| 1. |
- Eriksson, Jens, et al.
(författare)
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The influence of substrate morphology on thickness uniformity and unintentional doping of epitaxial graphene on SiC
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:24, s. 241607-
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Tidskriftsartikel (refereegranskat)abstract
- A pivotal issue for the fabrication of electronic devices on epitaxial graphene on SiC is controlling the number of layers and reducing localized thickness inhomogeneities. Of equal importance is to understand what governs the unintentional doping of the graphene from the substrate. The influence of substrate surface topography on these two issues was studied by work function measurements and local surface potential mapping. The carrier concentration and the uniformity of epitaxial graphene samples grown under identical conditions and on substrates of nominally identical orientation were both found to depend strongly on the terrace width of the SiC substrate after growth.
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| 2. |
- Jokubavicius, Valdas, et al.
(författare)
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Effects of source material on epitaxial growth of fluorescent SiC
- 2012
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Ingår i: Thin Solid Films. - : Elsevier. - 0040-6090 .- 1879-2731. ; 522, s. 7-10
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Tidskriftsartikel (refereegranskat)abstract
- The growth of fluorescent SiC using Fast Sublimation Growth Process was demonstrated using different types of SiC source materials. These were prepared by (i) high-temperature hot pressing, (ii) chemical vapor deposition and (iii) physical vapor transport. The optimized growth rates of 50 μm/h, 170 μm/h and 200 μm/h were achieved using the three types of sources, respectively. The best results in respect to growth rates are obtained using higher density sources. Fluorescent SiC layers with mirror-like morphology, very good crystal quality and yellowish or warm white light photoluminescence at room temperature were grown using all three types of the source materials.
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| 3. |
- Jokubavicius, Valdas, et al.
(författare)
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Geometrical Control of 3C and 6H-SiC Nucleation on Low Off-Axis Substrates
- 2011
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Ingår i: Materials Science Forum. - 0255-5476 .- 1662-9752. ; 679-680, s. 103-106
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Tidskriftsartikel (refereegranskat)abstract
- Growth of 3C or 6H-SiC epilayers on low off-axis 6H-SiC substrates can be mastered by changing the size of the on axis plane formed by long terraces in the epilayer using geometrical control. The desired polytype can be selected in thick (~200 µm) layers of both 6H-SiC and 3C-SiC polytypes on substrates with off-orientation as low as 1.4 and 2 degrees. The resultant crystal quality of the 3C and the 6H-SiC epilayers, grown under the same process parameters, deteriorates when lowering the off-orientation of the substrate.
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| 4. |
- Jokubavicius, Valdas, et al.
(författare)
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On stabilization of 3C-SiC using low off-axis 6H-SiC substrates
- 2012
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Konferensbidrag (refereegranskat)abstract
- Heteroepitaxial growth of 3C-SiC on 0.8 and 1.2 degree off-oriented 6H-SiC substrates was studied using a sublimation growth process. The 3C-SiC layers were grown at high growth rates with layer thickness up to 300 µm. The formation and the quality of 3C-SiC are influenced by the off-orientation of the substrate, the growth temperature (studied temperature range from 1750 oC to 1850oC), and the growth ambient (vacuum at 5*10-5 mbar and nitrogen at 5*10-1 mbar). The largest domains of 3C-SiC and the lowest number of double positioning boundaries were grown using nitrogen ambient and the highest growth temperature. The combined use of low off-axis substrate and high growth rate is a potential method to obtain material with bulk properties.
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| 5. |
- Ou, Yiyu, et al.
(författare)
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Characterization of donor-acceptor-pair emission in fluorescent 6H-SiC
- 2012
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Ingår i: Physica Scripta. - : Institute of Physics Publishing Ltd. - 0031-8949 .- 1402-4896. ; T148, s. 014003-
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Tidskriftsartikel (refereegranskat)abstract
- We investigated donor-acceptor-pair emission in N-B-doped 6H-SiC samples by using photoluminescence (PL) and angle-resolved PL. It is shown that n-type doping with concentrations larger than 10(18) cm(-3) is favorable for observing luminescence, and increasing nitrogen results in stronger luminescence. A dopant concentration difference greater than 4x10(18) cm(-3) is proposed to help achieve intense PL. Angular-dependent PL was observed that was attributed to the Fabry-Perot microcavity interference effect, and a strong luminescence intensity in a large emission angle range was also achieved. The results indicate that N-B-doped fluorescent SiC is a good wavelength converter in white LED applications.
