65 results on '"Andreas Waag"'
Search Results
2. A Combination of Ion Implantation and High‐Temperature Annealing: The Origin of the 265 nm Absorption in AlN
- Author
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Lukas Peters, Christoph Margenfeld, Jan Krügener, Carsten Ronning, and Andreas Waag
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carbon ,Surfaces and Interfaces ,high-temperature annealing ,Condensed Matter Physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,transmittance ,Materials Chemistry ,point defects ,ddc:530 ,ion implantation ,Dewey Decimal Classification::500 | Naturwissenschaften::530 | Physik ,Electrical and Electronic Engineering ,absorption ,AlN - Abstract
The commonly observed absorption around 265 nm in AlN is hampering the outcoupling efficiency of light-emitting diodes (LEDs) emitting in the UV-C regime. Carbon impurities in the nitrogen sublattice (CN) of AlN are believed to be the origin of this absorption. A specially tailored experiment using a combination of ion implantation of boron, carbon, and neon with subsequent high-temperature annealing allows to separate the influence of intrinsic point defects and carbon impurities regarding this absorption. Herein, the presented results reveal the relevance of the intrinsic nitrogen-vacancy defect VN. This is in contradiction to the established explanation based on CN defects as the defect causing the 265 nm absorption and will be crucial for further UV-LED improvement. Finally, in this article, a new interpretation of the 265 nm absorption is introduced, which is corroborated by density functional theory (DFT) results from the past decade, which are reviewed and discussed based on the new findings.
- Published
- 2022
3. Low‐Energy Hydrogen Ions Enable Efficient Room‐Temperature and Rapid Plasma Hydrogenation of TiO 2 Nanorods for Enhanced Photoelectrochemical Activity
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Xiaodan Wang, Leonhard Mayrhofer, Martin Keunecke, Sonia Estrade, Lluis Lopez‐Conesa, Michael Moseler, Andreas Waag, Lothar Schaefer, Weidong Shi, Xiangjian Meng, Junhao Chu, Zhiyong Fan, and Hao Shen
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Biomaterials ,General Materials Science ,General Chemistry ,Biotechnology - Published
- 2022
4. 3D GaN Fins as a Versatile Platform for a-Plane-Based Devices
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Andreas Waag, Achim Trampert, Irene Manglano Clavero, Christoph Margenfeld, Hao Zhou, Hendrik Spende, Frederik Steib, Hergo-Heinrich Wehmann, Hans-Jürgen Lugauer, Jana Hartmann, Lars Nicolai, Martin Strassburg, Angelina Jaros, Tobias Voss, Johannes Ledig, and Adrian Stefan Avramescu
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Free electron model ,Materials science ,02 engineering and technology ,Epitaxy ,01 natural sciences ,Article ,GaN fins, threading dislocation density, a-plane sidewalls, selective area MOVPE, marker layers ,Impurity ,0103 physical sciences ,ddc:6 ,Veröffentlichung der TU Braunschweig ,ddc:62 ,010302 applied physics ,business.industry ,Growth model ,Continuous mode ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Crystallographic defect ,Electronic, Optical and Magnetic Materials ,Template ,Optoelectronics ,ddc:621 ,0210 nano-technology ,business ,Material properties - Abstract
GaN fins on GaN-on-sapphire templates are fabricated by continuous mode selective area metalorganic vapor phase epitaxy. The fins exhibit high aspect ratios and smooth nonpolar a-plane sidewalls with an ultra-low threading dislocation density of a few 105 cm^-2 making them ideally suited for optoelectronic to electronic applications. A detailed analysis of the inner structure of GaN fins is provided by the help of marker layer experiments and correlation of results from fins fabricated under different growth conditions, leading to the development of a growth model to explain the final geometry and optical as well as electrical properties of these high aspect ratio fins. Distinctly different material properties for the central and outer parts of the fins are detected. Whereas the outer sidewalls represent high quality GaN surfaces with very low defect densities, a strong quenching of near band edge emission (NBE) in the central part of the fins is accompanied by heavy compensation of free electrons. A possible explanation is the incorporation of excessive point defects, like intrinsic defects or carbon impurity. The sidewall regions, however, prove to be highly suitable for device applications due to their strong NBE emission, low dislocation density and high free carrier concentration.
- Published
- 2018
5. Phosphor-converted white light from blue-emitting InGaN microrod LEDs
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Daniel Bichler, Frank Bertram, Peter Veit, Marcus Müller, Ion Stoll, Jana Hartmann, Jürgen Christen, Franz Zwaschka, Andreas Waag, Martin Mandl, Hans-Jürgen Lugauer, Benjamin Max, Martin Strassburg, Johanna Strube-Knyrim, Bianca Pohl-Klein, Barbara Huckenbeck, and Tilman Schimpke
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Materials science ,Phosphor ,Cathodoluminescence ,Gallium nitride ,02 engineering and technology ,Color temperature ,01 natural sciences ,law.invention ,chemistry.chemical_compound ,Optics ,law ,0103 physical sciences ,Materials Chemistry ,Metalorganic vapour phase epitaxy ,Electrical and Electronic Engineering ,Quantum well ,010302 applied physics ,business.industry ,Surfaces and Interfaces ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,chemistry ,Optoelectronics ,0210 nano-technology ,business ,Luminescence ,Light-emitting diode - Abstract
A uniform array of gallium nitride core-shell microrod (MR) light-emitting diode (LED) structures was grown by metalorganic vapor phase epitaxy. Defects and the quantum well (QW) luminescence in an individual rod were investigated by scanning tunneling electron microscopy (STEM) and STEM cathodoluminescence. Luminescence with different wavelength was detected from the quantum wells on the semipolar tip facets and the nonpolar sidewalls of the MRs. Furthermore, the MR array is processed into LED chips. The electro-optical characteristics of the devices are analyzed. Two separate emission bands are distinguished, which are attributed to the QWs on the semipolar tip facets and the nonpolar sidewalls, respectively. To obtain white LEDs, micrograin phosphors were developed which fit in between individual MRs. By using electrophoretic particle deposition, these phosphors are deposited onto the MR LED chips. Color coordinates, color temperature, and device efficiency are evaluated. Blue (top) and phosphor-converted white (bottom) microrod LEDs on 4″ wafer.
- Published
- 2016
6. Point Defect‐Induced UV‐C Absorption in Aluminum Nitride Epitaxial Layers Grown on Sapphire Substrates by Metal‐Organic Chemical Vapor Deposition
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Roland Zeisel, Rainer Lösing, Hans-Jürgen Lugauer, Axel Hoffmann, Andreas Waag, Felix Nippert, Christian Brandl, Matthew J. Davies, Marc Patrick Hoffmann, Martin Mandl, Nadine Tillner, and Christian Frankerl
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Materials science ,Photoluminescence ,Absorption spectroscopy ,Analytical chemistry ,Photoluminescence excitation ,Metalorganic vapour phase epitaxy ,Chemical vapor deposition ,Nitride ,Condensed Matter Physics ,Epitaxy ,Absorption (electromagnetic radiation) ,Electronic, Optical and Magnetic Materials - Abstract
Herein, the optical properties of aluminum nitride (AlN) epitaxial layers grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) are reported. The structures investigated in this study are grown at highly different degrees of supersaturation in the MOCVD process. In addition, both pulsed and continuous growth conditions are employed and AlN is deposited on nucleation layers favoring different polarities. The samples are investigated by photoluminescence (PL), photoluminescence excitation (PLE), and absorption spectroscopy and are found to vary significantly in absorption and emission characteristics. Two distinct absorption bands in the UV-C spectral range are observed and examined in greater detail, with either giving rise to a significant absorption coefficient of around 1000 cm−1. The corresponding defect transitions are identified by PL spectroscopy. Combined with secondary-ion mass spectrometry (SIMS) measurements, these absorption bands are allocated to the incorporation of carbon and oxygen impurities, depending on the applied growth conditions. Furthermore, similarities with other epitaxial growth techniques serving as basis for UV-C applications are highlighted. These results are highly relevant for a better understanding of absorption issues in AlN templates grown by various deposition techniques. In addition, consequences for the growth of efficient UV-C devices by MOCVD on sapphire substrates are outlined.
