Low-energy electron microscopy



Low-energy electron microscopy, or LEEM, is a technique used by surface scientists to study surface structure at mesoscopic scale on conducting and semiconducting materials. It is based on imaging the electrons elastically scattered from the surface. The high signal levels allow one to literally observe the motion of atomic steps on flat surfaces in real time during film deposition, exposure to reactive gases, or during heating and cooling. It is a powerful complement to scanning tunneling microscopy (STM), which has better spatial resolution than LEEM, but is usually less applicable to the study of dynamic processes.

High-energy (15-20 keV) electrons are directed at a sample and decelerated such that they interact with the surface at low energy (0-500 eV). The technique derives its surface sensitivity from the low energy of these electrons; electrons at these energies do not penetrate far into materials. These electrons reflect or diffract from the surface of the sample, are re-accelerated, and then focused onto an area detector (consisting typically of a microchannel plate detector, a phosphor screen and a CCD) to form an image of the surface. LEEM is unlike other electron microscopies in that the electron beam is not scanned or rastered across the surface. The entire region of interest is imaged at once.

Since it is an electron-based technique, it must be used in a high-vacuum (usually less than 10-7 Torr) environment. Typically, surface scientists use the instrument in ultra-high vacuum (less than 10-9 Torr) in which pristine, atomically flat surfaces can be studied.

The spatial resolution is of the order of 10 nanometers, and is limited by the aberrations in the electron optics used to image the surface onto the detector. On the other hand, on crystallographic surfaces the phase differences between electron waves scattered from different planes provide an atomic depth resolution.

The contrast mechanism in a LEEM instrument depends on the electron energy as well as the type of surface under study. At very low energies (less than a few electron volts) the electrons cannot penetrate into the surface, and they are all reflected. This gives an out-of-focus topographical image of the surface, and so is termed as mirror electron microscopy. Going to higher energies, the electrons diffracted at different angles can be selected by filtering the back-focal plane using an aperture. This results in modes of operation analogous to bright field and  dark field microscopies. The resulting contrast can be due to differences in the crystal symmetry and orientation, in the surface reconstruction, as well as chemical inhomogeneities on the surface.

LEEM is closely related to the low-energy electron diffraction (LEED) as both techniques are based on the same physical process (elastic back-scattering of electrons). Indeed, the LEEM instrument can be used to measure LEED patterns by an additional lens to image the diffraction plane instead of the image plane of the electron optics. In this way, simply by changing the settings of the lenses, one can switch from LEEM to LEED in a few seconds.

The technique was invented by Ernst Bauer, who is currently a professor at Arizona State University.

Recently, a version of the instrument which uses a spin polarized electron source has been developed. This version, called spin-polarized low-energy electron microscopy (SPLEEM), enables investigators to image magnetic domains on surfaces of materials and observe how they evolve as a function of time, temperature, film thickness, etc.