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Darakchieva, Vanya
Alternative names
Publications (10 of 107) Show all publications
Kuhne, P., Armakavicius, N., Stanishev, V., Herzinger, C. M., Schubert, M. & Darakchieva, V. (2018). Advanced Terahertz Frequency-Domain Ellipsometry Instrumentation for In Situ and Ex Situ Applications. IEEE Transactions on Terahertz Science and Technology, 8(3), 257-270
Open this publication in new window or tab >>Advanced Terahertz Frequency-Domain Ellipsometry Instrumentation for In Situ and Ex Situ Applications
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2018 (English)In: IEEE Transactions on Terahertz Science and Technology, ISSN 2156-342X, Vol. 8, no 3, p. 257-270Article in journal (Refereed) Published
Abstract [en]

We present a terahertz (THz) frequency-domain spectroscopic ellipsometer design that suppresses formation of standing waves by use of stealth technology approaches. The strategy to suppress standing waves consists of three elements geometry, coating, and modulation. The instrument is based on the rotating analyzer ellipsometer principle and can incorporate various sample compartments, such as a superconducting magnet, in situ gas cells, or resonant sample cavities, for example. A backward wave oscillator and three detectors are employed, which permit operation in the spectral range of 0.1–1 THz (3.3–33 cm−1 or 0.4–4 meV). The THz frequency-domain ellipsometer allows for standard and generalized ellipsometry at variable angles of incidence in both reflection and transmission configurations. The methods used to suppress standing waves and strategies for an accurate frequency calibration are presented. Experimental results from dielectric constant determination in anisotropic materials, and free charge carrier determination in optical Hall effect (OHE), resonant-cavity enhanced OHE, and in situ OHE experiments are discussed. Examples include silicon and sapphire optical constants, free charge carrier properties of two-dimensional electron gas in a group III nitride high electron mobility transistor structure, and ambient effects on free electron mobility and density in epitaxial graphene.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2018
Keywords
Ellipsometry, Frequency-domain analysis, Instruments, Measurement by laser beam, Coherence, Dielectrics, Laser beams
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-147883 (URN)10.1109/TTHZ.2018.2814347 (DOI)000431446900001 ()2-s2.0-85045191738 (Scopus ID)
Note

Funding agencies: Swedish Foundation for Strategic Research (SSF) [FFL12-0181, RIF14-055]; AForsk [13-318]; Swedish Research Council (VR) [2013-5580, 2016-00889]; Swedish Governmental Agency for Innovation Systems (VINNOVA Grant) [2011-03486]; Swedish Government Strategic 

Available from: 2018-05-18 Created: 2018-05-18 Last updated: 2018-05-30Bibliographically approved
Mock, A., Korlacki, R., Briley, C., Darakchieva, V., Monemar, B., Kumagai, Y., . . . Schubert, M. (2017). Band-to-band transitions, selection rules, effective mass, and excitonic contributions in monoclinic beta-Ga2O3. Physical Review B, 96(24), Article ID 245205.
Open this publication in new window or tab >>Band-to-band transitions, selection rules, effective mass, and excitonic contributions in monoclinic beta-Ga2O3
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 24, article id 245205Article in journal (Refereed) Published
Abstract [en]

We employ an eigenpolarization model including the description of direction dependent excitonic effects for rendering critical point structures within the dielectric function tensor of monoclinic beta-Ga2O3 yielding a comprehensive analysis of generalized ellipsometry data obtained from 0.75-9 eV. The eigenpolarization model permits complete description of the dielectric response. We obtain, for single-electron and excitonic band-to-band transitions, anisotropic critical point model parameters including their polarization vectors within the monoclinic lattice. We compare our experimental analysis with results from density functional theory calculations performed using the Gaussian-attenuation-Perdew-Burke-Ernzerhof hybrid density functional. We present and discuss the order of the fundamental direct band-to-band transitions and their polarization selection rules, the electron and hole effective mass parameters for the three lowest band-to-band transitions, and their excitonic contributions. We find that the effective masses for holes are highly anisotropic and correlate with the selection rules for the fundamental band-to-band transitions. The observed transitions are polarized close to the direction of the lowest hole effective mass for the valence band participating in the transition.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-144258 (URN)10.1103/PhysRevB.96.245205 (DOI)000419008900006 ()2-s2.0-85040165150 (Scopus ID)
Note

