| 1. |
- Alling, Björn, et al.
(författare)
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Theoretical investigation of cubic B1-like and corundum (Cr1−xAlx)2O3 solid solutions
- 2013
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Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 31:3
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Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations are employed to investigate the stability and properties of cubic rock-salt-like (Cr1−xAlx)2O3 solid solutions, stabilized by metal site vacancies as recently reported experimentally. It is demonstrated that the metal site vacancies can indeed be ordered in a way that gives rise to a suitable fourfold coordination of all O atoms in the lattice. B1-like structures with ordered and disordered metal site vacancies are studied for (Cr0.5Al0.5)2O3 and found to have a cubic lattice spacing close to the values reported experimentally, in contrast to fluorite-like and perovskite structures. The obtained B1-like structures are higher in energy than corundum solid solutions for all compositions, but with an energy offset per atom similar to other metastable systems possible to synthesize with physical vapor deposition techniques. The obtained electronic structures show that the B1-like systems are semiconducting although with smaller band gaps than the corundum structure.
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| 2. |
- Buchholt, Kristina, et al.
(författare)
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Growth and characterization of epitaxial Ti3GeC2 thin films on 4H-SiC(0001)
- 2012
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Ingår i: Journal of Crystal Growth. - : Elsevier. - 0022-0248 .- 1873-5002. ; 343:1, s. 133-137
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial Ti3GeC2 thin films were deposited on 4 degrees off-cut 4H-SiC(0001) using magnetron sputtering from high purity Ti, C, and Ge targets. Scanning electron microscopy and helium ion microscopy show that the Ti3GeC2 films grow by lateral step-flow with {11 (2) over bar0} faceting on the SiC surface. Using elastic recoil detection analysis, atomic force microscopy, and X-Ray diffraction the films were found to be substoichiometric in Ge with the presence of small Ge particles at the surface of the film.
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| 3. |
- Buchholt, Kristina, 1978-
(författare)
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Nanostructured materials for gas sensing applications
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this Thesis I have investigated the use of nanostructured films as sensing and contact layers for field effect gas sensors in order to achieve high sensitivity, selectivity, and long term stability of the devices in corrosive environments at elevated temperatures. Electrochemically synthesized Pd and Au nanoparticles deposited as sensing layers on capacitive field effect devices were found to give a significant response to NOx with small, or no responses to H2, NH3, and C3H6. Pt nanoparticles incorporated in a TiC matrix are catalytically active, but the agglomeration and migration of the Pt particles towards the substrate surface reduces the activity of the sensing layer. Magnetron sputtered epitaxial films from the Ti-Si-C and the Ti-Ge-C systems were grown on 4H-SiC substrates in order to explore their potential as high temperature stable ohmic contact materials to SiC based field effect gas sensors. Ti3SiC2 thin films deposited on 4H-SiC substrates were found to yield ohmic contacts to n-type SiC after a high temperature rapid thermal anneal at 950 ºC. Investigations on the growth mode of Ti3SiC2 thin films with varying Si content on 4H-SiC substrates showed the growth to be lateral step-flow with the propagation of steps with a height as small as half a unit cell. The amount of Si present during deposition leads to differences in surface faceting of the films and Si-supersaturation conditions gives growth of Ti3SiC2 films with the presence of TiSi2 crystallites. Current-voltage measurements of the as-deposited Ti3GeC2 films indicate that this material is also a promising candidate for achieving long term stable contact layers to 4H-SiC for operation at elevated temperatures in corrosive environments. Further investigations into the Ti-Ge-C system showed that the previously unreported solid solutions of (Ti,V)2GeC, (Ti,V)3GeC2 and (Ti,V)4GeC3 can be synthesized, and it was found that the growth of these films is affected by the nature of the substrate.
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| 4. |
- Buchholt, Kristina, et al.
(författare)
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Ohmic contact properties of magnetron sputtered Ti3SiC2 on n- and p-type 4H-silicon carbide
- 2011
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 98:4, s. 042108-
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial Ti3SiC2 (0001) thin film contacts were grown on doped 4H-SiC (0001) using magnetron sputtering in an ultra high vacuum system. The specific contact resistance was investigated using linear transmission line measurements. Rapid thermal annealing at 950 degrees C for 1 min of as-deposited films yielded ohmic contacts to n-type SiC with contact resistances in the order of 10(-4) Omega cm(2). Transmission electron microscopy shows that the interface between Ti3SiC2 and n-type SiC is atomically sharp with evidence of interfacial ordering after annealing. (c) 2011 American Institute of Physics.
