Thin PTCDA films on Si(0 0 1): 1. Growth mode

We have studied the interface and thin film formation of the organic molecular semiconductor 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA) on clean and on hydrogen passivated Si(0 0 1) surfaces. The studies were made by means of high resolution X-ray photoelectron spectroscopy (HRXPS), near edge X-ray absorption fine structure (NEXAFS), low energy electron diffraction (LEED), and atomic force microscopy (AFM). On the passivated surface the LEED pattern is somewhat diffuse but reveals that the molecules grow in several ordered domains with equivalent orientations to the substrate. NEXAFS shows that the molecules are lying flat on the substrate. The Si 2p XPS line shape is not affected when the film is deposited so it can be concluded that the interaction at the interface between PTCDA and the substrate is weak. The evolution of the film formation appears to be homogeneous for the first monolayer with a nearly complete coverage of flat lying molecules based on the XPS attenuation. For layer thickness of 0.5-2 monolayers (ML) the molecules start to form islands, attracting the molecules in between, leaving the substrate partly uncovered. For thicker films there is a Stranski-Krastanov growth mode with thick islands and a monolayer thick film in between. For the clean surface the ordering of the film is much lower and angle resolved photoelectron spectroscopy (ARPES) of the molecular orbitals have only a small dependence of the emission angle. NEXAFS shows that the molecules do not lie flat on the surface and also reveal a chemical interaction at the interface.     

Influence of the surface-termination of hexagonal SiC(0 0 0 1) on the temperature dependences of Ge growth modes and desorption

With the aim of comparing initial Ge adsorption and desorption modes on different surface terminations of 4H-SiC(0 0 0 1) faces, 3 × 3, √3×√3R30° (R3) and 6√3×6√3R30° (6R3) reconstructions, of decreasing Si surface richness, have been prepared by standard surface preparation procedures. They are controlled by reflection high energy electron diffraction (RHEED), low energy electron diffraction and photoemission. One monolayer of Ge has been deposited similarly at room temperature on each of these three surfaces, followed by the same set of isochronal heatings at increasing temperatures up to complete Ge desorption. At each step of heating, the structural and chemical status of the Ge ad-layer has been probed. Marked differences between the Si- (3 × 3 and R3) and C-rich (6R3) terminations have been obtained. Ge wetting layers are only obtained up to 400 °C on 3 × 3 and R3 surfaces in the form of a 4 × 4 reconstruction. The wetting is more complete on the R3 surface, whose atomic structure is the closest to an ideally Si-terminated 1 × 1 SiC surface. At higher temperatures, the wetting layer stage transiets in Ge polycrystallites followed by the unexpected appearance on the 3 × 3 surface of a more ordered Si island structure. It denotes a Si clustering of the initial Si 3 × 3 excess, induced by the presence of Ge. A phase separation mechanism between Si and Ge prevails therefore over alloying by Ge supply onto a such Si-terminated 3 × 3 surface. Conversely, no wetting is obtained on the 6R3 surface and island formation of exclusively pure Ge takes place already at low temperature. These islands exhibit a better epitaxial relationship characterized by Ge(1 1 1)//SiC(0 0 0 1) and Ge〈1 1 −2〉//SiC〈1 −1 0 0〉, ascertained by a clear RHEED spot pattern. The absence of any Ge-C bond signature in the X-ray photoelectron spectroscopy Ge core lines indicates a dominant island nucleation on heterogeneous regions of the surface denuded by the 6R3 graphite pavings. Owing to the used annealing cycles, the deposited Ge amount desorbs on the three surfaces at differentiated temperatures ranging from 950 to 1200 °C. These differences probably reflect the varying morphologies formed at lower temperature on the different surfaces. Considering all these results, the use of imperfect 6R3 surfaces appears to be suited to promote the formation of pure and coherent Ge islands on SiC.     

