| 1. |
- Balasubramanian, T, et al.
(författare)
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Surface electronic band structure and (A)over-bar surface state lifetimes at the Be(10(1)over-bar-0) surface : Experiment and theory
- 2001
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 64:20
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Tidskriftsartikel (refereegranskat)abstract
- The surface electronic band structure of the Be(10(1) over bar 0) surface is experimentally determined by angle-resolved photoemission and calculated by using density-functional theory. The experimental results agree well with the calculations, except for the fact that we were only able to resolve three surface states in the gap at (L) over bar, instead of four as predicted by the calculations. Through the temperature-dependent study, the phonon contribution subtracted width (h times inverse lifetime) of the shallow surface state at (A) over bar is found to be 51 +/- 8 meV. This is compared with the electron-electron interaction contribution to the width (53 meV) of the shallow surface state at A obtained from model potential calculations.
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| 2. |
- Boukhvalov, D W, et al.
(författare)
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Penetration of alkali atoms throughout a graphene membrane: theoretical modeling
- 2012
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Ingår i: NANOSCALE. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 4:5, s. 1749-1753
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Tidskriftsartikel (refereegranskat)abstract
- Theoretical studies of penetration of various alkali atoms (Li, Na, Rb, Cs) throughout a graphene membrane grown on a silicon carbide substrate are reported and compared with recent experimental results. Results of first principles modeling demonstrate a rather low (about 0.8 eV) energy barrier for the formation of temporary defects in the carbon layer required for the penetration of Li at a high concentration of adatoms, a higher (about 2 eV) barrier for Na, and barriers above 4 eV for Rb and Cs. Experiments prove migration of lithium adatoms from the graphene surface to the buffer layer and SiC substrate at room temperature, sodium at 100 degrees C and impenetrability of the graphene membrane for Rb and Cs. Differences between epitaxial and free-standing graphene for the penetration of alkali ions are also discussed.
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| 3. |
- Chen, Jr-Tai, et al.
(författare)
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Low thermal resistance of a GaN-on-SiC transistor structure with improved structural properties at the interface
- 2015
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 428, s. 54-58
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Tidskriftsartikel (refereegranskat)abstract
- The crystalline quality of AlGaN/GaN heterostructures was improved by optimization of surface pretreatment of the SiC substrate in a hot-wall metal-organic chemical vapor deposition reactor. X-ray photoelectron spectroscopy measurements revealed that oxygen- and carbon-related contaminants were still present on the SiC surface treated at 1200 °C in H2 ambience, which hinders growth of thin AlN nucleation layers with high crystalline quality. As the H2 pretreatment temperature increased to 1240 °C, the crystalline quality of the 105 nm thick AlN nucleation layers in the studied series reached an optimal value in terms of full width at half-maximum of the rocking curves of the (002) and (105) peaks of 64 and 447 arcsec, respectively. The improvement of the AlN growth also consequently facilitated a growth of the GaN buffer layers with high crystalline quality. The rocking curves of the GaN (002) and (102) peaks were thus improved from 209 and 276 arcsec to 149 and 194 arcsec, respectively. In addition to a correlation between the thermal resistance and the structural quality of an AlN nucleation layer, we found that the microstructural disorder of the SiC surface and the morphological defects of the AlN nucleation layers to be responsible for a substantial thermal resistance. Moreover, in order to decrease the thermal resistance in the GaN/SiC interfacial region, the thickness of the AlN nucleation layer was then reduced to 35 nm, which was shown sufficient to grow AlGaN/GaN heterostructures with high crystalline quality. Finally, with the 35 nm thick high-quality AlN nucleation layer a record low thermal boundary resistance of 1.3×10-8 m2 K/W, measured at an elevated temperature of 160 °C, in a GaN-on-SiC transistor structure was achieved.
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| 4. |
- Classen, Thomas, et al.
(författare)
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Hydrogen and Coordination Bonding Supramolecular Structures of Trimesic Acid on Cu(110)
- 2007
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 111:49, s. 12589-12603
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of trimesic acid (TMA) on Cu(110) has been studied in the temperature range between 130 and 550 K and for coverages up to one monolayer. We combine scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), reflection absorption infrared spectroscopy (RAIRS), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations to produce a detailed adsorption phase diagram for the TMA/Cu(110) system as a function of the molecular coverage and the substrate temperature. We identify a quite complex set of adsorption phases, which are determined by the interplay between the extent of deprotonation, the intermolecular bonding, and the overall energy minimization. For temperatures up to 280 K, TMA molecules are only partly deprotonated and form hydrogen-bonded structures, which locally exhibit organizational chirality. Above this threshold, the molecules deprotonate completely and form supramolecular metal−organic structures with Cu substrate adatoms. These structures exist in the form of single and double coordination chains, with the molecular coverage driving distinct phase transitions.
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| 5. |
- Coletti, Camilla, et al.
(författare)
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Surface morphology and structure of hydrogen etched 3C-SiC(001) on Si(001)
- 2006
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Ingår i: Silicon Carbide 2006 - Materials, Processing and Devices. - : Materials Research Society. ; , s. 131-136
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The surface of 3C-SiC(001) single-crystal epilayers grown on Si(001) substrates is well known to be inhomogeneous and defective. Therefore, the control and understanding at the atomic scale of 3C-SiC surfaces is a key issue. We study the effect of hydrogen etching at different temperatures on the morphology of 3C-SiC(001) surfaces by using Nomarksi optical microscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM). As-grown 3C-SiC(001) samples have been hydrogen etched in a horizontal hot-wall chemical vapor deposition (CVD) reactor at atmospheric pressure for different times and temperatures. Flat, high-quality surfaces presenting defined atomic terraces were observed within the 3C-SiC grain boundaries after etching at 1200°C for 30 minutes. Higher etching temperatures resulted in surfaces with step bunching and enlarged surface defects. Samples etched under the best conditions have been studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES).
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| 6. |
- Coletti, C., et al.
(författare)
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Surface studies of hydrogen etched 3C-SiC(001) on Si(001)
- 2007
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 91:6, s. 061914-
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Tidskriftsartikel (refereegranskat)abstract
- Themorphology and structure of 3C-SiC(001) surfaces, grown on Si(001) andprepared 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 andatomic steps are revealed by AFM. In ultrahigh vacuum asharp LEED pattern with an approximate (5×1) periodicity is observed. AES studies reveal a “bulklike” composition up to the nearsurface region and indicate that an overlayer consisting of aweakly bound silicon oxide monolayer is present.
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| 7. |
- Eklund, Per, et al.
(författare)
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Photoemission studies of Ti3SiC2 and nanocrystalline-TiC/amorphous-SiC nanocomposite thin films
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 74:4, s. 045417-
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Tidskriftsartikel (refereegranskat)abstract
- Photoemissionstudies using synchrotron radiation have been performed on epitaxial Ti3SiC2(0001)and compound nanocrystalline (nc-)TiC/amorphous (a-)SiC thin films deposited by magnetronsputtering. As-introduced samples were found to be covered by surfaceoxides, SiOx and TiOx. These oxides could be removed byin-situ annealing to ~1000 °C. For as-annealed Ti3SiC2(0001), surface Si wasobserved and interpreted as originating from decomposition of Ti3SiC2 throughSi out-diffusion. For nc-TiC/a-SiC annealed in situ to ~1000 °C, thesurface instead exhibited a dominant contribution from graphitic carbon, alsowith the presence of Si, due to C and Siout-diffusion from the a-SiC compound or from grain boundaries.
