Experimental X-ray Photoelectron Diffraction Research Articles

Structural determination of a W(001)c(2 x 2)-Ag surface by x-ray photoelectron diffraction with multiple-scattering analysis.

We have studied x-ray photoelectron diffraction (XPD) for Ag 3d emission from the W(001)c(2\ifmmode\times\else\texttimes\fi{}2)-Ag surface with both full multiple-scattering cluster (MSC) and single-scattering cluster analyses. Quantitative structural parameters are obtained for the top layer of the Ag-W alloy structure. Systematic reliability factor searches based on MSC have shown that the outermost layer of the W(001)c(2\ifmmode\times\else\texttimes\fi{}2)-Ag surface forms a rumpled Ag-W alloy structure, and the plane containing Ag atoms is 0.3\ifmmode\pm\else\textpm\fi{}0.1 \AA{} higher than the tungsten top layer. It has also been found that, in this system, there is a strong multiple-scattering effect in the photoelectron diffraction even for a high kinetic energy of 1115 eV, where single-scattering theory was generally assumed to be valid. This indicates that, for high Z and closely packed systems, the kinematical method is not suitable for the quantitative analysis of XPD experiments.

Inverted photoelectron diffraction: a new technique for surface structure determination

A reversal of the conventional X-ray photoelectron diffraction (XPD) experiment is described in which an incident electron beam is diffracted to reach different sites within a crystal with differing relative probabilities, which are expected to mirror those for the escape of photoelectrons from the same sites in the XPD experiment. The diffracted-electron flux arriving at each site is monitored by measuring the intensity of the characteristic X-ray emission following core ionisation by the incident electrons. This new approach potentially offers advantages over XPD in respect of minimum sample size and angular resolution in relation to equipment costs. Preliminary results from a GaAs crystal confirm that a clear correlation exists between results from the new technique and from XPD.

Local epitaxy of Ag on Bi2Sr2CaCu2O8+x(001).

Thin films of Ag have been deposited onto cleaved ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{x}}$(001) surfaces at room temperature. Ag 3d x-ray photoelectron diffraction experiments show very poor local order at coverages of up to 7 \AA{}. For higher Ag coverages, a distinct diffraction pattern is forming, indicative of local epitaxy in the form of two domains of Ag(110) patches with one diagonal of the rectangular surface unit cell aligned along the substrate a axis. Comparison with previously published scanning-tunneling-microscopy results [Y. S. Luo et al., Phys. Rev. B 46, 1114 (1992)] leads us to the conclusion that Ag epitaxy is promoted by local disruption of the substrate upon initial Ag deposition.

Nucleation and growth of gallium arsenide on silicon

We have studied the early stages of growth of GaAs on Si (001), misoriented by 4° towards [110], by migration-enhanced epitaxy (MEE) in a molecular beam epitaxy (MBE) system. We present results using in situ and ex situ analyses, reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD) and X-ray double crystal diffractometry (DCD) performed at each stage of growth. We show that the nucleation by MEE induces a surface roughness decreasing as the layer becomes thicker. XPD experiments at the onset of the growth show a stoichiometric GaAs without antiphase domains. The relaxation of stress for the layers deposited at low temperature (300°C) occurs via partial dislocation migration developing between them, stacking faults and microtwins. The post annealing of films with thicknesses less than 60 nm drives the formation of three-dimensional islands on the silicon surface. These islands develop (114) facets in the [ 1 1 ¯ 0 ] direction where the Ga migration is greater. The height over the base ratio of these islands is uniform and from Bauer's relation, we calculate the interface energy which can be correlated to the strain energy due to the dislocations near the interface. There are two reasons for the restruction of the surface. First, from a thermodynamical approach including surfaces and interface energies of the GaAs/Si system, we can demonstrate that the growth of GaAs/Si is a three-dimensional Volmer-Weber growth. Second, the bulk energy due to the compressive strain of a continuous pseudomorphic GaAs film is higher than that of an islanding GaAs where relaxation from the free surfaces of the islands occurs. The post annealing of films with thickness higher than 60 nm has a smoothing effect and can give a perfect two-dimensional (001) surface. Growth at higher temperatures (580°C) suppresses plane defects, creates a Lomer dislocation network at the interface and induces the 60° dislocations in the bulk. We perform X-ray DCD experiments on 90 nm thick layers. We observe, before annealing a compressive biaxial stress of about −1.9×10 8Pa which is the driving force for the elimination of the plane defects.

