On the Abnormal Conductivity of the Surface (001) of Alkali Halide Crystals
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On the Abnormal Conductivity of the Surface (001) of Alkali Halide Crystals V. P. Vlasova, A. E. Muslimova,*, and V. M. Kanevskya a Shubnikov
Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 117333 Russia * e-mail: [email protected] Received April 10, 2019; revised June 13, 2019; accepted June 13, 2019
Abstract—Surfaces (001) of alkali halide crystals (KCl, NaCl), which were obtained by cleavage in air and ultrahigh vacuum, have been studied by Auger electron spectroscopy. It is established that the surface obtained by cleavage in ultrahigh vacuum has an anomalously high conductivity, which is broken down during heating and depositing an atomic layer of metal (Au). DOI: 10.1134/S1063774519060269
INTRODUCTION The surface electric conductivity of a solid dielectric is known to be determined by the presence of a thin layer of adsorbed water [1]. The impurities presented on the surface dissociate during dissolution to form ions. Thus a fairly high electric conductivity (∼1 Ω–1) is generated. The surface conductivity of hydrophilic dielectrics, which adsorb water well, sharply increases (up to several orders of magnitude) when air humidity increases. Surface conductivity of hydrophobic dielectrics barely depends on relative humidity of air (up to dewfall). Therefore, the specific surface conductivity is assumed to depend on the properties of dielectric and the presence of contaminants and adsorbed water on its surface. Washing in solvents and high-temperature annealing are applied to reduce the surface conductivity. Alkali halide crystals (AHCs) belong to the most widewpread types of solid dielectrics. The great interest in them is related to their unique optical parameters (a wide transparency band), possibility for introducing color centers, specific chemical properties, etc. However, the low electrical conductivity of dielectric AHCs hinders significantly the study of the atomic structure of their surfaces applying electron and ion beams and scanning tunnel microscopy. Moreover, charged particles (even with a small kinetic energy) may change the surface structure. Previous experiments [2] on the scattering and diffraction of atomic beams from the LiF(001) surface showed the presence of diffraction peaks. However, these experiments cannot be used to estimate the surface cleanliness. The only conclusion made on the basis of measuring the distance between diffraction peaks was that the surface layer has the same structure
as the crystal bulk. The interest in the adsorption phenomena on NaCl and KCl, including water adsorption, resulted in a large number of works with application of various methods: mass spectrometry of the gases released upon heating [3] and cleavage [4], Auger electron spectroscopy and electron stimulated desorption [5], ellipsometry [6], low-energy electron diffraction (LEED) [4, 7–10], etc. Most of the studies were carried out on (001) cleavages (with subsequent surface processing) and (partial
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