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Growth of 3C-SiC and Graphene for Solar Water-Splitting Application
Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
2019 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

Silicon carbide (SiC) is regarded as an important semiconductor for a variety of applications including high-temperature, high-power and high-frequency devices. The most common polytypes of SiC are hexagonal (4H- or 6H-SiC) and cubic silicon carbide (3C-SiC), which differ from each other by the ordering of the Si–C bilayers along the c-axis crystal direction. Among different polytypes of SiC, 3C-SiC has attracted specific interest due to its prominent properties such as high electron mobility and low interface trap density in MOSFET devices. Moreover, with a relatively small bandgap of 2.36 eV and suitable conduction and valence band positions, 3C-SiC has also been considered as a promising material for solar water splitting application, which provides a completely renewable approach to convert solar energy into storable hydrogen fuel. However, the growth of high-quality 3C-SiC remains a great challenge for decades.

Graphene, a single layer of sp2-bonded carbon atoms, has shown outstanding electronic properties and becomes the most promising candidate for next-generation electronic and optoelectronic devices. Epitaxial growth of graphene on SiC substrates by sublimation of Si from SiC provides a feasible route to fabricate wafer-scale device-quality graphene. The most advantage of this method is that a variety of devices can be processed directly on graphene/SiC without any transfer process, which is needed in the case of graphene produced by exfoliation or CVD on metals. During past years, the growth of monolayer (ML) graphene on hexagonal SiC (6H-SiC, 4H-SiC) substrates has been extensively studied. However, it is challenging to grow large-area and uniform multilayer graphene on hexagonal SiC substrates due to the stepbunching issue during the sublimation growth.

Multilayer graphene has recently attracted great interest due to its tunable electronic properties for various electronic and optoelectronic applications. It has been shown that the electronic properties of multilayer graphene are strongly influenced by its stacking sequence. In particular, the rhombohedral stacking sequence (ABC stacking) has shown its potential to introduce a flat band energy dispersion at the K points of the Brillouin zone, which would result in many exotic phases of matter such as superconductivity. Among various SiC polytypes, 3CSiC is predicted to be the most suitable substrate for the epitaxial growth of rhombohedral multilayer graphene.

This thesis work mainly covers the sublimation growth of high-quality Si-face and C-face 3C-SiC on off-oriented 4H-SiC, exploring the proper parameter window for the growth of homogeneous graphene layers ranging from monolayer to multilayer on Si-face off-oriented 3C-SiC and the growth of graphene on C-face 3C-SiC, as well as the characterizations on 3CSiC and graphene. Moreover, as a proof of concept, photoelectrochemical (PEC) water splitting cells based on the Si-face and C-face 3C-SiC have been fabricated to study the conversion of solar energy into chemical fuel, hydrogen.

Firstly, the high-quality bulk-like Si-face and C-face 3C-SiC(111) were grown on 4- degree off-oriented 4H-SiC substrates by the sublimation epitaxy technique. The C-face sample exhibited a smoother surface with a step height of one-unit cell without the step bunching. In contrast, the Si-face 3C-SiC showed larger steps with a height of two-unit cells of 3C-SiC due to the pronounced step bunching. The cross-sectional studies showed that C-face 3C-SiC exhibited less polytype-transition layer than the Si-face sample. This would help the lateral enlargement of 3C-SiC domains. We also demonstrated that the crystalline quality of C-face 3C-SiC was comparable to the Si-face sample.

Secondly, we systematically studied the growth of monolayer and multilayer graphene on off-axis 3C-SiC(111). Taking advantage of the synergistic effect of periodic SiC step edges as graphene nucleation sites and the unique thermal decomposition energy of 3C-SiC steps, we demonstrated that the step bunching was fully eliminated during graphene growth on Si-face 3C-SiC and large-area monolayer, bilayer, and four-layer graphene were controllably obtained on high-quality off-axis Si-face 3C-SiC(111). The growth of uniform four-layer graphene over areas of tens of square micrometers was demonstrated. The electronic structures of multilayer graphene with different stacking sequences were systematically studied by experimental and theoretical analysis. It was demonstrated that the four-layer graphene exhibited rhombohedral stacking sequence, which introduced a flat band near the Fermi level. Moreover, the flat-band width and bandgap can be tuned by the interlayer spacing of graphene. In contrast, graphene layers grown on the off-axis C-face 3C-SiC(1̄1̄1̄) showed 1ML to 4ML graphene domains with large-area coverage over several of square micrometers and there was no buffer layer underneath. The low energy electron diffraction pattern collected on the monolayer graphene domain demonstrated four sets of graphene (1 x 1) spots, indicating the existence of rotational disorders within the monolayer graphene. To compare with graphene growth on the off-oriented 3C-SiC, the growth of graphene on off-oriented 4H-SiC epilayers was also explored. The 4HSiC epilayers were first grown on 4-degree off-oriented 4H-SiC substrates and periodically inclined step facets in-between terraces were induced on 4H-SiC epilayers due to the pronounced step bunching. The graphene grown on such step-structured surface of off-oriented 4H-SiC showed that the terraces were mainly covered by monolayer graphene and the buffer layer underneath it while on the step facets, graphene was strongly buckled and appeared to be largely decoupled from the surface.