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| 8. |
- Eriksson, Jens, et al.
(författare)
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The influence of substrate morphology on thickness uniformity and unintentional doping of epitaxial graphene on SiC
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:24, s. 241607-
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Tidskriftsartikel (refereegranskat)abstract
- A pivotal issue for the fabrication of electronic devices on epitaxial graphene on SiC is controlling the number of layers and reducing localized thickness inhomogeneities. Of equal importance is to understand what governs the unintentional doping of the graphene from the substrate. The influence of substrate surface topography on these two issues was studied by work function measurements and local surface potential mapping. The carrier concentration and the uniformity of epitaxial graphene samples grown under identical conditions and on substrates of nominally identical orientation were both found to depend strongly on the terrace width of the SiC substrate after growth.
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| 9. |
- Gauthier, Y., et al.
(författare)
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Surface structure and composition of the missing-row reconstruction of VC0.8(1 1 0) : A LEED, GIXRD and photoemission study
- 2007
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 601:16, s. 3383-3394
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Tidskriftsartikel (refereegranskat)abstract
- Low energy electron diffraction, grazing incidence X-ray diffraction and photoemission were used to decipher the detailed structural arrangement and chemical composition of the surface region of a transition metal carbide, VC0.8(1 1 0). In agreement with previous scanning tunneling microscopy (STM) studies, we find that the surface reconstructs with a ridge-and-valley grating structure along the [1 over(1, ¯) 0] direction resulting from {0 0 1} faceting for the (3 × 1) and the (4 × 1) phases. Both superstructures terminate on the vacuum side with a nearly stoïchiometric VC region due to C segregation, in contrast with the conclusions drawn from this previous STM study. However, the present experiments clearly show that these phases are metastable, and slow cooling results in a (1 × 1) surface, which is highly C depleted, similarly to the (1 0 0) face. © 2007 Elsevier B.V. All rights reserved.
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| 10. |
- Hetzel, M., et al.
(författare)
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Nanowire Reconstruction on the 4H-SiC(1-102) Surface
- 2007
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Ingår i: Silicon Carbide and Related Materials 2006. - : Trans Tech Publications Inc.. ; , s. 529-532
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Konferensbidrag (refereegranskat)abstract
- Ordered reconstruction phases on the 4H-SiC(1102) surface have been investigated usinglow-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunnelingmicroscopy (STM). After initial hydrogen etching, the samples were prepared by Si deposition andannealing in ultra-high vacuum (UHV). Two distinct reconstruction phases develop upon annealing,first with a (2×1), and at higher temperatures with a c(2×2) LEED pattern. After further annealingthe fractional order LEED spots vanish and a (1x1) pattern develops. For the (2×1) phase, STMmicrographs show that adatom chains develop on large flat terraces, which in view of AES consistof additional Si. These highly linear and equidistant chains represent a self-assembled well-orderedpattern of nanowires developing due to the intrinsic structure of the 4H-SiC(1102) surface. For thec(2×2) phase AES indicates a surface composition close to the bulk stoichiometry. For the (1×1)phase a further Si depletion is observed.
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| 11. |
- I Johansson, Leif, et al.
(författare)
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Detailed studies of graphene grown on C-face SiC
- 2012
-
Ingår i: Technical Proceedings of the 2012 NSTI Nanotechnology Conference and Expo, NSTI-Nanotech 2012. ; , s. 200-202
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Konferensbidrag (refereegranskat)abstract
- Graphene samples were grown on the C-face of SiC, at high temperature in a furnace and an Ar ambient, and were investigated using LEEM, XPEEM, LEED, XPS and ARPES. Formation of fairly large grains (crystallographic domains) of graphene exhibiting sharp (1x1) patterns in μ-LEED was revealed and that different grains showed different azimuthal orientations. Selective area constant initial energy photoelectron angular distribution patterns recorded showed the same results, ordered grains and no rotational disorder between adjacent layers. A grain size of up to a few μm was obtained on some samples.
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| 12. |
- Johansson, Leif, 1945-, et al.
(författare)
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A comparative photoemission study of polar and nonpolar SiC surfaces oxidized in N2O
- 2004
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 552:1-3, s. 251-259
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Tidskriftsartikel (refereegranskat)abstract
- Photoemission studies of oxidized SiC samples grown ex situ in N 2O, at a temperature of 900 °C, on the (0001), (0001̄), (112̄0) and (101̄0) surfaces are reported. Angle resolved data from the Si 1s and Si 2p core levels and the Si KL2,3L2,3 Auger transitions are analyzed and compared to data from a sample grown in O2 on the (0001) surface. The results show oxide growth and no oxy-nitride formation. The growth rate is found to be smallest for the Si-terminated (0001) surface and highest for the nonpolar (101̄0) surface. The presence of two oxidation states, Si+4 and a suboxide, are required to explain and model recorded Si 1s, Si 2p and Si KLL spectra. The SiO2 shift is found to be smaller on the (0001) surface than on the other three surfaces, which is attributed to an oxide thickness dependence of the shift. A layer attenuation model describes satisfactorily the intensity variations observed in the core level components versus electron emission angle when assuming the suboxide at the interface. Estimates made of the thickness of the oxide layers show that the oxidation rate for the (0001) surfaces is about half of that for the (101̄0) surface and that the oxidation rate for the (112̄0) and (0001̄) surfaces are similar but somewhat smaller than for the (101̄0) surface. The amount of suboxide is found to be smaller on the nonpolar than on the polar surfaces. © 2004 Elsevier B.V. All rights reserved.
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| 13. |
- Johansson, Leif, et al.
(författare)
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A photoemission study of polar and non-polar SiC surfaces oxidized in N2O.
- 2004
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Ingår i: SILICON CARBIDE AND RELATED MATERIALS 2003, PRTS 1 AND 2. - : Trans Tech Publications Inc.. ; , s. 1329-1332
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Konferensbidrag (refereegranskat)abstract
- Angle resolved photoemission studies of SiO2/SiC samples grown ex situ in N2O on polar and non-polar 4H-SiC surfaces are reported. Data from the Si 1s and Si 2p core levels and the Si KL2,3L2,3 Auger transitions are analyzed and compared to data from a sample grown in O-2 on the (0001) surface. The results show oxide growth without nitride or oxy-nitride formation. Presence of two oxidation states, SiO2 and a sub-oxide explains recorded Si 1s. Si 2p and Si KLL spectra. Estimates of the oxide layer thickness show that the oxidation rate is highest for the (10 (1) under bar0) surface, somewhat smaller and similar for the (11 (2) under bar0) and (000 (1) under bar) surfaces, and smaller by a factor of about two for the (0001) surface.
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| 14. |
- Johansson, Leif, et al.
(författare)
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Angle-resolved studies of SiO2/SiC samples
- 2002
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Ingår i: SILICON CARBIDE AND RELATED MATERIALS - 2002. - : Trans Tech Publications Inc.. ; , s. 539-542
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Konferensbidrag (refereegranskat)abstract
- Angle resolved studies of SiO2/SiC samples utilizing the Si 2p and Si 1s core levels and the Si KLL Auger transitions are reported. Samples with total oxide thicknesses from ca. 5 to 118 A are investigated. The data collected show that two oxidation states only, Si+1 and Si+4, are required to explain and model recorded Si 2p, Si 1s and Si KLL spectra. For all samples investigated the intensity variations observed in the core level components versus electron emission angle are found to be well described by a layer attenuation model when assuming that the sub-oxide, Si+1, is located at the interface. The SiO2 chemical shift is found to be larger in the Si 1s level than in the Si 2p level and moreover to depend on the thickness of the oxide layer.
