Semiconductor Physics Division Fachverband Halbleiterphysik (HL ...

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Semiconductor Physics Division (HL) Monday Lifting of the fundamental cavity mode polarization degeneracy in CdSe/ZnSSe-quantum dot monolithic microcavities — •Moritz Seyfried, Joachim Kalden, Kathrin Sebald, Jürgen Gutowski, Arne Gust, Carsten Kruse, and Detlef Hommel — Institue of Solid State Physics, University of Bremen, P.O. Box 330 440, D-28334 Bremen, Germany Lifting polarization degeneracy of the fundamental cavity mode in a quantum dot (QD)-based microcavity (MC) would allow to control the polarization state of the emitted photon in single-photon emitters. To realize devices in the blue to green spectral region, CdSe QDs were embedded in a monolithic ZnSSe based VCSEL structure grown by molecular beam epitaxy. Circular pillar MCs with diameters in the range from 500 nm up to 4000 nm were prepared by focused-ion-beam etching. Polarization dependent investigations of the fundamental cavity mode reveal a lifting of its degeneracy concerning the polarization. By an appropriate adjustment of the polarizer the two orthogonally polarized components of the fundamental mode can be observed individually with an energy splitting of up to 0.42 meV and quality factors of up to 7800. In any polarization orientation in between a superposition of both modes is detectable. Furthermore, an increase of the energy splitting with increasing pillar diameter is observed. As the MCs are nearly perfectly circular, a lifting of the mode degeneracy due to an elliptical cross-section of the pillars is very unlikely. Therefore, the existence of strain within the MCs as origin of the mode degeneracy is considered and tested by studying the influence of annealing. HL 4.5 Mon 11:15 POT 51 High quality junctions by interpenetration of vapor liquid solid grown nanostructures for microchip integration. — Seid Jebril 1 , Hanna Kuhlmann 1 , Sven Müller 2 , Carsten Ronning 3 , Lorenz Kienle 4 , Viola Duppel 5 , and •Rainer Adelung 1 — 1 Funktionale Nanomaterialien, CAU Kiel, Kaiserstr. 2, D-24143 Kiel — 2 Nanowires and thin films, II. Physikalisches Institut, Friedrich- Hund-Platz 1, D-37077 Göttingen — 3 Institute for Solid State Physics, Universität Jena, Max-Wien-Platz 1, D-07743 Jena — 4 Synthese und Realstruktur, CAU Kiel, Kaiserstr. 2, D-24143 Kiel — 5 MPI für Festkörperforschung, Heisenbergstr. 2, D-70569 Stuttgart The usability of nanostructures in electrical devices like gas sensors depends critically on the ability to form high quality contacts and junctions. For the fabrication of various nanostructures, vapor-liquid-solid (VLS) growth is a wide spread and very efficient technique. However, forming contacts with the VLS grown structures to utilize them in a device is still tedious, because either the substrate has to be epitaxial to the VLS material or a manual alignment is necessary. Here we demonstrate the contact formation by simply using the ability of individual crystals to interpenetrate each other during the straight forward VLS growth. This allows growing VLS structures directly on two neighboring gold circuit paths of a microchip; bridges over predefined gaps will be formed. Moreover, TEM investigations confirm the high quality of the crystalline junctions that allow demonstrations as UV and hydrogen-sensor. The VLS devices are compared with conventional produced [1] ZnO nanowires. [1] S. Jebril et al. Small,(2008)in press 15 min. break HL 4.6 Mon 11:45 POT 51 Zeitaufgelöste Spektroskopie von magnetischen Polaronen in CdMnSe/CdMgSe Quantengräben — •Tillmann Godde 1 , Irina I. Reshina 2 , Sergey V. Ivanov 2 , Ilya A. Akimov 1,2 , Dmitri R. Yakovlev 1,2 und Manfred Bayer 1 — 1 Experimentelle Physik II, Technische Universität Dortmund, D-44221 Dortmund, Germany — 2 Ioffe Physical-Technical Institute RAS, 194021 St. Petersburg, Rus- sia Ein magnetisches Polaron beschreibt eine lokale ferromagnetische Ausrichtung, die aus der starken Wechselwirkung des magnetischen Moments eines elektrischen Ladungsträgers mit den Spins lokalisierter magnetischer Ionen resultiert. Die Dynamik eines magnetischen Exziton-Polarons wurde erstmals in semimagnetischen Cd- MnSe/CdMgSe Quantengräben untersucht. Mittels zeitaufgelöster Photolumineszens-Spektroskopie auf einer Pikosekundenskala wurden die Abhängigkeiten der Bildungs- und Lebensdauer sowie die Energie