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| 5. |
- Buchholt, Kristina, et al.
(författare)
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Step-flow growth of nanolaminate Ti3SiC2 epitaxial layers on 4H-SiC(0 0 0 1)
- 2011
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Ingår i: SCRIPTA MATERIALIA. - : Elsevier Science B.V., Amsterdam.. - 1359-6462. ; 64:12, s. 1141-1144
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial Ti3SiC2(0 0 0 1) films were deposited on 4 degrees off-cut 4H-SiC(0 0 0 1) wafers using magnetron sputtering. A lateral step-flow growth mechanism of the Ti3SiC2 was discovered by X-ray diffraction, elastic recoil detection analysis, atomic force microscopy and electron microscopy. Helium ion microscopy revealed contrast variations on the Ti3SiC2 terraces, suggesting a mixed Si and Ti(C) termination. Si-rich growth conditions results in Ti3SiC2 layers with pronounced {1 1 (2) over bar 0) faceting and off-oriented TiSi2 crystallites, while stoichiometric growth yields truncated {1 (1) over bar 0 0) terrace edges.
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| 6. |
- Bugnet, M, et al.
(författare)
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Contribution of core-loss fine structures to the characterization of ion irradiation damages in the nanolaminated ceramic Ti3AlC2
- 2013
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 61:19, s. 7348-7363
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Tidskriftsartikel (refereegranskat)abstract
- The effect of low-energy ion irradiation on the nanolaminated Ti3AlC2 is investigated by means of X-ray diffraction, transmission electron microscopy, electron energy loss and X-ray absorption spectroscopy. The chemical sensitivity and local order probing from core-loss edges provide new insights into the structural modifications induced under irradiation. From the analysis of the C K energy loss near-edge structure and Al K X-ray absorption near-edge structure by ab initio calculations, the influence of the layered structure of this compound on the irradiation damage is demonstrated, and damage is found to be preferentially localized in the aluminum planes of the structure. On the basis of comparisons between calculations and experimental spectra, a structural model is proposed for the irradiated state. This study emphasizes the utility of core-loss fine structure analysis to enhance understanding of ion irradiation-induced damage in complex crystalline materials.
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| 7. |
- Cabioch, Thierry, et al.
(författare)
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Structural investigation of substoichiometry and solid solution effects in Ti2Al(C-x,N1-x)(y) compounds
- 2012
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Ingår i: Journal of the European Ceramic Society. - : Elsevier. - 0955-2219 .- 1873-619X. ; 32:8, s. 1803-1811
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Tidskriftsartikel (refereegranskat)abstract
- The milling, cold compaction and thermal annealing (4 h-1400 degrees C-Ar flow) of Ti, TiC, Al and AlN powders were used to produce Ti2Al(CxN(1-x))(y) compounds with x = 0, 0.25, 0.5, 0.75, 1 and 0.7 andlt;= y andlt;= 1. X-ray diffraction analysis, scanning electron microscopy observations combined with microanalysis confirmed the formation of the almost pure Ti(2)A/CxN(1-x) carbonitrides for y=1 whereas increasing amounts of titanium aluminides were formed when y decreases. Proportions of the different phases deduced from Rietveld refinements of the X-ray diffractograms indicate that no or very poor substoichiometry in carbon was possible in carbide whereas C and N deficiency can be achieved in nitrides and carbonitrides Ti2AlCxN(1-x). Electron Energy Loss Spectroscopy investigations confirm that carbonitrides can have at least 20% of vacancies on the C or N site. The a lattice parameter varies linearly with x whereas it is not the case for the c lattice parameter, its values being lower for the carbonitrides. Furthermore, a strong broadening of the carbonitrides XRD peaks is observed, a phenomenon that can be mainly attributed to C and N concentration gradients inside the samples.
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| 8. |
- Cabioch, Thierry, et al.