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| 15. |
- Johansson, Leif, et al.
(författare)
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Changes induced in the surface electronic structure of Be(0001) after Si adsorption
- 2002
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Ingår i: Surface Review and Letters. - 0218-625X. ; 9:2, s. 687-691
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Konferensbidrag (refereegranskat)abstract
- A study of effects induced in the Be 1s core level spectrum and in the surface band structure after Si adsorption on Be(0001) is reported. The changes in the Be 1s spectrum are quite dramatic. The number of resolvable surface components and the magnitude of the shifts do decrease and the relative intensities of the shifted components are drastically different compared to the clean surface. The surface band structure is also strongly affected after Si adsorption and annealing. At G the surface state is found to move down from 2.8 to 4.1 eV. The band also splits at around 0.5 Å-1 along both the G-K and G-M directions. At M and beyond K only one surface state is observed in the band gap instead of the two for the clean surface. Our findings indicate that a fairly small amount of Si in the outer atomic layers strongly modifies the electronic properties of these layers.
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| 16. |
- Johansson, Leif I., et al.
(författare)
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Multiple π-bands and Bernal stacking of multilayer graphene on C-face SiC, revealed by nano-Angle Resolved Photoemission
- 2014
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 4:4157
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Tidskriftsartikel (refereegranskat)abstract
- Only a single linearly dispersing π-band cone, characteristic of monolayer graphene, has so far been observed in Angle Resolved Photoemission (ARPES) experiments on multilayer graphene grown on C-face SiC. A rotational disorder that effectively decouples adjacent layers has been suggested to explain this. However, the coexistence of μm-sized grains of single and multilayer graphene with different azimuthal orientations and no rotational disorder within the grains was recently revealed for C-face graphene, but conventional ARPES still resolved only a single π-band. Here we report detailed nano-ARPES band mappings of individual graphene grains that unambiguously show that multilayer C-face graphene exhibits multiple π-bands. The band dispersions obtained close to the K-point moreover clearly indicate, when compared to theoretical band dispersion calculated in the framework of the density functional method, Bernal (AB) stacking within the grains. Thus, contrary to earlier claims, our findings imply a similar interaction between graphene layers on C-face and Si-face SiC.
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| 17. |
- Johansson, Leif I, et al.
(författare)
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Na induced changes in the electronic band structure of graphene grown on C-face SiC
- 2013
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Ingår i: Graphene. - : Scientific Research Publishing. - 2169-3439 .- 2169-3471. ; 2:1, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- Studies of the effects induced on the electron band structure after Na deposition, and subsequent heating, on a C-face 2 MLs graphene sample are reported. Na deposition shifts the Dirac point downwards from the Fermi level by about 0.5 eV due to electron doping. After heating at temperatures from around 120℃ to 300℃,thep-band appears considerably broadened. Collected Si 2p and Na 2p spectra then indicate Na intercalation in between the graphene layers and at the graphene SiC interface. The broadening is therefore interpreted to arise from the presence of two slightly shifted, but not clearly resolved,p-bands. Constant energy photoelectron distribution patterns, E(kx,ky);s, extracted from the clean 2MLs graphene C-face sample look very similar to earlier calculated distribution patterns for monolayer, but not Bernal stacked bilayer, graphene. After Na deposition the patterns extracted at energies below the Dirac point appear very similar so the doping had no pronounced effect on the shape or intensity distribution. At energies above the Dirac point the extracted angular distribution patterns show the flipped, “mirrored”, intensity distribution predicted for monolayer graphene at these energies. An additional weaker outer band is also discernable at energies above the Dirac point, which presumably is induced by the deposited Na.
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| 18. |
- Johansson, Leif I, et al.
(författare)
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Properties of epitaxial graphene grown on C-face SiC compared to Si-face
- 2014
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Ingår i: Journal of Materials Research. - : Cambridge University Press (CUP). - 0884-2914 .- 2044-5326. ; 29:3, s. 426-438
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial graphene of uniform thickness prepared on SiC is of great interest for various applications. On the Si-face, large area uniformity has been achieved, and there is a general consensus about the graphene properties. A similar uniformity has yet not been demonstrated on the C-face where the graphene has been claimed to be fundamentally different. A rotational disorder between adjacent graphene layers has been reported and suggested to explain why multilayer C-face graphene show the pi-band characteristic of monolayer graphene. Utilizing low energy electron microscopy, x-ray photoelectron electron microscopy, low energy electron diffraction, and photoelectron spectroscopy, we investigated the properties of C-face graphene prepared by sublimation growth. We observe the formation of micrometer-sized crystallographic grains of multilayer graphene and no rotational disorder between adjacent layers within a grain. Adjacent grains are in general found to have different azimuthal orientations. Effects on C-face graphene by hydrogen treatment and Na exposure were also investigated and are reported. Why multilayer C-face graphene exhibits single layer electronic properties is still a puzzle, however.
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| 19. |
- Johansson, Leif I, et al.
(författare)
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Reactions on the SiC(0001) root 3 x root 3 R30 degrees surface after Ti deposition and annealing
- 2011
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Ingår i: Physica status solidi. B, Basic research. - : John Wiley and Sons, Ltd. - 0370-1972 .- 1521-3951. ; 248:3, s. 667-673
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Tidskriftsartikel (refereegranskat)abstract
- The interactions of thin Ti layers deposited on the SiC(0001) root 3 x root 3 R30 degrees surface at room temperature and after annealing at temperatures from 450 to 1200 degrees C was investigated using photoemission and LEED. Chemically shifted components were revealed in the Si 2p spectrum and found to be more intense and pronounced than the shifted component in the C 1s spectrum after Ti deposition and annealing. The relative intensity of these shifted components were found to increase initially upon annealing at temperatures up to around 700 degrees C. At temperatures above 800 degrees C only the shifted component in the C 1s spectrum remained which indicate that only TiC then remains on the surface. At annealing temperatures of 600-700 degrees C formation of the ternary Ti(3)SiC(2) phase and an interface TiSi(x) layer is suggested from shifts and relative intensities observed for these components. That the formation and decomposition of the ternary phase occurs at a considerably lower temperature than earlier reported is attributed to the fact that we investigated the interaction of considerably thinner Ti layers with SiC substrates than in those earlier reported studies.
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| 20. |
- Johansson, Leif I, et al.
