Sharp Low Energy Electron Diffraction Research Articles

Nearly-grazing-incidence-high-temperature sputtering of Ruthenium(0001) surface

An atomically clean and ordered Ru(0001) surface with sharp low energy electron diffraction (LEED) spots, dispersing bands in angle-resolved photoemission spectroscopy, and the intensity-voltage (I-V) curves from LEED similar to the data existing in literature has been obtained by nearly-grazing-incidence-high-temperature sputtering at 973 K. This method of surface preparation is advantageous because it requires significantly lower temperature compared to 1800–1300 K used earlier, and also does not require any oxygen exposure. Both from the experiment as well as Monte Carlo calculations, the efficacy of the nearly-grazing incidence angle of sputtering (70°) is shown.

Surface relaxation and rumpling of Sn-doped β−Ga2O3(010)

We have studied the surface structure of single-crystal, wide-gap semiconductor $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}(010)$ using x-ray photoelectron diffraction (XPD), low-energy electron diffraction (LEED), and x-ray photoelectron spectroscopy (XPS). The XPS measurements show typical spectra for stoichiometric ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}(010)$. Annealing in vacuum produced a sharp ($1\ifmmode\times\else\texttimes\fi{}1$) LEED pattern, characteristic of the monoclinic structure. The XPD angular anisotropies were collected for the Ga $2{p}_{3/2}$ and O $1s$ core levels. Surface interlayer relaxation up to 8% of the bulk interplanar distance and 0.11--0.14 $\AA{}$ rumpling are observed at the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}(010)$ surface. At the surface, the oxygen atoms shift toward the vacuum with respect to the gallium atoms. The rumpling decreases to zero and and the interplanar distance reaches the bulk value of 1.52 $\AA{}$ by the sixth atomic layer. The surface structure agrees with that predicted by first-principles density functional theory calculations which, in addition, suggest a significant band gap narrowing of $\ensuremath{\approx}1$ eV in the surface layer, due to surface states spatially localized on surface oxygen atoms of ${\mathrm{O}}_{\mathrm{II}}$ type.

Surface Structural Analysis of the Layered Perovskite Ca1.9Sr0.1RuO4 by Low Energy Electron Diffraction I-V

Abstract – The atomic structure at surface of the layered perovskite Ca1.9Sr0.1RuO4 has been studied by Low Energy Electron Diffraction (LEED) I-V. The perovskite Ca1.9Sr0.1RuO4 of single crystal was cleaved in ultra high vacuum chamber (the pressure in the chamber was about 1x10-10 Torr). The experiments were conducted at room temperature (T=300 K). The sharp LEED pattern was observed which indicates that the surface of Ca1.9Sr0.1RuO4 is flat and it is a well ordered crystal. LEED I-V data, nine equivalent beams of the layered perovskite Ca1.9Sr0.1RuO4 were recorded at room temperature. LEED I-V calculation was performed to fit experimental data to obtain the surface atomic structure. The LEED I-V analysis reveals that in the surface of the layered perovskite Ca1.9Sr0.1RuO4 the RuO6 octahedra are rotated (in-plane rotation) alternating clockwise and counterclockwise. The in-plane rotation at the surface is 11 degree which is smaller than that in the bulk (13 degree). The Ru – O(1) bond-length at the surface is found to be 1.936 Å which is about the same as in the bulk (1.939 Å). The Ru – O(2) bond length at the surface is 1.863 Å which is much shorter than that in the bulk (2.040 Å). The volume of octahedral Ru-O6 at the surface is reduced by 9% with respect to the bulk. This finding shows that the atomic structure at surface of the layered perovskite Ca1.9Sr0.1RuO4is significantly different than that in the bulk. These lattice distortions strongly influence its electronic properties. Key words: Transition Metal Oxide; Perovskite; Surface Atomic Structure; LEED I-V

Growth and characterization of ultrathin epitaxial MnO film on Ag(001)

We present here a comprehensive growth procedure to obtain a well-ordered MnO(001) ultrathin film on Ag(001) substrate. Depending upon the oxygen partial pressure during the growth, different phases of manganese oxide have been detected by Low Energy Electron Diffraction (LEED) and X-ray Photoelectron Spectroscopic (XPS) studies. A modified growth scheme has been adopted to get well-ordered and stoichiometric MnO(001) ultrathin film. The detailed growth mechanism of epitaxial MnO film on Ag(001) has been studied step by step, using LEED and XPS techniques. Observation of sharp (1×1) LEED pattern with a low inelastic background, corresponds to a long-range atomic order with low defect densities indicating the high structural quality of the film. The Mn 2p and Mn 3s core-level spectra confirm the oxidation state as well as the stoichiometry of the grown MnO films. Apart from the growth optimization, the evolution of strain relaxation of the MnO(001) film with film thickness has been explored.