- Published
- 2020
7. Characterization of the internal properties of InGaN/GaN core-shell LEDs
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Xue Wang, Uwe Jahn, Hergo-Heinrich Wehmann, Michael Seibt, Henning Schuhmann, Andreas Waag, Sönke Fündling, and Johannes Ledig
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Materials science ,Current crowding ,Cathodoluminescence ,02 engineering and technology ,Electroluminescence ,Epitaxy ,01 natural sciences ,law.invention ,Optics ,Depletion region ,law ,0103 physical sciences ,Materials Chemistry ,Metalorganic vapour phase epitaxy ,Electrical and Electronic Engineering ,010302 applied physics ,business.industry ,Electron beam-induced current ,Surfaces and Interfaces ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Optoelectronics ,0210 nano-technology ,business ,Light-emitting diode - Abstract
In this paper, we present the structural, optical, and electrical characterization of InGaN/GaN core-shell micro-LED structures selectively grown by metal organic vapor phase epitaxy (MOVPE) in arranged arrays. In particular we analyze the core-shell geometry of the pillars, consisting of a fivefold InGaN/GaN multiple quantum well and a p-type layer on all sidewall and top facets. Additionally we analyze the optical properties of the structure and the active region with high spatial resolution by cathodoluminescence. Electron beam induced current measurements (EBIC) are performed using an SEM based manipulator setup, giving proof of the presence of a depletion region as well as the intended doping polarity of the n-type core and a p-type shell wrapped around the whole structure. Using a p–n–p configuration also current crowding is discussed by EBIC and electroluminescence is demonstrated by measuring emission patterns from single core–shell structures. Color overlay of SE image (grey, FOV = 12.7 μm) and EBIC image (red) of a cleaved core–shell LED pillar contacted by two probe tips at a substrate tilt of 30°.
- Published
- 2015
8. Growth mechanisms of GaN microrods for 3D core-shell LEDs: The influence of silane flow
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Andreas Waag, Jana Hartmann, Winfried Daum, Tilman Schimpke, Henning Schuhmann, Xue Wang, Markus Bähr, Wanja Dziony, Michael Seibt, Martin Dr. Straßburg, Johannes Ledig, Matin Sadat Mohajerani, Hergo-Heinrich Wehmann, Lorenzo Caccamo, and G. Lilienkamp
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Auger electron spectroscopy ,Materials science ,Passivation ,business.industry ,Nanotechnology ,Surfaces and Interfaces ,Condensed Matter Physics ,Silane ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,chemistry.chemical_compound ,Selective area epitaxy ,chemistry ,Transmission electron microscopy ,Materials Chemistry ,Optoelectronics ,Nanorod ,Growth rate ,Electrical and Electronic Engineering ,Thin film ,business - Abstract
The three dimensional growth of GaN structures as a basis for the fabrication of 3D GaN core-shell LEDs has attracted substantial attention in the past years. GaN nanorods or microrods with high aspect ratios can be grown by selective area epitaxy on a GaN buffer through a SiOx mask. It has been found earlier that silane substantially initiates vertical growth, with the exact underlying mechanisms being still unclear. Here, the influence of silane on the 3D GaN column growth was investigated by performing detailed growth experiments in combination with a thorough surface analysis in order to get insight into these mechanisms. The vertical growth rate is significantly enhanced by high silane fluxes, whereas the saturation of growth rate with the time is reduced. Thus, homogenous GaN columns with an aspect ratio of more than 35 could be achieved. A thin Si-rich layer on the non-polar m-plane facets of the columns has been detected using a combination of transmission electron microscopy, energy dispersive X-ray spectroscopy and Auger electron spectroscopy. This layer is suggested to be the reason for the increase in growth rate, modifying the effective collection range of the species along the sidewalls, and preventing the lateral growth.
- Published
- 2015
9. Surface photovoltage behavior of GaN columns
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Jiandong Wei, Hergo-Heinrich Wehmann, Jana Hartmann, Manal Ali Deeb, and Andreas Waag
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Kelvin probe force microscope ,Materials science ,business.industry ,Surface photovoltage ,Analytical chemistry ,Surfaces and Interfaces ,Condensed Matter Physics ,Epitaxy ,Isotropic etching ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Surface coating ,Etching (microfabrication) ,Materials Chemistry ,Optoelectronics ,Metalorganic vapour phase epitaxy ,Electrical and Electronic Engineering ,business ,Surface states - Abstract
GaN columns are grown by metal organic vapor phase epitaxy (MOVPE) with a high silane flow, which often leads to rough sidewalls and related problems during a subsequent shell overgrowth by e.g., InGaN/GaN quantum wells. In order to get more information on the surface of GaN columns before overgrowth, surface photovoltage (SPV) measurements have been performed by photo-assisted Kelvin probe force microscopy (KPFM) before and after chemical etching of the surface. Its electronic properties are demonstrated to be influenced by the silane injection during the samples' growth. Distinct differences are shown in the SPV behavior of GaN columns before and after etching with phosphoric acid. SPV decreases before etching, whereas after etching it increases like that on GaN bulk material. Silicon related surface states introduced during the growth are considered as the origin of the SPV behavior of the as grown samples. The change of this behavior after etching is attributed to the removal of the Si containing surface coating. Our investigations not only show the important role of silane injection during the growth on the SPV behavior of GaN columns, but it also reveals that the photo-assisted KPFM technique is an easy and quick method to analyze surfaces.
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- 2015
10. Growth and characterization of mixed polar GaN columns and core-shell LEDs
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Martin Mandl, Tilman Schimpke, Jana Hartmann, Andreas Waag, Xue Wang, Uwe Jahn, Martin Dr. Straßburg, Hergo-H. Wehmann, and Johannes Ledig
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Materials science ,Analytical chemistry ,Surfaces and Interfaces ,Condensed Matter Physics ,Epitaxy ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,law.invention ,Characterization (materials science) ,Metal ,Core shell ,law ,visual_art ,Phase (matter) ,Materials Chemistry ,visual_art.visual_art_medium ,Sapphire ,Polar ,Electrical and Electronic Engineering ,Light-emitting diode - Abstract
The growth mechanisms of selective area growth of GaN microcolumns by metal organic vapor phase epitaxy on patterned SiOx/sapphire templates, are investigated. Both Ga- and N-polar domains within a GaN column are detected. The growth system of mixed polar GaN columns is near reaction limitation, which is quantitatively determined by the Damkohler number. The major part of the mixed polar GaN columns is N-polar. However, the Ga-polar domains increase the vertical growth rate of the whole column. The strain status of the columns is almost totally relaxed. Core–shell LED structures were realized on the mixed polar GaN columns. The optical properties of the core–shell LEDs were characterized.
- Published
- 2015
11. A Highly Selective and Self-Powered Gas Sensor Via Organic Surface Functionalization of p-Si/n-ZnO Diodes
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Lorenzo Caccamo, Francisco Hernandez-Ramirez, Winfried Daum, Michael Moseler, Leonhard Mayrhofer, G. Lilienkamp, Hao Shen, Andreas Waag, J. Daniel Prades, Olga Casals, Martin W. G. Hoffmann, Universitat de Barcelona, and Publica
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Materials science ,Nanotechnology ,02 engineering and technology ,010402 general chemistry ,01 natural sciences ,density functional theory (DFT) ,hybrid materials ,General Materials Science ,Diode ,selective sensing ,self-powered ,Mechanical Engineering ,self-assembled monolayers ,Self-assembled monolayer ,Gas detectors ,Detectors de gasos ,021001 nanoscience & nanotechnology ,Highly selective ,0104 chemical sciences ,Semiconductors ,Mechanics of Materials ,Power consumption ,Modulation ,Surface modification ,0210 nano-technology ,Selectivity ,Hybrid material - Abstract
Selectivity and low power consumption are major challenges in the development of sophisticated gas sensor devices. A sensor system is presented that unifies selective sensor-gas interactions and energy-harvesting properties, using defined organic-inorganic hybrid materials. Simulations of chemical-binding interactions and the consequent electronic surface modulation give more insight into the complex sensing mechanism of selective gas detection.