Surface structure of Si(001) treated by hydrogen and argon electron cyclotron resonance plasmas

Si(001) surfaces subjected to H 2 or Ar ECR plasma irradiation are studied, in situ, from the standpoints of both impurity removal and induced crystallographic damage. The atomic cleanliness is checked by XPS (X-ray photoelectron spectroscopy) and UPS (ultra-violet photoelectron spectroscopy), while surface crystallographic information given by LEED and XPD (X-ray photoelectron diffraction) experiments. As an H ion-source, the ECR plant appears to be a convenient hydrogenation source, with low damage, able to passivate the surface in the usual hydrogenated LEED phases (dihydride 1 × 1 or monohydride 2 × 1) depending on the employed substrate temperature T. It presents nevertheless poor etching properties concerning the dioxide overlayer in our low plasma pressure domain (<5×10 −4 mbar). On the other hand, as an Ar ion source, the ECR plasma is more efficient to etch physically and clean, particularly at low working pressure and aided by a DC negative bias voltage and T s increase but suffers from more crystallographic perturbations checked by the LEED disappearance and quantified by the decrease of the anisotropy factor related to the XPD contrast. Finally, a procedure which combines exposures to the cleaning Ar ions followed by a refinement Si etching of the damaged overlayers using the H plasma allows the attainment of clean reconstructed 2 × 1 surfaces with processing temperatures limited at 500°C and suitable for subsequent epitaxial growths.

X-ray photoelectron diffraction as a tool for clean surface crystallography studies: the Pt(110)(2 × 1) reconstruction

For the first time, a clean metal surface reconstruction, viz. Pt(110)(2 × 1), is investigated using X-ray photoelectron diffraction (XPD). Comparison of the obtained experimental XPD azimuthal and polar curves with quasidynamical calculations reveals that the Pt(110)(2 × 1) reconstruction is, as expected, of the “missing row model” type and, furthermore, presents a first layer contraction estimated to be around 30%. The result is compared with previous LEED results.

X-ray photoelectron diffraction as a tool for clean surface crystallography studies: the Pt(110) (2 × 1) reconstruction

For the first time, a clean metal surface reconstruction, viz. Pt(110)(2 × 1), is investigated using X-ray photoelectron diffraction (XPD). Comparison of the obtained experimental XPD azimuthal and polar curves with quasidynamical calculations reveals that the Pt(110)(2 × 1) reconstruction is, as expected, of the “missing row model” type and, furthermore, presents a first layer contraction estimated to be around 30%. The result is compared with previous LEED results.

Elastic and inelastic electron mean free paths from x-ray photoelectron diffraction experiments

Angle-resolved detection of Pb4f72 photoelectrons from a Pb(110) crystal was used in conjunction with deposited K overlayers to determine the elastic and inelastic mean free paths of electrons at 1345 eV energy in K. The angle-resolved 4f72 intensity distribution along the [11̄0] azimuth showed large forward scattering enhancements whose attenuation by K overlayers was evaluated to yield an effective electron mean free path of 7.0 ± 1 Å. The inelastic and elastic mean free paths in K were determined as 45 ± 5 and 8 ± 4 Å, respectively. The high cross section for elastic scattering makes angle-resolved XPS (or Auger spectroscopy) a much more sensitive tool for detecting small coverages of adsorbates than its angle-integrated counterpart.

Polarity determination of the HgCdTe(111) surface by azimuthal X-ray photoelectron diffraction experiments

Azimuthal X-ray photoelectron diffraction (APD) experiments have been carried out on a sample of Hg0.78Cd0.22 Te (MCT) epitaxially grown by liquid phase epitaxy (LPE) on (111) CdTe. To our knowledge this is the first XPD study of the (111) face of a II-VI semiconductor. Reproducible diffraction features are seen for emission from core levels of Hg, Cd and Te both before and after minimum ion bombardment, with bombardment being done to remove the native oxide overlayer. Single scattering cluster (SSC) calculations have been carried out using a spherical wave (SW) model. From comparison between experiment and theory an absolute determination of the surface polarity is possible.

New evidence for segregation of Pt on Pt 50Ni 50(111) alloy surface from forward scattering photoelectron diffraction

It has been shown that on Pt 50 Ni 50 (111), Pt segregates to the surface and forms the uppermost layer. X-ray photoelectron diffraction experiments appear to be useful for investigating this segregation. We found that Ni 2p azimuthal plots exhibit stronger anisotropy than Pt 4f does. This is new evidence for Pt segregation at the surface. The results are interpreted in terms of multiple forward scattering. The calculations have been done for different Pt surface concentrations in order to estimate the degree of segregation.