des magnetischen Polarons in Magnetfeldern bis zu 7 T und bei Temperaturen von 1,9 bis 25 K bestimmt. Die Bildungsdauer der magnetischen Polaronen liegen in dem Bereich von 100-400 ps. Die Energie von 15 meV des Polarons wurde direkt durch eine selektive Anregungstechnik gemessen und durch weitere Methoden bestätigt. HL 4.7 Mon 12:00 POT 51 Accelerated, two-staged spin-lattice relaxation in (Zn,Mn)Se quantum wells — •Jörg Debus 1 , Vitalii Yu. Ivanov 2 , Andrei A. Maksimov 3 , Dmitri R. Yakovlev 1 , and Manfred Bayer 1 — 1 Experimentelle Physik II, Technische Universität Dortmund, 44221 Dortmund, Germany — 2 Institute of Physics, Polish Academy of Sciences, 02668 Warsaw, Poland — 3 Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia The dynamics of spin-lattice relaxation of the Mn ions in (Zn,Mn)Sebased diluted magnetic semiconductor quantum wells with low Mn concentration (≤ 2%) is studied by time-resolved photoluminescence. The quantum well magnetization is determined by single Mn ions as well as spin clusters with antiferromagnetically coupled Mn ions. Pair and triple clusters with next nearest neighbour ions acting as fast relaxing centers contribute to the energy transfer from the Mn system to the lattice via two-phonon Orbach transitions, resulting in a twostaged spin-lattice relaxation process. The efficiency of both processes is influenced by the Mn concentration, strength of applied magnetic field and optical excitation density. A further impact of next nearest neighbour spin clusters on the spin-phonon interactions is revealed in cusps in the static and dynamic quantum well magnetization at specific magnetic fields below 10 T. The cusps correspond to an additional cooling of the Mn spin temperature due to a crossing of Zeeman-splitted cluster spin levels. Consequently, the emission of resonant phonons causes an acceleration of the spin-lattice relaxation. HL 4.8 Mon 12:15 POT 51 Einfluss eines stöchiometriebedingten p-n-Übergangs auf die Diffusion in CdZnTe — •J. Kronenberg 1 , F. Wagner 1 , H. Wolf 1 , Th. Wichert 1 und ISOLDE Collaboration 2 — 1 Technische Physik, Universität des Saarlandes, 66123 Saarbrücken — 2 CERN, PH Department, 1211 Genf In früheren Studien wurde gezeigt, dass die Diffusion von Ag und Cu in CdTe ungewöhnliche Konzentrationsprofile aufweist [1]. Nach einseitiger Implantation und anschließender Diffusion unter Cd-Dampfdruck entsteht ein bzgl. der Probenmitte (Dicke ca. 800 µm) symmetrisches, peakförmiges Profil. Notwendig dazu ist Te-reiches, p-leitendes Material, das durch die thermische Behandlung unter Cd-Druck in Cdreiches, n-leitendes Material umgewandelt wird. Liegen bei der Diffusionstemperatur die Dotieratome hauptsächlich als positiv geladene, hoch mobile Zwischengitteratome vor, so spiegelt das Konzentrationsprofil das Profil der Fermienergie wider. Dabei zeigen dessen Flanken die aktuelle Position des p-n-Übergangs zwischen Te reichem und Cd reichem Material an. Mittlerweile wurden auch für Au und Na in CdTe nahezu dieselben Eigenschaften beobachtet. Auch Co und Ni zeigen ungewöhnliche Diffusionsprofile, unterscheiden sich jedoch deutlich von denen für Ag, Cu, Na und Au. Die beobachteten kastenförmigen Profile reichen von der implantierten Oberfläche bis hin zum jeweiligen p-n-Übergang. Ursachen für die Entstehung der kastenförmigen Profile werden diskutiert. Gefördert durch das BMBF, Projekt 05 KK7TS1 [1] H. Wolf et al., Phys. Rev. Lett. 94 (2005) 125901 HL 4.9 Mon 12:30 POT 51 Homo- and heteroepitaxial growth of ZnS — •Udo Roemer, Stefan Lautenschlaeger, Sebastian Eisermann, Oliver Graw, Joachim Sann, Melanie Pinnisch, Andreas Laufer, and Bruno K. Meyer — Ist phisics institute, Justus Liebig University Gießen ZnS in its zincblende structure has a direct bandgap of 3.6 eV at room temperature. As material for optoelectronic applications it possesses some advantages, for example the absence of crystal fields or piezoelectricity, compared to GaN or ZnO. So far there are only a handful of publications dealing the ZnS thin film growth, up to this work no homoepitaxial growth approach has been studied. We used both, ZnS single crystal substrates and GaP single crystal substrates to investigate the CVD growth of ZnS thin films. The grown epilayers have been studied using low temperature photoluminescence (PL), X-ray diffraction (XRD), Atomic Force Microscopy (AFM) and Secondary Ion Mass Spectroscopy (SIMS).
