We prepare NbN thin films by DC magnetron sputtering on [100] GaAs substrates, optimise their quality and demonstrate their use for efficient single photon detection in the near-infrared. The interrelation between the Nb:N content, growth temperature and crystal quality is established for 4−22nm thick films. Optimised films exhibit a superconducting critical temperature of 12.6±0.2K for a film thickness of 22 ± 0.5nm and 10.2 ± 0.2K for 4 ± 0.5nm thick films that are suitable for single photon detection. The optimum growth temperature is shown to be ∼ 475 • C reflecting a trade-off between enhanced surface diffusion, which improves the crystal quality, and arsenic evaporation from the GaAs substrate. Analysis of the elemental composition of the films provides strong evidence that the δ-phase of NbN is formed in optimised samples, controlled primarily via the nitrogen partial pressure during growth. By patterning optimum 4nm and 22nm thick films into a 100nm wide, 369µm long nanowire meander using electron beam lithography and reactive ion etching, we fabricated single photon detectors on GaAs substrates. Time-resolved studies of the photo-response, absolute detection efficiency and dark count rates of these detectors as a function of the bias current reveal maximum single photon detection efficiencies as high as 21 ± 2% at 4.3 ± 0.1K with ∼ 50k dark counts per second for bias currents of 98%I C at a wavelength of 950nm. As expected, similar detectors fabricated from 22nm thick films exhibit much lower efficiencies (0.004%) with very low dark count rates ≤ 3cps. The maximum lateral extension of a photo-generated hotspot is estimated to be 30±8nm, clearly identifying the low detection efficiency and dark count rate of the thick film detectors as arising from hotspot cooling via the heat reservoir provided by the NbN film. PACS numbers: 74.78.-w 74.25.Gz 78.67.Uh 85.25.Oj 85.25.-j 42.50.-p
Das Au lbri ngen du nner Harts toffkchic h ten i s t be i tri bolog isc h stark beanspruchten Werkzcugen, wie L. R. bei Dreh-tmd Umfornwerkzeugen, Stand der Technik. StandLeiterhdhungen der beschichteten Werkreugc in1 Vcrgleich 711 unbeschichteten Referen-Len sind vielfach hestiitigt [ I ) . In diese ni Z tisani me nhan g i 5 t in v ielen Fii I len .,Funk t i oils t rcnn ti ng ", d. h. eine Trennung der Aufgiiben von Werkstof'fvolumcn und Werkstoffoberllachc. cin Lentrales Schlagwort. Dabei wii-d davon ausgegangen, daf.3 tribologische und korrosive Beanspruchungen nur auf Oberfliichen wirken und darum nur d i e s mit einer geeigneten Schichl /.u schiitzen sind. Die Wahl der Stibstrntwerkstoffe oder eine Optiinierung des Schicht-Substrat-Vcrbundes stand oft nicht im Mittelpunkt der PVD -Schichtentwicklung. Betrachtungen zum Schicht-Substrat-Verbund sind jedoch unumgiinglich, wenn die Eigenschaften vtfn Schicht und Substrat stark von einander abweichen (z. B. Hnrtstolf'e auf ,.weichen" Leichtmetallen im Gegensatz zu Hartstoften auf hochfestcn Stahlsubstraten). Der Beitrag greift wescntliche Aspekte zur inechanisch-tribologischen und elektrochemischen Bewertung hartstoffbeschichteter Leichtmetalle am Beispiel PVD-CrN aul'.The application of' thin hard coatings on machining tools, e.g. drilling tools i s state of the art. The increase of lifetime of coated tools compared to uncoated tools i s well known. 111. In this context ,,separation of functions" i s an often used phrase, by meaning the separation of functions of the volume and the surface of materials. Going one step forward, from the point of view of tribology or corrosion this means,, you have ,,only" to protect the surface by using ;I ,,good" coating without looking at the material underneath. In the past the influence of substrate materials or the optimization of the system (substrate-coating) was not the main aim of PVD development. Looking at substrate and coating as a system is especially necessary, if the differences of the properties of substrate and coating are large (c.6. hard coatinglight metals). This paper shows different aspects of the tribological and electrochemical characterimtion oT PVD coated light metals (CrN coating).
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