| 1. |
- Buyanova, Irina, 1960-, et al.
(författare)
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Tunable laser spectroscopy of spin injection in ZnMnSe/ZnCdSe quantum structures
- 2002
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 81:12, s. 2196-
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Tidskriftsartikel (refereegranskat)abstract
- Magneto-optical spectroscopy in combination with tunable laser excitation is employed to study exciton spin alignment and injection in ZnMnSe/ZnCdSe quantum structures. This approach enables us to selectively create preferred spin orientation and to separately monitor subsequent spin injection from individual spin states, thus shedding light on a possible source of spin loss. It is shown that the limited spin polarization in a nonmagnetic quantum well due to spin injection from a ZnMnSe-based diluted magnetic semiconductor (DMS) is not caused by a limited degree of spin alignment in the DMS, which is in fact complete, but rather occurs during subsequent processes.
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| 2. |
- Cattelan, Mattia, et al.
(författare)
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Anodization study of epitaxial graphene : insights on the oxygen evolution reaction of graphitic materials
- 2019
-
Ingår i: Nanotechnology. - : Institute of Physics Publishing (IOPP). - 0957-4484 .- 1361-6528. ; 30:28
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Tidskriftsartikel (refereegranskat)abstract
- The photoemission electron microscopy and x-ray photoemission spectroscopy were utilized for the study of anodized epitaxial graphene (EG) on silicon carbide as a fundamental aspect of the oxygen evolution reaction on graphitic materials. The high-resolution analysis of surface morphology and composition quantified the material transformation during the anodization. We investigated the surface with lateral resolution amp;lt;150 nm, revealing significant transformations on the EG and the role of multilayer edges in increasing the film capacitance.
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| 3. |
- Chih-Wei, Chih-Wei, et al.
(författare)
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Direct epitaxial nanometer-thin InN of high structural quality on 4H-SiC by atomic layer deposition
- 2020
-
Ingår i: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 117:9
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Tidskriftsartikel (refereegranskat)abstract
- Indium nitride (InN) is a highly promising material for high frequency electronics given its low bandgap and high electron mobility. The development of InN-based devices is hampered by the limitations in depositing very thin InN films of high quality. We demonstrate growth of high-structural-quality nanometer thin InN films on 4H-SiC by atomic layer deposition (ALD). High resolution x-ray diffraction and transmission electron microscopy show epitaxial growth and an atomically sharp interface between InN and 4H-SiC. The InN film is fully relaxed already after a few atomic layers and shows a very smooth morphology where the low surface roughness (0.14nm) is found to reproduce sub-nanometer surface features of the substrate. Raman measurements show an asymmetric broadening caused by grains in the InN film. Our results show the potential of ALD to prepare high-quality nanometer-thin InN films for subsequent formation of heterojunctions.
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| 4. |
- Donchev, V., et al.
(författare)
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High-temperature excitons in GaAs quantum wells embedded in AlAs/GaAs superlattices
- 2000
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Ingår i: Vacuum. - 0042-207X .- 1879-2715. ; 58:2, s. 478-484
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Tidskriftsartikel (refereegranskat)abstract
- Photoluminescence (PL) spectra of GaAs quantum wells embedded in short-period AlAs/GaAs superlattices have been measured at 2 K and at room temperature. Two approaches have been applied in order to investigate the mechanisms of radiative recombination in these structures. In the first one, we studied the excitation density dependence of the PL intensity. In the second approach a line-shape analysis of the PL spectra is performed by means of a statistical model, which includes both free exciton, and free carrier recombinations. The fit based on this model reproduces with high accuracy the experimental spectra and allows to assess the relative contributions of excitons and free carriers to the radiative recombination process. The results of both approaches indicate the predominance of free excitons in the radiative recombination at room temperature.
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| 5. |
- Donchev, V., et al.
(författare)
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Photoluminescene line-shape analysis in quantum wells embedded in superlattices
- 2001
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Ingår i: Materials science & engineering. C, biomimetic materials, sensors and systems. - 0928-4931 .- 1873-0191. ; 15:1-2, s. 75-77
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Tidskriftsartikel (refereegranskat)abstract
- The temperature evolution of the main photoluminescence (PL) mechanisms, in GaAs quantum wells embedded in short-period AlAs/GaAs superlattices, is investigated. PL spectra are measured from 2 to 300 K. A detailed line-shape analysis of the PL peaks is performed by means of a statistical model, including both free exciton and free carrier recombination. The fits based on this model reproduce satisfactorily the experimental PL line shapes and allow to assess quantitatively the relative contributions of free excitons and free carriers to the radiative recombination at different temperatures. The results indicate the predominant role of free excitons in the radiative recombination up to room temperature, and are consistent with the mass action law. © 2001 Elsevier Science B.V. All rights reserved.
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| 6. |
- Giannazzo, F., et al.
(författare)
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Probing the uniformity of hydrogen intercalation in quasi-free-standing epitaxial graphene on SiC by micro-Raman mapping and conductive atomic force microscopy
- 2019
-
Ingår i: Nanotechnology. - : IOP PUBLISHING LTD. - 0957-4484 .- 1361-6528. ; 30:28
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, micro-Raman mapping and conductive atomic force microscopy (C-AFM) were jointly applied to investigate the structural and electrical homogeneity of quasi-free-standing monolayer graphene (QFMLG), obtained by high temperature decomposition of 4H-SiC(0001) followed by hydrogen intercalation at 900 degrees C. Strain and doping maps, obtained by Raman data, showed the presence of sub-micron patches with reduced hole density correlated to regions with higher compressive strain, probably associated with a locally reduced hydrogen intercalation. Nanoscale resolution electrical maps by C-AFM also revealed the presence of patches with enhanced current injection through the QFMLG/SiC interface, indicating a locally reduced Schottky barrier height (Phi(B)). The Phi(B) values evaluated from local I-V curves by the thermionic emission model were in good agreement with the values calculated for the QFMLG/SiC interface using the Schottky-Mott rule and the graphene holes density from Raman maps. The demonstrated approach revealed a useful and non-invasive method to probe the structural and electrical homogeneity of QFMLG for future nano-electronics applications.
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| 7. |
- Högberg, Hans, et al.
(författare)
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Reactive sputtering of CSx thin solid films using CS2 as precursor
- 2020
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Ingår i: Vacuum. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0042-207X .- 1879-2715. ; 182
-
Tidskriftsartikel (refereegranskat)abstract
- We deposit CSx thin solid films by reactive direct current magnetron sputtering of a C target in an argon plasma, using carbon disulfide (CS2) as a precursor to film growth. We investigate the influence of the partial pressure of the CS2 vapor introduced into the plasma on the composition, the chemical bonding structure, the structural, and the mechanical properties as determined by x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscopy (SEM), and nanoindentation for films deposited at 150 and 300 degrees C. The Raman and the XPS results indicate that S atoms are incorporated in mostly sp(2) bonded C network. These results agree with previous ab-initio theoretical findings obtained by modeling of the CSx compound by the Synthetic Growth Concept. The microstructure of the films as well as the results of their Raman characterization and the nano mechanical testing results all point out that with the increasing S content some spa bonding is admixed in the predominantly sp(2) bonded CSx network, leading to typical amorphous structure with short and interlocked graphene-like planes for S contents between 2% and 8%. We conclude that CSx thin solid films deposited by using CS2 as a precursor would be CSx films deposited at low temperature of similar to 150 degrees C and with an S content in the region of 6 at.% may be interesting candidates for applications as hard/elastic protective coatings.
