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Electron paramagnetic resonance study of defects in SiC
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Silicon carbide (SiC) is a wide bandgap semiconductor (energy gap of 3.26 eV and 3.03 eV for 4Hand 6H-SiC, respectively). With outstanding physical and electronic properties, SiC is a promising material for high-power, high-frequency and high-temperature applications. The electronic properties of a semiconductor are to a large extent determined by point defects in the crystal. As known from other semiconductors, defect control is crucially important for the successful device applications. Point defects can be impurities, such as the shallow nitrogen (N) donor or boron acceptor (the residual n- and p-type dopants in SiC), or intrinsic defects, such as vacancies, antisites, interstitials or combinations thereof. One of the key issues in the SiC technology is to develop semi insulating (SI) SiC substrates required for SiC MEtal Semiconductor Field Effect Transistors (MESFETs) and also for III-nitride based High Electron Mobility Transistors (HEMTs), to reduce the parasitic capacitance and to improve the device performance. For achieving the SI behavior the Fermi level should be pinned near the middle of the bandgap. This can be realized using defects with deep acceptor level(s) to compensate the residual shallow N donors which cause the natural ntype doping of as-grown SiC.

Vanadium (V) doped SI SiC has been developed since the 1990s. However, SiC MESFETs using V-doped SI SiC substrates are shown to have severe problems with electron trapping to eep levels in the SI substrates which causes reduction of the drain current and instability of the device performance. Since the beginning of this decade, V-free high-purity SI (HPSI) SiC substrates using intrinsic defects to compensate the N donors have been developed. The work in this thesis has been devoted to characterize defects in HPSI SiC using electron paramagnetic resonance (EPR). EPR detects transitions between energy levels split up by the interaction of unpaired electron spins (localized at the defect and neighboring atoms) with an applied magnetic field. Thanks to the sensitivity of the electron spins to their surroundings; especially to nearby nuclear spins that further splits the energy levels by the so-called hyperfine (hf) interaction, one can extract information on the structure and electronic configuration of a defect.

The work has been focused on (i) the identification of prominent defects, (ii) the determination of their energy levels and roles in the carrier compensation processes, (iii) the defect interaction and the stability of the SI properties at high temperatures, in order to identify the optimal defect(s) to be used for controlling the SI properties. EPR and ab initio supercell calculations have been the main tools for defect identification and all three common polytypes 3C-, 4H- and 6H-SiC of different conducting types (n-, p-type and SI) have been investigated. For determination of the energy levels in the bandgap, the combined results of EPR and photoexcitation EPR (photo-EPR), Deep Level Transient Spectroscopy (DLTS), the temperature dependence of the resistivity, and ab initio calculations have been evaluated. Annealing studies up to 1600 °C for samples with different defect compositions have been carried out for obtaining knowledge on the defect interaction and thermal stability of the SI properties as well as the change in resistivity, activation energy and defect concentration. Below is a short summary of the papers included in the thesis.

In paper 1, the identification of the neutrally charged divacancy (VCVSi 0) in 4H-SiC, by PR and ab initio calculations, is presented. The divacancy is a common defect in SiC and it is thought to play a role in carrier compensation in HPSI SiC. Annealing studies show that it is formed during migration of carbon vacancies (VC) and silicon vacancies (VSi) and in the studied samples it is thermally stable up to at least 1500 °C.

Paper 2 presents EPR identification of prominent defects in different types of HPSI 4H-SiC substrates grown by high-temperature chemical vapor deposition (HTCVD) and physical vapor transport (PVT), the determination of some of their deep acceptor levels and their roles in carrier compensation processes. VSi, VC, carbon antisite-vacany pair (CSiVC), and VCVSi were found to be the most common defects in different types of HPSI 4H-SiC. The samples could be grouped into three activation energy ranges Ea~0.8–0.9 eV, ~1.1–1.3 eV, and ~1.5 eV, and the possible defect levels related to these energies were discussed for each group. The samples with Ea~1.5 eV contain high concentrations of VC and VCVSi and low concentrations of VSi and as these samples had the most thermally stable SI properties, due to the increased thermal stability of VC when VSi is absent, we concluded that this defect composition is preferable.