Model NSR catalysts: Fabrication and reactivity of barium oxide layers on Cu(1 1 1)

The growth of barium oxide on a Cu(1 1 1) substrate, formed by the deposition of barium and its subsequent oxidation, yields stable BaO films which expose predominantly the BaO(1 0 0) surface. The interaction of the oxide films with common components of motor-vehicle exhaust gases (CO₂, H₂O, NO_x) has been studied using surface analytical techniques, including X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and reflection IR spectroscopy (RAIRS). The spectroscopic identification of Ba(OH)₂, BaCO₃ and Ba(NO₂)₂ phases is discussed, and the relative stabilities and decomposition mechanisms of these materials when supported on Cu(1 1 1) is revealed by a combination of TPD and XPS. BaO is shown to be resistant to reaction with pure NO and NO/O₂ mixtures, but exposure to NO₂ leads to the rapid formation of barium nitrite. The formation of the nitrite is proposed to be the first-step in the production of barium nitrate, which has previously been shown to be the main phase involved in NO_x storage and reduction (NSR) catalysis.     

Growth and annealing of zinc-blende CdSe thin films on GaAs (0 0 1) by molecular beam epitaxy

CdSe thin films have been grown on GaAs (0 0 1) substrates by molecular beam epitaxy (MBE). The effects of substrate temperature and annealing treatment on the structural properties of CdSe layers were investigated. The growth rate slightly decreases due to the accelerated desorption of Cd from CdSe surface with an increase in the temperature. The sample grown at 260 °C shows a polycrystalline structure with rough surface. As the temperature increases over 300 °C, crystalline CdSe (0 0 1) epilayers with zinc-blende structure are achieved and the structural quality is improved remarkably. The epilayer grown at 340 °C displays the narrowest full-width at half-maximum (FWHM) from (0 0 4) reflection in double-crystal X-ray rocking curve (DCXRC) and the smallest root-mean-square (RMS) roughness of 0.816 nm. Additionally, samples fabricated at 320 °C were annealed in air for 30 min to study the films’ thermal stability. X-ray diffraction (XRD) results indicate that the zinc-blende structure remains unchanged when the annealing temperature is elevated to 460 °C, meaning a good thermal stability of the cubic CdSe epilayers.     

Crystal growth of SrTiO3 films on H-terminated Si(1 1 1) with SrO buffer layers

Growth of epitaxial SrTiO₃ (STO) films has been examined on H-terminated Si(1 1 1) with SrO buffer layers. The epitaxial SrO buffer layers have reduced stress on H-terminated Si substrates. On the SrO buffer layers, the STO films grow epitaxially with triple domains at low temperature. Each STO domain has equivalent epitaxial relationship to SrO buffer layers, STO(1 1 0)∥SrO(1 1 1) and .     

Texture of molybdenum thin films on SrTiO3(1 0 0): A RHEED study

The structure of ultrathin Mo films on SrTiO₃(1 0 0) was studied by in situ reflection high-energy electron diffraction (RHEED). A different structure was observed for films less than 20 Å thick than for thicker films. These films were epitaxial and had a metastable structure. Thicker films had the dimensions of equilibrium bcc Mo(1 1 0). Relaxation processes transformed the metastable Mo into bcc Mo, resulting in the following orientation relationships between Mo and SrTiO₃: (1 1 0)[0 0 1]_bcc Mo ∥ (1 0 0)[0 0 1]_SrTiO3 and (1 1 0)[1 1 1]_bcc Mo ∥ (1 0 0)[0 1 1]_SrTiO3. The formation of such specific orientations is related to transformations via the Bain and Needle Path, respectively.     

