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Deep levels in SiC
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Silicon carbide (SiC) has been discussed as a promising material for high power bipolar devices for almost twenty years. Advances in SiC crystal growth especially the development of chemical vapor deposition (CVD) have enabled the fabrication of high quality material. Much progress has further been achieved in identifying minority charge carrier lifetime limiting defects, which may be attributed to structural defects, surface recombination or point defects located in the band gap of SiC.

Deep levels can act as recombination centers by interacting with both the valence and conduction band. As such, the defect levels reduce the minority charge carrier lifetime, which is of great importance in bipolar devices.

Impurities in semiconductors play an important role to adjust their semiconducting properties. Intentional doping can introduce shallow defect levels to increase the conductivity or deep levels for achieving semi-insulating (SI) SiC. Impurities, especially transition metals generate defect levels deep in the band gap of SiC, which trap charge carriers and thus reduce the charge carrier lifetime. Transition metals, such as vanadium, are used in SiC to compensate the residual nitrogen doping.

It has previously been reported that valence band edges of the different SiC polytypes are pinned to the same level and that deep levels related to transition metals can serve as a common reference level; this is known as the LANGER-HEINRICH (LH) rule.

Electron irradiation introduces or enhances the concentration of existing point defects, such as the carbon vacancy (VC) and the carbon interstitial (Ci). Limiting the irradiation energy, Eirr, below the displacement energy of silicon in the SiC lattice (Eirr < 220 keV), the generated defects can be attributed to carbon related defects, which are already created at lower Eirr. Ci are mobile at low temperatures and using low temperature heat treatments, the annealing behavior of the introduced Ci and their complexes can be studied.

Deep levels, which appear and disappear depending on the electrical, thermal and optical conditions prior to the measurements are associated with metastable defects. These defects can exist in more than one configuration, which itself can have different charge states. Capacitance transient investigations, where the defect’s occupation is studied by varying the depletion region in a diode, can be used to observe such occupational changes. Such unstable behavior may influence device performance, since defects may be electrically active in one configuration and inactive after transformation to another configuration.

This thesis is focused on electrical characterization of deep levels in SiC using deep level transient spectroscopy (DLTS). The first part, papers 1-4, is dedicated to defect studies of both impurities and intrinsic defects in as-grown material. The second part, consisting of papers 5-7, is dealing with the defect content after electron irradiation and the annealing behavior of the introduced deep levels.

In the first part, transition metal incorporation of iron (Fe) and tungsten (W) is discussed in papers 1 and 2, respectively. Fe and W are possible candidates to compensate the residual nitrogen doping in SiC. The doping with Fe resulted in one level in n-type material and two levels in p-type 4H-SiC. The capture process is strongly coupled to the lattice. Secondary ion mass spectrometry measurements detected the presence of B and Fe. The defects are suggested to be related to Fe and/or Fe-B-pairs.

Previous reports on tungsten doping showed that W gives rise to two levels (one shallow and one deep) in 4H- and only one deep level in 6H-SiC. In 3C-SiC, we detected two levels, one likely related to W and one intrinsic defect, labeled E1. The W related energy level aligns well with the deeper levels observed in 4H- and 6H-SiC in agreement with the LH rule.

The LH rule is observed from experiments to be also valid for intrinsic levels. The level related to the DLTS peak EH6=7 in 4H-SiC aligns with the level related to E7 in 6H-SiC as well as with the level related to E1 in 3C-SiC. The alignment suggests that these levels may originate from the same defect, probably the VC, which has been proposed previously for 4H- and 6H-SiC.

In paper 3, electrical characterization of 3C-layers grown heteroepitaxially on different SiC substrates is discussed. The material was of high quality with a low background doping concentration and SCHOTTKY diodes were fabricated. It was observed that nickel as rectifying contact material exhibits a similar barrier height as the previously suggested gold. A leakage current in the low nA range at a reverse bias of -2 V was achieved, which allowed capacitance transient measurements. One defect related to DLTS peak E1, previously presented in paper 2, was detected and suggested to be related to an intrinsic defect.

Paper 4 gives the evidence that chloride-based CVD grown material yields the same kind of defects as reported for standard CVD growth processes. However, for very high growth rates, exceeding 100 mm/h, an additional defect is observed as well as an increase of the Ti-concentration. Based on the knowledge from paper 2, the origin of the additional peak and the assumed increase of Ti-concentration can instead both be attributed to the deeper and the shallower level of tungsten in 4H-SiC, respectively.

