liu.seSearch for publications in DiVA
Change search
Link to record
Permanent link

Direct link
BETA
Gällström, Andreas
Alternative names
Publications (10 of 27) Show all publications
Gällström, A. (2015). Optical Characterization of Deep Level Defects in SiC. (Doctoral dissertation). Linköping: Linköping University Electronic Press
Open this publication in new window or tab >>Optical Characterization of Deep Level Defects in SiC
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Silicon Carbide (SiC) has long been considered a promising semiconductor material for high power devices, and has also recently found to be one of the emergent materials for quantum computing. Important for these applications are both the quality and purity of the crystal. In order to be able to engineer components (be it power devices or components for quantum computing), it is necessary to study and understand the behavior of various defects in the crystal.

Deep level defects can greatly influence the semiconducting properties, since they can act as recombination centers by interacting with both holes from the valence band and electrons from the conduction band. Because of this, they may be used to control the charge carrier life time. Besides influencing the electric properties of the materials, deep level defects are also of interest in the field of quantum computing. In this application, the deep level defects can be used as basic units for quantum information – so called qubits.

Deep level defects may also be classified based on their origin, i.e. impurity or intrinsic. An impurity consists of one or more foreign atoms, which means neither carbon nor silicon in the case of SiC. Impurities can be incorporated in the crystal during growth, or through implantation or diffusion. A defect is intrinsic when it does not involve foreign atoms, but instead imperfections in the perfect crystal structure, for example a vacancy, an anti-site or a combinations of these. Intrinsic defects can be created during growth or artificially, using for example electron irradiation.

This thesis is focused on characterization of several deep level defects in SiC using different optical techniques. The optical transitions investigated are in the near-infrared region.

Paper 1 focuses on the possibility to control the concentration of intrinsic defects through the cooling down procedure after high temperature annealing. The temperature of 2300°C is close to the bulk crystal growth temperature. It is shown that it is possible to control the concentration of the silicon vacancy (VSi) and UD-2 (later identified as the divacancy (VCVSi)) by the cooling  sequence. Both these defects have later been shown to be promising candidates as qubits and single photon emitters.

Paper 2 gives insight into the electronic structure of the unidentified deep level defect UD-4, which is believed to be of intrinsic origin. The defect is investigated in the polytypes 4H-, 6H-, and 15R-SiC, and the number of optical centers associated with UD-4 follows neither the number of inequivalent sites nor the possible configurations for pair-defects. There are two optical centers in 4H- and 6H-SiC, and three optical centers in 15R-SiC.

Paper 3 investigates several transition metals incorporated in SiC and the formation energies for different possible configurations. This is of importance since several impurity related deep level defects cannot be explained as purely substitutional defects, based on the fact that the number of optical centers does not follow the number of inequivalent sites. This is investigated in detail, and explained using an asymmetric split vacancy (ASV) model. It was found that the formation energy for some transition metals in ASV are lower than the transition metal in a substitutional configuration. Further on, it was shown that the formation energies for transition metals in ASV configurations depend strongly on what kinds of inequivalent sites the ASV can be described by and the lowest formation energy that is found for transition metals in ASV occupying two hexagonal sites.

In paper 4, the optical identification and electronic configuration of the commonly observed deep level defect tungsten (formerly known as UD-1) are reported. The electronic levels involved in the optical transitions of tungsten are deduced and described using group theory techniques.

Paper 5 shows that the above mentioned ASV model can be used to describe the properties of niobium in SiC. In the paper, the optical identification and properties are analyzed and investigated experimentally using photoluminescence, photoluminescence excitation spectroscopy and Zeeman spectroscopy.

In paper 6 the identification of molybdenum (formerly known as I-1) is reported including its electronic configuration. Molybdenum can be well described using the ASV model, and in this paper its local vibrational modes are also investigated in detail. It is shown that using the polarization dependence of local vibration replicas and a simplified defect molecule model, the estimated position of Mo in the ASV is in agreement with the theoretically predicted position reported in paper 3. The usefulness for molybdenum in SiC as a qubit is also investigated.

