However, only the first few layers of the material contribute to the diffraction in RHEED, so there are no diffraction conditions in the dimension perpendicular to the sample surface. The reciprocal lattices of bulk crystals consist of a set of points in 3D space. The Ewald's sphere analysis is similar to that for bulk crystals, however the reciprocal lattice for the sample differs from that for a 3D material due to the surface sensitivity of the RHEED process. The user must relate the geometry and spacing of the spots of a perfect pattern to the Ewald's sphere in order to determine the reciprocal lattice of the sample surface. The diffraction pattern at the screen relates to the Ewald's sphere geometry, so RHEED users can directly calculate the reciprocal lattice of the sample with a RHEED pattern, the energy of the incident electrons and the distance from the detector to the sample. Ewald's spheres show the allowed diffraction conditions for kinematically scattered electrons in a given RHEED setup. RHEED users construct Ewald's spheres to find the crystallographic properties of the sample surface. RHEED users also analyze dynamically scattered electrons with complex techniques and models to gather quantitative information from RHEED patterns. These electrons account for the high intensity spots or rings common to RHEED patterns. Users extract non-qualitative data from the kinematically diffracted electrons. Dynamic scattering occurs when electrons undergo multiple diffraction events in the crystal and lose some of their energy due to interactions with the sample. Some incident electrons undergo a single, elastic scattering event at the crystal surface, a process termed kinematic scattering. 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. Reflection high-energy electron diffraction ( RHEED) is a technique used to characterize the surface of crystalline materials.
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