| 1. |
- Pallon, J., et al.
(author)
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Ion beam evaluation of silicon carbide membrane structures intended for particle detectors
- 2016
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In: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier BV. - 0168-583X .- 1872-9584. ; 371, s. 132-136
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Journal article (peer-reviewed)abstract
- Thin ion transmission detectors can be used as a part of a telescope detector for mass and energy identification but also as a pre-cell detector in a microbeam system for studies of biological effects from single ion hits on individual living cells. We investigated a structure of graphene on silicon carbide (SiC) with the purpose to explore a thin transmission detector with a very low noise level and having mechanical strength to act as a vacuum window. In order to reach very deep cavities in the SiC wafers for the preparation of the membrane in the detector, we have studied the Inductive Coupled Plasma technique to etch deep circular cavities in 325 μm prototype samples. By a special high temperature process the outermost layers of the etched SiC wafers were converted into a highly conductive graphitic layer. The produced cavities were characterized by electron microscopy, optical microscopy and proton energy loss measurements. The average membrane thickness was found to be less than 40 μm, however, with a slightly curved profile. Small spots representing much thinner membrane were also observed and might have an origin in crystal defects or impurities. Proton energy loss measurement (also called Scanning Transmission Ion Microscopy, STIM) is a well suited technique for this thickness range. This work presents the first steps of fabricating a membrane structure of SiC and graphene which may be an attractive approach as a detector due to the combined properties of SiC and graphene in a monolithic materials structure.
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| 2. |
- Hens, Philip, et al.
(author)
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Large area buffer-free graphene on non-polar (001) cubic silicon carbide
- 2014
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In: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 80, s. 823-829
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Journal article (peer-reviewed)abstract
- Graphene is, due to its extraordinary properties, a promising material for future electronic applications. A common process for the production of large area epitaxial graphene is a high temperature annealing process of atomically flat surfaces from hexagonal silicon carbide. This procedure is very promising but has the drawback of the formation of a buffer layer consisting of a graphene-like sheet, which is covalently bound to the substrate. This buffer layer degenerates the properties of the graphene above and needs to be avoided. We are presenting the combination of a high temperature process for the graphene production with a newly developed substrate of (0 0 1)-oriented cubic silicon carbide. This combination is a promising candidate to be able to supply large area homogenous epitaxial graphene on silicon carbide without a buffer layer. We are presenting the new substrate and first samples of epitaxial graphene on them. Results are shown using low energy electron microscopy and diffraction, photoelectron angular distribution and X-ray photoemission spectroscopy. All these measurements indicate the successful growth of a buffer free few layer graphene on a cubic silicon carbide surface. On our large area samples also the epitaxial relationship between the cubic substrate and the hexagonal graphene could be clarified. (C) 2014 Elsevier Ltd. All rights reserved.
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| 3. |
- Jokubavicius, Valdas, et al.
(author)
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Lateral Enlargement Growth Mechanism of 3C-SiC on Off-Oriented 4H-SiC Substrates
- 2014
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In: Crystal Growth & Design. - : American Chemical Society (ACS). - 1528-7483 .- 1528-7505. ; 14:12, s. 6514-6520
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Journal article (peer-reviewed)abstract
- We introduce a 3C-SiC growth concept on off-oriented 4H-SiC substrates using a sublimation epitaxial method. A growth model of 3C-SiC layer development via a controlled cubic polytype nucleation on in situ formed on-axis area followed by a lateral enlargement of 3C-SiC domains along the step-flow direction is outlined. Growth process stability and reproducibility of high crystalline quality material are demonstrated in a series of 3C-SiC samples with a thickness of about 1 mm. The average values of full width at half-maximum of ω rocking curves on these samples vary from 34 to 48 arcsec indicating high crystalline quality compared to values found in the literature. The low temperature photoluminescence measurements also confirm a high crystalline quality of 3C-SiC and indicate that the residual nitrogen concentration is about 1–2 × 1016 cm–3. Such a 3C-SiC growth concept may be applied to produce substrates for homoepitaxial 3C-SiC growth or seeds which could be explored in bulk growth of 3C-SiC.
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| 4. |
- Syväjärvi, Mikael, et al.