(författare)
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Tailoring of the thermal expansion of Cr2(Alx,Ge1−x)C phases
- 2013
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Ingår i: Journal of the European Ceramic Society. - : Elsevier. - 0955-2219 .- 1873-619X. ; 33:4, s. 897-904
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Tidskriftsartikel (refereegranskat)abstract
- We report thermal expansion coefficients of the end members and solid-solution compounds in the Cr2(Alx,Ge1−x)C system. All samples studied were essentially phase-pure Cr2AlxGe1−xC except the Cr2GeC sample, which contained a substantial fraction of Cr5Ge3Cx. X-ray diffraction performed in the 25–800 °C temperature range shows that the in-plane thermal expansion remains essentially constant at about 14 ± 1 × 10−6 K−1 irrespective of Al content. The thermal expansion of the c axis decreases monotonically from 17 ± 1 × 10−6 K−1 for Cr2GeC to ∼12 ± 1 × 10−6 K−1 with increasing Al content. At around the Cr2(Al0.75,Ge0.25)C composition, the thermal expansion coefficients along the two directions are equal; a useful property to minimize thermal residual stresses. This study thus demonstrates that a solid-solution approach is a route for tuning a physical property like the thermal expansion. For completeness, we also include a structure description of the Cr5Ge3Cx phase, which has been reported before but is not well documented. Its space group is P63/mcm and its a and c lattice parameters are 7.14 Å and 4.88 Å, respectively. We also measured the thermal expansion coefficients of the Cr5Ge3Cx phase. They are found to be 16.3 × 10−6 K−1 and 28.4 × 10−6 K−1 along the a and c axes, respectively. Thus, the thermal expansion coefficients of Cr5Ge3Cx are highly anisotropic and considerably larger than those of the Cr2(Alx,Ge1−x)C phases.
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| 9. |
- Eklund, Per, et al.
(författare)
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Discovery of the Ternary Nanolaminated Compound Nb2GeC by a Systematic Theoretical-Experimental Approach
- 2012
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 109:3, s. 035502-
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Tidskriftsartikel (refereegranskat)abstract
- Since the advent of theoretical materials science some 60 years ago, there has been a drive to predict and design new materials in silicio. Mathematical optimization procedures to determine phase stability can be generally applicable to complex ternary or higher-order materials systems where the phase diagrams of the binary constituents are sufficiently known. Here, we employ a simplex-optimization procedure to predict new compounds in the ternary Nb-Ge-C system. Our theoretical results show that the hypothetical Nb2GeC is stable, and excludes all reasonably conceivable competing hypothetical phases. We verify the existence of the Nb2GeC phase by thin film synthesis using magnetron sputtering. This hexagonal nanolaminated phase has a and c lattice parameters of similar to 3.24 angstrom and 12.82 angstrom.
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| 10. |
- Eklund, Per, et al.
(författare)
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Epitaxial growth and electrical transport properties of Cr(2)GeC thin films
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:7, s. 075424-
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Tidskriftsartikel (refereegranskat)abstract
- Cr(2)GeC thin films were grown by magnetron sputtering from elemental targets. Phase-pure Cr(2)GeC was grown directly onto Al(2)O(3)(0001) at temperatures of 700-800 degrees C. These films have an epitaxial component with the well-known epitaxial relationship Cr(2)GeC(0001)//Al(2)O(3)(0001) and Cr(2)GeC(11andlt;(2) overbarandgt;0)//Al(2)O(3)(1andlt;(1)over barandgt;00) or Cr(2)GeC(11andlt;(2) over barandgt;0)//Al(2)O(3)(andlt;(1) over barandgt;2andlt;(1) over barandgt;0). There is also a large secondary grain population with (10andlt;(1)overbarandgt;3) orientation. Deposition onto Al(2)O(3)(0001) with a TiN(111) seed layer and onto MgO(111) yielded growth of globally epitaxial Cr(2)GeC(0001) with a virtually negligible (10andlt;(1) over barandgt;3) contribution. In contrast to the films deposited at 700-800 degrees C, the ones grown at 500-600 degrees C are polycrystalline Cr(2)GeC with (10andlt;(1) over barandgt;0)-dominated orientation; they also exhibit surface segregations of Ge as a consequence of fast Ge diffusion rates along the basal planes. The room-temperature resistivity of our samples is 53-66 mu Omega cm. Temperature-dependent resistivity measurements from 15-295 K show that electron-phonon coupling is important and likely anisotropic, which emphasizes that the electrical transport properties cannot be understood in terms of ground state electronic structure calculations only.
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