Polarization Controlled Light Emission from Nitride Based Pyramidal. Quantum Dots

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1 Polarization Controlled Light Emission from Nitride Based Pyramidal Quantum Dots Per Olof Holtz1, Chih-Wei Hsu1, Anders Lundskog1, Martin Eriksson1, K. Fredrik Karlsson1, Urban Forsberg1 and Erik Janzén1 1 Department of Physics, Chemistry and Biology (IFM), Linköping University, S , Linköping, Sweden InGaN quantum dots (QDs) formed on top of GaN pyramids have been fabricated by means of selective area growth (SAG) employing hot wall MOCVD. Upon regrowth of the GaN/AlN/SiC substrate with a SiN film patterned with µm-sized holes by means of standard UV lithography and RIE etching, the continued growth will evolve into epitaxially grown GaN based pyramids. In case of circular holes, symmetric GaN pyramids with six facets as a consequence of the wurtzite structure will form. Subsequently, these pyramids are overgrown by a thin and optically active InGaN quantum well. At the apices of the pyramids, nm-sized InGaN QDs will nucleate. Well-defined single emission lines with a sub-mev line width have been monitored by means of µ-photoluminescence (µpl) from these deterministic QDs. As a subsequent step, the holes were instead made asymmetric, in such a way that a rectangle was inserted between two semicircles. In that case, elongated GaN pyramids were formed with two of the six facets longer, resulting in InGaN ridges on top of these asymmetric pyramids. The primary emission line monitored originates from electron-hole pairs, i.e. a single neutral excitons. The emission energies are in the blue region (around 415 nm), but can be pre-defined by altering the growth conditions, such as the growth temperature, the well width and/or the In composition, for the InGaN layers. The excitonic emission from the extended InGaN QDs on top of the elongated pyramids exhibit a strong degree of linear polarization, with a typical polarization ratio >0.9 achieved for the case of an elongation of the pyramid base by 1µm. In this way, both site-control of the QDs on top of the pyramids, and control of the polarization directions for the linearly-polarized emissions and finally variable emission energies in a given wave length window can be achieved.

2 New Concepts and Materials for Solar Power Conversion Devices Wladek Walukiewicz Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA In the presentation we will discuss our work on materials suitable for novel approaches and concepts for high efficiency conversion of solar energy. The presentation will focus on three research areas: group III-nitrides for high efficiency multijunction solar cells, highly mismatched alloys for intermediate band solar cells and CdO based materials for transparent conductors.

3 Superconducting Nanowire Single Photon Detectors: Configuration and Material Aspects R. Cristiano 1, M. Ejrnaes 1, C. Camerlingo 1, L. Parlato 1,2, R. Arpaia 1,2,3, H. Myoron 4 and G. Pepe 1,2 1 CNR Instituto SPIN, UOS- Napoli, Via Cintia, Napoli, Italy 2 Dip.to di Scienze Fisiche, Università Federuco II di Napoli, Via Cinta, Napoli, Italy 3 Chalmers University of Technology, Dept Microtechnology and Nanoscience, Quantum Device Physics Laboratory, S Gothenburg, Sweden 4 Graduate School of Science and Engineering, Saitama University, Saitama, Japan Superconducting nanowire single-photon detectors (SNSPDs) for telecom wavelength (1550 nm) are currently of great interest mainly because of their application in the field of quantum optics, quantum key distribution and space telecommunication. We present the convenient use of a special configuration based on the parallel assembling of nanowires that turns to be useful for both the realization of large area SNSPD and to obtain high signal and signal-to-noise ration when ultra-narrow (<100 nm) nanowires are used in order to move toward mid-infrared and THz applications. We also discuss novel hybrid materials like NbN/NiCu and YBCO/LSMO bilayers for SNSPDs.

4 Plasma Synthesis of a-si:h Based Radial Junction Solar Cells or How to Optimize Light Trapping, Carrier Collection and Stability via a Low Cost Process Soumyadeep Misra, Martin Foldyna, Linwei Yu, and Pere Roca i Cabarrocas LPICM-CNRS, Ecole Polytechnique, 91128, Palaiseau, France Silicon nanowires offer a promising approach to achieve high performance thin film solar cells because of the excellent light trapping resulting from a combination of antenna and refractive index matching effects. Moreover, when used as the basis for a radial junction solar cell, they can boost the cell current by increasing the carrier collection efficiency. The most common approach to grow nanowires on top of low cost glass substrates is the vapor-liquid-solid method, where metal droplets assist the growth of crystalline nanowires. Over the past six years we have been developing a one pump down SiNW growth process based on plasma enhanced chemical vapor deposition combined with low melting point metals such as indium, tin and bismuth. The role of the plasma process and the liquid metal on the growth of the nanowires will be discussed in detail. Indeed, being the template for nanowires growth, controlling the distribution metal droplets (and therefore also nanowires) is very important to maximize the absorption and thus the short-circuit current of the cells. For this reason, a detailed parametric study has been carried out to optimize the density and size distribution of the catalyst droplets by changing plasma conditions. It has been found that the plasma conditions as well as the initial amount of Sn thermally evaporated on ZnO:Al coated Corning glass, have a significant impact on the density of the catalyst droplets and hence the nanowires fabricated on top of it. By optimizing those parameters the density of the nanowires has been varied in the range of 0.8*10 8 /cm 2 to 5.5*10 8 /cm 2. This optimization has allowed us to achieve a-si:h based radial junction solar cell on top of Sn catalyzed SiNWs with a V oc of 0.8 V, a J sc of 16.5 ma/cm 2, and a FF of 63%, resulting in a record power conversion efficiency of 8.2% with a remarkable stability, owing to extremely thin intrinsic a-si:h layer (<100 nm). These results will be discussed as well as perspectives they open for higher efficiency radial junction solar cells.

5 Gas and Particle Monitoring for Emission and Environment Control Anita Lloyd Spetz Linköping University, Sweden and University of Oulu, Finland Environmental hazards originate both from toxic gases and particulate matter. A new generation of SiC-FET sensors provide improved sensitivity, selectivity and long term stability and have been developed for environmental and emission control. Different sensing layers and choice of operation (cyclic) temperature, enables the SiC-FET sensors to monitor and control toxic emissions like CO, NO/NO 2, SO 2, H 2 S, VOCs and NH 3. A sensor system for control of small and medium sized power plants was commercialized ( Also particulate matter may pose danger to health depending on e.g. size and content. We develop smart packaging for detection of particles for example heating and detection of emitted gases can be used to identify content of the particles. Also electrical measurement of health status of cells during exposure to particles is developed as a method to investigate toxic effect of particles.