REVIEW OF SCIENTIFIC INSTRUMENTS 85, 043705 (2014)
Combining low-energy electron microscopy and scanning probe microscopy techniques for surface science: Development of a novel sample-holder F. Cheynis, F. Leroy, A. Ranguis, B. Detailleur,a) P. Bindzi, C. Veit,b) W. Bon, and P. Müller Aix-Marseille Université, CNRS, CINaM UMR 7325, 13288 Marseille, France
(Received 13 February 2014; accepted 3 April 2014; published online 22 April 2014) We introduce an experimental facility dedicated to surface science that combines Low-Energy Electron Microscopy/Photo-Electron Emission Microscopy (LEEM/PEEM) and variable-temperature Scanning Probe Microscopy techniques. A technical challenge has been to design a sample-holder that allows to exploit the complementary specifications of both microscopes and to preserve their optimal functionality. Experimental demonstration is reported by characterizing under ultrahigh vacuum with both techniques: Au(111) surface reconstruction and a two-layer thick graphene on 6HSiC(0001). A set of macros to analyze LEEM/PEEM data extends the capabilities of the setup. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4871437] I. INTRODUCTION
The recent interest for 2D materials (e.g., graphene) has brought forward a clear need for complementary surface-sensitive microscopies such as Low-Energy Electron Microscopy/Photo-Electron Emission Microscopy (LEEM/PEEM) and Scanning Probe Microscopy (SPM) techniques.1–3 LEEM/PEEM allows to characterize in real time a crystalline surface at a mesoscopic lateral scale (2–150 μm). SPM gives access to a lateral and vertical quantitative topography at a lateral scale of a few 10 nm and for an acquisition time of typically 10-15 min. To the best of our knowledge only three setups, combining under ultra-high vacuum (UHV), LEEM/PEEM and SPM exist around the world: I06 beamline at Diamond Light Source, UK,4 University of California, Davis, USA,5 and University of Montreal, Canada.6 Combining two microscopy techniques developed independently by two different manufacturers requires a specific design of the system. In this paper, a new UHV system dedicated to surface science is introduced. Its different components, including LEEM/PEEM and a variable-temperature SPM (vt-SPM) are described in Sec. II. Our innovative approach has been to design a sample-holder as a combination of the two original sample-holders so that no modifications of the microscopes are needed, preserving their optimal functionality. Demonstration of the capabilities of the system is made in Sec. III by characterizing, in a complementary approach, a same sample surface with the different available microscopy techniques. Two surfaces are characterized: Au(111) surface reconstruction and a two-layer thick graphene on 6H-SiC(0001). In Sec. IV, a freely available set of macros to analyze LEEM/PEEM data is introduced and extends the capabilities of the experimental setup. These macros allow for file handling (data browsing, conversion) and image processa) Present address: Aix-Marseille Université, CNRS, IBDML UMR 6216,
13288 Marseille, France.
b) Present address: IC2MP, UMR 7285-CNRS, Université de Poitiers, 86022
Poitiers Cedex, France. 0034-6748/2014/85(4)/043705/5/$30.00
ing (background removal, drift correction, region-of-interest intensity monitoring). Reviews of the different microscopy techniques are provided for further readings. II. EXPERIMENTAL FACILITY A. Setup overview
The complete UHV system installed at CINaM is illustrated in Fig. 1. It is composed of three main chambers including a LEEM/PEEM (label (a) in Fig. 1), a vt-SPM (label (b)), and a sample surface preparation chamber (label (c)). Up to four sample-holders can be stored in a dedicated UHV chamber (label (d)). The samples can be outgassed by radiative heating up to typically 850 K with a furnace (label (e)). The samples can be introduced under UHV conditions using one of the two loadlocks located on a straight axis towards either the LEEM/PEEM or the vt-SPM techniques (labels (f) and (g), resp.). B. LEEM
The electron microscope (LEEM III) was designed and manufactured by Elmitec Elektronenmikroskopie GmbH (Elmitec) based on the pioneering work of Bauer and Telieps.7 This non-scanning technique allows to study in real time (acquisition time in the range 0.1-10 s) and in situ surface dynamics at a mesoscopic scale (field-of-view in the range 2-150 μm). The LEEM III model shown in Fig. 1 is a combination of two techniques: LEEM and PEEM. The latter follows a photons-in (provided here by a short-arc UV Hg-lamp)/electrons-out scheme and the former an electronsin/electrons-out scheme. In the setup at CINaM, incoming electrons are generated by a hot-cathode Field Emission Gun made of a W(100) single-crystal covered by ZrO. The electron beam is adjusted in the illumination column using magnetic lenses. In both techniques, outcoming electrons are collected by the imaging column composed of magnetic lenses. Laterally averaged structural information is also available by
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D. Sample preparation and additional chambers
FIG. 1. 3D overview of the UHV system developed at CINaM and dedicated to surface science. (a) Low-Energy Electron Microscope/Photo-Electron Emission Microscopy (LEEM/PEEM) chamber, (b) variable-temperature Scanning Probe Microscopy (vt-SPM) chamber, (c) sample preparation chamber, (d) connection and storage chamber, (e) radiative furnace, (f) and (g) independent loadlocks.