- Published
- 2014
12. Heating of the Mn spin system by photoexcited holes in type-II (Zn,Mn)Se/(Be,Mn)Te quantum wells
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Dmitri R. Yakovlev, V. Yu. Ivanov, D. Dunker, Andreas Waag, A. A. Maksimov, Manfred Bayer, E. V. Filatov, J. Debus, and I. I. Tartakovskii
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Condensed Matter::Materials Science ,Photoluminescence ,Condensed matter physics ,Chemistry ,Relaxation (NMR) ,Heterojunction ,Magnetic semiconductor ,Condensed Matter Physics ,Spin (physics) ,Quantum well ,Excitation ,Electronic, Optical and Magnetic Materials ,Ion - Abstract
The efficiency of the Mn-spin system heating under pulsed laser excitation is studied in diluted magnetic semiconductor heterostructures Zn0.99 Mn0.01 Se/Be0.93 Mn0.07 Te with type-II band alignment by means of time-resolved photoluminescence and pump-probe reflectivity. An essential role in the heating is played by multiple spin-flip scatterings of a hole with localized spins of Mn2+ ions. The efficiency of the spin and energy transfer from photoexcited holes to Mn ions of the Zn0.99 Mn0.01 Se layer considerably depends on the hole lifetime in this layer. This lifetime can be limited by the hole relaxation into the Be0.93 Mn0.07 Te layers and is strongly sensitive to the excitation power and Zn0.99 Mn0.01 Se layer thickness. These dependences allow us to determine a characteristic time of about 20ps for the spin and energy transfer from photoexcited holes to the Mn spin system.
- Published
- 2014
13. Spin coherence of electrons and holes in ZnSe-based quantum wells studied by pump-probe Kerr rotation
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Laurens W. Molenkamp, Andreas Waag, O. A. Yugov, Manfred Bayer, E. A. Zhukov, A. Schwan, and Dmitri R. Yakovlev
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Physics ,Condensed matter physics ,Spin polarization ,Dephasing ,Electron ,Condensed Matter Physics ,Spin (physics) ,Fermi gas ,Anisotropy ,Quantum well ,Electronic, Optical and Magnetic Materials ,Coherence (physics) - Abstract
Spin coherence of resident electrons and holes is measured in ZnSe-based quantum wells by means of time-resolved Kerr rotation technique. At a temperature of 1.8 K spin dephasing time for localized electrons can be as long as 33 ns, and for holes of 0.8 ns. Electron spin precession is clearly observed in a wide temperature range up to 230 K. The electron spin dephasing becomes much shorter of 0.2 ns for the quantum well with a high-density electron gas. Using vector magnet all components of the g-factors tensor are evaluated for the electrons and heavy-holes, revealing strong anisotropy for the heavy-holes.
- Published
- 2014
14. Facile and Efficient Atomic Hydrogenation Enabled Black TiO 2 with Enhanced Photo‐Electrochemical Activity via a Favorably Low‐Energy‐Barrier Pathway
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Xiaodan Wang, Leonhard Mayrhofer, Sònia Estradé, G. Braeuer, Lluís López-Conesa, Michael Moseler, Lothar Schaefer, Hao Shen, Yuanjing Lin, Andreas Waag, Hao Zhou, Markus Hoefer, Francesca Peiró, and Zhiyong Fan
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Low energy ,Materials science ,Renewable Energy, Sustainability and the Environment ,Transmission electron microscopy ,Electron energy loss spectroscopy ,General Materials Science ,Density functional theory ,Photochemistry ,Electrochemistry - Published
- 2019
15. Vapour phase epitaxy of Cu 2 O on a‐plane Al 2 O 3
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Harald Scherg-Kurmes, Andrey Bakin, Andreas Waag, and Alexander Wagner
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Diffraction ,Chemistry ,Vapor pressure ,Impurity ,Phase (matter) ,Analytical chemistry ,chemistry.chemical_element ,Nanotechnology ,Metalorganic vapour phase epitaxy ,Atmospheric temperature range ,Condensed Matter Physics ,Epitaxy ,Copper - Abstract
We present a vapour phase based approach for the epitaxial growth of Cu2O with the utilization of elemental copper and oxygen as precursor materials. At present the implementation of MOCVD approach is hampered by the absence of the chemical source of Cu with high enough vapour pressure at reasonable temperatures. Moreover, the use of elemental precursors has the advantage of less impurity incorporation during growth opposed to chemical vapour based growth methods. The influence of process parameters on the growth is discussed. High resolution X-ray diffraction measurements reveal the growth of mainly the two (002) and (220) Cu2O orientations. Precise control of the growth parameters provides the reproducible growth of pure Cu2O phase without additional CuO formation. Epitaxial growth of Cu2O is demonstrated on a-plane Al2O3 in the temperature range of 730 °C–870 °C. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2013
16. Group III nitride core-shell nano- and microrods for optoelectronic applications
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Andreas Waag, Xiang Kong, Michael Binder, Christopher Kölper, Jürgen Christen, Martin Strassburg, Martin Mandl, Achim Trampert, F. Barbagini, Enrique Calleja, Frank Bertram, Xue Wang, Tilman Schimpke, and Johannes Ledig
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010302 applied physics ,Materials science ,Fabrication ,business.industry ,Nanotechnology ,02 engineering and technology ,Nitride ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Epitaxy ,01 natural sciences ,law.invention ,Solid-state lighting ,law ,0103 physical sciences ,Nano ,Optoelectronics ,General Materials Science ,Metalorganic vapour phase epitaxy ,0210 nano-technology ,business ,Layer (electronics) ,Light-emitting diode - Abstract
In the past few years, tremendous progress has been demonstrated on epitaxial growth and processing of group III nitride nano- and microrods (NAMs). This has also enabled the fabrication of optoelectronic devices based on NAMs as active elements. However, their efficiency is still far behind the performance of conventional GaN-based light emitting diodes (LEDs). This Review presents the most recent activities on the growth and processing of NAMs exhibiting a core–shell geometry, i.e. structures which consist of an active region shell layer wrapped around a three-dimensional (3D) core, which allow for an enormous increase in active area compared to planar technology. The most common growth approaches using metalorganic vapour phase epitaxy are described and evaluated with particular regard to their potential for solid state lighting applications. Examples for the unique properties of 3D NAMs are presented including their excellent crystalline quality. Furthermore, factors limiting the overall performance of 3D core–shell LEDs are revealed and the potential of overcoming these limitations are discussed. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2013
17. Higher than 90% internal quantum efficiency of photoluminescence in GaN:Si,Zn
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Matin Sadat Mohajerani, J. D. McNamara, Michael A. Reshchikov, Andreas Waag, Arne Behrends, and Andrey Bakin
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Quenching ,Materials science ,Photoluminescence ,Silicon ,business.industry ,chemistry.chemical_element ,Rate equation ,Condensed Matter Physics ,Acceptor ,chemistry ,Phenomenological model ,Optoelectronics ,Quantum efficiency ,Luminescence ,business - Abstract
Photoluminescence (PL) from high-quality GaN codoped with silicon and zinc was investigated in detail. The internal quantum efficiency (IQE) of PL was determined from the analysis of the dependencies of the PL intensity on the excitation intensity and temperature, and the simulation of these dependencies with a phenomenological model based on rate equations. The model reproduces an important phenomenon: the quenching of a recombination channel with a high IQE causes a rise in efficiency of all the other PL bands. Quantitative analysis of this phenomenon allows one to determine reliably the absolute IQE of PL. The absolute IQE of the PL in GaN co-doped with Si and Zn exceeds 90%, with the largest contribution coming from the blue luminescence band associated with the ZnGa acceptor. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2013