Funding Agencies|National Science Foundation (NSF) through the Center for Nanohybrid Functional Materials [EPS-1004094]; Nebraska Materials Research Science and Engineering Center [DMR-1420645]; Swedish Research Council [VR2013-5580, VR2016-00889]; Swedish Foundation for Strategic Research (SSF) [FFL12-0181, RIF14-055]; NSF [CMMI 1337856, EAR 1521428]; J. A. Woollam Foundation

Available from: 2018-01-12 Created: 2018-01-12 Last updated: 2018-01-31Bibliographically approved
Schubert, M., Korlacki, R., Knight, S., Hofmann, T., Schoeche, S., Darakchieva, V., . . . Higashiwaki, M. (2016). Anisotropy, phonon modes, and free charge carrier parameters in monoclinic beta-gallium oxide single crystals. PHYSICAL REVIEW B, 93(12), 125209
Open this publication in new window or tab >>Anisotropy, phonon modes, and free charge carrier parameters in monoclinic beta-gallium oxide single crystals
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2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 12, p. 125209-Article in journal (Refereed) Published
Abstract [en]

We derive a dielectric function tensor model approach to render the optical response of monoclinic and triclinic symmetry materials with multiple uncoupled infrared and far-infrared active modes. We apply our model approach to monoclinic beta-Ga2O3 single-crystal samples. Surfaces cut under different angles from a bulk crystal, (010) and ((2) over bar 01), are investigated by generalized spectroscopic ellipsometry within infrared and far-infrared spectral regions. We determine the frequency dependence of 4 independent beta-Ga2O3 Cartesian dielectric function tensor elements by matching large sets of experimental data using a point-by-point data inversion approach. From matching our monoclinic model to the obtained 4 dielectric function tensor components, we determine all infrared and far-infrared active transverse optic phonon modes with A(u) and B-u symmetry, and their eigenvectors within the monoclinic lattice. We find excellent agreement between our model results and results of density functional theory calculations. We derive and discuss the frequencies of longitudinal optical phonons in beta-Ga2O3. We derive and report density and anisotropic mobility parameters of the free charge carriers within the tin-doped crystals. We discuss the occurrence of longitudinal phonon plasmon coupled modes in beta-Ga2O3 and provide their frequencies and eigenvectors. We also discuss and present monoclinic dielectric constants for static electric fields and frequencies above the reststrahlen range, and we provide a generalization of the Lyddane-Sachs-Teller relation for monoclinic lattices with infrared and far-infrared active modes. We find that the generalized Lyddane-Sachs-Teller relation is fulfilled excellently for beta-Ga2O3.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2016
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-127273 (URN)10.1103/PhysRevB.93.125209 (DOI)000372412400003 ()
Note

Funding Agencies|National Science Foundation (NSF) through the Center for Nanohybrid Functional Materials [EPS-1004094]; Nebraska Materials Research Science and Engineering Center [DMR-1420645]; Swedish Research Council (VR) [2013-5580, 2010-3848]; Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program [2011-03486, 2014-04712]; Swedish Foundation for Strategic Research (SSF) [FFL12-0181, RIF14-055]; Linkoping Linnaeus Initiative on Nanoscale Functional Materials (LiLiNFM) - VR; University of Nebraska-Lincoln; J. A. Woollam Co., Inc.; J. A. Woollam Foundation; [CMMI 1337856]; [EAR 1521428]

Available from: 2016-04-20 Created: 2016-04-19 Last updated: 2016-05-10
Junige, M., Oddoy, T., Yakimova, R., Darakchieva, V., Wenger, C., Lupina, G., . . . Bartha, J. W. (2015). Atomic Layer Deposition of Al2O3 on NF3-pre-treated graphene. In: NANOTECHNOLOGY VII: . Paper presented at Conference on Nanotechnology VII. Society of Photo-optical Instrumentation Engineers (SPIE), 9519(951915)
Open this publication in new window or tab >>Atomic Layer Deposition of Al2O3 on NF3-pre-treated graphene
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2015 (English)In: NANOTECHNOLOGY VII, Society of Photo-optical Instrumentation Engineers (SPIE) , 2015, Vol. 9519, no 951915Conference paper, Published paper (Refereed)
Abstract [en]

