Symposium : K
Second generation (Al.,In)GaN laser diodes and complex optoelectronic devices
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| Nonpolar and semipolar laser diodes : Detlef Hommel | ||
| 09:05 | Progress in the Growth, Characterization and Device Performance for Nonpolar and Semipolar GaN-based Materials Authors : Robert M. Farrell Materials Department, University of California, Santa Barbara, CA 93106 Resume : Devices grown on c-plane GaN suffer from large internal electric fields due to discontinuities in spontaneous and piezoelectric polarization effects which cause charge separation between holes and electrons in quantum wells and limits the radiative recombination efficiency. Nonpolar GaN devices, such as in the m-plane , are free from polarization related electric fields since the polar c-axis is parallel to any heterointerfaces. Semipolar GaN-based devices have reduced electric fields and in some cases, such as , show a high propensity for Indium update for InGaN quantum wells.
In this talk, we present work on outstanding materials issues including: morphological stability with special emphasis on the role of substrate orientation and growth conditions; new evidence for dislocation-related strain relaxation in semipolar GaN-based heterostructures; unambiguous determination of the polarization cross-over in semipolar InGaN/GaN heterostructures; new detailed atom probe analysis of high performance GaN-based LEDs and laser diodes. Additionally, we update progress on nonpolar electron devices and nonpolar and semipolar LEDs and LDs including the achievement of high performance true blue (λ >450 nm) and true green (λ >520 nm) lasers on m-plane and semipolar (namely, ) GaN substrates. | 1 1 |
| 09:50 | Polarization Switching of the Optical Gain in Semipolar InGaN Quantum Wells Authors : Wolfgang G. Scheibenzuber*, Ulrich T. Schwarz* *Fraunhofer Institute for Applied Solid State Physics, Tullastrasse 72, D-79108 Freiburg, Germany Resume : Growth on semipolar crystal planes poses a possibility to reduce polarization fields and thereby enhance stimulated emission in (Al,In)GaN laser diodes. In this case the anisotropic optical properties of wurtzite (Al,In)GaN influence the optical eigenmodes of the laser waveguide and the optical gain. Experiments indicate that this anisotropy depends strongly on the indium content of the InGaN quantum wells.
We investigate the influence of birefringence and polarization switching on the optical gain of semipolar InGaN quantum wells depending on indium content and charge carrier concentration using self-consistent 6*6 k*p-band structure calculations. We compare two semipolar planes: The (11-22)- and the (20-21)-plane. In contrast to the (20-21)-plane, the dominant polarization of the optical gain in a quantum well on the (11-22)-plane can depend on both the indium content and the charge carrier concentration, as reported from experiments. These effects are explained by a detailed analysis of the wave function composition of the topmost valence bands in a semipolar quantum well.
Our results indicate that unlike the the (20-21)-plane, the (11-22)-plane offers the possibility to align the waveguide along a direction which allows mirror cleaving for laser diodes emitting in the green spectral range, which have a high indium content in their InGaN quantum wells. | 1 2 |
| 10:35 | Coffee break | |
| 10:50 | In-incorporation on semipolar surfaces for blue-green lasers Authors : Tim Wernicke, TU Berlin Simon Ploch, TU Berlin Jens Rass, TU Berlin Lukas Schade, IAF Freiburg Carsten Netzel, FBH Berlin Veit Hoffmann, FBH Berlin Arne Knauer, FBH Berlin Ulrich Schwarz, IAF Freiburg Markus Weyers, FBH Berlin Michael Kneissl, TU Berlin, FBH Berlin Resume : In order to extend the emission wavelength of nitride based semiconductor lasers into the green spectral range InGaN quantum wells (QWs) with high indium content need to be grown. QWs on the polar c-plane surface exhibit large polarization fields and strong carrier localization leading to a reduced density of states as can be seen by broadened emission lines. Growth on alternative surface orientations may solve some of the challenges for green emitters. However, it is not clear which orientation offers the best chance to obtain efficient laser diodes. Growth of QWs on different semipolar GaN substrates with reduced polarization, i.e., (10-12), (10-11), (20-21), and (11-22), was compared with growth of InGaN on c-plane and m-plane GaN substrates.
