Morphology and plasmonic properties of self-organized arrays of ...

40 CHAPTER 2. EXPERIMENTAL METHODSturbomolecularpumpsample holderAu crucible60° 60°LiF effusion cellFigure 2.1: Sketch of the experimental chamber (front view). The LiF effusion cell andthe Au crucible are visible. The sample holder can rotate and translate in the plane ofthe figure for the fine positioning during the MBE depositions.The deposition of insulating or metallic films is performed by means of the molecularbeam epitaxy (MBE) technique, which allows the formation of high quality crystallinefilms and nanostructures.The LiF source is composed by a boron nitride (h-BN) crucible, in which LiF lumpmaterial to be deposited is loaded. h-BN is a ceramic material with the same layeredstructure of graphite, from which it is sometimes called white graphite; it has a lowerelectric conductance with respect to graphite, but its thermal and chemical stability aremuch superior. The crucible is supported by a W filament, that also serves to its heatingbyJouleeffect. Itreachesatemperatureof≈ 700 ◦ C,sufficienttopromotethesublimationof the lump LiF crystals (the melting point of LiF is at 845 ◦ C). A screen of tantalum isused as collimator for the molecular beam and to prevent the chamber contamination andselectively expose the sample to the beam. The LiF source can achieve deposition ratesup to at least 6 nm/min, at base pressure in the 10 −7 mbar range. For this thesis, typicalrates of 1 nm/min were chosen, as explained in more detail in the next chapter. Thethickness of the deposited LiF layer was calibrated by ex-situ optical characterization.The Au source requires instead a different configuration, because of the higher Aumelting point (1068 ◦ C) and relatively low vapour pressure. In this case short sectionsof pure gold wires (99.99%, MaTecK) are stored in a crucible of molybdenum, which isthen heated by electron bombardment applying an high voltage with respect to a hotW filament. The crucible is supported by an alumina rod, which grants a good thermalinsulation, and an additional copper screen is used both as collimator and thermal shield.The Au evaporator allowed to achieve evaporation rates in the 0.4÷0.7 nm/min range,applying an overall power of ≈ 40 W to the Mo crucible. Typical pressures during growthwere in the 10 −7 mbar range, that, though high, were perfectly acceptable due to the lowreactivity of the involved material.Although the characterization of the samples is typically performed mostly ex-situ,it is possible to perform reflectivity measurements by means of an optical setup. The