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Characterizations of as grown and functionalized epitaxial grapheneg rown on SiC surfaces
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The superior electronic and mechanical properties of Graphene have promoted graphene to become one of the most promising candidates for next generation of electronic devices. Epitaxial growth of graphene by sublimation of Si from Silicon Carbide (SiC) substrates avoids the hazardous transfer process for large scale fabrication of graphene based electronic devices. Moreover, the operation conditions can potentially be extended to high temperatures, voltages and frequencies. This thesis is focused on characterizations of as grown and functionalized epitaxial graphene grown on both Si-face and C-face SiC. Synchrotron radiation-based techniques are employed for detailed investigations of the electronic properties and surface morphology of as grown and functionalized graphene.

Large area and homogeneous monolayer (ML) graphene has been possible to grow on SiC(0001) substrates by sublimation, but efforts to obtain multilayer graphene of similar quality have been in vain. A study of the transport behavior of silicon atoms through carbon layers was therefore performed for the purpose to gain a better understanding of the growth mechanism of graphene on Si-face SiC. It showed that a temperature of about 800°C is required for Si intercalation into the interface to take place. Intercalation of Si was found to occur only via defects and domain boundaries which probably is the reason to the limited growth of multilayer graphene. Annealing at 1000-1100°C induced formation of SiC on the surface and after annealing above 1200°C Si started to de-intercalate and desorb/sublimate.

Different alkali metals were found to affect graphene grown in SiC quite differently. Li started to intercalate already at room temperature by creating cracks and defects, while K, Rb and Cs were found unable to intercalate into the graphene/SiC interface. Effects induced by the alkali metal Na on graphene grown on both Si-face and C-face SiC were therefore studies. For the Si-face, partial intercalation of Na through graphene was observed on both 1 ML and 2 ML areas directly after Na deposition. Annealing at a temperature of about 75°C strongly promoted Na intercalation at the interface. The intercalation was confirmed to start at domain boundaries between 1 ML and 2 ML areas and at stripes/streaks on the 1 ML areas. Higher annealing temperature resulted in desorption of Na from the sample surface. Also for C-face graphene, a strong n-type doping was observed directly after Na deposition. Annealing at temperatures from around 120 to 300 °C was here found to result in a considerable π-band broadening, interpreted to indicate penetration of Na in between the graphene layers and at the graphene SiC interface.

The thermal stability of graphene based electronic devices can depend on the choice of contact material. Studies of the stability and effects induced by two commonly used metals (Pt and Al) on Si-face graphene were carried out after deposition and after subsequent annealing at different temperatures. Both Al and Pt were found to be good contact materials at room temperature. Annealing at respectively ~400 ºC and ~ 800 ºC was found to trigger intercalation of Al and Pt into the graphene/SiC interface, and induce quasi-free-standing bilayer electronic properties. Contacts of Pt can thus withstand higher temperatures than Al contacts. For Al inhomogeneous islands of different ordered phases were observed to form on the surface during annealing, while this was not the case for Pt. The initial single π-band structure was in the Al case restored after annealing at ~1200 ºC although some Al remained detectable from the sample. For Pt, the bilayer graphene electronic properties induced by intercalation were thermally stable up to 1200ºC. In the case of Al the stability and effects induced on C-face graphene were also investigated for comparison, and significant differences were revealed. An ordered Al-Si-C compound was found to form after annealing at temperatures between ca. 500ºC and 700ºC. Formation of this compound was accompanied with a large reduction of graphene in the surface region. Annealing at temperatures above 800°C resulted in a gradual decomposition of this compound and regrowth of graphene. No Al signal could be detected after annealing C-face graphene at 1000°C.

