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2.
  • Aartsen, M. G., et al. (author)
  • SEARCHES FOR TIME-DEPENDENT NEUTRINO SOURCES WITH ICECUBE DATA FROM 2008 TO 2012
  • 2015
  • In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 807:1
  • Journal article (peer-reviewed)abstract
    • In this paper searches for flaring astrophysical neutrino sources and sources with periodic emission with the IceCube neutrino telescope are presented. In contrast to time-integrated searches, where steady emission is assumed, the analyses presented here look for a time-dependent signal of neutrinos using the information from the neutrino arrival times to enhance the discovery potential. A search was performed for correlations between neutrino arrival times and directions, as well as neutrino emission following time-dependent light curves, sporadic emission, or periodicities of candidate sources. These include active galactic nuclei, soft gamma-ray repeaters, supernova remnants hosting pulsars, microquasars, and X-ray binaries. The work presented here updates and extends previously published results to a longer period that covers 4 years. of data from 2008 April 5 to 2012 May 16, including the first year of operation of the completed 86 string detector. The analyses did not find any significant time-dependent point sources of neutrinos, and the results were used to set upper limits on the neutrino flux from source candidates.
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3.
  • Aartsen, M. G., et al. (author)
  • Atmospheric and astrophysical neutrinos above 1 TeV interacting in IceCube
  • 2015
  • In: Physical Review D. - 1550-7998 .- 1550-2368. ; 91:2, s. 022001-
  • Journal article (peer-reviewed)abstract
    • The IceCube Neutrino Observatory was designed primarily to search for high-energy (TeV-PeV) neutLrinos produced in distant astrophysical objects. A search for. greater than or similar to 100 TeV neutrinos interacting inside the instrumented volume has recently provided evidence for an isotropic flux of such neutrinos. At lower energies, IceCube collects large numbers of neutrinos from the weak decays of mesons in cosmic-ray air showers. Here we present the results of a search for neutrino interactions inside IceCube's instrumented volume between 1 TeV and 1 PeV in 641 days of data taken from 2010-2012, lowering the energy threshold for neutrinos from the southern sky below 10 TeV for the first time, far below the threshold of the previous high-energy analysis. Astrophysical neutrinos remain the dominant component in the southern sky down to a deposited energy of 10 TeV. From these data we derive new constraints on the diffuse astrophysical neutrino spectrum, Phi(v) = 2.06(-0.3)(+0.4) x 10(-18) (E-v = 10(5) GeV)-2.46 +/- 0.12GeV-1 cm(-2) sr(-1) s(-1) for 25 TeV < E-v < 1.4 PeV, as well as the strongest upper limit yet on the flux of neutrinos from charmed-meson decay in the atmosphere, 1.52 times the benchmark theoretical prediction used in previous IceCube results at 90% confidence.
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4.
  • Aartsen, M. G., et al. (author)
  • Determining neutrino oscillation parameters from atmospheric muon neutrino disappearance with three years of IceCube DeepCore data
  • 2015
  • In: Physical Review D. - 1550-7998 .- 1550-2368. ; 91:7
  • Journal article (peer-reviewed)abstract
    • We present a measurement of neutrino oscillations via atmospheric muon neutrino disappearance with three years of data of the completed IceCube neutrino detector. DeepCore, a region of denser IceCube instrumentation, enables the detection and reconstruction of atmospheric muon neutrinos between 10 and 100 GeV, where a strong disappearance signal is expected. The IceCube detector volume surrounding DeepCore is used as a veto region to suppress the atmospheric muon background. Neutrino events are selected where the detected Cherenkov photons of the secondary particles minimally scatter, and the neutrino energy and arrival direction are reconstructed. Both variables are used to obtain the neutrino oscillation parameters from the data, with the best fit given by Delta m(32)(2) = 2.72(-0.20)(+0.19) x 10(-3) eV(2) and sin(2)theta(23) = 0.53(-0.12)(+0.09) (normal mass ordering assumed). The results are compatible, and comparable in precision, to those of dedicated oscillation experiments.
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5.
  • Aartsen, M. G., et al. (author)
  • Development of a general analysis and unfolding scheme and its application to measure the energy spectrum of atmospheric neutrinos with IceCube
  • 2015
  • In: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 75:3
  • Journal article (peer-reviewed)abstract
    • We present the development and application of a generic analysis scheme for the measurement of neutrino spectra with the IceCube detector. This scheme is based on regularized unfolding, preceded by an event selection which uses a Minimum Redundancy Maximum Relevance algorithm to select the relevant variables and a random forest for the classification of events. The analysis has been developed using IceCube data from the 59-string configuration of the detector. 27,771 neutrino candidates were detected in 346 days of livetime. A rejection of 99.9999 % of the atmospheric muon background is achieved. The energy spectrum of the atmospheric neutrino flux is obtained using the TRUEE unfolding program. The unfolded spectrum of atmospheric muon neutrinos covers an energy range from 100 GeV to 1 PeV. Compared to the previous measurement using the detector in the 40-string configuration, the analysis presented here, extends the upper end of the atmospheric neutrino spectrum by more than a factor of two, reaching an energy region that has not been previously accessed by spectral measurements.
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6.
  • Aartsen, M. G., et al. (author)
  • Energy reconstruction methods in the IceCube neutrino telescope
  • 2014
  • In: Journal of Instrumentation. - 1748-0221. ; 9, s. P03009-
  • Journal article (peer-reviewed)abstract
    • Accurate measurement of neutrino energies is essential to many of the scientific goals of large-volume neutrino telescopes. The fundamental observable in such detectors is the Cherenkov light produced by the transit through a medium of charged particles created in neutrino interactions. The amount of light emitted is proportional to the deposited energy, which is approximately equal to the neutrino energy for v(e) and v(mu) charged-current interactions and can be used to set a lower bound on neutrino energies and to measure neutrino spectra statistically in other channels. Here we describe methods and performance of reconstructing charged-particle energies and topologies from the observed Cherenkov light yield, including techniques to measure the energies of uncontained muon tracks, achieving average uncertainties in electromagnetic-equivalent deposited energy of similar to 15% above 10 TeV.
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7.
  • Aartsen, M. G., et al. (author)
  • Flavor Ratio of Astrophysical Neutrinos above 35 TeV in IceCube
  • 2015
  • In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 114:17
  • Journal article (peer-reviewed)abstract
    • A diffuse flux of astrophysical neutrinos above 100 TeV has been observed at the IceCube Neutrino Observatory. Here we extend this analysis to probe the astrophysical flux down to 35 TeV and analyze its flavor composition by classifying events as showers or tracks. Taking advantage of lower atmospheric backgrounds for showerlike events, we obtain a shower-biased sample containing 129 showers and 8 tracks collected in three years from 2010 to 2013. We demonstrate consistency with the (f(e) : f(mu) : f(tau))(circle plus) approximate to (1 : 1 : 1)(circle plus) flavor ratio at Earth commonly expected from the averaged oscillations of neutrinos produced by pion decay in distant astrophysical sources. Limits are placed on nonstandard flavor compositions that cannot be produced by averaged neutrino oscillations but could arise in exotic physics scenarios. A maximally tracklike composition of (0 : 1 : 0)(circle plus) is excluded at 3.3 sigma, and a purely showerlike composition of (1 : 0 : 0)(circle plus) is excluded at 2.3 sigma.
