| 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. |
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| 3. |
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| 4. |
- Rodner, Marius, 1991-, et al.
(författare)
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Iron oxide nanoparticle decorated graphene for ultra-sensitive detection of volatile organic compounds
- 2018
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Ingår i: Proceedings. - Basel Switzerland : MDPI. - 2504-3900. ; 2:13
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Tidskriftsartikel (refereegranskat)abstract
- It has been found that two-dimensional materials, such as graphene, can be used as remarkable gas detection platforms as even minimal chemical interactions can lead to distinct changes in electrical conductivity. In this work, epitaxially grown graphene was decorated with iron oxide nanoparticles for sensor performance tuning. This hybrid surface was used as a sensing layer to detect formaldehyde and benzene at concentrations of relevance in air quality monitoring (low parts per billion). Moreover, the time constants could be drastically reduced using a derivative sensor signal readout, allowing detection at the sampling rates desired for air quality monitoring applications.
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| 5. |
- 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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| 6. |
- Buyanova, Irina, 1960-, et al.
(författare)
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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
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 70:24, s. 245215-245219
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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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| 7. |
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| 8. |
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| 9. |
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| 10. |
- 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-
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Konferensbidrag (refereegranskat)
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| 11. |
- Davidsson, Joel, 1989-, et al.
(författare)
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Exhaustive characterization of modified Si vacancies in 4H-SiC
- 2022
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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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| 12. |
- Deminskyi, Petro, 1987-, et al.
(författare)
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Atomic layer deposition of InN using trimethylindium and ammonia plasma
- 2019
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Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Institute of Physics (AIP). - 0734-2101 .- 1520-8559. ; 37:2
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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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| 13. |
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| 14. |
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| 15. |
- Gilardoni, Carmem M., et al.
(författare)
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Spin-relaxation times exceeding seconds for color centers with strong spin-orbit coupling in SiC
- 2020
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Ingår i: New Journal of Physics. - : IOP PUBLISHING LTD. - 1367-2630. ; 22:10
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Tidskriftsartikel (refereegranskat)abstract
- Spin-active color centers in solids show good performance for quantum technologies. Several transition-metal defects in SiC offer compatibility with telecom and semiconductor industries. However, whether their strong spin-orbit coupling degrades their spin lifetimes is not clear. We show that a combination of a crystal-field with axial symmetry and spin-orbit coupling leads to a suppression of spin-lattice and spin-spin interactions, resulting in remarkably slow spin relaxation. Our optical measurements on an ensemble of Mo impurities in SiC show a spin lifetime T-1 of 2.4 s at 2 K.
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| 16. |
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| 17. |
- Gogova, Daniela, 1967-, et al.
(författare)
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High-quality 2? bulk-like free-standing GaN grown by HydrideVapour phase epitaxy on a Si-doped metal organic vapour phase epitaxial GaN template with an ultra low dislocation density
- 2005
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Ingår i: Japanese Journal of Applied Physics. - 0021-4922 .- 1347-4065. ; 44:3, s. 1181-1185
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Tidskriftsartikel (refereegranskat)abstract
- High-quality 2? crack-free free-standing GaN has been attained by hydride vapour phase epitaxial growth on a Si-doped MOVPE GaN template with a low dislocation density and subsequent laser-induced lift-off process. A low value of dislocation density of ~2.0 × 107cm-2 on the Ga-polar face was determined from cathodoluminescence images. X-ray diffraction (XRD) and low-temperature photoluminescence (PL) were exploited to investigate the structural and optical properties of the GaN material. The full width at half maximum value of XRD ?-scan of the free-standing GaN is 248 arcsec for the (1 0 1 4) reflection. The XRD and low-temperature PL mapping measurements consistently proved the high crystalline quality as well as the lateral homogeneity and the small residual stress of the material. Hence, the bulk-like free-standing GaN studied here is highly advantageous for being used as a lattice-constant and thermal-expansion-coefficient matched substrate for additional strain-free homoepitaxy of III-nitrides-based device heterostructures. The strain-free homoepitaxy will significantly reduce the defect density and thus, an improvement of the device performance and lifetime could be achieved. © 2005 The Japan Society of Applied Physics.
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| 18. |
- Hänninen, Tuomas, 1988-, et al.