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| 8. |
- Kakanakova-Gueorguieva, Anelia, et al.
(författare)
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MOCVD of AlN on epitaxial graphene at extreme temperatures
- 2021
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Ingår i: CrystEngComm. - : ROYAL SOC CHEMISTRY. - 1466-8033. ; 23:2, s. 385-390
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Tidskriftsartikel (refereegranskat)abstract
- The initial stages of metal organic chemical vapor deposition (MOCVD) of AlN on epitaxial graphene at temperatures in excess of 1200 degrees C have been rationalized. The use of epitaxial graphene, in conjunction with high deposition temperatures, can deliver on the realization of nanometer thin AlN whose material quality is characterized by the appearance of luminescent centers with narrow spectral emission at room temperature. It has been elaborated, based on our previous comprehensive ab initio molecular dynamics simulations, that the impact of graphene on AlN growth consists in the way it promotes dissociation of the trimethylaluminum, (CH3)(3)Al, precursor with subsequent formation of Al adatoms during the initial stages of the deposition process. The high deposition temperatures ensure adequate surface diffusion of the Al adatoms which is an essential factor in material quality enhancement. The role of graphene in intervening with the dissociation of another precursor, trimethylgallium, (CH3)(3)Ga, has accordingly been speculated by presenting a case of propagation of ultrathin GaN of semiconductor quality. A lower deposition temperature of 1100 degrees C in this case has better preserved the structural integrity of epitaxial graphene. Breakage and decomposition of the graphene layers has been deduced in the case of AlN deposition at temperatures in excess of 1200 degrees C.
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| 9. |
- Kakanakova-Gueorguieva, Anelia, et al.
(författare)
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Nanoscale phenomena ruling deposition and intercalation of AlN at the graphene/SiC interface
- 2020
-
Ingår i: Nanoscale. - : ROYAL SOC CHEMISTRY. - 2040-3364 .- 2040-3372. ; 12:37, s. 19470-19476
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Tidskriftsartikel (refereegranskat)abstract
- The possibility for kinetic stabilization of prospective 2D AlN was explored by rationalizing metal organic chemical vapor deposition (MOCVD) processes of AlN on epitaxial graphene. From the wide range of temperatures which can be covered in the same MOCVD reactor, the deposition was performed at the selected temperatures of 700, 900, and 1240 degrees C. The characterization of the structures by atomic force microscopy, electron microscopy and Raman spectroscopy revealed a broad range of surface nucleation and intercalation phenomena. These phenomena included the abundant formation of nucleation sites on graphene, the fragmentation of the graphene layers which accelerated with the deposition temperature, the delivery of excess precursor-derived carbon adatoms to the surface, as well as intercalation of sub-layers of aluminum atoms at the graphene/SiC interface. The conceptual understanding of these nanoscale phenomena was supported by our previous comprehensiveab initiomolecular dynamics (AIMD) simulations of the surface reaction of trimethylaluminum, (CH3)(3)Al, precursor with graphene. A case of applying trimethylindium, (CH3)(3)In, precursor to epitaxial graphene was considered in a comparative way.
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| 10. |
- Khranovskyy, Volodymyr, et al.
(författare)
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Light emission enhancement from ZnO nanostructured films grown on Gr/SiC substrates
- 2016
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Ingår i: Carbon. - : Pergamon Press. - 0008-6223 .- 1873-3891. ; 99, s. 295-301
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Tidskriftsartikel (refereegranskat)abstract
- We report on the application of a single layer graphene substrates for the growth of polycrystalline ZnO films with advanced light emission properties. Unusually high ultraviolet (UV) and visible (VIS) photoluminesce was observed from the ZnO/Gr/SiC structures in comparison to identical samples without graphene. The photoluminescence intensity depends non-monotonically on the films thickness, reaching its maximum for 150 nm thick films. The phenomena observed is explained as due to the dual graphene role: i) the dangling bond free substrate, providing growth of relaxed thin ZnO layers ii) a back reflector active mirror of the Fabry-Perot cavity that is formed. The reported results demonstrate the potential of two-dimensional carbon materials integration with light emitting wide band gap semiconductors and can be of practical importance for the design of future optoelectronic devices.
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| 11. |
- Le, Son Phuong, et al.
(författare)
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GaN-based light-emitting materials prepared by hot-wall metal-organic chemical vapor deposition
- 2022
-
Ingår i: Applied Physics A. - : SPRINGER HEIDELBERG. - 0947-8396 .- 1432-0630. ; 128:9
-
Tidskriftsartikel (refereegranskat)abstract
- GaN-based structures grown on SiC substrates by means of horizontal hot-wall metal-organic chemical vapor deposition (MOCVD) were systematically characterized, revealing high crystal quality. The hot-wall MOCVD grown GaN, doped by Mg and Si, respectively showed low-resistivity hole and electron transport, competitive with the state-of-the-art GaN. High concentrations of free holes (similar to 2 x 10(17) cm(-3)) were achieved for the as-grown Mg-doped GaN without thermal annealing, thanks to advantageous heating characteristics of the "hot-wall" reactor. The analysis of optical and electrical properties brought a picture, where Mg is the only impurity defining energy levels in the hot-wall MOCVD p-type doped GaN. Besides, InGaN/GaN light-emitting diodes employing such doped GaN materials in the carrier-transport layers were fabricated, resulting in high device performances. The devices exhibited bright electroluminescence with very narrow full widths at half maximum as well as negligible spectral shifts at high current levels (greater than or similar to 10 A/cm(2)). These results exemplified the rewards of the hot-wall MOCVD for development of high-quality nitrides-based structures, providing an attractive growth method to realize the demonstration of light-emitting devices with favorable properties.
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| 12. |
- Santangelo, Francesca, et al.
(författare)
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Real-time sensing of lead with epitaxial graphene-integrated microfluidic devices
- 2019
-
Ingår i: Sensors and actuators. B, Chemical. - : ELSEVIER SCIENCE SA. - 0925-4005 .- 1873-3077. ; 288, s. 425-431
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Tidskriftsartikel (refereegranskat)abstract
- Since even low concentrations of toxic heavy metals can seriously damage human health, it is important to develop simple, sensitive and accurate methods for their detection. Graphene, which is extremely sensitive to foreign species, is a key element in the development of a sensing platform where low concentrations of analyte have to be detected. This work discusses the proof of concept of a sensing platform for liquid-phase detection of heavy metals (e.g. Pb) based on epitaxial graphene sensors grown on Si-face 4H-SiC substrate (EG/SiC). The sensing platform developed includes a microfluidic chip incorporating all the features needed to connect and execute the Lab-on-chip (LOC) functions using 3D printing fast prototyping technology. Herein, we present the response of EG to concentrations of Pb2+ solutions ranging from 125 nM to 500 mu M, showing good stability and reproducibility over time and an enhancement of its conductivity with a Langmuir correlation between signal and Pb2+ concentration. Density functional theory (DFT) calculations are performed and clearly explain the conductivity changes and the sensing mechanism in agreement with the experimental results reported, confirming the strong sensitivity of the sensor to the lowest concentrations of the analyte. Furthermore, from the calibration curve of the system, a limit of detection (LoD) of 95 nM was extrapolated.
