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.

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.

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.

When the spectra corresponding to successively greater adsorbate coverages are superimposed, there is often a singular point through which they all pass. This is recognized as an isosbestic point, which is shown should be present whenever there is only a single adsorbate overlying the substrate. The lack of such a point consequently indicates the existence of multiple adsorbate species. These criteria are applied to the adsorption of O₂, H₂S and benzenethiol on Cu(410). For H₂S, two species are detected and assigned to surface and incorporated sulfur.

DC conductivity and Hall coefficient studies were made on MIS structures of n-type Pb_0.8Sn_0.2Te thin films grown by a flash evaporation technique in the temperature range 77–400 K. The decrease in R_H with positive gate field and increase in R_H with negative gate field have been attributed to the accumulation and depletion of charge carriers due to bending of bands. Mobility-temperature data have been analyzed in terms of various scattering mechanisms.

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.

UV photoemission, low energy electron diffraction and high-resolution electron energy loss spectroscopy (EELS) have been used in situ to study the nature of 1–25 Å of Au and Pd deposited on Si(111) at temperatures of 20 and 300 K. New information is obtained regarding the formation, nature and microstructure of these metallic layers. We describe and apply a theoretical analysis of EELS which allows the determination of DC transport properties in these ultrathin layers. All metallic layers exhibit significantly higher resistivities than expected in comparison to bulk metal silicides or metallic glass phases, and can be attributed to diffuse scattering of conduction electrons at the interface. The interaction and behavior of atomic hydrogen with these films as detected by EELS is also described and found to convey additional information about their microstructure.

The structure of thin silicon oxide films 5 nm in thickness, which were prepared by electron beam evaporation of SiO₂ glass onto a NaCl substrate, has been examined by high resolution electron microscopy and diffraction. Although the films which were prepared with substrate temperatures ranging from room up to 400°C gave rise to amorphous haloes, lattice fringes in areas 1–2 nm in extent were, however, seen in the micrographs. It is shown that the film is composed of α-quartz micro-crystallites. Crystals of α-cristobalite with sizes of several tens of nanometers appeared at a substrate temperature of 500°C. At a substrate temperature of 600°C, β-cristobalite crystals with sizes of several tens of nanometers appeared. The structural changes due to the substrate temperature were attributed to incorporation of sodium atoms from the substrate into the SiO₂ film.

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 $\text{ϵ}\text{= 1.94 −}\text{j}\text{0.87}$.

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.