Synthesis and characterization of surface oxide films on CoGa(100)

It has been shown that a Ga2O3 film forms on the surface of CoGa alloy crystals when exposed to oxygen (Pan, 2001 and Vlad, 2010). In this work we report the results of the characterization of surface oxides on CoGa(100) using X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and ion scattering spectroscopy (ISS). The oxides were synthesized using either O2 or NO2 as the oxidant at 300K or in excess of 700K. ISS scans showed that cobalt was always present in the top surface layer regardless of oxidation conditions. XPS showed that depending on the oxidant and the temperature, the composition of the oxide films vary depending on oxidation treatment, with some oxides being nearly all Ga2O3 and ordered with a sharp LEED pattern consisting of (2×1) domains rotated by 90º and others being Co–Ga mixed oxides that gave no diffraction pattern.

Quantum size effects in Bi films grown on GaAs (110)

Photoemission results, obtained with a helium-discharge lamp on the Bi/GaAs(110) interface shows strong modifications in the electronic structure of the low temperature deposited Bi overlayer upon annealing to room temperature, accompanied by developing of a sharp low energy electron-diffraction pattern. This behavior is consistent with the hypothesis of quantum size effects influence on growth morphology.

Interface and electronic characterization of thin epitaxial [formula omitted] films

The interface and electronic structure of thin (∼20–74 nm) Co 3 O 4 (1 1 0) epitaxial films grown by oxygen-assisted molecular beam epitaxy on MgAl 2 O 4 (1 1 0) single crystal substrates have been investigated by means of real and reciprocal space techniques. As-grown film surfaces are found to be relatively disordered and exhibit an oblique low energy electron diffraction (LEED) pattern associated with the O-rich CoO 2 bulk termination of the (1 1 0) surface. Interface and bulk film structure are found to improve significantly with post-growth annealing at 820 K in air and display sharp rectangular LEED patterns, suggesting a surface stoichiometry of the alternative Co 2 O 2 bulk termination of the (1 1 0) surface. Non-contact atomic force microscopy demonstrates the presence of wide terraces separated by atomic steps in the annealed films that are not present in the as-grown structures; the step height of ≈2.7 Å corresponds to two atomic layers and confirms a single termination for the annealed films, consistent with the LEED results. A model of the ( 1 × 1 ) surfaces that allows for compensation of the polar surfaces is presented.

Surface studies of hydrogen etched 3C-SiC(001) on Si(001)

The morphology and structure of 3C-SiC(001) surfaces, grown on Si(001) and prepared via hydrogen etching, are studied using atomic force microscopy (AFM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). On the etched samples, flat surfaces with large terraces and atomic steps are revealed by AFM. In ultrahigh vacuum a sharp LEED pattern with an approximate (5×1) periodicity is observed. AES studies reveal a “bulklike” composition up to the near surface region and indicate that an overlayer consisting of a weakly bound silicon oxide monolayer is present.

Growth of ultra-thin cerium oxide layers on Cu(1 1 1)

The reactive vacuum deposition of CeO 2 on Cu(1 1 1) surface in oxygen atmosphere provides high quality epitaxial ceria overlayers. We report the growth characteristics of Ce oxide, the structures, and the temperature stability of the oxide phases as investigated by low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy. We find that Ce oxide on the Cu(1 1 1) grows initially in the form of islands giving sharp hexagonal LEED pattern of the CeO 2(1 1 1) structure corresponding to the (1.5 × 1.5) structure. The CeO 2–Cu(1 1 1) films exhibited mixed valence states and temperature dependent CeO 2–Ce 2O 3 transition above 900 K due to the vacuum annealing. The transition progressed more rapidly at the surface, probably by formation of oxygen vacancies.

Study of phase transitions within alumina grown on top of CoAl (1 0 0) surface

The oxidation of CoAl(1 0 0) was investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), high resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM). In the case of alumina grown after oxidation with 15,000 L at room temperature, the characteristic EEL spectrum, along with the sharp (2 × 1) LEED pattern observed after annealing at 1000 K, showed that a stable well-ordered θ alumina was formed. The intermediate phase was found after heat treatment at 1150 K. At higher temperatures, the formation of the α-like alumina was observed. The comparative study of as-oxidation versus subsequent annealing of amorphous alumina at high-temperatures revealed a close similarity between the structures of alumina.