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| 13. |
- 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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| 14. |
- 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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| 15. |
- Schmidt, Susann, et al.
(författare)
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Reactive High Power Impulse Magnetron Sputtering of CFx Thin Films in Mixed Ar/CF4 and Ar/C4F8 Discharges
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The reactive high power impulse magnetron sputtering (HiPIMS) processes of C in Ar/tetrafluoromethane CF4 and Ar/octafluorocyclobutane (c-C4F8) have been characterized. Amorphous carbon fluoride (CFx) films were synthesized at deposition pressure and substrate temperature of 400 mPa and 110 oC, respectively. The CFx film composition was controlled in the range of 0.15 < x < 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 F plasmas show an abundance of CF+, C+, CF⁺₂, and CF⁺₃ (in this order) as corroborated by first-principles calculations. Only CF⁺₃ exceeded the Ar+ signal in a CF4 plasma. Two deposition regimes are found depending on the partial pressure of the F-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 F content can be attributed to the abundance of CF⁺₃ precursor species, weakening the C matrix.
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| 16. |
- Shtepliuk, Ivan, et al.
(författare)
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Clustering and Morphology Evolution of Gold on Nanostructured Surfaces of Silicon Carbide: Implications for Catalysis and Sensing
- 2021
-
Ingår i: ACS Applied Nano Materials. - : American Chemical Society (ACS). - 2574-0970. ; 4:2, s. 1282-1293
-
Tidskriftsartikel (refereegranskat)abstract
- A fundamental understanding of the behavior of gold (Au) nanostructures deposited on functional surfaces is imperative to discover and leverage interface-related phenomena that can boost the efficiency of existing electronic devices in sensorics, catalysis, and spintronics. In the present work, Au layers with nominal thickness of 2 nm were sputter-deposited on graphenized SiC substrates represented by buffer layer (BuL)/4H-SiC and monolayer epitaxial graphene (MLG)/4H-SiC. Morphometric analysis by means of scanning electron microscopy shows that Au on BuL self-assembles in nearly round-shaped plasmonically active islands, while on MLG, a fractal growth of considerably larger and ramified islands is observed. To correlate the experimentally established differences in surface morphology on the two types of graphenized substrates with energetics and kinetics of Au nanostructure growth, the deposit-substrate interaction strength was studied using density functional theory (DFT) calculations, molecular dynamics simulations, and optical measurements. The theoretical considerations involve participation of Au clusters with different sizes and energetics at the initial stages of the metal nanostructure formation. The results indicate that gold exhibits a considerably stronger interaction with BuL than with MLG, which can be considered as a key aspect for explaining the experimentally observed morphological differences. From the statistical analysis of Raman spectra, indications of Au intercalation of MLG are discussed. The current research shows that, due to its unique surface chemistry, buffer layer has peculiar affinity to gold when compared to other atomically flat surfaces, which is beneficial for boosting high-performance catalytic and sensing technologies based on low-dimensional materials.
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| 17. |
- Shtepliuk, Ivan, et al.
(författare)
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Electrochemical performance of gold-decorated graphene electrodes integrated with SiC
- 2023
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Ingår i: Microelectronic Engineering. - : Elsevier BV. - 0167-9317 .- 1873-5568. ; 278
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Tidskriftsartikel (refereegranskat)abstract
- Here we investigate the interface properties of gold (Au) decorated graphenized surfaces of 4H-SiC intended for electrochemical electrodes. These are fabricated using a two-step process: discontinuous Au layers with a nominal thickness of 2 nm are sputter-deposited onto 4H-SiC substrates with different graphenization extent—zero-layer graphene (ZLG) and monolayer epitaxial graphene) —followed by thermal annealing. By performing combined morphometric analysis, Raman mapping analysis, conductive atomic force microscopy, and electrochemical impedance spectroscopy measurements, we shed light on the relationship between physical processes (Au intercalation, particle re-shaping, and de-wetting) caused by thermal annealing and the intrinsic properties of graphenized SiC (vertical electron transport, charge-transfer properties, vibrational properties, and catalytic activity). We find that the impedance spectra of all considered structures exhibit two semicircles in the high and low frequency regions, which may be attributed to the graphene/ZLG/SiC (or Au/graphene/ZLG/SiC) and SiC/ZLG/graphene/electrolyte (or SiC/ZLG//Au/electrolyte) interfaces, respectively. An equivalent circuit model is proposed to estimate the interface carrier transfer parameters. This work provides an in-depth comprehension of the way by which the Au/2D carbon/SiC interaction strength influences the interface properties of heterostructures, which can be helpful for developing high performance catalytic and sensing devices.
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| 18. |
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| 19. |
- Shtepliuk, Ivan, et al.
(författare)
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Exploring the Interface Landscape of Noble Metals on Epitaxial Graphene
- 2021
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley-VCH Verlagsgesellschaft. - 1862-6300 .- 1862-6319. ; 218:17
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the interaction between noble metals (NMs) and epitaxial graphene is essential for the design and fabrication of novel devices. Within this framework, a combined experimental and theoretical investigation of the effect of vapor-deposited NM (silver [Ag] and gold [Au]) nanostructures on the vibrational and electronic properties of monolayer epitaxial graphene (MLG) on 4H-SiC is presented. Large sets of Raman scattering data are analyzed using supervised classification and statistical methods. This analysis enables identification of the specific Raman fingerprints of Au- and Ag-decorated MLG originating from different dispersion interactions and charge transfer at the metal nanostructure/MLG interface. It is found that Raman scattering spectra of Au-decorated MLG feature a set of allowed phonon modes similar to those in pristine MLG, whereas the stronger Ag physisorption triggers an activation of defect-related phonon modes and electron doping of MLG. A principal component analysis (PCA) and linear discriminant analysis (LDA) are leveraged to highlight the features in phonon dispersion of MLG that emanate from the NM deposition process and to robustly classify large-scale Raman spectra of metal-decorated graphene. The present results can be advantageous for designing highly selective sensor arrays on MLG patches decorated with different metals.
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| 20. |
- Shtepliuk, Ivan I., 1987-, et al.
(författare)
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Lead (Pb) interfacing with epitaxial graphene
- 2018
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 20:25, s. 17105-17116
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Tidskriftsartikel (refereegranskat)abstract
- Here, we report the electrochemical deposition of lead (Pb) as a model metal on epitaxial graphene fabricated on silicon carbide (Gr/SiC). The kinetics of electrodeposition and morphological characteristics of the deposits were evaluated by complementary electrochemical, physical and computational methods. The use of Gr/SiC as an electrode allowed the tracking of lead-associated redox conversions. The analysis of current transients passed during the deposition revealed an instantaneous nucleation mechanism controlled by convergent mass transport on the nuclei locally randomly distributed on epitaxial graphene. This key observation of the deposit topology was confirmed by low values of the experimentally-estimated apparent diffusion coefficient, Raman spectroscopy and scanning electron microscopy (SEM) studies. First principles calculations showed that the nucleation of Pb clusters on the graphene surface leads to weakening of the interaction strength of the metal-graphene complex, and only spatially separated Pb adatoms adsorbed on bridge and/or edge-plane sites can affect the vibrational properties of graphene. We expect that the lead adatoms can merge in large metallic clusters only at defect sites that reinforce the metal-graphene interactions. Our findings provide valuable insights into both heavy metal ion electrochemical analysis and metal electroplating on graphene interfaces that are important for designing effective detectors of toxic heavy metals.
