Low energy electron microscopy (LEEM)

SPELEEM applications , LEEM , energy filtered XPEEM , XAS-PEEM , microprobe-ARPES

What is LEEM?

LEEM is a parallel imaging technique based on the cathode lens, which uses elastically backscattered electrons to image crystalline surfaces and interfaces. Owing to the large electron backscattering cross section of most materials, LEEM is the ideal technique to image in video rate dynamic processes such as surface reconstructions, epitaxial growth, step dynamics, self-organization and others. LEEM enables to reach a lateral resolution of less than 10 nm. Most importantly, it offers several structure sensitive complementary imaging and diffraction methods to probe the surface.

Operating principle

A beam of high energy electrons (10-20 keV), decelerated by the retarding potential of the objective lens, impinges in normal incidence on the specimen surface, with energy in the range 0 to few hundred eV. The beam energy is varied by adjusting a bias voltage between sample and the emitter. The elastically backscattered electrons are then reaccelerated through the objective lens, following the inverse pathway. The objective produces a magnified image of the specimen in the beam separator, which is further magnified by several additional lenses in the imaging column of the instrument. This image is projected onto an imaging detector with microchannel plate and phosphorous screen, and finally acquired by a computer controlled CCD camera.  Along with real space imaging, a LEEM microscope is also capable of reciprocal space imaging. The first diffraction plane along the LEEM electron optics is first encountered in the backfocal plane of the objective lens. By changing the excitation of the lenses in the imaging column, it is straightforward to produce a magnified image of the diffraction pattern, Thus, a LEEM microscope can be used as a LEED. As a plus, diffraction measurements can be restricted to very small areas, thus enabling microprobe-LEED.

Mehods available in a LEEM microscope

Bright field and darkfield LEEM. The use of a contrast aperture positioned in the diffraction plane allows employing primary or secondary diffracted beams for imaging. When the primary diffracted beam (or "00" beam) is selected, we perform bright-field LEEM. Here, the contrast is purely structural (diffraction contrast) and depends on the local differences in diffraction amplitude for the different surface phases present on the sample. By selecting a secondary diffracted beam, a dark-field image of the surface is produced. Here all areas that contribute to the formation of the selected beam appear bright.

Commonly used contrast methods in LEEM are phase contrast and quantum size contrast. In the first, the height difference between terraces at different heights on the surface leads to a phase difference in the backscattered waves. Defocusing can convert such phase difference into an amplitude difference, allowing to image steps at surfaces. The second method is based on the interference of waves that are backscattered at the surface and at the interface of a thin film, producing maxima and minima in the backscattered intensity depending on the local thickness of the film.

MEM. In mirror electron microscopy (MEM) the surface is illuminated with electrons at very low energy, so that the electrons interact very weakly with the surface (this occurs at the transition MEM−LEEM). Under these conditions the contrast is due to work function differences and topography variations. MEM allows non-crystalline samples to be imaged.

µ−LEED. The SPELEEM is operated as a LEED instrument. The reflection of the e-beam by a crystalline surface results in the formation of a diffraction pattern in the back-focal plane of the objective lens. The lenses in the imaging column of the LEEM can be excited in such manner to project a magnified image of the diffraction pattern on the detector. The probed area can be restricted to either by inserting an aperture in the image plane in the input or exit side of the beam separator, respectively, thus revealing surface structure at length scales much shorter than in a conventional apparatus. For this reason, this operation mode is called microprobe-LEED, or microspot LEED or simply µ−LEED.

References and reviews

Low Energy Electron Microscopy;
E. Bauer;
Rep. Prog. Phys. 57, 895-938 (1994).
doi: 10.1088/0034-4885/57/9/002

LEEM basics;
E. Bauer;
Surf. Rev. Lett. 5, 1275-1286 (1998).
doi: 110.1142/S0218625X98001614

Trends in low energy electron microscopy;
M S Altman
Journal of Physics: Condensed Matter 22, 084017 (2010).
doi: 10.1088/0953-8984/22/8/084017

Ernst Bauer;
in Science of Microscopy, pp. 606-656.
edited by P. Hawkes and J. Spence, Kluwer/Springer Academic Publishers, 2007.

Last Updated on Wednesday, 03 July 2013 15:29