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H. Jönen,
U. Rossow,
H. Bremers,
L. Hoffmann,
M. Brendel,
A. D. Dräger,
S. Schwaiger, F. Scholz,
J. Thalmair,
J. Zweck,
A. Hangleiter
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ABSTRACT: We report on the optical properties of m-plane GaInN/GaN quantum wells (QWs). We found that the emission energy of GaInN QWs grown on m-plane SiC is significantly lower than on non-polar bulk GaN, which we attribute to the high density of stacking faults. Temperature and power dependent photoluminescence reveals that the GaInN QWs on SiC have almost as large internal quantum efficiencies as on bulk GaN despite the much higher defect density. Our results indicate that quantum-wire-like features formed by stacking faults intersecting the quantum wells provide a highly efficient light emission completely dominating the optical properties of the structures.
Applied Physics Letters 07/2011; 99(1):011901-011901-3. · 3.84 Impact Factor
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T. Wunderer,
M. Feneberg,
F. Lipski,
J. Wang,
R. A. R. Leute,
S. Schwaiger,
K. Thonke,
A. Chuvilin,
U. Kaiser,
S. Metzner, [......],
J. Christen,
G. J. Beirne,
M. Jetter,
P. Michler,
L. Schade,
C. Vierheilig,
U. T. Schwarz,
A. D. Dräger,
A. Hangleiter, F. Scholz
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ABSTRACT: Selective-area epitaxy is used to form three-dimensional (3D) GaN structures providing semipolar crystal facets. On full 2-in. sapphire wafers we demonstrate the realization of excellent semipolar material quality by introducing inverse GaN pyramids. When depositing InGaN quantum wells on such a surface, the specific geometry influences thickness and composition of the films and can be nicely modeled by gas phase diffusion processes. Various investigation methods are used to confirm the drastically reduced piezoelectric polarization on the semipolar planes. Complete electrically driven light-emitting diode test structures emitting in the blue and blue/green spectral regions show reasonable output powers in the milliwatt regime. Finally, first results of the integration of the 3D structures into a conventional laser design are presented.
physica status solidi (b) 02/2011; 248(3):549 - 560. · 1.32 Impact Factor
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ABSTRACT: Specially designed transmission electron microscopy (TEM) sample carriers have been developed to enable atomically resolved studies of the heat-induced evolution of adsorbates on graphene and their influence on electrical conductivity. Here, we present a strategy for graphene-based carrier realization, evaluating its design with respect to fabrication effort and applications potential. We demonstrate that electrical current can lead to very high temperatures in suspended graphene membranes, and we determine that current-induced cleaning of graphene results from Joule heating.
Journal of Physics D Applied Physics 01/2011; 44(5):055502. · 2.54 Impact Factor
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T Wunderer,
J Wang,
F Lipski,
S Schwaiger,
A Chuvilin,
U Kaiser,
S Metzner,
F Bertram,
J Christen,
S S Shirokov,
A E Yunovich, F Scholz
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ABSTRACT: Excellent semipolar GaN material quality can be ob-tained by growing inverse GaN pyramids on full 2 inch c-plane sapphire when combining the advantages of de-fect reduction via FACELO and selective area growth for faceted surfaces. The nearly defect free material is ob-tained by structuring the mask into a honeycomb pattern. When realizing full GaInN/GaN LED structures on those templates a relatively broad emission is observed during electroluminescence measurements. Furthermore, the dominant wavelength is found to be de-pendent on the applied current density. The pronounced shift is in good agreement with the wavelength shift along one single facet. Gas phase diffusion during the active area growth is believed to be the main reason for the varying indium content. The homogeneity could be influenced by changing the reactor pressure during the active area growth.
physica status solidi. 01/2010;
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ABSTRACT: In the present paper, we report the optical properties of the bicolor composite phosphor on the base of CaS and Ga2S3 and study the possibility using this composite to obtain a white light LED. In particular, the energy transfer between the green and red emission bands of the bicolor composite phosphor activated by Eu2+ ions was studied. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica Status Solidi (A) Applications and Materials 01/2009; 206(2):287 - 292. · 1.46 Impact Factor
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ABSTRACT: The analysis of the optical transmission spectra of semiconductor thin films on a transparent substrate is revisited. A new equation for the optical transmission is derived. Based on this equation and on the spacing of the interference pattern, a method is described and analyzed in detail for the determination of the film thickness, refractive index and absorption coefficient. As a notable feature, small values of the absorption coefficient can be determined as a fitting parameter at all wavelengths, regardless of strong interference. The method is compared to the frequently encountered envelope method of Manifacier et al and Swanepoel. The suitability of the method is illustrated on two GaN thin films grown on sapphire. Below the band gap, the index of refraction of GaN can be described by the equation n(λ(nm)) = 2.19 + 2.57(λ − 345)−1/2.