18. Determination of zinc concentration in GaN:Zn,Si from photoluminescence
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Stefan Kück, Matin Sadat Mohajerani, Arne Behrends, Michael A. Reshchikov, Andreas Waag, and Andrey Bakin
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Photoluminescence ,Materials science ,Silicon ,Doping ,Analytical chemistry ,chemistry.chemical_element ,Gallium nitride ,Zinc ,Chemical vapor deposition ,Condensed Matter Physics ,Acceptor ,chemistry.chemical_compound ,chemistry ,Luminescence - Abstract
Gallium nitride samples codoped with zinc and silicon were fabricated by metal organic chemical vapor deposition. Optical properties of these samples with lowlevel doped acceptor are investigated by temperature dependent photoluminescence (PL) measurements. PL spectrum shows three peaks labeled as near band edge (NBE), blue luminescence (BL), and yellow luminescence (YL). BL originates from ZnGa acceptors in GaN:Zn,Si. It is quenched above 250 K for all samples measured at excitation density of Pexc = 0.2 Wcm−2. The concentration of acceptors responsible for the BL band is estimated for relatively low Zn concentrations by comparing integrated PL intensity of NBE and Zn-related BL bands at 200 K (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2013
19. Zn doped GaN for single‐photon emission
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Arne Behrends, W. Schmunk, Johannes Ledig, Helmut Hofer, Andreas Waag, Ana Maria Racu, Andrey Bakin, Stefan Kück, M. Al-Suleiman, and Silke Peters
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Range (particle radiation) ,Materials science ,business.industry ,Chemical vapor deposition ,Electroluminescence ,Condensed Matter Physics ,law.invention ,Metal ,law ,Excited state ,visual_art ,visual_art.visual_art_medium ,Optoelectronics ,Metalorganic vapour phase epitaxy ,business ,Luminescence ,Light-emitting diode - Abstract
In this work we report on the optical investigation of Zn doped GaN films fabricated by metal organic chemical vapor deposition. The samples show bright emission in the blue spectral range around 2.9 eV when Si codoping is provided. This emission is suggested to be used for single-photon emission, thus the density of the Zn-Si pairs was drastically reduced leading to a decrease of the blue luminescence. For electrically excited single-photon sources these Zn-Si pairs have to be incorporated into LEDs, therefore we fabricated GaN-based nano-LEDs which show electroluminescence at 430 nm (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2012
20. Transparent conductive Ga-doped ZnO films fabricated by MOCVD
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Robert Walter, M. Al-Suleiman, Martin Strassburg, Hans-Jürgen Lugauer, Arne Behrends, Andreas Weimar, Alexander Wagner, Andrey Bakin, and Andreas Waag
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Fabrication ,Materials science ,business.industry ,Doping ,Surfaces and Interfaces ,Chemical vapor deposition ,Substrate (electronics) ,Condensed Matter Physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,law.invention ,law ,Sputtering ,Materials Chemistry ,Optoelectronics ,Metalorganic vapour phase epitaxy ,Electrical and Electronic Engineering ,business ,Electrical conductor ,Light-emitting diode - Abstract
Transparent conductive oxides (TCOs) are used for a variety of different applications, e.g., in solar cells and light emitting diodes (LEDs). Mostly, sputtering is used, which often results in a degradation of the underlying semiconductor material. In this work we report on a “soft” method for the fabrication of ZnO films as TCO layers by using metal organic chemical vapor deposition (MOCVD) at particularly low temperatures. The MOCVD approach has been studied focusing on the TCO key issues: fabrication temperature, morphology, optical, and electrical properties. Very smooth ZnO films with rms values down to 0.8 nm were fabricated at a substrate temperature of only 300 °C. Ga-doping is well controllable even for high carrier concentrations up to 2 × 1020 cm−3, which is above the Mott-density leading to metallic-like behavior of the films. Furthermore all films show excellent optical transparency in the visible spectral range. As a consequence, our MOCVD approach is well suited for the soft fabrication of ZnO-based TCO layers.
- Published
- 2012
21. Characterisation of 3D‐GaN/InGaN core‐shell nanostructures by transmission electron microscopy
- Author
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Ian Griffiths, Andreas Waag, Xue Wang, David Cherns, Martin Strassburg, Martin Mandl, and Hergo-Heinrich Wehman
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Nanostructure ,Materials science ,business.industry ,Scanning electron microscope ,Nucleation ,Crystal growth ,Nanotechnology ,Condensed Matter Physics ,Epitaxy ,Crystallographic defect ,Transmission electron microscopy ,Optoelectronics ,Metalorganic vapour phase epitaxy ,business - Abstract
Transmission and scanning electron microscopy have been used to characterise GaN/InGaN 3D nanostructures grown on patterned GaN/sapphire substrates by metal organic vapour phase epitaxy (MOVPE). It has been found that the growth of well ordered arrays of such nanostructures, containing multiple quantum wells on non-polar side-facets, can be achieved with a low density of defects. Growth changes and surface morphology play a major role in the nucleation of any defects present. The nanostructure morphology has been investigated and differing growth rates on adjacent facets studied. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2014
22. Traceable Nanomechanical Metrology of GaN Micropillar Array
- Author
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Muhammad Fahlesa Fatahilah, Andreas Waag, Min Xu, Jannick Langfahl-Klabes, Uwe Brand, Andre Felgner, Hutomo Suryo Wasisto, Frank Pohlenz, Prabowo Puranto, Klaas Strempel, Erwin Peiner, Feng Yu, and Zhi Li
- Subjects
010302 applied physics ,Materials science ,Nanometrology ,0103 physical sciences ,General Materials Science ,Nanotechnology ,02 engineering and technology ,Nanoindentation ,021001 nanoscience & nanotechnology ,0210 nano-technology ,Condensed Matter Physics ,01 natural sciences ,Metrology - Published
- 2018
23. GaN and ZnO nanostructures
- Author
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Shunfeng Li, Andreas Waag, Henning Riechert, Achim Trampert, Uwe Jahn, Arne Behrends, Jonas Lähnemann, Sönke Fündling, Stephan Merzsch, Ü. Sökmen, M. Al-Suleiman, Hergo-Heinrich Wehmann, and Andrey Bakin
- Subjects
Materials science ,Silicon ,Wide-bandgap semiconductor ,chemistry.chemical_element ,Gallium nitride ,Nanotechnology ,Chemical vapor deposition ,Condensed Matter Physics ,Epitaxy ,Electronic, Optical and Magnetic Materials ,chemistry.chemical_compound ,chemistry ,Silicon carbide ,Sapphire ,Nanorod - Abstract
GaN and ZnO are both wide band gap semiconductors with interesting properties concerning optoelectronic and sensor device applications. Due to the lack or the high costs of native substrates, alternatives like sapphire, silicon, or silicon carbide are taken, but the resulting lattice and thermal mismatches lead to increased defect densities which reduce the material quality. In contrast, nanostructures with high aspect ratio have lower defect densities as compared to layers. In this work, we give an overview on our results achieved on both ZnO as well as GaN based nanorods. ZnO nanostructures were grown by a wet chemical approach as well as by VPT on different substrates – even on flexible polymers. To compare the growth results we analyzed the structures by XRD and PL and show possible device applications. The GaN nano- and microstructures were grown by metal organic vapor phase epitaxy either in a self-organized process or by selective area growth for a better control of shape and material composition. Finally we take a look onto possible device applications, presenting our attempts, e.g., to build LEDs based on GaN nanostructures.
