| 511. |
- Kakanakova-Georgieva, Anelia, 1970-, et al.
(författare)
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Epitaxial growth of AlN layers on SiC substrates in a hot-wall MOCVD system
- 2002
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Ingår i: Phys. Stat. Sol. (c), Vol. 0, Issue 1. - : Wiley. ; , s. 205-208
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Konferensbidrag (refereegranskat)abstract
- In this study we report the successful growth of AlN and AlN/GaN on SiC substrates in a MOCVD process based on a hot-wall susceptor design. Different features of AlN growth are established depending on the total reactor pressure, temperature, off-cut SiC substrate orientation and V-to-III gas-flow ratio. The feasibility of the hot-wall MOCVD concept is demonstrated by the performance of AlN/GaN structures with state-of-the-art properties with strong potential for further optimization. A narrower X-ray rocking curve over the asymmetric 10.4 than the symmetric 00.2 reflection clearly underlines the high overall crystal quality of the GaN layers on AlN buffers grown in this type of MOCVD reactor.
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| 512. |
- Kakanakova-Georgieva, A, et al.
(författare)
-
Implementation of hot-wall MOCVD in the growth of high-quality GaN on SiC
- 2003
-
Ingår i: Materials Science Forum, Vols. 433-436. - : Trans Tech Publications. ; , s. 991-994
-
Konferensbidrag (refereegranskat)abstract
- This paper reports on the growth of high-quality GaN layers on SiC substrates by hotwall MOCVD. Use of AlN buffer with a thickness exceeding 50 nm is employed for the GaN deposition and it is found to encompass most of the misfit defects. A narrower X-ray rocking curve over asymmetric than over symmetric reflection is measured - full width at a half maximum (FWHM) of 350 arcsec vs. FWHM of 490 arcsec for 10.4 and 00.2 peaks, respectively, indicating high overall quality of the film. The free exciton photoluminescence emission peak has rather narrow FWHM of 5 meV. The typical thickness of the GaN layers is about 2 mum and they are completely depleted according to the capacitance-voltage profiling, which corresponds to estimated residual doping of less than 5x10(14) cm(-3). Only in some cases when the GaN layer is not depleted, deep level transient spectroscopy is performed and two deep traps with activation energies of 0.26 and 0.59 eV below the conduction band are measured.
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| 513. |
- Kakanakova-Georgieva, Anelia, 1970-, et al.
(författare)
-
Sublimation epitaxy of AlN on SiC : Growth morphology and structural features
- 2004
-
Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 273:1-2, s. 161-166
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Tidskriftsartikel (refereegranskat)abstract
- In order to study the development of individual AlN crystallites, sublimation epitaxy of AlN was performed on 4H-SiC, off-axis substrates in an inductively heated setup. Growth process variables like temperature, extrinsic nitrogen pressure and time were changed in an attempt to favor the lateral growth of individual AlN crystallites and thus open possibilities to prepare continuous patterns. Scanning and transmission electron microscopy and cathodoluminescence were used to obtain plan-view and cross-sectional images of the grown patterns and to study their morphology and structural features. The growth at 1900°C/200mbar results in AlN pattern consisting of individual single wurzite AlN crystallites with plate-like shape aligned along [1 1̄ 0 0] direction. The only defects these AlN crystallites contain are threading dislocations, some of which are terminated by forming half-loops. Because of the uniform distribution of the crystallites and their high structural perfection, this AlN pattern could represent interest as a template for bulk AlN growth. Alternative growth approaches to AlN crystallite formation are possible resulting in variation of the final AlN pattern structure. From a viewpoint of obtaining continuous patterns, the more favorable growth conditions involve applying of increased extrinsic gas pressure, 700 mbar in our case. © 2004 Elsevier B.V. All rights reserved.
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| 514. |
- Kakanakova-Georgieva, Anelia, 1970-, et al.
(författare)
-
Superior material properties of AlN on vicinal 4H-SiC
- 2006
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 100:3
-
Tidskriftsartikel (refereegranskat)abstract
- The crystal structure and optical properties of thick (>100 nm) AlN layers grown by hot-wall metalorganic chemical vapor deposition are characterized by infrared spectroscopic ellipsometry, cathodoluminescence, and transmission electron microscopy. The choice of substrates among the available SiC wafer polytype modifications (4H/6H) and misorientations (on-/off-axis cut) is found to determine the AlN defect interaction, stress homogeneity, and luminescence. The growth of thick AlN layers benefits by performing the epitaxy on off-axis substrates because, due to stacking faults, the propagation of threading defects in AlN layers is stopped in a narrow interface region. © 2006 American Institute of Physics.