(författare)
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Stacking of adjacent graphene layers grown on C-face SiC
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:12, s. 125405-
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Tidskriftsartikel (refereegranskat)abstract
- Graphene was grown on the C-face of nominally on-axis SiC substrates using high-temperature sublimation with Ar as the buffer inert gas. The results of studies of the morphology, thickness, and electronic structure of these samples using low-energy electronmicroscopy (LEEM), x-ray photoelectron emission microscopy, photoelectron spectroscopy, angle-resolved photoelectron spectroscopy (ARPES), and low-energy electron diffraction (LEED) are presented. The graphene thickness is determined to vary from 1 or 2 to 6 or 7 monolayers (MLs), depending on the specific growth conditions utilized. The formation of fairly large grains (i.e., crystallographic domains) of graphene exhibiting sharp 1 x 1 spots in micro-LEED is revealed. Adjacent grains are found to show different azimuthal orientations. Macro-LEED patterns recorded mimic previously published, strongly modulated, diffraction ring LEED patterns, indicating contribution from several grains of different azimuthal orientations. We collected selected area constant initial energy photoelectron angular distribution patterns that show the same results. When utilizing a small aperture size, one Dirac cone centered on each of the six K-points in the Brillouin zone is clearly resolved. When using a larger aperture, several Dirac cones from differently oriented grains are detected. Our findings thus clearly show the existence of distinct graphene grains with different azimuthal orientations; they do not show adjacent graphene layers are rotationally disordered, as previously reported for C-face graphene. The graphene grain size is shown to be different on the different samples. In some cases, a probing area of 400 nm is needed to detect the grains. On one sample, a probing area of 5 mu m can be used to collect a 1 x 1 LEED pattern from a multilayer graphene grain. ARPES is used to determine the position of the Dirac point relative to the Fermi level on two samples that LEEM shows have dominant coverage of 2 and 3 MLs of graphene, respectively. The Dirac point is found to be located within 75 meV of the Fermi level on both samples, which indicates that the electron carrier concentration induced in the second and third graphene layers on the C-face is less than similar to 4x10(11) cm(-2). Formation of patches of silicate is revealed on some samples, but the graphene formed on such nonhomogenous surfaces can contain fairly large ordered multilayer graphene grains.
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| 21. |
- Johansson, Leif I., et al.
(författare)
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The registry of graphene layers grown on SiC(000-1)
- 2012
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Ingår i: Materials Science Forum Vols 717 - 720. - : Trans Tech Publications Inc.. ; , s. 613-616
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Konferensbidrag (refereegranskat)abstract
- Graphene samples were grown on the C-face of SiC, at high temperature in a furnace andan Ar ambient, and were investigated using LEEM, XPEEM, LEED, XPS and ARPES. Formationof fairly large grains (crystallographic domains) of graphene exhibiting sharp 1x1 patterns in μ-LEED was revealed and that different grains showed different azimuthal orientations. Selective areaconstant initial energy photoelectron angular distribution patterns recorded showed the same results,ordered grains and no rotational disorder between adjacent layers. A grain size of up to a few μmwas obtained on some samples.
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| 22. |
- Johansson, Leif, et al.
(författare)
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Is the Registry Between Adjacent Graphene Layers Grown on C-Face SiC Different Compared to That on Si-Face SiC
- 2013
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Ingår i: Crystals. - : MDPI AG. - 2073-4352. ; 3:1, s. 1-13
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Tidskriftsartikel (refereegranskat)abstract
- Graphene grown on C-face SiC substrates using two procedures, high and low growth temperature and different ambients, was investigated using Low Energy Electron Microscopy (LEEM), X-ray Photo Electron Electron Microscopy (XPEEM), selected area Low Energy Electron Diffraction (μ-LEED) and selected area Photo Electron Spectroscopy (μ-PES). Both types of samples showed formation of μm-sized grains of graphene. The sharp (1 × 1) μ-LEED pattern and six Dirac cones observed in constant energy photoelectron angular distribution patterns from a grain showed that adjacent layers are not rotated relative to each other, but that adjacent grains in general have different azimuthal orientations. Diffraction spots from the SiC substrate appeared in μ-LEED patterns collected at higher energies, showing that the rotation angle between graphene and SiC varied. C 1s spectra collected did not show any hint of a carbon interface layer. A hydrogen treatment applied was found to have a detrimental effect on the graphene quality for both types of samples, since the graphene domain/grain size was drastically reduced. From hydrogen treated samples, μ-LEED showed at first a clear (1 × 1) pattern, but within minutes, a pattern containing strong superstructure spots, indicating the presence of twisted graphene layers. The LEED electron beam was found to induce local desorption of hydrogen. Heating a hydrogenated C-face graphene sample did not restore the quality of the original as-grown sample.
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| 23. |
- Johansson, Leif, et al.
(författare)
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Properties of the SiO2/SiC interface investigated by angle resolved studies of the Si 2p and Si 1s levels and the Si KLL Auger transitions
- 2003
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Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 529:3, s. 515-526
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Tidskriftsartikel (refereegranskat)abstract
- Angle resolved photoemission studies of the Si 2p and Si 1s core levels and the Si KL2,3L2,3 Auger transitions from SiO2/SiC samples are reported. Most samples investigated were grown in situ on initially clean and well ordered v3 × v3 reconstructed 4H-SiC(0 0 0 1) surfaces but some samples were grown ex situ using a standard dry oxidation procedure. The results presented cover samples with total oxide thicknesses from about 5 to 118 Å. The angle resolved data show that two oxidation states only, Si+1 and Si+4, are required to explain and model recorded Si 2p, Si 1s and Si KLL spectra. The intensity variations observed in the core level components versus electron emission angle are found to be well described by a layer attenuation model for all samples when assuming a sub-oxide (Si2O) at the interface with a thickness ranging from 2.5 to 4 Å. We conclude that the sub-oxide is located at the interface and that the thickness of this layer does not increase much when the total oxide thickness is increased from about 5 to 118 Å. The SiO2 chemical shift is found to be larger in the Si 1s level than in the Si 2p level and to depend on the thickness of the oxide layer. The SiO2 shift is found to be fairly constant for oxides less than about 10 Å thick, to increase by 0.5 eV when increasing the oxide thickness to around 25 Å and then to be fairly constant for thicker oxides. An even more pronounced dependence is observed in the Si KLL transitions where a relative energy shift of 0.9 eV is determined. The relative final state relaxation energy dR(2p) is determined from the modified Auger parameter. This yields a value of dR(2p) = -1.7 eV and implies, for SiO2/SiC, a "true" chemical shift in the Si 2p level of only ˜0.4 eV for oxide layers of up to 10 Å thick. © 2003 Elsevier Science B.V. All rights reserved.
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| 24. |
- Johansson, Leif, 1945-, et al.
(författare)
-
Synchrotron radiation studies of the SiO2/SiC(0001) interface
- 2004
-
Ingår i: JOURNAL OF PHYSICS-CONDENSED MATTER ISSN (0953-8984). - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; , s. S3423-S3434
-
Konferensbidrag (refereegranskat)abstract
- Two questions thought to have a significant effect on SiC-MOS device characteristics are treated. The existence of carbon clusters or carbon containing by-products and the existence of sub-oxides at the SiO2/SiC interface. Results of photoemission studies using synchrotron radiation of the interface of the Si-terminated surface of n-type SiC(0001) crystals are presented. The results show that no carbon clusters or carbon containing by-product can. be detected at the interface of in situ or ex situ grown samples with an oxide layer thickness larger than similar to10 Angstrom. The presence of sub-oxides at the SiO2/SiC interface was predicted in a theoretical calculation and has been revealed in Si 2p core level data by several groups. These results were not unanimous; significant differences in the number of sub-oxide and shifts were reported. A study also including the Si 1s core level and Si KLL Auger transitions was therefore made. These data show the presence of only one sub-oxide at the interface, assigned to Si1+ oxidation states. The SiO2 chemical shift is shown to exhibit a dependence on oxide thickness, similar to but smaller in magnitude than the thickness dependence earlier revealed for SiO2/Si.
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| 25. |
- Johansson, Leif, et al.
