| 1. |
- Schmidt, Susann, et al.
(författare)
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CF(x) thin solid films deposited by high power impulse magnetron sputtering: Synthesis and characterization
- 2011
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Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 206:4, s. 646-653
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Tidskriftsartikel (refereegranskat)abstract
- Fluorine containing amorphous carbon films (CF(x), 0.16 andlt;= x andlt;= 0.35) have been synthesized by reactive high power impulse magnetron sputtering (HiPIMS) in an Ar/CF(4) atmosphere. The fluorine content of the films was controlled by varying the CF(4) partial pressure from 0 mPa to 110 mPa at a constant deposition pressure of 400 mPa and a substrate temperature of 110 degrees C. The films were characterized regarding their composition, chemical bonding and microstructure as well as mechanical properties by applying elastic recoil detection analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy, and nanoindentation. First-principles calculations were carried out to predict and explain F-containing carbon thin film synthesis and properties. By geometry optimizations and cohesive energy calculations the relative stability of precursor species including C(2), F(2) and radicals, resulting from dissociation of CF4, were established. Furthermore, structural defects, arising from the incorporation of F atoms in a graphene-like network, were evaluated. All as-deposited CF(x) films are amorphous. Results from X-ray photoelectron spectroscopy and Raman spectroscopy indicate a graphitic nature of CF(x) films with x andlt;= 0.23 and a polymeric structure for films with x andgt;= 0.26. Nanoindentation reveals hardnesses between similar to 1 GPa and similar to 16 GPa and an elastic recovery of up to 98%.
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| 2. |
- Schmidt, Susann, et al.
(författare)
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Reactive high power impulse magnetron sputtering of CFx thin films in mixed Ar/C4F4 and Ar/C4F8 discharges
- 2013
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Ingår i: Thin Solid Films. - : Elsevier. - 0040-6090 .- 1879-2731. ; 542, s. 21-30
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Tidskriftsartikel (refereegranskat)abstract
- The reactive high power impulse magnetron sputtering processes of carbon in argon/tetrafluoromethane (CF4) and argon/octafluorocyclobutane (c-C4F8) have been characterized. Amorphous carbon fluoride (CFx) films were synthesized at deposition pressure and substrate temperature of 400 mPa and 110 degrees C, respectively. The CFx film composition was controlled in the range of 0.15 andlt; x andlt; 0.35 by varying the partial pressure of the F-containing gases from 0 mPa to 110 mPa. The reactive plasma was studied employing time averaged positive ion mass spectrometry and the resulting thin films were characterized regarding their composition, chemical bonding and microstructure as well as mechanical properties by elastic recoil detection analysis, X-ray photoelectron spectroscopy, transmission electron microscopy, nanoindentation, and water droplet contact angle measurements, respectively. The experimental results were compared to results obtained by first-principles calculations based on density functional theory. The modeling of the most abundant precursor fragment from the dissociation of CF4 and C4F8 provided their relative stability, abundance, and reactivity, thus permitting to evaluate the role of each precursor during film growth. Positive ion mass spectrometry of both fluorine plasmas shows an abundance of CF+, C+, CF2+, and CF3+ (in this order) as corroborated by first-principles calculations. Only CF3+ exceeded the Ar+ signal in a CF4 plasma. Two deposition regimes are found depending on the partial pressure of the fluorine-containing reactive gas, where films with fluorine contents below 24 at.% exhibit a graphitic nature, whereas a polymeric structure applies to films with fluorine contents exceeding 27 at.%. Moreover, abundant precursors in the plasma are correlated to the mechanical response of the different CFx thin films. The decreasing hardness with increasing fluorine content can be attributed to the abundance of CF3+ precursor species, weakening the carbon matrix.
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| 3. |
- Gueorguiev Ivanov, Ivan, et al.
(författare)
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Optical properties of the niobium centre in 4H, 6H, and 15R SiC
- 2013
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Ingår i: SILICON CARBIDE AND RELATED MATERIALS 2012. - : Trans Tech Publications. ; , s. 405-408
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Konferensbidrag (refereegranskat)abstract
- A set of lines in the photoluminescence spectra of 4H-, 6H-, and 15R-SiC in the near-infrared are attributed to Nb-related defects on the ground of doping experiments conducted with 4H-SiC. A model based on a an exciton bound at the Nb-centre in an asymmetric split vacancy configuration at a hexagonal site is proposed, which explains the structure of the luminescence spectrum and the observed Zeeman splitting of the lines.
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| 4. |
- Gällström, Andreas, et al.
(författare)
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Optical identification and electronic configuration of tungsten in 4H-and 6H-SiC
- 2012
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Ingår i: Physica. B, Condensed matter. - : Elsevier. - 0921-4526 .- 1873-2135. ; 407:10, s. 1462-1466
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Tidskriftsartikel (refereegranskat)abstract
- Several optically observed deep level defects in SiC are still unidentified and little is published on their behavior. One of the commonly observed deep level defects in semi-insulating SiC is UD-1. less thanbrgreater than less thanbrgreater thanThis report suggests that UD-1 is Tungsten related, based on a doping study and previously reported deep level transient spectroscopy data, as well as photo-induced absorption measurements. The electronic levels involved in the optical transitions of UD-1 are also deduced. The transitions observed in the photoluminescence of UD-1 are from a Gamma(C3v)(4), to two different final states, which transform according to Gamma(C3v)(5)circle plus Gamma(C3v)(6) and Gamma(C3v)(4), respectively.
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| 5. |
- Henry, Anne, et al.