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| 6. |
- Ou, Yiyu, et al.
(författare)
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Omnidirectional luminescence enhancement of fluorescent SiC via pseudoperiodic antireflective subwavelength structures
- 2012
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Ingår i: Optics Letters. - 0146-9592 .- 1539-4794. ; 37:18, s. 3816-3818
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Tidskriftsartikel (refereegranskat)abstract
- In the present work, an approach of fabricating pseudoperiodic antireflective subwavelength structures (ARS) on fluorescent SiC by using self-assembled etch mask is demonstrated. By applying the pseudoperiodic (ARS), the average surface reflectance at 6° incidence over the spectral range of 390–785 nm is dramatically suppressed from 20.5% to 1.62%, and the hydrophobic surface with a large contact angle of 98° is also achieved. The angle-resolved photoluminescence study presents a considerable omnidirectional luminescence enhancement with an integral intensity enhancement of 66.3% and a fairly preserved spatial emission pattern.
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| 7. |
- Ou, Y., et al.
(författare)
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Photoluminescence and Raman spectroscopy characterization of boron-and nitrogen-doped 6H silicon carbide
- 2012
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Ingår i: Silicon Carbide and Related Materials 2011. - : Trans Tech Publications Inc.. - 9783037854198 ; , s. 233-236
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Konferensbidrag (refereegranskat)abstract
- Nitrogen-boron doped 6H-SiC epilayers grown on low off-axis 6H-SiC substrates have been characterized by photoluminescence and Raman spectroscopy. The photoluminescence results show that a doping larger than 10 18 cm -3 is favorable to observe the luminescence and addition of nitrogen leads to an increased luminescence. A dopant concentration difference larger than 4×10 18 cm -3 is proposed to achieve intense photoluminescence. Raman spectroscopy further confirmed the doping type and concentrations for the samples. The results indicate that N-B doped SiC can serve as a good wavelength converter in white LEDs applications.
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| 8. |
- Sun, Jianwu, et al.
(författare)
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Low Temperature Photoluminescence Signature of Stacking Faults in 6H-SiC Epilayers Grown on Low Angle Off-axis Substrates
- 2012
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Konferensbidrag (refereegranskat)abstract
- The radiative recombination spectra of 6H-SiC epilayers grown on low angle (1.4° off-axis) substrates have been investigated by low temperature photoluminescence spectroscopy. Four different types of stacking faults have been identified, together with the presence of 3C-SiC inclusions. From the energy of the momentum-conserving phonons, four excitonic band gap energies have been found with Egx equal to 2.837, 2.698, 2.600 and 2.525 eV. These photoluminescence features, which give a rapid and non-destructive approach to identify stacking faults in 6H-SiC, provide a direct feedback to improve the material growth.
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| 9. |
- Vasiliauskas, Remigijus, et al.
(författare)
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Influence of twin boundary orientation on magnetoresistivity effect in free standing 3C–SiC
- 2012
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Ingår i: Materials letters (General ed.). - : Elsevier BV. - 0167-577X .- 1873-4979. ; 74, s. 203-205
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Tidskriftsartikel (refereegranskat)abstract
- Free standing 3C–SiC (111) samples with differently oriented twin boundaries were prepared using on-axis and slightly off-axis 6H–SiC substrates. The orientation of twin boundaries causes either an enhancement or suppression of the magnetoresistance mobility. The origin of carrier mobility difference is attributed to the specific structure of these defects. The height of the barriers created by twin boundaries was found to be 0.2 eV.
