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Sökning: LAR1:liu > Syväjärvi Mikael

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191.
  • Vasiliauskas, Remigijus, et al. (författare)
  • Impact of extended defects on Hall and magnetoresistivity effects in cubic silicon carbide
  • 2012
  • Ingår i: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 45:22, s. 225102-
  • Tidskriftsartikel (refereegranskat)abstract
    • From magnetoresistivity effect measurements the carrier mobility at room- temperature is 200 cm2/Vs in heteroepitaxially grown 3C-SiC on 6H-SiC by sublimation epitaxy. The main scattering mechanisms are found to be scattering by neutral impurities at low temperature and by phonons at higher temperature. The carrier concentration is in the range of 1016  cm-3, which corresponds to the concentration of residual doping by nitrogen acquired  from  photoluminescence  measurements.  Using  magnetoresistance  and  Hall mobility data we have created a simple model which quantifies the volume of the samples influenced by extended defects. A higher doping near extended defects is either not present in the samples or might be screened by the electrostatic field created by these defects.
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192.
  • Vasiliauskas, Remigijus, et al. (författare)
  • Influence of twin boundary orientation on magnetoresistivity effect in free standing 3C–SiC
  • 2012
  • Ingår i: Materials letters (General ed.). - : Elsevier BV. - 0167-577X .- 1873-4979. ; 74, s. 203-205
  • 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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193.
  • Vasiliauskas, Remigijus, et al. (författare)
  • Nucleation Control of Cubic Silicon Carbide on 6H- Substrates
  • 2012
  • Ingår i: Crystal Growth & Design. - : American Chemical Society (ACS). - 1528-7483 .- 1528-7505. ; 12:1, s. 197-204
  • Tidskriftsartikel (refereegranskat)abstract
    • The nucleation of cubic (3C) SiC on on-axis 6H-SiC was investigated in the temperature range 1500–1775 °C by the technique of sublimation epitaxy. We have studied two different cases: (i) the initial homoepitaxial growth of 6H-SiC followed by nucleation of 3C-SiC and (ii) nucleation of homoepitaxial 6H-SiC islands. The supersaturation in the growth cell was calculated using the modeled source to substrate temperature difference. We show that, at low temperature and supersaturation, growth of 6H-SiC commences in spiral growth mode, which prepares the surface for 3C-SiC nucleation. Provided the supersaturation is high enough, the 3C-SiC nucleates as two-dimensional islands on terraces of the homoepitaxial 6H-SiC. Detailed structural study indicates that the 3C-SiC began to grow on defect free surfaces. From the experimental and modeling results, we show that the growth parameter window for 3C-SiC is rather narrow. Deviation from it can result in 6H-SiC growth in spiral or 2D-nucleation mode, which suggests the importance of knowledge of supersaturation.
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194.
  • Vasiliauskas, Remigijus, et al. (författare)
  • Polytype Inclusions in Cubic Silicon Carbide
  • 2013
  • Ingår i: Silicon Carbide and Related Materials 2012. - : Trans Tech Publications. - 9783037856246 ; , s. 335-338
  • Konferensbidrag (refereegranskat)abstract
    • In this paper, we review our research on 6H-SiC polytype inclusions in 3C-SiC layers, which were grown on nominally on-axis 6H-SiC substrates using sublimation epitaxy. More than 90% coverage by 3C-SiC is typically achieved at growth temperature of 1775 degrees C. The main reason for the polytype inclusions to appear is local supersaturation non-uniformities over the sample surface which appear due to the temperature gradient and spiral growth nature of 6H-SiC. On the 6H-SiC spirals with small steps supersaturation is smaller and 3C-SiC nucleation and growth is diminished. Due to surface free energy and surface diffusion differences, polytype inclusions appear differently when 3C-SiC is grown on the Si- and C-faces. The 6H-SiC inclusions as well as twin boundaries act as neutral scattering centers and lower charge carrier mobility
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195.
  • Vasiliauskas, Remigijus, et al. (författare)
  • Polytype transformation and structural characteristics of 3C-SiC on 6H-SiC substrates
  • 2014
  • Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 395, s. 109-115
  • Tidskriftsartikel (refereegranskat)abstract
    • The 3C-SiC (111) was grown on on-axis 6H-SiC substrates in a temperature interval ranging from 1675oC where 3C-SiC nucleated, to 1825oC where coverage of the substrate by 3C-SiC was  nearly  100%.  The  6H-  to  3C-SiC  transformation  was  not  abrupt  and  two  different transitions could be observed. The first one occurs before or during 3C-SiC nucleation and consists  of 6H-,  3C-, 15R-SiC  and other  unresolved  stacking  sequences.  The second  one appears due to 6H-SiC and 3C-SiC competition  during the growth and results in non flat needle-like interface. A proposed model elucidates connection between four-fold twins nucleating at the 6H-/3C-SiC interface and the formation of depressions at the surface of the 3C-SiC layer.