Surface composition and structure of nickel ultra-thin films deposited on Pd(111)

Ultra-thin nickel films deposited on the Pd(111) surface were characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). Up to 3 ML coverage, a LEED (1 × 1) pattern with a diffuse background due to a random distribution of Ni atoms on the surface is observed. Annealing at 600 °C reduced the background drastically and sharp (1 × 1) spots appeared on the screen, but XPS showed no presence of nickel on the surface, indicating diffusion into the bulk. Annealing at 300 °C for 30 min yielded also a sharp (1 × 1) LEED pattern, and the XPS Ni/Pd intensity ratio decreased with annealing time. The comparison between experimental and theoretical XPD results indicated that the surface was covered partially by Ni islands and partially by a random Ni–Pd surface alloy.

Comparative study of oxidation on Cu and Cu 3Au surfaces with a hyperthermal O 2 molecular beam

Dissociative adsorption of hyperthermal O 2 molecules on Cu 3Au(1 0 0) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. Comparing the O-uptake curve on Cu 3Au to that on Cu, it was determined that the dissociative adsorption of O 2 implies a higher activation barrier and therefore less reactivity due to the Au alloying. The low-energy electron-diffraction (LEED) pattern of a c(2 × 2) for the clean surface turned into a sharp (1 × 1) LEED pattern during oxidation by a hyperthermal O 2 molecular beam, suggesting that O adsorption induces Cu-atom segregation on the surface.

Mechanism of low energy S + ion bombarded p-GaAs (100)

X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) have been used to characterize the chemical structure and site location of sulfur atom on p-GaAs (100) treated by bombardment of low energy S + ions over the range from 10 to 100 eV. S + ion bombardment resulted in the formation of Ga–S and As–S species on GaAs surface. It was found that the S + ions with energy above 50 eV were more effective in formation of Ga–S species, which assisted the GaAs (100) surface in reconstruction into an ordered (1 × 1) structure upon annealing. After taking into account physical damage due to the process of ion bombardment, we found that 50 eV was the optimal ion energy to form Ga–S species in the sulfur passivation of GaAs (100). The subsequent annealing process removed both donor and acceptor states that were introduced during the ion bombardment of GaAs, and resulted in a sharp (1 × 1) LEED pattern.

SiC pore surfaces: Surface studies of 4H–SiC(11¯02) and 4H–SiC(1¯102¯)

The morphology and atomic structure of 4H–SiC(11¯02) and 4H–SiC(1¯102¯) surfaces, i.e., the surfaces found in the triangular channels of porous 4H–SiC, have been investigated using atomic force microscopy, low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). After hydrogen etching, the surfaces show steps parallel and perpendicular to the c axis, yet drastically different morphologies for the two isomorphic orientations. Both surfaces immediately display a sharp LEED pattern. Together with the presence of oxygen in the AES spectra, this indicates the development of an ordered oxide. Both surfaces show an oxygen-free well-ordered surface after Si deposition and annealing.

Atomic Structure of Non-Basal-Plane SiC Surfaces: Hydrogen Etching and Surface Phase Transformations

AbstractA-plane (11-20) and diagonal cut (1-102) and (-110-2) surfaces of 4H-SiC have been investigated using atomic force microscopy (AFM), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM). After hydrogen etching the surfaces show large, flat terraces. On SiC(11-20) steps down to single atomic heights are observed. On the diagonal cut surfaces steps run parallel and perpendicular to the [-1101] direction, yet drastically different morphologies for the two isomorphic orientations are found. All surfaces immediately display a sharp LEED pattern. For SiC(1-102) and SiC(-110-2) the additional significant presence of oxygen in the AES spectra indicates the development of an ordered oxide. All three surfaces show an oxygen free, well ordered surface after Si deposition and annealing. A transformation between different surface phases is observed upon annealing.

Epitaxial growth of Al on Si(1 1 1) with Cu buffer layers

The structure of thin Al films grown on Si(1 1 1) with thin Cu buffer layers has been investigated using synchrotron radiation photoemission spectroscopy. A thin Cu(1 1 1) layer between the Si(1 1 1) substrate and an Al film may enhance quantum well effects in the Al film significantly. The strength of quantum well effects has been investigated qualitatively with respect to the thickness of the Cu buffer layer and to the Al film thickness. Deposition of Cu on Si(1 1 1)7 × 7 leads to formation of a disordered silicide layer in an initial regime before a well-ordered Cu(1 1 1) film is formed after deposition of the equivalent of 6 layers of Cu. In the regime below 6 layers of Cu the disorder is transferred to Al layers subsequently grown on top. The initial growth of up to 8 layers of Al on a well-ordered Si/Cu(1 1 1) layer leads to a disordered film due to the lattice mismatch between the two metals. When the Cu buffer layer and the Al over-layer are above 6 and 8 layers, respectively the Al film shows sharp low energy electron diffraction patterns and very sharp quantum well peaks in the valence band spectra signalling good epitaxial growth.