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8.
  • Aartsen, M. G., et al. (author)
  • Multipole analysis of IceCube data to search for dark matter accumulated in the Galactic halo
  • 2015
  • In: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 75:1
  • Journal article (peer-reviewed)abstract
    • Dark matter which is bound in the Galactic halo might self-annihilate and produce a flux of stable final state particles, e. g. high energy neutrinos. These neutrinos can be detected with IceCube, a cubic-kilometer sized Cherenkov detector. Given IceCube's large field of view, a characteristic anisotropy of the additional neutrino flux is expected. In this paper we describe a multipole method to search for such a large-scale anisotropy in IceCube data. This method uses the expansion coefficients of a multipole expansion of neutrino arrival directions and incorporates signal-specific weights for each expansion coefficient. We apply the technique to a high-purity muon neutrino sample from the Northern Hemisphere. The final result is compatible with the null-hypothesis. As no signal was observed, we present limits on the self-annihilation cross-section averaged over the relative velocity distribution down to 1.9x10(-23) cm(3) s(-1) for a dark matter particle mass of 700-1,000 GeV and direct annihilation into nu(nu) over bar. The resulting exclusion limits come close to exclusion limits from gamma-ray experiments, that focus on the outer Galactic halo, for high dark matter masses of a few TeV and hard annihilation channels.
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9.
  • Aartsen, M. G., et al. (author)
  • Observation of High-Energy Astrophysical Neutrinos in Three Years of IceCube Data
  • 2014
  • In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 113:10, s. 101101-
  • Journal article (peer-reviewed)abstract
    • A search for high-energy neutrinos interacting within the IceCube detector between 2010 and 2012 provided the first evidence for a high-energy neutrino flux of extraterrestrial origin. Results from an analysis using the same methods with a third year (2012-2013) of data from the complete IceCube detector are consistent with the previously reported astrophysical flux in the 100 TeV-PeV range at the level of 10(-8) GeV cm(-2) s(-1) sr(-1) per flavor and reject a purely atmospheric explanation for the combined three-year data at 5.7 sigma. The data are consistent with expectations for equal fluxes of all three neutrino flavors and with isotropic arrival directions, suggesting either numerous or spatially extended sources. The three-year data set, with a live time of 988 days, contains a total of 37 neutrino candidate events with deposited energies ranging from 30 to 2000 TeV. The 2000-TeV event is the highest-energy neutrino interaction ever observed.
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10.
  • Aartsen, M. G., et al. (author)
  • Search for a diffuse flux of astrophysical muon neutrinos with the IceCube 59-string configuration
  • 2014
  • In: Physical Review D. - 1550-7998 .- 1550-2368. ; 89:6, s. 062007-
  • Journal article (peer-reviewed)abstract
    • A search for high-energy neutrinos was performed using data collected by the IceCube Neutrino Observatory from May 2009 to May 2010, when the array was running in its 59-string configuration. The data sample was optimized to contain muon neutrino induced events with a background contamination of atmospheric muons of less than 1%. These data, which are dominated by atmospheric neutrinos, are analyzed with a global likelihood fit to search for possible contributions of prompt atmospheric and astrophysical neutrinos, neither of which have yet been identified. Such signals are expected to follow a harder energy spectrum than conventional atmospheric neutrinos. In addition, the zenith angle distribution differs for astrophysical and atmospheric signals. A global fit of the reconstructed energies and directions of observed events is performed, including possible neutrino flux contributions for an astrophysical signal and atmospheric backgrounds as well as systematic uncertainties of the experiment and theoretical predictions. The best fit yields an astrophysical signal flux for nu(mu) + (nu) over bar (mu) of E-2. Phi(E) = 0.25 x 10(-8) GeV cm(-2) s(-1) sr(-1), and a zero prompt component. Although the sensitivity of this analysis for astrophysical neutrinos surpasses the Waxman and Bahcall upper bound, the experimental limit at 90% confidence level is a factor of 1.5 above at a flux of E-2 . Phi(E) = 1.44 x 10(-8) GeV cm(-2) s(-1) sr(-1).
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11.
  • Aartsen, M. G., et al. (author)
  • Search for neutrino-induced particle showers with IceCube-40
  • 2014
  • In: Physical Review D. - 1550-7998 .- 1550-2368. ; 89:10, s. 102001-
  • Journal article (peer-reviewed)abstract
    • We report on the search for neutrino-induced particle showers, so-called cascades, in the IceCube-40 detector. The data for this search were collected between April 2008 and May 2009 when the first 40 IceCube strings were deployed and operational. Three complementary searches were performed, each optimized for different energy regimes. The analysis with the lowest energy threshold (2 TeV) targeted atmospheric neutrinos. A total of 67 events were found, consistent with the expectation of 41 atmospheric muons and 30 atmospheric neutrino events. The two other analyses targeted a harder, astrophysical neutrino flux. The analysis with an intermediate threshold of 25 TeV leads to the observation of 14 cascadelike events, again consistent with the prediction of 3.0 atmospheric neutrino and 7.7 atmospheric muon events. We hence set an upper limit of E-2 Phi(lim) <= 7.46 x 10(-8) GeV sr(-1) s(-1) cm(-2) (90% C.L.) on the diffuse flux from astrophysical neutrinos of all neutrino flavors, applicable to the energy range 25 TeV to 5 PeV, assuming an E-nu(-2) spectrum and a neutrino flavor ratio of 1: 1: 1 at the Earth. The third analysis utilized a larger and optimized sample of atmospheric muon background simulation, leading to a higher energy threshold of 100 TeV. Three events were found over a background prediction of 0.04 atmospheric muon events and 0.21 events from the flux of conventional and prompt atmospheric neutrinos. Including systematic errors this corresponds to a 2.7 sigma excess with respect to the background-only hypothesis. Our observation of neutrino event candidates above 100 TeV complements IceCube's recently observed evidence for high-energy astrophysical neutrinos.
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12.
  • Aartsen, M. G., et al. (author)
  • Search for non-relativistic magnetic monopoles with IceCube
  • 2014
  • In: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 74:7, s. 2938-
  • Journal article (peer-reviewed)abstract
    • The IceCube Neutrino Observatory is a large Cherenkov detector instrumenting of Antarctic ice. The detector can be used to search for signatures of particle physics beyond the Standard Model. Here, we describe the search for non-relativistic, magnetic monopoles as remnants of the Grand Unified Theory (GUT) era shortly after the Big Bang. Depending on the underlying gauge group these monopoles may catalyze the decay of nucleons via the Rubakov-Callan effect with a cross section suggested to be in the range of to . In IceCube, the Cherenkov light from nucleon decays along the monopole trajectory would produce a characteristic hit pattern. This paper presents the results of an analysis of first data taken from May 2011 until May 2012 with a dedicated slow-particle trigger for DeepCore, a subdetector of IceCube. A second analysis provides better sensitivity for the brightest non-relativistic monopoles using data taken from May 2009 until May 2010. In both analyses no monopole signal was observed. For catalysis cross sections of the flux of non-relativistic GUT monopoles is constrained up to a level of at a 90 % confidence level, which is three orders of magnitude below the Parker bound. The limits assume a dominant decay of the proton into a positron and a neutral pion. These results improve the current best experimental limits by one to two orders of magnitude, for a wide range of assumed speeds and catalysis cross sections.