In the second part of the thesis, studies of low-energy (200 keV) electron irradiated as-grown 4H-SiC were performed. In paper 5, bistable defects, labeled EB-centers, evolved in the DLTS spectrum after the annihilation of the irradiation induced defect levels related to DLTS peaks EH1, EH3 and the bistable M-center. In a detailed annealing study presented in paper 6, the partial transformation of M-centers into the EB-centers is discussed. The transition between the two defects (M-centers → EB-centers) takes place at rather low temperatures (T ≈ 400 oC), which suggests a mobile defect as origin. The M-center and the EB-centers are suggested to be related to Ci and/or Ci complex defects. The EB-centers anneal out at about 700 oC.

In paper 7, the DLTS peak EH5, which is observed after low- and high-energy electron irradiation is presented. The peak is associated with a bistable defect, labeled F-center. Configuration A exists unoccupied and occupied by an electron, whereas configuration B is only stable when filled by an electron. Reconfiguration temperatures for both configurations were determined and the reconfiguration energies were calculated from the transition kinetics. The reconfiguration B→A can also be achieved by minority charge carrier injection. The F-center is likely a carbon related defect, since it is already present after low-energy irradiation.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press , 2011. , 50 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1388
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-70356ISBN: 978-91-7393-100-7 (print)OAI: oai:DiVA.org:liu-70356DiVA: diva2:438448
Public defence
2011-10-06, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 09:15 (English)
Opponent
Supervisors
Available from: 2011-09-02 Created: 2011-09-02 Last updated: 2015-09-22Bibliographically approved
List of papers
1. Deep levels in iron doped n- and p-type 4H-SiC
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2011 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 110, 123701-1-123701-5 p.Article in journal (Refereed) Published
Abstract [en]

Deep levels were detected in Fe-doped n- and p-type 4H-SiC using deep level transient spectroscopy (DLTS). One defect level (EC 0.39 eV) was detected in n-type material. DLTS spectra of p-type 4H-SiC show two dominant peaks (EV + 0.98 eV and EV + 1.46 eV). Secondary ion mass spectrometry measurements confirm the presence of Fe in both n- and p-type 4H-SiC epitaxial layers. The majority capture process for all the three Fe-related peaks is multi-phonon assisted. Similar defect behavior in Si indicates that the observed DLTS peaks are likely related to Fe and Fe-B pairs.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2011
Keyword
crystal microstructure, vacancies, defects, radiation effects, semiconductors
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-70352 (URN)10.1063/1.3669401 (DOI)000298639800044 ()
Note
funding agencies|Swedish Research Council (VR)||Swedish Energy Agency||Available from: 2011-09-02 Created: 2011-09-02 Last updated: 2017-12-08Bibliographically approved
2. Deep levels in tungsten doped n-type 3C-SiC
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2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 98, no 15, 152104- p.Article in journal (Refereed) Published
Abstract [en]

Tungsten was incorporated in SiC and W related defects were investigated using deep level transient spectroscopy. In agreement with literature, two levels related to W were detected in 4H-SiC, whereas only the deeper level was observed in 6H-SiC. The predicted energy level for W in 3C-SiC was observed (E-C-0.47 eV). Tungsten serves as a common reference level in SiC. The detected intrinsic levels align as well: E1 (E-C-0.57 eV) in 3C-SiC is proposed to have the same origin, likely V-C, as EH6/7 in 4H-SiC and E7 in 6H-SiC, respectively.