(författare)
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Silicon carbonitride thin films deposited by reactive high power impulse magnetron sputtering
- 2018
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Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 335, s. 248-256
-
Tidskriftsartikel (refereegranskat)abstract
- Amorphous silicon carbonitride thin films for biomedical applications were deposited in an industrial coating unit from a silicon target in different argon/nitrogen/acetylene mixtures by reactive high power impulse magnetron sputtering (rHiPIMS). The effects of acetylene (C2H2) flow rate, substrate temperature, substrate bias voltage, and HiPIMS pulse frequency on the film properties were investigated. Low C2H2 flow rates (<10 sccm) resulted in silicon nitride-like film properties, seen from a dense morphology when viewed in cross-sectional scanning electron microscopy, a hardness up to ∼22 GPa as measured by nanoindentation, and Si-N bonds dominating over Si-C bonds in X-ray photoelectron spectroscopy core-level spectra. Higher C2H2 flows resulted in increasingly amorphous carbon-like film properties, with a granular appearance of the film morphology, mass densities below 2 g/cm3 as measured by X-ray reflectivity, and a hardness down to 4.5 GPa. Increasing substrate temperatures and bias voltages resulted in slightly higher film hardnesses and higher compressive residual stresses. The film H/E ratio showed a maximum at film carbon contents ranging between 15 and 30 at.% and at elevated substrate temperatures from 340 °C to 520 °C.
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| 19. |
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| 20. |
- Ivanov, Ivan Gueorguiev, 1955-, et al.
(författare)
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Effective-mass approximation for shallow donors in uniaxial indirect band-gap crystals and application to 4H-SiC
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 73:4
-
Tidskriftsartikel (refereegranskat)abstract
- The effective-mass theory is applied for description of the electronic states of shallow donors in indirect band-gap uniaxial crystals, which have three different components of the electron effective-mass tensor, and two different components of the tensor of the dielectric constant. The Hamiltonian in the resulting Schrödinger equation for the envelope function has D2h symmetry and, after proper parametrization, a nonvariational numerical method is used for its solution. Two particular cases of D∞h symmetry are identified and discussed separately. The comparison between theory and experiment for the 4H polytype of silicon carbide is revised using the least-squares method to determine the binding energies of the ground state of the most shallow nitrogen donor in this material, its valley-orbit split-off counterpart, and the mean value of the dielectric constant, and completed with calculation of the theoretical transition probabilities. In addition, the lowest-lying binding energies of the states, between which optical transitions are allowed, are calculated on a grid of values of the two parameters describing the anisotropy and the tabulated values can be used for interpolation to describe other materials. © 2006 The American Physical Society.
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| 21. |
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| 22. |
- Ivanov, Ivan Gueorguiev, 1955-, et al.
(författare)
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Temperature Dependence and Selective Excitation of the Phosphorus Related Photoluminescence in 4H-SiC
- 2009
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Ingår i: Materials Science Forum, Vols. 615-617. - : Trans Tech Publications. - 9780878493340 ; , s. 263-266
-
Konferensbidrag (refereegranskat)abstract
- The paper presents experimental data on the temperature dependence and the excitation properties of the phosphorus-related photoluminescence in 4H SiC. Two main sets of phonon replicas can be observed with selective excitation, which are attributed to two of the no-phonon lines observed in the spectrum. Some of the excited states are also attributed to one of the no-phonon lines on the ground of the selectively excited spectra. A tentative explanation of the observed features in terms of multiple bound excitons is proposed.
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| 23. |
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| 24. |
- Ivanov, Ivan Gueorguiev, 1955-, et al.
(författare)
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Wave-function symmetry and the properties of shallow P donors in 4H SiC
- 2009
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Ingår i: Silicon Carbide and Related Materials 2007, Pts 1 and 2. - : Trans Tech Publications Inc.. ; , s. 445-448, s. 445-448
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Konferensbidrag (refereegranskat)abstract
- A new investigation on the optical properties of the phosphorus-bound excitons is presented. Arguments are given in favor of the possibility of degenerate donor state for phosphorus substituting Si atom on hexagonal site. On the base of a simple model, it is shown that the experimental spectra also provide evidence in favor of this possibility. The possibility for violation of the Haynes rule in the case of phosphorus donors on the two inequivalent sites is indicated.