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| 21. |
- Shtepliuk, Ivan, et al.
(författare)
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Interplay between thin silver films and epitaxial graphene
- 2020
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Ingår i: Surface & Coatings Technology. - : ELSEVIER SCIENCE SA. - 0257-8972 .- 1879-3347. ; 381
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Tidskriftsartikel (refereegranskat)abstract
- Thin Ag films, with nominal thickness in the range 2 to 30 nm, are deposited using direct current magnetron sputtering and film morphology is studied by means of plan-view scanning electron microscopy. We find that for 2 mn nominal thickness the film surface consists of isolated circular nanoscale islands, which become interconnected as further material is deposited, leading to a continuous film at a nominal thickness of 30 nm. Our experimental findings are discussed in the context of the density functional theory results, which show that van der Waals forces dominate the interaction between Ag and epitaxial graphene. We also performed micro-Raman analysis and we find that the G and 2D modes of epitaxial graphene exhibit a red-shift upon Ag-layer deposition; which is interpreted as a result of charge transfer at the Ag/graphene interface. Moreover, we observed a pronounced enhancement of the G peak amplitude and area irrespective of the film nominal thickness and morphology, which we attribute to a combination of the charge transfer and plasmonic resonance effects. Our observations provide a critical information on the interaction between Ag and epitaxial graphene, which can be useful to design electronic and sensing devices based on Ag-epitaxial graphene hybrids.
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| 22. |
- Shtepliuk, Ivan, et al.
(författare)
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Manipulation of epitaxial graphene towards novel properties and applications
- 2020
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Ingår i: MATERIALS TODAY-PROCEEDINGS. - : ELSEVIER. - 2214-7853. ; , s. 37-45
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Konferensbidrag (refereegranskat)abstract
- The integration of epitaxial graphene on 4H-SiC with different metals may allow tunability of electronic and optical properties of graphene, enabling novel high-performance devices. Here we present a Raman spectroscopy study on epitaxial graphene decorated with electrodeposited Pb and Li adatoms and with magnetron sputtered 5 nm-thick Ag nano-island films. We find that the presence of metals on the epitaxial graphene surface generates defects and induces n-type doping, which is evidenced by the observation of the defect related Raman modes (namely D, D and D + G) and systematic red-shift of the main characteristic modes of graphene. In-depth statistical analysis of the Raman data before and after metal deposition complemented by density functional theory (DFT) calculations allowed to link the interaction strength between the three selected metals and graphene with the metal-induced changes in the vibrational/electronic properties of graphene. Large-area uniform electron doping of epitaxial graphene and surface-enhanced Raman scattering (SERS) effect are reached by room temperature deposition of Ag nano-island films. Very promising results have been obtained from graphene subjected to electrochemical intercalation by Li, which can serve as prerequisites of the construction of Li batteries. The strong interaction between Li or Pb with graphene implies the possibility to exploit the epitaxial graphene as an efficient material for energy storage or for heavy metal sensing, while predominant van der Waals interaction between Ag and graphene favors the formation of extremely thin silver coatings towards two-dimensional metal systems. The present results give better understanding of the nature of epitaxial graphene response to metal deposition and can be useful to design high-performance energy storage devices, optical sensors and heavy metal detection systems. (C) 2019 Elsevier Ltd. All rights reserved.
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| 23. |
- Shtepliuk, Ivan, et al.
(författare)
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Probing the uniformity of silver-doped epitaxial graphene by micro-Raman mapping
- 2020
-
Ingår i: Physica. B, Condensed matter. - : ELSEVIER. - 0921-4526 .- 1873-2135. ; 580
-
Tidskriftsartikel (refereegranskat)abstract
- We present a Raman spectroscopy study on epitaxial graphene decorated with thin Ag films (2-15 nm), which are deposited using magnetron sputtering. We find that the presence of Ag on the graphene surface induces doping, the uniformity and efficiency of which is determined by Ag nominal thickness. Deposition of Ag films with thicknesses up to 5 nm favors the effective electron transfer from Ag to epitaxial graphene. A significant redshift and broadening of the 2D peak are observed with increasing the Ag-layer thickness above 5 nm, which is indicative of large strain and doping fluctuations. We also observe a non-trivial linear growth of 2D/G peak intensity ratio with increasing D/G ratio for all Ag-decorated samples, which is explained by increase of peak amplitude due to surface enhanced Raman scattering and charged impurity-induced screening caused by the presence of Ag on the graphene surface.
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| 24. |
- Shtepliuk, Ivan, et al.
(författare)
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Raman probing of hydrogen-intercalated graphene on Si-face 4H-SiC
- 2019
-
Ingår i: Materials Science in Semiconductor Processing. - : ELSEVIER SCI LTD. - 1369-8001 .- 1873-4081. ; 96, s. 145-152
-
Tidskriftsartikel (refereegranskat)abstract
- We report the results of in-depth Raman study of quasi-free-standing monolayer graphene on the (0001) Si- face of 4H-SiC, which contains similar to 0.1-2.10(11) cm(-2) sp(3) defects that have been introduced by hydrogen intercalation. The nature of the intercalation-induced defects is elucidated and ascribed to the formation of the C-H bonds. At the higher intercalation temperature in the formed monolayer graphene the defect-related Raman scattering displays a great enhancement and new spectral features attributed to D and D+D modes appear. Comprehensive statistical analysis of the Raman data enabled us to estimate the homogeneity of the Raman scattering processes and to separate strain and doping effects. Analysis of the compressive strain and carrier density maps revealed that the intercalation temperature of 900 degrees C and intercalation time of 1 h are more favorable conditions for conversion of the buffer layer to uniformly relaxed and p-doped monolayer graphene in comparison to annealing at 1100 degrees C for 30 min.
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| 25. |
- Shtepliuk, Ivan, et al.
(författare)
-
Silver nanoparticle array on weakly interacting epitaxial graphene substrate as catalyst for hydrogen evolution reaction under neutral conditions
- 2021
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 119:15
-
Tidskriftsartikel (refereegranskat)abstract
- The paucity of research on hydrogen evolution reaction (HER) under neutral conditions, which is a more sustainable way to produce H-2 compared to acidic and alkaline HER, encourages the development of efficient catalytic materials and devices and deeper investigation of the mechanisms behind neutral HER. We present an electrode concept for facilitating HER under neutral conditions. The concept entails the use of annealing-reshaped silver (Ag) nanoparticle array on monolayer epitaxial graphene (MEG) on 4H-SiC. Measurements of HER performance show more positive onset potential of the cathodic HER for Ag-decorated MEG compared to that for pristine MEG, indicating improved water dissociation at Ag/MEG electrodes. Complementary morphological characterization, absorption measurements, and Raman mapping analysis enable us to ascribe the enhanced catalytic performance of electrodes decorated with 2 nm thick annealed Ag on the synergetic effect originating from simultaneous water reduction on circular Ag nanoparticles of 31 nm in diameter and on compressively strained Ag-free graphene regions. The overall results pave the way toward development of stable van der Waals heterostructure electrodes with a tunable metal-carbon interaction for fast HER under neutral conditions. (C) 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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| 26. |
- Shtepliuk, Ivan, et al.