A series of quantum wells grown at different temperatures revealed a strong dependence of the indium incorporation on the growth surface orientations. The indium incorporation is highest for (10-11) followed by (11-22) and c-plane surfaces. Micro PL mapping on sub micrometer scale revealed strongly reduced fluctuations of emission intensity and wavelength for all semipolar and nonpolar orientations. (11-22) shows the best homogeneity and FWHM together with the lowest RMS surface roughness. Also the amplified spontaneous emission under high power stripe excitation from QWs embedded in a waveguide was highest for this orientation. Although the (11-22) has not the highest In-incorporation, it showed the best overall performance. | 1 3 |
| 11:55 | GaN-based laser structure integrating three-dimensional semipolar InGaN QWs within a planar c-oriented waveguide Authors : R.A.R. Leute, T. Wunderer, F. Lipski, F. Scholz; Institute of Optoelectronics, Ulm University; J. Biskupek, A. Chuvilin, L. Lechner, U. Kaiser; Central Facility of Electron Microscopy, Ulm University; M. Brendel, A.D. Dräger, and A. Hangleiter; Institute of Applied Physics, Technische Universität Braunschweig Resume : Selective area epitaxy has been established as means to grow three-dimensional GaN structures of high crystal quality with semipolar surfaces on conventional two-inch c-oriented sapphire substrates. The reduced piezoelectric fields on these surfaces show great promise to lead to improved device performance. InGaN/GaN MQWs as well as complete LED structures have been realized on these surfaces.
Gradients in QW thickness and In-composition, which can be explained by gas-phase diffusion, however lead to broadened emission spectra which could be beneficial for LED lighting applications, yet are unfavorable for a laser diode.
We minimize this effect by successfully reducing the size of the 3D structures below the micrometer range with a periodicity down to 235 nm using e-beam lithography and SiO2 sputter coating. While narrowing the full-width at half maximum of the QW emission, this allows us at the
same time to integrate these semipolar QWs within the waveguide structure of a conventional laser diode. Furthermore the periodic modulation of the refractive index caused by the SiO2 mask could potentially be utilized for a distributed feedback device. The three-dimensional active layer is planarized with GaN grown under 2D conditions and embedded within AlGaN cladding layers for optical confinement. We present detailed structural analyses (including TEM) as well as the results from optical characterization methods (including gain analyses). | 1 4 |
| 12:15 | Comparable indium incorporation in polar and nonpolar GaInN quantum wells for long-wavelength lasers Authors : H. Jönen, H. Bremers, U. Rossow, A. Schwiegel, M. Brendel, A. D. Dräger, and A. Hangleiter Institut für Angewandte Physik, Technische Universität Braunschweig, 38106 Braunschweig, Germany S. Schwaiger, and F. Scholz Institut für Optoelektronik, Universität Ulm, 89069 Ulm, Germany Resume : While GaInN based violet and blue lasers are commercially available, several problems occur on the way towards the green spectral region. Among others the high indium contents needed for green emission result in high piezoelectric fields which dramatically reduce the oscillator strength. One promising approach to avoid the negative influence of polarization fields is to grow on nonpolar surfaces, i.e. the a-plane or the m-plane of the wurtzite structure. However, the optimum growth conditions for nonpolar layers may significantly differ from growth on c-plane. In this contribution we compare GaInN multiple quantum wells on c-plane, m-plane and a-plane surfaces and show that the In incorporation efficiency is identical for all three surfaces. Our samples were grown in a low pressure MOVPE system on bulk GaN substrates, (HVPE) GaN templates or foreign substrates. High resolution X-ray diffraction measurements were used to determine lattice constants, strain states and finally the In concentration in the quantum wells. Applying the same growth conditions in the QWs we find identical growth rates and In concentrations for c-plane and m-plane in good agreement with optical experiments, in particular for homoepitaxial growth. For heteroepitaxial GaInN QWs we find strong evidence that the optical properties are dominated by the high density of stacking faults. Preliminary experiments on a-plane also indicate that the In incorporation is very similar to that on c-plane and m-plane. | 1 5 |
