Surface Stoichiometries Research Articles

High-resolution electron-energy-loss spectroscopy studies of GaAs (100) surfaces

High-resolution electron-energy-loss spectroscopy (HREELS) has been used to study the vibrational spectra of chemisorbed atomic H on the GaAs(100)c(2×8), (4×6), (4×1), and c(8×2) surface structures. Our results show that both As–H and Ga–H species are present on the GaAs(100)c(2×8) surfaces even at very low exposures. This result is notable because it indicates that the c(2×8) structure is terminated with less than a full monolayer of As atoms. Furthermore, HREELS results indicate that a wide range of surface stoichiometries exists for this surface with the Ga–H mode actually slightly stronger than the As–H mode at the higher annealing temperatures. On the c(8×2) structure the HREELS data show almost exclusively Ga–H with little or no As–H. Strong similarities are observed between the (4×1) and the c(8×2) surfaces, particularly after sufficient annealing, which minimizes the presence of the more As-rich (1×6) domains that often accompanies the (4×1) structure. Several model structures incorporating dimer vacancies are discussed in relation to the HREELS data.

DETERMINATION OF THE METAL PARTICLE SIZE OF SUPPORTED Pt, Rh, AND Ir CATALYSTS. A CALIBRATION OF HYDROGEN CHEMISORPTION BY EXAFS

Hydrogen chemisorption measurements of highly dispersed Pt, Rh, and Ir catalysts yielded H/M values exceeding unity. These results cannot be used straightforwardly to determine the average metal particle size, because the H/M stoichiometry on the surface is unknown. EXAFS measurements were performed to determine the metal particle size and thereby, to calibrate the hydrogen chemisorption results. The high hydrogen chemisorption values can be explained best by assuming H/M surface stoichiometries exceeding unity. The adsorption stoichiometry differs among Pt, Rh, and Ir in the order H/Pt <H/Rh <H/Ir, analogous to the order in stability of the corresponding metal polyhydride complexes.

Effects of chemical state on quantitative AES of metal–sulfur compounds

AbstractA quantitative approach to AES is illustrated for various metal–sulfur compounds involving: (a) intensity measurements via integral areas of background subtracted N(E) spectra (including sample probe potential biasing to improve background subtraction); (b) low primary electron dose, pulse‐count spectrum acquisition to minimize primary electron beam damage and charging artifacts; (c) corrections to spectral intensity for variations in electron backscattering and inelastic Auger electron mean free path for each metal–sulfur compound matrix; (d) XPS quantitative analysis to correct for differences in surface vs. bulk stoichiometries. Although the above approach results in improved analytical precisions, a residual difference of greater than a factor of 2 remains in the Auger elemental sensitivity factor for sulfur (LMM) in corresponding metal sulfate vs. sulfide compounds. This suggests a chemical state dependent Auger electron yield (core level ionization cross section times the Auger transition probability). Accurate AES quantification, therefore, may require chemical state specific sensitivity factors, particularly for valence shell transitions.

On the adjustability of the “abrupt” heterojunction band-gap discontinuity

There is growing evidence that the offset in the band edge between two different semiconductors is intimately linked to the chemical and geometric structure at the interface. By conducting experiments sensitive on an atomic scale, we focus on the relation of the nanostructure changes to the electronic barrier formation for heterojunctions between Ge, GaAs, and AlAs. We demonstrate that there is not a single value for the distribution of band gap differences between valence band and conduction band offsets. Attempting to apply the commonly used Anderson electron affinity rule (i.e., ΔE c = Δ χ ), shows no systematic behavior of band-gap discontinuities as measured by photoelectron spectroscopy. The discontinuity depends not on a simple scaling parameter such as electron affinity or LCAO bond strength, but rather depends on the precise processing and substrate conditions prior to and during the heterostructure growth. Examples are presented of dependences on crystallographic orientation, semiconductor crystallinity, and growth parameters (such as deposition sequence and rate, substrate temperature and preparation). The link between the chemistry and structure at the heterojunction interface is demonstrated by measurements of the evolution of GaAs/Ge(100) “band” offsets as the nanostructure changes during growth for a variety of starting GaAs surface stoichiometries and reconstructions. Such chemical rearrangements are controlled by local, interatomic interactions at the free surfaces, neither bulk, equilibrium thermodynamic parameters (such as enthalpy of formation) nor free surface parameters (reconstruction, or surface stoichiometry) are important. Rather, the free surface and interface phase diagrams must be understood to give proper consideration to the stable or even metastable compounds which form. In order to accommodate such energy considerations as surface-induced strain and electrostatic dipole forces, the “abrupt” epitaxial heteroj unction will have an interface whose extent may be greater than a single atomic layer even for nonpolar orientations of compound semiconductors. With this understanding, we consider the possibilities for experimentally adjusting the band edge discontinuity. Atomic site selection during growth, creation of stabilizing phases and other growth and processing possibilities are also explored.

Quantitative comparison of direct and derivative AES with XPS of metal sulfides

AbstractIntegral area measurements of Auger intensities for various metal sulfides give comparable quantitative results to those obtained from derivative peak‐to‐peak heights. Highest precisions for sulfur sensitivity factors were obtained by: (a) intensity measurements involving integration of background‐subtracted N(E) lineshapes, (b) matrix corrections for electron mean free path and backscattering factor, (c) the use of XPS‐derived surface stoichiometries rather than assumed bulk stoichiometries for the metal sulfide standards. Despite the inherent inaccuracies of XPS quantitation, the XPS results help to correct for surface deviations from bulk stoichiometry owing to surface contamination, facile S surface oxidation, and preferential S loss usually observed during Ar+ sputter cleaning. The integral area Auger measurement techniques are particularly promising for electron‐beam sensitive materials which often cannot be characterized using high primary electron doses and conventional energy‐analyzer potential modulation differentiation.

X-ray photoelectron spectroscopy of TiO 2 and other titanate electrodes and various standard Titanium oxide materials: Surface compositional changes of the TiO 2 electrode during photoelectrolysis

Surface compositional changes were observed for TiO 2 single crystal electrodes used for photoelectrolysis of water. Surface stoichiometries of several types of TiO 2, SrTiO 3 and BaTiO 3 electrodes were characterized by XPS and compared with a variety of titanium, titanium oxide and titanium hydride standard materials. Reduction of the electrode surface in a hydrogen atmosphere results in an oxygen deficient surface composition. Photoelectrolysis at current densities of 10–15 mA cm 2 for periods up to 8 h appears to return the electrode surface to a nearly stoichiometric oxygen-to-metal ratio. Reduction of the titanium oxide surfaces was also observed by exposure to an argon ion beam. Analysis of the electrode surface by a combination of XPS and ion-sputter profiling was still possible by simultaneous analysis of standard materials.

Single-crystal-aluminum Schottky-barrier diodes prepared by molecular-beam epitaxy (MBE) on GaAs

Metal-semiconductor surface barriers were formed by epitaxially grown Al(001) on GaAs(001) with MBE at room temperature. The diodes exhibit nearly ideal electrical characteristics. It is also demonstrated that different barrier heights may be achieved by growing Al layers on MBE GaAs surfaces with different surface stoichiometries.

Geometry and interactions of hydrogen outside a platinum (111) surface

A pair potential description of surface H-H interactions is shown to explain recent experimental information on surface stoichiometries and heats of adsorption and desorption.