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Optical Characterization of Deep Level Defects in SiC
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
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. , 42 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1674
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-117978ISBN: 978-91-7519-059-4 (print)OAI: oai:DiVA.org:liu-117978DiVA: diva2:812600
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
List of papers
1. Influence of Cooling Rate after High Temperature Annealing on Deep Levels in High-Purity Semi-Insulating 4H-SiC
Open this publication in new window or tab >>Influence of Cooling Rate after High Temperature Annealing on Deep Levels in High-Purity Semi-Insulating 4H-SiC
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2007 (English)In: Materials Science Forum, vol. 556-557, Trans Tech Publications , 2007, 371- p.Conference paper (Refereed)
Abstract [en]

The influence of different cooling rates on deep levels in 4H-SiC after high temperature annealing has been investigated. The samples were heated from room temperature to 2300°C, followed by a 20 minutes anneal at this temperature. Different subsequent cooling sequences down to 1100°C were used. The samples have been investigated using photoluminescence (PL) and IV characteristics. The PL intensities of the silicon vacancy (VSi) and UD-2, were found to increase with a faster cooling rate.

Place, publisher, year, edition, pages
Trans Tech Publications, 2007
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-38676 (URN)10.4028/www.scientific.net/MSF.556-557.371 (DOI)45293 (Local ID)45293 (Archive number)45293 (OAI)
Conference
ECSCRM 2006
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2015-05-19
2. The Electronic Structure of the UD-4 defect in 4H, 6H and 15R SiC
Open this publication in new window or tab >>The Electronic Structure of the UD-4 defect in 4H, 6H and 15R SiC
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2009 (English)In: Materials Science Forum, Vols. 600-603, Trans Tech Publications , 2009, 397-400 p.Conference paper (Refereed)
Abstract [en]

The photoluminescence (PL) of the UD-4 defect is observed in semi-insulating bulk 4H, 6H and 15R SiC. In 4H and 6H SiC the UD-4 defect consists of two families of no-phonon (NP) lines, Ua and Ub, and in 15R SiC it consists of three families, Ua, Ub and U15R. The Ua family in 4H, 6H and 15R all show similar temperature behavior with higher energy NP lines becomming observable at higher temperatures. In the case of the Ub and U15R families, a luminescence line with lower energy than the prominent luminescence line appears at higher temperatures. The polarization and Zeeman measurements suggest that the defect has C3v symmetry.

Place, publisher, year, edition, pages
Trans Tech Publications, 2009
Series
, Materials Science Forum, ISSN 1662-9752 ; 600-603
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-41968 (URN)10.4028/www.scientific.net/MSF.600-603.397 (DOI)59428 (Local ID)59428 (Archive number)59428 (OAI)
Conference
ICSCRM 2007, Otsu, Japan October 14-19, 2007
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2015-05-19
3. Asymmetric Split-Vacancy Defects in SiC Polytypes: A Combined Theoretical and Electron Spin Resonance Study
Open this publication in new window or tab >>Asymmetric Split-Vacancy Defects in SiC Polytypes: A Combined Theoretical and Electron Spin Resonance Study
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2011 (English)In: Physical Review Letters, ISSN 0031-9007, Vol. 107, no 19, 195501- p.Article in journal (Refereed) Published
Abstract [en]

Transition metal defects were studied in different polytypes of silicon carbide (SiC) by ab initio supercell calculations. We found asymmetric split-vacancy (ASV) complexes for these defects that preferentially form at only one site in hexagonal polytypes, and they may not be detectable at all in cubic polytype. Electron spin resonance study demonstrates the existence of ASV complex in niobium doped 4H polytype of SiC.

Place, publisher, year, edition, pages
American Physical Society, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-72653 (URN)10.1103/PhysRevLett.107.195501 (DOI)000297006100005 ()
Note
Funding Agencies|Swedish Foundation for Strategic Research||Swedish Research Council||Swedish Energy Agency||Swedish National Infrastructure for Computing|SNIC 011/04-8SNIC001-10-223|Knut and Alice Wallenberg Foundation||Available from: 2011-12-02 Created: 2011-12-02 Last updated: 2015-05-19
4. Optical identification and electronic configuration of tungsten in 4H-and 6H-SiC
Open this publication in new window or tab >>Optical identification and electronic configuration of tungsten in 4H-and 6H-SiC
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2012 (English)In: Physica. B, Condensed matter, ISSN 0921-4526, Vol. 407, no 10, 1462-1466 p.Article 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
Keyword
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: 2015-09-22
5. Optical properties and Zeeman spectroscopy of niobium in silicon carbide
Open this publication in new window or tab >>Optical properties and Zeeman spectroscopy of niobium in silicon carbide
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(English)Manuscript (preprint) (Other academic)
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 at hexagonal site. Using this concept we propose a model considering Nb-bound exciton, the recombination of which is responsible for the observed luminescence. The exciton energy is estimated using firstprinciples 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 explained on the ground 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 hexagonal site in the studied SiC polytypes behaves like a deep acceptor.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117972 (URN)
Available from: 2015-05-19 Created: 2015-05-19 Last updated: 2015-05-19Bibliographically approved
6. A defect center for quantum computing: Mo in SiC
Open this publication in new window or tab >>A defect center for quantum computing: Mo in SiC
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The electronic structure and vibrational properties of molybdenum (Mo) in SiC are analyzed and investigated in detail. Mo is considered as occupying the silicon-carbon divacancy in the so-called asymmetric split vacancy (ASV) configuration. Group-theoretical considerations within this model are used to explain the experimental results (optical properties and behavior in magnetic field). The vibrational properties of the defect are studied using simple the “defect molecule” model with parameters determined phenomenologically from the experimental data. The position of Mo in the ASV configuration deduced from this model is shown to be in good agreement with the earlier reported data from ab initio supercell calculations. The usefulness of molybdenum in SiC in quantum computing is investigated, and it shown that Mo is a highly promising candidate for quantum computing.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-117973 (URN)
Available from: 2015-05-19 Created: 2015-05-19 Last updated: 2015-05-19Bibliographically approved
7. 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
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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

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