(författare)
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Understanding Graphene Response to Neutral and Charged Lead Species: Theory and Experiment
- 2018
-
Ingår i: Materials. - : MDPI. - 1996-1944. ; 11:10
-
Tidskriftsartikel (refereegranskat)abstract
- Deep understanding of binding of toxic Lead (Pb) species on the surface of two-dimensional materials is a required prerequisite for the development of next-generation sensors that can provide fast and real-time detection of critically low concentrations. Here we report atomistic insights into the Lead behavior on epitaxial graphene (Gr) on silicon carbide substrates by thorough complementary study of voltammetry, electrical characterization, Raman spectroscopy, and Density Functional Theory (DFT). It is verified that the epitaxial graphene exhibits quasi-reversible anode reactions in aqueous solutions, providing a well-defined redox peak for Pb species and good linearity over a concentration range from 1 nM to 1 mu M. The conductometric approach offers another way to investigate Lead adsorption, which is based on the formations of stable charge-transfer complexes affecting the p-type conductivity of epitaxial graphene. Our results suggest the adsorption ability of the epitaxial graphene towards divalent Lead ions is concentration-dependent and tends to saturate at higher concentrations. To elucidate the mechanisms responsible for Pb adsorption, we performed DFT calculations and estimated the solvent-mediated interaction between Lead species in different oxidative forms and graphene. Our results provide central information regarding the energetics and structure of Pb-graphene interacting complexes that underlay the adsorption mechanisms of neutral and divalent Lead species. Such a holistic understanding favors design and synthesis of new sensitive materials for water quality monitoring.
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| 27. |
- Shtepliuk, Ivan, et al.
(författare)
-
Understanding of the Electrochemical Behavior of Lithium at Bilayer-Patched Epitaxial Graphene/4H-SiC
- 2022
-
Ingår i: Nanomaterials. - : MDPI AG. - 2079-4991. ; 12:13
-
Tidskriftsartikel (refereegranskat)abstract
- Novel two-dimensional materials (2DMs) with balanced electrical conductivity and lithium (Li) storage capacity are desirable for next-generation rechargeable batteries as they may serve as high-performance anodes, improving output battery characteristics. Gaining an advanced understanding of the electrochemical behavior of lithium at the electrode surface and the changes in interior structure of 2DM-based electrodes caused by lithiation is a key component in the long-term process of the implementation of new electrodes into to a realistic device. Here, we showcase the advantages of bilayer-patched epitaxial graphene on 4H-SiC (0001) as a possible anode material in lithium-ion batteries. The presence of bilayer graphene patches is beneficial for the overall lithiation process because it results in enhanced quantum capacitance of the electrode and provides extra intercalation paths. By performing cyclic voltammetry and chronoamperometry measurements, we shed light on the redox behavior of lithium at the bilayer-patched epitaxial graphene electrode and find that the early-stage growth of lithium is governed by the instantaneous nucleation mechanism. The results also demonstrate the fast lithium-ion transport (~4.7–5.6 × 10−7 cm2·s−1) to the bilayer-patched epitaxial graphene electrode. Raman measurements complemented by in-depth statistical analysis and density functional theory calculations enable us to comprehend the lithiation effect on the properties of bilayer-patched epitaxial graphene and ascribe the lithium intercalation-induced Raman G peak splitting to the disparity between graphene layers. The current results are helpful for further advancement of the design of graphene-based electrodes with targeted performance.
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| 28. |
- Vagin, Mikhail, et al.
(författare)
-
Bidirectional Hydrogen Electrocatalysis on Epitaxial Graphene
- 2022
-
Ingår i: ACS Omega. - : American Chemical Society (ACS). - 2470-1343. ; 7:15, s. 13221-13227
-
Tidskriftsartikel (refereegranskat)abstract
- The climate change due to human activities stimulates the research on new energy resources. Hydrogen has attracted interest as a green carrier of high energy density. The sustainable production of hydrogen is achievable only by water electrolysis based on the hydrogen evolution reaction (HER). Graphitic materials are widely utilized in this technology in the role of conductive catalyst supports. Herein, by performing dynamic and steady-state electrochemical measurements in acidic and alkaline media, we investigated the bidirectional electrocatalysis of the HER and hydrogen oxidation reaction (HOR) on metal- and defect-free epigraphene (EG) grown on 4H silicon carbide (4HSiC) as a ground level of structural organization of general graphitic materials. The absence of any signal degradation illustrates the high stability of EG. The experimental and theoretical investigations yield the coherent conclusion on the dominant HER pathway following the Volmer-Tafel mechanism. We ascribe the observed reactivity of EG to its interaction with the underlying SiC substrate that induces strain and electronic doping. The computed high activation energy for breaking the O-H bond is linked to the high negative overpotential of the HER. The estimated exchange current of HER/HOR on EG can be used in the evaluation of complex electrocatalytic systems based on graphite as a conducing support.
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| 29. |
- Wang, Xiangjun, et al.
(författare)
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Growth and characterization of Na0.5K0.5NbO3 thin films on polycrystalline Pt80Ir20 substrates
- 2002
-
Ingår i: Journal of Materials Research. - 0884-2914 .- 2044-5326. ; 17:5, s. 1183-1191
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Tidskriftsartikel (refereegranskat)abstract
- Na0.5K0.5NbO3 thin films have been deposited onto textured polycrystalline Pt80Ir20 substrates using radio frequency magnetron sputtering. Films were grown in off- and on-axis positions relative to the target at growth temperatures of 500-700 degreesC and sputtering pressures of 1-7 Pa. The deposited films were found to be textured, displaying a mixture of two orientations (001) and (101). Films grown on-axis showed a prefered (001) orientation, while the off-axis films had a (101) orientation. Scanning electron microscopy showed that the morphology of the films was dependent on the substrate position and sputtering pressure. The low-frequency (10 kHz) dielectric constants of the films were found to be in the range of approximately 490-590. Hydrostatic piezoelectric measurements showed that the films were piezoelectric in the as-deposited form with a constant up to 14.5 pC/N.
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| 30. |
- Yazdi, Gholamreza, et al.
(författare)
-
Effect of epitaxial graphene morphology on adsorption of ambient species
- 2019
-
Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 486, s. 239-248
-
Tidskriftsartikel (refereegranskat)abstract
- This work illustrates the impact of atmospheric gases on the surface of epitaxial graphene. The different rate of adsorption on different parts of graphene samples provides a concrete evidence that the surface morphology of graphene plays a significant role in this process. The uneven adsorption occurs only on the surface of the monolayer graphene and not on bilayer graphene. The second monolayer is distinguished and verified by the phase contrast mode of atomic force microscopy and the low energy electron microscopy, respectively. Raman spectroscopy is used to study the strain on the surface of graphene; results indicate that monolayer and bilayer graphene exhibit different types of strain. The bilayer is under more compressive strain in comparison with monolayer graphene that hinders the process of adsorption. However, the wrinkles and edges of steps of the bilayer are under tensile strain, hence, facilitate adsorption. Samples were subjected to X-ray photoelectron spectroscopy which confirms that the adsorbates on the epitaxial graphene are carbon clusters with nitrogen and oxygen contamination. For reversing the adsorption process the samples are annealed and a method for the removal of these adsorbates is proposed.