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| 25. |
- Janzén, Erik, 1954-, et al.
(författare)
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Defects in SiC
- 2004
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 26. |
- Janzén, Erik, 1954-, et al.
(författare)
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Defects in SiC
- 2008
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Ingår i: Defects in Microelectronic Materials and Devices. - : Taylor and Francis LLC. ; , s. 770-
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Uncover the Defects that Compromise Performance and Reliability As microelectronics features and devices become smaller and more complex, it is critical that engineers and technologists completely understand how components can be damaged during the increasingly complicated fabrication processes required to produce them.A comprehensive survey of defects that occur in silicon-based metal-oxide semiconductor field-effect transistor (MOSFET) technologies, this book also discusses flaws in linear bipolar technologies, silicon carbide-based devices, and gallium arsenide materials and devices. These defects can profoundly affect the yield, performance, long-term reliability, and radiation response of microelectronic devices and integrated circuits (ICs). Organizing the material to build understanding of the problems and provide a quick reference for scientists, engineers and technologists, this text reviews yield- and performance-limiting defects and impurities in the device silicon layer, in the gate insulator, and/or at the critical Si/SiO2 interface. It then examines defects that impact production yield and long-term reliability, including:Vacancies, interstitials, and impurities (especially hydrogen)Negative bias temperature instabilitiesDefects in ultrathin oxides (SiO2 and silicon oxynitride)Take A Proactive Approach The authors condense decades of experience and perspectives of noted experimentalists and theorists to characterize defect properties and their impact on microelectronic devices. They identify the defects, offering solutions to avoid them and methods to detect them. These include the use of 3-D imaging, as well as electrical, analytical, computational, spectroscopic, and state-of-the-art microscopic methods. This book is a valuable look at challenges to come from emerging materials, such as high-K gate dielectrics and high-mobility substrates being developed to replace Si02 as the preferred gate dielectric material, and high-mobility substrates
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| 27. |
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| 28. |
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| 29. |
- Kakanakova-Georgieva, Anelia, 1970-, et al.
(författare)
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Uniform hot-wall MOCVD epitaxial growth of 2 inch AlGaN/GaN HEMT structures
- 2007
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Ingår i: Journal of Crystal Growth, Vol. 300. - : Elsevier BV. - 0022-0248. ; , s. 100-103
-
Konferensbidrag (refereegranskat)abstract
- The hot-wall metalorganic chemical vapor deposition (MOCVD) concept has been applied to the growth of AlxGa1-xN/GaN high electron mobility transistor (HEMT) device heterostructures on 2 inch 4H-SiC wafers. Due to the small vertical and horizontal temperature gradients inherent to the hot-wall MOCVD concept the variations of all properties of a typical HEMT heterostructure are very small over the wafer: GaN buffer layer thickness of 1.83 μm±1%, Al content of the AlxGa1-xN barrier of 27.7±0.1%, AlxGa1-xN barrier thickness of 25 nm±4%, sheet carrier density of 1.05×1013 cm-2±4%, pinch-off voltage of -5.3 V±3%, and sheet resistance of 449 Ω±1%.
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| 30. |
- Kamiyama, S., et al.
(författare)
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Extremely high quantum efficiency of donor-acceptor-pair emission in N-and-B-doped 6H-SiC
- 2006
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 99:9
-
Tidskriftsartikel (refereegranskat)abstract
- High-efficiency visible light emission in N-and-B-doped 6H-SiC epilayers was observed in photoluminescence measurements at room temperature. The orange-yellow light emission due to the recombination of donor-acceptor pairs (DAPs) has a broad spectrum with a peak wavelength of 576 nm and a full width at half maximum of 110 nm at 250 K. The high B concentration of more than 1018 cm-3 improves the emission efficiency of the DAP recombination at a high temperature. Compared with the photoluminescence spectrum of GaN at 10 K, a high quantum efficiency of 95% was estimated for the highly B-doped sample. From time-resolved photoluminescence measurements, a DAP recombination time of 5.0 ms was obtained, which is in good agreement with the calculated value by the rate equation with the assumption of a 95% internal quantum efficiency. This is quite promising as a light-emitting medium by optical pumping, as well as monolithic light sources combined with nitride-based light-emitting diodes grown on the DA-doped SiC epilayer. © 2006 American Institute of Physics.