Place, publisher, year, edition, pages
American Institute of Physics, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-67976 (URN)10.1063/1.3579527 (DOI)000289580800030 ()
Note
Original Publication: Franziska Beyer, Carl Hemmingsson, Andreas Gällström, Stefano Leone, Henrik Pedersen, Anne Henry and Erik Janzén, Deep levels in tungsten doped n-type 3C-SiC, 2011, APPLIED PHYSICS LETTERS, (98), 15, 152104. http://dx.doi.org/10.1063/1.3579527 Copyright: American Institute of Physics http://www.aip.org/ Available from: 2011-05-04 Created: 2011-05-04 Last updated: 2017-12-11
3. Deep levels in hetero-epitaxial as-grown 3C-SiC
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2010 (English)In: AIP Conference Proceedings, Vol. 1292, 2010, 63-66 p.Conference paper, Published paper (Refereed)
Abstract [en]

3C-SiC grown hetero-epitaxially on 4H- or 6H-SiC using a standard or a chloride-based CVD process were electrically characterized using IV, CV and DLTS. The reverse leakage current of the Au-Schottky diodes was  reduced to lower than 10-8 A at -2V by a thermal oxidation step using UV-light illumination at 200oC. The Schottky barrier height of the Ni and Au contacts were determined by IV measurement to be ØB = 0.575  eV and ØB = 0.593 eV, respectively, for a contact diameter of about 150 mm. One dominant DLTS peak was observed in the 3C-epilayers independently of the substrate at about EC0:60 eV which is attributed to W6-level in 3C-SiC. This deep level is thought to be related to an intrinsic defect.

Keyword
Deep levels, 3C-SiC, hetero-epitaxial, chloride-based CVD
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-64425 (URN)10.1063/1.3518312 (DOI)978-073540847-0 (ISBN)
Conference
E-MRS Symposium F on 2010 Wide Bandgap Cubic Semiconductors: From Growth to Devices, 2010
Available from: 2011-01-24 Created: 2011-01-24 Last updated: 2015-09-22Bibliographically approved
4. Defects in 4H-SiC Layers Grown by Chloride-based Epitaxy
Open this publication in new window or tab >>Defects in 4H-SiC Layers Grown by Chloride-based Epitaxy
2009 (English)In: Materials Science Forum Vols. 615-617 / [ed] Amador Pérez-Tomás, Trans Tech Publications , 2009, 373- p.Conference paper, Published paper (Refereed)
Abstract [en]

Chloride-based 4H-SiC epitaxial layers were investigated by DLTS, MCTS and PL. The DLTS spectra of the as grown samples showed dominance of the Z1/2 and the EH6/7 peaks. For growth rates exceeding 100 µm/h, an additional peak occurred in the DLTS spectra which can be assigned to the UT1 defect. The shallow and the deep boron complexes as well as the HS1 defect are observed in MCTS measurements. The PL spectra are completely dominated by the near band gap (NBG) emission. No luminescence from donor-acceptor pair occurred. The PL line related to the D1 centre was weakly observed. In the NBG region nitrogen bound exciton (N-BE) and free exciton (FE) related lines could be seen. The addition of chlorine in the growth process gives the advantage of high growth rates without the introduction of additional defects.

Place, publisher, year, edition, pages
Trans Tech Publications, 2009
National Category
Natural Sciences Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-45290 (URN)10.4028/www.scientific.net/MSF.615-617.373 (DOI)80732 (Local ID)80732 (Archive number)80732 (OAI)
Conference
European Conference on Silicon Carbide and Related Materials, 7-11 September, Barcelona, Spain
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2015-03-11Bibliographically approved
5. Bistable defects in low-energy electron irradiated n-type 4H-SiC
Open this publication in new window or tab >>Bistable defects in low-energy electron irradiated n-type 4H-SiC
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2010 (English)In: PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, ISSN 1862-6254, Vol. 4, no 8-9, 227-229 p.Article in journal (Refereed) Published
Abstract [en]

Epitaxial n-type 4H-SiC layers were irradiated at room temperature by low-energy electrons. During the annihilation process of the irradiation induced defects EH I and EH3, three new bistable centers, labeled EB centers, were detected in the DLTS spectrum. The reconfigurations of the EB centers (I -andgt; II and II -andgt; I) take place at room temperature with a thermal reconfiguration energy of about 0.95 eV. The threshold energy for moving the Si atom from its site in the SiC crystal structure is higher than the applied irradiation energy; therefore, the EB centers are attributed to carbon related complex defects.