A similar study is presented in paper 4 of different types of HPSI 6H-SiC substrates grown by HTCVD. The samples could be grouped into two activation energy ranges Ea~0.6-0.7 eV and ~1.0-1.2 eV. VC, CSiVC and VCVSi were found to be the prominent defects and the relationship between their energy levels and the activation energies was discussed. The  materials were still SI after annealing up to 1600°C although the activation energies were lowered. The (+|0) level of VC was also specifically studied by photo-EPR and determined to be located at ~1.47 eV above the valence band, similar to 4H-SiC.

The content of Paper 3 concerns an EPR study of two defects, labeled L5 and L6, in electron irradiated n-type 3C-SiC. The L5 defect could be related to the neutrally charged divacancy as it shows some features similar to the divacancy in 4H-SiC. The L5 defect anneals out at low temperatures (~200°C) and could possibly be carbon interstitial related.

Paper 5 presents an attempt to study the energy levels of VC by photo-EPR without the usual interference from other defect levels. By using pure free-standing n-type 4H-SiC epilayers with very low defect concentrations and low-energy electron (200 keV) irradiation we could combine photo-EPR and DLTS to study energy levels related to VC.VC+ and VC- could be detected simultaneously and from the study we concluded that the (+|0) is located at ~EC–1.77 eV and suggested that the (0|−) and (1−|2−) levels are located at ~EC–0.8 eV and ~ EC–1.0 eV, respectively.

The investigation in paper 6 concerns the identification of the EI4 EPR center in 4H- and 6HSiC. Based on detailed studies of the hf interactions, the annealing behavior and ab initio supercell calculations we believe the corresponding defect is a complex between a carbon vacancy-carbon antisite and a carbon vacancy at the third neighbor site of the antisite in the neutral charge state, (VC-CSiVC)0. It could be directly involved in carrier compensation in some samples before it anneals out (at ~850 °C in irradiated samples or higher temperatures in as-grown sample) and also seems to be an intermediate state in the formation of the divacancy.

In Paper 7, an EPR study of a radiation-induced defect, labeled LE5, in 4H- and 6H-SiC is presented. The observation of the LE5 spectra in samples irradiated at low temperatures (77-100 K) indicates that it is a primary defect. From the low symmetry (C1), the Si hf structures, and the low anneal-out temperature (~600-750 °C) we suggested that the defect may be a complex involving a silicon antisite (SiC) perturbed by a nearby defect.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2010. , 39 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1319
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-56586ISBN: 978-91-7393-380-3 (print)OAI: oai:DiVA.org:liu-56586DiVA: diva2:320515
Public defence
2010-06-11, Planck, fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Note
In the list of included papers is the title of paper VII "A primary complex defect in electron-irradiated 3C-, 4H- and 6H-SiC" but in manuscript the title is "Silicon antisite related defects in electron-irradiated p-type 4H- and 6H-SiC".Available from: 2010-05-25 Created: 2010-05-25 Last updated: 2010-11-04Bibliographically approved
List of papers
1. Divacancy in 4H-SiC
Open this publication in new window or tab >>Divacancy in 4H-SiC
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2006 (English)In: Physical review letters / publ. by the American Physical Society, Vol. 96, 055501-1- p.Article in journal (Refereed) Published
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-36458 (URN)31399 (Local ID)31399 (Archive number)31399 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2011-12-29
2. Defects and carrier compensation in semi-insulating 4H-SiC substrates
Open this publication in new window or tab >>Defects and carrier compensation in semi-insulating 4H-SiC substrates
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2007 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 75, no 15Article in journal (Refereed) Published
Abstract [en]