- Published
- 2010
24. GaN and LED structures grown on pre‐patterned silicon pillar arrays
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Achim Trampert, Ü. Sökmen, Peter Hinze, Uwe Jahn, Hergo-Heinrich Wehmann, Shunfeng Li, Sönke Fündling, Erwin Peiner, Richard Neumann, Stephan Merzsch, Henning Riechert, Thomas Weimann, and Andreas Waag
- Subjects
Materials science ,Plasma etching ,Silicon ,business.industry ,Nanowire ,chemistry.chemical_element ,Cathodoluminescence ,Condensed Matter Physics ,law.invention ,chemistry ,law ,Optoelectronics ,Nanorod ,Metalorganic vapour phase epitaxy ,business ,Nanopillar ,Light-emitting diode - Abstract
GaN nanorods (or nanowires) have attracted great interest in a variety of applications, e.g. high-efficiency light emitting diodes, monolithic white light emission and optical interconnection due to their superior properties. In contrast to the mostly investigated self-assembled growth of GaN nanorods, we performed GaN nanorod growth by pre-patterning of the Si substrates. The pattern was transferred to Si substrates by photolithography and cryo-temperature inductively-coupled plasma etching. These Si templates then were used for further GaN nanorod growth by metal-organic vapour phase epitaxy (MOVPE). The low temperature AlN nucleation layer had to be optimized since it differs from its 2D layer counterpart on the surface area and orientations. We found a strong influence of diffusion processes, i.e. the GaN grown on top of the Si nanopillars can deplete the GaN around the Si pillars. Transmission electron microscopy measurements demonstrated clearly that the threading dislocations bend to the side facets of the pyramidal GaN nanostructures and terminate. Cathodoluminescence measurements reveal a difference of In composition and/or thickness of InGaN quantum wells on the different facets of the pyramidal GaN nanostructures. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2010
25. Three-dimensionally structured silicon as a substrate for the MOVPE growth of GaN nanoLEDs
- Author
-
Achim Trampert, Peter Hinze, Andreas Waag, Uwe Jahn, Shunfeng Li, Erwin Peiner, Ü. Sökmen, Henning Riechert, Thomas Weimann, Stephan Merzsch, Hergo-Heinrich Wehmann, and Sönke Fündling
- Subjects
Materials science ,Indium nitride ,Silicon ,business.industry ,chemistry.chemical_element ,Heterojunction ,Cathodoluminescence ,Gallium nitride ,Surfaces and Interfaces ,Condensed Matter Physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,chemistry.chemical_compound ,Optics ,chemistry ,Materials Chemistry ,Optoelectronics ,Nanorod ,Metalorganic vapour phase epitaxy ,Electrical and Electronic Engineering ,business ,Nanopillar - Abstract
Three-dimensionally patterned Si(111) substrates are used to grow GaN based heterostructures by metalorganic vapour phase epitaxy, with the goal of fabricating well controlled, defect reduced GaN-based nanoLEDs. In contrast to other approaches to achieve GaN nanorods, we employed silicon substrates with deep etched nanopillars to control the GaN nanorods growth by varying the size and distance of the Si pillars. The small footprint of GaN nanorods grown on Si pillars minimise the influence of the lattice mismatched substrate and improve the material quality. For the Si pillars an inductively coupled plasma dry-etching process at cryogenic temperature has been developed. An InGaN/GaN multi quantum well (MQW) structure has been incorporated into the GaN nanorods. We found GaN nanostructures grown on top of the silicon pillars with a pyramidal shape. This shape results from a competitive growth on different facets as well as from surface diffusion of the growth species. Spatially resolved optical properties of the structures are analysed by cathodoluminescence. Strongly spatial-dependent MQW emission spectra indicate the growth rate differences on top of the rods.
- Published
- 2009
26. Cathodoluminescence of single ZnO nanorod heterostructures
- Author
-
Bernard Piechal, Andrey Bakin, F. Donatini, Jinkyoung Yoo, Gyu-Chul Yi, Abdelhamid El-Shaer, Andreas Waag, A. C. Mofor, and Le Si Dang
- Subjects
Materials science ,business.industry ,Piezoelectric polarization ,Heterojunction ,Cathodoluminescence ,Substrate (electronics) ,Orders of magnitude (numbers) ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Blueshift ,Optics ,Optoelectronics ,Nanorod ,business ,Quantum well - Abstract
Optical properties of ZnO-based single nanorods are probed by cathodoluminescence (CL) measurements at T = 5 K. We observe a variation of the ZnO near band edge CL by three orders of magnitude along the nanorod axis, accompanied by a spectral blueshift of 10-30 meV. This indicates a rather poor structural quality of the nanorod bottom part, close to the substrate. ZnO/ZnMgO quantum wells grown on top of ZnO nanorods are found to exhibit much stronger confinement effects as compared to their two-dimensional counterparts, suggesting a reduced spontaneous and piezoelectric polarization effects.
- Published
- 2007
27. Electrical characterization of ZnO nanorods
- Author
-
Thomas Weimann, A. C. Mofor, E. Schlenker, Andreas Waag, Andrey Bakin, B. Postels, Hergo-H. Wehmann, and Peter Hinze
- Subjects
Nanostructure ,Materials science ,business.industry ,Schottky diode ,Nanotechnology ,Substrate (electronics) ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Electrical resistivity and conductivity ,Electrode ,Optoelectronics ,Nanorod ,business ,Ohmic contact ,Electron-beam lithography - Abstract
Zinc oxide (ZnO) nanorods were grown by a wet chemical approach and by vapor phase transport. To explore the electrical properties of individual nanostructures current-voltage (I-V) characteristics were obtained by using an atomic force microscope (AFM) with a conductive tip or by detaching the nanorods from the growth substrate, transferring them to an isolating substrate and contacting them with evaporated Ti/Au electrodes patterned by electron-beam lithography. The AFM-approach only yields a Schottky diode behavior, while the Ti/Au forms ohmic contacts to the ZnO. For the latter method the obtained I-V curves reveal a resistivity of the nanorods in the order of 10 -5 Q cm which is unusually low for undoped ZnO. We therefore assume the existence of a highly conductive surface channel.
- Published
- 2007
28. ZnMgO‐ZnO quantum wells embedded in ZnO nanopillars: Towards realisation of nano‐LEDs
- Author
-
A. C. Mofor, E. Schlenker, Andrey Bakin, M. Al-Suleiman, Andreas Waag, and Abdelhamid El-Shaer
- Subjects
Fabrication ,Materials science ,Sapphire ,Nanotechnology ,Heterojunction ,Thin film ,Condensed Matter Physics ,Epitaxy ,Quantum well ,Nanopillar ,Molecular beam epitaxy - Abstract
ZnO thin films, ZnMgO/ZnO heterostructures and ZnO nanostructures were fabricated using molecular beam epitaxy (MBE), vapour phase transport (VPT) and an aqueous chemical growth approach (ACG). The possibility to employ several fabrication techniques is of special importance for the realization of unique device structures. MBE was implemented for ZnO-based layer and heterostructure growth. Pronounced RHEED oscillations were used for growth control and optimisation, resulting in high quality ZnO and Zn1–xMgxO epilayers and heterostructures, as well as ZnMgO/ZnO quantum wells on sapphire and SiC substrates. A novel advanced VPT approach is developed and sapphire, SiC, ZnO epitaxial layers, and even plastic and glass were implemented as substrates for ZnO growth. The VPT fabrication of ZnO nanopillars, leading to well aligned, c-axis oriented nanopillars with excellent quality and purity is demonstrated. Successful steps were made towards device fabrication on ZnO basis. The nanopillar fabrication technique is combined with MBE technology: MBE-grown ZnMgO/ZnO quantum well structures were grown on ZnO nanopillars presenting significant progress towards nano-LEDs realization. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2007
29. Time-resolved optically-detected magnetic resonance of II-VI diluted-magnetic-semiconductor heterostructures
- Author
-
S. M. Ryabchenko, V. Yu. Ivanov, Andreas Waag, Marek Godlewski, G. Karczewski, and Dmitri R. Yakovlev
- Subjects
Condensed matter physics ,Condensed Matter::Other ,Chemistry ,Exciton ,Relaxation (NMR) ,Analytical chemistry ,Spin–lattice relaxation ,Heterojunction ,Surfaces and Interfaces ,Magnetic semiconductor ,Condensed Matter Physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Materials Chemistry ,Electrical and Electronic Engineering ,Luminescence ,Quantum well ,Microwave - Abstract
Time-resolved optically-detected magnetic resonance (ODMR) technique was used to study spin dynamics of Mn 2+ ions in (Zn,Mn)Se- and (Cd,Mn)Te-based diluted magnetic semiconductor quantum wells. Times of spin-lattice relaxation have been measured directly from a dynamical shift of exciton luminescence lines after a pulsed impact of 60 GHz microwave radiation.