Finally, the PEC water splitting performance based on the Si-face and C-face 3C-SiC was systematically studied. It was found that the SiC surface polarity played an important role in the PEC performance. The influence of both Si-face and C-face on surface proton transfer was investigated. It was demonstrated that the Si-face SiC was more energy-favorable, thus making oxygen evolution reaction operate at a very low overpotential. Furthermore, the PEC watersplitting performance was significantly enhanced by using NiO/3C-SiC p-n junction as a photoanode. A high photovoltage of 1.0 V, a photocurrent density of 1.01 mA/cm-2 at 0.55 V versus reversible hydrogen electrode (VRHE), a low onset potential of 0.20 VRHE and a high fill factor of 57% were demonstrated in the PEC water splitting cell under AM1.5G 100 mW cm-2 illumination.

Ort, förlag, år, upplaga, sidor
Linköping: Linköping University Electronic Press, 2019. , s. 42
Serie
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2003
Nationell ämneskategori
Den kondenserade materiens fysik
Identifikatorer
URN: urn:nbn:se:liu:diva-159100DOI: 10.3384/diss.diva-159100ISBN: 9789176850213 (tryckt)OAI: oai:DiVA.org:liu-159100DiVA, id: diva2:1338851
Disputation
2019-09-12, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (Engelska)
Opponent
Handledare
Forskningsfinansiär
Vetenskapsrådet, 621-2014-5461Vetenskapsrådet, 2018-04670Forskningsrådet Formas, 2016-00559Stiftelsen för internationalisering av högre utbildning och forskning (STINT), CH2016-6722ÅForsk (Ångpanneföreningens Forskningsstiftelse), 16-399Stiftelsen Olle Engkvist Byggmästare, 189-0243Tillgänglig från: 2019-07-24 Skapad: 2019-07-24 Senast uppdaterad: 2019-09-09Bibliografiskt granskad
Delarbeten
1. A comparative study of high-quality C-face and Si-face 3C-SiC(1 1 1) grown on off-oriented 4H-SiC substrates
Öppna denna publikation i ny flik eller fönster >>A comparative study of high-quality C-face and Si-face 3C-SiC(1 1 1) grown on off-oriented 4H-SiC substrates
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2019 (Engelska)Ingår i: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 52, nr 34Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We present a comparative study of the C-face and Si-face of 3C-SiC(111) grown on off-oriented 4H-SiC substrates by the sublimation epitaxy. By the lateral enlargement method, we demonstrate that the high-quality bulk-like C-face 3C-SiC with thickness of ~1 mm can be grown over a large single domain without double positioning boundaries (DPBs), which are known to have a strongly negative impact on the electronic properties of the material. Moreover, the C-face sample exhibits a smoother surface with one unit cell height steps while the surface of the Si-face sample exhibits steps twice as high as on the C-face due to step-bunching. High-resolution XRD and low temperature photoluminescence measurements show that C-face 3C-SiC can reach the same high crystalline quality as the Si-face 3C-SiC. Furthermore, cross-section studies of the C- and Si-face 3C-SiC demonstrate that in both cases an initial homoepitaxial 4H-SiC layer followed by a polytype transition layer are formed prior to the formation and lateral expansion of 3C-SiC layer. However, the transition layer in the C-face sample is extending along the step-flow direction less than that on the Si-face sample, giving rise to a more fairly consistent crystalline quality 3C-SiC epilayer over the whole sample compared to the Si-face 3C-SiC where more defects appeared on the surface at the edge. This facilitates the lateral enlargement of 3C-SiC growth on hexagonal SiC substrates.