Electron paramagnetic resonance (EPR) studies revealed that vacancies (VC and VSi), carbon vacancy-antisite pairs (VC CSi) and the divacancy (VC VSi) are common defects in high-purity semi-insulating (HPSI) 4H-SiC substrates. Their concentrations and some of their deep acceptor levels were estimated by EPR and photoexcitation EPR. The commonly observed thermal activation energies, Ea ∼0.8-0.9 eV, ∼1.1 eV, ∼1.25-1.3, and ∼1.5 eV, as determined from the temperature dependence of the resistivity, in different types of HPSI substrates were associated to different deep acceptor levels of VSi, VC, VC CSi, and VC VSi. The annealing behavior of these vacancy-related defects and their interaction at high temperatures (up to 1600°C) in HPSI materials were studied. Carrier compensation processes were proposed to explain the observed change of the thermal activation energy due to high temperature annealing. VC and VC VSi were suggested to be suitable defects for controlling the SI properties whereas the incorporation of VSi and VC CSi during the crystal growth or processing should be avoided for achieving stable HPSI materials. © 2007 The American Physical Society.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-38665 (URN)10.1103/PhysRevB.75.155204 (DOI)45266 (Local ID)45266 (Archive number)45266 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2017-12-13
3. Electron paramagnetic resonance study on n-type electron-irradiated 3C-SiC
Open this publication in new window or tab >>Electron paramagnetic resonance study on n-type electron-irradiated 3C-SiC
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2008 (English)In: PROCEEDINGS OF THE 17TH INTERNATIONAL VACUUM CONGRESS/13TH INTERNATIONAL CONFERENCE ON SURFACE SCIENCE/INTERNATIONAL CONFERENCE ON NANOSCIENCE AND TECHNOLOGY, BRISTOL, ENGLAND: IOP PUBLISHING LTD , 2008Conference paper, Published paper (Refereed)
Abstract [en]

Electron Paramagnetic Resonance (EPR) was used to study defects in n-type 3C-SiC films irradiated by 3-MeV electrons at room temperature with a dose of 2x10(18) cm(-2). After electron irradiation, two new EPR spectra with an effective spin S = 1, labeled L5 and L6, were observed. The L5 center has C-3v symmetry with g = 2.004 and a fine-structure parameter D = 436.5 x 10(-4) cm(-1). The L5 spectrum was only detected under light illumination and it could not be detected after annealing at similar to 550 C. The principal z-axis of the D tensor is parallel to the < 111 >-directions, indicating the location of spins along the Si-C bonds. Judging from the symmetry and the fact that the signal was detected under illumination in n-type material, the L5 center may be related to the divacancy in the neutral charge state. The L6 center has a C-2v-symmetry with an isotropic g-value of g=2.003 and the fine structure parameters D=547.7 x 10(-4) cm-1 and E=56.2 x 10(-4) cm(-1). The L6 center disappeared after annealing at a rather low temperature (similar to 200 degrees C), which is substantially lower than the known annealing temperatures for vacancy-related defects in 3C-SiC. This highly mobile defect may be related to carbon interstitials.

Place, publisher, year, edition, pages
BRISTOL, ENGLAND: IOP PUBLISHING LTD, 2008
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-42572 (URN)10.1088/1742-6596/100/4/042032 (DOI)000275655200080 ()66159 (Local ID)66159 (Archive number)66159 (OAI)
Conference
17th International Vacuum Congress/13th International Conference on Surface Science/Internatinal Conference on Nanoscience and Technology
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2014-08-22
4. Intrinsic Defects in HPSI 6H-SiC: an EPR Study
Open this publication in new window or tab >>Intrinsic Defects in HPSI 6H-SiC: an EPR Study
2009 (English)In: Materials Science Forum, Vols. 600-603, Trans Tech Publications , 2009, 381-384 p.Conference paper, Published paper (Refereed)
Abstract [en]

High-purity, semi-insulating 6H-SiC substrates grown by high-temperature chemical vapor deposition were studied by electron paramagnetic resonance (EPR). The carbon vacancy (VC), the carbon vacancy-antisite pair (VCCSi) and the divacancy (VCVSi) were found to be prominent defects. The (+|0) level of VC in 6H-SiC is estimated by photoexcitation EPR (photo-EPR) to be at ~ 1.47 eV above the valence band. The thermal activation energies as determined from the temperature dependence of the resistivity, Ea~0.6-0.7 eV and ~1.0-1.2 eV, were observed for two sets of samples and were suggested to be related to acceptor levels of VC, VCCSi and VCVSi. The annealing behavior of the intrinsic defects and the stability of the SI properties were studied up to 1600°C.