imaging diffraction plane as in a standard LEED technique. A review of these techniques can be found in Ref. 8. In LEEM mode (PEEM mode, resp.), a standard lateral resolution below 10 nm (∼30 nm, resp.) is achieved. In most cases, direct morphological information cannot be obtained except, in LEEM mode, for vicinal surfaces where atomic steps can be visualized and for ultrathin films (few ML) where quantum-based contrast can occur. The samples can be radiatively heated up to typically 850 K and flashed using electron bombardment up to 1900– 2000 K. The sample stage can also be cooled down to 150 K using liquid nitrogen. Sample temperature is monitored with a WRe5-WRe26 thermocouple spot-welded on the sampleholder and close to the rear side of the sample. The LEEM chamber is equipped with a Knudsen evaporation source and an additional electron beam evaporator is also available. The entire LEEM chamber has a base pressure below 2 × 10−10 Torr. C. Variable-temperature scanning probe microscopy
The vt-SPM developed by Omicron NanoTechnology GmbH (Omicron) is a VT AFM XA 650 model. This equipment is a combination of a Scanning Tunneling Microscope (STM) and an Atomic Force Microscope (AFM) using optical deflection detection. These microscopy techniques allow to image at a nanoscale the sample surface with fieldof-views in the range of a few nanometers to a few micrometers. For reduced field-of-views, lateral atomic resolution can be achieved. Quantitative out-of-plane information is also available. Reviews and perspectives of SPM techniques can be found in Ref. 9. Standard image acquisition time is of ∼10 min. In the microscope, the sample can be imaged between room temperature and up to 650-700 K. Radiative heating is monitored using a Ni–CrNi thermocouple integrated to the sample stage. SPM chamber base pressure is in the 5 × 10−10 Torr range. The STM imaging mode has been used to demonstrate the operativity of the instrument (Sec. III). Standard chemically etched W tips available from Omicron have been used after preparation on the sample surface using voltage pulses and/or indentation.
The UHV system developed at CINaM is equipped with a surface preparation chamber. Samples can be outgassed and/or flashed up to 1900–2000 K. Annealing under partial gas pressure (e.g., O2 , H2 , Ar) can be achieved using three independent gas-lines, each connected to a micro-leak valve. Sample surface can be cleaned by Ar ion sputtering (ISE 5, Omicron NanoTechnology GmbH). Thin films are deposited using a Knudsen evaporation cell. An additional electron beam evaporator is also available. The monitoring of the film thickness is possible using a quartz crystal microbalance. An Auger-Electron Spectrometer (single-pass spectrometer outfitted with a cylindrical mirror analyzer, model ESA 100, STAIB Instrumente GmbH) allows to determine the sample surface chemical composition and possible contaminations. Residual gas can also be analyzed by a quadrupole mass spectrometer (Prisma 80, Pfeiffer Vacuum) from 1 to 80 amu. To connect the LEEM/PEEM chamber and the preparation/SPM chamber axis, an autonomously pumped chamber is used. The functionality of this chamber is twofold: it is used to connect the aforementioned techniques and to store up to four samples on a carousel under UHV conditions (base pressure 3 × 10−10 Torr). For samples requiring only LEEM/PEEM characterizations, radiative degassing up to typically 850 K can be achieved in a furnace located in-line between the LEEM loadlock and the LEEM chamber. E. Custom-designed holders
In the setup described in Ref. 5, the authors chose to work with a standard ELMITEC sample-holder and to modify the STM sample stage. Our approach is based on a Elmitec sample-holder base acting as a receptacle for a Omicron sample plate. The reason for this approach is to preserve the microscope functionality. This places stringent requirements on the sample-holder design. For instance, in situ imaging of surface dynamics in LEEM/PEEM needs heating and temperature measurements. Additionally, for SPM measurements, the sample-holder must fit to an existing highly engineered vibration-isolation stage to achieve optimized performance. Practically [Figs. 2(a), 2(c), and 2(e)], the sample is fixed on a Omicron SPM plate using spot-welded Ta wires or a spotwelded windowed Ta foil. This plate is in turn mounted on a modified Elmitec cartridge base made of Ti including a receptacle for the Omicron sample plate. The modifications include on each side a lateral slide made of Mo bridge-designed support and Ta covering foil tighten together with Mo screws. For this LEEM/SPM compatible sample-holder, no Mo cap is used contrary to standard Elmitec sample-holders. The use of smoothly bent Ta wires or of a windowed Ta foil to fix the sample maintains electrostatic arcs sufficiently rare when approaching a sample to its 2 mm-working distance under a 20 kV voltage. The sample temperature on this sample-holder is measured using a pyrometer (IGAQ 10-LO, Impac). Further improvements may include the use of a thermocouple junction spot-welded on one of the Mo bridge-designed supports. Sample transfer to the different chambers of the setup is realized using rods equipped with a rotating transfer tip
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FIG. 2. (a), (c), and (e) 3D views and picture of the custom-designed sampleholder developed at CINaM allowing to produce state-of-the-art pictures of the same sample using both LEEM/PEEM and SPM techniques. (b), (d), and (f) 3D views and picture of the custom-designed SPM tip-holder developed at CINaM to insert into UHV Omicron SPM probes using Elmitec transfer rods. Using this tip-holder STM probes can be prepared by flash heating with DC current and/or Ar-ion bombardment.