Graphene grown on C-face SiC has attracted high interest since its mobility has been reported to be one order of magnitude higher compared to Si-face graphene. C-face graphene has moreover been claimed to be fundamentally different compared to Si-face graphene. A rotational disorder between adjacent graphene layers has been suggested that effectively decouples the graphene layers and result in monolayer electronic properties of multilayer C-face graphene. The domain/grain size is typically much smaller for C-face graphene and the number of graphene layers less uniform than on Si-face graphene. Using LEEM and micro-LEED we showed that there is no rotational disorder between adjacent layers within the domains/grains but that they had different azimuthal orientations. Using nano-APRES, we recently also revealed that multilayer Cface graphene show multiple π-bands and Bernal stacking, similar to multilayer Si-face graphene.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. , 32 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1689
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-120893DOI: 10.3384/diss.diva-120893ISBN: 978-91-7685-998-8 (print)OAI: oai:DiVA.org:liu-120893DiVA: diva2:849345
Public defence
2015-10-08, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2015-08-28 Created: 2015-08-28 Last updated: 2016-08-31Bibliographically approved
List of papers
1. Si intercalation/deintercalation of graphene on 6H-SiC(0001)
Open this publication in new window or tab >>Si intercalation/deintercalation of graphene on 6H-SiC(0001)
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2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 4, 045418- p.Article in journal (Refereed) Published
Abstract [en]

The intercalation and deintercalation mechanisms of Si deposited on monolayer graphene grown on SiC(0001) substrates and after subsequent annealing steps are investigated using low-energy electron microscopy (LEEM), photoelectron spectroscopy (PES), and micro-low-energy electron diffraction (mu-LEED). After Si deposition on samples kept at room temperature, small Si droplets are observed on the surface, but no intercalation can be detected. Intercalation is revealed to occur at an elevated temperature of about 800. C. The Si is found to migrate to the interface region via defects and domain boundaries. This observation may provide an answer to the problem of controlling homogeneous bi-/multilayer graphene growth on nearly perfect monolayer graphene samples prepared on SiC(0001). Likewise, Si penetrates more easily small monolayer graphene domains because of the higher density of domain boundaries. Upon annealing at 1000-1100 degrees C, formation of SiC on the surface is revealed by the appearance of a characteristic surface state located at about 1.5 eV below the Fermi level. A streaked mu-LEED pattern is also observed at this stage. The SiC formed on the surface is found to decompose again after annealing at temperatures higher than 1200 degrees C.

Place, publisher, year, edition, pages
American Physical Society, 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-74640 (URN)10.1103/PhysRevB.85.045418 (DOI)000298988000005 ()
Note

Funding Agencies|EU||VR Linnaeus||

Available from: 2012-02-03 Created: 2012-02-03 Last updated: 2017-12-08
2. Changes in structural and electronic properties of graphene grown on 6H-SiC(0001) induced by Na deposition
Open this publication in new window or tab >>Changes in structural and electronic properties of graphene grown on 6H-SiC(0001) induced by Na deposition
2012 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 111, no 8, 083711- p.Article in journal (Refereed) Published
Abstract [en]

The effects of Na deposited on monolayer graphene on SiC(001) were investigated by synchrotron-based photoelectron spectroscopy and angle resolved photoelectron spectroscopy. The experimental results show that Na prefers to adsorb on the graphene layer after deposition at room temperature. Nonetheless, part of the Na atoms are able to intercalate in between the graphene and the buffer layer and some go even further into the substrate interface as indicated by the shift of the bulk SiC component in the C 1s and Si 2p core level spectra. The ARPES spectrum exhibits a lowering of the Dirac point indicating increased n-type doping of the monolayer graphene induced by the deposited Na atoms. Upon subsequently heating the sample, we found that a slightly elevated temperature is essential in order to promote Na intercalation. A fully Na intercalation at the graphene-SiC interface is obtained after heating at a temperature of about 75 degrees C. The intercalated Na decouples the buffer layer and transforms it into a second graphene layer so two pi-bands are observed in the ARPES spectra. Interestingly, the two bands show different locations of the Dirac point but both exhibit linear dispersion in the vicinity of the (K) over bar point and not the hyperbolic dispersion observed for AB stacked bi-layer graphene. When heating the sample to about 125 degrees C or higher, Na is found to leave the interface and the second graphene layer is transformed back to the carbon buffer layer.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-77872 (URN)10.1063/1.4704396 (DOI)000303598800062 ()
Note
Funding Agencies|EU||Available from: 2012-05-31 Created: 2012-05-31 Last updated: 2017-12-07
3. Detailed studies of Na intercalation on furnace-grown graphene on 6H-SiC(0001)
Open this publication in new window or tab >>Detailed studies of Na intercalation on furnace-grown graphene on 6H-SiC(0001)
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2013 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 613, 88-94 p.Article in journal (Refereed) Published
Abstract [en]