Growth and structure of vanadium oxide on anatase (1 0 1) terraces

The interaction of vanadium oxide with epitaxial anatase films exposing (1 0 1) terraces was characterized. The TiO₂ films were grown on vicinal LaAlO₃ (1 1 0) substrates by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE); reflection high energy and low energy electron diffraction (RHEED and LEED) indicated that the films exposed (1 0 1) terraces of the anatase TiO₂ polymorph. When a vanadium oxide monolayer was deposited onto the anatase surface by OPA-MBE at 725 K, only (1 × 1) RHEED and LEED patterns were observed. The V X-ray photoelectron spectroscopy (XPS) peak intensities indicated that the monolayer wetted the anatase surface and so the diffraction patterns were attributed to an epitaxial vanadia layer. Analysis of the vanadium oxide monolayer by X-ray and ultraviolet photoelectron spectroscopies revealed that the V was predominantly 5+. When the vanadia coverage was increased at 725 K, Auger electron spectra showed only very slow attenuation of the anatase Ti peaks while spots began to develop in RHEED patterns recorded along the LaAlO₃ direction; both indicative of 3-D cluster formation. In the orthogonal direction, the RHEED patterns showed unusual diagonal streaks. Meanwhile, the (1 × 1) LEED pattern persisted even after 30 nm of vanadia was deposited. This was attributed to gaps between the 3-D clusters exposing the epitaxial monolayer. Core level XPS spectra of the 3-D clusters revealed a broad V 2p_3/2 peak that was centered at the position expected for V⁴⁺ but could be deconvoluted into three peaks corresponding to V³⁺, V⁴⁺, and V⁵⁺. It is shown that crystallographic shear that accommodates such variations in the oxygen content of V oxides can lead to the diagonal streaks in RHEED patterns recorded along the LaAlO₃ [0 0 1] direction even as the pattern in the orthogonal direction shows sharp transmission spots. The results show that vanadia growth on anatase (1 0 1) proceeds through the Stranski-Krastanov mode with a strong vanadia-titania interaction stabilizing a dispersed vanadia monolayer. The results are compared with previous data for vanadia growth on anatase (0 0 1) where smooth, epitaxial VO₂ films grow ad infinitum.     

Initial stages of MnAs/GaAs(0 0 1) epitaxy studied by RHEED azimuthal scans

We study the nucleation phase of molecular beam epitaxy of (hexagonal) MnAs on (cubic) GaAs (0 0 1) using reflection high-energy electron diffraction (RHEED) azimuthal scans. The nucleation proceeds from a non-reconstructed initial stage through randomly oriented small nuclei and two orientation stages to the final single-phase epitaxial orientation. The fascinatingly complex nucleation process contains elements of both Volmer-Weber and Stranski-Krastanov growth. The measurement demonstrates the potential of high-resolution RHEED techniques to assess details of the surface structure during epitaxy.     

RT growth of acetonitrile and acrylonitrile on Si(0 0 1)-2 × 1 studied by XPS and LEED

In this work we show the adsorption of acetonitrile (CH₃CN) and acrylonitrile (CH₂CHCN) on Si(0 0 1)-2 × 1 at room temperature by increasing the molecular doses. Especially, by means of XPS and LEED data, we stress the action of these molecules on the silicon surface locating the dangling-bonds quasi-saturation within 10 L. The shortage of nitrogen XPS signal and some anomalies in carbon spectra point to an invading action from a traditional X-ray source (Al-K_α line) against chemisorbed molecules. In particular, we think that a long exposure to this radiation could break carbon-silicon bonds changing some adsorption geometries and making desorb molecular fragments.     

Ultra-thin Si overlayers on the TiO2 (1 1 0)-(1 × 2) surface: Growth mode and electronic properties

The growth of thin subnanometric silicon films on TiO₂ (1 1 0)-(1 × 2) reconstructed surfaces at room temperature (RT) has been studied in situ by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS), Auger electron and electron-energy-loss spectroscopies (AES and ELS), quantitative low energy electron diffraction (LEED-IV), and scanning tunneling microscopy (STM). For Si coverage up to one monolayer, a heterogeneous layer is formed. Its composition consists of a mixture of different suboxides SiO_x (1 < x ? 2) on top of a further reduced TiO₂ surface. Upon Si coverage, the characteristic (1 × 2) LEED pattern from the substrate is completely attenuated, indicating absence of long-range order. Annealing the SiO_x overlayer results in the formation of suboxides with different stoichiometry. The LEED pattern recovers the characteristic TiO₂ (1 1 0)-(1 × 2) diagram. LEED I-V curves from both, substrate and overlayer, indicate the formation of nanometric sized SiO_x clusters.     

