Surface electron microscopy with low energy electrons

LEEM, SPLEEM, SPELEEM, PEEM etc

Surfaces and thin films have been among the most important subjects in science and technology during the last few decades. Considerable understanding has been gained in this period by laterally averaging measurement techniques, but simultaneously it has become increasingly evident that many problems can be solved only by laterally resolving methods (surface microsopy). Because of this Ernst Bauer's main efforts in the eighties/nineties were directed at the further development of cathode lens electron microscopy into multi-method surface electron microscopy with slow electrons.

The starting point was Low Energy Electron Microscopy (LEEM), which he invented in 1962. It came to fruition in 1985. One of the great advantages of this method is the rapid image acquisition that allows the study of surface processes such as adsorption, phase transitions, chemical reactions, thin film growth etc. with 10 nm lateral resolution and atomic depth resolution.

Combination possibilities with other techniques make LEEM a unique surface imaging method. The combination of LEEM with LEED (Low Energy Electron Diffraction) is one of the advantages of LEEM over other cathode lens imaging methods such as PEEM. The possibilities of LEED are greatly enhanced in a LEEM instrument. In many cases a characteristic LEED pattern can be obtained from a small surface feature (one micron in diameter or less) and its crystal structure, orientation, shape and the composition can be identified by this selected area LEED. When compared to a standard LEED system, LEED in a LEEM instrument has the advantage that the positions of the LEED spots on a flat surface do not change with energy. This makes it easy to take, for example, I (V) curves and to identify facets. Other advantages are: the (00) beam intensity can be measured at normal incidence, the LEED pattern can be studied from 0 eV upwards, with proper illumination optics large transfer widths can be obtained and the imaging lenses allow large magnifications of the LEED pattern so that the instrument can also be used for SPALEED.

A special version of LEEM that uses spin-polarized electrons for illumination a Spin-Polarized Low Energy Electron Microscopy (SPLEEM) was developed in the late eighties and used successfully already for a decade. It became important in recent years in connection with the developments in magnetic thin films for sensor and information storage. SPLEEM is based on the fact that in ferromagnetic materials in addition to the structural contrast there is also a magnetic contrast proportional to P•M (P polarization of the electron beam, M magnetization of the sample). Therefore SPLEEM allows imaging of the magnetic domain structure and of the magnetization direction simultaneously with the crystal structure and topography of the films. It is not element-specific but allows a direct correlation between magnetic structure, microstructure and crystal structure via ordinary LEEM and LEED.

The instrument development of LEEM revived other surface imaging methods such as Mirror Electron Microscopy (MEM) and all forms of emission microscopy, foremost Photoelectron Microscopy (PEEM), which could be done now on surfaces in UHV or in well-defined gaseous environment. MEM is very useful for non-crystalline specimens and for imaging of magnetic and electric fields in front of the specimen. PEEM also does not require a crystalline specimen. It can be used to image not only the surface but also magnetic fields in front of it. For many applications laboratory light sources are sufficient. The intense light from free electron lasers opens up the field of low energy threshold PEEM. The increasing availability and brightness of synchrotron radiation has extended PEEM into the X-ray range up to 1000 eV (XPEEM). XPEEM allows chemical identification on the 10 nm scale. Circular and linear polarized synchrotron light allows element-selective magnetic imaging, which makes use of circular and linear magnetic dichroism and has successfully been done in a LEEM instrument. In this imaging method the photon-excited secondary electrons are used for imaging so that no energy filter is needed.      

These combination possibilities were used to extend the LEEM technique to Spectroscopic Photo Emission and Low Energy Electron Microscopy (SPELEEM). This instrument was developed in Ernst Bauer's group under the name "Spectroscopic LEEM" at the Technical University Clausthal in Germany in the early nineties by adding an imaging energy filter. Its power for spectromicroscopy and magnetic imaging was first demonstrated in the early 1990ies at the synchrotron radiation source BESSY 1 using linearly polarized light and circularly polarized light, respectively. From summer 1996 until summer 1999 the microscope has been operating in the synchrotron radiation light source ELETTRA. 2000 an improved SPELEEM was installed in a special beamline in ELLETRA called "Nanospectroscopy beamline". It demonstrated not only the usefulness of the energy filter in XPEEM but also of the combination of XPEEM with LEEM and LEED. The instrument can be used alternately for XPEEM, LEEM, LEED, MEM and other imaging modes, depending upon the particular problem studied. The combination of these imaging, diffraction and spectroscopy modes allows a comprehensive characterization of the specimen. This is of particular importance when the chemical identification of structural features is necessary for the understanding of a surface or thin film process. In addition, PED (photoelectron diffraction) and ARUPS (angle-resolved photoelectron spectroscopy) of small selected areas give local atomic configuration and band structure information, respectively.