Place, publisher, year, edition, pages
John Wiley and Sons, Ltd, 2010
Keyword
crystal microstructure, vacancies, defects, radiation effects, semiconductors
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-60692 (URN)10.1002/pssr.201004249 (DOI)000282541400015 ()
Available from: 2010-11-01 Created: 2010-10-22 Last updated: 2015-09-22Bibliographically approved
6. Annealing behavior of the EB-centers and M-center in low-energy electron irradiated n-type 4H-SiC
Open this publication in new window or tab >>Annealing behavior of the EB-centers and M-center in low-energy electron irradiated n-type 4H-SiC
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2011 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 109, no 10, 103703- p.Article in journal (Refereed) Published
Abstract [en]

After low-energy electron irradiation of epitaxial n-type 4H-SiC with a dose of 5 x 10(16) cm(-2), the bistable M-center, previously reported in high-energy proton implanted 4H-SiC, is detected in the deep level transient spectroscopy (DLTS) spectrum. The annealing behavior of the M-center is confirmed, and an enhanced recombination process is suggested. The annihilation process is coincidental with the evolvement of the bistable EB-centers in the low temperature range of the DLTS spectrum. The annealing energy of the M-center is similar to the generation energy of the EB-centers, thus partial transformation of the M-center to the EB-centers is suggested. The EB-centers completely disappeared after annealing temperatures higher than 700 degrees C without the formation of new defects in the observed DLTS scanning range. The threshold energy for moving Si atom in SiC is higher than the applied irradiation energy, and the annihilation temperatures are relatively low, therefore the M-center, EH1 and EH3, as well as the EB-centers are attributed to defects related to the C atom in SiC, most probably to carbon interstitials and their complexes.

Place, publisher, year, edition, pages
American Institute of Physics, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-69907 (URN)10.1063/1.3586042 (DOI)000292115900079 ()
Note
Original Publication: Franziska Beyer, Carl Hemmingsson, Henrik Pedersen, Anne Henry, Erik Janzén, J. Isoya, N. Morishita and T. Ohshima, Annealing behavior of the EB-centers and M-center in low-energy electron irradiated n-type 4H-SiC, 2011, Journal of Applied Physics, (109), 10, 103703. http://dx.doi.org/10.1063/1.3586042 Copyright: American Institute of Physics http://www.aip.org/ Available from: 2011-08-09 Created: 2011-08-08 Last updated: 2017-12-08
7. Influence of background concentration induced field on the emission rate signatures of an electron trap in zinc oxide Schottky devices
Open this publication in new window or tab >>Influence of background concentration induced field on the emission rate signatures of an electron trap in zinc oxide Schottky devices
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2010 (English)In: JOURNAL OF APPLIED PHYSICS, ISSN 0021-8979, Vol. 107, no 10Article in journal (Refereed) Published
Abstract [en]

Various well-known research groups have reported points defects in bulk zinc oxide (ZnO) [N-D (intrinsic): 10(14)-10(17) cm(-3)] naming oxygen vacancy, zinc interstitial, and/or zinc antisite having activation energy in the range of 0.32-0.22 eV below conduction band. The attribution is probably based on activation energy of the level which seems not to be plausible in accordance with Vincent et al., [J. Appl. Phys. 50, 5484 (1979)] who suggested that it was necessary to become vigilant before interpreting the data attained for a carrier trap using capacitance transient measurement of diodes having ND greater than 10(15) cm(-3). Accordingly the influence of background free-carrier concentration, ND induced field on the emission rate signatures of an electron point defect in ZnO Schottky devices has been investigated by means of deep level transient spectroscopy. A number of theoretical models were tried to correlate with the experimental data to ascertain the mechanism. Consequently Poole-Frenkel model based on Coulomb potential was found consistent. Based on these investigations the electron trap was attributed to Zn-related charged impurity. Qualitative measurements like current-voltage and capacitance-voltage measurements were also performed to support the results.

Place, publisher, year, edition, pages
American Institute of Physics, 2010
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-57405 (URN)10.1063/1.3428426 (DOI)000278182400083 ()
Note
Original Publication: Hadia Noor, P Klason, Sadia Muniza Faraz, Omer Nour, Qamar Ul Wahab, Magnus Willander and M Asghar, Influence of background concentration induced field on the emission rate signatures of an electron trap in zinc oxide Schottky devices, 2010, JOURNAL OF APPLIED PHYSICS, (107), 10, 103717. http://dx.doi.org/10.1063/1.3428426 Copyright: American Institute of Physics http://www.aip.org/ Available from: 2010-06-18 Created: 2010-06-18 Last updated: 2014-01-15

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