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Techniques

Surface science depends on complementary techniques to elucidate the structure and chemistry of surfaces, interfaces and thin films.

RHEED - reflection high energy electron diffraction.

Commonly used as an in situ monitoring technique for MBE. The grazing incidence electron beam does not interfere with the evaporation sources. Scattering from a crystalline surface produces a diffraction pattern on a phosphor screen whose features are characteristic of the periodicity of the crystal, and the periodicity of the surface reconstruction in particular. MADGEneto uses a compact electron gun running at 12.5 kV while Morbius includes and electron gun with 30 kV capability.

LEED - low energy electron diffraction.

Instead of high energy grazing incidence electrons, in LEED one uses low energy (100 eV or so) electrons at normal incidence. These are diffracted through large angles by the target crystal and in a modern LEED optics, the diffracted electrons are accelerated back past the miniature electron gun on to a rear-view phosphor screen.

Electron spectroscopies - AES, XPS, UPS, IPES, ARPES.

In electron spectroscopies, we hit the surface with some excitation (photons or electrons) and measure the energy spectrum of ejected electrons. This tells us about the chemical elements present near the surface, or about the band structure of the material, including surface-localised states. Electron energy spectra are measured with a concentric hemispherical analyser (CHA). Morbius is designed to accomodate an Omicron EA125 or VSW HA100 CHA. We also have a cylindrical mirror analyser (CMA) commonly used for Auger electron spectroscopy (AES). Surface specificity arises from the short inelastic mean free path of electrons in solids at energies from a few eV to a few keV.

AES allows chemical analysis of surfaces. An incident electron beam (energy ~ 5 keV) is used to excite Auger transitions in near-surface atoms. The energies of emitted Auger electrons are characteristic of different elements.

Incident X-ray photons can also generate photoelectrons which are energy-analysed, the basis of X-ray photoelectron spectroscopy (XPS). Peaks in the energy spectrum are characteristic of the core levels of individual elements near the surface of the sample (the basic equation is: electron energy = photon energy - core level binding energy - work function). In both AES and XPS the local chemical environment of an atom can shift the binding energy slightly, giving these techniques sensitivity to local chemical bonding too.

STM - scanning tunnelling microscopy.

In STM, an atomically sharp probe is brought to within ~1 nm of an electrically conducting surface. A bias voltage between probe and surface allows a current to flow by quantum tunnelling. The probe is scanned across the surface in a raster pattern to build up an image: typically the tunnel current is kept constant and the z motion of scanner as its x and y coordinates are scanned form the image. STM allows surfaces to be imaged with atomic resolution, but bear in mind that because (1) it is sensitive to the "electron clouds" around atoms and (2) the shapes of these clouds is not known a priori at a surface, it is not generally possible to infer atomic positions from STM images without a lot more supporting evidence. Roughly speaking: we can see blobs with atomic spacing, but without some other evidence we don't know what those blobs represent. Here are a few slides about STM. The MADGEneto system has an Omicron STM-1 (a truly classic instrument from the 1990s, probably the earliest widely successful commercial UHV STM) with custom-built electronics and software.