The SPELEEM is unique multi-method instrument, which combines microscopy and spectroscopy with high spatial and energy resolution, respectively, with LEED and energy-selected photoelectron angular distribution from selected small areas. With one instrument it is now possible to determine the structure and the morphology by LEEM/LEED/PED and the local chemical composition and chemical state by XPEEM/XPS on the same area of the surface on the 10 nm-scale. This combination of imaging, diffraction and spectroscopy modes is presently possible only with this instrument. The spatial resolution allows the separation of particles with sizes smaller than 25 nm, and peak shifts of less than 0.2 eV can be measured.

The high brightness of third generation synchrotron radiation sources has opened the door to chemical and magnetic surface imaging with resolutions in the 10 nm range. Eight other synchrotron radiation sources: SLS in Switzerland, Spring-8 in Japan, Diamond in Great Britain, Maxlab in Sweden, ALBA in Spain, BNL in USA, SSRF in China, Synchrotron Thailand, are also equipped with SPELEEM instruments.

The success of the instrument developments in Bauer’s group in Technical University Clausthal has led to the commercial production of these instruments (Elmitec) and stimulated several other groups to develop similar instruments for surface imaging with low energy electrons, resulting in a variety of commercial instruments. The first version of the very successful IS-PEEM of FOCUS was built by a member of Bauer’s group based on a flange-on LEEM, which he had developed beforehand in his group. The original Staib Instruments PEEM was designed by a member of the LEEM discussion group in Clausthal. The predecessor of the SPECS LEEM was a system similar to Bauer’s original LEEM. Thus the Ernst Bauer’s group in Clausthal has become the cradle of modern surface electron microscopy with low energy electrons. Today there are hundreds of the various versions of these instruments in the world and are further developed, continuously broadening their application range, for example in imaging and spectroscopy in reciprocal space (k-space imaging and spectroscopy), in particular also spin-resolved.

To conclude: Surface electron microscopy using low energy electrons started with the invention of LEEM.

References:

General: Book: Ernst Bauer "Surface Microscopy with Low Energy Electrons", Springer, 2014.

History: E. Bauer: Surface electron microscopy: the first thirty years, Surf. Sci. 299/300 (1994) 102-115.

LEEM:

E. Bauer: Low Energy Electron Microscopy, Rep. Prog. Phys. 57 (1994) 895-938. 

E. Bauer: LEEM basics, Surf. Rev. Lett. 5 (1998) 1275-1286.

Ernst Bauer: LEEM and SPLEEM, in: Science of Microscopy, edit by P. Hawkes and J. Spence (Kluwer/Springer Academic Publishers, 2007) pp. 606-656.

Ernst Bauer: LEEM, SPLEEM and SPELEEM, in: Science of Microscopy, edit by P. Hawkes and J. Spence (Springer Business Media, 2018) in print.

PEEM:

E. Bauer: Photoelectron Microscopy, J. Phys. Condens. Matter 13 (2001) 11391-11405.  

E. Bauer: A brief history of PEEM, J. Electron. Spectrosc. Relat. Phenom. 185 (2012) 314– 322.

SPLEEM:

R. Zdyb and E. Bauer:  Spin-resolved unoccupied electronic band structure from quantum size oscillations in the reflectivity of slow electrons from ultrathin ferromagnetic crystals, Phys. Rev. Lett. 88 (2002) 166403-1-4.

E. Bauer: Spin-Polarized Low Energy Electron Microscopy (SPLEEM), in: Magnetic Microscopy of Nanostructures, eds. H. Hopster and H.P. Oepen (Springer, Berlin, 2005) p.111-136.

E. Bauer: Spin-Polarized Low Energy Electron Microscopy, in: Novel techniques for characterizing magnetic materials, ed. Y. Zhu (Kluwer Academic Publ., Boston, 2005) p. 361-379.

E. Bauer: Spin-polarized low energy electron microscopy in: The Handbook of Magnetism and Advanced Magnetic Materials, edit. by H. Kronmueller and S. Parkin (John Wiley&Sons, Chichester, 2007) Vol.3, pp 1470-1482.

SPELEEM and XPEEM:

E. Bauer and T. Schmidt: Multi-Method High Resolution Surface Analysis with Slow Electrons, in: Surf. Rev. Lett. 5 (1998) 1287-1296.

E. Bauer: Photoelectron spectromicroscopy: present and future, J. Electron Spectrosc. Relat. Phenom. 114-116 (2001) 976-987.

E. Bauer and T. Schmidt: Multi-Method High Resolution Surface Analysis with Slow Electrons, in: High Resolution Imaging and Spectroscopy of Materials, ed. by F. Ernst and M. Ruehle (Springer, Berlin Heidelberg 2003) 363-390. 

A. Locatelli and E. Bauer: Recent advances in chemical and magnetic imaging of surfaces and interfaces by XPEEM, J. Phys. Condens. Matter, (2008) 82202-82024.

E. Bauer, K.M. Man, A. Pavlovska, A. Locatelli, T.O. Menteş, M.A. Niño, M.S.

Altman: Fe3S4 (Greigite) formation by vapor-solid reaction, J. Mater. Chem A 2 (2014) 1903-1913.