In paper 7, two different intrinsic nearest pair-neighbor defects are reported: UD-2 (VCVSi) and UD-0 (tentatively assigned as the VCCSi). Their optical properties are analyzed together with their creation and annihilation properties.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. p. 42
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1674
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117978 (URN)978-91-7519-059-4 (ISBN)
Public defence
2015-06-03, Nobel (BL32), Fysikhuset, Campus Valla, Linköping, 09:15 (English)
Opponent
Supervisors
Available from: 2015-05-19 Created: 2015-05-19 Last updated: 2015-05-19Bibliographically approved
Gällström, A., Magnusson, B., Tien Son, N., Ivanov, I. G. & Janzén, E. (2015). Optical identification of intrinsic nearest-neighbor defects in SiC.
Open this publication in new window or tab >>Optical identification of intrinsic nearest-neighbor defects in SiC
Show others...
2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

The optical signature of two types of intrinsic nearest-neighbor defects in SiC is observed in 4H- and 6H-SiC. The first optical signature belong to a defect previously known as UD-2 and identified as the divacancy pair, and the second – to a defect referred to here as UD-0, an unidentified defect. In both these defects, the number of optical centers is equal to the number of possible configurations for nearest-neighbor pairs in the unit cells of these polytypes. The polarization of all optical transitions is investigated. The formation of the two defects by means of electron irradiation and subsequent annealing in samples with different Fermi levels is studied, too. The observed transitions are investigated using group-theoretical analysis and UD-0 is tentatively assigned to the carbon-vacancy carbonantisite pair, based on energy positions of the lines and spin configuration.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117974 (URN)
Available from: 2015-05-19 Created: 2015-05-19 Last updated: 2015-05-19Bibliographically approved
Gällström, A., Magnusson, B., Leone, S., Kordina, O., Son, N. T., Ivády, V., . . . Ivanov, I. G. (2015). Optical properties and Zeeman spectroscopy of niobium in silicon carbide. Physical Review B. Condensed Matter and Materials Physics, 92(7), 1-14, Article ID 075207.
Open this publication in new window or tab >>Optical properties and Zeeman spectroscopy of niobium in silicon carbide
Show others...
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 7, p. 1-14, article id 075207Article in journal (Refereed) Published
Abstract [en]

The optical signature of niobium in the low-temperature photoluminescence spectra of three common polytypes of SiC (4H, 6H, and 15R) is observed and confirms the previously suggested concept that Nb occupies preferably the Si-C divacancy with both Si and C at hexagonal sites. Using this concept we propose a model considering a Nb-bound exciton, the recombination of which is responsible for the observed luminescence. The exciton energy is estimated using first-principles calculation and the result is in very good agreement with the experimentally observed photon energy in 4H SiC at low temperature. The appearance of six Nb-related lines in the spectra of the hexagonal 4H and 6H polytypes at higher temperatures is tentatively explained on the grounds of the proposed model and the concept that the Nb center can exist in both C1h and C3v symmetries. The Zeeman splitting of the photoluminescence lines is also recorded in two different experimental geometries and the results are compared with theory based on phenomenological Hamiltonians. Our results show that Nb occupying the divacancy at the hexagonal site in the studied SiC polytypes behaves like a deep acceptor.

Place, publisher, year, edition, pages
American Physical Society, 2015
National Category
Theoretical Chemistry Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-117972 (URN)10.1103/PhysRevB.92.075207 (DOI)000362204100001 ()
Note

At the time for thesis presentation publication was in status: Manuscript

Funding Agencies|Knut and Alice Wallenberg Foundation; Lendulet program of the Hungarian Academy of Sciences; Hungarian OTKA Project [K101819]; Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Tomsk State University Academic D. I. Mendeleev Fund Program [8.1.18.2015]

Available from: 2015-05-19 Created: 2015-05-19 Last updated: 2017-12-04Bibliographically approved
Trinh, X. T., Gällström, A., Nguyen, S. T., Leone, S., Kordina, O. & Janzén, E. (2013). Electron Paramagnetic Resonance Studies of Nb in 6H-SiC. In: Materials Science Forum (Volumes 740 - 742): . Paper presented at ECSCRM 2012, St Petersburg, Russia (pp. 385-388). Trans Tech Publications Inc.
Open this publication in new window or tab >>Electron Paramagnetic Resonance Studies of Nb in 6H-SiC
Show others...
2013 (English)In: Materials Science Forum (Volumes 740 - 742), Trans Tech Publications Inc., 2013, p. 385-388Conference paper, Published paper (Refereed)
Abstract [en]