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13.
  • Aartsen, M. G., et al. (author)
  • Search for Prompt Neutrino Emission from Gamma-Ray Bursts with IceCube
  • 2015
  • In: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 805:1
  • Journal article (peer-reviewed)abstract
    • We present constraints derived from a search of four years of IceCube data for a prompt neutrino flux from gammaray bursts (GRBs). A single low-significance neutrino, compatible with the atmospheric neutrino background, was found in coincidence with one of the 506 observed bursts. Although GRBs have been proposed as candidate sources for ultra-high-energy cosmic rays, our limits on the neutrino flux disfavor much of the parameter space for the latest models. We also find that no more than similar to 1% of the recently observed astrophysical neutrino flux consists of prompt emission from GRBs that are potentially observable by existing satellites.
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14.
  • Aartsen, M. G., et al. (author)
  • SEARCHES FOR EXTENDED AND POINT-LIKE NEUTRINO SOURCES WITH FOUR YEARS OF ICECUBE DATA
  • 2014
  • In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 796:2, s. 109-
  • Journal article (peer-reviewed)abstract
    • We present results on searches for point-like sources of neutrinos using four years of IceCube data, including the first year of data from the completed 86 string detector. The total livetime of the combined data set is 1373 days. For an E-2 spectrum, the observed 90% C. L. flux upper limits are similar to 10(-12) TeV-1 cm(-2) s(-1) for energies between 1 TeV and 1 PeV in the northern sky and similar to 10(-11) TeV-1 cm(-2) s(-1) for energies between 100 TeV and 100 PeV in the southern sky. This represents a 40% improvement compared to previous publications, resulting from both the additional year of data and the introduction of improved reconstructions. In addition, we present the first results from an all-sky search for extended sources of neutrinos. We update the results of searches for neutrino emission from stacked catalogs of sources and test five new catalogs; two of Galactic supernova remnants and three of active galactic nuclei. In all cases, the data are compatible with the background-only hypothesis, and upper limits on the flux of muon neutrinos are reported for the sources considered.
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15.
  • Aartsen, M. G., et al. (author)
  • Searches for small-scale anisotropies from neutrino point sources with three years of IceCube data
  • 2015
  • In: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 66, s. 39-52
  • Journal article (peer-reviewed)abstract
    • Recently, IceCube found evidence for a diffuse signal of astrophysical neutrinos in an energy range of similar to 60 TeV to the PeV-scale [1]. The origin of those events, being a key to understanding the origin of cosmic rays, is still an unsolved question. So far, analyses have not succeeded to resolve the diffuse signal into point-like sources. Searches including a maximum-likelihood-ratio test, based on the reconstructed directions and energies of the detected down- and up-going neutrino candidates, were also performed on IceCube data leading to the exclusion of bright point sources. In this paper, we present two methods to search for faint neutrino point sources in three years of IceCube data, taken between 2008 and 2011. The first method is an autocorrelation test, applied separately to the northern and southern sky. The second method is a multipole analysis, which expands the measured data in the northern hemisphere into spherical harmonics and uses the resulting expansion coefficients to separate signal from background. With both methods, the results are consistent with the background expectation with a slightly more sparse spatial distribution, corresponding to an underfluctuation. Depending on the assumed number of sources, the resulting upper limit on the flux per source in the northern hemisphere for an E-2 energy spectrum ranges from similar to 1.5. 10(-8) GeV/cm(2) s(-1), in the case of one assumed source, to similar to 4. 10(-10) GeV/cm(2) s(-1), in the case of 3500 assumed sources.
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16.
  • Aartsen, M. G., et al. (author)
  • Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector
  • 2013
  • In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 342:6161, s. 947-
  • Journal article (peer-reviewed)abstract
    • We report on results of an all-sky search for high-energy neutrino events interacting within the IceCube neutrino detector conducted between May 2010 and May 2012. The search follows up on the previous detection of two PeV neutrino events, with improved sensitivity and extended energy coverage down to about 30 TeV. Twenty-six additional events were observed, substantially more than expected from atmospheric backgrounds. Combined, both searches reject a purely atmospheric origin for the 28 events at the 4 sigma level. These 28 events, which include the highest energy neutrinos ever observed, have flavors, directions, and energies inconsistent with those expected from the atmospheric muon and neutrino backgrounds. These properties are, however, consistent with generic predictions for an additional component of extraterrestrial origin.
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17.
  • Aartsen, M. G., et al. (author)
  • Improvement in fast particle track reconstruction with robust statistics
  • 2014
  • In: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 736, s. 143-149
  • Journal article (peer-reviewed)abstract
    • The IceCube project has transformed 1 km(3) of deep natural Antarctic ice into a Cherenkov detector Muon neutrinos are detected and their direction is inferred by mapping the light produced by the secondary muon track inside the volume instrumented with photomultipliers. Reconstructing the muon track from the observed light is challenging due to noise, light scattering in the ice medium, and the possibility of simultaneously having multiple muons inside the detector, resulting from the large flux of cosmic ray muons. This paper describes work on two problems: (1) the truck reconstruction problem, in which, given a set of observations, the goal is to recover the track of a muon; and (2) the coincident event problem, which is to determine how many muons are active in the detector during a time window. Rather than solving these problems by developing more complex physical models that are applied at later stages of the analysis, our approach is to augment the detector's early reconstruction with data filters and robust statistical techniques. These can be implemented at the level of on-line reconstruction and, therefore, improve all subsequent reconstructions. Using the metric of median angular resolution, a standard metric for track reconstruction, we improve the accuracy in the initial reconstruction direction by 13%. We also present improvements in measuring the number of muons in coincident events: we can accurately determine the number of muons 98% of the time.
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18.
  • Aartsen, M. G., et al. (author)
  • The IceProd framework : Distributed data processing for the IceCube neutrino observatory
  • 2015
  • In: Journal of Parallel and Distributed Computing. - : Elsevier BV. - 0743-7315 .- 1096-0848. ; 75, s. 198-211
  • Journal article (peer-reviewed)abstract
    • IceCube is a one-gigaton instrument located at the geographic South Pole, designed to detect cosmic neutrinos, identify the particle nature of dark matter, and study high-energy neutrinos themselves. Simulation of the IceCube detector and processing of data require a significant amount of computational resources. This paper presents the first detailed description of IceProd, a lightweight distributed management system designed to meet these requirements. It is driven by a central database in order to manage mass production of simulations and analysis of data produced by the IceCube detector. IceProd runs as a separate layer on top of other middleware and can take advantage of a variety of computing resources, including grids and batch systems such as CREAM, HTCondor, and PBS. This is accomplished by a set of dedicated daemons that process job submission in a coordinated fashion through the use of middleware plugins that serve to abstract the details of job submission and job management from the framework. (C) 2014 Elsevier Inc. All rights reserved.
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19.
