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Low Energy Electron Diffraction

The objective lens is shared with other types of emission microscopes. At the heart of any LEEM there are several electrostatic and magnetic lenses [ 42 ]. Electron lenses are well-developed and used in a variety of charged-particle instrumentation. The preference of electrostatic[ 43 ] vs magnetostatic lenses is mostly a question of convenience. While magnetic lens have a slight edge in aberrations, they are usually more bulky and difficult to cool unless the coils are outside vacuum.

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They also rotate the electron spin and the image they form, although this effect can be avoided using self-canceling doublets. The objective lens decelerates the electrons to a few eV close to the sample. Obviously, having the sample close to ground potential is convenient from the experimental point of view. But it is not compatible with magnetic lenses, which have their central tube section at ground. Electrostatic lenses do not have this limitation.

The most popular commercial designs see Fig. In these instruments the illuminating and imaging sections and the objective and sample sections are housed in separate but connected vacuum chambers. The magnetic coils surround lengths of thin vacuum tubing at ground. As the connection between the sample area and the columns is through a small opening several mm , differentially pumping permits imaging under pressures in the 10 mbar range while keeping the detector and electron emitter at UHV.

There are also fully electrostatic instruments with the exception of the beam separator, see Fig. The use of electrostatic lenses makes for compact instruments that can be mounted on a standard flange e. Such systems comprise one commercial design Elmitec IV[ 16 ] and a few research systems[ 41 , 44 ].

As all the electrostatic lenses are in the sample vacuum chamber, this type of LEEM usually is more demanding in terms of background pressure. Three types of electron sources are used in current LEEM instruments: thermionic, field emission and photoemission sources. The most popular thermionic source is based on a LaB single crystal emitter. It has a long lifetime, is inexpensive and quite sturdy in a UHV environment[ 45 ]. The main limitations are the energy spread of about 0. More recently, cold field emission CFE sources are gaining in popularity.

(1) Overview

The energy spread for cold field emission is 0. Finally, GaAs sources are the only spin-polarized electron sources presently used for magnetic imaging i.

The energy spread is even smaller than cold field emission sources 0. A GaAs electron source[ 46 ] consist of a GaAs cathode conditioned to negative electron affinity NEA from which electrons are emitted when illuminated by circularly polarized light from a diode laser. The need to properly prepare the GaAs surface cleaning it, and then depositing Cs and oxygen together with the requirement of extreme vacuum well below 10 mbar for reasonable lifetimes of the prepared cathode make this electron source quite demanding.

Recent designs focus the laser beam to a micrometer area and provide brightness comparable with thermal sources[ 47 ]. The spin direction of the electrons is perpendicular to the GaAs cathode. A spin manipulator between the GaAs cathode and the condenser optics is used to turn the spin direction to any desired orientation relative to the sample[ 48 , 39 ]. The condenser lenses are used to obtain a parallel beam of electrons before reaching the sample by providing a demagnified image of the source at the objective backfocal plane.

Several condenser lenses provide for maximum flexibility, although some recent designs use a minimalist approach of a single condenser lens[ 19 ].

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Apertures can reduce the area down to about 0. All objectives are thus composed of an accelerating lens and a subsequent electrostatic or magnetostatic lens for focusing see the inset in Fig. As shown in Fig. Several objective designs have been tried, see inset of Figure. Nearly all the spherical and chromatic aberrations that limit the final instrument resolution originate at this objective system.

The quantification of the objective aberrations has been studied in detail[ 38 ] due to the current interest in the aberration correction of LEEM instruments. The beam separator function is to separate the electrons going into the sample from the electrons leaving the sample.

In its simplest form, it consists of a square or circular magnetic dipole with two parallel pole plates excited by a coil. A limitation of this design is that a dipole field focuses the electrons that move in a plane normal to the magnetic field. This means that the focusing properties are very different in the two axis of the electron beam. Most of the current systems are based on a 60 deflector, which comprises an array of dipole fields[ 50 ] see Fig. More recently, 90 deflectors are starting to gain ground[ 40 ] see Fig.

They allow stacking the optics vertically, a particular advantage for aberration correction where an additional beam separator is needed [ 18 ]. They are constructed from a central square dipole magnet, surrounded by one[ 40 ] or more rings[ 33 ] at different magnetic potentials, or by additional electrostatic round lenses[ 51 ].

After the last imaging lens, the electron distribution is amplified by microchannel plates before impinging on a phosphor screen. Outside of vacuum, a digital camera is used to record and store images of the screen. The channel-plate-based detectors are delicate and typically do not have a uniform response across their area. The main operational danger of a LEEM lies in the microchannel plates: excessive electron flux can damage the plates and the phosphor, giving rise to dead areas in the images.

Moreover, runaway events in the detector can cause damage even with no electron beam. Specially, when switching from real space mode to a diffraction pattern, care has to be exercised not to exceed the maximum current of the channel plate. Controlling the image intensity safely is essential for the current detectors.

In some instruments, the preferred way to control intensity is changing the illuminating beam current, while other instruments change the channel plate voltage. Direct imaging sensors without channel plates are being tested but present challenges in detector area and vacuum compatibility [ 52 ].

Additional capability comes from the ability to filter the electrons emitted from the surface by their energy. In its simplest form, LEEM makes images from elastically reflected electrons. The inelastically emitted electrons are then an unwanted background in the images and in LEED patterns. But the inelastic electrons also contain information, as do electrons in photoemission.

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An energy filter extracts information from these electrons. Two types of energy filters are being used. One type decelerates and then bends the electron beam, which disperses it by energy. A recent design takes advantage of the energy dispersion within the beam separator itself to provide energy discrimination without extra optical elements other than an aperture[ 40 ]. Two modes of operation are common.

SURFACE ANALYSIS | Low-Energy Electron Diffraction

In the first, the dispersive plane of the energy filter is imaged onto the detector. Converting image intensity versus position into a count rate vs. An aperture in the beam separator is used to select the analysis region, analogous to selected-area LEED. In the second mode, an aperture in the dispersive plane is used to select a given energy bandpass.

The passed electrons are then formed into an energy filtered image. Changing the sample potential changes the electron energy that passes through the filter's aperture to the detector.