The effects induced by Na deposited on furnace grown graphene on SiC(0001) and after subsequent annealing are investigated using LEEM, mu-LEED, mu-PES and XPEEM. Intercalation in between carbon layers and at the interface is observed to occur both on the 1 ML and 2 ML areas directly after Na deposition. Annealing at a temperature around 100 degrees C is found to strongly promote Na intercalation. Exposure to the electron beam or the focused synchrotron radiation in the LEEM/XPEEM is also found to promote the intercalation, which is confirmed to begin at domain boundaries between the 1 ML and 2 ML areas, and also as stripe/streak-like features on the 1 ML areas. The XPEEM data show Na adsorption on the surface and intercalation at the interface to be quite non-uniform. When annealing at higher temperatures Na starts to de-intercalate and leave the sample, but Na is still detectable on the sample after annealing at 240 degrees C.

Place, publisher, year, edition, pages
Elsevier, 2013
Keyword
Epitaxial graphene on SiC, Sodium intercalation, LEEM, XPEEM, mu-LEED, mu-PES
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-94593 (URN)10.1016/j.susc.2013.03.011 (DOI)000319180600014 ()
Note

Funding Agencies|Swedish Research Council|621-2011-4252|ESF||EU||

Available from: 2013-06-27 Created: 2013-06-27 Last updated: 2017-12-06
4. High thermal stability quasi-free-standing bilayer graphene formed on 4H-SiC(0 0 0 1) via platinum intercalation
Open this publication in new window or tab >>High thermal stability quasi-free-standing bilayer graphene formed on 4H-SiC(0 0 0 1) via platinum intercalation
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2014 (English)In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 79, 631-635 p.Article in journal (Refereed) Published
Abstract [en]

Influences on electronic structure induced by platinum (Pt) deposited on monolayer graphene grown on SiC(0 0 0 1) are investigated by photoelectron spectroscopy (PES), selected area low energy electron diffraction (μ-LEED) and angle resolved photoelectron spectroscopy (ARPES) techniques at the MAX Laboratory. Stable monolayer graphene electronic properties are observed after Pt deposition and after annealing at temperatures below 600 °C. At ⩾600 °C platinum silicide forms at the graphene/SiC interface. Annealing at 900 °C results in an efficient decoupling of the carbon buffer layer from the SiC substrate and transformation into a second graphene layer. At this stage a quasi-free standing bi-layer graphene sample is obtained. The new superstructure spots then appearing in μ-LEED pattern suggest formation of an ordered platinum silicide at the interface. This silicide is found to be stable even after annealing at temperature up to 1200 °C.

Place, publisher, year, edition, pages
Elsevier, 2014
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-111494 (URN)10.1016/j.carbon.2014.08.027 (DOI)
Available from: 2014-10-20 Created: 2014-10-20 Last updated: 2017-12-05Bibliographically approved
5. Effects of Al on epitaxial graphene grown on 6H-SiC(0001)
Open this publication in new window or tab >>Effects of Al on epitaxial graphene grown on 6H-SiC(0001)
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2014 (English)In: Materials Research Express, ISSN 2053-1591, Vol. 1, no 1, 1-13 p., 015606Article in journal (Refereed) Published
Abstract [en]