  • Aartsen, M. G., et al. (author)
  • First Observation of PeV-Energy Neutrinos with IceCube
  • 2013
  • In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 111:2, s. 021103-
  • Journal article (peer-reviewed)abstract
    • We report on the observation of two neutrino-induced events which have an estimated deposited energy in the IceCube detector of 1.04 +/- 0.16 and 1.14 +/- 0.17 PeV, respectively, the highest neutrino energies observed so far. These events are consistent with fully contained particle showers induced by neutral-current nu(e,mu,tau) ((nu) over bar (e,mu,tau)) or charged-current nu(e) ((nu) over bar (e)) interactions within the IceCube detector. The events were discovered in a search for ultrahigh energy neutrinos using data corresponding to 615.9 days effective live time. The expected number of atmospheric background is 0.082 +/- 0.004(stat)(-0.057)(+0.041)(syst). The probability of observing two or more candidate events under the atmospheric background-only hypothesis is 2.9 x 10(-3) (2.8 sigma) taking into account the uncertainty on the expected number of background events. These two events could be a first indication of an astrophysical neutrino flux; the moderate significance, however, does not permit a definitive conclusion at this time.
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20.
  • Aartsen, M. G., et al. (author)
  • IceCube search for dark matter annihilation in nearby galaxies and galaxy clusters
  • 2013
  • In: Physical Review D. - 1550-7998 .- 1550-2368. ; 88:12
  • Journal article (peer-reviewed)abstract
    • We present the results of a first search for self-annihilating dark matter in nearby galaxies and galaxy clusters using a sample of high-energy neutrinos acquired in 339.8 days of live time during 2009/10 with the IceCube neutrino observatory in its 59-string configuration. The targets of interest include the Virgo and Coma galaxy clusters, the Andromeda galaxy, and several dwarf galaxies. We obtain upper limits on the cross section as a function of the weakly interacting massive particle mass between 300 GeV and 100 TeV for the annihilation into b (b) over bar, W+(W) over bar (-), tau(+)tau(-), mu(+)mu(-) , and nu(nu) over bar. A limit derived for the Virgo cluster, when assuming a large effect from subhalos, challenges the weakly interacting massive particle interpretation of a recently observed GeV positron excess in cosmic rays.
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21.
  • Aartsen, M. G., et al. (author)
  • Measurement of Atmospheric Neutrino Oscillations with IceCube
  • 2013
  • In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 111:8, s. 081801-
  • Journal article (peer-reviewed)abstract
    • We present the first statistically significant detection of neutrino oscillations in the high-energy regime (> 20 GeV) from an analysis of IceCube Neutrino Observatory data collected in 2010 and 2011. This measurement is made possible by the low-energy threshold of the DeepCore detector (similar to 20 GeV) and benefits from the use of the IceCube detector as a veto against cosmic-ray-induced muon background. The oscillation signal was detected within a low-energy muon neutrino sample (20-100 GeV) extracted from data collected by DeepCore. A high-energy muon neutrino sample (100 GeV-10 TeV) was extracted from IceCube data to constrain systematic uncertainties. The disappearance of low-energy upward-going muon neutrinos was observed, and the nonoscillation hypothesis is rejected with more than 5 sigma significance. In a two-neutrino flavor formalism, our data are best described by the atmospheric neutrino oscillation parameters vertical bar Delta m(32)(2)vertical bar = (2.3(-0.5)(+0.6)) x 10(-3) eV(2) and sin(2) (2 theta(23)) > 0.93, and maximum mixing is favored.
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22.
  • Aartsen, M. G., et al. (author)
  • Measurement of the cosmic ray energy spectrum with IceTop-73
  • 2013
  • In: Physical Review D. - 1550-7998 .- 1550-2368. ; 88:4, s. 042004-
  • Journal article (peer-reviewed)abstract
    • We report on the measurement of the all-particle cosmic ray energy spectrum with the IceTop air shower array in the energy range from 1.58 PeV to 1.26 EeV. The IceTop air shower array is the surface component of the IceCube Neutrino Observatory at the geographical South Pole. The analysis was performed using only information from IceTop. The data used in this work were taken from June 1, 2010 to May 13, 2011. During that period the IceTop array consisted of 73 stations, compared to 81 in its final configuration. The measured spectrum exhibits a clear deviation from a single power law above the knee around 4 PeV and below 1 EeV. We observe spectral hardening around 18 PeV and steepening around 130 PeV.
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23.
  • Aartsen, M. G., et al. (author)
  • Observation of the cosmic-ray shadow of the Moon with IceCube
  • 2014
  • In: Physical Review D. - 1550-7998 .- 1550-2368. ; 89:10, s. 102004-
  • Journal article (peer-reviewed)abstract
    • We report on the observation of a significant deficit of cosmic rays from the direction of the Moon with the IceCube detector. The study of this "Moon shadow" is used to characterize the angular resolution and absolute pointing capabilities of the detector. The detection is based on data taken in two periods before the completion of the detector: between April 2008 and May 2009, when IceCube operated in a partial configuration with 40 detector strings deployed in the South Pole ice, and between May 2009 and May 2010 when the detector operated with 59 strings. Using two independent analysis methods, the Moon shadow has been observed to high significance (> 6 sigma) in both detector configurations. The observed location of the shadow center is within 0.2 degrees of its expected position when geomagnetic deflection effects are taken into account. This measurement validates the directional reconstruction capabilities of IceCube.
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24.
  • Aartsen, M. G., et al. (author)
  • Probing the origin of cosmic rays with extremely high energy neutrinos using the IceCube Observatory
  • 2013
  • In: Physical Review D. - 1550-7998 .- 1550-2368. ; 88:11
  • Journal article (peer-reviewed)abstract
    • We have searched for extremely high energy neutrinos using data taken with the IceCube detector between May 2010 and May 2012. Two neutrino-induced particle shower events with energies around 1 PeV were observed, as reported previously. In this work, we investigate whether these events could originate from cosmogenic neutrinos produced in the interactions of ultrahigh energy cosmic rays with ambient photons while propagating through intergalactic space. Exploiting IceCube's large exposure for extremely high energy neutrinos and the lack of observed events above 100 PeV, we can rule out the corresponding models at more than 90% confidence level. The model-independent quasidifferential 90% C. L. upper limit, which amounts to E-2 phi(nu e)+(nu mu)+(nu tau) = 1.2 x 10(-7) GeV cm(-2) s(-1) sr(-1) at 1 EeV, provides the most stringent constraint in the energy range from 10 PeV to 10 EeV. Our observation disfavors strong cosmological evolution of the highest energy cosmic-ray sources such as the Fanaroff-Riley type II class of radio galaxies.
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25.
  • Aartsen, M. G., et al. (author)
  • SEARCH FOR TIME-INDEPENDENT NEUTRINO EMISSION FROM ASTROPHYSICAL SOURCES WITH 3 yr OF IceCube DATASearch for time-independent neutrino emission from astrophysical sources with 3 yr of icecube data
  • 2013
  • In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 779:2, s. 132-
  • Journal article (peer-reviewed)abstract
    • We present the results of a search for neutrino point sources using the IceCube data collected between 2008 April and 2011 May with three partially completed configurations of the detector: the 40-, 59-, and 79-string configurations. The live-time of this data set is 1040 days. An unbinned maximum likelihood ratio test was used to search for an excess of neutrinos above the atmospheric background at any given direction in the sky. By adding two more years of data with improved event selection and reconstruction techniques, the sensitivity was improved by a factor of 3.5 or more with respect to the previously published results obtained with the 40-string configuration of IceCube. We performed an all-sky survey and a dedicated search using a catalog of a priori selected objects observed by other telescopes. In both searches, the data are compatible with the background-only hypothesis. In the absence of evidence for a signal, we set upper limits on the flux of muon neutrinos. For an E-2 neutrino spectrum, the observed limits are (0.9-5) x 10(-12) TeV-1 cm(-2) s(-1) for energies between 1 TeV and 1 PeV in the northern sky and (0.9-23.2) x 10(-12) TeV-1 cm(-2) s(-1) for energies between 10(2) TeV and 10(2) PeV in the southern sky. We also report upper limits for neutrino emission from groups of sources that were selected according to theoretical models or observational parameters and analyzed with a stacking approach. Some of the limits presented already reach the level necessary to quantitatively test current models of neutrino emission.