Measurement Science and Technology 08/2008; 19(10):105303. · 1.49 Impact Factor
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ABSTRACT: GaInN quantum wells with reduced piezoelectric field deposited epitaxially on the {101} facets of selectively grown GaN stripes show distinct polarization properties of the emitted light. Systematic electro- and photoluminescence studies demonstrate that the light is linearly polarized parallel to the stripes, representing the 〈110〉 direction of the GaN crystal. Our model calculations show that this is a consequence of the strain-induced valence-band splitting depending on the crystal orientation. The polarization ratio is calculated and compared to measured values.
Journal of Applied Physics 03/2007; 101(5):053530-053530-6. · 2.17 Impact Factor
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ABSTRACT: Optically detected magnetic resonance measurements of the effective g-value (g*) of electrons in type-I GaxIn1-xAs/InP quantum wells are presented. The observation of spin resonance on the circularily polarized luminescence is explained in terms of recombination in a one-side p-modulation doped quantum well and spin thermalization. Quantum confinement changes the electron effective g-value. It further induces a strongly anisotropic g-tensor. A calculation for the g*||-component agrees well with the experimental data, but the anisotropy can be explained only qualitatively.
Physica Scripta 01/2007; 1994(T54):100. · 1.20 Impact Factor
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ABSTRACT: Direction and strength of piezoelectric built-in fields of GaInN quantum wells have been experimentally determined. The quantum wells have been grown either on the conventional {0001} crystal plane of GaN or on {101} facets of selectively grown GaN stripes. The emission peak position of the electric-field-dependent photoluminescence can be modeled and yields value and sign of the piezoelectric field dependent on the strain of the quantum wells. On the semipolar {101} facets, the quantum wells show a much weaker field (−0.1 MV/cm) compared to quantum wells grown on polar {0001} planes (−1.9 MV/cm), consistent with theoretic predictions.
Applied Physics Letters 12/2006; 89(24):242112-242112-3. · 3.84 Impact Factor
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ICMOVPE-XIII 2006, Miyazaki, Japan; 05/2006
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ABSTRACT: The optical properties of spontaneously ordered GaInP2 grown by MOVPE are studied by means of photoluminescence, time-resolved photoluminescence, and polarization dependent photoluminescence excitation spectroscopy. Ordered samples grown on a substrate which is misoriented 6° toward [111]B show clear excitonic features both in luminescence and absorption. In excitation spectra both valence band edges are clearly resolved and are shown to be polarization dependent as predicted by band structure calculations. In these samples an additional non-excitonic low-energy photoluminescence emission is observed the peak energy of which depends strongly on excitation density and temperature and which shows extremely long luminescence lifetimes. A density of states model for ordered GaInP2 is proposed which allows to explain the results obtained in the last few years. The density of states model is discussed with respect to the microstructure of the ordered alloy as known from TEM investigations.
physica status solidi (b) 02/2006; 193(1):213 - 229. · 1.32 Impact Factor
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ABSTRACT: Small-signal properties of 650-nm vertical-cavity surface-emitting lasers (VCSELs) with different oxide aperture sizes were measured. A small diameter VCSEL of 3.5 mum has a maximum resonance frequency of 5.7 GHz. The photon density determines the maximum resonance frequency. Modeling also indicates higher photon densities in the small VCSEL due to better thermal behavior
IEEE Photonics Technology Letters 02/2006; · 2.19 Impact Factor
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ABSTRACT: We present measurements on an aperiodic device-specific longitudinal-mode pattern in InGaN laser diodes. The characteristic shape of this pattern occurs only if the laser is driven slightly above threshold; in addition, it tunes with temperature at exactly the same rate as the cavity modes. By careful selection of the collection optics and averaging ten rapid scans over 15 min in a high-resolution Fourier transform spectrometer, we could exclude possible explanations like beating of mode families, self-pulsation, or external reflections. A naive simulation of the longitudinal modes profiting from their individual "gain profile" along the cavity suggests that we see the signature of quantum-well thickness fluctuations.
IEEE Photonics Technology Letters 10/2005; · 2.19 Impact Factor
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Sven-Silvius Schad,
B. Neubert,
C. Eichler,
M. Scherer,
F. Habel,
M. Seyboth, F. Scholz,
D. Hofstetter,
P. Unger,
W. Schmid,
C. Karnutsch,
K. Streubel
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ABSTRACT: Different experimental and simulation techniques aiming at a better understanding of lateral mode absorption in light-emitting diodes (LEDs) are presented in this paper. A measurement of transmitted power versus propagation distance allows us to derive the absorption losses of LED layer structures at their emission wavelength. Two models for the observed intensity distribution are presented: one is based on scattering, whereas the other relies on selective absorption. Both models were applied to InGaN-on-sapphire-based LED structures. Material absorption losses of 7 cm<sup>-1</sup> for the scattering model and 4 cm<sup>-1</sup> for the absorbing-layer model were obtained. Furthermore, these values are independent of the emission wavelength of the layer structure in the 403-433-nm range. The losses are most likely caused by a thin highly absorbing layer at the interface to the substrate. In a second step, interference of the modal field profile with the absorbing layer can be used to determine its thickness (d=75 nm) and its absorption coefficient (α ≈ 3900 cm<sup>-1</sup>). This method has also been tested and applied on AlGaInP-based layer structures emitting at 650 nm. In this case, the intensity decay of α=30 cm<sup>-1</sup> includes a contribution from the absorbing substrate.
