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ABSTRACT: Photonic crystals (PhCs) have now been firmly established as an efficient means for light extraction from light emitting diodes (LEDs). We analyze the diffraction properties from thin GaN micro-cavity LEDs with hexagonal lattices that feature three guided TE modes only. In contrast to common design rules, we find that high order diffraction contributes significantly to the light extraction and increases the directionality of the emitted light. The implementation of the PhC leads to an enhancement in light extraction by a factor of up to 1.8 and the directionality of the light is greatly improved with a radiant intensity enhancement factor of 4.3, which can only be explained by the higher order diffraction that has been hitherto neglected. Furthermore, we show that higher order diffraction contributes significantly to the high azimuthal extraction uniformity we observe, suggesting that the use of quasi-crystal lattices is not necessary. We use a model including mode absorption where each in-plane angle of the guided modes is treated separately in order to explain the experimental results.
IEEE Journal of Quantum Electronics 01/2010; · 1.88 Impact Factor
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ABSTRACT: A large variety of nanostructures including surface roughening, photonic crystals and plasmonics has been applied to light-emitting diodes in order to improve their performance. This paper gives an overview of the different approaches and the obtained results and compares their relevance for commercial LED applications, focussing on solid-state lighting.
IEEE/LEOS Winter Topicals Meeting Series, 2009; 02/2009
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ABSTRACT: We report directional light extraction from AlGaInP thin-film resonant cavity light emitting diodes (RCLEDs) with shallow photonic crystals (PhCs). Diffraction of guided modes into the light extraction cone enhances the light extraction by a factor of 2.6 compared to unstructured RCLEDs, where the farfields still show higher directionality than Lambertian emitters. The external quantum efficiency is 15.5% to air and 26% with encapsulation, respectively. The PhC-RCLEDs are also more stable to a temperature induced wavelength shift than unstructured RCLEDs.
Applied Physics Letters 12/2008; 93(23):231109-231109-3. · 3.84 Impact Factor
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ABSTRACT: We investigate the use of photonic crystals for light extraction from high-brightness thin-film AlGaInP light-emitting diodes with different etch depths, lattice constants, and two types of lattices (hexagonal and Archimedean). Both simulations and experimental results show that the extraction of high order modes with a low effective index neff is most efficient. The highest external quantum efficiency without encapsulation is 19% with an Archimedean A7 lattice with reciprocal lattice constant G = 1.5 k0, which is 47% better than an unstructured reference device.
Applied Physics Letters 07/2008; 93(4):041105-041105-3. · 3.84 Impact Factor
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ABSTRACT: The monolithic integration of pump lasers and optically pumped vertical-external-cavity surface-emitting lasers is demonstrated. An innovative contacting scheme for the pump lasers offers high design flexibility and scalability. First devices at 1000 nm generate output powers of 2.5 W in pulsed and 0.65 W in continuous-wave operation.
IEEE Photonics Technology Letters 01/2008; · 2.19 Impact Factor
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ABSTRACT: Facet temperature changes in broad-area red-emitting high-power AlGaInP lasers are analyzed by means of micro-Raman spectroscopy. Measurements as a function of injection current demonstrate that the temperature at the laser output facet rises linearly with optical output power. Temperature profile measurements across the laser stripe show a strong correlation between near field intensity, facet temperature, and catastrophic optical damage (COD). Additionally, temperature-power analyses reveal that a critical facet temperature is needed to induce COD. The consistent results produced by complementary measurement techniques indicate that absorption of stimulated photons at the laser facet is the major source of facet heating.
Applied Physics Letters 07/2007; 91(4):041115-041115-3. · 3.84 Impact Factor
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ABSTRACT: Thin-film light-emitting diodes (LEDs) belong to the most successful LED concepts for achieving high efficiencies. The incorporation of buried microreflectors with inclined facets prevents the light generation under the top contact and bondpad and offers an additional light extraction scheme. As a result, an external quantum efficiency of 50% could be demonstrated at a wavelength of 650 nm, and a luminous efficiency of more than 100 lm/W could be achieved in the wavelength range from 595 to 620 nm
IEEE Photonics Technology Letters 06/2007; · 2.19 Impact Factor
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ABSTRACT: The enhancement in external quantum efficiency of a red light-emitting diode (LED) from photonic crystal (PhC) hole patterns was investigated. A red LED was chosen because its epitaxial layers are relatively free from defects as compared to GaN-based LEDs. The peak emission wavelength was 642 nm, and a triangular-lattice PhC was designed with a hole diameter to lattice distance ratio of 0.5. The lattice distance to wavelength ratio (a/lambda) was varied from 0.2 to 4.6 in order to evaluate the enhancement in the external quantum efficiency. An improvement in efficiency greater than 75% was obtained for a/lambda between 0.6 and 2.0. This improvement of the optical characteristics occurred with unchanged electrical properties
IEEE Photonics Technology Letters 10/2006; · 2.19 Impact Factor
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ABSTRACT: The authors present a detailed investigation of defects generated during catastrophic optical damage (COD) in high-power 650 nm AlGaInP lasers using microphotoluminescence (μ- PL ) mapping, focused ion beam (FIB) microscopy, and deep-etching techniques. High-resolution μ- PL images demonstrated that during COD, nonradiative dark line defects (DLDs) originate from the front mirror of the laser and propagate in several branches into the laser perpendicular to the output facet. Furthermore, FIB microscopy identified the epitaxial layers affected by COD, revealing that DLDs are confined to the active region. In addition, deep etching confirmed that these defects have a noncrystalline nature.
