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Electron Paramagnetic Resonance studies of negative-U centers in AlGaN and SiC
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Silicon (Si) is the most commonly used n-type dopant in AlGaN, but the conductivity of Si-doped AlxGa1-xN was often reported to drop abruptly at high Al content (x>0.7) and the reason was often speculated to be due to either compensation by deep levels or self-compensation of the so-called DX (or negative-U) center. Understanding the electronic structure of Si and carrier compensation processes is the essential for improving the n-type doping of high-Al-content AlxGa1-xN. In our studies of Si-doped AlGaN layers grown by metal-organic chemical vapor deposition, Electron Paramagnetic Resonance (EPR) was used to study the electronic structure of Si in high-Al-content AlxGa1-xN.

From the temperature dependence of the concentration of the Si donor on the neutral charge state Ed determined by EPR, we showed that Si already forms a stable DX center in AlxGa1-xN with x ~0.77. However, with the Fermi level locating only ~3 meV below Ed, Si still behaves as a shallow donor and high conductivity at room temperature could be achieved in Al0.77Ga0.23N:Si layers. In samples with the concentration of the residual oxygen (O) impurity larger than that of Si, we observed no carrier compensation by O in Al0.77Ga0.23N:Si layers, suggesting that at such Al content, O does not seem to hinder the n-type doping in the material. The result is presented in paper 1.

In paper 2, we determined the dependence of the EDX level of Si on the Al content in AlxGa1-xN:Si layers (0.79≤x≤1) with the Si concentration of ~2×1018 cm-3 and the concentrations of residual O and C impurities of about an order of magnitude lower (~1÷2×1017 cm-3). We found the coexistence of two DX centers (stable and metastable ones) of Si in AlxGa1-xN for x≥0.84. For the stable DX center, abruptly deepening of EDX with increasing of the Al content for x≥0.83 was observed, explaining the drastic decrease of the conductivity as often reported in previous transport studies. For the metastable DX center, the EDX level remains close to Ed for x=0.84÷1 (~11 meV for AlN).

The Z1/Z2 defect is the most common deep level revealed by Deep Level Transient Spectroscopy (DLTS) in 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD). It has previously been shown by DLTS to be a negative-U system which is more stable with capturing two electrons. The center is also known to be the lifetime killer in asgrown CVD material and, therefore, attracts much attention. Despite nearly two decades of intensive studies, including theoretical calculations and different experimental techniques, the origin of the Z1/Z2 center remains unclear. EPR is known to be a powerful method for defect identification, but a direct correlation between EPR and DLTS is difficult due to different requirements on samples for each technique. Using high n-type 4H-SiC CVD free-standing layers irradiated with lowenergy (250 keV) electrons, which mainly displace carbon atoms creating C vacancies, C interstitials and their associated defects, it was possible to increase the irradiation dose, allowing the application of EPR and DLTS on the same samples. Combining EPR, DLTS and supercell calculations, we identified the negatively charged carbon vacancy at the quasi-cubic (k) site and observed clear negative-U behaviors of the negative carbon vacancies at both hexagonal (h) and k sites. Our results showed that the Z1/Z2 center is related to the (2-|0) level of VC and its higher-lying levels Z1 and Z2 are related to the (-|0) levels of VC at the h and k sites, respectively. The result is presented in paper 3.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. , 28 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1697
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-112408DOI: 10.3384/lic.diva-112408ISBN: 978-91-7519-168-3 (print)OAI: oai:DiVA.org:liu-112408DiVA: diva2:765995
Presentation
2014-12-16, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2014-11-25 Created: 2014-11-25 Last updated: 2015-04-17Bibliographically approved
List of papers
1. Negative-U behavior of the Si donor in Al0.77Ga0.23N
Open this publication in new window or tab >>Negative-U behavior of the Si donor in Al0.77Ga0.23N
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2013 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, no 4, 042101- p.Article in journal (Refereed) Published
Abstract [en]

Electron paramagnetic resonance (EPR) spectrum of a shallow donor is observed at low temperatures in darkness in Si-doped Al0.77Ga0.23N epitaxial layers grown on 4H-SiC substrates. It is shown from the temperature dependence of the donor concentration on the neutral donor state measured by EPR that Si is a DX (or negative-U) center but behaves as a shallow donor due to a small separation of only ∼3 meV between the neutral state Ed and the lower-lying negative state EDX. The neutral state is found to follow the effective mass theory with Ed ∼ 52–59 meV.

Keyword
aluminium compounds, effective mass, gallium compounds, III-V semiconductors, impurity states, paramagnetic resonance, semiconductor epitaxial layers, silicon, wide band gap semiconductors
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-96762 (URN)10.1063/1.4816266 (DOI)000322406600040 ()
Available from: 2013-08-26 Created: 2013-08-26 Last updated: 2017-12-06
2. Stable and metastable Si negative-U centers in AlGaN and AlN
Open this publication in new window or tab >>Stable and metastable Si negative-U centers in AlGaN and AlN
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2014 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, no 16, 162106-1-162106-4 p.Article in journal (Refereed) Published
Abstract [en]

Electron paramagnetic resonance studies of Si-doped AlxGa1−xN (0.79 ≤ x ≤ 1.0) reveal two Si negative-U (or DX) centers, which can be separately observed for x ≥ 0.84. We found that for the stable DX center, the energy |EDX| of the negatively charged state DX, which is also considered as the donor activation energy, abruptly increases with Al content for x ∼ 0.83–1.0 approaching ∼240 meV in AlN, whereas EDX remains to be close to the neutral charge state Ed for the metastable DX center (∼11 meV below Ed in AlN).

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2014
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-112407 (URN)10.1063/1.4900409 (DOI)000344363000034 ()
Available from: 2014-11-25 Created: 2014-11-25 Last updated: 2017-12-05Bibliographically approved
3. Negative-U carbon vacancy in 4H-SiC: Assessment of charge correction schemes and identification of the negative carbon vacancy at the quasicubic site
Open this publication in new window or tab >>Negative-U carbon vacancy in 4H-SiC: Assessment of charge correction schemes and identification of the negative carbon vacancy at the quasicubic site
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2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 23, 235209-1-235209-13 p.Article in journal (Refereed) Published
Abstract [en]

The carbon vacancy (VC) has been suggested by different studies to be involved in the Z1/Z2 defect-a carrier lifetime killer in SiC. However, the correlation between the Z1/Z2 deep level with VC is not possible since only the negative carbon vacancy (V−C) at the hexagonal site, V−C(h), with unclear negative-U behaviors was identified by electron paramagnetic resonance (EPR). Using freestanding n-type 4H-SiC epilayers irradiated with low energy (250 keV) electrons at room temperature to introduce mainly VC and defects in the C sublattice, we observed the strong EPR signals of V−C(h) and another S = 1/2 center. Electron paramagnetic resonance experiments show a negative-U behavior of the two centers and their similar symmetry lowering from C3v to C1h at low temperatures. Comparing the 29Si and 13C ligand hyperfine constants observed by EPR and first principles calculations, the new center is identified as V−C(k). The negative-U behavior is further confirmed by large scale density functional theory supercell calculations using different charge correction schemes. The results support the identification of the lifetime limiting Z1/Z2 defect to be related to acceptor states of the carbon vacancy.

Place, publisher, year, edition, pages
American Physical Society, 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-103821 (URN)10.1103/PhysRevB.88.235209 (DOI)000331754500005 ()
Available from: 2014-01-28 Created: 2014-01-28 Last updated: 2017-12-06Bibliographically approved

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Trinh, Xuan Thang

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