  •  
26.
  • Aartsen, M. G., et al. (author)
  • South Pole glacial climate reconstruction from multi-borehole laser particulate stratigraphy
  • 2013
  • In: Journal of Glaciology. - 0022-1430 .- 1727-5652. ; 59:218, s. 1117-1128
  • Journal article (peer-reviewed)abstract
    • The IceCube Neutrino Observatory and its prototype, AMANDA, were built in South Pole ice, using powerful hot-water drills to cleanly bore >100 holes to depths up to 2500 m. The construction of these particle physics detectors provided a unique opportunity to examine the deep ice sheet using a variety of novel techniques. We made high-resolution particulate profiles with a laser dust logger in eight of the boreholes during detector commissioning between 2004 and 2010. The South Pole laser logs are among the most clearly resolved measurements of Antarctic dust strata during the last glacial period and can be used to reconstruct paleoclimate records in exceptional detail. Here we use manual and algorithmic matching to synthesize our South Pole measurements with ice-core and logging data from Dome C, East Antarctica. We derive impurity concentration, precision chronology, annual-layer thickness, local spatial variability, and identify several widespread volcanic ash depositions useful for dating. We also examine the interval around similar to 74 ka recently isolated with radiometric dating to bracket the Toba (Sumatra) supereruption.
  •  
27.
  •  
28.
  • Emmoth, Birger, et al. (author)
  • In-situ measurements of carbon and deuterium deposition using the fast reciprocating probe in TEXTOR
  • 2009
  • In: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 390-91, s. 179-182
  • Journal article (peer-reviewed)abstract
    • Silicon samples were exposed in the scrape-off layer of the TEXTOR plasma using a fast reciprocating probe, with the aim of studying carbon deposition and deuterium retention during Dynamic Ergodic Divertor (DED) operation. Separate samples were exposed for 300 ms at the flat-top phase of neutral beam heated discharges. The exposure conditions were varied on a shot-to-shot basis by external magnetic perturbations generated by the DED in the m/n = 3/1, DC regime, base configuration. Nuclear Reaction Analysis (NRA) was used to characterise collector sample surfaces after their exposure. Enhanced concentrations of both carbon and deuterium (C 3-10 x 10(16) at./cm(2), D 8-60 x 10(15) at./cm(2)) were found. The D/C ratio was less than unity which indicates that most of the carbon and deuterium were co-deposited. Carbon e-folding lengths of about 2 cm were found on both toroidal sides of the probe independent of DED perturbations.
  •  
29.
  • Monakhov, E. V., et al. (author)
  • Formation of a double acceptor center during divacancy annealing in low-doped high-purity oxygenated Si
  • 2002
  • In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 65:23
  • Journal article (peer-reviewed)abstract
    • Deep-level transient spectroscopy studies of electronic defect levels in 7-MeV proton-irradiated n-type float-zone Si with a doping of (3-5)x10(12) cm(-3) and oxygen content of similar to10(16)-10(17) cm(-3) have been performed. The thermal stability of the irradiation-induced defects has been investigated for temperatures up to 400 degreesC. It has been found that annealing of the divacancy-related levels, the singly negative, V-2(0/-), and the doubly negative, V-2(-/=), charge states at 220-300 degreesC results in the formation of a new center with singly negative, X(0/-), and doubly negative, X(-/=), charge states. The new center anneals out at 325-350 degreesC during isochronal treatment for 15 min. The capture kinetics studies reveal that the electron capture cross section of X(0/-) is larger than that of V-2(0/-) while the capture cross section of X(-/=) is close to that of V-2(-/=). The transformation of V-2(0/-) and V-2(-/=) into X(0/-) and X(-/=) is very efficient with only a small loss in the peak amplitudes, and the position of the energy levels are close to those of V-2. Hence, it is tempting to suggest that the atomic configuration of the X center is closely related to that of V-2, and a possible identification of X may be the divacancy-oxygen center (V2O).
  •  
30.
  • Monakhov, E. V., et al. (author)
  • Laplace transform transient spectroscopy study of a divacancy-related double acceptor centre in Si
  • 2003
  • In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 15:39, s. S2771-S2777
  • Journal article (peer-reviewed)abstract
    • Radiation-induced divacancy-related levels in high-purity oxygen-enriched n-type silicon have been studied with the use of deep level transient spectroscopy (DLTS) and Laplace-DLTS. It has been shown that heat treatment at 250degreesC results in a shift of the divacancy (V-2)-related peaks observed by 'standard' DLTS. Using Laplace-DLTS it is demonstrated that the shift is due to annealing of V-2 and formation of a new acceptor centre. The new centre has presumably two negative charge states: singly and doubly negative. The formation of the new centre holds a close one-to-one correlation with the annealing of V-2, indicating that the new centre is a result of divacancy interaction with an impurity or a defect. The close position of the electronic levels of the new centre to that of V-2 suggests a similar electronic and microscopic structure of the new centre to V-2, and a tentative identification is a divacancy-oxygen centre.
  •  
31.
  • Svensson, B. G., et al. (author)
  • Defects and diffusion in high purity silicon for detector applications
  • 2004
  • In: Conference on Photo-Responsive Materials, Proceedings. - : Wiley-VCH Verlagsgesellschaft. - 3527405526 ; , s. 2250-2257
  • Conference paper (peer-reviewed)abstract
    • In this contribution we review some recent results on defects occurring after irradiation and thermal treatment of silicon detectors for ionizing radiation. In particular, the annealing of the prominent divacancy centre and the concurrent growth of a new double acceptor centre, assigned to the divacancy-oxygen pair, is treated in detail. The detectors were fabricated using oxygenated high purity float zone (FZ) silicon wafers of n-type in order to improve the radiation hardness. The results obtained have important implication on our current understanding of the degradation of silicon detectors at high radiation fluences and suggest that a new concept for optimised impurity/defect engineering needs to be developed.
  •  
32.
  • Aberg, D., et al. (author)
  • Nitrogen deactivation by implantation-induced defects in 4H-SiC epitaxial layers
  • 2001
  • In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 78:19, s. 2908-2910
  • Journal article (peer-reviewed)abstract
    • Ion implantation causes free charge carrier reduction due to damage in the crystalline structure. Here, nitrogen-doped 4H silicon carbide (n type) epitaxial layers have been investigated using low ion doses in order to resolve the initial stage of the charge carrier reduction. It was found that the reduction of free carriers per ion-induced vacancy increases with increasing nitrogen content. Nitrogen is suggested to be deactivated through reaction with migrating point defects, and silicon vacancies or alternatively interstitials are proposed as the most likely candidates.