(författare)
-
The surface band structure of Li/Be(10(1)over-bar0)
- 2002
-
Ingår i: Surface review and letters. - 0218-625X. ; 9:3-4, s. 1493-1496
-
Tidskriftsartikel (refereegranskat)abstract
- A photoemision study of the surface states on Be(10 (1) over bar0) after Li adsorption at room temperature is reported. The surface band structure was mapped along four high symmetry directions of the SBZ. Fairly large shifts in the surface band locations were obtained but all surface states observed experimentally after Li adsorption were found to correspond to Be-derived states and no Li-derived surface states could be identified. The surface state bands located close to the Fermi level (E-F) were found to be affected the most and it is suggested that one surface state band which on the clean surface is located above E-F is pulled down below E-F after Li adsorption.
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| 26. |
- Karlsson, P.G., et al.
(författare)
-
Ultrathin ZrO2 films on Si-rich SiC(0 0 0 1)-(3 × 3) : Growth and thermal stability
- 2007
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 601:11, s. 2390-2400
-
Tidskriftsartikel (refereegranskat)abstract
- The growth and thermal stability of ultrathin ZrO2 films on the Si-rich SiC(0 0 0 1)-(3 × 3) surface have been explored using photoelectron spectroscopy (PES) and X-ray absorption spectroscopy (XAS). The films were grown in situ by chemical vapor deposition using the zirconium tetra tert-butoxide (ZTB) precursor. The O 1s XAS results show that growth at 400 °C yields tetragonal ZrO2. An interface is formed between the ZrO2 film and the SiC substrate. The interface contains Si in several chemically different states. This gives evidence for an interface that is much more complex than that formed upon oxidation with O2. Si in a 4+ oxidation state is detected in the near surface region. This shows that intermixing of SiO2 and ZrO2 occurs, possibly under the formation of silicate. The alignment of the ZrO2 and SiC band edges is discussed based on core level and valence PES spectra. Subsequent annealing of a deposited film was performed in order to study the thermal stability of the system. Annealing to 800 °C does not lead to decomposition of the tetragonal ZrO2 (t-ZrO2) but changes are observed within the interface region. After annealing to 1000 °C a laterally heterogeneous layer has formed. The decomposition of the film leads to regions with t-ZrO2 remnants, metallic Zr silicide and Si aggregates. © 2007 Elsevier B.V. All rights reserved.
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| 27. |
- Payer, Dietmar, et al.
(författare)
-
Toward Mechanical Switching of Surface-Adsorbed [2]Catenane by in Situ Copper Complexation
- 2007
-
Ingår i: Journal of the American Chemical Society. - Washington, DC, USA : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 129, s. 15662-15667
-
Tidskriftsartikel (refereegranskat)abstract
- Using scanning tunneling microscopy (STM), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS), we demonstrate that a free [2]catenane consisting of two interlocking 30-membered rings (cat-30) can be deposited on a Ag(111) surface by vacuum sublimation without decomposition. The deposited cat-30 molecules self-organize as ordered dimer chain structures at the surface, presumably via intermolecular π−π stacking. An in situ addition of Cu atoms to the surface-adsorbed catenanes induces a drastic change in the molecular organization, i.e., from the dimer chain structure to isolated species. The nitrogen core level spectra suggest that the cat-30 phenanthroline units coordinate with Cu, indicating that the free catenane has been transformed into a Cu-complexed [2]catenane. Since it is known that the two interlocked macrocyclic rings of the free ligand cat-30 completely rearrange, i.e., circumrotate, upon complexation to copper, our results reveal that when adsorbed on the silver surface, the two macrocyclic rings of the free [2]catenane can glide within one another so as to generate the corresponding copper complex by in situ Cu complexation.
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| 28. |
- Soubatch, Serguei, et al.
(författare)
-
Atomic Structure of Non-Basal-Plane SiC Surfaces : Hydrogen Etching and Surface Phase Transformations
- 2006
-
Ingår i: Silicon Carbide 2006 - Materials, Processing and Devices. - : Materials Research Society. - 1558998675 ; , s. 271-282
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A-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.
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| 29. |
- ul-Hassan, Jawad, et al.
(författare)
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Control of epitaxial graphene thickness on 4H-SiC(0001) and bufferlayer removal through hydrogen intercalation
- 2012
-
Ingår i: Materials Science Forum (Volumes 717 - 720). - : Trans Tech Publications Inc.. ; , s. 605-608
-
Konferensbidrag (refereegranskat)abstract
- We report graphene thickness, uniformity and surface morphology dependence on thegrowth temperature and local variations in the off-cut of Si-face 4H-SiC on-axis substrates. Thetransformation of the buffer layer through hydrogen intercalation and the subsequent influence onthe charge carrier mobility are also studied. A hot-wall CVD reactor was used for in-situ etching,graphene growth in vacuum and the hydrogen intercalation process. The number of graphene layersis found to be dependent on the growth temperature while the surface morphology also depends onthe local off-cut of the substrate and results in a non-homogeneous surface. Additionally, the influence of dislocations on surface morphology and graphene thickness uniformity is also presented.
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| 30. |
- Virojanadara, Chariya, et al.
(författare)
-
A diagonal cut through the SiC bulk unit cell : Structure and composition of the 4H-SiC(1-102) surface
- 2008
-
Ingår i: Applied Physics Letters. - College Park, MD, USA : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 92:6, s. 061902-
-
Tidskriftsartikel (refereegranskat)abstract
- The atomic and electronic structure of 4H-SiC(102) surfaces was investigated using low-energy electron diffraction, scanning tunneling microscopy, and photoelectron spectroscopy. Three well ordered phases can be prepared by Si deposition and annealing. The (2×1) phase is Si enriched and terminated by an ordered array of Si-adatom chains which contribute an anisotropic electronic surface state. The c(2×2) phase has a surface composition close to SiC bulk and possesses adatoms in the periodicity of the superlattice. At high temperatures, a (1×1) phase develops with a graphitelike composition.
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| 31. |
- Virojanadara, Chariya, et al.
(författare)
-
A low-energy electron microscopy and x-ray photo-emission electron microscopy study of Li intercalated into graphene on SiC(0001)
- 2010
-
Ingår i: New Journal of Physics. - : Institute of Physics (IoP) and Deutsche Physikalische Gesellschaft. - 1367-2630. ; 12:125015
-
Tidskriftsartikel (refereegranskat)abstract
- The effects induced by the deposition of Li on 1 and 0 ML graphene grown on SiC(0001) and after subsequent heating were investigated using low-energy electron microscopy (LEEM) and x-ray photo-emission electron microscopy (XPEEM). For 1 ML samples, the collected photoelectron angular distribution patterns showed the presence of single pi-cones at the six equivalent K-points in the Brillouin zone before Li deposition but the presence of two pi-cones (pi-bands) after Li deposition and after heating to a few hundred degrees C. For 0 ML samples, no pi-band could be detected close to the Fermi level before deposition, but distinct pi-cones at the K-points were clearly resolved after Li deposition and after heating. Thus Li intercalation was revealed in both cases, transforming the carbon buffer layer (0 ML) to graphene. On 1 ML samples, but not on 0 ML, a (root 3 x root 3) R30 degrees diffraction pattern was observed immediately after Li deposition. This pattern vanished upon heating and then wrinkles/cracks appeared on the surface. Intercalation of Li was thus found to deteriorate the quality of the graphene layer, especially for 1 ML samples. These wrinkles/cracks did not disappear even after heating at temperatures andgt;= 500 degrees C, when no Li atoms remained on the substrate.
