LEED. Instruments for Low Energy Electron Diffraction and AES. SPECTALEED LEED / Auger SpectaView MCP-LEED MBD-LEED SPA-LEED

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1 LEED Instruments for Low Energy Electron Diffraction and AES SPECTALEED LEED / Auger SpectaView MCP-LEED MBD-LEED SPA-LEED

2 Versatility for Structural and Chemical Analysis 15 years experience in the production and development of LEED systems enables OMICRON to offer a variety of LEED instruments and supporting software with both excellent performance and reliability. SPEC CTALEED Control Unit PC SPECTALEED The SPECTALEED family ranges from basic instruments for crystal preparation with optional AES capability, to advanced applications. These include investigation of insulators or sensitive samples using beam currents down to 0.1 na, dedicated Spot Profile Analysis (SPA-LEED), I/V measurements, real-time Video LEED and MBD LEED. Most commonly, LEED is used to determine the surface structure after its preparation during a multitechnique experiment involving other surface analysis methods. Structural characteristics of the sample, for instance terrace width, step height and facets may be investigated by the analysis of spot profiles and I/V curves. In addition, dynamic processes such as the structural phase transitions which take place during adsorption, can also be monitored and analysed in real-time using the SpectaView Video-LEED package. Surface sensitive chemical analysis is made available through the use of Auger The SPECTALEED Control Unit offers LEED and AES with PC controlled data acquisition in one single compact unit with integrated Lock-in amplifier. Electron Spectroscopy (A AES). In this case, the SPECTAL LEED is employed as a Retarding Field Analyser (RFA) in conjunction with the DATAuger software. SPECTALEED delivers exceptionally sharp and bright LEED patterns with high resolution in k-space (typical transfer r width of 300 Å at 100 ev), for easy observation even under conditions of high ambient light. Non-magnetic gold plated molybdenum grids provide a high transmission (82 %) and 2

3 a uniform work function. The wide viewing angle (102 ) and minimal shadowing of the screen by the miniature electron gun give a maximum visible LEED pattern. The integral miniature electron source operates down to 5 ev, and is designed for minimised light emission - a very important feature for LEED I/V analysis. The standard thoriated tungsten filament allows the operation at high background pressures and in aggressive atmospheres, e.g. oxygen. An optionally available LaB 6 filament yields a perfectly circular spot with high beam current and extremely low stray light, due to the low emission temperature. High brightness and perfect geometry LEED patterns are achieved with the SPECTALEED even down to very low energies. W(100) at E = ev C 60 at E = 7.9 ev The SPECTALEED Control Unit uses active HV modules for all elements of the electron gun to produce either a parallel beam ( ev) for LEED or a focused beam (250 µm spot size at 3 kev) for AES. Beam current and energy can be monitored without biasing the sample. Lens settings for a fixed focus range from ev beam energy are supplied as standard. The SPECTALEED Control Unit is used for all LEED instruments. 23

4 The MCP-LEED uses a fringe-field correction plate to produce a perfect LEED pattern on the plane channelplate. MBD-LEED with 3 pre-aligned, integrated miniature evaporators. MCP-LEED The highly sensitive channelplate version of the SPECTALEED reverse view LEED optics is specially designed for operation using low primary beam currents. Such low beam currents may be demanded in applications where the beam current levels experienced in standard LEED applications (typically in the microampere range) may possibly lead to damage of delicate samples or cause rapid charging of insulator surfaces. With the MCP-LEED, the low beam current in the nanoampere range enables the structural study of inherently sensitive or problematic samples, such as insulators or single crystals with weakly bound adsorbates, without complications caused by deterioration due to beam-induced effects. The primary beam current may be further reduced, down to the picoampere range, by means of a second channelplate. NaCl(100) with 5nA beam current 1) Al 2 O 3 (0001) at 232 ev, beam current of 30 na. MBD-LEED Organic molecules (DCNQI) on Ag(110) during deposition in UHV at 340 K (electron energy: 14.6 ev). Spots produced by the restructuring of molecules into a denser packing are marked yellow 2). The Molecular-Beam-Deposition-LEED is a new instrument for the in-situ observation of thin film growth during the evaporation process. It combines 3 miniature Knudsen cells with the SPECTALEED for simultaneous and independent evaporation of up to three different materials. The Knudsen cells are integrated into the LEED optics so as to cause minimum reduction of the LEED viewing area and no disturbance of the LEED pattern. Low light emission is maintained even at the maximum evaporation temperature of approx. 700 C. The quartz glass crucibles are prealigned onto the sample and are easily exchanged and refilled. The design geometry prevents contamination of the LEED grids by the evaporant. - Ultra low beam current na - Single or double channelplate - Viewing angle 70 - High stability/low noise channelplate control unit - Temperature range C - Crucible capacity 15 mm 3 (max. 50 mm 3 ) - Evaporator control unit with up to 3 independent T-controllers, programmable via RS ) Courtesy of: J. Vogt and H. Weiss, Univ. of Magdeburg 2) Courtesy of: C. Seidel, H. Kopf and H. Fuchs, Univ. of Muenster

