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19 th International Symposium on Space Terahertz Technology P11-2 Epitaxial ultra-thin NbN films grown on sapphire dedicated for superconducting mixers B. Guillet 1 , R. Espiau de Lamaestre 2 , P. Odier 3 , M.P. Chauvat 4 , P. Ruterana 4 , L. Méchin 1 , J.C. Villégier 5 1 GREYC, CNRS, ENSICAEN & Université Caen Basse Normandie, 6 Bd Maréchal Juin, 14050 Caen, France 2 CEA-LETI, MINATEC, 17 Avenue des Martyrs, 38054 Grenoble Cedex 9, France 3 Institut Néel, CNRS-Grenoble, 38042 Grenoble, France 4 SIFCOM, CNRS, ENSICAEN & Université Caen Basse Normandie, 6 Bd Maréchal Juin, 14050 Caen, France 5 CEA-DRFMC, MINATEC, Superconducting Device Group, 17 Avenue des Martyrs, 38054 Grenoble Cedex 9, France Sputtered niobium nitride (NbN) films have been considered as a good candidate for rapid single flux quantum electronics based on Josephson Junctions, for THz mixers based on Hot Electron Bolometric effect or for single photon detection applications (SSPD). Applications would benefit from higher quality films (epitaxial growth) with low concentration of defects, such as grain boundaries or twins, whose nature and concentration depend on the deposition conditions and the substrate. Efforts devoted to grow NbN aim at improving their critical temperature and critical current density, while keeping their thickness in the 3 to 5 nm range and Tc above 10 K, which insure a large bandwidth and large SNR detection at 4K. Choice of substrate is critical: for applications, MgO wafers and R-plane sapphire are usually considered as best choice. However, growing NbN on either M-plane or A-plane orientations of sapphire wafers, 3 inch in diameter, can help improving the film quality and fabrication yield. NbN thin films were grown by reactive DC magnetron sputtering at about 600°C and passivated by an AlN layer 1.5 nm thick deposited in-situ at room temperature. Growth on M-plane is shown to be better than on other sapphire orientations, including R-plane. NbN layer critical temperature reaches 13.3 K. Their properties are uniform on the 3 inch wafer, for a film thickness of 4.4 nm measured by X-ray reflectivity. We also obtained promising results on NbN growth on silicon wafers by using either an epitaxial YSZ/CeO 2 buffer layer grown ex-situ by PLD or a thin NbMgO buffer sputtered in-situ. Transport properties of NbN grown on those various substrates have been correlated to their crystallographic microstructure, examined by both symmetric and asymmetric X ray diffraction, high resolution transmission electron microscopy (HRTEM), spectroscopical ellipsometry, atomic force microscopy (AFM). These results will be presented in the framework of HEB and SSPD applications. Epitaxial multilayers NbN/MgO/NbN on M-plane sapphire have been also studied. Applications of these tunnel junctions as superconductor-insulator-superconductor (SIS) mixers have been considered. 152
19 th International Symposium on Space Terahertz Technology P11-3 Terahertz emission from ZnSe nano-dot surface Shan He a,b , Xiaoshu Chen a , Xiaojun Wu a , Fuli Zhao a , Reng Wang c , Weizheng Fang c , Shuhong Hu c , Ning Dai c and *Gang Wang a a State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-Sen University, Guangzhou 510275, P. R. China b School of Chemistry and Chemical Engineering,, Sun Yat-Sen University, Guangzhou 510275, P. R. China c National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, P. R. China, 200083 *email: stswangg@mail.sysu.edu.cn ABSTRACT As shown in Figure 1 (Fig. 1) ZnSe nano dots are fabricated on the surface of orientation ZnSe using femtosecond laser ablation technique. In this work, we studied the THz generation properties of the ZnSe nano-dots by electro-optic detection configuration. Three THz radiation mechanisms are observed in experiments: current surge effect (drift current), Photo Dember effect (diffusion current) and optical rectification. Compared with bulk ZnSe, the ZnSe nano dots generated much higher THz radiation power at the same experimental condition. When the ZnSe nano-dots covered 10% of total radiation surface, the radiation power is about two times stronger than that from the bulk bare sample (Fig. 2). Basicly, there are two kinds of mechanisms occur predominantly for the THz radiation of ZnSe nano dots: the first is lighting-rod effect that fields tend to concentrate at the tips of protrusions on surface; the second is local-plasmon effect that collective oscillation of electrons occurs in these protrusions. These two effects make the total surface electric field largely enhanced. In this study, we attribute the THz radiation enhancement phenomenon of ZnSe nano dots to the surface field enhancement effect. Using a simple hemispheriod model, we also obtained the enhancement factor of ZnSe nano dots. Keywords: THz radiation, surface field enhancement, surface nano-dot, ZnSe. Fig. 1. SEM micrograph of ZnSe nano-dots. Fig. 2. THz fluence as a function of pump fluence for nano-dot and bulk ZnSe samples. 153
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