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| 32. |
- Virojanadara, Chariya, et al.
(författare)
-
Adsorption of metastable molecular oxygen on SiC(0001)-root 3 x root 3
- 2002
-
Ingår i: Materials Science forum, Vols. 389-393. ; , s. 697-700
-
Konferensbidrag (refereegranskat)abstract
- Photoemission studies of initial oxygen adsorption on the SiC (0001)-root3xroot 3 surface at different temperatures are reported. After exposures of 0.1 L to 10 L with the sample at room temperature or cooled to ca. 100 K structures appear in the 0 Is spectrum that are identified to originate from metastable oxygen. Adsorption of metastable molecular oxygen is suggested from the similarities obtained with earlier findings on Si (111)-7x7.
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| 33. |
- Virojanadara, Chariya, et al.
(författare)
-
Atomic and Electronic Structure of the (2x1) and c(2x2) 4H-SiC(1(1)over-bar02) Surfaces
- 2008
-
Ingår i: SILICON CARBIDE AND RELATED MATERIALS 2007, PTS 1 AND 2. - : Trans Tech Publications Inc.. ; , s. 291-296
-
Konferensbidrag (refereegranskat)abstract
- The atomic and electronic structure of 4H-SiC(1 1 02) surfaces were investigated usingscanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and photoemission(PES). Two well ordered phases existing on this surface, i.e. (2×1) and c(2×2) are discussed. The(2×1) phase consists of a Si adlayer which is topped by an array of ordered Si-nanowires withelectronic states confined to one dimension. For the c(2×2) phase STM indicates the presence ofadatoms and PES a surface composition close to bulk SiC stoichiometry. A detailed atomic modelfor this c(2×2) phase is proposed.
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| 34. |
- Virojanadara, Chariya, et al.
(författare)
-
Buffer layer free large area bi-layer graphene on SiC(0001)
- 2010
-
Ingår i: SURFACE SCIENCE. - : Elsevier BV. - 0039-6028. ; 604:2, s. L4-L7
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of hydrogen exposures on monolayer graphene grown on the silicon terminated SiC(0 0 0 1) surface is investigated using photoelectron spectroscopy (PES), low-energy electron microscopy (LEEM) and micro low-energy electron diffraction (mu-LEED). Exposures to ionized hydrogen are shown to have a pronounced effect on the carbon buffer (interface) layer. Exposures to atomic hydrogen are shown to actually convert/transform the monolayer graphene plus carbon buffer layer to bi-layer graphene, i.e. to produce carbon buffer layer free bi-layer graphene on SiC(0 0 0 1). This process is shown to be reversible, so the initial monolayer graphene plus carbon buffer layer situation is recreated after heating to a temperature of about 950 degrees C. A tentative model of hydrogen intercalation is suggested to explain this single to bi-layer graphene transformation mechanism. Our findings are of relevance and importance for various potential applications based on graphene-SiC structures and hydrogen storage.
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| 35. |
- Virojanadara, Chariya, et al.
(författare)
-
Core level photoelectron spectroscopy studies of a √7 × √7 R19° reconstructed Au/4H-SiC(0 0 0 1¯) surface
- 2005
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 585:3, s. 163-169
-
Tidskriftsartikel (refereegranskat)abstract
- A study of surface and interface properties of thin Au layers deposited on SiC(0001¯) surfaces is reported. Two reconstructions were prepared, a Si-rich 2 × 2 and a C-rich 3 × 3 surface, before Au deposition and subsequent annealing at different temperatures. For the Si-rich 2 × 2 surface a stable v7 × v7 R19° reconstruction is obtained after Au deposition and annealing at temperatures between 400 and 850 °C. On this surface two surface shifted Si2p components are revealed and the Au4f spectra clearly indicate silicide formation. The variations in relative intensity for the different core level components with electron emission angle suggest: -formation of an ordered silicide layer on the surface, -excess Au ("bulk Au") to form a layer underneath the ordered silicide, -and silicide formation also at the interface between "bulk Au" and SiC. The "bulk Au" component is found to decrease rapidly with annealing temperature. This decrease is due to Au diffusion into the SiC sample as confirmed by annealing at similar temperatures of Au films deposited onto deliberately oxidized SiC(0001¯) surfaces. For the C-rich 3 × 3 surface the evolution of the core level spectra after Au deposition and annealing is shown to be distinctly different than for the Si-rich 2 × 2 and no new stable reconstruction is observed. © 2005 Elsevier B.V. All rights reserved.
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| 36. |
- Virojanadara, Chariya, et al.
(författare)
-
Effects of nitrogenmonoxide on 4H-SiC(0 0 0 1)
- 2003
-
Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 530:1-2, s. 17-25
-
Tidskriftsartikel (refereegranskat)abstract
- Effects induced by nitrogenmonoxide (NO) exposures on the v3 × v3 R30° reconstructed 4H-SiC(0 0 0 1) surface are reported. NO exposures from 0.3 to 1 × 106 L at a substrate temperature of 800 °C are investigated. Recorded Si2p spectra show three shifted components, besides the bulk SiC peak. These are assigned to originate from SiO2, N-Si-O and Si3N4/Si+1 (since we cannot distinguish between Si3N4 and an Si+1 oxidation state). It can be concluded that SiO2 does grow on top of N-Si-O and that Si3N4/Si+1 is located at the interface. Two N1s components are observed after NO exposures. A main one, located at around 398.05 eV, assigned to originate from Si3N4 and a weaker one suggested to correspond to N-Si-O bonding. The assignments are made using Si2p and N1s spectra collected after NH3 and O2 exposures under similar conditions. No graphite like carbon or carbon by-product at the interface can be detected after large NO or O2 exposures. © 2003 Elsevier Science B.V. All rights reserved.
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| 37. |
- Virojanadara, Chariya, et al.
(författare)
-
Electronic and atomic structure of the 4 H-SiC (1 over(1, ¯) 0 2) -c (2 × 2) surface
- 2008
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 602:2, s. 525-533
-
Tidskriftsartikel (refereegranskat)abstract
- The (1 over(1, ¯) 0 2) orientated plane of hexagonal silicon carbide of the 4H polytype consists of a periodic arrangement of stripes with alternating bond configuration on a nanometer scale. The two stripe configurations of the bulk truncated surface have an atomic structure very close to the carbon-face SiC basal plane and the cubic SiC(1 0 0) surface, respectively. The structural and electronic properties of the c(2 × 2) reconstruction on the 4 H-SiC (1 over(1, ¯) 0 2) surface were investigated using photoemission spectroscopy (PES), scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). The core level photoemission spectra reveal two surface shifted Si2p components and one shifted C1s component in addition to the SiC bulk peaks. In accordance with the periodicity observed in LEED, atomically resolved STM micrographs show a c(2 × 2) arrangement of bright features which are accounted as Si adatoms. The electronic structure of this SiC (1 over(1, ¯) 0 2) -c (2 × 2) phase is experimentally determined by angle resolved PES studies of the valence band revealing four surface states. Based on the experimental observations and a comparison to similar phases on other SiC surfaces, a tentative surface model can be developed which consists of Si adatoms in so-called H3 sites on the basal-plane type stripes and carbon dimers in Si bridging configuration on the cubic stripes of the bulk truncated surface. © 2007 Elsevier B.V. All rights reserved.
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| 38. |
- Virojanadara, Chariya, et al.