- Published
- 2007
30. Growth kinetics and properties of ZnO/ZnMgO hetero‐ structures grown by radical‐source molecular beam epitaxy
- Author
-
S. V. Ivanov, Andrey Bakin, Andreas Waag, S. B. Listoshin, T. V. Shubina, and Abdelhamid El-Shaer
- Subjects
Fabrication ,Reflection high-energy electron diffraction ,Growth kinetics ,Chemistry ,Analytical chemistry ,Nanotechnology ,Heterojunction ,Growth rate ,Condensed Matter Physics ,Stoichiometry ,Molecular beam epitaxy - Abstract
A phenomenological approach to quantitative description of Zn(Mg)O growth by radical-source molecular beam epitaxy, based on the experimental studies of RHEED intensity oscillations, has been developed. It allows a precise control of growth rate, composition and stoichiometry at any growth temperature, Along with optimization of a growth initiation procedure on a c-sapphire, it is necessary condition for fabrication of high quality ZnO epilayers and ZnO/ZnMgO heterostructures in a wide Mg composition range. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2007
31. Stimulated emission from ZnO nanorod arrays
- Author
-
H. Priller, Heinz Kalt, R. Kling, Hong Jin Fan, Claus F. Klingshirn, Robert Hauschild, Holger Lange, Andreas Waag, and Margit Zacharias
- Subjects
Materials science ,Photoluminescence ,Scattering ,business.industry ,Physics::Optics ,Plasma ,Condensed Matter Physics ,Condensed Matter::Materials Science ,Optics ,Polariton ,Optoelectronics ,Nanorod ,Stimulated emission ,business ,Lasing threshold ,Excitation - Abstract
We discuss the time-resolved photoluminescence (PL) spectra of single ZnO nanorods taken at excitation fluences above and below the laser threshold. In the latter case, P-band emission related to polariton-polariton scattering is observed for certain rod geometries while stimulated emission occurs within the electron-hole plasma band. We calculate the intensity distribution of low-order waveguide modes as well as their energy dependence for given nanorod geometries to discuss their relevance with respect to nanorod lasing and polariton propagation. Additional finite-element analysis confirms that a gold layer formed at the nanorod-substrate interface under certain growth conditions leads to an enhancement of confinement within the resonator. (2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2006
32. Catalyst‐free vapor‐phase transport growth of vertically aligned ZnO nanorods on 6H‐SiC and (11‐20)Al 2 O 3
- Author
-
Juergen Christen, A. C. Mofor, D. Fuhrmann, Abdelhamid El-Shaer, Andreas Waag, Andreas Hangleiter, Andrey Bakin, and Frank Bertram
- Subjects
Growth pressure ,Nanostructure ,Materials science ,Fabrication methods ,Vapor phase ,Nanorod ,Nanotechnology ,Graphite ,Condensed Matter Physics ,Luminescence ,Catalysis - Abstract
ZnO nanostructures are expected to pave the way for many interesting applications in optoelectronics, spin electronics gas sensor technology and biomedicine. Fabrication methods, especially for nanorods have been based mostly on catalyst-assisted growth methods that employ metal-organic sources and other contaminating agents like graphite to grow ZnO nanorods at relatively high temperatures. We report on the growth of ZnO nanorods on 6H-SiC and (11-20)Al2O3 using purely elemental sources, without catalysis and at relatively low temperatures and growth pressure in a specially designed vapor-phase transport system. ZnO nanorods with widths of 80-900 nm and lengths of 4–12 µm were obtained. Nanorod concentrations of up to 109 cm–2 with homogenous luminescence and high purity were noted. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2006
33. A study of ZnMnO as a material for magneto‐ and spin‐electronics
- Author
-
J. Eisenmenger, F. Reuss, Th. Mueller, Uwe Siegner, Abdelhamid El-Shaer, A. C. Mofor, Andrey Bakin, Paul J. Ziemann, Andreas Waag, Wolfgang Limmer, and H. Ahlers
- Subjects
Paramagnetism ,Magnetization ,Materials science ,Condensed matter physics ,Transition metal ,Spintronics ,Ferromagnetism ,Doping ,Nanotechnology ,Substrate (electronics) ,Condensed Matter Physics ,Epitaxy - Abstract
Due to the large spin moment exhibited by the transition metal Mn, it is being extensively investigated as a candidate element for producing ferromagnetic ZnO for magnetoelectronic and spintronic applications. However, ferromagnetism in ZnMnO is being controversially discussed. We review our investigations on epitaxially grown ZnMnO layers and present findings on Mn-implanted ZnO layers. The results of our investigations on the magnetic properties of the fabricated ZnMnO show that it is not trivial to confirm ferromagnetism in ZnMnO grown on sapphire because of ferromagnetic signal produced by the substrate. After a deduction of the substrate contribution to magnetisation data, both epitaxially doped ZnO layers and Mn-implanted bulk and epitaxial layers show paramagnetic behaviour. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2006
34. CBE growth of high-quality ZnO epitaxial layers
- Author
-
M. Kreye, A. C. Mofor, Michael Heuken, J. Stoimenos, Jürgen Bläsing, Béla Pécz, Alois Krost, Andrey Bakin, Abdelhamid El-Shaer, and Andreas Waag
- Subjects
Materials science ,Photoluminescence ,business.industry ,Surface finish ,Condensed Matter Physics ,Epitaxy ,Chemical beam epitaxy ,Electronic, Optical and Magnetic Materials ,Full width at half maximum ,Optics ,Surface roughness ,Optoelectronics ,Wafer ,business ,High-resolution transmission electron microscopy - Abstract
Further improvements on the recently reported novel approach to zinc oxide Chemical Beam Epitaxy (CBE) are presented. Hydrogen peroxide is employed as a very efficient novel oxidant. ZnO layers with a thickness from 100 nm to 600 nm were grown on c-sapphire using a MgO buffer. PL-mapping as well as conductivity mapping shows a good uniformity across the 2 inch ZnO-on-sapphire epiwafers. The measured surface roughness for the best layers is as low as 0.26 nm. HRXRD measurements of the obtained ZnO layers show excellent quality of the single crystalline ZnO. The FWHM of the HRXRD (0002) rocking curves measured for the 2 inch ZnO-on-sapphire wafers is as low as 27 arcsec with a very high lateral homogeneity across the whole wafer. Plane view HRTEM observations reveal the very good quality of the ZnO films. The results indicate that CBE is a suitable technique to fabricate ZnO of very high structural quality, which can eventually be used as an alternative to bulk ZnO substrates.
- Published
- 2006
35. Picosecond kinetics of magnetization in optically excited (Zn,Mn)Se quantum wells
- Author
-
A. V. Akimov, Laurens W. Molenkamp, A. V. Scherbakov, Manfred Bayer, I. A. Merkulov, Andreas Waag, and Dmitri R. Yakovlev
- Subjects
Magnetization ,Chemistry ,Picosecond ,Excited state ,Atomic physics ,Condensed Matter Physics ,Spin (physics) ,Excitation ,Quantum well ,Electronic, Optical and Magnetic Materials ,Magnetic field ,Ion - Abstract
The magnetization kinetics in (Zn,Mn)Se/(Zn,Be)Se quantum wells is studied on a ps-time scale after pulsed laser excitation. The magnetization induced by an external magnetic field is reduced by up to 30% within about 100 ps due to spin and energy transfer from photocarriers to magnetic Mn ions. A model based on spin-momentum coupling in the valence band is suggested for explaining this transfer.
- Published
- 2006
36. Aqueous chemical growth and patterning of ZnO nanopillars on different substrate materials
- Author
-
Andreas Waag, B. Postels, M. Kreye, D. Fuhrmann, Andreas Hangleiter, and Hergo-Heinrich Wehmann
- Subjects
chemistry.chemical_classification ,Materials science ,Photoluminescence ,Aqueous solution ,chemistry ,Nanotechnology ,Polymer ,Condensed Matter Physics ,Luminescence ,Lithography ,Nanopillar - Abstract
Aqueous chemical growth (ACG) is a low-temperature approach that is only weakly influenced by the substrate and allows for the growth of ZnO nanopillars on various substrates. ACG is an efficient way to generate wafer-scale and densely packed arrays of ZnO nanopillars even on polymer materials. Photoluminescence (PL) characterisation clearly shows a comparatively strong band-edge luminescence even at room temperature that is accompanied with a rather weak visible luminescence in the yellow / orange spectral range. We introduce a rather simple postgrowth lithographic technique. Patterning of ZnO nanopillars even on layered conducting and flexible substrate materials using ACG as a low-temperature growth technique is demonstrated. The economical potential for future applications and devices using ZnO nanopillar arrays is discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2006
37. Temperature-dependent polarized luminescence of exciton polaritons in a ZnO film
- Author
-
Th. Gruber, C. Kirchner, Andreas Waag, T. V. Shubina, O. V. Nekrutkina, Bo Monemar, Karl Fredrik Karlsson, and A. A. Toropov
- Subjects
Condensed Matter::Quantum Gases ,Photoluminescence ,Materials science ,Condensed matter physics ,Condensed Matter::Other ,Linear polarization ,Exciton ,Physics::Optics ,Mineralogy ,Surfaces and Interfaces ,Chemical vapor deposition ,Exciton-polaritons ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Epitaxy ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,Materials Chemistry ,Polariton ,Electrical and Electronic Engineering ,Luminescence - Abstract
We report on the studies of linearly polarized photoluminescence (PL) in a (0001) oriented ZnO epitaxial film, grown by metal organic chemical vapor deposition on a GaN template. The emission of mixed longitudinal-transverse exciton polariton modes was observed up to 130 K that evidences polaritonic nature of the excitonic spectrum up to this elevated temperature.
