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| 2. |
- Carlsson, Patrick, 1975-
(författare)
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Electron paramagnetic resonance study of defects in SiC
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 3. |
- Carlsson, Patrick, 1975-, et al.
(författare)
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Electron paramagnetic resonance study on n-type electron-irradiated 3C-SiC
- 2008
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Ingår i: PROCEEDINGS OF THE 17TH INTERNATIONAL VACUUM CONGRESS/13TH INTERNATIONAL CONFERENCE ON SURFACE SCIENCE/INTERNATIONAL CONFERENCE ON NANOSCIENCE AND TECHNOLOGY. - BRISTOL, ENGLAND : IOP PUBLISHING LTD.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 4. |
- Carlsson, Patrick, 1975-, et al.
(författare)
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EPR and ab initio calculation study on the EI4 center in 4H and 6H-SiC
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 82:23, s. 235203-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Carlsson, Patrick, 1975-, et al.
(författare)
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Intrinsic Defects in HPSI 6H-SiC : an EPR Study
- 2009
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Ingår i: Materials Science Forum, Vols. 600-603. - : Trans Tech Publications. ; , s. 381-384
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 6. |
- Carlsson, Patrick, 1975-, et al.
(författare)
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Photo-EPR Studies on Low-Energy Electron-irradiated 4H-SiC
- 2009
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Ingår i: Materials Science Forum, Vols. 615-617. - Materials Science Forum Vols. 615-617 : Trans Tech Publications. - 9780878493340 ; , s. 401-404
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Konferensbidrag (refereegranskat)abstract
- Photoexcitation electron paramagnetic resonance (photo-EPR) was used to determine deep levels related to the carbon vacancy (VC) in 4H-SiC. High-purity free-standing n-type 4H-SiC epilayers with concentration of intrinsic defects (except the photo-insensitive SI1 center) below the detection limit of EPR were irradiated with low-energy (200 keV) electrons to create mainly VC and defects related to the C sublattice. The simultaneous observation of and signals, their relative intensity changes and the absence of other defects in the sample provide a more straight and reliable interpretation of the photo-EPR results. The study suggests that the (+|0) level of VC is located at ~EC–1.77 eV in agreement with previously reported results and its single and double acceptor levels may be at ~ EC–0.8 eV and ~ EC–1.0 eV, respectively.
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| 7. |
- Carlsson, Patrick, 1975-, et al.
(författare)
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The EI4 EPR centre in 6H SiC
- 2010
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Ingår i: Physica Scripta, Vol. T141. - : IOP Publishing. ; , s. 014013-
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Konferensbidrag (refereegranskat)abstract
- We present the results of our recent electron paramagnetic resonance (EPR) studies of the EI4 EPR centre in electron-irradiated high-purity semi-insulating 6H SiC. Higher signal intensities and better resolution compared with previous studies have enabled a more detailed study of the hyperfine (hf) structure. Based on the observed hf structure due to the interaction with Si and C neighbours, the effective spin S = 1, the C-1h-symmetry and the annealing behaviour, we suggest a carbon vacancy-carbon antisite complex in the neutral charge state, VCVCCSi0, with the vacancies and the antisite in the basal plane, as a new defect model for the centre.
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| 8. |
- Gällström, Andreas, 1978-, et al.
(författare)
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Influence of Cooling Rate after High Temperature Annealing on Deep Levels in High-Purity Semi-Insulating 4H-SiC
- 2007
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Ingår i: Materials Science Forum, vol. 556-557. - : Trans Tech Publications. ; , s. 371-
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 9. |
- Hahn, S., et al.
(författare)
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Contact-Less Electrical Defect Characterization of Semi-Insulating 6H-SiC Bulk Material
- 2009
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Ingår i: Materials Science Forum Vols. 600-603. - : Trans Tech Publications. ; , s. 405-408
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Konferensbidrag (refereegranskat)abstract
- The novel technique microwave detected photo induced current transient spectroscopy (MD-PICTS) was applied to semi-insulating 6H-SiC in order to investigate the properties of inherent defect levels. Defect spectra can be obtained in the similar way to conventional PICTS and DLTS. However, there is no need for contacting the samples, which allows for non-destructive and spatially resolved electrical characterization. This work is focused on the investigation of semi-insulating 6H-SiC grown under different C/Si-ratios. In the corresponding MD-PICTS spectra several shallow defect levels appear in the low temperature range. However the peak assignment needs further investigation. Additionally different trap reemission dynamics are obtained for higher temperatures, which are supposed to be due to different compensation effects.
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| 10. |
- Janzén, Erik, 1954-, et al.
(författare)
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The Silicon vacancy in SiC
- 2009
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Konferensbidrag (refereegranskat)abstract
- A model is presented for the silicon vacancy in SiC. The previously reported photoluminescence spectra in 4H and 6H SiC attributed to the silicon vacancy are in this model due to internal transitions in the negative charge state of the silicon vacancy. The magnetic resonance signals observed are due to the initial and final states of these transitions.
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