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| 31. |
- Karhu, Robin, 1987-, et al.
(författare)
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Long Charge Carrier Lifetime in As-Grown 4H-SiC Epilayer
- 2016
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Ingår i: Materials Science Forum. - : Trans Tech Publications. - 0255-5476 .- 1662-9752. ; 858, s. 125-128
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Tidskriftsartikel (refereegranskat)abstract
- Over 150 μm thick epilayers of 4H-SiC with long carrier lifetime have been grown with a chlorinated growth process. The carrier lifetime have been determined by time resolved photoluminescence (TRPL), the lifetime varies a lot between different areas of the sample. This study investigates the origins of lifetime variations in different regions using deep level transient spectroscopy (DLTS), low temperature photoluminescence (LTPL) and a combination of KOH etching and optical microscopy. From optical microscope images it is shown that the area with the shortest carrier lifetime corresponds to an area with high density of structural defects.
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| 32. |
- Kasic, A., et al.
(författare)
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Characterization of crack-free relaxed GaN grown on 2″ sapphire
- 2005
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 98:7, s. 73525-
-
Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the growth of high-quality and virtually strain-free bulklike GaN by hydride vapor-phase epitaxy in a vertical atmospheric-pressure reactor with a bottom-fed design. The 300‐μm-thick GaN layer was grown on a 2″ (0 0 0 1) sapphire substrate buffered with a ∼ 2‐μm-thick GaN layer grown by metal-organic chemical-vapor deposition. During the cool down process to room temperature, cracking was induced in the sapphire substrate, thereby allowing the bulklike GaN layer to relax without provoking cracking of itself. The crystalline quality and the residual strain in the 2″ GaN wafer were investigated by various characterization techniques. The lateral homogeneity of the wafer was monitored by low-temperature photoluminescence mapping. High-resolution x-ray diffraction and photoluminescence measurements proved the high crystalline quality of the material grown. The position of the main near-band-gap photoluminescence line and the phonon spectra obtained from infrared spectroscopic ellipsometry show consistently that the 2″ crack-free GaN is virtually strain-free over a diameter of approximately 4 cm.
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| 33. |
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| 34. |
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| 35. |
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| 36. |
- Monemar, Bo, 1942-, et al.
(författare)
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Growth of thick GaN layers by hydride vapor phase epitaxy
- 2005
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Ingår i: Journal of Ceramic Processing Research. - 1229-9162. ; 6:2, s. 153-162
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we describe recent experimental work on the growth of thick GaN layers (up to >300 μm) on sapphire with hydride vapour phase epitaxy (HVPE), the removal of the sapphire substrate by the laser liftoff technique, and the properties of these thick GaN layers. Two different growth setups were used, one horizontal and one vertical system. Specific conditions in the growth procedure, like gas flow pattern, growth rate and the use of buffer layers, strongly influence the properties of the grown layers. Important defect problems are cracking (both during and after growth), and the generation of dislocations and surface pits. A large bowing is also observed for thick layers, depending very much on the properties of the initially grown material. For growth of thick layers excessive parasitic growth of GaN upstream of the substrate has to be avoided. Laser liftoff is demonstrated to be a feasible process to remove the sapphire substrate, causing the GaN surface bowing to decrease and revert from convex to concave. The threading dislocation density of 300 μm thick GaN layers is found to be about 107 cm-2, rather independent of the type of buffer layer employed. It reduces further in thicker layers. The pit density varies with growth conditions, it can be reduced if the parasitic growth is avoided. The bowing is a serious problem, since the layers have to be polished to make the surface epi-ready. The XRD rocking curve widths measured seem to correlate with the bowing of the layers, a reduction by about a factor two is often observed when the substrate is removed. Optical characterisation like photoluminescence (PL) and ir spectroscopic ellipsometry (IRSE) is very useful to monitor strain in the layers, as well as impurities and point defects. Residual shallow donors are related to O and Si, shallow acceptors are mainly of intrinsic origin, i.e. complexes with the Ga vacancy.
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| 37. |
- Nagy, Roland, et al.
