Applications of Surface Science最新文献

The following facts, and many others, concerning III–V (e.g., GaAs, InP) Schottly barriers can be understood in terms of Fermi-level pinning by interfacial antisite defects (sheltered by vacancies) at semiconductor/metal contacts: (i) the barrier heights are almost independent of the metal in the contact; (ii) the surface Fermi levels can be pinned at sub-monolayer coverages and the pinning energies are almost unaffected by changes of stoichiometry or crystal structure; (iii) the schottky barrier heigh for n-InP with Cu, Ag, or Au is ⋍0.5 eV, but changes to ⋍0.1 eV when reactive metal contacts (Fe, Ni, or Al) are employed because the antisite defects are dominated by P vacancies; and (iv) the dependence on alloy composition or alloys of AlAs, GaAs, GaP, InAs, and GaAs is extremely complex — owing to the dependence of the binding energy for the cation-on-anion-site deep level on alloy composition. Fermi-level pinning by Si dangling bonds at Si/transition-metal silicide interfaces accounts for the following facts: (i) the barrier heights are independent of the transition-metal, to within ⋍0.3 eV; (ii) on the 0.1 eV scale there are chemical trends in barrier heights for n-Si, with the heights decreasing in the order Pt, Pd, and Ni; (iii) barriers form at low metallic coverage, (iv) barrier heights are independent of silicide crystal structure or stoichiometry to ± 0.1 eV; and (v) the barrier heights for n-Si and p-Si add up to approximately the energy of the band gap.

The Chadi total energy minimization technique is probably the simplest, physically realistic approach for determining the surface geometries of solids and has proved highly successful in predicting the surface relaxation and reconstruction of a wide variety of covalent solids. One of the basic assumptions of this method has been that the tight-binding parameters, which describe the dependence of the electronic energy of the system upon the surface configuration of atoms, simply vary as the inverse square of the appropriate interatomic distance. More recent work, however, has shown that there are now strong grounds for doubting the validity of this 1/d² approximation, and has suggested that a better representation of the spatial dependence of the LCAO model Hamiltonian parameters might be obtained from self-consistent bandstructure calculations performed at different lattice constants. The purpose of this paper is to assess the relative merit of these two alternative models by employing them in a direct determination of some of the lattice dynamical properties of silicon, and to discuss the implications of the results of these calculations for surface structure analyses within the Chadi formalism.

LEED fine structure is found in LEED intensity curves at low energies below emergence thresholds for new diffracted beams. The fine structure is due to the interaction between a reflected beam and a beam which is totally internally reflected by the surface barrier. The fine structure may be strongly affected by chemisorption and this provides a useful technique for surface structural studies. The effects of surface roughening or smoothing, reconstruction and disordered adsorption can be clearly distinguished. Examples of the application of this technique to the oxidation of low index planes of Cu, Ni and W are discussed. It is concluded that LEED fine structure analysis provides valuable information which supplements that obtainable from other compatible surface science techniques.

Ultraviolet inverse photoemission is an exciting new technique for band mapping of empty electronic states in solids and at surfaces. It includes the energy region between the Fermi and the vacuum level inaccessible by ordinary photoemission. Applications discussed in this paper include band mapping in nickel, spin-resolved studies of iron, and chemisorption of NO and CO on palladium.

Hall mobility (μ_H) and field effect mobility (μ_FE) studies were carried out on PbTe films of different thickness grown on KCl (100) substrates by the hot wall epitaxy (HWE) technique. The Hall mobility was obtained using the standard Van der Pauw technique. The diffused scattering mobility, μ_D, due to size effect was calculated and compared with μ_H. A large discrepancy between μ_H and μ_D was explained on the basis of a residual mobility contribution which was attributed to the scattering due to grain boundaries, dislocations, cleavage steps and other surface effects. For AC field effect studies an MIS structure with a thin mica spacer between the film surface and metal electrode was used. The field effect mobility, μ_FE, was obtained at different temperatures from 98 to 156 K in the frequency range of 40 to 200 kHz. The variation of μ_FE with frequency was found to be largely due to the relaxation of fast surface states having time constants from 1.84 to 0.96 μs in the above temperature range. The activation energy and capture cross-section of these surface states were calculated to be 0.02 eV and 10⁻¹⁹ cm² respectively. Unlike the Hall mobility, the effective DC field effect mobility derived from the experimental results was found to be independent of film thickness.

The coadsorption of oxygen and caesium on silver has been studied by means of UPS and TPD. Adsorption of oxygen on a caesiated surface occurs in two stages, with a sticking coefficient, s, of nearly one in the first stage. In the second stage, the sticking coefficient is lower, but still higher than on the clean surface. After adsorption, the oxygen is present in two states, which are identified in TPD. These are ascribed to oxygen atoms adsorbed directly adjacent to Cs atoms, and those more distant from the Cs. Shifts in the Cs 5p lines are consistent with this assignment. The relevance to industrial catalysis of partial oxidation reactions over silver (e.g. epoxidation of ethylene) is discussed.

The secondary ion yields of polytetrafluoroethylene (PTFE) and polyethylene (PE) were compared with that of Al⁺ emitted from Al₂O₃ using mixed samples of their powders. The surface compositions of mixtures were measured by means of X-ray photoemission spectroscopy (XPS). Secondary ion mass spectrometry (SIMS) measurements with 1 keV Ar⁺ showed that the secondary ion yields of CF⁺ from PTFE and C₂H⁺₃ from PE were ∼ 20 and ∼ 4 times larger than that of Al⁺ from Al₂O₃, respectively. The sputtering yield of PTFE was also examined by weight loss measurements; the results suggest that the large secondary ion yield of PTFE is primarily due to its large sputtering yield.

Transition metal silicides were made by the vacuum thermal annealing of 20 nm metal layers on silicon substrates. The samples were then argon sputtered at 2 keV in a UHV Auger system till the silicide-silicon interface was reached. The interface was irradiated with electrons and the Auger spectrum recorded at regular time intervals. Metal atoms which had diffused into the silicon were found to be moving towards the surface under electron bombardment. A surface silicide layer 1–2 nm thick was formed by these migrating metal atoms. Pd, Pt and Cr silicides on (100) and (111) silicon surfaces were observed. The electron induced migration was much stronger for the Pd₂SiSi interface than for either the Pt₂SiSi or CrSi₂Si interface.

We investigate the scattering of electrons in a quasi-two-dimensional electron gas at the AlGaAs/GaAs interface, off ionized impurities located in AlGaAs. We use multiple-scattering (t-matrix) techniques to calculate the mobility and bound state energy of the electrons.

The advantages of spectroellipsometry include relative instrumentational simplicity, sensitivity to composition, density, and microstructure of thin films, submonolayer sensitivity to adsorbates, overlayers, and interfaces, and the capability of providing this information nondestructively, in real time, and in any transparent ambient. In this paper I discuss present trends and likely future directions for instrumentation, data analysis, and applications, and illustrate present capabilities by various examples.