Defects in unintentionally Nb-doped 6H-SiC grown by high-temperature chemical vapor deposition were studied by electron paramagnetic resonance (EPR). An EPR spectrum with a hyperfine (hf) structure consisting of ten equal-intensity lines was observed. The hf structure is identified to be due to the hf interaction between an electron spin S=1/2 and a nuclear spin of 93Nb. The hf interaction due to the interaction three nearest Si neighbors was also observed, suggesting the involvement of the C vacancy (VC) in the defect. Only one EPR spectrum was observed in 6H-SiC polytype. The obtained spin-Hamiltonian parameters are similar to that of the Nb-related EPR defect in 4H-SiC, suggesting that the EPR center in 6H-SiC is the NbSiVC complex in the neutral charge state, NbSiVC0. Photoexcitation EPR experiments suggest that the single negative charge state of the NbSiVC complex is located at ~1.3 eV below the conduction band minimum.

Place, publisher, year, edition, pages
Trans Tech Publications Inc., 2013
Keywords
Electron Paramagnetic Resonance (EPR), Nb, Split-Vacancy Complex
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-91730 (URN)10.4028/www.scientific.net/MSF.740-742.385 (DOI)000319785500090 ()
Conference
ECSCRM 2012, St Petersburg, Russia
Available from: 2013-04-30 Created: 2013-04-30 Last updated: 2013-08-23
Gueorguiev Ivanov, I., Gällström, A., Leone, S., Kordina, O., Tien Son, N., Henry, A., . . . Janzén, E. (2013). Optical properties of the niobium centre in 4H, 6H, and 15R SiC. In: SILICON CARBIDE AND RELATED MATERIALS 2012: . Paper presented at 9th European Conference on Silicon Carbide and Related Materials (ECSCRM 2012) (pp. 405-408). Trans Tech Publications, 740-742
Open this publication in new window or tab >>Optical properties of the niobium centre in 4H, 6H, and 15R SiC
Show others...
2013 (English)In: SILICON CARBIDE AND RELATED MATERIALS 2012, Trans Tech Publications , 2013, Vol. 740-742, p. 405-408Conference paper, Published paper (Refereed)
Abstract [en]

A set of lines in the photoluminescence spectra of 4H-, 6H-, and 15R-SiC in the near-infrared are attributed to Nb-related defects on the ground of doping experiments conducted with 4H-SiC. A model based on a an exciton bound at the Nb-centre in an asymmetric split vacancy configuration at a hexagonal site is proposed, which explains the structure of the luminescence spectrum and the observed Zeeman splitting of the lines.

Place, publisher, year, edition, pages
Trans Tech Publications, 2013
Keywords
transition metals; niobium; photoluminescence; Zeeman effect
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-96515 (URN)10.4028/www.scientific.net/MSF.740-742.405 (DOI)000319785500095 ()
Conference
9th European Conference on Silicon Carbide and Related Materials (ECSCRM 2012)
Available from: 2013-08-21 Created: 2013-08-20 Last updated: 2014-10-08
Son Tien, N., Trinh, X. T., Gällström, A., Leone, S., Kordina, O., Janzén, E., . . . Gali, A. (2012). Electron paramagnetic resonance and theoretical studies of Nb in 4H- and 6H-SiC. Journal of Applied Physics, 112(8), 083711
Open this publication in new window or tab >>Electron paramagnetic resonance and theoretical studies of Nb in 4H- and 6H-SiC
Show others...
2012 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 112, no 8, p. 083711-Article in journal (Refereed) Published
Abstract [en]

High purity silicon carbide (SiC) materials are of interest from high-power high temperature applications across recent photo-voltaic cells to hosting solid state quantum bits, where the tight control of electrically, optically, and magnetically active point defects is pivotal in these areas. 4H- and 6H-SiC substrates are grown at high temperatures and the incorporation of transition metal impurities is common. In unintentionally Nb-doped 4H- and 6H-SiC substrates grown by high-temperature chemical vapor deposition, an electron paramagnetic resonance (EPR) spectrum with C-1h symmetry and a clear hyperfine (hf) structure consisting of ten equal intensity hf lines was observed. The hf structure can be identified as due to the interaction between the electron spin S - 1/2 and the nuclear spin of Nb-93. Additional hf structures due to the interaction with three Si neighbors were also detected. In 4H-SiC, a considerable spin density of similar to 37.4% was found on three Si neighbors, suggesting the defect to be a complex between Nb and a nearby carbon vacancy (V-C). Calculations of the Nb-93 and Si-29 hf constants of the neutral Nb on Si site, Nb-Si(0), and the Nb-vacancy defect, NbSiVC0, support previous reported results that Nb preferentially forms an asymmetric split-vacancy (ASV) defect. In both 4H- and 6H-SiC, only one Nb-related EPR spectrum has been observed, supporting the prediction from calculations that the hexagonal-hexagonal defect configuration of the ASV complex is more stable than others.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-86385 (URN)10.1063/1.4759362 (DOI)000310597500056 ()
Note