commercially available at Elmitec. The base of the Elmitec sample-holder remaining unchanged, the sample can still be heated radiatively or using electron bombardment. To transfer the sample to the vt-SPM stage, a wobblestick is used to remove/insert the sample plate from the Elmitec base sampleholder. Following the same principle of unmodified Elmitec sample-holder base, a dedicated holder for Omicron SPM probes has been designed [Figs. 2(b), 2(d), and 2(f)] to allow for loading under UHV and for in situ W tip preparation by DC current heating ( 1000 K) to remove W oxides resulting from the chemical etching.10–12 III. SYSTEM CAPABILITIES: Au(111) SURFACE AND GRAPHENE ON 6H-SiC(0001)
To demonstrate the functionality of the setup and the complementarity of the microscopy techniques, we present results obtained using both LEEM/PEEM and STM on two sample surfaces: Au(111) substrate and graphene on 6HSiC(0001). A. Au(111) surface and herringbone reconstruction
The Au(111) sample is a 5N 9 mm-circular 1 mm-thick single crystal provided by Mateck GmbH with a polishing residual roughness below 0.03 μm and an orientation accuracy better than 0.1◦ . It is mounted on a Omicron Mo plate using two spot-welded Ta wires. The as-delivered crystal has been prepared by typically 20–30 repeated cycles of Ar-ion bombardment (PAr = 4-5 × 10−7 Torr, VAr = 1 kV, ISample
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FIG. 3. (a)-(c) LEEM, (d) LEED, √ and√(e) and (f) STM characterization of a Au(111) surface showing 22 3 × 22 3 (herringbone) surface reconstruction. The field-of-view (FOV) is (a) 50 μm, (b) 25 μm, (c) 5 μm. The electron energy is (a) 15 eV, (b) and (c) 15.6 eV. In (a) and (b), the dashed white line is a guide-to-the-eye for the mean orientation of step-bunches. In (c), wide dark lines are step-bunches. (d) LEED pattern of √ the Au(111) surface with addi√ tional diffracted spots from the 22 3 × 22 3 reconstruction. The dashed circle √ is a zoom√in of a (1,0) spot of the Au(111) LEED pattern to highlight the 22 3 × 22 3 additional spots. The electron energy is 27.1 eV. (e) STM image of the Au(111) surface (Vtip = −0.7 V, Itun. = 0.4 nA, z = 0.8 nm). (f) [Same scanned area as in (e)] represents the remainder of the local height divided by the Au(111) interlayer spacing and highlights the herringbone reconstruction (z = 0.24 nm).
= 8–9 μA for 45-60 min)/Annealing (∼500 ◦ C for 20– 30 min). At the end of this process, the Au(111) exhibits a very high crystalline quality as demonstrated by LEEM [Figs. 3(a)–3(c)], LEED [Fig. 3(d)], and STM [Figs. 3(e)3(f)]. In particular, LEED and √ STM techniques clearly evi√ dence the occurrence of the 22 3 × 22 3 (also called herringbone) surface reconstruction encountered in the case of a clean Au(111) surface.13–15 In a standard LEEM, the details of the herringbone reconstruction cannot be observed. This illustrates the benefit of a complementary approach based on LEEM/PEEM and SPM for surface science studies requiring real-time acquisition and atomic resolution. Recent LEEM developments of aberration correction improve the microscope lateral resolution and allows to identify the herringbone reconstruction16 without bridging the gap towards atomic resolution. At large lateral scales [Figs. 3(a) and 3(b)], the vicinality of the crystal is evidenced as step-bunches tend to have a common direction. Defects are also visible as dark and bright spots but with a low areal density (∼10−2 μm−2 ). Bright spots are thought to be remaining Au hillocks formed during ion bombardment. For a FOV of 5 μm, monoatomic steps are observable as later confirmed by STM images. Fig. 3(e) evidences a step of a height of 2.4 ± 0.1 Å close to the Au(111) interlayer spacing of 2.35 Å. Fig. 3(f) [same scanned area as in Fig. 3(e)] represents the remainder of the
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local height divided by the Au(111) interlayer spacing17 to highlight the herringbone reconstruction. Geometrical dimensions of the reconstruction are in agreement with previous studies from the literature (longitudinal periodicity ∼30 nm, kink-to-kink lateral distance 6.6 nm, and height variation ∼0.2 Å).14, 17 STM image shown in Fig. 3(e) has been flattened and processed to obtain Fig. 3(f) using WSxM.18 B. Graphene on 6H-SiC(0001)
The graphene sample has been grown at CRHEA (CNRS, Valbonne, France) by propane chemical vapor deposition on a 6H-SiC(0001) substrate (Tanke Blue).19–21 The sample has been exposed for 5 min at a temperature of 1450 ◦ C to a gas pressure of 800 mbar obtained from laminar flows of propane (5 sccm), argon (104 sccm), and hydrogen (3 × 103 sscm). During temperature ramp-up and cooling down, the sample has remained under hydrogen and argon. Prior to the LEEM/STM characterizations shown in Fig. 4, the graphene sample has been degassed for several hours at ∼600 ◦ C under UHV and flashed for few minutes at >1200 ◦ C. A clear contrast is evidenced in Figs. 4(a) and 4(b) between two types of domains appearing as bright and dark areas. Topographic details such as vicinality, terrace width and steps are also visible. It is now well-established that LEEM enables to determine the local graphene thickness using electron reflectivity curves interpreted in terms of electron wave interferences resulting from the graphenelayer-associated quantum well.22 The number of minima in the reflectivity curves in the range of electron energies 0-6 eV determines the number of local graphene layers [Fig. 4(c)]. This sample is mainly made of a 1 ML-thick graphene and the second ML has already started to grow. Structural information available from the LEED pattern [Fig. 4(d)] evidences a R30◦ – structure of the graphene with respect to the SiC substrate. STM images of the graphene surface are shown in Figs. 4(e) and 4(f). A total step height of 2.6 ± 0.2 Å is visible between the lowest and the highest terraces in Fig. 4(e) and match the standard SiC bilayer thickness. An intermediate terrace of limited extension decorates locally the main bilayer step. The inset in Fig. 4(e) shows the height profile along the black line and an interpretation of the local layer stacking matching the measured height steps. From the small scale STM image of Fig. 4(f) and its corresponding FFT shown as an inset, a mean in-plane distance of 2.4 ± 0.2 Å can be determined which is the expected value of graphene lateral lattice parameter.23 IV. A MACRO COLLECTION FOR LEEM DATA ANALYSIS
A collection of macros have been developed using the environment of Igor Pro commercial software (Wavemetrics, Inc.24 ) to extend the LEEM data analysis tools provided by Elmitec acquisition software Uview2000. The current implementations of the LEEM/PEEM acquisition software analysis tools usually require that the users develop their own advanced image processing macros. Despite the fact that the reported macros are based on widely spread tech-
FIG. 4. (a) LEEM image of the graphene on 6H-SiC(0001) (FOV = 10 μm and the electron energy is 20 eV). (b) Zoom in of LEEM image (a) exhibiting two distinctive domain contrasts. A slight step contrast is also visible. (c) Electron reflectivity curves of the two labelled area in (b) allowing to determine the local graphene thickness. (d) LEED pattern of the imaged area showing the R30◦ – structure of the graphene with respect to the SiC substrate. The electron energy is 34 eV. (e) and (f) STM images of the graphene surface. (e) 80 × 80 nm2 (Vtip = 4.1 V, Itun. = 0.8 nA, z = 0.9 nm). The inset shows the height profile along the black line and an interpretation of the local layer stacking matching the measured height steps. (f) 5 × 5 nm2 image on the lowest terrace shown in (e) (Vtip = 1.7 V, Itun. = 5.0 nA, z = 0.2 nm). The inset is the 2D FFT of the image allowing to determine graphene in-plane atomic distances.