Influence of boron on the initial stages of Si molecular beam epitaxy on Si(1 1 1) studied by reflection high-energy electron diffraction

The influence of boron as a function of coverage on molecular beam epitaxial Si growth on Si(1 1 1) surface was systematically studied by reflection high-energy electron diffraction. At boron coverage above 0.3 monolayer regular oscillations occur with a period typically for two-bilayers, whereas at lower boron coverage a transient behaviour with irregular intensity oscillations was observed in the initial growth stages. This behaviour can be attributed to a superposition of a bilayer and a two-bilayer dominated growth mode. The appearance of these two growth modes is discussed in terms of an initial surface defect-induced nucleation of bilayer-high Si islands and the formation of two-bilayers-high Si islands on top of the van der Waals like boron-covered surface, respectively. We suggest that these surface defects are of intrinsic nature, and their number only depends on the amount of boron at the surface. The oscillations become regular during further Si deposition with a bilayer period, indicating a diminishing influence of the layer/substrate interface on the growth processes.     

Initial growth of platinum on oxygen-covered Ni(1 1 0) surfaces

The initial growth of Pt on the Ni(1 1 0)-(3 × 1)-O and NiO(1 1 0) surfaces has been studied by coaxial impact collision ion scattering spectroscopy (CAICISS), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). Prior to Pt deposition, the atomic structure of the near-surface regions of the Ni(1 1 0)-(3 × 1)-O and NiO(1 1 0) structures were studied using CAICISS, finding changes to the interlayer spacings due to the adsorption of oxygen. Deposition of Pt on the Ni(1 1 0)-(3 × 1)-O surface led to a random substitutional alloy in the near-surface region at Pt coverages both below and in excess of 1 ML. In contrast, when the surface was treated with 1800 L of atomic oxygen in order to form a NiO(1 1 0) surface, a thin Pt layer was formed upon room temperature Pt deposition. XPS and LEED data are presented throughout to support the CAICISS observations.     

Epitaxial growth of MnO/Ag(0 0 1) films

Among the 3d transition metal monoxides, MnO plays a particular role due to the high spin ground state of the 3d⁵ configuration of Mn²⁺. Therefore, MnO could be an excellent model system to investigate many kinds of spin dependent or magnetic interactions between electrons in the surface of an antiferromagnetic solid. But in practice, most experimental investigations on crystalline MnO(0 0 1) surfaces suffer from the bad structural quality of the surface of cleaved bulk crystals. In this paper, we report on the epitaxial growth of MnO/Ag(0 0 1) surfaces, which is proved to be a method to produce crystalline MnO(0 0 1) surfaces of high structural order.     

Epitaxial growth of cobalt clusters on the bare and the oxide film grown on NiAl(1 0 0) surfaces

The structure of the nano-sized cobalt clusters on bare NiAl(1 0 0) and an oxidized NiAl(1 0 0) surfaces have been investigated by AES, LEED and RHEED. The deposition of Co onto bare NiAl(1 0 0) at room temperature led to small crystalline Co grains and surface asperities of substrate. The latter is likely induced by replacement of surface Al, Ni atoms by Co deposit. At 800 K Co particles aggregate to form clusters, but incorporation of Co into bulk NiAl(1 0 0) could occur upon annealing at 900 K. On the other hand, pure face-centered cubic (fcc) phase of Co crystallites of ≈1 nm in diameter with inclusion of smaller-sized particles (D < 1 nm) are observed on Θ-Al₂O₃ after Co deposition at room temperature. After annealing the Co nano-clusters grow larger at expense of small particles (D ≈ 3 nm), where the [1 1 0] and [−1 1 0] axis of the Co(0 0 1) facets are parallel to the [1 0 0] and [0 1 0] directions of (0 0 1)oxide, respectively. The in-plane lattice constant of Co clusters is ca. 4% larger than that of bulk Co, yielding less strain at the Co/oxide interface. A 15° ± 10% random orientation of the normal to (0 0 1) facet of Co clusters with respect to (0 0 1)oxide surface was deduced from the “arc”-shape reflection spots in RHEED. These results suggest that both orientation and phase of Co clusters are strongly affected by the nature and structure of oxide surface.     