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196.
  • Vasiliauskas, Remigijus (författare)
  • Sublimation Growth and Performance of Cubic Silicon Carbide
  • 2012
  • 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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197.
  • Vasiliauskas, Remigijus, et al. (författare)
  • Sublimation Growth and Structural Characterization of 3C-SiC on Hexagonal and Cubic SiC Seeds
  • 2010
  • Ingår i: Materials Science Forum, Vols. 645-648. - : Transtec Publications; 1999. ; , s. 175-178
  • Konferensbidrag (refereegranskat)abstract
    • Epitaxial growth of cubic silicon carbide on 6H-SiC substrates, and 6H-SiC substrates with (111) 3C-SiC buffer layer, deposited by vapour liquid solid mechanism, was compared. The morphological details of the grown layers were studied by optical microscopy and their microstructure by transmission electron microscopy. The influence of the substrate on the nucleation of 3C-SiC, the initial homoepitaxial 6H-SiC nucleation before 3C-SiC as well as the formation of defects, are discussed.
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198.
  • Vasiliauskas, Remigijus, 1982-, et al. (författare)
  • Two-dimensional nucleation of cubic and 6H silicon carbide
  • 2009
  • Ingår i: ECSCRM2008,2008. - Materials Science Forum Vols. 615-617 : Trans Tech Publications. - 9780878493340 ; , s. 189-192
  • Konferensbidrag (refereegranskat)abstract
    • The initial stage of heteroepitaxial growth of 3C-SiC and homoepitaxial growth of 6H-SiC on nominal 6H-SiC on-axis substrates has been studied. Before 3C-SiC starts to nucleate, 6H-SiC grows in a step-flow growth mode due to a slight off-orientation of the substrate surface already at about 1500oC. In the 1650-1700oC temperature interval 3C-SiC nucleates as 2D islands. A distance away from the 3C-SiC island 6H-SiC grows in step-flow mechanism. In the vicinity of the 3C-SiC islands the 6H-SiC growth steps start to change direction and even split into two steps with the equal height of 0.5 nm, which is approaching the unit cell size of cubic SiC. When the supersaturation is lower in comparison with the conditions for 3C-SiC growth, there is only formation of 6H-SiC, i.e. homoepitaxial growth. The growth mode of 6H-SiC is dependent on temperature. At the lowest temperature there is spiral growth while at higher temperature 2D nucleation is preferred.
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199.
  • Via, Francesco La, et al. (författare)
  • New Approaches and Understandings in the Growth of Cubic Silicon Carbide
  • 2021
  • Ingår i: Materials. - : MDPI. - 1996-1944 .- 1996-1944. ; 14:18
  • Forskningsöversikt (refereegranskat)abstract
    • In this review paper, several new approaches about the 3C-SiC growth are been presented. In fact, despite the long research activity on 3C-SiC, no devices with good electrical characteristics have been obtained due to the high defect density and high level of stress. To overcome these problems, two different approaches have been used in the last years. From one side, several compliance substrates have been used to try to reduce both the defects and stress, while from another side, the first bulk growth has been performed to try to improve the quality of this material with respect to the heteroepitaxial one. From all these studies, a new understanding of the material defects has been obtained, as well as regarding all the interactions between defects and several growth parameters. This new knowledge will be the basis to solve the main issue of the 3C-SiC growth and reach the goal to obtain a material with low defects and low stress that would allow for realizing devices with extremely interesting characteristics.
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200.
  • Virojanadara, Chariya, et al. (författare)
  • Homogeneous large-area graphene layer growth on 6H-SiC(0001)
  • 2008
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:24, s. 245403-
  • Tidskriftsartikel (refereegranskat)abstract
    • Homogeneous large-area graphene monolayers were successfully prepared ex situ on 6H-SiC(0001). The samples have been studied systematically and the results are compared with those from a sample cut from the same wafer and prepared by in situ heating. The formation of smaller graphene flakes was found on the in situ prepared sample, which is in line with earlier observations. Distinctly different results are observed from the ex situ graphene layers of different thicknesses, which are proposed as a guideline for determining graphene growth. Recorded C 1s spectra consisted of three components: bulk SiC, graphene (G), and interface (I), the latter being a 6 root 3 layer. Extracted intensity ratios of G/I were found to give a good estimate of the thickness of graphene. Differences are also revealed in micro low energy electron diffraction images and electron reflectivity curves. The diffraction patterns were distinctly different from a monolayer thickness up to three layers. At a larger thickness only the graphitelike spot was visible. The electron reflectivity curve showed a nice oscillation behavior with kinetic energy and as a function of the number of graphene layers. The graphene sheets prepared were found to be very inert and the interface between the substrate and the layer(s) was found to be quite abrupt. No free Si could be detected in or on the graphene layers or at the interface.
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