Ort, förlag, år, upplaga, sidor
Biopress Ltd, 2019
Nationell ämneskategori
Den kondenserade materiens fysik
Identifikatorer
urn:nbn:se:liu:diva-159101 (URN)10.1088/1361-6463/ab2859 (DOI)000475964100002 ()
Anmärkning

Funding agencies:  Swedish Research Council (Vetenskapsradet) [621-2014-5461, 2018-04670, 2016-05362, 621-2014-5825]; Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS) [2016-00559]; Swedish Foundation for International Cooperation

Tillgänglig från: 2019-07-24 Skapad: 2019-07-24 Senast uppdaterad: 2019-08-07
2. Elimination of step bunching in the growth of large-area monolayer and multilayer graphene on off-axis 3CSiC (111)
Öppna denna publikation i ny flik eller fönster >>Elimination of step bunching in the growth of large-area monolayer and multilayer graphene on off-axis 3CSiC (111)
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2018 (Engelska)Ingår i: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 140, s. 533-542Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Multilayer graphene has exhibited distinct electronic properties such as the tunable bandgap for optoelectronic applications. Among all graphene growth techniques, thermal decomposition of SiC is regarded as a promising method for production of device-quality graphene. However, it is still very challenging to grow uniform graphene over a large-area, especially multilayer graphene. One of the main obstacles is the occurrence of step bunching on the SiC surface, which significantly influences the formation process and the uniformity of the multilayer graphene. In this work, we have systematically studied the growth of monolayer and multilayer graphene on off-axis 3CSiC(111). Taking advantage of the synergistic effect of periodic SiC step edges as graphene nucleation sites and the unique thermal decomposition energy of 3CSiC steps, we demonstrate that the step bunching can be fully eliminated during graphene growth and large-area monolayer, bilayer, and four-layer graphene can be controllably obtained on high-quality off-axis 3CSiC(111) surface. The low energy electron microscopy results demonstrate that a uniform four-layer graphene has been grown over areas of tens of square micrometers, which opens the possibility to tune the bandgap for optoelectronic devices. Furthermore, a model for graphene growth along with the step bunching elimination is proposed.

Ort, förlag, år, upplaga, sidor
Elsevier, 2018
Nationell ämneskategori
Materialkemi
Identifikatorer
urn:nbn:se:liu:diva-151054 (URN)10.1016/j.carbon.2018.08.042 (DOI)000450120200057 ()
Anmärkning

Funding agencies: Swedish Research Council (Vetenskapsradet) [621-2014-5461, 621-2014-5825]; Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS) [2016-00559]; Swedish Foundation for International Cooperation in Research and Higher 

Tillgänglig från: 2018-09-12 Skapad: 2018-09-12 Senast uppdaterad: 2019-07-24
3. Flat-Band Electronic Structure and Interlayer Spacing Influence in Rhombohedral Four-Layer Graphene
Öppna denna publikation i ny flik eller fönster >>Flat-Band Electronic Structure and Interlayer Spacing Influence in Rhombohedral Four-Layer Graphene
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2018 (Engelska)Ingår i: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 18, nr 9, s. 5862-5866Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

The stacking order of multilayer graphene significantly influences its electronic properties. The rhombohedral stacking sequence is predicted to introduce a flat band, which has high density of states and the enhanced Coulomb interaction between charge carriers, thus possibly resulting in superconductivity, fractional quantum Hall effect, and many other exotic phases of matter. In this work, we comprehensively study the effect of the stacking sequence and interlayer spacing on the electronic structure of four-layer graphene, which was grown on a high crystalline quality 3C-SiC(111) crystal. The number of graphene layers and coverage were determined by low energy electron microscopy. First-principles density functional theory calculations show distinctively different band structures for ABAB (Bernal), ABCA (rhombohedral), and ABCB (turbostratic) stacking sequences. By comparing with angle-resolved photoelectron spectroscopy data, we can verify the existence of a rhombohedral stacking sequence and a nearly dispersionless electronic band (flat band) near the Fermi level. Moreover, we find that the momentum width, bandgap, and curvature of the flat-band region can be tuned by the interlayer spacing, which plays an important role in superconductivity and many other exotic phases of matter. © 2018 American Chemical Society.

Ort, förlag, år, upplaga, sidor
American Chemical Society, 2018
Nyckelord
flat-band; Graphene; interlayer spacing; rhombohedral stacking; superconductor
Nationell ämneskategori
Den kondenserade materiens fysik
Identifikatorer
urn:nbn:se:liu:diva-151307 (URN)10.1021/acs.nanolett.8b02530 (DOI)30136852 (PubMedID)2-s2.0-85052867510 (Scopus ID)
Tillgänglig från: 2018-09-17 Skapad: 2018-09-17 Senast uppdaterad: 2019-07-24

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