Place, publisher, year, edition, pages
Trans Tech Publications, 2009
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-41959 (URN)10.4028/www.scientific.net/MSF.600-603.381 (DOI)59418 (Local ID)59418 (Archive number)59418 (OAI)
Conference
12th International Conference on Silicon Carbide and Related Materials, ICSCRM 2007; Otsu; Japan
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2014-09-12
5. Deep levels in low-energy electron-irradiated 4H-SiC
Open this publication in new window or tab >>Deep levels in low-energy electron-irradiated 4H-SiC
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2009 (English)In: PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, ISSN 1862-6254, Vol. 3, no 4, 121-123 p.Article in journal (Refereed) Published
Abstract [en]

Deep levels introduced by low-energy (200 keV) electron irradiation in n-type 4H-SiC epitaxial layers grown by chemical vapour deposition were studied by deep level transient spectroscopy (DLTS) and photoexcitation electron paramagnetic resonance (photo-EPR). After irradiation, several DLTS levels, EH1, EH3, Z(1/2), EH5 and EH6/7, often reported in irradiated 4H-SiC, were observed. In irradiated freestanding films from the same wafer, the EPR signals of the carbon vacancy in the positive and negative charge states, V-C(+) and V-C(-), respectively, can be observed simultaneously under illumination with light of certain photon energies. Comparing the ionization energies obtained from DLTS and photo-EPR, we suggest that the EH6/7 (at similar to E-C - 1.6 eV) and EH5 (at similar to E-C - 1.0 eV) electron traps may be related to the single donor (+ vertical bar 0) and the double acceptor (1- vertical bar 2-) level of V-C, respectively. Judging from the relative intensity of the DLTS signals, the EH6/7 level may also be contributed to by other unidentified defects.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-19391 (URN)10.1002/pssr.200903060 (DOI)
Available from: 2009-06-23 Created: 2009-06-22 Last updated: 2015-03-11
6. EPR and ab initio calculation study on the EI4 center in 4H and 6H-SiC
Open this publication in new window or tab >>EPR and ab initio calculation study on the EI4 center in 4H and 6H-SiC
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2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 23, 235203- p.Article in journal (Refereed) Published
Abstract [en]

We present new results from electron paramagnetic resonance (EPR) studies of the EI4 EPR center in 4H- and 6H-SiC. The EPR signal of the EI4 center was found to be drastically enhanced in electron-irradiated high-purity semi-insulating materials after annealing at 700-750°C. Strong EPR signals of the EI4 center with minimal interferences from other radiation-induced defects in irradiated high-purity semiinsulating materials allowed our more detailed study of the hyperfine (hf) structures. An additional large-splitting 29Si hf structure and 13C hf lines of the EI4 defect were observed. Comparing the data on the defect formation, the hf interactions and the annealing behavior obtained from EPR experiments and from ab initio supercell calculations of different carbon-vacancy related complexes, we suggest a complex between a carbon vacancy-carbon antisite and a carbon vacancy at the third neighbor site of the antisite in the neutral charge state, (VC-CSiVC)0, as a new defect model for the EI4 center.

Place, publisher, year, edition, pages
American Physical Society, 2010
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-56584 (URN)10.1103/PhysRevB.82.235203 (DOI)000286767600003 ()
Note
Original Publication: Patrick Carlsson, Tien Son Nguyen, A. Gali, J. Isoya, N. Morishita, T. Ohshima, B. Magnusson and Erik Janzen, EPR and ab initio calculation study on the EI4 center in 4H and 6H-SiC, 2010, Physical Review B Condensed Matter, (82), 23, 235203. http://dx.doi.org/10.1103/PhysRevB.82.235203 Copyright: American Physical Society http://www.aps.org/Available from: 2010-05-25 Created: 2010-05-25 Last updated: 2017-12-12
7. Silicon antisite related defects in electron-irradiated p-type 4H- and 6H-SiC
Open this publication in new window or tab >>Silicon antisite related defects in electron-irradiated p-type 4H- and 6H-SiC
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The electron paramagnetic resonance (EPR) LE5 centers were previously observed in electron-irradiated p-type 4H- and 6H-SiC but have not been identified due to lack of experimental data. In this study, two different Si hyperfine (hf) structures of the LE5 centers have been detected and the corresponding hf tensors have been determined. One structure is due to a very anisotropic hf interaction with one Si atom and the other structure to the hf interaction with two neighboring Si atoms in the basal plane. The obtained g values and Si hf constants are in good agreement with calculated parameters reported for antisite pairs in 4H-SiC. Based on the similarity in the spin-Hamiltonian parameters, the LE5 centers may be the antisite pairs in the positive charge state.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-56585 (URN)
Available from: 2010-05-25 Created: 2010-05-25 Last updated: 2010-05-25

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