| 1. |
- Johansson, Leif I, et al.
(författare)
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Li induced effects in the core level and pi-band electronic structure of graphene grown on C-face SiC
- 2015
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Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Institute of Physics (AIP). - 0734-2101 .- 1520-8559. ; 33:6
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Tidskriftsartikel (refereegranskat)abstract
- Studies of the effects induced in the electronic structure after Li deposition, and subsequent heating, on graphene samples prepared on C-face SiC are reported. The as prepared graphene samples are essentially undoped, but after Li deposition, the Dirac point shifts down to 1.2 eV below the Fermi level due to electron doping. The shape of the C 1s level also indicates a doping concentration of around 10(14) cm(-2) after Li deposition, when compared with recent calculated results of core level spectra of graphene. The C 1s, Si 2p, and Li 1s core level results show little intercalation directly after deposition but that most of the Li has intercalated after heating at 280 degrees C. Heating at higher temperatures leads to desorption of Li from the sample, and at 1030 degrees C, Li can no longer be detected on the sample. The single pi-band observable from multilayer C-face graphene samples in conventional angle resolved photoelectron spectroscopy is reasonably sharp both on the initially prepared sample and after Li deposition. After heating at 280 degrees C, the p-band appears more diffuse and possibly split. The Dirac point becomes located at 0.4 eV below the Fermi level, which indicates occurrence of a significant reduction in the electron doping concentration. Constant energy photoelectron distribution patterns extracted from the as prepared graphene C-face sample and also after Li deposition and heating at 280 degrees C look very similar to earlier calculated distribution patterns for monolayer graphene. (C) 2015 Author(s).
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| 2. |
- Nuala, M.Caffrey, et al.
(författare)
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Structural and electronic properties of Li-intercalated graphene on SiC(0001)
- 2016
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Ingår i: Physical Review B: covering condensed matter and materials physics. - : American Physical Society. - 2469-9950. ; 93:19, s. 195421-1-195421-9
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the structural and electronic properties of Li-intercalated monolayer graphene on SiC(0001) using combined angle-resolved photoemission spectroscopy and first-principles density functional theory. Li intercalates at room temperature both at the interface between the buffer layer and SiC and between the two carbon layers. The graphene is strongly n-doped due to charge transfer from the Li atoms and two pi bands are visible at the (K) over bar point. After heating the sample to 300 degrees C, these pi bands become sharp and have a distinctly different dispersion to that of Bernal-stacked bilayer graphene. We suggest that the Li atoms intercalate between the two carbon layers with an ordered structure, similar to that of bulk LiC6. An AA stacking of these two layers becomes energetically favourable. The pi bands around the (K) over bar point closely resemble the calculated band structure of a C6LiC6 system, where the intercalated Li atoms impose a superpotential on the graphene electronic structure that opens gaps at the Dirac points of the two pi cones.
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| 3. |
- Polley, Craig Michael, 1984, et al.
(författare)
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Origin of the π -band replicas in the electronic structure of graphene grown on 4H -SiC(0001)
- 2019
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9969 .- 2469-9950. ; 99:11
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Tidskriftsartikel (refereegranskat)abstract
- The calculated electronic band structure of graphene is relatively simple, with a Fermi surface consisting only of six Dirac cones in the first Brillouin zone-one at each (K) over bar. In contrast, angle-resolved photoemission measurements of graphene grown on SiC(0001) often show six satellite Dirac cones surrounding each primary Dirac cone. Recent studies have reported two further Dirac cones along the (Gamma) over bar-(K) over bar line, and argue that these are not photoelectron diffraction artifacts but real bands deriving from a modulation of the ionic potential in the graphene layer. Here we present measurements using linearly polarized synchrotron light which show all of these replicas as well as several additional ones. Using information obtained from dark corridor orientations and angular warping, we demonstrate that all but one of these additional features-including those previously assigned as real initial-state bands-are possible to explain by simple final-state photoelectron diffraction.
