl. Phys. Lett., 93, 081907 veröffentlicht.
Abstract:
We compare the spontaneous emission of C- and M-plane GaN quantum dots embedded in AlN. C-plane dots are characterized by an intense emission with an exceptionally long decay time up to room temperature.
In contrast, M-plane dots exhibit a much weaker emission with a very short decay time. In addition, the emission of the C-plane dots temporally evolves on a timescale of seconds, while the emission of the M-plane dots is stable over time. These findings are correlated with the different growth mode and microstructure of C- and M-plane GaN quantum dots.
Wir haben nun Zugriff auf ein Ellipsometer, das als gemeinsames Gerät des CFN eingerichtet wurde.
Um die Zusammenarbeit im Rahmen des Projekted A2 des CFN zu vertiefen, werden wir die Diplomanden Philipp Burger und Daniel Rülke zusammen mit den Gruppen von Dr. Michael Hetterich und Prof. Dr. Heinz Kalt betreuen. Herr Burger und Herr Rülke werden an InAs Quantenpunkten für optoelektronische Anwendungen arbeiten.
Das Team wächst mit Philipp Ganz als Diplomand weiter. Herr Ganz wird Cu-doped GaN untersuchen.
Unser Artikel mit dem Titel "Line defects of M-plane GaN grown on gamma-LiAlO2 by plasma-assisted molecular beam epitaxy" von Ikai Lo, Chia-Ho Hsieh, Yen-Liang Chen, Wen-Yuan Pang, Yu-Chi Hsu, Jih-Chen Chiang, Ming-Chi Chou, Jenn-Kai Tsai, und D. M. Schaadt wurde in in Appl. Phys. Lett., 92, 202106 veröffentlicht.
Abstract:
The edge and threading dislocations of M-plane GaN epilayers grown on gamma-LiAlO2 have been studied by high-resolution transmission electron microscope. We found that edge dislocations were grown in [1-100] direction while threading dislocations were generated along a1 or -a2 axes. We also observed a single stacking fault in the M-plane GaN epilayer.
Christian Barth ist der Gruppe als weiterer Diplomand beigetreten.
Er wird eine Anlage zur elektrischen Spektroskopie von Halbleitern in Elektrolytlösungen einrichten.
Mathieu Helfrich ist nun zweiter Diplomand in der Gruppe.
Er wird am Wachstum von InAs Quantenpunkten auf vorstrukturierten Substraten arbeiten.
Im folgenden Sommersemester 2008 wird Dr. Daniel Schaadt zusammen mit Dr. Michael Hetterich die Vorlesung "Halbleiterphysik" halten.
Des Weiteren wird Dr. Daniel Schaadt auch als Betreuer im Hauptseminar "Halbleiter-Nanostrukturen" mitarbeiten.
Dr. Marco Schowalter von der Universität Bremen besucht uns, um mit Dr. Dongzhi Hu GaInNAs Proben zur Charakterisierung mittels Tansmissionselektronenmikroskopie herzustellen.
Wir haben einen Laser mit Frequenzverdopplung angeschafft, der in einem vom CFN gemeinsam benutzten Photolumineszenzlabor eingerichtet wird und uns erlauben wird, Nitride im Ultravioletten zu untersuchen.
Die Gruppe wächst weiter. Mareike Trunk hat als Diplomandin angefangen und wird am Wachstum von Nitriden für photoelektrolytische Anwendungen arbeiten.
Als Teil der Zusammenarbeit mit den Gruppen von Prof. Ikai Lo und Prof. Mitch Chou aus Taiwan wird uns Herr Yen-Liang Chen für ein Jahr als Austauschdoktorand besuchen.
Ralf Schuber hat als erster Doktorand angefangen.
Er wird sich mit piezoelektrischen Feldern in Nitridnanostrukturen beschäftigen.
Im folgenden Wintersemester 2007/08 wird Dr. Daniel Schaadt zusammen mit Prof. Dr. Heinz Kalt die Vorlesung "Halbleiteroptik" halten. Des Weiteren wird Dr. Daniel Schaadt auch das Kapitel "Halbleiternanostrukturen" in der CFN-Ringvorlesung "Grundlagen der Nanotechnologie I" unterrichten.
Wir haben eine Zusammenarbeit mit den Grupppen von Prof. Ikai Lo und Prof. Mitch Chou von der National Sun Yat-Sen Universität in Kaohsiung, Taiwan, vereinbart. Wir werden am Wachstum von Nitriden auf LiAlO2 Substraten arbeiten.
Wir haben nun Zugriff auf ein Röntgendiffraktometer, das als gemeinsames Gerät des CFN eingerichtet wurde.
Unser Artikel mit dem Titel "Polarization-dependent beam switch based on an M-plane GaN/AlN distributed Bragg reflector" von D. M. Schaadt, O. Brandt, S. Ghosh, T. Flissikowski, U. Jahn, und H. T. Grahn wurde in Appl. Phys. Lett., 90, 231117 veröffentlicht.
Abstract:
The authors demonstrate a two-color distributed Bragg reflector (DBR) consisting of 20 periods of alternating [1-100]-oriented (M-plane) AlN and GaN layers grown on LiAlO2 by molecular-beam epitaxy. Due to the birefringent nature of GaN and AlN, the wavelength region of the stop band depends on the polarization state of the incoming light beam (parallel or perpendicular) with respect to the c axis of the wurtzite crystal structure.
In the wavelength range, where the transmittance for one polarization direction and the reflectance for the orthogonal polarization direction are both high, the DBR can be used as a beam switch or polarization filter.
Unser Artikel mit dem Titel "Three-dimensional magnetic flux-closure domain patterns in MnAs thin films on GaAs(001)" von R. Engel-Herbert, T. Hesjedal, und D. M. Schaadt wurde in J. Appl. Phys., 101, 09K103 veröffentlicht.
Abstract:
The magnetic microstructure of single-crystalline MnAs films on GaAs(001) has been investigated. Magnetic force microscopy (MFM) reveals a three-dimensional magnetization pattern that is in disagreement with the simple domain picture observed by surface-sensitive magnetic imaging.
Here, we present a consistent micromagnetic picture of MnAs thin films in the ferromagnetic stripe phase, which appears in the course of the phase transition. A number of equilibrium magnetization patterns of the stripes are found that are, in fact, based on flux-closure domain patterns in the basal plane of MnAs.
The simulation of a stripe array yields excellent agreement with the measured surface magnetization. The experimentally observed stray field contrast was confirmed by MFM contrast simulations based on these equilibrium magnetization patterns.
Die neue unabhängige Nachwuchsgruppe wurde eingerichtet. Das Team startet mit den Mitgliedern Dr. Daniel M. Schaadt (Leiter), Dr. Dongzhi Hu (Postdoktorandin) und Heinrich Reimer (Techniker). Wir haben zur Zeit Zugriff auf zwei Molekularstrahlanlagen (eine für III-As-Wachstum und eine für IIb-VI-Wachstum), einen Hall-Messplatz sowie eine thermische Verdampfungsanlage zur Herstellung von Kontakten. Im ersten Jahr werden wir eine weitere MBE-Anlage zum Wachstum von Nitriden sowie weitere Charakterisierungsanlagen einrichten.