(författare)
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Quantum Properties of Dichroic Silicon Vacancies in Silicon Carbide
- 2018
-
Ingår i: Physical Review Applied. - 2331-7019. ; 9:3
-
Tidskriftsartikel (refereegranskat)abstract
- Although various defect centers have displayed promise as either quantum sensors, single photon emitters, or light-matter interfaces, the search for an ideal defect with multifunctional ability remains open. In this spirit, we study the dichroic silicon vacancies in silicon carbide that feature two well-distinguishable zero-phonon lines and analyze the quantum properties in their optical emission and spin control. We demonstrate that this center combines 40% optical emission into the zero-phonon lines showing the contrasting difference in optical properties with varying temperature and polarization, and a 100% increase in the fluorescence intensity upon the spin resonance, and long spin coherence time of their spin-3/2 ground states up to 0.6 ms. These results single out this defect center as a promising system for spin-based quantum technologies.
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| 38. |
- Nguyen, Son Tien, 1953-, et al.
(författare)
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Common point defects in as-grown ZnO substrates studied by optical detection of magnetic resonance
- 2008
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Ingår i: Journal of Crystal Growth, Vol. 310. - : Elsevier BV. - 0022-0248. ; , s. 1006-1009
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Konferensbidrag (refereegranskat)abstract
- Defects in as-grown commercial zinc oxide (ZnO) substrates were studied by photoluminescence and optical detection of magnetic resonance (ODMR). In addition to the Zn vacancy and shallow donor centers, we observed several ODMR centers with spin S=1/2, labeled LU1-LU4. Among these, the axial LU3 and non-axial LU4 centers were detected in all studied samples. The ODMR signals of LU3/LU4 were found to be drastically increased after electron irradiation. The preliminary result indicates that these common ODMR centers in as-grown ZnO are related to intrinsic defects. © 2007 Elsevier B.V. All rights reserved.
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| 39. |
- Nguyen, Son Tien, et al.
(författare)
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Ligand hyperfine interactions at silicon vacancies in 4H-SiC
- 2019
-
Ingår i: Journal of Physics. - : IOP PUBLISHING LTD. - 0953-8984 .- 1361-648X. ; 31:19
-
Tidskriftsartikel (refereegranskat)abstract
- The negative silicon vacancy (V-Si(-)) in SiC has recently emerged as a promising defect for quantum communication and room-temperature quantum sensing. However, its electronic structure is still not well characterized. While the isolated Si vacancy is expected to give rise to only two paramagnetic centers corresponding to two inequivalent lattice sites in 4H-SiC, there have been five electron paramagnetic resonance (EPR) centers assigned to V-Si(-) in the past: the so-called isolated no-zero-field splitting (ZFS) V-Si(-) center and another four axial configurations with small ZFS: T-V1a, T-V2a, T-V1b, and T-V2b. Due to overlapping with Si-29 hyperfine (hf) structures in EPR spectra of natural 4H-SiC, hf parameters of T-V1a have not been determined. Using isotopically enriched 4H-(SiC)-Si-28, we overcome the problems of signal overlapping and observe hf parameters of nearest C neighbors for all three components of the S = 3/2 T-V1a and T-V2a centers. The obtained EPR data support the conclusion that only T-V1a and T-V2a are related to V-Si(-) and the two configurations of the so-called isolated no-ZFS V-Si(-) center, V-Si(-) (I) and V-Si(-) (II), are actually the central lines corresponding to the transition I-1/2 amp;lt;-amp;gt; I + 1/2 of the T-V2a and T-V1a centers, respectively.
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| 40. |
- Nguyen, Son Tien, 1953-, et al.
(författare)
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Magnetic resonance studies of defects in electron-irradiated ZnO substrates
- 2007
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Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 401-402, s. 507-510
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Tidskriftsartikel (refereegranskat)abstract
- Optical detection of magnetic resonance (ODMR) was used to study defects in electron-irradiated ZnO substrates. In addition to the shallow donor and the Zn vacancy, several ODMR centers with an effective electron spin were detected. Among these, the axial LU3 and non-axial LU4 centers are shown to be dominating recombination centers. The annealing behavior of radiation-induced defects was studied and possible defect models are discussed.
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| 41. |
- Nguyen, Son Tien, 1953-, et al.