5 Let's Talk About Software SpectaView SpectaView is a user-friendly Video-LEED system for high-speed imaging and I(V)- and I(t)-measurements, consisting of software (Windows NT), frame grabber and low light CCD camera. The Video-LEED image is displayed in real-time. The high rate of data acquisition required for the study of dynamic processes is achieved by data storage rates in excess of 8 frames per second. Data analysis may be carried out either directly, or from the saved images. Up to 18 I(V) curves can be measured simultaneously in either automatic, template or manual spot-tracking modes. The enhanced image presentation includes online brightness and contrast control. DATAuger Software The DATAuger software makes full use of the multitasking ability of Windows NT. Easy to use routines for AES in analogue mode allow experiments consisting of up to 10 sequential scans with independent setting of all experimental parameters. Operating values of the oscillator and Lock-in amplifier may be set individually for each scan. In addition to the normal data acquisition mode, Fast Scan enables rapid selection and setting of optimum sample position and focus. Peak Mode displays a bar graph and the signal value at a fixed energy setting. Data processing routines include 3 and 5 point smoothing, Savitsky Golay smoothing, together with differentiation, integration, background subtraction and addition or subtraction of scans. CCD Fully software controlled AES data acquisition with DATAuger. The spectrum of silver was acquired with the SPEC- TALEED using a 3.5 kev/ 30 µa primary beam and a grid modulation of 5 V pp (resolution 0.5%). Frame grabber and DAC/ADC card allow full PC control of the LEED control unit and fast acquisition of I(V), I(t) and image data. - I(V) and I(t)-measurements - Automatic/manual/template spot tracking - 25 fps real-time display (8 fps stored) - PCI Frame grabber, 512 x 512 pixel, 256 grey levels - Low light CCD (CCIR, analogue out, nm) - 12 bit DAC/ADC for energy/beam current (PCI-board) - Active cross-talk compensation for high sensitivity - Fully computer controlled spectra acquisition - Regulated high-stability filament supply - Easy adjustment with Fast Scan and Peak modes - Control unit with integrated Lock-in/oscillator - PCI interface board (Windows NT) 25

6 Latest version of the SPA-LEED, redesigned in close collaboration with the University of Hannover. SPA-LEED achieves excellent resolution and high dynamic range by scanning the diffraction pattern relative to the detector by means of a patented octopole detector. SPA-LEED Conventional LEED is most commonly used to judge surface perfection, identify adsorbateinduced superstructures and determine the surface structure as derived from I/V curves. Spot Profile Analysis (SPA-LEED) was pioneered by Prof. Henzler and his group at the University of Hannover. The SPA-LEED is manufactured under exclusive licence by OMICRON. SPA-LEED: Au-induced faceting of Si(001)3). SPA-LEED pattern measured during homoepitaxial growth of Si on Si(111) in H2 at: p = 5 x 10-7 mbar, quantitative analysis of lateral and vertical lattice constants, terrace/island size and height distributions, ordering parameters in phase transitions, and diffusion barriers at step edges. SPA-LEED can be applied in-situ during epitaxial growth processes, chemical reactions, surface diffusion, ordering transitions, reconstruction, etc. Observation of intensity modulations and spot profile allows precise character-isation of growth processes. T = 520 C, E = 75.6 ev.4) Spot Profile Analysis is a dedicated, high performance LEED application which exploits detailed structural information contained in the width and shape of the LEED spots, to show any deviation from the ideal lattice. SPA-LEED is designed to give maximum resolution in k-space, thus extending the detection limit to a typical average terrace width of about 50 nm and a mosaic angle of down to Applications of SPA-LEED include the classification and analysis of point defects, facets, terraces, islands, steps, horizontal and vertical roughness, thermal or static disorder, domains, and phase transitions. The technique enables precise 26 With SPA-LEED the conventional LEED pattern is only visible on a part of the LEED screen, since a section of the screen area is occupied by an electron multiplier positioned behind a 100 µm aperture. Heart of the SPA-LEED is an octopole which is used to scan the diffracted electrons relative to the SPA-LEED detector. In this way individual LEED spots can be quantitatively analysed with excellent sensitivity and signal-to-noise ratio. The dynamic range is 100 times higher than in conventional LEED, and resolution in k-space, determined by the transfer width of > 100 nm, is significantly improved. Thus very low concentrations of defects can be detected and the nature of their structure can be investigated. 3) Courtesy of: F. J. Meyer zu Heringdorf, H. Minoda and M. H. v. Hoegen, Univ. of Hannover 4) Courtesy of: M. H. v. Hoegen and A. Golla, Univ. of Hannover