niques, sharing these tools might be of interest for the community. The overview of the graphical user interface (GUI) is shown in Fig. 5. This set of macros is freely available to the LEEM/PEEM user community25 considering that the authors will be pleased that the current report be cited. The GUI is divided into three parts. The most outfitted component concerns Elmitec movie films (.dav) or image sequences (.dat sequence). These files are opened with a dedicated button. Browsing of the raw data is then possible. The contrast adjustment panel developed by Wavemetrics can be used. Upon browsing, each picture is loaded and contrast adjusted. Standard file handling procedures are implemented such as Extract (e.g., to select certain images of the original data and create a new movie) and Convert (to generate a .mov or a .avi movie for presentations). Image processing includes polynomial background removal (Backgrnd) using Igor Pro built-in procedure and drift correction (Drift. Corr.). For the latter, two approaches are implemented. The first is based on an automated image-to-image intercorrelation. The second approach requires that the user samples different
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the graphene sample and S. Curiotto (CINaM) for fruitful discussions. The authors also would like to thank T. Leoni, C. Becker, and L. Masson (CINaM) for their help in getting started with the STM. This work has been funded by ANR PNano grant DéFIS (ANR 08-Nano-036), ANR grant LOTUS (ANR-13-BS04-0004), PACA region grants (Démouillage, Nanosurf), Aix Marseille Université via FEDER and CIM PACA frameworks and internal grants (Nanoélasticité, Couplage LEEM/STM). 1 H.
FIG. 5. Overview of the graphical user interface (GUI) developed for the analysis of LEEM (Elmitec) data in the environment provided by Igor Pro commercially available at Wavemetrics, Inc.24
reference positions for a given detail on the surface under consideration. The drift is then linearly interpolated between these positions and a new movie is generated. A particle analysis procedure (Part. Anal.) is also available. An arbitrary number of particles (i.e., black, resp. white, areas on a white, resp. black, background) are characterized (area and x, y position) as a function of the movie frame number. Manual and Iterated built-in thresholding methods are implemented to detect automatically the current position of a given particle. Finally, a procedure to measure the intensity of a given region-of-interest along the image sequence is implemented (IROI ). This is particularly useful for reflectivity curves. The second and third components of the GUI enable to display in Igor Pro .mov movies and various types of individual images obtained from Elmitec acquisition software (.dat) or in standard formats (.tiff, .png, and .jpeg). V. CONCLUSION
We have successfully connected under UHV conditions two complementary surface microscopies without any modification of the original design. A sample-holder has been specifically developed allowing to characterize the same sample surface with both techniques and a set of macros to analyses LEEM/PEEM raw data have been implemented. The imaging capabilities of the setup have been demonstrated. ACKNOWLEDGMENTS
The authors acknowledge A. Michon and M. Portail (CRHEA, CNRS, Valbonne, France) for the fabrication of
Hibino, H. Kageshima, and M. Nagase, J. Phys. D: Appl. Phys. 43, 374005 (2010). 2 Y. Cui, Q. Fu, and X. Bao, Phys. Chem. Chem. Phys. 12, 5053–5057 (2010). 3 J. M. Wofford, E. Starodub, A. L. Walter, S. Nie, A. Bostwick, N. C. Bartelt, K. Thürmer, E. Rotenberg, K. F. McCarty, and O. D. Dubon, New J. Phys. 14(5), 053008 (2012). 4 S. S. Dhesi, S. A. Cavill, A. Potenza, H. Marchetto, R. A. Mott, P. Steadman, A. Peach, E. L. Shepherd, X. Ren, U. H. Wagner, and R. Reininger, AIP Conf. Proc. 1234, 311 (2010). 5 C. L. H. Devlin, D. N. Futuba, A. Loui, J. D. Shine, and S. Chiang, Mater. Sci. Eng. B 96, 215–220 (2002). 6 See https://wiki.umontreal.ca/display/brano/STM-LEEM for an overview of the experimental setup available at the University of Montreal, Canada; online accessed 8 October 2013. 7 W. Telieps and E. Bauer, Ultramicroscopy 17(1), 57–65 (1985). 8 E. Bauer, Rep. Prog. Phys. 57, 895 (1994). 9 Adv. Funct. Mater. 23(20), 2461–2660 (2013), guest edited by Sergei Kalinin. 10 A.-D. Müller, F. Müller, M. Hietschold, F. Demming, J. Jersch, and K. Dickmann, Rev. Sci. Instrum. 70, 3970 (1999). 11 A.-S. Lucier, H. Mortensen, Y. Sun, and P. Grütter, Phys. Rev. B 72, 235420 (2005). 12 T. Hagedorn, M. E. Ouali, W. Paul, D. Oliver, Y. Miyahara, and P. Grütter, Rev. Sci. Instrum. 82, 113903 (2011). 