(författare)
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Recombination centers in as-grown and electron-irradiated ZnO substrates
- 2007
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 102:9
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Tidskriftsartikel (refereegranskat)abstract
- Optical detection of magnetic resonance (ODMR) was used to study defects in ZnO substrates irradiated with 3 MeV electrons at room temperature. The Zn vacancy and some other ODMR centers were detected. Among these, the Zn vacancy and two other centers, labeled as LU3 and LU4, were also commonly observed in different types of as-grown ZnO substrates. The LU3 and LU4 are related to intrinsic defects and act as dominating recombination centers in irradiated and as-grown ZnO. © 2007 American Institute of Physics.
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| 42. |
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| 43. |
- Shi, Yuchen, et al.
(författare)
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A comparative study of high-quality C-face and Si-face 3C-SiC(1 1 1) grown on off-oriented 4H-SiC substrates
- 2019
-
Ingår i: Journal of Physics D. - : Biopress Ltd. - 0022-3727 .- 1361-6463. ; 52:34
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Tidskriftsartikel (refereegranskat)abstract
- We present a comparative study of the C-face and Si-face of 3C-SiC(111) grown on off-oriented 4H-SiC substrates by the sublimation epitaxy. By the lateral enlargement method, we demonstrate that the high-quality bulk-like C-face 3C-SiC with thickness of ~1 mm can be grown over a large single domain without double positioning boundaries (DPBs), which are known to have a strongly negative impact on the electronic properties of the material. Moreover, the C-face sample exhibits a smoother surface with one unit cell height steps while the surface of the Si-face sample exhibits steps twice as high as on the C-face due to step-bunching. High-resolution XRD and low temperature photoluminescence measurements show that C-face 3C-SiC can reach the same high crystalline quality as the Si-face 3C-SiC. Furthermore, cross-section studies of the C- and Si-face 3C-SiC demonstrate that in both cases an initial homoepitaxial 4H-SiC layer followed by a polytype transition layer are formed prior to the formation and lateral expansion of 3C-SiC layer. However, the transition layer in the C-face sample is extending along the step-flow direction less than that on the Si-face sample, giving rise to a more fairly consistent crystalline quality 3C-SiC epilayer over the whole sample compared to the Si-face 3C-SiC where more defects appeared on the surface at the edge. This facilitates the lateral enlargement of 3C-SiC growth on hexagonal SiC substrates.
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| 44. |
- Shi, Yuchen, et al.
(författare)
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Elimination of step bunching in the growth of large-area monolayer and multilayer graphene on off-axis 3CSiC (111)
- 2018
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Ingår i: Carbon. - : Elsevier. - 0008-6223 .- 1873-3891. ; 140, s. 533-542
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Tidskriftsartikel (refereegranskat)abstract
- Multilayer graphene has exhibited distinct electronic properties such as the tunable bandgap for optoelectronic applications. Among all graphene growth techniques, thermal decomposition of SiC is regarded as a promising method for production of device-quality graphene. However, it is still very challenging to grow uniform graphene over a large-area, especially multilayer graphene. One of the main obstacles is the occurrence of step bunching on the SiC surface, which significantly influences the formation process and the uniformity of the multilayer graphene. In this work, we have systematically studied the growth of monolayer and multilayer graphene on off-axis 3CSiC(111). Taking advantage of the synergistic effect of periodic SiC step edges as graphene nucleation sites and the unique thermal decomposition energy of 3CSiC steps, we demonstrate that the step bunching can be fully eliminated during graphene growth and large-area monolayer, bilayer, and four-layer graphene can be controllably obtained on high-quality off-axis 3CSiC(111) surface. The low energy electron microscopy results demonstrate that a uniform four-layer graphene has been grown over areas of tens of square micrometers, which opens the possibility to tune the bandgap for optoelectronic devices. Furthermore, a model for graphene growth along with the step bunching elimination is proposed.
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| 45. |
- ul-Hassan, Jawad, et al.