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| 515. |
- Karlsson, L, et al.
(författare)
-
Growth, microstructure, and mechanical properties of arc evaporated TiCxN1-x (0 <= x <= 1) films
- 2000
-
Ingår i: Surface & Coatings Technology. - 0257-8972 .- 1879-3347. ; 126:1
-
Tidskriftsartikel (refereegranskat)abstract
- TiCxN1-x films with x ranging from 0 to 1 were grown by arc evaporation by varying the flow ratio between the reactive gases. The substrates were cemented carbide inserts (WC-6 wt.% Co) which were negatively biased at 400 V, resulting in a deposition temperature of similar to 550 degrees C. The film composition, as measured by glow discharge optical emission spectroscopy, was found to vary almost linearly with the gas flow ratio. Cross-sectional transmission electron microscopy in combination with X-ray diffraction (XRD) showed that the films were of single-phase NaCl-structure with a dense columnar microstructure. The intrinsic stress analyzed using the XRD sin(2)psi method, was found to have a maximum of - 5.9 GPa in the composition range of 0.4 less than or equal to x less than or equal to 0.7 which correlated with a maximum in XRD peak broadening due to inhomogeneous strains. The hardness and Young's modulus of the as-deposited TiCxN1-x films were measured by the nanoindentation technique. A maximum in hardness of 45 GPa was found at the same composition range (0.4 Ix I 0.7) as the intrinsic stress maximum. The hardness for x = 0 (TiN) and x = 1 (TiC) were found to be 28 and 36 GPa, respectively. The Young's modulus was constant similar to 610 GPa for films with compositions up to x = 0.6, thereafter it decreased to 540 GPa at x = 1. The increase in intrinsic stress with increasing carbon content is suggested to be due to increased stability of defects created from the collision cascade or/and by a change in the defect structure itself. The fact that hardness showed a maximum at the same composition as residual stress and FWHM indicates that obstruction on dislocation movement has a major influence on the hardness of these films. (C) 2000 Published by Elsevier Science S.A. All rights reserved.
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| 516. |
- Karlsson, L., et al.
(författare)
-
Influence of residual stresses on the mechanical properties of TiCxN1-x (x = 0, 0.15, 0.45) thin films deposited by arc evaporation
- 2000
-
Ingår i: Thin Solid Films. - 0040-6090 .- 1879-2731. ; 371:1, s. 167-177
-
Tidskriftsartikel (refereegranskat)abstract
- The influence of residual stress state and composition on the mechanical properties of arc evaporated TiCxN1-x thin films been investigated. By controlling the flow ratios of the reactive gases, N2 and CH4, films with compositions x = 0 (TIN), x to approximately 0.15, and x to approximately 0.45 have been grown on cemented carbide substrates. The residual stress state was altered through variations in the negative substrate bias over the range 20 V=Vs=820 V. The intrinsic stress, sint, measured by the X-ray diffraction (XRD) sin2? method was compressive and increased with decreasing Vs and increasing x. The latter behavior is suggested to be due to increased effective stability of defect complexes when the carbon content increases. Maximum stress level was between -6 and -7 GPa and limited by interior cracking of the films. The increase in intrinsic stress was accompanied by an increase in XRD peak broadening due to inhomogeneous strains. The hardness, H, and Young's modulus, E, of as-deposited films were measured using the nanoindentation technique. Apparently linear correlations between sint and H were found for each film composition where H increased with x. The maximum H, 44 GPa, was thus obtained for the x to approximately 0.45 film with sint = -5.5 GPa. The lowest hardness for this composition was 35 GPa for a film with sint = -2.7 GPa. For the TiN films, a similar variation in hardness of 33 GPa at sint = -5.8 to 26 GPa at sint = -1.2 GPa was obtained. E was constant at approximately 610 GPa for most of the films, with a slight decrease in the films with the lowest sint values.
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| 517. |
- Karlsson, Lennart, et al.