13 U. Harten, A. M. Lahee, J. P. Toennies, and C. Wöll, Phys. Rev. Lett. 54, 2619 (1985). 14 C. Wöll, S. Chiang, R. J. Wilson, and P. H. Lippel, Phys. Rev. B 39, 7988 (1989). 15 J. V. Barth, H. Brune, G. Ertl, and R. J. Behm, Phys. Rev. B 42, 9307 (1990). 16 T. Schmidt, H. Marchetto, P. Lvesque, U. Groh, F. Maier, D. Preikszas, P. Hartel, R. Spehr, G. Lilienkamp, W. Engel, R. Fink, E. Bauer, H. Rose, E. Umbach, and H.-J. Freund, Ultramicroscopy 110, 1358–1361 (2010). 17 V. Repain, J. M. Berroir, S. Rousset, and J. Lecoeur, Europhys. Lett. 47(4), 435 (1999). 18 I. Horcas, R. Fernández, J. M. Gómez-Rodríguez, J. Colchero, J. GómezHerrero, and A. M. Baro, Rev. Sci. Instrum. 78(1), 013705 (2007). 19 A. Michon, S. Vézian, A. Ouerghi, M. Zielinski, T. Chassagne, and M. Portail, Appl. Phys. Lett. 97, 171909 (2010). 20 A. Michon, S. Vézian, E. Roudon, D. Lefebvre, M. Zielinski, T. Chassagne, and M. Portail, J. Appl. Phys. 113, 203501 (2013). 21 A. Michon, L. Largeau, A. Tiberj, J.-R. Huntzinger, O. Mauguin, S. Vézian, D. Lefebvre, F. Cheynis, F. Leroy, P. Müller, T. Chassagne, M. Zielinski, and M. Portail, Mater. Sci. Forum 740–742, 117 (2013). 22 H. Hibino, H. Kageshima, F. Maeda, M. Nagase, Y. Kobayashi, and H. Yamaguchi, Phys. Rev. B 77, 075413 (2008). 23 P. Mallet, F. Varchon, C. Naud, L. Magaud, C. Berger, and J.-Y. Veuillen, Phys. Rev. B 76, 041403 (2007). 24 I. Wavemetrics, see http://www.wavemetrics.com/ for details on IGOR Pro software; online accessed 8 October 2013. 25 F. Cheynis, see www.cinam.univ-mrs.fr/cheynis/leem-igor to download the set of macros to analyze LEEM/PEEM data acquired using Elmitec acquisition software Uview2000.
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Combining low-energy electron microscopy and scanning probe microscopy techniques for surface science: Development of a novel sample-holder F. Cheynis, F. Leroy, A. Ranguis, B. Detailleur,a) P. Bindzi, C. Veit,b) W. Bon, and P. Müller Aix-Marseille Université, CNRS, CINaM UMR 7325, 13288 Marseille, France
(Received 13 February 2014; accepted 3 April 2014; published online 22 April 2014) We introduce an experimental facility dedicated to surface science that combines Low-Energy Electron Microscopy/Photo-Electron Emission Microscopy (LEEM/PEEM) and variable-temperature Scanning Probe Microscopy techniques. A technical challenge has been to design a sample-holder that allows to exploit the complementary specifications of both microscopes and to preserve their optimal functionality. Experimental demonstration is reported by characterizing under ultrahigh vacuum with both techniques: Au(111) surface reconstruction and a two-layer thick graphene on 6HSiC(0001). A set of macros to analyze LEEM/PEEM data extends the capabilities of the setup. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4871437] I. INTRODUCTION
The recent interest for 2D materials (e.g., graphene) has brought forward a clear need for complementary surface-sensitive microscopies such as Low-Energy Electron Microscopy/Photo-Electron Emission Microscopy (LEEM/PEEM) and Scanning Probe Microscopy (SPM) techniques.1–3 LEEM/PEEM allows to characterize in real time a crystalline surface at a mesoscopic lateral scale (2–150 μm). SPM gives access to a lateral and vertical quantitative topography at a lateral scale of a few 10 nm and for an acquisition time of typically 10-15 min. To the best of our knowledge only three setups, combining under ultra-high vacuum (UHV), LEEM/PEEM and SPM exist around the world: I06 beamline at Diamond Light Source, UK,4 University of California, Davis, USA,5 and University of Montreal, Canada.6 Combining two microscopy techniques developed independently by two different manufacturers requires a specific design of the system. In this paper, a new UHV system dedicated to surface science is introduced. Its different components, including LEEM/PEEM and a variable-temperature SPM (vt-SPM) are described in Sec. II. Our innovative approach has been to design a sample-holder as a combination of the two original sample-holders so that no modifications of the microscopes are needed, preserving their optimal functionality. Demonstration of the capabilities of the system is made in Sec. III by characterizing, in a complementary approach, a same sample surface with the different available microscopy techniques. Two surfaces are characterized: Au(111) surface reconstruction and a two-layer thick graphene on 6H-SiC(0001). In Sec. IV, a freely available set of macros to analyze LEEM/PEEM data is introduced and extends the capabilities of the experimental setup. These macros allow for file handling (data browsing, conversion) and image processa) Present address: Aix-Marseille Université, CNRS, IBDML UMR 6216,