(författare)
-
Electronic structure study of reconstructed Au-SiC(0 0 0 1) surfaces
- 2006
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 600:2, s. 436-441
-
Tidskriftsartikel (refereegranskat)abstract
- A study of surface and interface properties of reconstructed Au-SiC(0 0 0 1) surfaces is reported. Two reconstructions were prepared on SiC(0 0 0 1), a v3 × v3R30° and a Si-rich 3 × 3, before Au deposition and subsequent annealing at different temperatures. For the Si-rich 3 × 3 surface the existence of three stable reconstructions 2v3 × 2v3R30°, 3 × 3 and 5 × 5 are revealed after deposition of Au layers, 4-8 Å thick, and annealing at progressively higher temperatures between 500 and 950 °C. For the 2v3 surface two surface shifted Si 2p components are revealed and the Au 4f spectra clearly indicate silicide formation. The variation in relative intensity for the different core level components with photon energy suggests formation of an ordered silicide layer with some excess Si on top. Similar core level spectra and variations in relative intensity with photon energy are obtained for the 3 × 3 and 5 × 5 phases but the amount of excess Si on top is observed to be smaller and an additional weak Si 2p component becomes discernable. For the v3 surface the evolution of the core level spectra after Au deposition and annealing is shown to be distinctly different than for the Si-rich 3 × 3 surface and only one stable reconstruction, a 3 × 3 phase, is observed at similar annealing temperatures. © 2005 Elsevier B.V. All rights reserved.
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| 39. |
- Virojanadara, Chariya, et al.
(författare)
-
Epitaxial graphene on 6H-SiC and Li intercalation
- 2010
-
Ingår i: PHYSICAL REVIEW B. - : American Physical Society. - 1098-0121. ; 82:20, s. 205402-
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of lithium (Li) exposures on monolayer graphene grown on the silicon-terminated SiC(0001) surface is investigated using low-energy electron microscopy, photoelectron spectroscopy, and micro-low-energy electron diffraction. After Li deposition, islands or Li droplets are observed on the surface, and are found to coalesce together with time. Formation of a dipole layer at the interface, interpreted to originate from Li-Si bonding, is observed directly after Li deposition, and manifested by a 2 eV shift of the C 1s and Si 2p bulk SiC peaks. This indicates that Li atoms penetrate through the graphene and carbon buffer layer directly after deposition at room temperature since three pi bands are then moreover observed at the K point, instead of the single pi band for monolayer graphene. The existence of three pi bands is interpreted as a mixture of bilayer and monolayer graphene plus a difference in doping levels due to an uneven distribution of Li atoms. Li gives rise to electron doping of the graphene and results in a lowering of the Dirac point. After annealing to a few hundred degrees Celsius, a more even Li distribution and intercalation is obtained since then two distinct pi bands appear at the K point.
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| 40. |
- Virojanadara, Chariya, et al.
(författare)
-
High energy photoemission investigations of SiO2/SiC samples
- 2001
-
Ingår i: Applied Surface Science. - 0169-4332 .- 1873-5584. ; 172:3-4, s. 253-259
-
Tidskriftsartikel (refereegranskat)abstract
- Chemically etched and directly load-locked SiO2/SiC samples are investigated using a photon energy of 3.0 keV. Si 2p and C 1s spectra recorded at different electron emission angles each show two components originating from SiC, SiO2 and graphite like carbon, respectively. The relative intensity of these are extracted and compared to calculated intensity variations. For the samples investigated, best agreement between experimental and calculated intensity variations is obtained when assuming a graphite like layer on top of the oxide. No graphite like carbon at the SiO2/SiC interface was detected, even on a sample for which the graphite like carbon contribution at the surface corresponds to a layer thickness of only 0.05 angstrom. The energy separation between the oxide and carbide components in the Si 2p spectrum was monitored before and after Ar+ sputtering cycles and before and after in situ heating. The separation increased directly upon sputtering while only in situ heating does not affect it. We suggest that defects induced by the sputtering give rise to the increase, observed in the energy separation.
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| 41. |
- Virojanadara, Chariya, et al.
(författare)
-
Homogeneous large-area graphene layer growth on 6H-SiC(0001)
- 2008
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:24, s. 245403-
-
Tidskriftsartikel (refereegranskat)abstract
- Homogeneous large-area graphene monolayers were successfully prepared ex situ on 6H-SiC(0001). The samples have been studied systematically and the results are compared with those from a sample cut from the same wafer and prepared by in situ heating. The formation of smaller graphene flakes was found on the in situ prepared sample, which is in line with earlier observations. Distinctly different results are observed from the ex situ graphene layers of different thicknesses, which are proposed as a guideline for determining graphene growth. Recorded C 1s spectra consisted of three components: bulk SiC, graphene (G), and interface (I), the latter being a 6 root 3 layer. Extracted intensity ratios of G/I were found to give a good estimate of the thickness of graphene. Differences are also revealed in micro low energy electron diffraction images and electron reflectivity curves. The diffraction patterns were distinctly different from a monolayer thickness up to three layers. At a larger thickness only the graphitelike spot was visible. The electron reflectivity curve showed a nice oscillation behavior with kinetic energy and as a function of the number of graphene layers. The graphene sheets prepared were found to be very inert and the interface between the substrate and the layer(s) was found to be quite abrupt. No free Si could be detected in or on the graphene layers or at the interface.
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| 42. |
- Virojanadara, Chariya, et al.
(författare)
-
Large homogeneous mono-/bi-layer graphene on 6H-SiC(0001) and buffer layer elimination
- 2010
-
Ingår i: JOURNAL OF PHYSICS D-APPLIED PHYSICS. - : Iop Publishing Ltd. - 0022-3727 .- 1361-6463. ; 43:37, s. 374010-
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 43. |
- Virojanadara, Chariya, et al.
(författare)
-
Letter: Interfacial investigation of in situ oxidation of 4H-SiC
- 2001
-
Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 472:1-2
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- 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 recorded from the in situ prepared SiO2/4H-SiC samples. Oxidation was made at a pressure of ~10-3 Torr in flowing oxygen and at substrate temperatures from 600°C to 950°C. The highest oxidation rate was obtained at 800°C when ˜25 Å thick SiO2 layers were prepared. The surface related C 1s components observed on the clean reconstructed 4H-SiC(0001) surfaces were found to disappear after oxidation. No carbon or carbon containing by-product at the interface or in the oxide were possible to observe for the films grown.
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| 44. |
- Virojanadara, Chariya, et al.
(författare)
-
Metastable oxygen adsorption on SiC(0 0 0 1)-v3 × v3 R30°
- 2002
-
Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 519:1-2, s. 73-78
-
Tidskriftsartikel (refereegranskat)abstract
- Initial oxygen adsorption at different temperatures on the SiC(0 0 0 1)- v3 × v3 R30° surface has been studied using photoemission. Oxygen exposures with the sample at 800 °C results in formation of stable oxides. However, after small exposures (0.1-10 L) with the sample at room temperature or cooled to ˜100 K additional structures appear in the O 1s spectrum that are identified to originate from metastable oxygen. Similar additional structures were recently revealed on Si(111)-7 × 7 and suggested to originate from adsorption of metastable molecular oxygen in an ins-paul configuration. © 2002 Elsevier Science B.V. All rights reserved.
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| 45. |
- Virojanadara, Chariya, et al.
