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Cathodoluminescence System


Figure 2 : (a) Photograph of FEI Quanta 250 FEG SEM with SPARC CL optics box attached (Chalmers, Sweden). (b) Inside the chamber, a piezoelectric mirror positioning system is mounted, which is used to position a parabolic mirror in four dimensions ( x , y , pitch, and yaw). (c) Photograph taken from the bottom of the mirror. (d) Schematic overview of the setup showing the different detection schemes: a spectrometer for 2D CL imaging spectroscopy, a PMT for fast wavelength fi ltered or panchromatic imaging, and a CCD imaging detector for angle-resolved measurements. (e) Graphical representation of angle-resolved detection of CL on a 2D CCD array (image by Tremani). Photographs in (b, c) by Henk-Jan Boluijt.


in frequency and momentum, allowing the electron to couple to a wide variety of optical modes [ 4 ]. Additionally, the excitation probability is related to the local density of optical states (LDOS), a key quantity in optics [ 5 ]. Although this type of CL generation is always present, the excitation probabilities are relatively low (10 -4 photons per electron), requiring a sensitive detection system and careful sample design. At typical scanning electron microscopy (SEM) currents the sample is bombarded by ~10 10 electrons per second leading to an emission of 10 6 photons per second of coherent CL signal, which is enough for spectroscopy purposes. Hence, such a probe is very useful for studying nanophotonics below the diff raction limit for light.


Materials and Methods T e SPARC system is a sensitive, modular CL system that is built as a plug-in for a scanning electron microscope (SEM). It was developed at the FOM Institute AMOLF (Amsterdam, T e Netherlands), and the technology was then transferred to DELMIC at the end of 2011. Figure 2a shows a photograph of a system installed on a FEI Quanta 250 FEG SEM (Chalmers University of Technology). The system has a motorized light collection system mounted in the SEM chamber ( Figure 2b ), in which the micropositioning of a high-quality aluminum paraboloid mirror is performed by piezomotors connected to a titanium leaf-spring system. T e mirror can be moved laterally ( x , y ) and can be rotated in two directions (tilt, yaw) (see Figure 2c ).


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The alignment in the vertical direction can be done with the SEM stage. Such micropositioning of the paraboloid is necessary to obtain a well-aligned system in which the CL beam coming from the mirror is parallel, ensuring optimal light collection in the optics box outside of the SEM ( Figure 2d ). Furthermore, the mirror has a large acceptance angle (1.46π sr) to maximize light collection. T e combination of these two features gives the system a high sensitivity and allows detection of faint signals such as the coherent forms of CL. T e parallel CL beam can be directed to a variety of detectors: Czerny-Turner spectrographs with charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) detectors for hyperspectral analysis and photomultiplier tubes for wavelength filtered imaging and/or panchromatic (unfi ltered) fast imaging applica- tions. T e detectors can be optimized for the wavelength range of interest (ultraviolet, visible, near infrared).


In addition to the spectral imaging in the more conven- tional CL imaging mode, the SPARC platform can also perform angle-resolved imaging [ 6 , 7 ]. In angle-resolved mode the CL beam is projected onto a 2D CCD array allowing angle-resolved detection of the emission ( Figure 2e ), that is, the emission intensity as function of azimuthal angle φ and zenithal angle θ . By using the proper coordinate transformation, the data collected on the CCD can be converted to units of emitted power per steradian. To obtain spectral selectivity in this


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