13288 Marseille, France.
b) Present address: IC2MP, UMR 7285-CNRS, Université de Poitiers, 86022
Poitiers Cedex, France. 0034-6748/2014/85(4)/043705/5/$30.00
ing (background removal, drift correction, region-of-interest intensity monitoring). Reviews of the different microscopy techniques are provided for further readings. II. EXPERIMENTAL FACILITY A. Setup overview
The complete UHV system installed at CINaM is illustrated in Fig. 1. It is composed of three main chambers including a LEEM/PEEM (label (a) in Fig. 1), a vt-SPM (label (b)), and a sample surface preparation chamber (label (c)). Up to four sample-holders can be stored in a dedicated UHV chamber (label (d)). The samples can be outgassed by radiative heating up to typically 850 K with a furnace (label (e)). The samples can be introduced under UHV conditions using one of the two loadlocks located on a straight axis towards either the LEEM/PEEM or the vt-SPM techniques (labels (f) and (g), resp.). B. LEEM
The electron microscope (LEEM III) was designed and manufactured by Elmitec Elektronenmikroskopie GmbH (Elmitec) based on the pioneering work of Bauer and Telieps.7 This non-scanning technique allows to study in real time (acquisition time in the range 0.1-10 s) and in situ surface dynamics at a mesoscopic scale (field-of-view in the range 2-150 μm). The LEEM III model shown in Fig. 1 is a combination of two techniques: LEEM and PEEM. The latter follows a photons-in (provided here by a short-arc UV Hg-lamp)/electrons-out scheme and the former an electronsin/electrons-out scheme. In the setup at CINaM, incoming electrons are generated by a hot-cathode Field Emission Gun made of a W(100) single-crystal covered by ZrO. The electron beam is adjusted in the illumination column using magnetic lenses. In both techniques, outcoming electrons are collected by the imaging column composed of magnetic lenses. Laterally averaged structural information is also available by
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D. Sample preparation and additional chambers
FIG. 1. 3D overview of the UHV system developed at CINaM and dedicated to surface science. (a) Low-Energy Electron Microscope/Photo-Electron Emission Microscopy (LEEM/PEEM) chamber, (b) variable-temperature Scanning Probe Microscopy (vt-SPM) chamber, (c) sample preparation chamber, (d) connection and storage chamber, (e) radiative furnace, (f) and (g) independent loadlocks.
imaging diffraction plane as in a standard LEED technique. A review of these techniques can be found in Ref. 8. In LEEM mode (PEEM mode, resp.), a standard lateral resolution below 10 nm (∼30 nm, resp.) is achieved. In most cases, direct morphological information cannot be obtained except, in LEEM mode, for vicinal surfaces where atomic steps can be visualized and for ultrathin films (few ML) where quantum-based contrast can occur. The samples can be radiatively heated up to typically 850 K and flashed using electron bombardment up to 1900– 2000 K. The sample stage can also be cooled down to 150 K using liquid nitrogen. Sample temperature is monitored with a WRe5-WRe26 thermocouple spot-welded on the sampleholder and close to the rear side of the sample. The LEEM chamber is equipped with a Knudsen evaporation source and an additional electron beam evaporator is also available. The entire LEEM chamber has a base pressure below 2 × 10−10 Torr. C. Variable-temperature scanning probe microscopy
The vt-SPM developed by Omicron NanoTechnology GmbH (Omicron) is a VT AFM XA 650 model. This equipment is a combination of a Scanning Tunneling Microscope (STM) and an Atomic Force Microscope (AFM) using optical deflection detection. These microscopy techniques allow to image at a nanoscale the sample surface with fieldof-views in the range of a few nanometers to a few micrometers. For reduced field-of-views, lateral atomic resolution can be achieved. Quantitative out-of-plane information is also available. Reviews and perspectives of SPM techniques can be found in Ref. 9. Standard image acquisition time is of ∼10 min. In the microscope, the sample can be imaged between room temperature and up to 650-700 K. Radiative heating is monitored using a Ni–CrNi thermocouple integrated to the sample stage. SPM chamber base pressure is in the 5 × 10−10 Torr range. The STM imaging mode has been used to demonstrate the operativity of the instrument (Sec. III). Standard chemically etched W tips available from Omicron have been used after preparation on the sample surface using voltage pulses and/or indentation.
The UHV system developed at CINaM is equipped with a surface preparation chamber. Samples can be outgassed and/or flashed up to 1900–2000 K. Annealing under partial gas pressure (e.g., O2 , H2 , Ar) can be achieved using three independent gas-lines, each connected to a micro-leak valve. Sample surface can be cleaned by Ar ion sputtering (ISE 5, Omicron NanoTechnology GmbH). Thin films are deposited using a Knudsen evaporation cell. An additional electron beam evaporator is also available. The monitoring of the film thickness is possible using a quartz crystal microbalance. An Auger-Electron Spectrometer (single-pass spectrometer outfitted with a cylindrical mirror analyzer, model ESA 100, STAIB Instrumente GmbH) allows to determine the sample surface chemical composition and possible contaminations. Residual gas can also be analyzed by a quadrupole mass spectrometer (Prisma 80, Pfeiffer Vacuum) from 1 to 80 amu. To connect the LEEM/PEEM chamber and the preparation/SPM chamber axis, an autonomously pumped chamber is used. The functionality of this chamber is twofold: it is used to connect the aforementioned techniques and to store up to four samples on a carousel under UHV conditions (base pressure 3 × 10−10 Torr). For samples requiring only LEEM/PEEM characterizations, radiative degassing up to typically 850 K can be achieved in a furnace located in-line between the LEEM loadlock and the LEEM chamber. E. Custom-designed holders
In the setup described in Ref. 5, the authors chose to work with a standard ELMITEC sample-holder and to modify the STM sample stage. Our approach is based on a Elmitec sample-holder base acting as a receptacle for a Omicron sample plate. The reason for this approach is to preserve the microscope functionality. This places stringent requirements on the sample-holder design. For instance, in situ imaging of surface dynamics in LEEM/PEEM needs heating and temperature measurements. Additionally, for SPM measurements, the sample-holder must fit to an existing highly engineered vibration-isolation stage to achieve optimized performance. Practically [Figs. 2(a), 2(c), and 2(e)], the sample is fixed on a Omicron SPM plate using spot-welded Ta wires or a spotwelded windowed Ta foil. This plate is in turn mounted on a modified Elmitec cartridge base made of Ti including a receptacle for the Omicron sample plate. The modifications include on each side a lateral slide made of Mo bridge-designed support and Ta covering foil tighten together with Mo screws. For this LEEM/SPM compatible sample-holder, no Mo cap is used contrary to standard Elmitec sample-holders. The use of smoothly bent Ta wires or of a windowed Ta foil to fix the sample maintains electrostatic arcs sufficiently rare when approaching a sample to its 2 mm-working distance under a 20 kV voltage. The sample temperature on this sample-holder is measured using a pyrometer (IGAQ 10-LO, Impac). Further improvements may include the use of a thermocouple junction spot-welded on one of the Mo bridge-designed supports. Sample transfer to the different chambers of the setup is realized using rods equipped with a rotating transfer tip
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FIG. 2. (a), (c), and (e) 3D views and picture of the custom-designed sampleholder developed at CINaM allowing to produce state-of-the-art pictures of the same sample using both LEEM/PEEM and SPM techniques. (b), (d), and (f) 3D views and picture of the custom-designed SPM tip-holder developed at CINaM to insert into UHV Omicron SPM probes using Elmitec transfer rods. Using this tip-holder STM probes can be prepared by flash heating with DC current and/or Ar-ion bombardment.