| 12:35 | Lunch | |
| Substrate and new devices : Tadek Suski | ||
| 14:00 | GaN substrates grown by ammonothermal method Authors : R. Dwilinski, R. Doradzinski, J. Garczynski, L. Sierzputowski, R. Kucharski, M. Zając Ammono Sp. z o.o., Czerwonego Krzyża 2/31, 00-377 Warsaw, Poland M. Rudzinski, W. Strupinski, Institute of Electronic Materials Technology, Wólczyńska 133, 01-919 Warsaw, Poland J. Serafinczuk, Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology Janiszewskiego 11/17, 50-372 Wrocław, Poland R. Kudrawiec Institute of Physics, Warsaw University of Technology, Wybrzeże Wyspiańskiego 27, 50 370 Wrocław, Poland Resume : Recently, large interest has been devoted to ammonothermal method. In this technique GaN containing feedstock is dissolved in one zone of the high pressure autoclave, then transported by convection in the temperature gradient to the second zone, where GaN is crystallized on native seeds due to the supersaturation of the solution. The typical pressures and temperatures applied are 0.1÷ 0.3 GPa and 500 °C ÷ 600 ºC, respectively. AMMONO company invented and developed the ammonobasic environment of the method, where mineralizersin the form of alkali metals or their amides were added into autoclave in order to enhance GaN solubility.
This communication shows the properties of GaN substrates (polar C-plane, non-polar M-plane, semipolar) produced at AMMONO company and their application in homoepitaxial layer deposition. The crucial result, showing 2-inch c-plane GaN seed crystals and their structural and electrical properties will be presented. In the crystals considered, a low dislocation density (5 x 10E+3 cm-2) and wide spectrum of electrical properties is attained. At the same time the crystal lattice is extremely flat, and the (0002) rocking curve is very narrow (FWHM=16 arcsec).
GaN epilayers deposited on any type of substrate (polar, non-polar or semi-polar) exhibit also very good structural properties and intrinsic narrow free exciton lines, indicating usufullness of ammonothermal substrates for the epitaxy of GaN-based optoelectronic devices. | 2 1 |
| 14:45 | Bulk GaN substrates for laser diodes technology Authors : Izabella Grzegory Institute of High Pressure Physics Polish Academy of Sciences ul. Sokołowska 29/37, 01-142 Warsaw, Poland Resume : The GaN – AlN – InN material system is extremely attractive for laser diodes (LDs) because due to direct energy gaps of its constituents covering very large spectral range of 0.7 to 6.5eV. In particular the GaN-based LDs emitting red, blue and green light can be constructed. There are however serious technological problems related to thermodynamic properties of GaN, AlN and InN, making the construction of these devices relatively difficult. Due to extremely high melting temperatures (especially AlN) and pressures (especially InN), it is not possible to growth large high quality crystals of III-V nitrides by methods based on cooling of their stoichiometric melts. Therefore, the methods requiring less extreme conditions have to be developed. For GaN, the Hydride Vapor Phase Epitaxy (HVPE) on heterogeneous substrates (GaAs and sapphire) is most widely used for production of laser quality free standing GaN wafers. The GaN wafers which can be used as substrates for epitaxy of laser diode structures have to fulfill several quality criteria the most important of which are: low dislocation density (< 106cm-2), good electrical conductivity, optimum surface orientation (usually with a small miscut to a low index crystallographic plane) and size. Excellent quality crystals were recently grown by ammonothermal method.
A special role of HVPE method in development of laser quality quasi-bulk GaN substrates and bulk GaN crystallization itself will be emphasized. Bulk GaN crystals grown by HVPE are used as substrates with various orientations for research as well as seed crystals at the initial stages of development of ammonothermal and liquid solution growth methods. However the crystal lattice of free standing GaN crystals grown by HVPE on sapphire or on GaN-sapphire substrates is usually strongly deformed due to lattice and thermal mismatch of GaN and Al2O3. This deformation (bowing) is detrimental for further use of these crystals as substrates for epitaxy but also as seeds for bulk growth including the growth by HVPE itself.
