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ABSTRACTSurface diffusion has been studied in the Ag/Ge (111) and Cs/Si (100) systems using UHV-SEM based techniques, biassed secondary electron imaging (b-SEI), Micro-AES and RHEED. Ag and Cs were deposited through a mask of holes held close to the substrate at room temperature, and annealed at higher temperatures Ta. Under certain conditions, the Ag and Cs patches split into two distinct regions with different sub-Monolayer (ML) coverages θ, observable using b-SEI; the sensitivity for Cs/Si (100) is below 0.5% ML. These patch widths were measured as f(ø,Ta,t), and effective diffusion coefficients extracted. Both systems were modelled using coupled diffusion equations, involving adatoms in two different surface phases. Comparison with experiment yields activation energies for adsorption (Ea) and diffusion (Ed).
Shovkatjon Akhunov, Ilkhomjon Saydumarov, Shamsiddin Urolov
et al.
In this work, we studied secondary ion emission of quarts resonator surface coated silver under bombardment with multiply charged ions of Csq+ and Siq+. For this purpose, the Csq+ and Siq+ ion sources were developed and tested. The experiments were carried out using a modernized magnetic mass spectrometer MI 1201 which has included Csq+ and Siq+ ion sources. The bombarding multiply charged ions had an energy range of 1–10 keV per unit charge. As a result of prolonged ion bombardment, the silver film was etched away, exposing a transitional chromium layer that had been applied to improve adhesion of the silver coating to the quartz. At the boundary of the transition from the silver layer to chromium, cluster ions consisting of cesium, silver, and chromium atoms were observed in the mass spectra, and with further etching, the chromium ion became the main peak in the spectrum. The obtained data demonstrates that the yield of secondary atomic and molecular ions was increasing with growing of number of atoms in bombarded multiply charged ions and this is very distinct for light elements with large ionization potentials.
Measurement of the energy dependence of the yields of Na+, K+, and Cs+ ions formed when beams of the corresponding alkali atoms M hit a Si(111) surface and of Na+ for Na striking a Pt surface at a temperature T are those expected for equilibrium for thermal kinetic energies E of the incident atoms. Above a threshold energy of 0.5–1.0 eV the yields rise rapidly to maxima greater than 0.1 and then remain approximately constant as the energy increases to 100 eV. They are nearly independent of T in the range 300–1100 K. The results are represented well by a classical model having (1) a rate of electron transfer that varies exponentially with the distance z of M from the surface, (2) potentials that slow the incoming atom down as it nears the surface, and (3) energy transfer by elastic two-body collisions with the surface represented as hard cubes, each having the mass of an integral number n of surface atoms. At z=5 Å the rate on Si for Na and K is 1012.5±0.2 s−1 with n equal to 3 and 5, respectively, while for Cs it is three times less with n equal to 20. For Na on Pt it is 1012.9±0.1 s−1 with n equal to 1.
The development of a high-intensity Cs sputter negative ion source has been completed. The source is equipped with a spherical tungsten ionizer and a target wheel with 18 sample positions. The wheel is separated from the hot sputter region by a gate valve in order to change the samples rapidly and reduce cross contamination. A drive system can push the selected sample into the sputter position and pull it back to the wheel. A metal-ceramic bonded ring is used to support all the parts at sputter potential. The ring is protected from vapor depositions by a specially designed shielding structure, resulting in reliable insulation between the ionizer and the sputter samples. No insulation trouble has occurred during the work for over one year. Beams of 10-μA BeO−, 5-μA Al−, 4.5-μA Fe−, and 350-μA C−, etc. have been delivered. The normalized beam emittance ranges from (2–4) πmm mrad MeV1/2. The memory effect has been evaluated. More than 1000 and 500 times of attenuation for C− and BeO− are observed, respectively, in 10 min after a sample change. The ionization efficiency for 12C− is higher than 5.4% for a graphite sample.
In this work we study the adsorption of Cs on S-covered Si(100)-(2 × 1) and Si(100)-(1 × 1) surfaces, as well as the adsorption of S on Cs-covered Si(100)-(2 × 1). The experiment was performed in an ultrahigh vacuum (UHV) chamber with low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and work function (WF) measurements. Predeposited S increases the binding energy and the maximum amount of Cs that can be deposited on the surface. The presence of S inhibits the pattern of the characteristic WF curve of Cs on clean Si(100)-(2 × 1), i.e. an initial decrease to a minimum value, Φmin, followed by an increase toward the value Φmax of metallic Cs. The WF, instead, decreases to a value close to that of saturated Cs on clean Si(100)-(2 × 1), where it forms a plateau. This is characteristic of the covalent bonding of Cs with the semiconductor substrate. Independently of the sequence of Cs and S deposition, (a) the transition Si(100)-(2 × 1) → Si(100)-(1 × 1) occurs when ΘS > 0.5 ML, and (b) the sites of Cs and S remain the same, with the Cs atoms residing between the S atoms. Heating of the S/Cs/Si composite surfaces to ~ 650 K causes a reorganization of the Cs and S adatoms in a tendency to form a Cs–S complex. The issue of site preference for Cs and S adatoms has been discussed in detail in the structural models provided.