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| 31. |
- Armakavicius, Nerijus, et al.
(författare)
-
Resolving mobility anisotropy in quasi-free-standing epitaxial graphene by terahertz optical Hall effect
- 2021
-
Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 172, s. 248-259
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, we demonstrate the application of terahertz-optical Hall effect (THz-OHE) to determine directionally dependent free charge carrier properties of ambient-doped monolayer and quasi-free-standing-bilayer epitaxial graphene on 4H–SiC(0001). Directionally independent free hole mobility parameters are found for the monolayer graphene. In contrast, anisotropic hole mobility parameters with a lower mobility in direction perpendicular to the SiC surface steps and higher along the steps in quasi-free-standing-bilayer graphene are determined for the first time. A combination of THz-OHE, nanoscale microscopy and optical spectroscopy techniques are used to investigate the origin of the anisotropy. Different defect densities and different number of graphene layers on the step edges and terraces are ruled out as possible causes. Scattering mechanisms related to doping variations at the step edges and terraces as a result of different interaction with the substrate and environment are discussed and also excluded. It is suggested that the step edges introduce intrinsic scattering in quasi-free-standing-bilayer graphene, that is manifested as a result of the higher ratio between mean free path and average terrace width parameters. The suggested scenario allows to reconcile existing differences in the literature regarding the anisotropic electrical transport in epitaxial graphene.
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| 32. |
- Asada, Satoshi, et al.
(författare)
-
Calibration on wide-ranging aluminum doping concentrations by photoluminescence in high-quality uncompensated p-type 4H-SiC
- 2017
-
Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 111:7
-
Tidskriftsartikel (refereegranskat)abstract
- Previous work has shown that the concentration of shallow dopants in a semiconductor can be estimated from the photoluminescence (PL) spectrum by comparing the intensity of the bound-to-the- dopant exciton emission to that of the free exciton. In this work, we study the low-temperature PL of high-quality uncompensated Al-doped p-type 4H-SiC and propose algorithms for determining the Al-doping concentration using the ratio of the Al-bound to free-exciton emission. We use three different cryogenic temperatures (2, 41, and 79 K) in order to cover the Al-doping range from mid 10(14) cm(-3) up to 10(18) cm(-3). The Al-bound exciton no-phonon lines and the strongest free-exciton replica are used as a measure of the bound-and free-exciton emissions at a given temperature, and clear linear relationships are obtained between their ratio and the Al-concentration at 2, 41, and 79 K. Since nitrogen is a common unintentional donor dopant in SiC, we also discuss the criteria allowing one to determine from the PL spectra whether a sample can be considered as uncompensated or not. Thus, the low-temperature PL provides a convenient non-destructive tool for the evaluation of the Al concentration in 4H-SiC, which probes the concentration locally and, therefore, can also be used for mapping the doping homogeneity. Published by AIP Publishing.
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| 33. |
- Astner, Thomas, et al.
(författare)
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Vanadium in silicon carbide: telecom-ready spin centres with long relaxation lifetimes and hyperfine-resolved optical transitions
- 2024
-
Ingår i: QUANTUM SCIENCE AND TECHNOLOGY. - : IOP Publishing Ltd. - 2058-9565. ; 9:3
-
Tidskriftsartikel (refereegranskat)abstract
- Vanadium in silicon carbide (SiC) is emerging as an important candidate system for quantum technology due to its optical transitions in the telecom wavelength range. However, several key characteristics of this defect family including their spin relaxation lifetime (T1), charge state dynamics, and level structure are not fully understood. In this work, we determine the T1 of an ensemble of vanadium defects, demonstrating that it can be greatly enhanced at low temperature. We observe a large spin contrast exceeding 90% and long spin-relaxation times of up to 25 s at 100 mK, and of order 1 s at 1.3 K. These measurements are complemented by a characterization of the ensemble charge state dynamics. The stable electron spin furthermore enables high-resolution characterization of the systems' hyperfine level structure via two-photon magneto-spectroscopy. The acquired insights point towards high-performance spin-photon interfaces based on vanadium in SiC.
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| 34. |
- Bakoglidis, Konstantinos, et al.
(författare)
-
Rolling performance of carbon nitride-coated bearing components in different lubrication regimes
- 2017
-
Ingår i: Tribology International. - : Pergamon Press. - 0301-679X .- 1879-2464. ; 114, s. 141-151
-
Tidskriftsartikel (refereegranskat)abstract
- The performance of carbon nitride (CN) coated roller bearings is investigated, using a micropitting rig. The rolling performance is evaluated using Stribeck test, with a continuously varying rolling speed (0.2 - 2 m/s). Rolling contact fatigue tests with constant speeds (0.5, 1, 2, and 3.5 m/s) are also conducted in order to study the high-cycle performance of the rollers. The obtained Stribeck curve shows that the presence of coatings eliminates run-in, resulting in low friction coefficients (similar to 0.08). Raman spectroscopy, performed at the wear tracks, reveals that CNx maintain stable chemical state. Coatings show abrasion although the wear rate is not detrimental for the performance of the rollers, since a CNx to-steel contact is retained during the entire rolling contact fatigue test.
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| 35. |
- Bosma, Tom, et al.
(författare)
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Identification and tunable optical coherent control of transition-metal spins in silicon carbide
- 2018
-
Ingår i: NPJ QUANTUM INFORMATION. - : SPRINGERNATURE. - 2056-6387. ; 4
-
Tidskriftsartikel (refereegranskat)abstract
- Color centers in wide-bandgap semiconductors are attractive systems for quantum technologies since they can combine long-coherent electronic spin and bright optical properties. Several suitable centers have been identified, most famously the nitrogen-vacancy defect in diamond. However, integration in communication technology is hindered by the fact that their optical transitions lie outside telecom wavelength bands. Several transition-metal impurities in silicon carbide do emit at and near telecom wavelengths, but knowledge about their spin and optical properties is incomplete. We present all-optical identification and coherent control of molybdenum-impurity spins in silicon carbide with transitions at near-infrared wavelengths. Our results identify spin S= 1/2 for both the electronic ground and excited state, with highly anisotropic spin properties that we apply for implementing optical control of ground-state spin coherence. Our results show optical lifetimes of similar to 60 ns and inhomogeneous spin dephasing times of similar to 0.3 mu S, establishing relevance for quantum spin-photon interfacing.
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| 36. |
- Bouhafs, Chamseddine, et al.
(författare)
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Multi-scale investigation of interface properties, stacking order and decoupling of few layer graphene on C-face 4H-SiC
- 2017
-
Ingår i: Carbon. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0008-6223 .- 1873-3891. ; 116, s. 722-732
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, we report a multi-scale investigation using several nano-, micro and macro-scale techniques of few layer graphene (FLG) sample consisting of large monolayer (ML) and bilayer (BL) areas grown on C-face 4H-SiC (000-1) by high-temperature sublimation. Single 1 x 1 diffraction patterns are observed by micro-low-energy electron diffraction for ML, BL and trilayer graphene with no indication of out-of-plane rotational disorder. A SiOx layer is identified between graphene and SiC by X-ray photoelectron emission spectroscopy and reflectance measurements. The chemical composition of the interface layer changes towards SiO2 and its thickness increases with aging in normal ambient conditions. The formation mechanism of the interface layer is discussed. It is shown by torsion resonance conductive atomic force microscopy that the interface layer causes the formation of non-ideal Schottky contact between ML graphene and SiC. This is attributed to the presence of a large density of interface states. Mid-infrared optical Hall effect measurements revealed Landau-level transitions in FLG that have a square-root dependence on magnetic field, which evidences a stack of decoupled graphene sheets. Contrary to previous works on decoupled C-face graphene, our BL and FLG are composed of ordered decoupled graphene layers without out-of-plane rotation. (C) 2017 Elsevier Ltd. All rights reserved.
