L.I. Johansson

Electronic and structural properties of transition-metal carbide and nitride surfaces

Abstract Findings concerning the electronic and structural properties of clean surfaces of transition-metal carbides and nitrides are reviewed. Topics treated are: investigations of the occupied and unoccupied bulk band structure and of occurrence of surface and vacancy-induced states, occurrence of chemical shifts and surface shifts in core electron level binding energies, occurrence of surface relaxations/reconstructions on the low-index surfaces and of vacancy concentration gradients in the surface region. A considerable amount of information concerning these topics has been collected to date in both experimental and theoretical investigations. These results are summarized and discussed. The results also show, however, that much remains to be done before a detailed systematic knowledge about the structural and electronic properties and their interrelation has been achieved for this class of compounds.

Oxidation of cerium and titanium studied by photoelectron spectroscopy

Abstract Cerium and titanium are examples of reactive metals that form protective oxide films on their surfaces upon exposure to oxygen. In order to study the building up of these oxide films we have undertaken photoemission measurements with X-ray and ultraviolet radiation (ESCA and UPS). Cerium and titanium levels were studied using ESCA on freshly in situ evaporated metal films. These levels, as well as the O 1s level, were studied after different oxygen exposures. In order to vary the probing depth, spectra have been recorded at two different electron emission angles. All measurements were performed at room temperature. Effects of oxygen exposures upon core levels were quite different in the two metals. In the Ce 3d spectrum a strong peak related to the oxide was observed at an exposure of only 2 L. Such a strong peak was not observed in the Ti 2p spectrum even after an exposure of 3000 L. The valence band spectrum of Ti as observed both by UPS and ESCA measurements, however, changed significantly at much smaller exposures.

Morphology, Atomic and Electronic Structure of 6H‐SiC(0001) Surfaces

Recent findings concerning primarily the √3 x √3 and 6√3 x 6√3 reconstructed surfaces of 6H-SiC(0001) are reviewed. First, the morphology of some different types of 6H-SiC crystals is discussed. The scanning tunneling microscopy (STM) and atomic force microscopy (AFM) results presented show that surfaces with a morphology suitable for surface investigations can be prepared using sublimation- or hydrogen-etching. Then results obtained concerning the atomic and electronic structure for the reconstructed surfaces, prepared using an ex situ method for oxide removal and in situ heating, are presented. For the √3 x √3 reconstruction, recent STM and photoelectron spectroscopy (PES) data are discussed in view of available theoretical results. The STM images presented are shown to be consistent with a structural model of Si or C adatoms in threefold symmetric sites. The theoretical results favor Si adatoms in T4 sites as the optimal configuration for this reconstruction. However, the surface shifted components extracted in studies of the C 1s and Si 2p core levels and the location of a surface state band mapped out in angle resolved experiments cannot be explained using this structural model. At present, there is no structural model that satisfactorily can explain all experimental findings for the √3 x √3 reconstruction. A monocrystalline graphite overlayer on top of bulk-terminated or √3 x √3-reconstructed SiC has previously been proposed to explain the 6√3 x 6√3-reconstructed surface. However, STM and PES results are presented that unambiguously show that there is no graphite on the surface when a well developed 6√3 x 6√3 low-energy electron diffraction (LEED) pattern is observed. The STM images recorded during the gradual development of the 6√3 x 6√3 surface show growing fractions of pseudo-periodic 6 x 6 and 5 x 5 reconstructions. These reconstructed regions dominate on the surface, but small √3 x √3-reconstructed regions are still present when a well developed 6√3 x 6√3 LEED pattern is observed. It is shown that the 6 x 6,5 x 5 LEED pattern can be fully explained by scattering from surfaces with a mixture of 6 x 6, 5 x 5 and √3 x √3 reconstructions. Due to the complexity of the STM data, no structural model is proposed for the 6 x 6 and 5 x 5 reconstructions. STM and PES results are presented showing that graphitization of the surface is obtained only after heating at higher temperatures than that required for observing a well developed 6√3 x 6√3 LEED pattern. The STM images then show that the graphite appears as a monocrystalline overlayer on top of the 6 x 6 reconstruction and not on bulk-terminated or √3 x √3-reconstructed SiC(0001).