In this work it is shown that final (in free standing GaN) bowing of {0001} planes depends on the coupling between the crystal and the substrate (induced by type and filling factor of the mask), temperature regime during HVPE growth and shape of the crystallization front. It is also shown that at optimized conditions, 2 inch free standing GaN crystals with c-plane curvature radii exceeding 10 m. can be obtained
High Pressure Solution Growth (HPSG) is one of a few methods giving single crystalline GaN which is successfully used for fabrication of laser diodes. This is possible due to extremely low dislocation density and high, very uniform electrical conductivity of this material. The problem of limited size of spontaneously grown high pressure GaN crystals is being solved by the use of seed crystals grown by HVPE. In this way the 1-2 inch (0001) oriented pressure grown crystals with dislocation density significantly lower than in the seed can be obtained.
In this work the growth and physical properties of HNPS crystals deposited on and separated from “home made” 2 inch GaN substrates grown by HVPE with c-planes curvature radii varying from 2 to 20 m are studied. In particular the influence of c-plane bowing in the initial HVPE substrate on quality, rate and mode of growth from solution is analyzed. It is shown that curvature of c-planes of the 0.3-0.6mm thick HNPS GaN crystal – substrate system follows the one of the substrate whereas the curvature of the HNPS crystal separated from the substrate becomes much smaller (R > 100m can be achieved) which indicates elastic and reversible character of deformation in the high pressure material. Flat c-planes free standing HNPS GaN crystals with diameters between 1 and 2 inches will be shown and characterized. | 2 2 |
| 15:30 | Coffee break | |
| 15:50 | CW BLUE SUPERLUMINESCENT LIGHT EMITTING DIODES BASED ON GAN Authors : Eric Feltin1, Marco Rossetti2, Julien Dorsaz2, Raffaele Rezzonico2, Marcus Duelk2, Christian Velez2, Antonino Castiglia3, Gatien Cosendey3, Jean-François Carlin3, and Nicolas Grandjean3 1 NOVAGAN s.a.r.l., Chemin de Mornex 5, CH-1003 Lausanne, Switzerland 2 EXALOS AG, Wagistrasse 21, CH-8952 Schlieren, Switzerland 3 Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland Resume : Superluminescent light emitting diodes (SLEDs) are devices combining the beam directionality of laser diodes (LDs) with the broadband emission of light emitting diodes (LEDs). SLEDs have a highly directional emission similar to that of LDs thanks to the use of a waveguide structure. However, SLEDs do not feature an optical cavity; hence they exhibit a broad and smooth spectrum similar to that of LEDs. Thanks to these specific properties, short-wavelength SLEDs could have a major impact in medical or biomedical and in projection applications.
We report now on the demonstration of CW blue SLEDs based on the emission of InGaN QWs embedded in an AlInGaN waveguiding structure. Narrow ridge-waveguide devices realized by standard processing techniques and with extremely low facet reflectivity. In order to prevent laser action the output facet is not perpendicular to the ridge like in an LD but tilted by a few degrees. For comparison, LDs were fabricated with the same structure and size.
The SLEDs show single lateral mode CW output powers > 35 mW. At high current, the SLED emission is not formed by a multimode structure typical of LDs but exhibits a smooth Gaussian peak with a linewidth of 4-5 nm. Tuning the composition of the active region, SLEDs spanning all the spectral range between 410 and 445 nm could be realized. The light output is highly directional and results in a coupling into single mode fibers > 50%.
This work was supported by the NCCR Quantum Photonics research instrument of the Swiss National Science Foundation and the CTI-KTI project “Blue SLED”. | 2 3 |
| 16:35 | Toward nitride quantum dot lasers Authors : D. Hommel University of Bremen Resume : Quantum dots (QD) are of great interest in optoelectronic devices due to their strong confinement and efficient radiative recombination. Applications span from laser diodes with low current threshold to single photon emitters for quantum cryptography. Due to their high exciton binding energy InGaN/GaN-based QDs are promising candidates for laser diodes in the green spectral region as well as for single photons on demand at room temperature.