(författare)
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Photoluminescence of 8H-SiC
- 2013
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Ingår i: Silicon Carbide and Related Materials 2012. - : Trans Tech Publications. - 9783037856246 ; , s. 347-350
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Konferensbidrag (refereegranskat)abstract
- 8H-SiC epilayers grown on small 8H-SiC Lely platelets are investigated optically using photoluminescence spectroscopy. At low temperature the near band gap emission detected in the 2.78 to 2.67 eV range contains sharp lines associated to nitrogen-bound-exciton recombination. Three different no-phonon lines are detected accompanied by their phonon replicas. Free-exciton replicas are also observed which allows the determination of the excitonic band gap. The binding energy of the bound excitons can thus be determined and the ionization energies of the three nitrogen levels in 8H-SiC are estimated and found to be rather shallow compared to the values for other hexagonal polytypes. Additional bound-exciton lines are observed when the experimental photoluminescence temperature is increased.
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| 6. |
- Ivanov, Ivan Gueorguiev, et al.
(författare)
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High-Resolution Raman and Luminescence Spectroscopy of Isotope-Pure (SiC)-Si-28-C-12, Natural and C-13 - Enriched 4H-SIC
- 2014
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Ingår i: Silicon Carbide and Related Materials 2013, PTS 1 AND 2. - : Trans Tech Publications Inc.. ; , s. 471-474
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Konferensbidrag (refereegranskat)abstract
- The optical properties of isotope-pure (SiC)-Si-28-C-12, natural SiC and enriched with C-13 isotope samples of the 4H polytype are studied by means of Raman and photoluminescence spectroscopies. The phonon energies of the Raman active phonons at the Gamma point and the phonons at the M point of the Brillouin zone are experimentally determined. The excitonic bandgaps of the samples are accurately derived using tunable laser excitation and the phonon energies obtained from the photoluminescence spectra. Qualitative comparison with previously reported results on isotope-controlled Si is presented.
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| 7. |
- Ivanov, Ivan Gueorguiev, et al.
(författare)
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Ionization energy of the phosphorus donor in 3C-SiC from the donor-acceptor pair emission
- 2010
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Ingår i: JOURNAL OF APPLIED PHYSICS. - : American Institute of Physics. - 0021-8979 .- 1089-7550. ; 108:6, s. 063532-
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Tidskriftsartikel (refereegranskat)abstract
- Donor-acceptor pair luminescence of P-Al and N-Al pairs in 3C-SiC is analyzed. The structures in the spectra corresponding to recombination of pairs at intermediate distances are fitted with theoretical spectra of type I (P-Al pairs) and type II (N-Al pairs). It is shown that in the regions chosen for fitting the line positions obey the equation (h) over bar omega(R)=E-G-E-D-E-A+e(2)/epsilon R, where (h) over bar omega(R) is the energy of the photon emitted by recombination of a pair at a distance R, e is the electron charge, epsilon is the static dielectric constant, and E-G, E-D, and E-A are the electronic band gap and the donor and acceptor ionization energies, respectively. The fits yield the values E-G-E-D-E-A for the N-Al (2094 meV) and P-Al (2100.1 meV) cases. Using the known value of the nitrogen ionization energy, 54.2 meV, phosphorus ionization energy of 48.1 meV is obtained. Identification of the sharp lines corresponding to recombination of close pairs in the P-Al spectrum is suggested.
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| 8. |
- Ivanov, Ivan Gueorguiev, et al.
(författare)
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Layer-number determination in graphene on SiC by reflectance mapping
- 2014
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 77, s. 492-500
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Tidskriftsartikel (refereegranskat)abstract
- We report a simple, handy and affordable optical approach for precise number-of-layers determination of graphene on SiC based on monitoring the power of the laser beam reflected from the sample (reflectance mapping) in a slightly modified micro-Raman setup. Reflectance mapping is compatible with simultaneous Raman mapping. We find experimentally that the reflectance of graphene on SiC normalized to the reflectivity of bare substrate (the contrast) increases linearly with similar to 1.7% per layer for up to 12 layers, in agreement with theory The wavelength dependence of the contrast in the visible is investigated using the concept of ideal fermions and compared with existing experimental data for the optical constants of graphene. We argue also that the observed contrast is insensitive to the doping condition of the sample, as well as to the type of sample (graphene on C- or Si-face of 4H or 6H SiC, hydrogen-intercalated graphene). The possibility to extend the precise layer counting to similar to 50 layers makes reflectivity mapping superior to low-energy electron microscopy (limited to similar to 10 layers) in quantitative evaluation of graphene on the C-face of SiC. The method is applicable for graphene on other insulating or semiconducting substrates. (C) 2014 Elsevier Ltd. All rights reserved.
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| 9. |
- Ivanov, Ivan Gueorguiev, et al.
(författare)
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Resonant ionization of shallow donors in electric field
- 2014
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Ingår i: Physica Scripta. - : Institute of Physics Publishing (IOPP). - 0031-8949 .- 1402-4896. ; 89:8, s. 085802-
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we report on our experimental observations of the resonant ionization of a phosphorus donor in silicon in a homogeneous electric field, which is expressed in the sudden rise of the conductivity of the sample at a low temperature when the electric field approaches the critical value of ∼3.2 MV m-1. The effect is discussed in terms of the field-induced interaction of the states using a simplified model based on the effective-mass theory. The results from our model are qualitatively similar to the previously published advanced model base, which is based on the first principles; this predicts the ionization thresholds at approximate fields of 2.45 and 3.25 MV m-1, the latter being in very good agreement with our experiment. The possibility of observing more than one resonance is also discussed.
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| 10. |
- Jokubavicius, Valdas, et al.
(författare)
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Lateral Enlargement Growth Mechanism of 3C-SiC on Off-Oriented 4H-SiC Substrates
- 2014
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Ingår i: Crystal Growth & Design. - : American Chemical Society (ACS). - 1528-7483 .- 1528-7505. ; 14:12, s. 6514-6520
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Tidskriftsartikel (refereegranskat)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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