Semiconductor Physics Division (HL) Monday HL 5: Photovoltaic Time: Monday 10:15–13:15 Location: POT 151 HL 5.1 Mon 10:15 POT 151 Spatially resolved photoluminescence measurements for a comparative analysis of CuInS2 and Cu(In,Ga)S2 thin films — •Florian Heidemann 1 , Levent Gütay 1 , Rudolf Brüggemann 1 , Saoussen Merdes 3 , Alexander Meeder 2 , and Gottfried H. Bauer 1 — 1 Institute of Physics, CvO University Oldenburg, Germany — 2 SULFURCELL Solartechnik GmbH, Berlin, Germany — 3 Helmholtz-Zentrum Berlin, Germany At present chalcopyrite semiconductors like Cu(In,Ga)Se2 and Cu(In,Ga)S2 are most promising absorbers for thin film solar cells. Due to their grainy structure and inhomogeneous growth they show considerable degrees of spatial inhomogeneities in structural, optical and optoelectronic properties in the length scale of grain sizes. To analyze these locally fluctuating magnitudes we have performed spectrally resolved luminescence measurements with a high lateral resolution (≤ 1µm) in a confocal microscope setup. This makes the determination of the spatial variation in the splitting of the quasi-Fermi levels (µ = EF n − EF p) as well as the local absorbance of the material possible. A comparison of these properties, which are crucial for the solar light conversion efficiency, is made for CuInS2 and Cu(In,Ga)S2 absorber layers for data obtained from statistically representative scan areas. The results show that an increase in bandgap and the mean µ due to an incorporation of gallium does not come along with a decrease in the variation of µ over the absorber layer. A quantification of the lateral patterns of constant µ by Minkowski-operations indicates similar pattern sizes in the range of a few µm for the analyzed samples. HL 5.2 Mon 10:30 POT 151 Diffusion processes and chemical changes at the (Zn,Mg)O/ CuIn(S,Se)2 interface caused by RF magnetron sputtering deposition — •Felix Erfurth 1 , Benjamin Hußmann 1 , Lothar Weinhardt 1 , Achim Schöll 1 , Friedrich Reinert 1 , Eberhard Umbach 1,2 , Thomas Niesen 3 , Jörg Palm 3 , Sven Visbeck 3 , Alexander Grimm 4 , Iver Lauermann 4 , and Reiner Klenk 4 — 1 Universität Würzburg, Experimentelle Physik II, 97074 Würzburg — 2 Forschungszentrum Karlsruhe GmbH, 76133 Karlsruhe — 3 Avancis GmbH & Co. KG, 81739 München — 4 Helmholtz Zentrum Berlin für Materialien und Energie, 14109 Berlin (Zn,Mg)O buffer layers on Cu(In,Ga)(S,Se)2 thin film solar cells are a promising alternative to CdS buffer layers. Although in the past the radio frequency magnetron sputtering deposition on (Zn,Mg)O has been empirically optimized to reach high efficiencies, only little is known about the influence of the sputter parameters on the electronic and chemical structure of the absorber/buffer interface. We have used in-situ X-ray Photoelectron Spectroscopy (XPS) and X-ray induced Auger Electron Spectroscopy (XAES) to analyze the influence of different sputter parameters and to compare samples prepared by different groups. All samples show the formation of indium oxide and zinc - sulfur/selenide bonds at the buffer/absorber interface. Moreover, the hydroxide/oxide ratio at the interface is strongly increased as compared to the respective ratio in the bulk. Further studies showed a diffusion of sodium to the surface for thin (Zn,Mg)O layers. HL 5.3 Mon 10:45 POT 151 Mikroskopische Lumineszenzeigenschaften von polykristallinen CuInS2-Dünnschichten — •Anja Dempewolf 1 , Frank Bertram 1 , Alexander Franke 1 , Thomas Hempel 1 , Jürgen Christen 1 , Joachim Klaer 2 , Frank Wünsch 2 und Thomas Schedel-Niedrig 2 — 1 Institut für Experimentelle Physik, Otto-von- Guericke-Universität Magdeburg — 2 Helmholtz-Zentrum Berlin für Materialien und Energie Die Lumineszenzeigenschaften von sowohl unpassivierten als auch SixN- und ZnO/CdS-passivierten polykristallinen CuInS2- Dünnschichten (CIS) für die Anwendung als Absorbermaterial in Solarzellen wurden mittels orts-, zeit- und spektral aufgelöster Kathodolumineszenz (KL) und spektral aufgelöster Photolumineszenz (PL) untersucht. Die auf einem Glassubstrat mit gesputtertem Mo-Kontakt durch ein RTP-Verfahren (rapid thermal processing) hergestellten CIS-Schichten besitzen eine Dicke von etwa 2,1 µm und weisen eine körnige Oberflächenmorphologie auf. Das integrale Tieftemperatur- KL-Spektrum (5 K) des CIS wird durch eine exzitonische Lumines- zenz um 1,5 eV dominiert. In lokal aufgelösten Spektren lässt sich in einem Energiebereich von 1,35 eV bis 1,56 eV eine Vielzahl von einzelnen Emissionslinien identifizieren, die vor allem gebundenen Exzitonen und Donator-Akzeptor-Übergängen zugeschrieben werden. Allgemein zeigen die untersuchten Schichten in der Intensität und Emissionsenergie eine inhomogene Lumineszenzverteilung. So weichen die Lumineszenzeigenschaften individueller Körner als auch verschiedener Facetten eines einzelnen Korns signifikant voneinander ab. HL 5.4 Mon 11:00 POT 151 Spectral Response of CuIn1−xGaxSe2 Heterodiodes Operated at Constant VOC and Constant ISC Compared with Traditionally Recorded Spectral Quantum Yield — •Sven Burdorf, Rudolf Brüggemann, and Gottfried Heinrich Bauer — Institute of Physics, Carl von Ossietzky University Oldenburg D-26111 Oldenburg, F.R. Germany Traditional spectral response experiments in solar cells, such as quantum yields show the dependence of the excess carrier contribution and respective recombination on the depth of the device in terms of the profile of the optical generation. However, this depth information is masked by the condition of current continuity that is met by contributions of minority as well as majority carriers across the entire depth of the device and commonly the information on local properties is not reflected straightforwardly. In our approach - analogously to the concept of the constant photocurrent method (CPM) - we have adjusted either constant VOC (c-Voc) or ISC (c-Isc) by recording the spectral photon fluxes necessary for these conditions. In particular in Voc operation excess carriers recombine exclusively within the device and thus the signal is more sensitive against recombination. Our results of the c-Voc and c-Isc experiments for CIGSe-heterodiodes show significant differences particularly in the short-wavelength regimes. The comparison of experimental results with numerical modeling shows that this difference is growing larger with increasing interface recombination. 15 min. break HL 5.5 Mon 11:30 POT 151 Performance of InGaAsP/InGaAs tandem solar cells with an InGaAs/GaAsSb tunnel junction — •Erol Sagol, Nadine Szabo, Ulf Seidel, Christian Höhn, Klaus Schwarzburg, and Thomas Hannappel — Helmholtz-Zentrum Berlin, Glienicker Str. 100, 14109 Berlin-Germany Three types of state-of-the-art III-V triple-junction solar cells have already surpassed the 40% efficiency mark, despite having non-optimized band gaps. But still considerably higher efficiencies can be achieved with a four-junction configuration, which has optimized band gaps around 1.9, 1.4, 1.0 and 0.7 eV. This can be realized with a mechanically stacked GaAs-based GaInP/GaAs tandem and an InPbased InGaAsP/InGaAs tandem cell. For this purpose, we grew In- GaAsP/InGaAs tandem solar cells lattice-matched to InP by MOVPE. The InGaAs bottom cell (0.73 eV) and the InGaAsP top cell (1.03 eV) were connected with a tunnel junction, which was composed of highly doped n-InGaAs and p-GaAsSb layers. In order to evaluate the performance of the tunnel junction, separate devices were grown without the photoactive layers. High current densities of several thousand A/cm2 were achieved already in the bias regime of several 100 mV. Hence, voltage losses in the tunnel diode should not be of concern for the solar cell even under extreme concentration ratios (> 1000 suns). Our results show that the contribution of such a low band gap InGaAsP/InGaAs tandem bottom cell, reaching efficiencies above 10% under GaAs, is considerably higher than a conventional germanium subcell. HL 5.6 Mon 11:45 POT 151 3D photonic crystal interlayers for mircomorph thin film silicon tandem cell — Andreas Bielawny 1 , •Johannes Üpping1 , Paul T. Miclea 1 , Ralf B. Wehrspohn 1 , Carsten Rockstuhl 2 , Falk Lederer 2 , Marius Peters 3 , Lorenz Steidl 4 , Rudolf Zentel 4 , Seung-Mo Lee 5 , Mato Knez 5 , Andreas Lambertz 6 , and Reinhard Carius 6 — 1 Institute of Physics, mikroMD, University of Halle Wittenberg — 2 Institue of Physics, Solid States Optics, University of Jena — 3 Freiburg Centre for Material Research, University of Freiburg — 4 Dept. of Chemistry, Pharmacy and Earth Sci-
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