  •  
33.
  • Aberg, D., et al. (author)
  • Nitrogen passivation by implantation-induced point defects in 4H-SiC epitaxial layers
  • 2001
  • In: Applied Surface Science. - 0169-4332 .- 1873-5584. ; 184:04-jan, s. 263-267
  • Journal article (peer-reviewed)abstract
    • Ion implantation causes damage to the crystal lattice resulting in the loss of free charge carriers. In this study, low dose implantations using different ions and implantation doses are made to resolve the initial carrier loss in nitrogen-doped epitaxial layers. A strong dependence of compensation on nitrogen concentration is seen, showing that nitrogen is passivated by implantation-induced point defects. An activation energy of 3.2 eV for the dissociation of the passivated nitrogen center is obtained.
  •  
34.
  •  
35.
  • Alfieri, G., et al. (author)
  • Defect energy levels in hydrogen-implanted and electron-irradiated n-type 4H silicon carbide
  • 2005
  • In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 98:11
  • Journal article (peer-reviewed)abstract
    • Using deep level transient spectroscopy (DLTS), we have studied the energy position and thermal stability of deep levels in nitrogen doped 4H-SiC epitaxial layers after 1.2 MeV proton implantation and 15 MeV electron irradiation. Isochronal annealing was performed at temperatures from 100 to 1200 degrees C in steps of 50 degrees C. The DLTS measurements, which were carried out in the temperature range from 120 to 630 K after each annealing step, reveal the presence of ten electron traps located in the energy range of 0.45-1.6 eV below the conduction band edge (E-c). Of these ten levels, three traps at 0.69, 0.73, and 1.03 eV below E-c, respectively, are observed only after proton implantation. Dose dependence and depth profiling studies of these levels have been performed. Comparing the experimental data with computer simulations of the implantation and defects profiles, it is suggested that these three new levels, not previously reported in the literature, are hydrogen related. In particular, the E-c-0.73 eV level displays a very narrow depth distribution, confined within the implantation profile, and it originates most likely from a defect involving only one H atom.
  •  
36.
  • Ayedh, H. M., et al. (author)
  • Controlling the carbon vacancy concentration in 4H-SiC subjected to high temperature treatment
  • 2016
  • In: 16th International Conference on Silicon Carbide and Related Materials, ICSCRM 2015. - : Trans Tech Publications. - 9783035710427 ; , s. 414-417
  • Conference paper (peer-reviewed)abstract
    • The carbon vacancy (VC) is the major charge carrier lifetime limiting-defect in 4H-SiC epitaxial layers and it is readily formed during elevated heat treatments. Here we describe two ways for controlling the VC concentration in 4H-SiC epi-layer using different annealing procedures. One set of samples was subjected to high temperature processing at 1950 °C for 3 min, but then different cooling rates were applied. A significant reduction of the VC concentration was demonstrated by the slow cooling rate. In addition, elimination of the VC’s was also established by annealing a sample, containing high VC concentration, at 1500 °C for a sufficiently long time. Both procedures clearly demonstrate the need for maintaining thermodynamic equilibrium during cooling.
  •  
37.
  • Ayedh, H. M., et al. (author)
  • Controlling the carbon vacancy in 4H-SiC by thermal processing
  • 2018
  • In: ECS Transactions. - : Electrochemical Society Inc.. - 1938-6737 .- 1938-5862. ; , s. 91-97
  • Conference paper (peer-reviewed)abstract
    • The carbon vacancy (Vc) is perhaps the most prominent point defect in silicon carbide (SiC) and it is an efficient charge carrier lifetime killer in high-purity epitaxial layers of 4H-SÌC. The Vc concentration needs to be controlled and minimized for optimum materials and device performance, and an approach based on post-growth thermal processing under C-rich ambient conditions is presented. It utilizes thermodynamic equilibration and after heat treatment at 1500 °C for 1 h, the Vc concentration is shown to be reduced by a factor-25 relative to that in as-grown state-of-the-art epi-layers. Concurrently, a considerable enhancement of the carrier lifetime occurs throughout the whole of >40 urn thick epi-layers. 
  •  
38.
  • Ayedh, H. M., et al. (author)
  • Elimination of carbon vacancies in 4H-SiC employing thermodynamic equilibrium conditions at moderate temperatures
  • 2015
  • In: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 107:25
  • Journal article (peer-reviewed)abstract
    • The carbon vacancy (VC) is a major point defect in high-purity 4H-SiC epitaxial layers limiting the minority charge carrier lifetime. In layers grown by chemical vapor deposition techniques, the VC concentration is typically in the range of 1012cm-3, and after device processing at temperatures approaching 2000 °C, it can be enhanced by several orders of magnitude. In the present study, both as-grown layers and a high-temperature processed one have been annealed at 1500 °C and the VC concentration is demonstrated to be strongly reduced, exhibiting a value of only a few times 1011cm-3 as determined by deep-level transient spectroscopy measurements. The value is reached already after annealing times on the order of 1 h and is evidenced to reflect thermodynamic equilibrium under C-rich ambient conditions. The physical processes controlling the kinetics for establishment of the VC equilibrium are estimated to have an activation energy below ∼3 eV and both in-diffusion of carbon interstitials and out-diffusion of VC’s are discussed as candidates. This concept of VC elimination is flexible and readily integrated in a materials and device processing sequence.
  •  
39.
  • Ayedh, H. M., et al. (author)
  • Elimination of carbon vacancies in 4H-SiC epi-layers by near-surface ion implantation : Influence of the ion species
  • 2015
  • In: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 118:17
  • Journal article (peer-reviewed)abstract
    • The carbon vacancy (VC) is a prevailing point defect in high-purity 4H-SiC epitaxial layers, and it plays a decisive role in controlling the charge carrier lifetime. One concept of reducing the VC-concentration is based on carbon self-ion implantation in a near surface layer followed by thermal annealing. This leads to injection of carbon interstitials (Ci's) and annihilation of VC's in the epi-layer "bulk". Here, we show that the excess of C atoms introduced by the self-ion implantation plays a negligible role in the VC annihilation. Actually, employing normalized implantation conditions with respect to displaced C atoms, other heavier ions like Al and Si are found to be more efficient in annihilating VC's. Concentrations of VC below ∼2 × 1011 cm-3 can be reached already after annealing at 1400 °C, as monitored by deep-level transient spectroscopy. This corresponds to a reduction in the VC-concentration by about a factor of 40 relative to the as-grown state of the epi-layers studied. The negligible role of the implanted species itself can be understood from simulation results showing that the concentration of displaced C atoms exceeds the concentration of implanted species by two to three orders of magnitude. The higher efficiency for Al and Si ions is attributed to the generation of collision cascades with a sufficiently high energy density to promote Ci-clustering and reduce dynamic defect annealing. These Ci-related clusters will subsequently dissolve during the post-implant annealing giving rise to enhanced Ci injection. However, at annealing temperatures above 1500 °C, thermodynamic equilibrium conditions start to apply for the VC-concentration, which limit the net effect of the Ci injection, and a competition between the two processes occurs.
  •  
40.