Funding Agencies|Swedish Energy Agency||Swedish Research Council VR/Linne Environment LiLI-NFM, FP7|270197|NHDP|TAMOP-4.2.1/B-09/1/KMR-2010-0002|Swedish National Infrastructure for Computing||Knut and Alice Wallenberg Foundation||

Available from: 2012-12-14 Created: 2012-12-14 Last updated: 2017-12-06
Gällström, A., Magnusson, B., Beyer, F., Gali, A., Son, N. ., Leone, S., . . . Janzén, E. (2012). Electronic Configuration of Tungsten in 4H-, 6H-, and 15R-SiC. In: Materials Science Forum Vols 717 - 720: . Paper presented at 14th International Conference on Silicon Carbide and Related Materials (ICSCRM 2011), 11-16 September 2011, Cleveland, OH, USA (pp. 211-216). Trans Tech Publications Inc., 717-720
Open this publication in new window or tab >>Electronic Configuration of Tungsten in 4H-, 6H-, and 15R-SiC
Show others...
2012 (English)In: Materials Science Forum Vols 717 - 720, Trans Tech Publications Inc., 2012, Vol. 717-720, p. 211-216Conference paper, Published paper (Refereed)
Abstract [en]

A commonly observed unidentified photoluminescence center in SiC is UD-1. In this report, the UD-1 center is identified to be tungsten related. The identification is based on (i) a W-doping study, the confirmation of W in the samples was made using deep level transient spectroscopy (DLTS), (ii) the optical activation energy of the absorption of UD-1 in weakly n-type samples corresponds to the activation energy of the deep tungsten center observed using DLTS. The tungsten-related optical centers are reported in 4H-, 6H-, and 15R-SiC. Further, a crystal field model for a tungsten atom occupying a Si-site is suggested. This crystal field model is in agreement with the experimental data available: polarization, temperature dependence and magnetic field splitting.

Place, publisher, year, edition, pages
Trans Tech Publications Inc., 2012
Keywords
deep level defect; PL; transition metal; Crystal Field Model
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-87570 (URN)10.4028/www.scientific.net/MSF.717-720.211 (DOI)000309431000049 ()978-3-03785-419-8 (ISBN)
Conference
14th International Conference on Silicon Carbide and Related Materials (ICSCRM 2011), 11-16 September 2011, Cleveland, OH, USA
Available from: 2013-01-18 Created: 2013-01-18 Last updated: 2015-09-22
Son, N. T., Ivady, V., Gali, A., Gällström, A., Leone, S., Kordina, O. & Janzén, E. (2012). Identification of Niobium in 4H-SiC by EPR and ab Initio Studies. In: Materials Science Forum Vols 717 - 720: . Paper presented at 14th International Conference on Silicon Carbide and Related Materials (ICSCRM 2011), 11-16 September 2011, Cleveland, OH, USA (pp. 217-220). Trans Tech Publications Inc., 717-720
Open this publication in new window or tab >>Identification of Niobium in 4H-SiC by EPR and ab Initio Studies
Show others...
2012 (English)In: Materials Science Forum Vols 717 - 720, Trans Tech Publications Inc., 2012, Vol. 717-720, p. 217-220Conference paper, Published paper (Refereed)
Abstract [en]

In unintentionally Nb-doped 4H-SiC grown by high-temperature chemical vapor deposition (HTCVD), an electron paramagnetic resonance (EPR) center with C-lh symmetry and an electron spin S=1/2 was observed. The spectrum shows a hyperfine structure consisting of ten equal-intensity hyperfine (hf) lines which is identified as due to the hf interaction between the electron spin and the nuclear spin of Nb-93. An additional hf structure due to the interaction with two equivalent Si neighbors was also observed. Ab initio supercell calculations of Nb in 4H-SiC suggest that Nb may form a complex with a C-vacancy (V-C) resulting in an asymmetric split-vacancy (ASV) defect, Nb-Si-V-C. Combining results from EPR and supercell calculations, we assign the observed Nb-related EPR center to the hexagonal-hexagonal configuration of the AVS defect in the neutral charge state, (Nb-Si-V-C)(0).