(författare)
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The influence of thermal annealing on residual stresses and mechanical properties of arc-evaporated TiCxN1−x (x=0, 0,15 and 0,45) thin films
- 2002
-
Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 50:20, s. 5103-5114
-
Tidskriftsartikel (refereegranskat)abstract
- We report the stress relaxation behavior of arc-evaporated TiCxN1−x thin films during isothermal annealing between 350 and 900°C. Films with x=0, 0,15 and 0,45 each having an initial compressive intrinsic stress σint = -5.4 GPa were deposited by varying the substrate bias Vs and the gas composition. Annealing above the deposition temperature leads to a steep decrease in the magnitude of σint to a saturation stress value, which is a function of the annealing temperature. The corresponding apparent activation energies for stress relaxation are Ea=2.4, 2.9, and 3.1 eV, for x=0, 0,15 and 0,45 respectively. TiC0.45N0.55 films with a lower initial stress σint = -3 GPa obtained using a high substrate bias, show a higher activation energy Ea=4.2 eV.In all the films, stress relaxation is accompanied by a decrease in defect density indicated by the decreased width of X-ray diffraction peaks and decreased strain contrast in transmission electron micrographs. Correlation of these results with film hardness and microstructure measurements indicates that the stress relaxation is a result of point-defect annihilation taking place both during short-lived metal-ion surface collision cascades during deposition, and during post-deposition annealing by thermally activated processes. The difference in Ea for the films of the same composition deposited at different Vs suggests the existence of different types of point-defect configurations and recombination mechanisms.
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| 518. |
- Kashiwaya, Shun, et al.
(författare)
-
Formation of Ti2AuN from Au-Covered Ti2AlN Thin Films: A General Strategy to Thermally Induce Intercalation of Noble Metals into MAX Phases
- 2020
-
Ingår i: Crystal Growth & Design. - : AMER CHEMICAL SOC. - 1528-7483 .- 1528-7505. ; 20:6, s. 4077-4081
-
Tidskriftsartikel (refereegranskat)abstract
- Thermally induced intercalation of noble metals into non-van der Waals ceramic compounds presents a method to produce a new class of layered materials. We recently demonstrated an exchange reaction of Au with A layers of MAX phase carbides with plentiful combinations of A and M elements. Here, we report the first substitution of Al with Au in a Ti2AlN MAX phase nitride at an elevated temperature without destroying the original layered structure. These results bolster the generalization of the Au intercalation for the A elements in MAX phases with diverse combinations of M, A, and X elements. Furthermore, we propose crucial factors to achieve the exchange reaction: there should be a chemical potential for the A element to dissolve in or react with noble metals to intercalate; the noble metals should be inert to the initial metal carbides/nitrides; and it is necessary to choose the reaction temperature that allows balanced interdiffusion of the noble metals and A elements.
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| 519. |
- Kashiwaya, Shun, et al.
(författare)
-
Synthesis of goldene comprising single-atom layer gold
- 2024
-
Ingår i: Nature Synthesis. - : Nature Publishing Group. - 2731-0582.
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Tidskriftsartikel (refereegranskat)abstract
- The synthesis of monolayer gold has so far been limited to free-standingseveral-atoms-thick layers, or monolayers confned on or inside templates.Here we report the exfoliation of single-atom-thick gold achieved throughwet-chemically etching away Ti3C2 from nanolaminated Ti3AuC2, initiallyformed by substituting Si in Ti3SiC2 with Au. Ti3SiC2 is a renown MAX phase,where M is a transition metal, A is a group A element, and X is C or N. Ourdeveloped synthetic route is by a facile, scalable and hydrofuoric acid-freemethod. The two-dimensional layers are termed goldene. Goldene layerswith roughly 9% lattice contraction compared to bulk gold are observedby electron microscopy. While ab initio molecular dynamics simulationsshow that two-dimensional goldene is inherently stable, experiments showsome curling and agglomeration, which can be mitigated by surfactants.X-ray photoelectron spectroscopy reveals an Au 4f binding energy increaseof 0.88 eV. Prospects for preparing goldene from other non-van der WaalsAu-intercalated phases, including developing etching schemes,are presented.
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| 520. |
- Kashiwaya, Shun, et al.
(författare)
-
Synthesis of single-atom-thick gold layers
- 2024
-
Ingår i: NATURE SYNTHESIS. - : SPRINGERNATURE. - 2731-0582.
-
Tidskriftsartikel (refereegranskat)abstract
- Monolayer gold could exhibit properties of benefit to various applications, but has been challenging to synthesize. Now, the exfoliation of two-dimensional single-atom-thick gold layers - termed goldene - is achieved through wet-chemically etching away Ti3C2 from Ti3AuC2, a nanolaminated MAX-phase. Goldene shows lattice contraction and an increase in the gold 4f binding energy compared with the bulk.
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