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| 37. |
- Bouhafs, Chamseddine, 1984-
(författare)
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Structural and Electronic Properties of Graphene on 4H- and 3C-SiC
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Graphene is a one-atom-tick carbon layer arranged in a honeycomb lattice. Graphene was first experimentally demonstrated by Andre Geim and Konstantin Novoselov in 2004 using mechanical exfoliation of highly oriented pyrolytic graphite (exfoliated graphene flakes), for which they received the Nobel Prize in Physics in 2010. Exfoliated graphene flakes show outstanding electronic properties, e.g., very high free charge carrier mobility parameters and ballistic transport at room temperature. This makes graphene a suitable material for next generation radio-frequency and terahertz electronic devices. Such applications require fabrication methods of large-area graphene compatible with electronic industry. Graphene grown by sublimation on silicon carbide (SiC) offers a viable route towards production of large-area, electronic-grade material on semi-insulating substrate without the need of transfer. Despite the intense investigations in the field, uniform wafer-scale graphene with very high-quality that matches the properties of exfoliated graphene has not been achieved yet. The key point is to identify and control how the substrate affects graphene uniformity, thickness, layer stacking, structural and electronic properties. Of particular interest is to understand the effects of SiC surface polarity and polytype on graphene properties in order to achieve large-area material with tailored properties for electronic applications. The main objectives of this thesis are to address these issues by investigating the structural and electronic properties of epitaxial graphene grown on 4HSiC and 3C-SiC substrates with different surface polarities. The first part of the thesis includes a general description of the properties of graphene, bilayer graphene and graphite. Then, the properties of epitaxial graphene on SiC by sublimation are detailed. The experimental techniques used to characterize graphene are described. A summary of all papers and contribution to the field is presented at the end of Part I. Part II consists of seven papers.
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| 38. |
- Buyanova, Irina, 1960-, et al.
(författare)
-
Direct experimental evidence for unusual effects of hydrogen on the electronic and vibrational properties of GaNxP1−x alloys : a proof for a general property of dilute nitrides
- 2004
-
Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 70:24, s. 245215-245219
-
Tidskriftsartikel (refereegranskat)abstract
- Direct experimental evidence for dramatic effects of hydrogen incorporation on the electronic structure and lattice properties of GaNxP1−x alloys is presented. By employing photoluminescence excitation spectroscopy, postgrowth hydrogenation is shown to reopen the band gap of the GaNP alloys and to efficiently reduce the N-induced coupling between the conduction band states. By Raman spectroscopy, these effects are shown to be accompanied by hydrogen-induced breaking of the Ga-P bond in the alloy, evident from disappearance of the corresponding vibrational mode. According to the performed Raman and x-ray diffraction measurements, the hydrogenation is also found to cause a strong expansion of the GaNP lattice, which changes the sign of strain from tensile in the as-grown GaNP epilayers to compressive in the posthydrogenated structures, due to the formation of complexes between N and H.
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| 39. |
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| 40. |
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| 41. |
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| 42. |
- Choubina, Tatiana, 1950-, et al.
(författare)
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Slow light in GaN
- 2008
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Ingår i: 16th Int. Symp. ¿Nanostructures: Physics and Technology,2008. ; , s. 257-
-
Konferensbidrag (refereegranskat)
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| 43. |
- Cilibrizzi, Pasquale, et al.
(författare)
-
Ultra-narrow inhomogeneous spectral distribution of telecom-wavelength vanadium centres in isotopically-enriched silicon carbide
- 2023
-
Ingår i: Nature Communications. - : NATURE PORTFOLIO. - 2041-1723. ; 14:1
-
Tidskriftsartikel (refereegranskat)abstract
- Spin-active quantum emitters have emerged as a leading platform for quantum technologies. However, one of their major limitations is the large spread in optical emission frequencies, which typically extends over tens of GHz. Here, we investigate single V4+ vanadium centres in 4H-SiC, which feature telecom-wavelength emission and a coherent S = 1/2 spin state. We perform spectroscopy on single emitters and report the observation of spin-dependent optical transitions, a key requirement for spin-photon interfaces. By engineering the isotopic composition of the SiC matrix, we reduce the inhomogeneous spectral distribution of different emitters down to 100 MHz, significantly smaller than any other single quantum emitter. Additionally, we tailor the dopant concentration to stabilise the telecom-wavelength V4+ charge state, thereby extending its lifetime by at least two orders of magnitude. These results bolster the prospects for single V emitters in SiC as material nodes in scalable telecom quantum networks. Several solid-state defect platforms have been proposed for application as a spin-photon interface in quantum communication networks. Here the authors report spin-selective optical transitions and narrow inhomogeneous spectral distribution of V centers in isotopically-enriched SiC emitting in the telecom O-band.
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| 44. |
- Csore, A., et al.
(författare)
-
Fluorescence spectrum and charge state control of divacancy qubits via illumination at elevated temperatures in 4H silicon carbide
- 2022
-
Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 105:16
-
Tidskriftsartikel (refereegranskat)abstract
- Divacancy in its neutral charge state () in 4H silicon carbide (SiC) is a leading quantum bit (qubit) contender. Owing to the lattice structure of 4H SiC, four different VCVSi configurations can be formed. The ground and the optically accessible excited states of configurations exhibit a high-spin state, and the corresponding optical transition energies are around ≈1.1eV falling in the near-infrared wavelength region. Recently, photoluminescence (PL) quenching has been experimentally observed for all configurations in 4H SiC at cryogenic temperatures. It has been shown that is converted to and it remains in this shelving dark state at cryogenic temperatures until photoexcitation with the threshold energies or above is applied to convert back to . In this study, we demonstrate both in experiments and theory that the threshold energy for reionization is temperature dependent. We carry out density functional theory (DFT) calculations in order to investigate the temperature dependent reionization spectrum, i.e., the spectrum of the process. We find that simultaneous optical reionization and qubit manipulation can be carried out at room temperature with photoexcitation at the typical excitation wavelength used for readout of the divacancy qubits in 4H SiC, in agreement with our experimental data. We also provide the analysis of the PL spectrum of , characteristic for each configuration in 4H SiC, using the Huang-Rhys theory, and find that one configuration in 4H SiC stands out in terms of the strength of coherent emission among the four configurations.
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| 45. |
- Davidsson, Joel, 1989-, et al.