7 Si/Si(111) (0,0) spot Intensity ring" which identifies a regular distribution of islands on the surface. Coverage [Bilayers] Spot intensity oscillations and spot profile measure-ments during epitaxial growth processes give precise measures of the growing coverage and the growth characteristics, e.g. increasing roughness. The SPA-LEED is operated by the LEED control electronics in combination with the SPA-LEED Control Unit. The SPA-LEED data acquisition and evaluation software features an easy-to- handle setup routine. Rate meter and oscilloscope functions are displayed on the monitor, thus enabling interactive optimisation of peak shape, peak intensity, scan range and scan area. The auto-scan menu allows individual measurement routines to be defined and run consecutively. Electron beam energy, beam current and frequency signal from a quartz micro balance can be recorded for the control of layer growth. The software features many useful data analysis routines including calibration of the deflection voltage in Percentage of Brillouin-Zone, linear or logarithmic count rate display with scaling to the primary current or user defined rescaling. Zoom, smoothing, line scans, coloured 3D view, contour plots, comparison facility for two or more scans, evaluation of integral and peak intensities and FWHM of individual spots are also available. Operation is assisted by a comprehensive manual plus an electronic Help menu. SPA-LEED analysis of a diffraction spot of 5 ML Si on H-terminated Si(111), revealing a Henzler ring which identifies a regular distribution of islands on the surface. - Transfer width > 1000 Å at 95.4 ev - Beam current range 500 pa - 10 µa, LaB 6 filament - Angular resolution better than Deflection Scan ± 30, viewing angle ± 11 - Channeltron detector for > 8 x 10 6 cps - 16 bit DACs for deflection, and 32 bit counter - Fixed focus range ev - Fully software controlled (2 ISA PC slots, DOS) - Control unit for octopol deflector, channeltron supply 27

8 How to contact us Headquarters: Limburger Str. 75 D Taunusstein Germany Tel. +49 (0) / Fax +49 (0) / or Taiwan: Omega Scientific Taiwan Ltd Tel. (02) Fax (02) UK: Omicron Surface Science Ltd Tel Fax France: Omicron Eurl Tel Fax Italy:, Roma Tel. (06) Fax (06) Switzerland: Tel Fax Sweden: Advanced Vacuum AB Tel Fax Headquarters USA: Omicron NanoTechnology USA Tel. (952) Fax (952) USA (East): Omicron NanoTechnology, E. Regional Office Tel. (724) Fax (724) USA (West): Omicron NanoTechnology, W. Regional Office Tel. (303) Fax (303) Japan: ULVAC-PHI, Inc. Tel Fax South Korea: Woosin Cryovac Ltd Tel. (031) Fax (031) Singapore: Research Instruments Pte Ltd Tel Fax India: Mack International Tel. (022) Fax (022) China: Omicron China Office Tel. (010) Fax (010) Australia: Thomson Scientific Instruments Pty Ltd Tel. (03) Fax (03) BRAZIL: BOC DO BRASIL LTDA Tel. (011) Fax (011) Specifications SPECTALEED MBD-LEED MCP-LEED Mgax. beam current x x x > 10 µa for WTh, > 30 µa for LaB6 Transfer width x x x > 250 Å at E = 100 ev Fixed focus range x x x ev Viewing angle x x 102 x (70 for MCP-LEED) AES resolution x x better than 0.5 % (FWHM) Filament types x x x Thoriated tungsten; LaB6 crystal Number of grids x x x 3 (AES with MCP-LEED) x x 4 (AES with SPECTALEED) Channelplates x single or double channelplate Magnetic shielding x x x mu-metal (stainless steel cylinder for mu-metal chambers) Bakeout temperature x x x 220 C Standard length (mm)* x x x 203 (8 ) or 254 (10 ) flange-sample Mounting flange type x x x NW 150 CF (8 OD), tapped holes Optional lin. retraction x x x 45 mm (up to 140 mm on request) Note: SPA-LEED Specifications and technical details described inside * Special lengths on request SPECTALEED, MCP-LEED, MBD-LEED SPA-LEED (MCP and MBD only) Rotary drive for linear retraction 120 a= Optics to sample distance 3 grids: d=26 mm 4 grids: d=22mm mu-metal shield a Ø Ø or NW 150 CF 8" OD Mounting Flange (tapped holes) View from vacuum side All dimensions in [mm] Power Supply NG LEED Beam energy range ev Retard energy range Filament supply Monitor outputs (0-10 V) Remote control (0-10 V) AES (optional) ev (for AES) emission regulated (switchable) beam current (10 na µa) / beam energy (full range) beam energy / retard energy Integrated Lock-in with oscillator 212 Screen H.V. 54 Octopoles 36 Gun & Grid CP CC 36 CR Ø Sample 203 View from vacuum side All dimensions in [mm] Technical alterations reserved 301-V06/10.02 Printed by Druckerei und Verlag Klaus Koch GmbH