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| 10. |
- Vasiliauskas, Remigijus
(författare)
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Sublimation Growth and Performance of Cubic Silicon Carbide
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Silicon carbide (SiC) is a wide band gap semiconductor satisfying requirements to replace silicon in devices operating at high power and high frequency at high temperature, and in harsh environments. Hexagonal polytypes of SiC, such as 6H-SiC and 4H-SiC are available on the power device markets. However, the cubic SiC (3C-SiC) polytype is still not industrially used, essentially due to the lack of 3CSiC substrates. This is mainly because of a high density of defects appearing in the crystals. Thus, it is critical to understand material growth and defect formation, and learn to control their appearance. Ensuring, that growth methods capable of large scale industrial production can be applied.The aim of this work was to develop operation conditions for fabrication of 3C-SiC crystals via understanding fundamentals of the growth process and to explore structural and electrical properties of the grown material, including its suitability for substrate applications. The physical vapor transport or sublimation process has already shown a capability to produce substantial quantities of large area and high quality hexagonal SiC substrates. In the present study a similar growth principle, but in a different geometry, namely sublimation epitaxy, was applied. Using this method very high growth rates (up to 1 mm/h) can be achieved for hexagonal polytypes while maintaining high material quality. Additionally, the growth process does not require expensive or hazardous materials, thus making the method very attractive for industrial use.When growing 3C-SiC directly on 6H-SiC, the substrate roughness does not have significant influence on the yield and quality of 3C-SiC. This is mostly due to the growth of homoepitaxial 6H-SiC which appears before the 3C-SiC. Structural characterization showed that 3C-SiC grown directly on 6HSiC exhibited the highest quality as compared with other substrate preparation, such as annealing or deposition of a 3C-SiC buffer layer. Thus, further investigation was devoted to the growth of 3C-SiC on 6H-SiC substrates.The parameter window for the growth of 3C-SiC is quite narrow. The temperature interval is from ~1675oC, where the material starts to nucleate, to ~1850oC, where an uncontrolled growth process begins. Si-rich conditions (high Si/C ratio) and high supersaturation are needed in the growth chamber for preferable 3C-SiC nucleation. Deviation from these parameters leads to the growth of homoepitaxial 6HSiC in spiral or 2D island mode along with cubic SiC with high defect density.Nucleation is the most important step in the growth process. The growth on 6H-SiC substrates commences from homoepitaxial 6H-SiC growth in spiral mode, which makes the surface perfect for 3CSiC nucleation. At temperature of ~1675oC the supersaturation is high enough and the 3C-SiC nucleation initiates in two-dimensional islands on the 6H-SiC spiral terraces. Control of the homoepitaxial 6H-SiC growth is a key element in the growth of 3C-SiC.SiC is a polar material having surfaces terminated by either silicon or carbon atoms, called Si- and C-face, respectively. The growth is different on both faces due to the different free surface energies. The lower surface free energy on the C-face causes more uniform nucleation of 3C-SiC and thereafter more uniform twinned domain distribution. Additionally, calculations showed that increase of growth temperature from 1675oC to 1775oC does not change the supersaturation ratio on the C-face due to a much higher surface diffusion length. This results in appearance of pits in the 3C-SiC layer with a 6H-SiC spiral. The pits were not observed on Si-face material as the supersaturation ratio was much higher. Pits formed in the early stages of growth were overgrown more effectively during the later stages.Characterization by transmission electron microscopy showed that transformation from 6H-SiC to 3C-SiC is not abrupt and can appear in two different modes. The first one is forming a few micrometers of polytypic transition zone consisting predominantly of 15R-, 6H- and 3C-SiC. The second one appears due to a competition between 3C-SiC and 6H-SiC resulting in a step-like intermixing zone between these polytypes. Four-fold twins were observed, which resulted in depressions at the surface of 3C-SiC. These defects expand proportionally to the layer thickness, thus drastically reducing usable area of thick layers.Electrical measurements revealed carrier mobility ~200 cm2/Vs at room temperature and the dominant charge carrier scattering is by neutral centers and phonons. The neutral centers originate from extended defects, such as 6H-SiC inclusions, stacking faults and twin boundaries. By growing 3C-SiC on atomically flat and vicinal substrates a preferential orientation of twin boundaries (TBs) was achieved. The mobility was higher in the material with twin boundaries parallel to the current flow, and lower when twin boundaries were perpendicular to the current flow. This was less pronounced at higher temperature as relatively fewer carriers have to overcome barriers created by TBs.Finally, the substrate capability of the 3C-SiC (111) was demonstrated by growth of a monolayer graphene, which was compared with graphene grown on hexagonal SiC poytypes. The quality of the graphene in terms of thickness uniformity and pit appearance was the best when grown on 3C-SiC. The lower quality on hexagonal substrates was attributed to a more difficult process control which is due to the more complex crystal structure.
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