- Published
- 2005
38. Spin‐lattice relaxation in heteromagnetic nanostructures
- Author
-
A. V. Akimov, Andreas Waag, Dmitri R. Yakovlev, L. Hansen, Laurens W. Molenkamp, Alexey V. Scherbakov, and W. Ossau
- Subjects
Nanostructure ,Semiconductor ,Condensed matter physics ,Spintronics ,Chemistry ,business.industry ,Spin–lattice relaxation ,Spin diffusion ,Charge (physics) ,Spin (physics) ,business ,Ion - Abstract
The rapidly expanding research in Spintronics, the electronics utilizing the electron spin instead of its charge, is driven by the very interesting potential applications. The actual task is to develop principles for the spin manipulations in spintronic devices. In this Report we suggest and verify experimentally a concept of heteromagnetic semiconductor structures. It is based on spin diffusion between layers of the nanostructure with different magnetic properties and allows controlling the spin-switching rate for magnetic ions. A ten times increase of spin-lattice relaxation rate of magnetic Mn-ions is achieved in (Zn,Mn)Se/(Be,Mn)Te heteromagnetic structures with an inhomogeneous distribution of Mn-ions. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2004
39. Spin and energy transfer between magnetic ions and freecarriers in diluted‐magnetic semiconductor heterostructures
- Author
-
A. V. Akimov, Manfred Bayer, Alexey V. Scherbakov, W. Ossau, A. A. Maksimov, Dmitri R. Yakovlev, Andreas Waag, M. K. Kneip, I. I. Tartakovskii, Laurens W. Molenkamp, and D. Keller
- Subjects
Condensed matter physics ,Phonon ,Chemistry ,Lattice (order) ,Energy transfer ,Non-equilibrium thermodynamics ,Heterojunction ,Magnetic semiconductor ,Plasma ,Ion - Abstract
In this paper we give a brief overview of our studies on dynamical processes in diluted-magnetic-semiconductor heterostructures based on (Zn,Mn)Se and (Cd,Mn)Te. Presence of free carriers is an important factor which determines the energy- and spin transfer in a coupled systems of magnetic ions, lattice (the phonon system) and carriers. We report also new data on dynamical response of magnetic ions interacting with photogenerated electron-hole plasma. (Zn,Mn)Se/(Zn,Be)Se structures with relatively high Mn content of 11% provide spin-lattice relaxation time of about 20 ns, which is considerably shorter then the characterictic times of nonequilibrium phonons ranging to 1 μs. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
- Published
- 2004
40. Comparison of linear and nonlinear optical spectra of various ZnO epitaxial layers and of bulk material obtained by different experimental techniques
- Author
-
Th. Gruber, Claus F. Klingshirn, Heinz Kalt, Andreas Waag, Takafumi Yao, J. Brückner, H. Priller, and H. J. Ko
- Subjects
Condensed Matter::Materials Science ,Photoluminescence ,Chemistry ,Analytical chemistry ,Photoluminescence excitation ,Stimulated emission ,Metalorganic vapour phase epitaxy ,Electron hole ,Condensed Matter Physics ,Spectroscopy ,Epitaxy ,Electronic, Optical and Magnetic Materials ,Molecular beam epitaxy - Abstract
We investigate ZnO epitaxial layers grown by MBE (Molecular Beam Epitaxy) and MOVPE (Metal Organic Vapor Phase Epitaxy) techniques. The samples show similar optical behavior in temperature dependent photoluminescence measurements, reflection and photoluminescence excitation spectroscopy in the low density regime. High excitation measurements show different behavior. While the MBE sample leads to stimulated emission from the exciton-exciton-scattering, an electron hole plasma is formed in the MOVPE sample which leads to stimulated emission at higher excitation intensities. The gain value measured by the variable stripe length method is much higher for the MBE grown sample.
- Published
- 2004
41. Optical anisotropy of non-common-atom quantum wells and dots: effects of interface symmetry reduction
- Author
-
Sergei Ivanov, Andreas Waag, A. A. Toropov, S. V. Sorokin, O. V. Nekrutkina, T. V. Shubina, G. Landwehr, and Dmitry Solnyshkov
- Subjects
Photoluminescence ,Condensed matter physics ,Condensed Matter::Other ,Chemistry ,Linear polarization ,Physics::Optics ,Heterojunction ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Electron localization function ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,Chemical bond ,Quantum dot ,Quantum well ,Molecular beam epitaxy - Abstract
We report on the investigations of in-plane optical anisotropy in non-common-atom heterostructures: ZnSe/BeTe perfect quantum wells (QWs) and CdSe/BeTe rough QWs and quantum dots. A noticeable linear polarization of photoluminescence (PL) with respect to the in-plane [1–10] and [110] crystal axes was observed in the ZnSe/BeTe QWs with equivalent ZnTe-type interfaces due to the reduction of QW symmetry, induced by unintentional formation of BeSe chemical bonds at a “BeTe-ZnSe” interface. The BeSe bond concentration and, hence, the polarization degree depend on the Te/Be flux ratio during molecular beam epitaxy growth of the samples. Strongly linearly polarized (up to 80%) PL was detected in the CdSe/BeTe structures, evidencing QW-like flat symmetry of the emitting sites of carrier localization.
- Published
- 2003
42. MOCVD Growth of ZnO for Optoelectronic Applications
- Author
-
Andreas Waag, C. Kirchner, Th. Gruber, Klaus Thonke, and Rolf Sauer
- Subjects
Materials science ,Photoluminescence ,Annealing (metallurgy) ,business.industry ,Exciton ,Doping ,Chemical vapor deposition ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Hall effect ,Optoelectronics ,Metalorganic vapour phase epitaxy ,Emission spectrum ,business - Abstract
In this paper we report on the metal-organic chemical vapour deposition of ZnO layers. We focus on heteroepitaxial growth on c-plane Al 2 O 3 and the influence of the VI/II ratio during growth on the properties of the layers. The layer quality has been investigated by HRXRD, PL and reflectivity measurements. Under optimized growth conditions the photoluminescence is dominated by strong near band edge emission lines with half widths below 4 meV and the excitonic signals are clearly visible in reflectivity measurements. Hall effect measurements indicate an n-type background doping in the 10 17 cm -3 range, and mobilities of more than 100 cm 2 /Vs can be reached. Moreover, a post-growth annealing step is found to improve the quality of the layers grown under suboptimal conditions. The problem of strain will be addressed and a tensile strain can be confirmed in the heteroepitaxial ZnO layers.
- Published
- 2002
43. Diffusion of Carriers Induced by Exchange Interaction with Magnetic-Ion System in (Zn, Mn)Se/(Zn, Be)Se Quantum Wells
- Author
-
Laurens W. Molenkamp, D. Keller, F. Pulizzi, Dmitri R. Yakovlev, Jan C. Maan, W. Ossau, Andreas Waag, Th. Gruber, and Peter C. M. Christianen
- Subjects
Electron mobility ,Zeeman effect ,Photoluminescence ,Condensed matter physics ,Chemistry ,Diffusion ,Energy level splitting ,Exchange interaction ,Magnetic semiconductor ,Condensed Matter Physics ,Molecular physics ,Electronic, Optical and Magnetic Materials ,symbols.namesake ,symbols ,Quantum well - Abstract
We studied the diffusion of carriers in dilute magnetic semiconductors for conditions when an additional force caused by a spatially inhomogeneous exchange potential is active. The spatially inhomogeneous exchange potential arises from a creation of photocarriers in the Zn 0.988 Mn 0.012 Se/ Zn 0.94 Be 0.06 Se quantum well. The photocarriers enhance the spin temperature of the Mn ions by a spin-flip scattering with the magnetic ions, which results in a reduction of the giant Zeeman splitting. The gaussian like intensity profile of the exciting laser beam causes a locally varying exchange potential for the carriers. A strong influence of the gradient in the exchange potential on the diffusion of the carriers was found. The diffusion of the carriers was examined by a spatially resolved photoluminescence technique.