Place, publisher, year, edition, pages
Trans Tech Publications Inc., 2012
Keywords
Transition metal impurity; split-vacancy defect; electron paramagnetic resonance; ab initio calculations
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-87571 (URN)10.4028/www.scientific.net/MSF.717-720.217 (DOI)000309431000050 ()
Conference
14th International Conference on Silicon Carbide and Related Materials (ICSCRM 2011), 11-16 September 2011, Cleveland, OH, USA
Available from: 2013-01-18 Created: 2013-01-18 Last updated: 2016-06-02
Ivanov, I. G., Gällström, A., Coble, R., Devaty, R. P., Choyke, W. J. & Janzén, E. (2012). Investigation of intrinsic carbon-related defects in 4H-SiC by selective-excitation photoluminescence spectroscopy. In: Materials Science Forum Vols 717 - 720: . Paper presented at 14th International Conference on Silicon Carbide and Related Materials (ICSCRM 2011), 11-16 September 2011, Cleveland, OH, USA (pp. 259-262). Trans Tech Publications Inc., 717-720
Open this publication in new window or tab >>Investigation of intrinsic carbon-related defects in 4H-SiC by selective-excitation photoluminescence spectroscopy
Show others...
2012 (English)In: Materials Science Forum Vols 717 - 720, Trans Tech Publications Inc., 2012, Vol. 717-720, p. 259-262Conference paper, Published paper (Refereed)
Abstract [en]

Emission of carbon-related defects is investigated by means of selectively-excited photoluminescence in high purity 4H-SiC electron-irradiated with very low dose. Two new centers with clearly associated phonon replicas are observed, one of which is tentatively assigned to the carbon split interstitial at the hexagonal site. The temperature dependence of the spectrum is also studied and indicates that at least some of the observed luminescence lines arise from recombination of excitons bound to isoelectronic centers.

Place, publisher, year, edition, pages
Trans Tech Publications Inc., 2012
Keywords
carbon aggregates; carbon split interstitial; di-carbon antisite; photoluminescence
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-87572 (URN)10.4028/www.scientific.net/MSF.717-720.259 (DOI)000309431000060 ()
Conference
14th International Conference on Silicon Carbide and Related Materials (ICSCRM 2011), 11-16 September 2011, Cleveland, OH, USA
Available from: 2013-01-18 Created: 2013-01-18 Last updated: 2014-10-20
Gällström, A., Magnusson, B., Beyer, F., Gali, A., Son Tien, N., Leone, S., . . . Janzén, E. (2012). Optical identification and electronic configuration of tungsten in 4H-and 6H-SiC. Physica. B, Condensed matter, 407(10), 1462-1466
Open this publication in new window or tab >>Optical identification and electronic configuration of tungsten in 4H-and 6H-SiC
Show others...
2012 (English)In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 407, no 10, p. 1462-1466Article in journal (Refereed) Published
Abstract [en]

Several optically observed deep level defects in SiC are still unidentified and little is published on their behavior. One of the commonly observed deep level defects in semi-insulating SiC is UD-1. less thanbrgreater than less thanbrgreater thanThis report suggests that UD-1 is Tungsten related, based on a doping study and previously reported deep level transient spectroscopy data, as well as photo-induced absorption measurements. The electronic levels involved in the optical transitions of UD-1 are also deduced. The transitions observed in the photoluminescence of UD-1 are from a Gamma(C3v)(4), to two different final states, which transform according to Gamma(C3v)(5)circle plus Gamma(C3v)(6) and Gamma(C3v)(4), respectively.

Place, publisher, year, edition, pages
Elsevier, 2012
Keywords
Deep level defect, PL, Transition metal
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-77521 (URN)10.1016/j.physb.2011.09.062 (DOI)000303149600003 ()
Available from: 2012-05-28 Created: 2012-05-22 Last updated: 2017-12-07
Organisations

Search in DiVA

Show all publications