commercially available at Elmitec. The base of the Elmitec sample-holder remaining unchanged, the sample can still be heated radiatively or using electron bombardment. To transfer the sample to the vt-SPM stage, a wobblestick is used to remove/insert the sample plate from the Elmitec base sampleholder. Following the same principle of unmodified Elmitec sample-holder base, a dedicated holder for Omicron SPM probes has been designed [Figs. 2(b), 2(d), and 2(f)] to allow for loading under UHV and for in situ W tip preparation by DC current heating ( 1000 K) to remove W oxides resulting from the chemical etching.10–12 III. SYSTEM CAPABILITIES: Au(111) SURFACE AND GRAPHENE ON 6H-SiC(0001)
To demonstrate the functionality of the setup and the complementarity of the microscopy techniques, we present results obtained using both LEEM/PEEM and STM on two sample surfaces: Au(111) substrate and graphene on 6HSiC(0001). A. Au(111) surface and herringbone reconstruction
The Au(111) sample is a 5N 9 mm-circular 1 mm-thick single crystal provided by Mateck GmbH with a polishing residual roughness below 0.03 μm and an orientation accuracy better than 0.1◦ . It is mounted on a Omicron Mo plate using two spot-welded Ta wires. The as-delivered crystal has been prepared by typically 20–30 repeated cycles of Ar-ion bombardment (PAr = 4-5 × 10−7 Torr, VAr = 1 kV, ISample
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FIG. 3. (a)-(c) LEEM, (d) LEED, √ and√(e) and (f) STM characterization of a Au(111) surface showing 22 3 × 22 3 (herringbone) surface reconstruction. The field-of-view (FOV) is (a) 50 μm, (b) 25 μm, (c) 5 μm. The electron energy is (a) 15 eV, (b) and (c) 15.6 eV. In (a) and (b), the dashed white line is a guide-to-the-eye for the mean orientation of step-bunches. In (c), wide dark lines are step-bunches. (d) LEED pattern of √ the Au(111) surface with addi√ tional diffracted spots from the 22 3 × 22 3 reconstruction. The dashed circle √ is a zoom√in of a (1,0) spot of the Au(111) LEED pattern to highlight the 22 3 × 22 3 additional spots. The electron energy is 27.1 eV. (e) STM image of the Au(111) surface (Vtip = −0.7 V, Itun. = 0.4 nA, z = 0.8 nm). (f) [Same scanned area as in (e)] represents the remainder of the local height divided by the Au(111) interlayer spacing and highlights the herringbone reconstruction (z = 0.24 nm).
= 8–9 μA for 45-60 min)/Annealing (∼500 ◦ C for 20– 30 min). At the end of this process, the Au(111) exhibits a very high crystalline quality as demonstrated by LEEM [Figs. 3(a)–3(c)], LEED [Fig. 3(d)], and STM [Figs. 3(e)3(f)]. In particular, LEED and √ STM techniques clearly evi√ dence the occurrence of the 22 3 × 22 3 (also called herringbone) surface reconstruction encountered in the case of a clean Au(111) surface.13–15 In a standard LEEM, the details of the herringbone reconstruction cannot be observed. This illustrates the benefit of a complementary approach based on LEEM/PEEM and SPM for surface science studies requiring real-time acquisition and atomic resolution. Recent LEEM developments of aberration correction improve the microscope lateral resolution and allows to identify the herringbone reconstruction16 without bridging the gap towards atomic resolution. At large lateral scales [Figs. 3(a) and 3(b)], the vicinality of the crystal is evidenced as step-bunches tend to have a common direction. Defects are also visible as dark and bright spots but with a low areal density (∼10−2 μm−2 ). Bright spots are thought to be remaining Au hillocks formed during ion bombardment. For a FOV of 5 μm, monoatomic steps are observable as later confirmed by STM images. Fig. 3(e) evidences a step of a height of 2.4 ± 0.1 Å close to the Au(111) interlayer spacing of 2.35 Å. Fig. 3(f) [same scanned area as in Fig. 3(e)] represents the remainder of the
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local height divided by the Au(111) interlayer spacing17 to highlight the herringbone reconstruction. Geometrical dimensions of the reconstruction are in agreement with previous studies from the literature (longitudinal periodicity ∼30 nm, kink-to-kink lateral distance 6.6 nm, and height variation ∼0.2 Å).14, 17 STM image shown in Fig. 3(e) has been flattened and processed to obtain Fig. 3(f) using WSxM.18 B. Graphene on 6H-SiC(0001)
The graphene sample has been grown at CRHEA (CNRS, Valbonne, France) by propane chemical vapor deposition on a 6H-SiC(0001) substrate (Tanke Blue).19–21 The sample has been exposed for 5 min at a temperature of 1450 ◦ C to a gas pressure of 800 mbar obtained from laminar flows of propane (5 sccm), argon (104 sccm), and hydrogen (3 × 103 sscm). During temperature ramp-up and cooling down, the sample has remained under hydrogen and argon. Prior to the LEEM/STM characterizations shown in Fig. 4, the graphene sample has been degassed for several hours at ∼600 ◦ C under UHV and flashed for few minutes at >1200 ◦ C. A clear contrast is evidenced in Figs. 4(a) and 4(b) between two types of domains appearing as bright and dark areas. Topographic details such as vicinality, terrace width and steps are also visible. It is now well-established that LEEM enables to determine the local graphene thickness using electron reflectivity curves interpreted in terms of electron wave interferences resulting from the graphenelayer-associated quantum well.22 The number of minima in the reflectivity curves in the range of electron energies 0-6 eV determines the number of local graphene layers [Fig. 4(c)]. This sample is mainly made of a 1 ML-thick graphene and the second ML has already started to grow. Structural information