| 1. |
- Nilsson, Daniel, et al.
(författare)
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On the behavior of the silicon donor in conductive AlxGa1-xN (0.63≤x≤1) layers
- 2015
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Ingår i: Physica status solidi. B, Basic research. - : Wiley-VCH Verlagsgesellschaft. - 0370-1972 .- 1521-3951. ; 252:6, s. 1306-1310
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the silicon donor behavior in intentionally silicon doped AlxGa1-xN (0.63≤x≤1) grown by hot-wall metal-organic chemical vapor deposition. Efficient silicon doping was obtained for lower Al contents whereas the conductivity drastically reduces for AlGaN layers with Al content in the range x~0.84-1. Degradation of the structural quality and compensation by residual O and C impurities were ruled out as possible explanations for the reduced conductivity. By combining frequency dependent capacitance-voltage and electron paramagnetic resonance measurements we show that the Si donors are electrically active and that the reduced conductivity can be explained by the increased activation energy caused by the sharp deepening of the Si DX– state..
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| 2. |
- Trinh, Thang, et al.
(författare)
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Background risk of breast cancer and the association between physical activity and mammographic density
- 2015
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Ingår i: Breast Cancer Research. - : BIOMED CENTRAL LTD. - 1465-5411 .- 1465-542X. ; 17
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Tidskriftsartikel (refereegranskat)abstract
- Introduction High physical activity has been shown to decrease the risk of breast cancer, potentially by a mechanism that also reduces mammographic density. We tested the hypothesis that the risk of developing breast cancer in the next 10 years according to the Tyrer-Cuzick prediction model influences the association between physical activity and mammographic density. Methods We conducted a population-based cross-sectional study of 38,913 Swedish women aged 4074 years. Physical activity was assessed using the validated web-questionnaire Active-Q and mammographic density was measured by the fully automated volumetric Volpara method. The 10-year risk of breast cancer was estimated using the Tyrer-Cuzick (TC) prediction model. Linear regression analyses were performed to assess the association between physical activity and volumetric mammographic density and the potential interaction with the TC breast cancer risk. Results Overall, high physical activity was associated with lower absolute dense volume. As compared to women with the lowest total activity level (<40 metabolic equivalent hours [MET-h] per day), women with the highest total activity level (>= 50 MET-h/day) had an estimated 3.4 cm(3) (95% confidence interval, 2.3-4.7) lower absolute dense volume. The inverse association was seen for any type of physical activity among women with <3.0% TC 10-year risk, but only for total and vigorous activities among women with 3.0-4.9% TC risk, and only for vigorous activity among women with >= 5.0% TC risk. The association between total activity and absolute dense volume was modified by the TC breast cancer risk (P-interaction = 0.05). As anticipated, high physical activity was also associated with lower non-dense volume. No consistent association was found between physical activity and percent dense volume. Conclusions Our results suggest that physical activity may decrease breast cancer risk through reducing mammographic density, and that the physical activity needed to reduce mammographic density may depend on background risk of breast cancer.