(författare)
-
Oxidation states present on SiC (0001) after oxygen exposure
- 2002
-
Ingår i: Materials Science Forum, Vols. 389-393. ; , s. 701-704
-
Konferensbidrag (refereegranskat)abstract
- Core level photoemission experiments using synchrotron radiation were performed in order to study the properties of the SiO2/SiC interface on Si-terminated 4H-SiC surfaces after different oxygen exposures. The surfaces were oxidized gradually from 1 - 1.2x10(6) L at both room temperature (RT) and at 800degreesC. Recorded Si 2p and C Is spectra show only two oxidation states, Si+1 and Si+4, at both temperatures and no carbon containing by-product at the interface after the largest exposure investigated, 1.2x10(6) L. A clean and well ordered 3 surface is shown to be possible to re-create by in situ heating even after the largest oxygen exposures made at 800degreesC.
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| 46. |
- Virojanadara, Chariya, et al.
(författare)
-
Oxidation studies of 4H-SiC(0001) and (0001)
- 2002
-
Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 505:1-3, s. 358-366
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Tidskriftsartikel (refereegranskat)abstract
- The results of a photoemission study of Si- and C-terminated 4H SiC surfaces after different oxygen exposures are presented and discussed. The surfaces were oxidized gradually from 1 to 1.2 x 10(6) L at both room temperature and at 800 degreesC. Recorded Si 2p and C 1s spectra show at both temperatures only two oxidations states, Si1- and Si4- for the Si-terminated surface and Si2+ and Si4+ for the C-terminated surface, For the Si-terminated surface, no carbon containing by-product can be detected at the interface or at the surface after the largest exposure investigated. For the C-terminated surface, oxygen exposures are shown to affect the surface related carbon components quite strongly and the Si2+ oxidation state is interpreted to originate from a mixture of Si-O-C bonding, The surface/interface related carbon decreases dramatically after the largest exposure investigated but is not eliminated as on the Si-terminated surface. For the latter, a clean and well ordered root3 surface is shown to be possible to re-create by in situ heating even after the largest oxygen exposures made. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 47. |
- Virojanadara, Chariya, 1977-, et al.
(författare)
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Oxidation Studies of Non-Polar 4H-SiC Surfaces
- 2004
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Ingår i: SILICON CARBIDE AND RELATED MATERIALS 2003, PRTS 1 AND 2. - : Trans Tech Publications Inc.. ; , s. 1321-1324
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Konferensbidrag (refereegranskat)abstract
- The results of a photoemission study of clean and oxidized non-polar (1010) and (1120) surfaces of 4H-SiC crystals are reported. The effects induced in the core levels and valence bands upon initial oxidation were investigated. The surfaces were oxidized gradually from 1 L to 10(6) L while keeping the substrate at a temperature of 800degreesC. Recorded Si 2p spectra show three oxidation states for both surfaces and these are interpreted to originate from Si+1, Si+2 and Si+4, respectively. This is quite different compared to earlier results for the polar surfaces where only Si+4 and one sub-oxide were revealed on each surface. It is concluded that the Si+4 oxide (SiO2) grow as a layer on top of the Si+1 and Si+2 sub-oxides that are located at the interface. The surface/interface related carbon is found to decrease dramatically, but not to be totally eliminated, after the large oxygen exposures made
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| 48. |
- Virojanadara, Chariya, et al.
(författare)
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Photoemission study of clean and oxidized nonpolar 4H-SiC surfaces
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 68:12
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Tidskriftsartikel (refereegranskat)abstract
- The results of a photoemission study of clean and oxidized nonpolar (10 (1) over bar0) and (11 (2) over bar0) surfaces of 4H-SiC crystals are reported. For the clean surfaces prominent surface shifted components are revealed in the C 1s spectra, although with different relative strengths for the two surfaces. In the Si 2p spectra only one weak surface related feature is observed. These observations suggest carbon enrichment on both surfaces. Valence band spectra show a semiconducting behavior for both surfaces but also distinct differences. The lower lying subband, originating mainly from C 2s states, is for example found to be considerably weaker on the (10 (1) over bar0) than on the (11 (2) over bar0) surface. Observed differences are suggested to originate from differences in surface carbon arrangement and bonding. It is concluded, from the absence of a graphitelike C 1s peak, that the clean surfaces are not graphitized. The effects induced in the core levels and valence bands upon initial oxidation were investigated. The surfaces were oxidized gradually from 1 to 10(6) L while keeping the substrate at a temperature of 800 degreesC. Recorded Si 2p spectra show three oxidation states for both surfaces, and these are interpreted to originate from Si+1, Si+2, and Si+4, respectively. This is quite different compared to earlier results for the polar surfaces where only Si+4 and one suboxide were revealed on each surface. It is concluded that the Si+4 oxide (SiO2) grow as a layer on top of the Si+1 and Si+2 suboxides that are located at the interface. The surface/interface related carbon is found to decrease dramatically, but not to be totally eliminated, after the large oxygen exposures made.
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| 49. |
- Virojanadara, Chariya, et al.
(författare)
-
Photoemission study of Si-rich 4H-SiC surfaces and initial SiO2 SiC interface formation
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:19
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Tidskriftsartikel (refereegranskat)abstract
- Photoemission studies of Si-rich polar and nonpolar 4H-SiC surfaces before and after oxygen exposures are reported. For the clean Si-rich (0001) -3×3 surface, three prominent surface-shifted components are revealed in the Si 2p spectrum. This observation agrees well with the structural model suggested for this surface but disagrees with earlier results where only two surface-shifted Si 2p components were identified. Also for the other three Si-rich surfaces investigated three similar surface components are revealed although with different relative strengths. The C 1s spectrum exhibits only one sharp bulk peak for the clean Si-rich surfaces. This is different compared to earlier results from the same surfaces prepared by in situ heating only. The effects induced upon initial oxidation of these Si-rich surfaces are investigated. Recorded Si 2p spectra show only one suboxide, Si+1 and Si+2, for the polar (0001) and (000 1¯) surfaces, respectively, besides the fully developed Si+4 oxide (Si O2). For the nonpolar surfaces two suboxide, Si+1 and Si+2, are observed. Similarities and differences compared to earlier findings are discussed. Valence band spectra collected from clean surfaces, before Si deposition, are presented for the nonpolar surfaces. The presence of a sharp structure at binding energies of about 2.0 and 2.7 eV for the (10 1¯ 0) and the (11 2¯ 0) surfaces, respectively, is observed. This structure shows no dispersion with photon energy and is very sensitive to oxygen exposures and is therefore tentatively suggested to be a surface resonance state. © 2005 The American Physical Society.
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| 50. |
- Virojanadara, Chariya, et al.
(författare)
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Schottky barrier height studies of Au/4H-SiC(0001) using photoemission and synchrotron radiation
- 2002
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Ingår i: Journal of Electronic Materials. - : Springer Science and Business Media LLC. - 0361-5235 .- 1543-186X. ; 31:12, s. 1353-1356
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Tidskriftsartikel (refereegranskat)abstract
- The Schottky barrier height (SBH) of Au on 4H-SiC(0001) has been studied using photoemission and synchrotron radiation. The Au was deposited in-situ on clean and well-ordered root3 X roots R30degrees reconstructed SiC surfaces prepared by in situ heating at similar to950degreesC. The SBH was determined from the shift observed in the Si 2p core level, in addition to the initial band bending determined for the clean surface. The results were compared with values obtained by electrical, capacitance-voltage (C-V), and current-voltage (I-V) characterization methods. A favorable comparison between the three independent, SBH determination methods was found.
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