| 1. |
- Kakanakova-Gueorguie, Anelia, et al.
(författare)
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Silicon and oxygen in high-Al-content AlGaN: incorporation kinetics and electron paramagnetic resonance study
- 2014
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Ingår i: Gettering and Defect Engineering in Semiconductor Technology XV. - : Trans Tech Publications Inc.. ; , s. 441-445
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Konferensbidrag (refereegranskat)abstract
- The high-Al-content AlxGa1-xN alloys, xgreater than0.70, and AlN is the fundamental wide-band-gap material system associated with the technology development of solid-state LEDs operating at the short wavelengths in the deep-UV (lambda less than 280 nm). Yet, their properties are insufficiently understood. The present study is intended to bring elucidation on the long-time debated and much speculated Si transition from shallow donor in GaN to a localized deep DX defect in AlxGa1-xN alloys with increasing Al content. For that purpose electron paramagnetic resonance is performed on a particular selection of high-Al-content epitaxial layers of Al0.77Ga0.23N, alternatively Al0.72Ga0.28N, alloy composition.
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| 2. |
- Kawahara, Koutarou, et al.
(författare)
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Quantitative comparison between Z1∕2 center and carbon vacancy in 4H-SiC
- 2014
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 115:14, s. 143705-
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Tidskriftsartikel (refereegranskat)abstract
- In this study, to reveal the origin of the Z(1/2) center, a lifetime killer in n-type 4H-SiC, the concentrations of the Z(1/2) center and point defects are compared in the same samples, using deep level transient spectroscopy (DLTS) and electron paramagnetic resonance (EPR). The Z(1/2) concentration in the samples is varied by irradiation with 250 keV electrons with various fluences. The concentration of a single carbon vacancy (V-C) measured by EPR under light illumination can well be explained with the Z(1/2) concentration derived from C-V and DLTS irrespective of the doping concentration and the electron fluence, indicating that the Z(1/2) center originates from a single V-C.
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| 3. |
- Nilsson, Daniel, et al.
(författare)
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Highly Si-doped Al0.72Ga0.28N layers: n-type conductivity bound by the process temperature
- 2014
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Establishing n- and p- type conductivity via intentional doping in epitaxial layers is fundamental to any semiconductor material system and its relevant device applications. Process parameters such as temperature, precursor gas-flow-rates and pressure may all control intentional doping in metal-organic chemical vapour deposition (MOCVD) of semiconductor materials. The incorporation of impurities such as carbon and oxygen may also be affected by the same process parameters, and the concentration of these impurities has a direct impact on the electrical, optical and thermal properties of epitaxial layers, as has been observed in the MOCVD of technologically-important III-V semiconductor materials such as AlGaAs and GaN.
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| 4. |
- Szállás, A., et al.
(författare)
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Characterization of the nitrogen split interstitial defect in wurtzite aluminum nitride using density functional theory
- 2014
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 116:11, s. 113702-1-113702-5
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Tidskriftsartikel (refereegranskat)abstract
- We carried out Heyd-Scuseria-Ernzerhof hybrid density functional theory plane wave supercell calculations in wurtzite aluminum nitride in order to characterize the geometry, formation energies, transition levels, and hyperfine tensors of the nitrogen split interstitial defect. The calculated hyperfine tensors may provide useful fingerprint of this defect for electron paramagnetic resonance measurement.
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| 5. |
- Szasz, K., et al.
(författare)
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Theoretical and electron paramagnetic resonance studies of hyperfine interaction in nitrogen doped 4H and 6H SiC
- 2014
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 115:7, s. 073705-
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Tidskriftsartikel (refereegranskat)abstract
- Motivated by recent experimental findings on the hyperfine signal of nitrogen donor (N-C) in 4H and 6H SiC, we calculate the hyperfine tensors within the framework of density functional theory. We find that there is negligible hyperfine coupling with Si-29 isotopes when NC resides at h site both in 4H and 6H SiC. We observe measurable hyperfine coupling to a single Si-29 at k site in 4H SiC and k(1) site in 6H SiC. Our calculations unravel that such Si-29 hyperfine coupling does not occur at k(2) site in 6H SiC. Our findings are well corroborated by our new electron paramagnetic resonance studies in nitrogen doped 6H SiC.
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| 6. |
- Trinh, Xuan Thang
(författare)
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Electron Paramagnetic Resonance studies of negative-U centers in AlGaN and SiC
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 7. |
- Trinh, Xuan Thang, et al.
(författare)
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Identification of the negative carbon vacancy at quasi-cubic site in 4H-SiC by EPR and theoretical calculations
- 2014
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Ingår i: Silicon Carbide and Related Materials 2013, PTS 1 AND 2. - : Trans Tech Publications Inc.. ; , s. 285-288
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Konferensbidrag (refereegranskat)abstract
- In freestanding n-type 4H-SiC epilayers irradiated with low-energy (250 keV) electrons at room temperature, the electron paramagnetic resonance (EPR) spectrum of the negative carbon vacancy at the hexagonal site, V-C(-)(h), and a new signal were observed. From the similarity in defect formation and the spin-Hamiltonian parameters of the two defects, the new center is suggested to be the negative C vacancy at the quasi-cubic site, V-C(-)(k). The identification is further supported by hyperfine calculations.
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| 8. |
- Trinh, Xuan Thang, et al.
(författare)
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Stable and metastable Si negative-U centers in AlGaN and AlN
- 2014
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 105:16, s. 162106-1-162106-4
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Tidskriftsartikel (refereegranskat)abstract
- 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).
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