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| 3. |
- Trinh, Thang
(författare)
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Determinants of breast cancer risk : focusing on mammographic density
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Breast cancer is the most common cancer and also one of the leading causes of cancer death among women worldwide. Since most known factors associated with breast cancer risk are difficult to influence, the potential of lifestyle factors, which are modifiable, in breast cancer prevention has recently been emphasised. Studies have shown a reduced risk of breast cancer among women who are more physically active, and an increased risk among women with higher alcohol consumption or cigarette smoking. These lifestyle factors have been hypothesised to influence breast cancer risk through a mechanism that involves mammographic density, one of the strongest risk factors of the disease. Moreover, whether such associations might be modified by the women’s background risk of breast cancer is unclear. We therefore used data from the KARMA (KARolinska MAmmography) study to investigate the potential influence of background breast cancer risk on the association between physical activity (Study I, n = 38,913), alcohol consumption (Study II, n = 53,060), and cigarette smoking (Study III, n = 53,728) and mammographic density. These lifestyle factors were assessed using self-administrated web-based questionnaires. Mammographic density was estimated using the fully-automated volumetric Volpara method and expressed as absolute dense volume, non-dense volume, and per cent dense volume. The Tyrer-Cuzick (TC) prediction model was used to estimate the individual background risk of developing breast cancer in the next 10 years. In Study I, higher levels of physical activity were associated with a lower absolute dense breast volume and non-dense (adipose) breast volume, but a higher per cent dense breast volume among all women. After taking the TC 10-year risk of breast cancer into consideration, an association with lower absolute density was seen for all types of physical activity among women at low (< 3.0%) TC risk, for total and vigorous activities among women at moderate (3.0-4.9%) TC risk, and only for vigorous activity among women at high (≥ 5.0%) TC risk. In Study II, among all women we found an overall association between alcohol consumption and absolute and per cent dense breast volumes. Furthermore, alcohol consumption was only associated with a higher absolute dense volume among high-risk women. In Study III, current cigarette smoking was associated with lower absolute and per cent dense volumes, and this association was not found to be modified by TC 10-year background breast cancer risk. However, with respect to breast cancer risk, this finding should be viewed in light of the carcinogenic effects of cigarette smoking. In the last study, we used prospective cohort data of 58,441 Swedish women of whom 522 developed invasive breast cancer. Overall, women with higher alcohol consumption had an increased risk of breast cancer compared to those with no alcohol consumption. After taking the TC background 10-year risk of breast cancer into account, alcohol consumption was only associated with breast cancer risk among women at moderate background risk.
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| 4. |
- Trinh, Xuan Thang, et al.
(författare)
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Electron paramagnetic resonance studies of carbon interstitial related defects in 4H-SiC
- 2015
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In n-type 4H-SiC grown by chemical vapor deposition and irradiated by low-energy (250 keV) electrons, an electron paramagnetic resonance center, labeled EI8a, was observed at room temperature. A short anneal at temperatures in the range of 300-500 °C in darkness changes EI8a to a new center, labeled EI8b, which can be converted back by illumination at room temperature. We show that EI8a and EI8b are the two different configurations of the same defect, labeled EI8, with C1h symmetry and an electron spin S=1/2. The EI8 center is stable up to ~650 °C and annealed out at ~800 °C. Based on the observed hyperfine structures due to the hyperfine interaction between the electron spin and the nuclear spins of four 29Si atoms and three 13C atoms, the EI8 center is suggested to be related to a carbon interstitial cluster.
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| 5. |
- Trinh, Xuan Thang
(författare)
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Electron Paramagnetic Resonance Studies of Point Defects in AlGaN and SiC
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Point defects in semiconductor materials are known to have important influence on the performance of electronic devices. For defect control, knowledge on the model of defects and their properties is required. Information on defects, such as the symmetry and the localization of spins, is essential for identification of defects and understanding their electronic structure. Such information can be obtained from Electron Paramagnetic Resonance (EPR). In many cases, the energy levels of defects can be determined from photoexcitation EPR (photo-EPR) or temperature dependence of the EPR signal. The thesis contains six papers, focusing on the identification and electronic structure investigation of defects and impurities in AlxGa1-xN (x~0.7-1) and silicon carbide (SiC) using EPR in combination with other electrical characterizations and density functional theory calculations.The two first