(author)
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Cubic silicon carbide as a potential photovoltaic material
- 2016
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In: Solar Energy Materials and Solar Cells. - : Elsevier. - 0927-0248 .- 1879-3398. ; 145, s. 104-108
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Journal article (peer-reviewed)abstract
- In this work we present a significant advancement in cubic silicon carbide (3C-SiC) growth in terms of crystal quality and domain size, and indicate its potential use in photovoltaics. To date, the use of 3C-SiC for photovoltaics has not been considered due to the band gap of 2.3 eV being too large for conventional solar cells. Doping of 3C-SiC with boron introduces an energy level of 0.7 eV above the valence band. Such energy level may form an intermediate band (IB) in the band gap. This IB concept has been presented in the literature to act as an energy ladder that allows absorption of sub-bandgap photons to generate extra electron-hole pairs and increase the efficiency of a solar cell. The main challenge with this concept is to find a materials system that could realize such efficient photovoltaic behavior. The 3C-SiC bandgap and boron energy level fits nicely into the concept, but has not been explored for an IB behavior. For a long time crystalline 3C-SiC has been challenging to grow due to its metastable nature. The material mainly consists of a large number of small domains if the 3C polytype is maintained. In our work a crystal growth process was realized by a new approach that is a combination of initial nucleation and step-flow growth. In the process, the domains that form initially extend laterally to make larger 3C-SiC domains, thus leading to a pronounced improvement in crystalline quality of 3C-SiC. In order to explore the feasibility of IB in 3C-SiC using boron, we have explored two routes of introducing boron impurities; ion implantation on un-doped samples and epitaxial growth on un-doped samples using pre-doped source material. The results show that 3C-SiC doped with boron is an optically active material, and thus is interesting to be further studied for IB behavior. For the ion implanted samples the crystal quality was maintained even after high implantation doses and subsequent annealing. The same was true for the samples grown with pre-doped source material, even with a high concentration of boron impurities. We present optical emission and absorption properties of as-grown and boron implanted 3C-SiC. The low-temperature photoluminescence spectra indicate the formation of optically active deep boron centers, which may be utilized for achieving an IB behavior at sufficiently high dopant concentrations. We also discuss the potential of boron doped 3C-SiC base material in a broader range of applications, such as in photovoltaics, biomarkers and hydrogen generation by splitting water.
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| 5. |
- Karlsson, M., et al.
(author)
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Wafer-scale epitaxial graphene on SiC for sensing applications
- 2015
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In: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE - The International Society for Optics and Photonics. - 9781628418903
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Conference paper (peer-reviewed)abstract
- The epitaxial graphene-on-silicon carbide (SiC-G) has advantages of high quality and large area coverage owing to a natural interface between graphene and SiC substrate with dimension up to 100 mm. It enables cost effective and reliable solutions for bridging the graphene-based sensors/devices from lab to industrial applications and commercialization. In this work, the structural, optical and electrical properties of wafer-scale graphene grown on 2'™'™ 4H semi-insulating (SI) SiC utilizing sublimation process were systemically investigated with focus on evaluation of the graphene'™s uniformity across the wafer. As proof of concept, two types of glucose sensors based on SiC-G/Nafion/Glucose-oxidase (GOx) and SiC-G/Nafion/Chitosan/GOx were fabricated and their electrochemical properties were characterized by cyclic voltammetry (CV) measurements. In addition, a few similar glucose sensors based on graphene by chemical synthesis using modified Hummer'™s method were also fabricated for comparison.
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| 6. |
- Shi, Yuchen, et al.
(author)
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Epitaxial Graphene Growth on the Step-Structured Surface of Off-Axis C-Face 3C-SiC(1¯1¯1¯)
- 2020
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In: Physica Status Solidi (B) Basic Research. - : Wiley. - 0370-1972 .- 1521-3951. ; 257:6
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Journal article (peer-reviewed)abstract
- Graphene layers grown on the C-face SiC exhibit quite different structural and electronic properties compared with those grown on the Si-face SiC. Herein, the growth and structural properties of graphene on the off-axis C-face 3C-SiC((Formula presented.)) are studied. The as-grown 4° off-axis 3C-SiC((Formula presented.)) exhibits highly periodic steps with step height of ≈0.75 nm and terrace width of ≈50 nm. After annealing at 1800 °C under 850 mbar argon atmosphere, relatively uniform large graphene domains can be grown. The low-energy electron microscopy (LEEM) results demonstrate that one monolayer (ML) to four-ML graphene domains are grown over several micrometers square, which enables us to measure micro low-energy electron diffraction (μ-LEED) on the single graphene domain. The μ-LEED pattern collected on the monolayer domain mainly exhibits four sets of graphene (1 × 1) spots, indicating the presence of graphene grains with different azimuthal orientations in the same graphene sheet. Raman spectra collected on the graphene domains show rather small D peaks, indicating the presence of less defects and higher crystalline quality of the graphene layers grown on the C-face off-axis 3C-SiC((Formula presented.)).
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| 7. |
- Grivickas, V., et al.
(author)
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Carrier lifetimes and influence of in-grown defects in N-B Co-doped 6H-SiC
- 2014
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In: IOP Conference Series. - : Institute of Physics Publishing (IOPP). ; 56:1, s. 012004-
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Conference paper (peer-reviewed)abstract
- The thick N-B co-doped epilayers were grown by the fast sublimation growth method and the depth-resolved carrier lifetimes have been investigated by means of the free-carrier absorption (FCA) decay under perpendicular probe-pump measurement geometry. In some samples, we optically reveal in-grown carbon inclusions and polycrystalline defects of substantial concentration and show that these defects slow down excess carrier lifetime and prevent donor-acceptor pair photo luminescence (DAP PL). A pronounced electron lifetime reduction when injection level approaches the doping level was observed. It is caused by diffusion driven non-radiative recombination. However, the influence of surface recombination is small and insignificant at 300 K.
