Two types of diffraction contribute to RHEED patterns. The lines that can be observed are Kikuchi Lines. The bright spots indicate where many electrons reach the detector. A RHEED pattern obtained from electron diffraction from a clean TiO2 (110) surface. Video 1 depicts a metrology instrument recording the RHEED intensity oscillations and deposition rate for process control and analysis.įigure 2. Users characterize the crystallography of the sample surface through analysis of the diffraction patterns. Some of the electron waves created by constructive interference collide with the detector, creating specific diffraction patterns according to the surface features of the sample. The diffracted electrons interfere constructively at specific angles according to the crystal structure and spacing of the atoms at the sample surface and the wavelength of the incident electrons. Atoms at the sample surface diffract (scatter) the incident electrons due to the wavelike properties of electrons. The glancing angle of incident electrons allows them to escape the bulk of the sample and to reach the detector. In the RHEED setup, only atoms at the sample surface contribute to the RHEED pattern. The reflected (specular) beam follows the path from the sample to the detector. The sample surface diffracts electrons, and some of these diffracted electrons reach the detector and form the RHEED pattern. Electrons follow the path indicated by the arrow and approach the sample at angle θ. Systematic setup of the electron gun, sample and detector/CCD components of a RHEED system. Figure 1 shows the most basic setup of a RHEED system.įigure 1. The electrons interfere according to the position of atoms on the sample surface, so the diffraction pattern at the detector is a function of the sample surface. Incident electrons diffract from atoms at the surface of the sample, and a small fraction of the diffracted electrons interfere constructively at specific angles and form regular patterns on the detector. The electron gun generates a beam of electrons which strike the sample at a very small angle relative to the sample surface. Low-energy electron diffraction (LEED) is also surface sensitive, but LEED achieves surface sensitivity through the use of low energy electrons.Ī RHEED system requires an electron source (gun), photoluminescent detector screen and a sample with a clean surface, although modern RHEED systems have additional parts to optimize the technique. Transmission electron microscopy, another common electron diffraction method samples mainly the bulk of the sample due to the geometry of the system, although in special cases it can provide surface information. RHEED systems gather information only from the surface layer of the sample, which distinguishes RHEED from other materials characterization methods that also rely on diffraction of high-energy electrons. The energies of emitted Auger electrons are again characteristic of different elements.Reflection high-energy electron diffraction ( RHEED) is a technique used to characterize the surface of crystalline materials. An incident electron beam (energy ~ 5 keV) is used to excite Auger transitions in near-surface atoms. Similar in concept to XPS, AES also allows chemical analysis of surfaces. For example, at a partly oxidised GaAs surface, Ga bonded to As shows and Ga bonded to oxygen show slightly different binding energies in XPS. Perhaps the most useful is the 'chemical shift': the core level binding energy is modified slightly (~ 1 eV) by the local chemical bonding environment of the atom. The basic equation is: electron energy = photon energy - core level binding energy - work function, but actually other small shifts occur as well. Peaks in the energy spectrum are characteristic of the core levels of individual elements near the surface of the sample. XPS - X-ray photoelectron spectroscopy.Ĭommonly used chemical analysis technique: incident X-ray photons generate photoelectrons which are energy-analysed. All of the non-MBE chambers in the group are equipped with LEED systems. 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. Instead of high energy grazing incidence electrons, in LEED one uses low energy (100 eV or so) electrons at normal incidence. Both MBE chambers in the group are equipped with RHEED systems. 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. The grazing incidence electron beam does not interfere with the effusion sources. RHEED - reflection high energy electron diffraction.Ĭommonly used as an in situ monitoring technique for MBE.
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