Graphene has been considered for a variety of applications including novel nanoelectronic device concepts. However, the deposition of ultra-thin high-k dielectrics on top of graphene has still been challenging due to graphenes lack of dangling bonds. The formation of large islands and leaky films has been observed resulting from a much delayed growth initiation. In order to address this issue, we tested a pre-treatment with NF3 instead of XeF2 on CVD graphene as well as epitaxial graphene monolayers prior to the Atomic Layer Deposition (ALD) of Al2O3. All experiments were conducted in vacuo; i. e. the pristine graphene samples were exposed to NF3 in the same reactor immediately before applying 30 (TMA-H2O) ALD cycles and the samples were transferred between the ALD reactor and a surface analysis unit under high vacuum conditions. The ALD growth initiation was observed by in-situ real-time Spectroscopic Ellipsometry (irtSE) with a sampling rate above 1Hz. The total amount of Al2O3 material deposited by the applied 30 ALD cycles was cross-checked by in-vacuo X-ray Photoelectron Spectroscopy (XPS). The Al2O3 morphology was determined by Atomic Force Microscopy (AFM). The presence of graphene and its defect status was examined by in-vacuo XPS and RAMAN Spectroscopy before and after the coating procedure, respectively.

Place, publisher, year, edition, pages
Society of Photo-optical Instrumentation Engineers (SPIE), 2015
Series
Proceedings of SPIE, ISSN 0277-786X
Keywords
Atomic Layer Deposition (ALD); Aluminium(III) oxide (Al2O3); graphene; in-vacuo pre-treatment; Nitrogen trifluoride (NF3); in-situ real-time Spectroscopic Ellipsometry (irtSE); in-vacuo X-ray Photoelectron Spectroscopy (XPS); Atomic Force Microscopy (AFM)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-120482 (URN)10.1117/12.2181242 (DOI)000357932000016 ()978-1-62841-642-8 (ISBN)
Conference
Conference on Nanotechnology VII
Available from: 2015-08-11 Created: 2015-08-11 Last updated: 2015-08-11
Knight, S., Schoeche, S., Darakchieva, V., Kuhne, P., Carlin, J.-F. -., Grandjean, N., . . . Hofmann, T. (2015). Cavity-enhanced optical Hall effect in two-dimensional free charge carrier gases detected at terahertz frequencies. Optics Letters, 40(12), 2688-2691
Open this publication in new window or tab >>Cavity-enhanced optical Hall effect in two-dimensional free charge carrier gases detected at terahertz frequencies
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2015 (English)In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 40, no 12, p. 2688-2691Article in journal (Refereed) Published
Abstract [en]

The effect of a tunable, externally coupled Fabry-Perot cavity to resonantly enhance the optical Hall effect signatures at terahertz frequencies produced by a traditional Drude-like two-dimensional electron gas is shown and discussed in this Letter. As a result, the detection of optical Hall effect signatures at conveniently obtainable magnetic fields, for example, by neodymium permanent magnets, is demonstrated. An AlInN/GaN-based high-electron mobility transistor structure grown on a sapphire substrate is used for the experiment. The optical Hall effect signatures and their dispersions, which are governed by the frequency and the reflectance minima and maxima of the externally coupled Fabry-Perot cavity, are presented and discussed. Tuning the externally coupled Fabry-Perot cavity strongly modifies the optical Hall effect signatures, which provides a new degree of freedom for optical Hall effect experiments in addition to frequency, angle of incidence, and magnetic field direction and strength. (C) 2015 Optical Society of America

Place, publisher, year, edition, pages
Optical Society of America, 2015
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-120042 (URN)10.1364/OL.40.002688 (DOI)000356234300004 ()26076237 (PubMedID)
Note

Funding Agencies|J.A. Woollam Foundation; National Science Foundation [DMR-1420645, EPS-1004094]; Swedish Agency for Innovation Systems [2011-03486, 2014-04712]; Swedish Foundation for Strategic Research [FFL12-0181]; Swedish Research Council [2013-5580]

Available from: 2015-07-06 Created: 2015-07-06 Last updated: 2017-12-04
Khromov, S., Persson, P. O., Wang, X., Yoshikawa, A., Monemar, B., Rosén, J., . . . Darakchieva, V. (2015). Correlation between switching to n-type conductivity and structural defects in highly Mg-doped InN. Applied Physics Letters, 106(23), Article ID 232102.
Open this publication in new window or tab >>Correlation between switching to n-type conductivity and structural defects in highly Mg-doped InN
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2015 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 106, no 23, article id 232102Article in journal (Refereed) Published
Abstract [en]

The effect of Mg doping on the microstructure of InN epitaxial films in relation to their free-charge carrier properties has been investigated by transmission electron microscopy (TEM) and aberration corrected scanning TEM. We observe a direct correlation between Mg concentration and the formation of stacking faults. The threading dislocation density is found to be independent of Mg concentration. The critical Mg concentration for the on-set of stacking faults formation is determined and found to correlate with the switch from p- to n-type conductivity in InN. Potential mechanisms involving stacking faults and point defect complexes are invoked in order to explain the observed conductivity reversal. Finally, the stacking faults are structurally determined and their role in the reduction of the free electron mobility in highly doped InN: Mg is discussed. (C) 2015 AIP Publishing LLC.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-120046 (URN)10.1063/1.4922301 (DOI)000356170900016 ()
Note