  • Ayedh, H. M., et al. (author)
  • Formation and annihilation of carbon vacancies in 4H-SiC
  • 2016
  • In: 16th International Conference on Silicon Carbide and Related Materials, ICSCRM 2015. - : Trans Tech Publications. - 9783035710427 ; , s. 331-336
  • Conference paper (peer-reviewed)abstract
    • The carbon vacancy (VC) is a major point defect in high-purity 4H-SiC epitaxial layers limiting the minority charge carrier lifetime. In layers grown by chemical vapor deposition techniques, the VC concentration is typically in the range of 1012 cm-3 and after device processing at temperatures approaching 2000 °C, it can be enhanced by several orders of magnitude. In the present contribution, we show that the cooling rate after high-temperature processing has a profound influence on the resulting VC concentration where a slow rate promotes elimination of VC. Further, isochronal annealing of as-grown and as-oxidized epi-layers protected by a carbon-cap was undertaken between 800 °C and 1600 °C. The results reveal that thermodynamic equilibrium of VC is established rather rapidly at moderate temperatures, reaching a VC concentration of only a few times 1011 cm-3 after 40 min at 1500 °C. Hence, the concept of eliminating VC’s by annealing at moderate temperatures under C-rich equilibrium conditions shows great promise and enables reannealing of high-temperature processed wafers, in contrast to the procedures commonly used today to eliminate VC. In-diffusion of carbon interstitials and out-diffusion of VC’s are discussed as the kinetics processes establishing the thermodynamic equilibrium.
  •  
41.
  • Ayedh, H. M., et al. (author)
  • Formation of carbon vacancy in 4H silicon carbide during high-temperature processing
  • 2014
  • In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 115:1, s. 012005-
  • Journal article (peer-reviewed)abstract
    • As-grown and pre-oxidized silicon carbide (SiC) samples of polytype 4H have been annealed at temperatures up to 1950 degrees C for 10 min duration using inductive heating, or at 2000 degrees C for 30 s using microwave heating. The samples consisted of a n-type high-purity epitaxial layer grown on 4 degrees off-axis < 0001 > n(+)-substrate and the evolution of the carbon vacancy (V-C) concentration in the epitaxial layer was monitored by deep level transient spectroscopy via the characteristic Z(1/2) peak. Z(1/2) appears at similar to 0.7 eV below the conduction band edge and arises from the doubly negative charge state of V-C. The concentration of V-C increases strongly after treatment at temperatures >= 1600 degrees C and it reaches almost 10(15)cm(-3) after the inductive heating at 1950 degrees C. A formation enthalpy of similar to 5.0 eV is deduced for V-C, in close agreement with recent theoretical predictions in the literature, and the entropy factor is found to be similar to 5 k (k denotes Boltzmann's constant). The latter value indicates substantial lattice relaxation around V-C, consistent with V-C being a negative-U system exhibiting considerable Jahn-Teller distortion. The microwave heated samples show evidence of non-equilibrium conditions due to the short duration used and display a lower content of V-C than the inductively heated ones. Finally, concentration-versus-depth profiles of V-C favour formation in the "bulk" of the epitaxial layer as the prevailing process and not a Schottky type process at the surface.
  •  
42.
  • Ayedh, H. M., et al. (author)
  • Formation of D-Center in p-type 4H-SiC epi-layers during high temperature treatments
  • 2017
  • In: 11th European Conference on Silicon Carbide and Related Materials, ECSCRM 2016. - : Trans Tech Publications Inc.. - 9783035710434 ; , s. 262-265
  • Conference paper (peer-reviewed)abstract
    • The current work is devoted to studying the evolution of deep level defects in the lower half of the 4H-SiC bandgap after high temperature processing and ion implantation. Two as-grown and pre-oxidized 4H-SiC sets of samples have been thermally treated at temperatures up to 1950 °C for 10 min duration using RF inductive heating. Another set of as grown samples was implanted by 4.2 MeV Si ions at room temperature (RT) with different doses (1- 4×108 cm-2). The so-called “D-center” at EV+0.6 eV dominates and forms after the elevated heat treatments, while it shows no change after the ion implantations (EV denotes the valence band edge). In contrast, the concentration of the so-called HK4 level at EV+1.44 eV increases with the implantation dose, whereas it anneals out after heat treatment at ≥ 1700 °C.
  •  
43.
  • Ayedh, H. M., et al. (author)
  • Isothermal treatment effects on the carbon vacancy in 4H silicon carbide
  • 2015
  • In: Mater. Sci. Forum. - : Trans Tech Publications, Ltd.. - 9783038354789 ; , s. 351-354
  • Conference paper (peer-reviewed)abstract
    • The carbon vacancy (VC) is a minority carrier lifetime controlling defect in 4H-SiC and it is formed during high temperature treatment. In this study, we have performed heat treatment on two sets of n-type 4H-SiC epitaxial samples. The first set was isothermally treated at 1850 °C to follow the evolution of VC as a function of time. The VC concentration is not affected by changing the duration. Samples of the other set were treated at 1950 °C for 10 min, but with different cooling rates and a reduction of the VC concentration was indeed demonstrated by lowering the cooling rate. The VC concentration in the slow-cooled sample is about 2 times less than in the fast-cooled one, reflecting a competition between equilibrium conditions and the cooling rate.
  •  
44.
  • Ayedh, H. M., et al. (author)
  • Kinetics modeling of the carbon vacancy thermal equilibration in 4H-SiC
  • 2018
  • In: International Conference on Silicon Carbide and Related Materials, ICSCRM 2017. - : Trans Tech Publications. - 9783035711455 ; , s. 233-236
  • Conference paper (peer-reviewed)abstract
    • The carbon vacancy (VC) is a major limiting-defect of minority carrier lifetime in n-type 4H-SiC epitaxial layers and it is readily formed during high temperature processing. In this study, a kinetics model is put forward to address the thermodynamic equilibration of VC, elucidating the possible atomistic mechanisms that control the VC equilibration under C-rich conditions. Frenkel pair generation, injection of carbon interstitials (Ci’s) from the C-rich surface, followed by recombination with VC’s, and diffusion of VC’s towards the surface appear to be the major mechanisms involved. The modelling results show a close agreement with experimental deep-level transient spectroscopy (DLTS) depth profiles of VC after annealing at different temperatures.
  •  
45.
  • Ayedh, H. M., et al. (author)
  • Thermodynamic equilibration of the carbon vacancy in 4H-SiC : A lifetime limiting defect
  • 2017
  • In: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 122:2
  • Journal article (peer-reviewed)abstract
    • The carbon vacancy (V-C) is a prominent defect in as-grown 4H-SiC epitaxial layers for high power bipolar devices. V-C is electrically active with several deep levels in the bandgap, and it is an efficient "killer" of the minority carrier lifetime in n-type layers, limiting device performance. In this study, we provide new insight into the equilibration kinetics of the thermodynamic processes governing the V-C concentration and how these processes can be tailored. A slow cooling rate after heat treatment at similar to 2000 degrees C, typically employed to activate dopants in 4H-SiC, is shown to yield a strong reduction of the V-C concentration relative to that for a fast rate. Further, post-growth heat treatment of epitaxial layers has been conducted over a wide temperature range (800-1600 degrees C) under C-rich surface conditions. It is found that the thermodynamic equilibration of V-C at 1500 degrees C requires a duration less than 1 h resulting in a V-C concentration of only similar to 10(11) cm(-3), which is, indeed, beneficial for high voltage devices. In order to elucidate the physical processes controlling the equilibration of V-C, a defect kinetics model is put forward. The model assumes Frenkel pair generation, injection of carbon interstitials (C-i's) from the C-rich surface (followed by recombination with V-C's), and diffusion of V-C's towards the surface as the major processes during the equilibration, and it exhibits good quantitative agreement with experiment.