The epitaxial growth of Pd electrodeposition on Au(1 0 0) studied by LEED and RHEED

The epitaxial growth of Pd adlayers electrochemically deposited onto Au(1 0 0) has been studied by LEED, RHEED and AES. For the first 6 ML, the Pd deposits grow pseudomorphically on Au(1 0 0) with a lateral expansion of 4.5% with respect to bulk Pd. The strain in the expanded commensurate (1 × 1) Pd layers on Au(1 0 0) begins to be relieved at the Pd coverage between 6 and 9 ML range by formation of a compressed Pd film with respect to Au(1 0 0) surface and the compression increases continuously with thickness. At ca. 20 ML Pd the lattice constant of the film approaches to the bulk Pd and three-dimensional Pd islands develop since around 30 ML coverage. No superstructure due to the Pd-Au surface alloy can be found for coverages from monolayer up to 30 ML Pd on Au(1 0 0). A c(2 × 2) phase has been observed on the Pd-deposited Au(1 0 0) electrodes, which is ascribed to an ordered Cl adlayers adsorbed on Pd adlayers rather than a Pd-Au surface alloy.     

Me-PTCDI thin film deposition on InSb(1 1 1)A

The evolution of N,N′-dimethylperylene-3,4,9,10-dicarboxyimide (Me-PTCDI) thin films formed by vapour deposition on InSb(1 1 1)A substrates has been studied by X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED). XPS studies of the Me-PTCDI covered surface indicate that no significant interaction occurs at sub-monolayer coverage when compared to multilayer Me-PTCDI films. HREELS studies suggest only a weak interaction as evidenced by very small changes in the frequencies of several molecular vibrational modes. LEED patterns show the Me-PTCDI overlayer adopts a structure commensurate with the underlying InSb(1 1 1)A substrate surface and that can be rationalised by van der Waals intermolecular energy calculations for the Me-PTCDI unit cell. The results are consistent with a weak interaction at the Me-PTCDI/InSb interface, the formation of the commensurate structure being sufficient to overcome the small energetic penalty associated with deviation from the calculated intermolecular interaction energy minimum.     

CuPc molecules adsorbed on Au(1 1 0)-(1 × 2): growth morphology and evolution of valence band states

We present the growth morphology, the long-range ordering, and the evolution of the valence band electronic states of ultrathin films of copper phthalocyanine (CuPc) deposited on the Au(1 1 0)-(1 × 2) reconstructed surface, as a function of the organic molecule coverage. The low energy electron diffraction patterns present a (5 × 3) reconstruction from the early adsorption stages. High-resolution UV photoelectron spectroscopy data show the disappearance of the Au surface states related to the (1 × 2) reconstruction, and the presence of new electronic features related to the molecule-substrate interaction and to the CuPc molecular states. The CuPc highest occupied molecular orbital gradually emerges in the valence band, while the interface electronic states are quenched, upon increasing the coverage.     

Ultra-thin film growth of titanium dioxide on W(1 0 0)

We have employed low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy to follow the epitaxial growth of thin films of TiO₂ on W(1 0 0). The films were grown both by metal vapour deposition of titanium onto the substrate in UHV with subsequent annealing in a low partial pressure of oxygen, and by metal vapour deposition in a low partial pressure of oxygen. LEED patterns showed the characteristic patterns of (1 1 0) oriented rutile. A systematic spot splitting was observed and attributed to a stepped surface. The calculated step height was found to be in good agreement with that expected for rutile TiO₂(1 1 0), 3.3 Å. Titanium core level shifts were used to identify oxidation states as a function of film thickness allowing the interpretation in terms of a slightly sub-stoichiometric interface layer in contact with the substrate. In combination with the LEED patterns, the film structure is therefore determined to be (1 1 0) oriented rutile with a comparable level of stoichiometry to UHV prepared bulk crystals. The ordered step structure indicates considerable structural complexity of the surface.     

Anisotropic kinetics on growing Ge(0 0 1) surfaces

Reflection high-energy electron diffraction (RHEED), reflectance difference spectroscopy (RDS), and scanning tunneling microscopy (STM) have been used to study the anisotropic kinetics on the growing Ge(0 0 1) surface. While switching of dimer direction in alternate (2 × 1)/(1 × 2) layers causes the bilayer-period oscillations in RD response, RHEED oscillations are governed by variations in surface step densities. We show that the RHEED oscillations are strongly affected by the growth front morphology: when the growth front becomes distributed over several layers, the transition from bilayer- to monolayer-period occurs in RHEED oscillations.