Aluminum was deposited on epitaxial monolayer-grown graphene on SiC(0001). The effects of annealing up to 1200 °C on the surface and interface morphology, chemical composition, and electron band structure were analyzed in situ by synchrotron-based techniques at the MAX Laboratory. After heating at around 400 °C, Al islands or droplets are observed on the surface and the collected Si 2p, Al 2p, and C 1s core levels spectra indicate Al intercalation at the graphene SiC interface. Also, the original single π -band splits into two, indicating decoupling of the carbon buffer layer and the formation of a quasi-free-standing bilayer-like electronic structure. Further heating at higher temperatures from 700 to 900 °C yields additional chemical reactions. Broader core level spectra are then observed and clear changes in the π -bands near the Dirac point are detected. More electron doping was detected at this stage since one of the π -bands has shifted to about 1.1 eV below the Fermi level. Different ordered phases of (7x7), (4x4), (1x1)Al , and (1x1)G were also observed on the surface in this temperature range. The original single π π-band was restored after heating at ~1200°C, although an Al signal was still able to be detected.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2014
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-120851 (URN)10.1088/2053-1591/1/1/015606 (DOI)
Available from: 2015-08-28 Created: 2015-08-28 Last updated: 2016-08-31Bibliographically approved
6. Effects of aluminum on epitaxial graphene grown on C-face SiC
Open this publication in new window or tab >>Effects of aluminum on epitaxial graphene grown on C-face SiC
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2015 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, no 19, 195306- p.Article in journal (Refereed) Published
Abstract [en]

The effects of Al layers deposited on graphene grown on C-face SiC substrates are investigated before and after subsequent annealing using low energy electron diffraction (LEED), photoelectron spectroscopy, and angle resolved photoemission. As-deposited layers appear inert. Annealing at a temperature of about 400 degrees C initiates migration of Al through the graphene into the graphene/SiC interface. Further annealing at temperatures from 500 degrees C to 700 degrees C induces formation of an ordered compound, producing a two domain root 7 x root 7R19 degrees LEED pattern and significant changes in the core level spectra that suggest formation of an Al-Si-C compound. Decomposition of this compound starts after annealing at 800 degrees C, and at 1000 degrees C, Al is no longer possible to detect at the surface. On Si-face graphene, deposited Al layers did not form such an Al-Si-C compound, and Al was still detectable after annealing above 1000 degrees C.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:liu:diva-119250 (URN)10.1063/1.4921462 (DOI)000355005600036 ()
Note

Funding Agencies|Swedish Research Council [621-2011-4252]; Linnaeus Grant

Available from: 2015-06-12 Created: 2015-06-12 Last updated: 2017-12-04
7. Na induced changes in the electronic band structure of graphene grown on C-face SiC
Open this publication in new window or tab >>Na induced changes in the electronic band structure of graphene grown on C-face SiC
2013 (English)In: Graphene, ISSN 2169-3439, Vol. 2, no 1, 1-7 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Scientific Research Publishing, 2013
Keyword
Graphene on C-Face SiC; Graphene Band Structure; Na Intercalation; Constant Energy Photoelectron Angular Distribution Patterns
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-103822 (URN)10.4236/graphene.2013.21001 (DOI)
Available from: 2014-01-28 Created: 2014-01-28 Last updated: 2016-07-01Bibliographically approved
8. Is the Registry Between Adjacent Graphene Layers Grown on C-Face SiC Different Compared to That on Si-Face SiC
Open this publication in new window or tab >>Is the Registry Between Adjacent Graphene Layers Grown on C-Face SiC Different Compared to That on Si-Face SiC
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2013 (English)In: Crystals, ISSN 2073-4352, Vol. 3, no 1, 1-13 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
C-face graphene; layer registry; large grain sizes; sublimation growth; hydrogen treatment
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-91408 (URN)10.3390/cryst3010001 (DOI)
Available from: 2013-04-24 Created: 2013-04-24 Last updated: 2017-12-06
9. Multiple π-bands and Bernal stacking of multilayer graphene on C-face SiC, revealed by nano-Angle Resolved Photoemission
Open this publication in new window or tab >>Multiple π-bands and Bernal stacking of multilayer graphene on C-face SiC, revealed by nano-Angle Resolved Photoemission
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2014 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 4, no 4157Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Nature Publishing Group, 2014
Keyword
Electronic properties and materials, Graphene, Stacking
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-105279 (URN)10.1038/srep04157 (DOI)000331885900004 ()
Funder
Swedish Research Council, 621-2011-4252Swedish Research Council, 621-2011-4249Swedish Foundation for Strategic Research , 10-0026
Available from: 2014-03-14 Created: 2014-03-14 Last updated: 2017-12-05Bibliographically approved

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