- Published
- 2002
44. Magnetic Characterisation of Highly Doped MBE Grown Be1?xMnxTe and Bulk Zn1?xMnxTe
- Author
-
Andreas Waag, Tomasz Dietl, Le Van Khoi, Lasse H Hansen, Laurens W. Molenkamp, Maciej Sawicki, and David Ferrand
- Subjects
Curie–Weiss law ,Condensed matter physics ,Chemistry ,Doping ,Magnetic semiconductor ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,Magnetization ,Hysteresis ,Nuclear magnetic resonance ,Ferromagnetism ,Curie temperature ,Condensed Matter::Strongly Correlated Electrons ,Molecular beam epitaxy - Abstract
Magnetic properties of two new classes of highly p-type doped II-VI diluted magnetic semiconductors (DMS): thin layers of Be 1-x Mn x Te:N and bulk Zn 1-x Mn x Te:P are presented. Despite the fact that the samples are insulating, we observe clear indications of the low temperature ferromagnetic correlation, namely: (i) positive Curie-Weiss temperature (T CW 2 K), and (ii) open hysteresis loops at temperatures below T CW . A qualitative analysis of the results favours those models of the carrier-mediated ferromagnetism in DMS which allow for the coupling of only a part of the available spins.
- Published
- 2002
45. Elliptically Polarized Luminescence of Spin-Oriented Carriers Recombining at Anisotropic Type-II Interface in ZnSe/BeTe Quantum Wells
- Author
-
Dmitri R. Yakovlev, Laurens W. Molenkamp, Andreas Waag, V. P. Kochereshko, W. Ossau, G. Landwehr, E. L. Ivchenko, L. Hansen, and A. V. Platonov
- Subjects
Photoluminescence ,Condensed matter physics ,Chemistry ,Linear polarization ,Electric field ,Elliptical polarization ,Condensed Matter Physics ,Anisotropy ,Polarization (waves) ,Circular polarization ,Quantum well ,Electronic, Optical and Magnetic Materials - Abstract
The polarization properties of indirect photoluminescence (PL) in ZnSe/BeTe quantum well structures with a type-II band alignment were studied experimentally and analyzed theoretically. The linear polarization degree appears with increasing density of photoexcitation. It is caused by an electric field along the structure growth axis induced by a spatial separation of photocarriers. The PL circular polarization degree was induced by magnetic fields applied along the structure growth axis. Its maximum value was limited by the in-plane optical anisotropy of the optical matrix elements at the type-II heterointerface. In-plane oriented magnetic fields induce a linear polarization degree that is determined by a non-zero in-plane component of the heavy-hole g-factor in the BeTe layers.
- Published
- 2002
46. Raman Scattering and Luminescence in Semimagnetic CdSe/ZnSe Nanostructures with Quantum Discs
- Author
-
A. A. Toropov, G. Landwehr, Sergei Ivanov, Andreas Waag, I. I. Reshina, and D. N. Mirlin
- Subjects
Photoluminescence ,Condensed matter physics ,Condensed Matter::Other ,Chemistry ,Exciton ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Resonance (particle physics) ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,symbols.namesake ,X-ray Raman scattering ,Spin wave ,Spin crossover ,symbols ,Raman spectroscopy ,Raman scattering - Abstract
Resonant Raman scattering and photoluminescence were studied in semimagnetic CdSe/ZnSe structures with self-organized flat quantum discs of CdZnMnSe composition. Different kinds of magnetic excitations were observed in Raman scattering under sharp resonance with excitons localized in the discs. They include spin-flip transitions from donor-bound electrons in the discs and collective multiple spin-flip transitions in the system of Mn 2+ ions.
- Published
- 2002
47. Trionic Gain in ?-Doped ZnSe Quantum Wells
- Author
-
Dmitri R. Yakovlev, Fritz Henneberger, Andreas Waag, Joachim Puls, and G. V. Mikhailov
- Subjects
Condensed Matter::Quantum Gases ,Photoluminescence ,Condensed Matter::Other ,Chemistry ,Exciton ,Electron ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,Charge carrier ,Stimulated emission ,Atomic physics ,Trion ,Excitation ,Quantum well - Abstract
This paper reports on the observation of optical gain due to charged excitons (trions) in quantum wells. The specific optical coupling of the trion gives rise to stimulated emission on the low-energy wing of the trion photoluminescence line without degeneracy and inversion in the total particle numbers. Gain values as large as 10 4 cm -1 are found for excitation intensities of a few kW/cm. Higher injection levels lead to carrier heating which limits the available gain. A calculation of the absorption-gain crossover based on a kinetically determined equilibrium of excitons, trions and electrons with a common carrier gas temperature describes the experimental data well.
- Published
- 2002
48. Interface Properties and in-Plane Linear Photoluminescence Polarization in Highly Excited Type-II ZnSe/BeTe Heterostructures
- Author
-
Andreas Waag, S. V. Zaitsev, Dmitri R. Yakovlev, I. I. Tartakovskii, P. S. Dorozhkin, G. Landwehr, V. D. Kulakovskii, A. A. Maksimov, L. Hansen, and W. Ossau
- Subjects
Photoluminescence ,Condensed matter physics ,Linear polarization ,Chemistry ,business.industry ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Polarization (waves) ,Electronic, Optical and Magnetic Materials ,Photoexcitation ,Condensed Matter::Materials Science ,Optics ,Excited state ,Electric field ,Anisotropy ,business ,Excitation - Abstract
Properties of interfaces with no-common atom in type-II ZnSe/BeTe heterostructures are studied with polarization spectroscopy. Radiative recombination and its polarization anisotropy have been found to depend crucially on the interface configuration and excitation power. Effects of external and internal electric fields on the in-plane polarization are studied both numerically and experimentally at low and high levels of photoexcitation density.
- Published
- 2002
49. Interface Effects in Type-II CdSe/BeTe Quantum Dots
- Author
-
A. A. Toropov, S. V. Sorokin, Galia Pozina, G. Landwehr, Magnus Willander, Dmitry Solnyshkov, Andreas Waag, Bo Monemar, R. N. Kyutt, J. P. Bergman, T. V. Shubina, Sergei Ivanov, and Alexander A. Lebedev
- Subjects
Photoluminescence ,Condensed matter physics ,Quantum dot ,Linear polarization ,Chemistry ,Electron ,Electronic structure ,Condensed Matter Physics ,Polarization (waves) ,Electron localization function ,Symmetry (physics) ,Electronic, Optical and Magnetic Materials - Abstract
We report on optical and structural studies of the interface symmetry in CdSe/BeTe multiple-layer structures containing self-assembled quantum dots. Temperature and decay behavior of the broad photoluminescence (PL) band is consistent with the type-II transitions involving deeply localized electron states. Large linear in-plane polarization of the PL (up to 80%) is observed, implying the C-2v (or lower) symmetry of the individual places of the electron localization.
- Published
- 2002
50. Trions in ZnSe-Based Quantum Wells Probed by 50 T Magnetic Fields
- Author
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Andreas Waag, Nikolai A. Gippius, Alexander B. Dzyubenko, W. Ossau, N. Miura, Dmitri R. Yakovlev, G. V. Astakhov, Scott A. Crooker, and A. Yu. Sivachenko
- Subjects
Condensed matter physics ,Condensed Matter::Other ,Chemistry ,Energy level splitting ,Binding energy ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Molecular physics ,Electronic, Optical and Magnetic Materials ,Magnetic field ,Singlet state ,Triplet state ,Trion ,Saturation (magnetic) ,Quantum well - Abstract
Singlet and triplet states of negatively charged excitons (trions) in ZnSe/(Zn,Be,Mg)Se quantum wells have been studied by means of photoluminescence in pulsed magnetic fields up to 50 T. Singlet state binding energies, measured for different well widths ranging from 48 to 190 A, show a monotonic increase with growing magnetic fields with a tendency to saturation. This behavior is in qualitative agreement with results of model calculations. Quantitatively, the binding energy of singlet states is underestimated by about 50%. The triplet state assigned to the dark triplet becomes detectable in magnetic fields above 15 T. A crossover of the triplet and singlet states is expected in magnetic fields of about 50 T.
- Published
- 2001
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