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| 8. |
- Jokubavičius, Valdas
(author)
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Sublimation Growth of 3C-SiC : From Thick Layers to Bulk Material
- 2016
-
Doctoral thesis (other academic/artistic)abstract
- Silicon carbide (SiC) is a semiconductor material which holds high promises for various device applications. It can be obtained in different crystal structures called polytypes. The most common ones are hexagonal (6H- and 4H-SiC) and cubic (3C-SiC) silicon carbide. The 6H- and 4H-SiC single crystal substrates are commercially available, while technologies for the growth of 3C-SiC are still under development. The unique 3C-SiC properties like isotropy, narrower bandgap (2.4 eV) compared to hexagonal polytypes (about 3 eV) and high electron mobility make it better over hexagonal counterparts for some semiconductor applications, for example, metal oxide semiconductor field effect transistors (MOSFETs). However, due to lack of high quality material, the full potential of 3C-SiC in device applications has not been revealed. In addition, it has properties suitable to explore new concepts in efficient photovoltaics or solar driven hydrogen generation by water splitting.There is a need for 3C-SiC seeds to grow large 3C-SiC crystals by the widely used Physical Vapor Transport (PVT) technique. In case of hexagonal SiC polytypes such seeds were produced by the Lely method during which hexagonal SiC crystals spontaneously nucleate on the inner walls of a crucible. However, the formation of 3C-SiC using the Lely method is rarely observed. Therefore, the 3C-SiC has to be heteroepitaxially grown on silicon or hexagonal SiC substrates. Silicon is an inexpensive material with very high crystalline quality. However, due to almost 20% mismatch in lattice parameters and 8% difference in thermal expansion coefficient there is a high density of structural defects formed at the 3C-SiC/Si interface. In contrast, the 3C-SiC/hexagonal SiC material system does not encounter such problems, but there are other challenges like polytype stability or formation of structural defects called double positioning boundaries (DPBs).This thesis work mainly focuses on the growth of 3C-SiC on hexagonal SiC substrates using sublimation epitaxy. The research covers the development of growth process for thick (~1 mm) 3C-SiC layers, advancement of the growth process to eliminate DPBs and growth of bulk material using thick 3C-SiC layers as seeds. The 3C-SiC was grown on off-oriented hexagonal SiC substrates. The surfaces of such substrates contain high density of steps. Therefore, they have mostly been used for the growth of homoepitaxial hexagonal layers or bulk crystals via step flow mechanism. However, as demonstrated in this thesis, under special conditions the 3C-SiC with high crystalline quality can also be grown on off-oriented hexagonal substrates. The stability window for the growth of hexagonal and cubic polytypes on nominally on-axis hexagonal SiC substrates is also explored. Moreover, it is demonstrated how the temperature profile inside the graphite crucible is influenced by the change in thermal insulation properties and how such change results in enhanced polytype stability during the growth of thick SiC layers. In addition, different sources for sublimation epitaxial growth of doped SiC layers were analyzed to gain further understanding of new parameter windows.As a part of this thesis, a sublimation etching of 6H-, 4H- and 3C-SiC polytypes is presented using two different etching arrangements in vacuum (10-5 mbar) and Ar ambient. It is demonstrated that this technique can be used to remove residual scratches on the surface as well as to obtain various surface step structures which could be used for the growth of graphene nanostructures.
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| 9. |
- Murugesan, Murali, 1979, et al.
(author)
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Carbon fiber solder matrix composite for thermal management of high power electronics
- 2014
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In: Journal of Materials Chemistry. - : Royal Society of Chemistry (RSC). - 1364-5501 .- 0959-9428 .- 2050-7534 .- 2050-7526. ; 2:35, s. 7184-7187
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Journal article (peer-reviewed)abstract
- A carbon fiber based tin–silver–copper alloy matrix composite (CF-TIM) was developed via electrospinning of a mesophase pitch with polyimide and carbonization at 1000 °C, followed by sputter coating with titanium and gold, and alloy infiltration. The carbonized fibers, in film form, showed a thermal conductivity of [similar]4 W m−1 K−1 and the CF-TIM showed an anisotropic thermal conductivity of 41 ± 2 W m−1 K−1 in-plane and 20 ± 3 W m−1 K−1 through-plane. The thermal contact resistance of the CF-TIM was estimated to be below 1 K mm2 W−1. The CF-TIM showed no reduction in effective through-plane thermal conductivity after 1000 temperature cycles, which indicates the potential use of CF-TIM in thermal management applications.
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| 10. |
- Ou, Haiyan, et al.
(author)
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Advances in wide bandgap SiC for optoelectronics
- 2014
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In: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 87:3, s. 58-
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Journal article (peer-reviewed)abstract
- Silicon carbide (SiC) has played a key role in power electronics thanks to its unique physical properties like wide bandgap, high breakdown field, etc. During the past decade, SiC is also becoming more and more active in optoelectronics thanks to the progress in materials growth and nanofabrication. This paper will review the advances in fluorescent SiC for white light-emitting diodes, covering the polycrystalline doped SiC source material growth, single crystalline epitaxy growth of fluorescent SiC, and nanofabrication of SiC to enhance the extraction efficiency for fluorescent SiC based white LEDs.
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