Journal of Lightwave Technology 11/2004; 22(10):2323- 2332. · 2.78 Impact Factor
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ABSTRACT: 900 mW of continuous-wave power has been achieved in AlInGaP tapered lasers at 640 nm. Good beam quality and high device efficiency are obtained. Superluminescent diodes show strongly increased spectral width while retaining the good beam properties.
Lasers and Electro-Optics, 2004. (CLEO). Conference on; 06/2004
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ABSTRACT: Quenching of photocurrent spectroscopy and thermally stimulated currents has been investigated in undoped AlGaN layers and GaN/AlGaN heterostructures grown by metalorganic vapor phase epitaxy on sapphire substrates. The quenching was induced by an additional illumination as well as by an excitation with light below bandgap energy, respectively. In GaN layers, mainly the thermal defect emission band between 210 and 300 K is involved in the quenching processes and there are strong indications of a reduction of the band by incorporation of aluminum. In AlGaN layers, a decrease of quenching with increasing Al content is observed in accordance to a reduction of the height of the thermal emission between 210 and 300 K in these layers. These results are supported by quenching experiments in AlGaN/GaN heterostructures. © 2004 American Institute of Physics.
Applied Physics Letters 04/2004; 84(18):3498-3500. · 3.84 Impact Factor
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ABSTRACT: Pulsed lasing operation of a 670 nm AlGaInP-based oxide-confined vertical-cavity surface-emitting laser (VCSEL) at high temperatures is demonstrated. At +120°C heatsink temperature output power exceeded 0.5 mW and at +160°C 25 μW output power was achieved
Electronics Letters 12/2003; · 0.96 Impact Factor
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ABSTRACT: The composition and thickness of GaInN quantum wells (QWs) determines the luminescence properties of GaInN/GaN-based optoelectronic devices. The In distribution is characterized by considerable inhomogeneities on two spatial scales. Variations of locally averaged In concentrations and QW thickness on a “large” scale of several 10 nm along the QW occur in addition to small clusters with high In concentrations up to 100% and sizes of only a few nanometers. In the present work, we investigated the influence of the growth rate during metal organic vapor phase epitaxy (MOVPE) growth on the In distribution. The composition of GaInN single QWs was determined by evaluating high-resolution transmission electron microscopy (HRTEM) lattice-fringe images. At low GaInN deposition rates, a significant increase of the inhomogeneity of the In distribution compared to a sample grown with a high rate was found. A good correlation between experimental and calculated photoluminescence energies is achieved. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 11/2003;
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ABSTRACT: Quenching effects induced by additional below-bandgap illumination in undoped semi-insulating GaN were investigated using optical admittance spectroscopy (OAS) and photocurrent (PC) spectroscopy as well as optically excited, thermally stimulated currents (TSC). In OAS and PC, a decrease of defect-related signals due to the quenching light was observed. The thermal quenching of the defect band between 2.7 and 3.3 eV shows a good agreement with thermal emissions as measured by TSC, indicating the same defects cause the optical transitions in OAS/PC and the thermal transitions in TSC. The thermal emission in the temperature region between 250 and 300 K, which is responsible for the thermal quenching of the blue band (BB) in OAS, also shows an optical quenching under below-bandgap excitation. © 2003 American Institute of Physics.
Applied Physics Letters 06/2003; 82(23):4083-4085. · 3.84 Impact Factor
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ABSTRACT: The reduction of the active region of semiconductor lasers to quasi-zero dimensions has different effects on static and dynamic laser parameters as already discussed in the last two decades. Most prominent effects due to thermodynamics of low dimensional electron-hole plasmas are the threshold reduction and improved temperature stability of lasers with low dimensional active regions. Based on the changed density of states also some interesting device properties with respect to modulation response and beam quality of a laser are expected but not yet reached. This obvious discrepancy between theoretical expectations and experimental results must be related to additional effects such as carrier transport and relaxation. After a brief review of nano-fabrication aspects of dot lasers by self assembled methods as well as by lithography based implantation and etching methods, also laser device properties based on thermodynamics in low dimensional systems will be discussed. Strong emphasis will be put on carrier dynamics (transport, recombination, and relaxation). On the footing of a rate equation approach we discuss the static and dynamic properties of quantum dot lasers as a function of the dot array filling factor. Also some applications and device properties will be discussed. Based on periodic dot arrays, gain coupled dot distributed feedback (DFB) lasers can be realized which result in an improved side mode suppression ratio of the laser emission. From modulation experiments, extremely low dynamic chirp of dot lasers can be observed making quantum dot lasers promising candidates for high speed communication in the long wavelength range, if modulation response limitations can be solved.
SINGLE QUANTUM DOTS: FUNDAMENTALS, APPLICATIONS AND NEW CONCEPTS. 01/2003; 90:185--235.