papers concern EPR studies of silicon (Si) in AlGaN alloys. Due to its direct and wide band gap which can be tailored from 3.4 eV for GaN to 6.2 eV for AlN, high-Al-content wurtzite AlxGa1-xN (x≥0.7) has been considered as a promising material for fabrication of compact, high-efficiency and non-toxic deep ultraviolet light-emitting diodes (LEDs) and laser diodes (LDs) for replacing low-efficiency and toxic mercury lamps in water/air purification and sterilization. Si is commonly used for n-type doping in AlGaN and AlN, 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 carrier compensation by other deep levels or Si itself when it transforms from a shallow donor to a DX (or negative-U) center which acts as an acceptor. In paper 1, 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 the neutral charge state, Ed, Si still behaves as a shallow donor. Negligible carrier compensation by oxygen (O) in Al0.77Ga0.23N:Si layers was observed, suggesting that at such Al content, O does not seem to hinder the n-type doping in the material. In paper 2, we found the coexistence of two Si DX centers, the stable DX1 and the metastable DX2, in AlxGa1-xN for x≥0.84. For the stable DX1 center, abrupt deepening of the energy level of the negative charge state DX–, EDX, which determines the ionization energy Ea of the Si donor, with increasing of the Al content for x≥0.83 was observed. The dependence of Ea on the Al content in AlxGa1-xN:Si layers (0.79≤x≤1) was determined. The results explain the drastic decrease of the conductivity as often reported for AlxGa1-xN:Si in previous transport studies. For the metastable DX2 center, we found that the EDX level remains close to Ed for x=0.84÷1.SiC is a wide band-gap semiconductor having high-thermal conductivity, high breakdown field, and large saturated electron drift velocity which are essential properties for high-voltage and high-power devices. In paper 3, the identification of niobium (Nb) in 4Hand 6H-SiC grown by high-temperature chemical vapor deposition (CVD) by EPR and theoretical calculations is presented. We showed that the incorporated Nb formed asymmetric split-vacancy defect (NbSiVC) in which Nb locates in a divacancy, closer to the Si vacancy, and prefers only the hexagonal-hexagonal configuration. In papers 4 and 5, we present the identification and the electronic structure of the negative-U Z1/Z2 center in 4HSiC. The Z1/Z2 defect is known to be the most common deep level revealed by Deep Level Transient Spectroscopy (DLTS) in 4H-SiC epitaxial layers grown by CVD. The center is also known to be the lifetime killer in as-grown CVD material and, therefore, attracts much attention. Using high-doped n-type free-standing 4H-SiC layers irradiated with low-energy (250 keV) electrons, which mainly displace carbon atoms creating C vacancies (VC), C interstitials and their associated defects, it was possible to increase the irradiation dose and, hence, the defect concentration, allowing the application of EPR and DLTS on the same samples. In paper 4, using EPR, photo-EPR, DLTS and capacitance-voltage measurements, we 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 hexagonal (h) and quasi-cubic (k) sites, respectively. In paper 5, combining EPR and supercell calculations, the negatively charged VC at the k-site was identified. We obtained the excellent agreement in the energy levels of Z1/Z2 determined by DLTS and energy levels of VC calculated by supercell calculations and observed clear negative-U behaviors of the negatively charged VC at both k and h-sites by EPR measurements, consolidating our assignment of the Z1/Z2 levels to the negatively charged states of VC. In paper 6, we studied a defect related to displaced C atoms in n-type 4H-SiC irradiated by low-energy electrons. In irradiated layers, we observed an EPR center at room temperature. After annealing at temperatures in the range of 300-500 °C, this center transforms to a second configuration which is observed in darkness and can be changed back to the first configuration under illumination. Based on the observed 29Si and 13C hyperfine structures, two observed configurations of the EPR center were suggested to be related to different configurations of a carbon interstitial cluster. The annealing, bistable behaviors and energy levels of this EPR center are discussed.
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| 6. |
- Zeglio, Erica, et al.
(författare)
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Conjugated Polyelectrolyte Blends for Electrochromic and Electrochemical Transistor Devices
- 2015
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Ingår i: Chemistry of Materials. - : AMER CHEMICAL SOC. - 0897-4756 .- 1520-5002. ; 27:18, s. 6385-6393
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Tidskriftsartikel (refereegranskat)abstract
- Two self-doped conjugated polyelectrolytes, having semiconducting and metallic behaviors, respectively, have been blended from aqueous solutions in order to produce materials with enhanced optical and electrical properties. The intimate blend of two anionic conjugated polyelectrolytes combine the electrical and optical properties of these, and can be tuned by blend stoichiometry. In situ conductance measurements have been done during doping of the blends, while UV vis and EPR spectroelectrochemistry allowed the study of the nature of the involved redox species. We have constructed an accumulation/depletion mode organic electrochemical transistor whose characteristics can be tuned by balancing the stoichiometry of the active material.
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