available from the LEED pattern [Fig. 4(d)] evidences a R30◦ – structure of the graphene with respect to the SiC substrate. STM images of the graphene surface are shown in Figs. 4(e) and 4(f). A total step height of 2.6 ± 0.2 Å is visible between the lowest and the highest terraces in Fig. 4(e) and match the standard SiC bilayer thickness. An intermediate terrace of limited extension decorates locally the main bilayer step. The inset in Fig. 4(e) shows the height profile along the black line and an interpretation of the local layer stacking matching the measured height steps. From the small scale STM image of Fig. 4(f) and its corresponding FFT shown as an inset, a mean in-plane distance of 2.4 ± 0.2 Å can be determined which is the expected value of graphene lateral lattice parameter.23 IV. A MACRO COLLECTION FOR LEEM DATA ANALYSIS
A collection of macros have been developed using the environment of Igor Pro commercial software (Wavemetrics, Inc.24 ) to extend the LEEM data analysis tools provided by Elmitec acquisition software Uview2000. The current implementations of the LEEM/PEEM acquisition software analysis tools usually require that the users develop their own advanced image processing macros. Despite the fact that the reported macros are based on widely spread tech-
FIG. 4. (a) LEEM image of the graphene on 6H-SiC(0001) (FOV = 10 μm and the electron energy is 20 eV). (b) Zoom in of LEEM image (a) exhibiting two distinctive domain contrasts. A slight step contrast is also visible. (c) Electron reflectivity curves of the two labelled area in (b) allowing to determine the local graphene thickness. (d) LEED pattern of the imaged area showing the R30◦ – structure of the graphene with respect to the SiC substrate. The electron energy is 34 eV. (e) and (f) STM images of the graphene surface. (e) 80 × 80 nm2 (Vtip = 4.1 V, Itun. = 0.8 nA, z = 0.9 nm). The inset shows the height profile along the black line and an interpretation of the local layer stacking matching the measured height steps. (f) 5 × 5 nm2 image on the lowest terrace shown in (e) (Vtip = 1.7 V, Itun. = 5.0 nA, z = 0.2 nm). The inset is the 2D FFT of the image allowing to determine graphene in-plane atomic distances.
niques, sharing these tools might be of interest for the community. The overview of the graphical user interface (GUI) is shown in Fig. 5. This set of macros is freely available to the LEEM/PEEM user community25 considering that the authors will be pleased that the current report be cited. The GUI is divided into three parts. The most outfitted component concerns Elmitec movie films (.dav) or image sequences (.dat sequence). These files are opened with a dedicated button. Browsing of the raw data is then possible. The contrast adjustment panel developed by Wavemetrics can be used. Upon browsing, each picture is loaded and contrast adjusted. Standard file handling procedures are implemented such as Extract (e.g., to select certain images of the original data and create a new movie) and Convert (to generate a .mov or a .avi movie for presentations). Image processing includes polynomial background removal (Backgrnd) using Igor Pro built-in procedure and drift correction (Drift. Corr.). For the latter, two approaches are implemented. The first is based on an automated image-to-image intercorrelation. The second approach requires that the user samples different
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the graphene sample and S. Curiotto (CINaM) for fruitful discussions. The authors also would like to thank T. Leoni, C. Becker, and L. Masson (CINaM) for their help in getting started with the STM. This work has been funded by ANR PNano grant DéFIS (ANR 08-Nano-036), ANR grant LOTUS (ANR-13-BS04-0004), PACA region grants (Démouillage, Nanosurf), Aix Marseille Université via FEDER and CIM PACA frameworks and internal grants (Nanoélasticité, Couplage LEEM/STM). 1 H.
FIG. 5. Overview of the graphical user interface (GUI) developed for the analysis of LEEM (Elmitec) data in the environment provided by Igor Pro commercially available at Wavemetrics, Inc.24
reference positions for a given detail on the surface under consideration. The drift is then linearly interpolated between these positions and a new movie is generated. A particle analysis procedure (Part. Anal.) is also available. An arbitrary number of particles (i.e., black, resp. white, areas on a white, resp. black, background) are characterized (area and x, y position) as a function of the movie frame number. Manual and Iterated built-in thresholding methods are implemented to detect automatically the current position of a given particle. Finally, a procedure to measure the intensity of a given region-of-interest along the image sequence is implemented (IROI ). This is particularly useful for reflectivity curves. The second and third components of the GUI enable to display in Igor Pro .mov movies and various types of individual images obtained from Elmitec acquisition software (.dat) or in standard formats (.tiff, .png, and .jpeg). V. CONCLUSION
We have successfully connected under UHV conditions two complementary surface microscopies without any modification of the original design. A sample-holder has been specifically developed allowing to characterize the same sample surface with both techniques and a set of macros to analyses LEEM/PEEM raw data have been implemented. The imaging capabilities of the setup have been demonstrated. ACKNOWLEDGMENTS
The authors acknowledge A. Michon and M. Portail (CRHEA, CNRS, Valbonne, France) for the fabrication of
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