Funding Agencies|Swedish Research Council (VR) [2008-405, 2012-4359, 2013-5580, 642-2013-8020]; ERC [258509]; Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program [2011-03486]; Swedish Foundation for Strategic Research (SSF) [FFL12-0181]; National Basic Research Program of China [2012CB619300]; National Natural Science Foundation of China [61225019, 61376060]

Available from: 2015-07-06 Created: 2015-07-06 Last updated: 2017-12-04
Ben Sedrine, N., Zukauskaite, A., Birch, J., Jensen, J., Hultman, L., Schoeche, S., . . . Darakchieva, V. (2015). Infrared dielectric functions and optical phonons of wurtzite YxAl1-xN (0 less than= x less than= 0.22). Journal of Physics D: Applied Physics, 48(41), 415102
Open this publication in new window or tab >>Infrared dielectric functions and optical phonons of wurtzite YxAl1-xN (0 less than= x less than= 0.22)
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2015 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 48, no 41, p. 415102-Article in journal (Refereed) Published
Abstract [en]

YAlN is a new member of the group-III nitride family with potential for applications in next generation piezoelectric and light emitting devices. We report the infrared dielectric functions and optical phonons of wurtzite (0001) YxAl1-xN epitaxial films with 0 less than= x less than= 0.22. The films are grown by magnetron sputtering epitaxy on c-plane Al2O3 and their phonon properties are investigated using infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The infrared-active E-1(TO) and LO, and the Raman active E-2 phonons are found to exhibit one-mode behavior, which is discussed in the framework of the MREI model. The compositional dependencies of the E-1(TO), E-2 and LO phonon frequencies, the high-frequency limit of the dielectric constant, epsilon(infinity), the static dielectric constant, epsilon(0), and the Born effective charge Z(B) are established and discussed.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2015
Keywords
YAlN; phonons; infrared dielectric function
National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-122192 (URN)10.1088/0022-3727/48/41/415102 (DOI)000362007100004 ()
Note

Funding Agencies|Swedish Research Council (VR) [2013-5580]; Swedish Governmental Agency for Innovation Systems (VINNOVA) [2011-03486]; Swedish Foundation for Strategic Research (SSF) [2012FFL12-0181]; National Science Foundation [MRSEC DMR-0820521, EPS-1004094]; Stiftelsen Lars Hiertas Minne [FO2013-0587]; [RECI/FIS-NAN/0183/2012 (FCOMP-01-0124-FEDER-027494)]

Available from: 2015-10-26 Created: 2015-10-23 Last updated: 2017-12-01
Bouhafs, C., Darakchieva, V., Persson, I., Tiberj, A., Persson, P. O., Paillet, M., . . . Yakimova, R. (2015). Structural properties and dielectric function of graphene grown by high-temperature sublimation on 4H-SiC(000-1). Journal of Applied Physics, 117(8), 085701
Open this publication in new window or tab >>Structural properties and dielectric function of graphene grown by high-temperature sublimation on 4H-SiC(000-1)
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2015 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, no 8, p. 085701-Article in journal (Refereed) Published
Abstract [en]

Understanding and controlling growth of graphene on the carbon face (C-face) of SiC presents a significant challenge. In this work, we study the structural, vibrational, and dielectric function properties of graphene grown on the C-face of 4H-SiC by high-temperature sublimation in an argon atmosphere. The effect of growth temperature on the graphene number of layers and crystallite size is investigated and discussed in relation to graphene coverage and thickness homogeneity. An amorphous carbon layer at the interface between SiC and the graphene is identified, and its evolution with growth temperature is established. Atomic force microscopy, micro-Raman scattering spectroscopy, spectroscopic ellipsometry, and high-resolution cross-sectional transmission electron microscopy are combined to determine and correlate thickness, stacking order, dielectric function, and interface properties of graphene. The role of surface defects and growth temperature on the graphene growth mechanism and stacking is discussed, and a conclusion about the critical factors to achieve decoupled graphene layers is drawn. (C) 2015 AIP Publishing LLC.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-117253 (URN)10.1063/1.4908216 (DOI)000351132500070 ()
Note