Photoemission study on the Sb-induced reconstruction of the Si(1 1 2) surface

This study investigated the Sb-induced reconstruction of the Si(1 1 2)2 × 1 surface using low energy electron diffraction (LEED) and synchrotron radiation photoelectron spectroscopy (SRPES). When Sb was deposited on the clean Si(1 1 2) surface at 300 °C, a sharp hexagonal LEED pattern with weak “ n/5” fractional spots was observed, which is very similar to one of the Sb/Si(1 1 1)1 × 1 surfaces. The Si 2p and Sb 4d core level photoemission spectra were measured. The analysis results of the SRPES showed that the Si 2p core-level spectrum is composed of two surface-related components: a surface component at +0.2 eV and a small surface-related component at –0.2 eV with respect to the bulk component. In addition, the Sb 4d core-level spectrum was well reproduced with one doublet component. The results revealed that Sb atoms were adsorbed at unique sites on a Si(1 1 1)1 × 1 surface. On the basis of the nanofacet structure model of the Sb/Si(1 1 2) surface suggested by a previous scanning tunneling microscopy (STM) study, the structural model of the Sb/Si(1 1 2) surface was compared with the LEED pattern and the SRPES results.

Reactive growth of NiO ultrathin films on Pd(1 0 0): a multitechnique approach

Reactive growth of NiO ultrathin films on Pd(1 0 0) has been performed by evaporating metallic Ni in an oxygen atmosphere. The evolution of the ultrathin film is followed by means of low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD) and scanning tunnelling microscopy (STM). The first monolayer (ML) of the deposited oxide develops as a completely wetting 2D overlayer with a c(4 × 2) periodicity, as shown by sharp LEED patterns and atomically resolved STM images. Further NiO deposition results in 3D NiO(1 0 0) islands formation on-top the c(4 × 2) superstructure, as shown by STM images and XPD data. Two layers thick islands are shown to be pseudomorphic to the substrate, i.e. characterised by in-plane compressive strain and interlayer expansion. LEED, XPD and STM give independent yet converging evidence that partial strain relaxation occurs within the third monolayer.

X-ray standing wave study of the Sr/Si( [formula omitted])-( [formula omitted]) surface

Sub-monolayer surface phases of Sr on Si(0 0 1) have been studied with low-energy electron diffraction (LEED) and X-ray standing waves (XSW). A (3 × 1) phase was observed after depositing 0.6–0.8 ML Sr on room-temperature Si(0 0 1). Annealing at 750–800 °C caused a portion of the Sr to desorb and resulted in a sharp (2 × 3) LEED pattern. Normal Si(0 0 4) and off-normal Si(0 2 2) and Si(1 1 1) XSW measurements made on the (2 × 3) phase indicate that Sr atoms must sit at either cave or bridge sites. The XSW results also suggest that if a sufficiently low anneal temperature is used, the (2 × 3) phase co-exists with short-range ordered regions of Sr atoms located at valley-bridge sites.

Surface Characterization and Electrochemical Behavior of Well-Defined Pt−Pd{111} Single-Crystal Surfaces: A Comparative Study Using Pt{111} and Palladium-Modified Pt{111} Electrodes

Two Pd−Pt{111} single-crystal alloy surfaces have been prepared with 6.25% and 25% Pd bulk composition. Their surface structure and composition have been characterized both in ultrahigh vacuum (UHV) using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and low-energy ion scattering (LEIS) and electrochemically using cyclic voltammetry (CV). Comparison has been made with the voltammetric response of Pt{111} and Pt{111} modified with a monolayer of palladium [Pd−Pt{111}]. The alloy crystals when clean and thermally annealed in UHV gave rise to sharp (1 × 1) LEED patterns. AES and LEIS revealed that a surface enrichment in palladium took place in agreement with an earlier study using polycrystalline Pd−Pt alloy samples. Gentle ion etching of the thermally equilibrated surface (to remove the selvedge region) followed by AES and LEIS analysis established that the bulk composition was in precise agreement with the nominal bulk composition expected on the basis of the amount of palladi...