(författare)
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Quasi-free-standing monolayer and bilayer graphene growth on homoepitaxial on-axis 4H-SiC(0001) layers
- 2015
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 82:C, s. 12-23
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Tidskriftsartikel (refereegranskat)abstract
- Quasi-free-standing monolayer and bilayer graphene is grown on homoepitaxial layers of 4H-SiC. The SiC epilayers themselves are grown on the Si-face of nominally on-axis semi-insulating substrates using a conventional SiC hot-wall chemical vapor deposition reactor. The epilayers were confirmed to consist entirely of the 4H polytype by low temperature photoluminescence. The doping of the SiC epilayers may be modified allowing for graphene to be grown on a conducing substrate. Graphene growth was performed via thermal decomposition of the surface of the SiC epilayers under Si background pressure in order to achieve control on thickness uniformity over large area. Monolayer and bilayer samples were prepared through the conversion of a carbon buffer layer and monolayer graphene respectively using hydrogen intercalation process. Micro-Raman and reflectance mappings confirmed predominantly quasi-free-standing monolayer and bilayer graphene on samples grown under optimized growth conditions. Measurements of the Hall properties of Van der Pauw structures fabricated on these layers show high charge carrier mobility (> 2000 cm(2)/Vs) and low carrier density (
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| 46. |
- Wagner, Matthias, 1969-, et al.
(författare)
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Photoluminescence upconversion in 4H-SiC
- 2002
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 81:14, s. 2547-
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Tidskriftsartikel (refereegranskat)abstract
- Efficient photoluminescence upconversion is observed in 4H-SiC samples containing both the UD-3 defect and the titanium impurity. In this process, the titanium photoluminescence emission with no-phonon (NP) lines at 2.848 eV (A0) and 2.789 eV (B0) can be excited by tuning the laser to the NP line of UD-3 at 1.356 eV. In samples containing either only UD-3 or only titanium, a different photoluminescence upconversion process can be observed, which occurs at photon energies higher than ~1.5 eV without exhibiting sharp features. At least one of the two processes generates both free electrons and free holes and can, therefore, be a candidate for an important recombination channel.
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| 47. |
- Winters, Michael, 1986, et al.
(författare)
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Assessment of H-intercalated graphene for microwave FETs through material characterization and electron transport studies
- 2015
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 81:1, s. 96-104
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial graphene is grown on semi-insulating (SI) 4H-SiC in a hot wall CVD reactor by graphitization and in-situ intercalation with (H)ydrogen. A holistic material characterization is performed in order to ascertain the number of layers, layer uniformity, and electron transport properties of the epi-layers via electronic test structures and Raman spectroscopy. Bilayer graphene field effect transistors (GFETs) are fabricated using a full electron beam lithography (EBL) process which is optimized for low contact resistances of r(c)
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| 48. |
- Yakimova, Rositsa, 1942-, et al.
(författare)
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Optical and Morphological Features of Bulk and Homoepitaxial ZnO
- 2006
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Ingår i: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 39, s. 247-256
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Tidskriftsartikel (refereegranskat)abstract
- ZnO substrate crystals from two different sources, and epitaxial layers have been studied by SEM, AFM, photoluminescence (T=2-135K) and EPR. Although fabricated by the same growth principle, i.e. the hydrothermal technique, the substrates differ in terms of purity and structural quality. In the PL spectra of all samples the dominating emission originates from the donor bound exciton (BE) recombination positioned at about 3361 meV. The temperature dependence of the spectra confirms the assignment of the free exciton emission in the purest sample, the line at 3376 meV evolves into a broad peak at higher temperatures, probably including both A and B excitons. Another FE-related emission appears as a shoulder on the high-energy side of FEA,B above 40 K. It is expected and associated with the crystal-field split-off counterpart of the valence band. Free-exciton related emission in the less pure sample can only be seen if the temperature is above 45 K. At T=135K all bound excitons are quenched and the spectrum in both samples consists of the free exciton no-phonon lines and their replicas. However, the emission from the pure samples is several orders of magnitude stronger than that from the other sample, which indicates strong non-radiative quenching of the excitons in the latter sample. The EPR measurements reveal a possible scenario of impurity re-arrangement, e.g. annealing at 950 °C may dissociate existing complexes and release Fe as isolated ions. The AFM and SEM investigations of an epilayer grown by MOCVD on one of the studied substrates have indicated growth instabilities and structural irregularities, thus pointing to the need for substrate quality and epitaxial process optimization.
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