Growth of 3C-SiC and Graphene for Solar Water-Splitting Application

Sammanfattning: 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.