Low-energy electron diffraction

Abstract

Many investigations in metallurgy and physics require a knowledge of the chemical and physical state of the surface of a solid. For example, junction effects at semiconductor interfaces depend sensitively upon the surface states, which, in turn, depend upon the surface symmetry and chemistry; rigorous studies of epitaxial overgrowth must necessarily be performed on surfaces with known atomic arrangements. The techniques previously available for such studies have all had serious attendant disadvantages. The most commonly used method was that of reflection high-energy electron diffraction (RREED), which has proved extremely powerful. However, it requires an atomically smooth surface if the diffraction pattern is not to contain confusing features which result from surface asperities and undulations. Another powerful structural method is field-ion microscopy (FIM), but this has among its disadvantages the fact that it can cause progressive removal of the surface being studied. X-ray-fluorescence analysis is of great value for chemical identification of a material but it is not sufficiently selective to give information about the chemical species of atoms residing on a surface. If attention is to be directed at the true surface of the material, i.e. at the first few atomic layers only, then it is essential that the clean surface be maintained in an environment that will not contaminate it for at least the duration of a measurement. In practice, this usually means that the surface has to be cleaned and observed while it is in an ultra-high-vacuum (URV) system. In this way, the condensation of gas from the atmosphere at the surface can be kept below 1 monolayer in 1000 s if the total pressure is kept below 10^-9 torr (0.13 μN/m²). This is a reasonable time for most experimental observations. The majority of the previous structural studies have paid no such, rigorous attention to pressure and to surface cleanliness and, consequently, are of doubtful value in studies of surface condition and effects that depend upon surface structure.Many investigations in metallurgy and physics require a knowledge of the chemical and physical state of the surface of a solid. For example, junction effects at semiconductor interfaces depend sensitively upon the surface states, which, in turn, depend upon the surface symmetry and chemistry; rigorous studies of epitaxial overgrowth must necessarily be performed on surfaces with known atomic arrangements. The techniques previously available for such studies have all had serious attendant disadvantages. The most commonly used method was that of reflection high-energy electron diffraction (RREED), which has proved extremely powerful. However, it requires an atomically smooth surface if the diffraction pattern is not to contain confusing features which result from surface asperities and undulations. Another powerful structural method is field-ion microscopy (FIM), but this has among its disadvantages the fact that it can cause progressive removal of the surface being studied. X-ray-fluorescence analysis is of great value for chemical identification of a material but it is not sufficiently selective to give information about the chemical species of atoms residing on a surface. If attention is to be directed at the true surface of the material, i.e. at the first few atomic layers only, then it is essential that the clean surface be maintained in an environment that will not contaminate it for at least the duration of a measurement. In practice, this usually means that the surface has to be cleaned and observed while it is in an ultra-high-vacuum (URV) system. In this way, the condensation of gas from the atmosphere at the surface can be kept below 1 monolayer in 1000 s if the total pressure is kept below 10^-9 torr (0.13 μN/m²). This is a reasonable time for most experimental observations. The majority of the previous structural studies have paid no such, rigorous attention to pressure and to surface cleanliness and, consequently, are of doubtful value in studies of surface condition and effects that depend upon surface structure.