Applied Physics Letters 10/2006; · 3.84 Impact Factor
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ABSTRACT: The authors report on quantitative investigations of the voltage drop across the heterostructure layer sequence of an operating AlGaInP light-emitting diode via Kelvin probe force microscopy for different external biases between −2.0 and +1.86 V. In the low voltage regime, most of the voltage drops in the active layer. For bias voltages above +1.5 V, however, they found an additional voltage drop on the p side of the device, which reduces the power efficiency of the light-emitting diode.
Applied Physics Letters 09/2006; 89(10):103522-103522-3. · 3.84 Impact Factor
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ABSTRACT: We have realized a 76 MHz white-light differential transmission spectroscopy system. The technique employs a Ti:sapphire laser oscillator and a tapered fiber to generate a white-light continuum spanning almost the full visible to near-infrared spectral range. Using acousto-optical modulation and subsequent lock-in detection, transient relative transmission changes as small as 10(-5) are detected. The method is applied to study the ultrafast gain dynamics of the active layer of a vertical-external-cavity surface-emitting laser based on a multiple-quantum-well structure.
Optics Letters 05/2006; 31(8):1157-9. · 3.40 Impact Factor
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ABSTRACT: AlGaInP thin-film resonant cavity light-emitting diodes (RCLEDs) show an improved performance compared to standard red emitting RCLEDs. External quantum efficiencies at 650 nm of 23% and 18% with and without encapsulation, respectively, have been obtained for devices showing a maximum emission in the normal direction. Thanks to the high angle-averaged reflectivity of the bottom hybrid mirror, a strong photon recycling effect occurs in these structures. The decrease of the absorption with increasing injection level reduces photon recycling and increases extraction of lateral guided modes. The redirection of part of the emission from the vertical to the lateral direction with increasing current density is reflected in the evolution of the far-field radiation pattern.
IEEE Photonics Technology Letters 02/2006; · 2.19 Impact Factor
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ABSTRACT: The internal quantum efficiency of (AlxGa1−x)0.5 In0.5P light-emitting devices (LEDs), with an emission wavelength ranging from 650 to 560 nm, is determined by means of a model that takes into account the radiative and nonradiative recombination in the active layer, the diffusive leakage of carriers into the confining layers, and the influence of photon recycling on the light extraction efficiency. The evaluation is based on measurements of the external quantum efficiency of the LEDs as a function of the operating current and temperature. The analysis provides the wavelength dependence of both the nonradiative recombination as well as the carrier leakage.
Journal of Applied Physics 10/2005; 98(8):086101-086101-3. · 2.17 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: The internal quantum efficiency of light emitting diodes (LEDs) based on the AlGaInP material system was analysed. This study formulated a comparatively simple rate equation model to gain a quantitative understanding of the contributions of the different loss mechanisms. On the experimental side a series of LED structures was grown with multiple (Al<sub>x</sub>Ga<sub>1-x</sub>)<sub>0.5</sub>In<sub>0.5</sub>P quantum wells embedded in In<sub>0.5</sub>Al<sub>0.5</sub>P confining layers. The emission wavelength was adjusted varying the aluminum content, from 650 nm down to 560 nm in the green spectral range. Simple test structures were processed and the optical output power measured over a certain range of operating currents and temperatures. On the other hand, in the computation of the external quantum efficiency it is important to note that due to re-absorption and re-emission of light in the active layer ('photon recycling') the extraction efficiency becomes dependent on the internal quantum efficiency. This effect was accounted for by performing ray-tracing simulations for these specific LED structures. For small internal efficiencies the extraction efficiency decreases by a factor of 3. Including this effect in the model a good fit to the experimental data over a wide temperature and current range could be achieved.
Numerical Simulation of Optoelectronic Devices, 2004. NUSOD '04. Proceedings of the 4th International Conference on; 09/2004
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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: An electrically conductive omnidirectional reflector (ODR) is demonstrated in an AlGaInP light-emitting diode (LED). The ODR serves as p-type contact and comprises the semiconductor, a metal layer and an intermediate low-refractive index dielectric layer. The dielectric layer is perforated by an array of AuZn microcontacts thus enabling electrical conductivity. It is shown that the ODR significantly increases light extraction from an AlGaInP LED as compared to a reference LED employing a distributed Bragg reflector (DBR). External quantum efficiencies of 18% and 11% are obtained for the ODR- and the DBR-LED, respectively.
IEEE Electron Device Letters 12/2003; · 2.85 Impact Factor
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Lasers and Electro-Optics, 2003. CLEO '03. Conference on; 07/2003
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ABSTRACT: In this paper, we give an overview of light-emitting diodes (LEDs)
with radial tapers. Light is generated in the very center of a
circularly symmetrical structure and is outcoupled at a tapered ring.
Encapsulated devices with an emission wavelength of 980 nm achieve
wallplug efficiencies of 48%. Non-encapsulated InGaAlP-based
red-emitting LEDs show quantum efficiencies of 13%. A new device design
combines the taper with a wafer-scale soldering technique promising a
feasible fabrication method
IEEE Journal of Selected Topics in Quantum Electronics 04/2002; · 3.78 Impact Factor
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ABSTRACT: This paper reviews the recent progress of AlGaInP high brightness
light-emitting diodes. After the discussion of some basic material
properties and the general problem of light extraction we will discuss
several approaches of high efficiency devices
IEEE Journal of Selected Topics in Quantum Electronics 04/2002; · 3.78 Impact Factor
