Applications of Surface Science最新文献

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.

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.

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.

A new medium energy electron diffraction (MEED) system, which can observe both back scattering (BSMEED) and forward scattering (RMEED) patterns from the same surface of crystalline solids has been designed. Results obtained from MgO(001) and GaAs(001) indicate that MEED has the following favourable characteristics: (1) Unlike LEED, post-acceleration of the diffracted beam with a grid is not necessary for MEED. The diffraction pattern can be seen directly on a fluorescent screen. (2) Collimation and focusing of the incident beam are readily achieved and sharp diffraction spots are observed. (3) Unlike RHEED, a strictly flat surface is not necessary for MEED. Relatively rough, but clean surfaces can produce diffraction patterns. (4) From Kikuchi lines or bands it is possible to determine the precise orientation of the crystal surface. (5) By changing the energy of the incident beam it is possible to observe the surface layer both in two and three dimensions.

ZnTe films of about 15 nm thickness were prepared by vacuum deposition from ZnTe power onto NaCl substrates heated to 300°C. Zinc oxide particles with a size of 5 nm which were formed on the surface of ZnTe crystals showed no epitaxial relationship with the latter. Tellurium crystals appeared on certain parts of the ZnTe crystals with specific epitaxial relationships such as $\text{(0}\text{1}\text{1}\text{1}\text{)[10}\text{1}\text{0]}\text{Te}\text{(111)[110]}\text{ZnTe and}\text{(0}\text{1}\text{1}\text{1}\text{)[10}\text{1}\text{0]}\text{Te}\text{(0001)[10}\text{1}\text{0]}\text{ZnTe}$. Oxidation of the (110) surfaces of ZnTe crystals are slower than for any other plane. Images of the (110) films at an initial stage of oxidation contained faint superlattice fringes of about 0.85 nm spacing. On the basis of the matching between the HREM images and computer simulation images, the origin of the fringes was attributed to a periodic array of Zn vacancies introduced by the diffusion of Zn atoms to the surface of the ZnTe crystal.

Reduction in the scattering losses of ZrO₂, Al₂O₃, CeO₂ and Ta₂O₅ waveguides fabricated on BK-7 glass substrates using ion-assisted evaporation techniques has been achieved. The lowest losses have been observed for an Al₂O₃ ion-assisted film using 1200 eV O₂¹. These losses are of the order of 2–5 dB cm⁻¹ and compare with > 15–20 dB cm¹ for evaporated guides produced without ion assistance.

The resonant reaction $\text{H}\text{(}{}^{19}\text{F}\text{,αγ)}{}^{16}\text{O}{}^{\mathrm{\ast }}$ was used to study the hydrogen content and distribution with depth in a-Si : H films. The films were prepared in a DC magnetron by glow discharge decomposition of silane at a pressure of 1 Pa. The results showed that a surface hydrogen peak was present for a film prepared at 310°C but disappeared when the films were prepared at 370°C. The total hydrogen content was one half of that calculated using infrared absorption spectroscopy. RBS was used to determine the film density and scanning electron microscopy was used to investigate microstructure.

The electrical and optical properties of LiAl_xB_1−x thin films deposited at 350°C by the Sandwich-Type Evaporation Method are measured. These films are metallic with a resistivity of (1–4) × 10⁵ ohm cm at room temperature. In addition, we have developed a unique method. named the Brewster Angle Method (BAM), for obtaining the optical constants of thin films for LiAl_xB_1−x. In this method reflection coefficients are measured on the surface of thin films at any angle of incidence using a HeNe laser beam. The optical constants are calculated from the Brewster angle, φ_B, at the minimum value of reflection coefficient $\text{|}\text{R}{}_{\mathrm{<mtext>B</mtext>}}\text{|}{}_{\mathrm{<mtext>min</mtext>}}$. For a typical LiAl_xB_1−x film the optical constants are $\text{n}\text{= 1.42 −}\text{j}\text{0.31}$ and the complex dielectric constants are $\text{ϵ}\text{= 1.94 −}\text{j}\text{0.87}$.

The two-dimensional ordering in second stage alkali-metal-graphite intercalation compounds is investigated. It is shown that all hexagonal incommensurate metal lattices form commensurate superlattices. Using this, the observed orientational ordering is modelled by the simple condition that the experimental structure is a superlattice with a local minimum (varying the incommensurate lattice dimension) of the average distance of the metal atom from its nearest graphite hexagon centre. This approach is used in a detailed study of the diffraction pattern of the low temperature phase of second stage rubidium graphite.

A review is given on recent progress in tetrahedrally-bonded amorphous semiconductors and their technological applications to optoelectronic devices. First, some significant advantages of these materials are pointed out, and tangible instances are demonstrated from current technological topics. The present state of the art in optoelectronic device development with this new kind of thin film is then reviewed, and the technical data are summarized and discussed.