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| 4. |
- Virojanadara (Jacobi), Chariya, 1977-
(författare)
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Studies of Surface and Interface Properties of 4H-SiC/Si02
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis work is focused on photoemission studies of clean and oxidized 4H-SiC surfaces since 4H-SiC has been considered the most promising for device applications. Oxide layers can be easily grown by standard wet and dry oxidation methods but to obtain good interface properties (i.e. low defect densities) is a major problem for the utilization of SiC. It had been claimed that formation of carbon containing by-products at the interface was a major problem. Therefore the first effort made was to determine if any carbon byproducts could be detected at the interface of SiO2/SiC(0001) samples prepared by ex-situ dry oxidation. The results obtained showed no carbon by-products at the interface but a weak contribution from graphite like carbon on the surface of the samples. This surface carbon could however shadow the presence of carbon byproducts at the interface. It was estimated that a 0.12 Å thick graphite layer would have been required at the interface in order to be detected in the experiments performed. To avoid the surface carbon contamination oxide layers were then instead prepared by in-situ oxidation. The results obtained from such samples, where the oxide thickness was varied from about 5 to 25 Å, showed no carbon containing by-products at the interface, the surface, or in the oxide layers. The presence of an intermediate oxide located at the interface and interpreted to correspond to si+1 oxidation states was instead revealed. This suboxide was suggested to be a factor that could contribute to the interface defect density. The observation of only one suboxide (Si+1)was, however, in conflict with results reported by other research groups. They claimed the formation of foursuboxides (Si+1, si+2, Si+3 Si+4) in oxidation studies of SiC(0001), i.e. similar as on elemental Si surfaces.Therefore an additional effort was made to resolve this issue using higher photon energies. This allowed us to study also Si 1s core level and Si KLL Auger spectra on SiO2/SiC(0001) samples where the oxide thickness was varied from about 5 to 120 Å. These results confirmed that the presence of only one suboxide was possible to detect and that it was located at the interface. It was moreover found that the SiO2 oxide chemical shift exhibited a dependence on the oxide thickness. The shift was found to be fairly constant for oxides less than ca 10 Å thick, to increase by about 0.5 eV when increasing the oxide thickness to around 25 Å and then to gradually saturate for thicker oxides. In the O 1s core level no changes in the line shape could be observed with oxide thickness or after different oxygen exposures when keeping the substrate at an elevated temperature.However, slight differences in the O 1s line shape were reveled after small exposures made at room temperature (RT) and at elevated temperature. After exposures at 800°C the O 1s spectrum exhibited only a slightly asymmetric main peak originating from formation of stable oxides. After exposures at RT and even more pronounced at LN temperature the O 1s spectrum exhibited additional structures at lower and higher binding energies and a considerably broader main peak. To model these spectra three additional components had to be used. From their exposure and time-decay dependence they were identified to originate from metastable oxygen.Experiments using a nitrogen containing gas (NO and N2O) for the oxidation of SiC at a substratetemperature of 800°C were also performed. The reason was that the use of a nitrogen containing gas had been claimed to improve the interface properties. For NO exposed samples it was found that the amount of suboxide (Si+1) at the interface was reduced and replaced by a Si3N4. No significant difference in C 1s spectra recorded after NO and 02 exposure could be revealed, however. This suggested that carbon byproducts do not appear to be a main reason for the interface defect density for SiO2/SiC (0001) samples grown in NO. For oxide samples grown using N2O no N 1s signal could be detected and only the Si+1 suboxide was observed besides the fully developed oxide, SiO2. The absence of nitrogen at the interface for these samples was suggested to be due to that the substrate temperature selected was too low to efficiently dissociate N2O.Included in the thesis are also photoemission studies of the polar (0001̅ ) and nonpolar (101̅ 0) and (11 2̅ 0)surfaces. For the clean polar (0001̅ ) surface the C1s spectra showed that the surface was carbon rich and composed of at least two carbon layers on the surface. This surface carbon was found to be not completely eliminated after the largest oxygen exposures made (106 L). This was opposite to the observations made for the polar (0001) surface at similar exposures. Only one suboxide, with a different chemical shift suggesting it to originate from Si+2 oxidation states, was observed in the Si 2p spectra. It was interpreted to originate from Si-O-C bonding due to the excesses surface/interface carbon present on the (0001̅ ) surface. Also the clean nonpolar surfaces were found to be carbon rich (Si depleted) or rearranged in such a way that the carbon atoms move outward and Si atoms move inward. After similarly large oxygen exposures as for the polar surfaces a weak contribution from carbon by-products was found to remain at the interface for the nonpolar surfaces. The Si 2p data recorded showed the presence of two suboxides (Si+1 and Si+2) besides Si02. This seemed reasonable and consistent with the earlier observations since the ideal (i.e. bulk truncated) nonpolar surfaces are composed of both silicon and carbon atoms. Studies of oxide samples identically prepared on the four different surfaces were carried out. The results showed the oxidation rate to be highest on the (101̅ 0) surface, somewhat lower on the (0001̅ ) and (11 2̅ 0) surfaces, and about a factor of two lower on the (0001) surface.The SiO2/SiC interface and surface properties of Si-rich SiC surfaces, prepared by Si deposition, was also investigated. After Si deposition the Si 2p data collected showed three prominent surface shifted peaks for both the polar and nonpolar surfaces indicating Si surface enrichment. The C 1s spectrum then showed only one sharp bulk SiC peak. After smaller oxygen exposures recorded core level spectra showed that the Si adlayers, created by the Si deposition, were not completely oxidized but that the C 1s spectrum was affected. The C 1s peak was broadened and a small asymmetry on the high binding side could be detected. After large oxygen exposures, i.e. when the Si adlayers were fully oxidized, the presence of the same suboxide(s) as for the corresponding surface prepared without Si deposition was(were) detected for all four surfaces.
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