  •  
46.
  • Azarov, A. Yu, et al. (author)
  • Annealing of ion implanted CdZnO
  • 2012
  • In: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 45:23, s. 235304-
  • Journal article (peer-reviewed)abstract
    • We have studied the effect of the Cd content on the recovery of ion-induced damage in wurtzite CdxZn1-xO (x <= 0.05) films and compared with that in pure wurtzite ZnO and rock-salt CdO.200 keV Au+ and 55 keV Ar+ ion implants were performed at room temperature in the dose range of 5 x 1014-6.5 x 1015 cm-2. Rutherford backscattering/channelling spectrometry was used to characterize the damage evolution in the course of annealing (600-900 degrees C in air). A complex defect annealing behaviour is revealed in CdZnO as a function of annealing temperature, Cd content and ion dose. In particular, defects in the low dose implanted CdZnO films can be effectively removed at 800 degrees C, while the high dose implantation results in the formation of defects stable at least up to 900 degrees C. Moreover, annealing of the CdZnO films is accompanied by Cd loss at the surface for temperatures exceeding 800 degrees C. In contrast, CdO exhibits a typical damage accumulation behaviour for metals and semiconductors with high degree of ionicity, resulting in saturation and extended defect formation at high ion doses. These extended defects in pure ZnO and CdO, formed either directly during implantation or by reconstruction during post-implant annealing, are substantially more stable compared with small defects which can be efficiently removed at 700 degrees C and 600 degrees C for ZnO and CdO, respectively.
  •  
47.
  • Azarov, A. Yu., et al. (author)
  • Damage accumulation and annealing behavior in high fluence implanted MgZnO
  • 2012
  • In: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier BV. - 0168-583X .- 1872-9584. ; 272, s. 426-429
  • Journal article (peer-reviewed)abstract
    • Molecular beam epitaxy grown MgxZn1-xO (x <= 0.3) layers were implanted at room temperature with 150 keV Er-166(+) ions in a fluence range of 5 x 10(15-)3 x 10(16) cm(-2). Evolution of ion-induced damage and structural changes were studied by a combination of Rutherford backscattering spectrometry, nuclear reaction analysis and time-of-flight elastic recoil detection analysis. Results show that damage production enhances in both Zn- and O-sublattices with increasing the Mg content in the MgZnO. However, MgZnO as well as pure ZnO exhibits a high degree of dynamic annealing and MgZnO can not be amorphized even at the highest ion fluence used. Annealing of heavily damaged ZnO leads to a strong surface erosion and thinning of the film. Increasing the Mg content suppresses the surface evaporation in high fluence implanted MgZnO but leads to a strong surface decomposition accompanied with a Mg-rich surface layer formation during post-implantation annealing.
  •  
48.
  • Azarov, A. Yu., et al. (author)
  • Effect of composition on damage accumulation in ternary ZnO-based oxides implanted with heavy ions
  • 2010
  • In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 108:3, s. 033509-
  • Journal article (peer-reviewed)abstract
    • Thin films of wurtzite MgxZn1-xO (x <= 0.3) grown by molecular beam epitaxy and wurtzite CdxZn1-xO (x <= 0.05) grown by metal organic chemical vapor deposition were implanted at room temperature with 150 keV Er+ ions and 200 keV Au+ ions in a wide dose range. Damage accumulation was studied by Rutherford backscattering/channeling spectrometry. Results show that the film composition affects the damage accumulation behavior in both MgZnO and CdZnO dramatically. In particular, increasing the Mg content in MgZnO results in enhanced damage accumulation in the region between the bulk and surface damage peaks characteristically distinguished in the pure ZnO. However, the overall damage accumulation in MgZnO layers, as well as in pure ZnO, exhibits saturation with increasing ion dose and MgZnO cannot be amorphized even at the highest ion dose used (3 X 10(16) Er/cm(2)). Increasing the Cd content in CdZnO affects the saturation stage of the damage accumulation and leads to an enhancement of damage production in both Cd and Zn sublattices. (C) 2010 American Institute of Physics. [doi:10.1063/1.3467532]
  •  
49.
  • Azarov, A. Yu, et al. (author)
  • Effect of implanted species on thermal evolution of ion-induced defects in ZnO
  • 2014
  • In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 115:7, s. 073512-
  • Journal article (peer-reviewed)abstract
    • Implanted atoms can affect the evolution of ion-induced defects in radiation hard materials exhibiting a high dynamic annealing and these processes are poorly understood. Here, we study the thermal evolution of structural defects in wurtzite ZnO samples implanted at room temperature with a wide range of ion species (from B-11 to Bi-209) to ion doses up to 2 x 10(16) cm(-2). The structural disorder was characterized by a combination of Rutherford backscattering spectrometry, nuclear reaction analysis, and transmission electron microscopy, while secondary ion mass spectrometry was used to monitor the behavior of both the implanted elements and residual impurities, such as Li. The results show that the damage formation and its thermal evolution strongly depend on the ion species. In particular, for F implanted samples, a strong out-diffusion of the implanted ions results in an efficient crystal recovery already at 600 degrees C, while co-implantation with B (via BF2) ions suppresses both the F out-diffusion and the lattice recovery at such low temperatures. The damage produced by heavy ions (such as Cd, Au, and Bi) exhibits a two-stage annealing behavior where efficient removal of point defects and small defect clusters occurs at temperatures similar to 500 degrees C, while the second stage is characterized by a gradual and partial annealing of extended defects. These defects can persist even after treatment at 900 degrees C. In contrast, the defects produced by light and medium mass ions (O, B, and Zn) exhibit a more gradual annealing with increasing temperature without distinct stages. In addition, effects of the implanted species may lead to a nontrivial defect evolution during the annealing, with N, Ag, and Er as prime examples. In general, the obtained results are interpreted in terms of formation of different dopant-defect complexes and their thermal stability.
  •  
50.
  • Azarov, A. Yu., et al. (author)
  • Structural damage in ZnO bombarded by heavy ions
  • 2010
  • In: Vacuum. - : Elsevier BV. - 0042-207X .- 1879-2715. ; 84:8, s. 1058-1061
  • Journal article (peer-reviewed)abstract
    • The effect of implantation parameters on damage build-up in ZnO bombarded with Bi and Er ions is studied by Rutherford backscattering/channelling spectrometry. The results show that the damage accumulation behaviour in ZnO is different dramatically from that in other semiconductors. In particular, a variation of implantation parameters, such as collision cascade density, sample temperature and ion flux, has only a minor influence on the damage accumulation in the crystal bulk for the case of such heavy ions. Moreover, an intermediate damage peak, between the surface and bulk defect peaks, is observed for all the irradiation conditions studied. The cascade density affects the behaviour of this intermediate peak with increasing ion dose.
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