Funding Agencies|Marie Curie actions [264613-NetFISiC]; Swedish Research Council (VR) [2011-4447, 2013-5580]; Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program [2011-03486]; Swedish foundation for strategic research (SSF) [FFL12-0181]; FP7 EU project Nano-Rf [FP7-ICT-2011-8]; French ANR under the Grafonics Project [ANR-10-NANO-0004]; European Union Seventh Framework Programme under Graphene Flagship [604391]; Knut and Alice Wallenbergs foundation

Available from: 2015-04-22 Created: 2015-04-21 Last updated: 2017-12-04
Xie, M., Schubert, M., Lu, J., Persson, P. O., Stanishev, V., Hsiao, C.-L., . . . Darakchieva, V. (2014). Assessing structural, free-charge carrier, and phonon properties of mixed-phase epitaxial films: The case of InN. Physical Review B. Condensed Matter and Materials Physics, 90(19), 195306
Open this publication in new window or tab >>Assessing structural, free-charge carrier, and phonon properties of mixed-phase epitaxial films: The case of InN
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 19, p. 195306-Article in journal (Refereed) Published
Abstract [en]

We develop and discuss appropriate methods based on x-ray diffraction and generalized infrared spectroscopic ellipsometry to identify wurtizte and zinc-blende polymorphs, and quantify their volume fractions in mixed-phase epitaxial films taking InN as an example. The spectral signatures occurring in the azimuth polarization (Muller matrix) maps of mixed-phase epitaxial InN films are discussed and explained in view of polymorphism (zinc-blende versus wurtzite), volume fraction of different polymorphs and their crystallographic orientation, and azimuth angle. A comprehensive study of the structural, phonon and free electron properties of zinc-blende InN films containing inclusions of wurtzite InN is also presented. Thorough analysis on the formation of the zinc-blende and wurtzite phases is given and the structural evolution with film thickness is discussed in detail. The phonon properties of the two phases are determined and discussed together with the determination of the bulk free-charge carrier concentration, and electron accumulation at the mixed-phase InN film surfaces.

Place, publisher, year, edition, pages
American Physical Society, 2014
National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:liu:diva-113012 (URN)10.1103/PhysRevB.90.195306 (DOI)000345423000003 ()
Note

Funding Agencies|Swedish Research Council (VR) [2013-5580]; Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program [2011-03486]; Swedish Foundation for Strategic Research (SSF) [FFL12-0181]; Linkoping Linnaeus Initiative on Nanoscale Functional Materials (LiLiNFM) by VR; National Science Foundation [MRSEC DMR-0820521]; University of Nebraska-Lincoln; J. A. Woollam Co., Inc.; J. A. Woollam Foundation

Available from: 2015-01-12 Created: 2015-01-08 Last updated: 2018-03-08
Xie, M., Sedrine, B., Hong, L., Monemar, B., Schöche, S., Hofmann, T., . . . Nanishi, Y. (2014). Effect of Mg doping on the structural and free-charge carrier properties of InN. Journal of Applied Physics, 115(16), 163504
Open this publication in new window or tab >>Effect of Mg doping on the structural and free-charge carrier properties of InN
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2014 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, no 16, p. 163504-Article in journal (Refereed) Published
Abstract [en]

We study the structural and free-charge carrier properties of two sets of InN films grown by molecular beam epitaxy doped with different Mg concentrations from 1x1018 cm-3 to 3.9x1021 cm-3. We determine the effect of Mg doping on surface morphology, lattice parameters, structural characteristics and carrier properties. We show that infrared spectroscopic ellipsometry can be used to evidence successful p-type doping in InN, which is an important issue in InN. High resolution X-ray diffraction, combined with atomic force microscopy measurements reveals a drastic decrease in structural quality of the film for Mg concentrations above 1020 cm-3, accompanied with a significant increase in surface roughness. In addition, a decrease of the c-lattice parameter and an increase of the a-lattice parameter are found with increasing Mg concentration. Different contributions to the strain are discussed and it is suggested that the incorporation of Mg leads to a change of growth mode and generation of tensile growth strain. At high Mg concentrations zinc-blende InN inclusions appear which are suggested to originate from higher densities of stacking faults. Infrared spectroscopic ellipsometry analysis shows a reduced LPP-coupling, manifested as a characteristic dip in the IRSE data, and qualitatively different broadening behavior for Mg concentrations between 1.1x1018 cm−3 and 2.9x1019 cm−3 indicate the existence of a p-type conducting bulk InN layer for these Mg concentrations.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-85564 (URN)10.1063/1.4871975 (DOI)000335228400017 ()
Available from: 2012-11-23 Created: 2012-11-23 Last updated: 2017-12-07Bibliographically approved
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