(författare)
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Exhaustive characterization of modified Si vacancies in 4H-SiC
- 2022
-
Ingår i: Nanophotonics. - : Walter de Gruyter. - 2192-8606 .- 2192-8614. ; 11:20, s. 4565-4580
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Tidskriftsartikel (refereegranskat)abstract
- The negatively charged silicon vacancy (V-Si(-)) in silicon carbide is a well-studied point defect for quantum applications. At the same time, a closer inspection of ensemble photoluminescence and electron paramagnetic resonance measurements reveals an abundance of related but so far unidentified signals. In this study, we search for defects in 4H-SiC that explain the above magneto-optical signals in a defect database generated by automatic defect analysis and qualification (ADAQ) workflows. This search reveals only one class of atomic structures that exhibit silicon-vacancy-like properties in the data: a carbon anti-site (C-Si) within sub-nanometer distances from the silicon vacancy only slightly alters the latter without affecting the charge or spin state. Such a perturbation is energetically bound. We consider the formation of V-Si(-) + C-Si; up to 2 nm distance and report their zero phonon lines and zero field splitting values. In addition, we perform high-resolution photoluminescence experiments in the silicon vacancy region and find an abundance of lines. Comparing our computational and experimental results, several configurations show great agreement. Our work demonstrates the effectiveness of a database with high-throughput results in the search for defects in quantum applications.
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| 46. |
- Deminskyi, Petro, 1987-, et al.
(författare)
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Atomic layer deposition of InN using trimethylindium and ammonia plasma
- 2019
-
Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Institute of Physics (AIP). - 0734-2101 .- 1520-8559. ; 37:2
-
Tidskriftsartikel (refereegranskat)abstract
- Indium nitride (InN) is a low bandgap, high electron mobility semiconductor material of interest to optoelectronics and telecommunication. Such applications require the deposition of uniform crystalline InN thin films on large area substrates, with deposition temperatures compatible with this temperature-sensitive material. As conventional chemical vapor deposition (CVD) struggles with the low temperature tolerated by the InN crystal, the authors hypothesize that a time-resolved, surface-controlled CVD route could offer a way forward for InN thin film deposition. In this work, the authors report atomic layer deposition of crystalline, wurtzite InN thin films using trimethylindium and ammonia plasma on Si(100). They found a narrow atomic layer deposition window of 240-260 degrees C with a deposition rate of 0.36 A/cycle and that the flow of ammonia into the plasma is an important parameter for the crystalline quality of the film. X-ray diffraction measurements further confirmed the polycrystalline nature of InN thin films. X-ray photoelectron spectroscopy measurements show nearly stoichiometric InN with low carbon level (amp;lt;1 at. %) and oxygen level (amp;lt;5 at. %) in the film bulk. The low carbon level is attributed to a favorable surface chemistry enabled by the NH3 plasma. The film bulk oxygen content is attributed to oxidation upon exposure to air via grain boundary diffusion and possibly by formation of oxygen containing species in the plasma discharge. Published by the AVS.
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| 47. |
- Donchev, V., et al.
(författare)
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Optical properties of thick GaInAs(Sb)N layers grown by liquid-phase epitaxy
- 2017
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Ingår i: 19TH INTERNATIONAL SCHOOL ON CONDENSED MATTER PHYSICS (ISCMP): ADVANCES IN NANOSTRUCTURED CONDENSED MATTER: RESEARCH AND INNOVATIONS. - : IOP PUBLISHING LTD.
-
Konferensbidrag (refereegranskat)abstract
- We present an experimental and theoretical study of GaInAs(Sb)N layers with thickness around 2 mu m, grown by liquid-phase epitaxy (LPE) on n-type GaAs substrates. The samples are studied by surface photovoltage (SPV) spectroscopy and by photoluminescence spectroscopy. A theoretical model for the band structure of Sb-containing dilute nitrides is developed within the semi-empirical tight-binding approach in the sp(3)d(5)s*s(N) parameterisation and is used to calculate the electronic structure for different alloy compositions. The SPV spectra measured at room temperature clearly show a red shift of the absorption edge with respect to the absorption of the GaAs substrate. The shifts are in agreement with theoretical calculations results obtained for In, Sb and N concentrations corresponding to the experimentally determined ones. Photoluminescence measurements performed at 300K and 2 K show a smaller red shift of the emission energy with respect to GaAs as compared to the SPV results. The differences are explained by a tail of slow defect states below the conduction band edge, which are probed by SPV, but are less active in the PL experiment.
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| 48. |
- Donchev, V., et al.
(författare)
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Photoluminescence study of AlAs/GaAs superlattices containing enlarged wells
- 2000
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Ingår i: Thin Solid Films. - 0040-6090 .- 1879-2731. ; 364:1, s. 224-227
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Tidskriftsartikel (refereegranskat)abstract
- Photoluminescence (PL) spectra of MBE grown short-period AlAs/GaAs superlattices with one or two enlarged wells (5 and 12 nm) have been measured at 2 K. Sharp PL peaks corresponding to excitonic transitions between the lowest electron and heavy-hole states in the enlarged wells are observed. The excitonic transition energies are calculated by means of an envelope function based model, taking into account the exciton binding energies. The model incorporates a smooth potential at the interfaces, which is represented by a diffusion potential, the diffusion length being a parameter. The calculated and experimentally observed excitonic transition energies agree well if diffusion lengths of 3.5 and 4.5 monolayers are considered in the samples with and without a buffer layer, respectively. These values are consistent with the complicated nature of the growth kinetics and mechanisms of quantum heterostructures. The PL spectra reveal also complicated structures connected with the superlattice. Their qualitative discussion confirms the smooth potential model. Thus, an attempt is made to extend the analysis of complicated AlAs/GaAs heterostructures towards real interfaces, which is essential for advanced device fabrication.
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| 49. |
- Egilsson, T., et al.
(författare)
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Excitation properties of hydrogen-related photoluminescence in 6H-SiC
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 62:11, s. 7162-7168
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the excitation properties of a well-known hydrogen-related bound exciton (H-BE) photoluminescence (PL) in 6H-SiC. In the case of the so-called primary H-BE's, photoluminescence excitation (PLE) spectroscopy reveals several excited states that have not been reported previously. In order to explain these states we propose a pseudodonor model. The primary H-BE's are thus regarded as donors where strongly localized holes serve as the positive cores. From a comparison between the PLE spectra of the three different primary H-BE's corresponding to the three inequivalent substitutional lattice sites in 6H-SiC, we attempt to distinguish between the hexagonal and cubic lattice sites. We have also investigated the dependence of the optically induced quenching of the H-BE PL on the energy of the exciting light. We observe that the quenching of the H-BE PL is only efficient when the exciting light has energy above the threshold for phonon-assisted free-exciton (FE) formation or when its energy coincides with the energy needed for resonant absorption into the H-BE states. When creating FE's, we observe different types of behavior depending on the initial conditions. We argue that our results are best explained by the existence of two configurations of the same charge state of the H defect, namely a stable one: A (giving rise to the H-BE PL), and a metastable one: B (not revealed in the PL spectrum). The recombination of excitons bound at these two configurations can give rise to the transformations A?B and B?A. The existence of the B configuration is revealed through the effect of the B?A process on the temporal changes of the H-BE PL.
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| 50. |
- Egilsson, T., et al.
(författare)
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Excitation spectra of nitrogen bound excitons in 4H- and 6H-SiC
- 2002
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 91:3, s. 2028-2032
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Tidskriftsartikel (refereegranskat)abstract
- We report photoluminescence excitation spectra of the nitrogen (N) donor bound excitons (BE) in 4H- and 6H-SiC. The spectra reveal several excited states of the N-BEs. An attempt is made in the article to classify the N-BE states according to a simple shell model. © 2002 American Institute of Physics.
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