Applications of Surface Science最新文献

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.

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.

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 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.

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.

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.

Several recent developments and future prospects in the use of core-level X-ray photoelectron diffraction (XPD) for surface structure studies are considered. Experiments involving both standard X-ray sources and synchrotron radiation are discussed.

A straightforward technique is described whereby the edges of two contiguous films can be precisely aligned without overlap so that the contact area between them is defined by the thickness times the width of the film. In this way junction areas as small as 0.02 μm² can be achieved with very simple photolithographic techniques. Electron micrographic, electrical and surface studies have been carried out on junctions of this type and preliminary results are reported.

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.

Growth processes of semiconducting indium nitride prepared by radiofrequency sputtering of a metallic target in a reactive nitrogen plasma have been studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Contrary to previous reports progressive nitridation of the target surface is observed [1] and is found to control film morphology and growth rate. Samples prepared from fully nitrided targets are exclusively c-axis oriented and polycrystalline, while other wurtzite planes persist when target nitridation is incomplete. The target process involved here requires the partial pressure of atomic nitrogen to be 10⁻⁴ Torr in the RF discharge region close to the target. Surface migration processes are also dependent on the target state. These phenomena are analyzed within the context of the structure zone model [2] while considering a unified approach to crystal growth habit [3].