Nevertheless, special attention has to be maid to the growth of such quantum dot structures in order to ensure their stability after overgrowth. In case of the InGaN/GaN system dots grown on GaN surfaces by the Stransky-Krastanov mode are unstable under overgrowth [1]. InGaN dots are often dissolved during GaN cap layer deposition and results obtained wrongly interpreted. Therefore, a new growth procedure had to be found to obtain stable InGaN QDs in a GaN matrix [2, 3]. Based on extended growth studies and careful scanning transmission electron microscopy argument will be given that spinodal phase separation plays an important role in the InGaN dot formation. A breakthrough in identifying the InGaN QDs without destroying structures was obtained by Z-contrast imaging [4].
Although the density of the overgrown InGaN dots can be changed roughly within one order of magnitude their density is too small and stacking is needed to obtain sufficient modal gain for lasing. Using the variable stripe length method the modal gain for a three-fold QD-stack is 50 cm-1. This is a rather low value but taking into account the dot density the gain per dot could be estimated to 10-9 cm-1, a rather high value comparable to that for InAs and CdSe quantum dots. 3 – 7-fold stacks have been incorporated into complete nitride laser structures grown on sapphire and bulk nitride substrates.
Emission from a single InGaN QD was observed in photo- and electroluminescence up to 150 K [7, 8] which make the material ideal as sources for photons-on-demand at elevate temperatures.
Following the development of monolithic microcavities [9] wide gap QDs have been successfully incorporated and studied in detail [10, 11]. Micro-PL spectra reveal distinct sharp emission lines attributed to single InGaN QDs. Air posted micropillars prepared by focused ion beam show a transverse mode structure in good agreement with theoretical calculations. Electroluminescence from a fully monolithic nitride cavity structure with AlInN/GaN DBRs, 5-λ cavity, single InGaN QD layer and intracavity contacts will be reported.
[1] A. Pretorius et. al. “Structural investigation of growth and dissolution of InGaN nano-islands grown by molecural beam epitaxy” J. Cryst. Growth 310 (2008) 748
[2] T. Yamaguchi et. al. „A novel approach for the growth of InGaN quantum dots” phys. stat. sol. (c) 3 (2006) 3955
[3] C. Tessarek et. al. “Improved capping layer growth towards increased stability of InGaN quantum dots “ phys. stat. sol. (c) 6 (2009) S561
[4] A. Rosenauer et. al. “Evidence for indium rich islands in InGaN by composition mapping using scanning tunnelling electron microscopy Z-contrast imaging” Phys. Rev. Lett., submitted
[5] J. Kalden et. al. „Optical properties and modal gain of InGaN quantum dot stacks“ phys. stat. sol. (c) 6 (2009) S593
[6] T. Aschenbrenner et. al. “Incorporation of QD ensembles in separate confinement heterostructures for long wavelength emission” phys. stat. sol. (c) 6 (2009) S921
[7] K. Sebald et. al. „Optical properties of single InGaN quantum dots up to 150 K” phys. stat. sol. (c) 3 (2006) 3864
[8] J. Kalden et. al. „Electroluminescence form a single InGaN quantum dot in the green spectral region up to 150 K” Nanotechn. 21 (2010) 015204
[9] H. Lohmeyer et. al. “Resonant modes in monolithic nitride pillar microcavities” Eur. Phys. J. B 48 (2005) 291
[10] K. Sebald et. al. “Optical properties of InGaN quantum dots in monolithic pillar microcavities” Appl. Phys. Lett., in press
[11] H. Dartsch et. Al. “Electroluminescence from InGaN quantum dots in a monolithically grown GaN/AlInN cavity “ IC-MOVE 2010, proc.: J. Cryst. Growth | 2 4 |
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