Surface Core Level Shifts of the Lanthanide Metals Ce58-Lu71: A Comprehensive Experimental Study

Synchrotron radiation excited photoelectron spectra of the 4f emission region for all lanthanide metals Ce58 to Lu71 (except Pm61) have been recorded. Photon energies ranging from 30 eV to 200 eV have been used to excite the 4f electrons with maximum surface sensitivity. The 4f emission spectra of all metals studied, with the exception of Ce, show 4f emission from the surface layer atoms and the bulk atoms as clearly separated structures with different binding energies. For Ce no unambiguous separation of the 4f structure into bulk and surface emission could be made. We fitted the experimental spectra with calculated 4f emission spectra obtained by adding a bulk and a shifted surface multiplet for each metal. The intensity ratios within the multiplets were obtained from an atomic calculation utilizing the intermediate coupling scheme. The intensity ratio between bulk and surface emission (IB/IS) and the magnitude of the surface core level shift (SCS) were used as fit parameters. The deduced SCSs are all positive (towards higher binding energy), 0.40 eV-0.77 eV, and in fair agreement with calculated SCSs using the Johansson-Martensson-Rosengren model. For the trivalent lanthanides we found a systematic variation of the SCSs through the series, which can be attributed to differences in the actual electron distribution between s, p and d-like valence states. For the trivalent Sm we found the surface to be completely divalent and deduced a surface shift of > 0.46 eV, being the energy necessary to change the surface valency.

ESCA studies of core level shifts and valence band structure in nonstoichiometric single crystals of titanium carbide

Abstract Core level shifts and valence band structure from six single crystals of TiC x have been studied by X-ray photoelectron spectroscopy (ESCA). The

Large homogeneous mono-/bi-layer graphene on 6H-SiC(0001) and buffer layer elimination

In this paper we discuss and review results of recent studies of epitaxial growth of graphene on silicon carbide. The presentation is focused on high quality, large and uniform layer graphene growth on the SiC(0 0 0 1) surface and the results of using different growth techniques and parameters are compared. This is an important subject because access to high-quality graphene sheets on a suitable substrate plays a crucial role for future electronics applications involving patterning. Different techniques used to characterize the graphene grown are summarized. We moreover show that atomic hydrogen exposures can convert a monolayer graphene sample on SiC(0 0 0 1) to bi-layer graphene without the carbon buffer layer. Thus, a new process to prepare large, homogeneous stable bi-layer graphene sheets on SiC(0 0 0 1) is presented. The process is shown to be reversible and should be very attractive for various applications, including hydrogen storage.

Analysis of the Formation Conditions for Large Area Epitaxial Graphene on SiC Substrates

We are aiming at understanding the graphene formation mechanism on different SiC polytypes (6H, 4H and 3C) and orientations with the ultimate goal to fabricate large area graphene (up to 2 inch) with controlled number of monolayers and spatial uniformity. To reach the objectives we are using high-temperature atmospheric pressure sublimation process in an inductively heated furnace. The epitaxial graphene is characterized by ARPES, LEEM and Raman spectroscopy. Theoretical studies are employed to get better insight of graphene patterns and stability. Reproducible results of single layer graphene on the Si-face of 6H and 4H-SiC polytypes have been attained. It is demonstrated that thickness uniformity of graphene is very sensitive to the substrate miscut.

Design and performance of a spherical grating monochromator used at MAX I

We report on the performance of a grazing incidence spherical grating monochromator installed at MAX I and designed to cover the photon energy range from about 15-200 eV with high resolving power. It is intended mainly for angle-resolved photoemission work. Therefore, both refocusing optics, to obtain a small spot size at the sample, and a higher order light suppressor, to reduce the content of higher orders, have been incorporated in the design. The theoretically calculated energy resolution is presented and compared to the resolution obtained in photoabsorption measurements of gas-phase He, Ne, Kr, Xe, and N2. The possibility to reduce the influence of higher orders is illustrated by photoemission data collected on Si (100). The experimental results show that the monochromator fulfills the expected design goals. (Less)

Interfacial investigation of in situ oxidation of 4H-SiC

An in situ oxidation study of v3 × v3 R30° reconstructed 4H-SiC(001) surfaces is reported. An intermediate oxidation state (interpreted to be Si+1) is revealed in core level photoemission spectra r ...

Surface state on the SiC(0001)-(√3 × √3) surface

An angle resolved photoemission study of a surface state on the SiC(0001)-(√3 × √3) surface is reported. Experiments carried out on the 6H and 4H polytypes give essentially identical results. A surface state band with semiconducting occupation is observed, centered around 1.0 eV above the valence band maximum (VBM) and with a width of about 0.2 eV. Recently calculated results for a Si-adatom-induced √3 × √3 reconstruction give a metallic surface state band centered about 1.2 eV above the VBM and with a width of 0.35 eV. The dispersion determined experimentally is smaller than calculated but exhibits the same trend, the surface state disperses downwards towards the VBM with increasing parallel wavevector component along both the \qGM and \qGK directions of the √3 × √3 surface Brillouin zone. The VBM is determined to be located at about 2.3(±0.2) eV below the Fermi level. The results indicate that Si adatoms on top of an outermost SiC bilayer may be an inadequate structural model for explaining recent experimental findings for the SiC(0001)-(√3 × √3) surface.