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| 2. |
- Hellsvik, J., et al.
(författare)
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Tuning order-by-disorder multiferroicity in CuO by doping
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:1, s. 014437-
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Tidskriftsartikel (refereegranskat)abstract
- The high Curie temperature multiferroic compound CuO has a quasidegenerate magnetic ground state that makes it prone to manipulation by the so-called "order-by-disorder" mechanism. First principle computations supplemented with Monte Carlo simulations and experiments show that isovalent doping allows us to stabilize the multiferroic phase in nonferroelectric regions of the pristine material phase diagram with experiments reaching a 250% widening of the ferroelectric temperature window with 5% of Zn doping. Our results allow us to validate the importance of a quasidegenerate ground state on promoting multiferroicity on CuO at high temperatures and open a path to the material engineering of multiferroic materials. In addition we present a complete explanation of the CuO phase diagram and a computation on the incommensurability in excellent agreement with experiment without free parameters.
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| 3. |
- Monemar, Bo, et al.
(författare)
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Evidence for Two Mg Related Acceptors in GaN
- 2009
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Ingår i: PHYSICAL REVIEW LETTERS. - 0031-9007 .- 1079-7114. ; 102:23, s. 235501-
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Tidskriftsartikel (refereegranskat)abstract
- The optical signatures of Mg-related acceptors in GaN have been revisited in samples specifically grown on bulk GaN templates, to avoid strain broadening of the optical spectra. Bound-exciton spectra can be studied in these samples for Mg concentrations up to [Mg]approximate to 2x10(19) cm(-3). Contrary to previous work it is found that instabilities in the photoluminescence spectra are not due to unstable shallow donors, but to unstable Mg-related acceptors. Our data show that there are two Mg-related acceptors simultaneously present: the regular (stable) substitutional Mg acceptor, and a complex acceptor which is unstable in p-GaN.
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| 4. |
- Monemar, Bo, 1942-, et al.
(författare)
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Optical signatures of dopants in GaN
- 2006
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Ingår i: Materials Science in Semiconductor Processing. - : Elsevier BV. - 1369-8001 .- 1873-4081. ; 9:1-3, s. 168-174
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Tidskriftsartikel (refereegranskat)abstract
- The characteristic optical spectra for shallow donors and acceptors in GaN are discussed. The most accurate photoluminescence (PL) data are obtained from samples grown on freestanding GaN substrates, where strain shifts are absent and a low spectroscopic line width is obtained. Recent PL data for excitons bound to the O and Si donors are discussed in some detail, giving accurate values for the binding energies and excited bound donor states. The Mg-acceptor is the most important one for p-doping, but the related optical spectra are controversial. We show that there are two acceptors present in Mg-doped GaN, with two different acceptor bound exciton peaks, and also two corresponding lower energy donor-acceptor pair spectra. We give tentative evidence for their interpretation. © 2006 Elsevier Ltd. All rights reserved.
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| 5. |
- Monemar, Bo, 1942-, et al.
(författare)
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Recombination of free and bound excitons in GaN
- 2008
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 245:9, s. 1723-1740
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Tidskriftsartikel (refereegranskat)abstract
- We report on recent optical investigations of free and bound exciton properties in bulk GaN. In order to obtain reliable data it is important to use low defect density samples of low doping. We have used thick GaN layers (of the order of 1 mm) grown by halide vapour phase epitaxy (HVPE) with a residual doping down to <1016 cm-3 in this work. With such samples all polarisation geometries could also easily be exploited. The influence of the surface states on the photoluminescence (PL) experiments is analysed, it is concluded that surface recombination plays an important role for the free exciton (FE) recombination. The electronic structure of the FEs is discussed in detail, including the influence of spin-exchange and polariton effects, and compared with polarised PL spectra at 2 K. The detailed structure of excited states from the PL spectra is discussed, but further data are needed to fully explain all the peaks observed. The polarized FE spectra at room temperature allow a determination of the bandgap as 3.437 eV at 290 K, assuming an exciton binding energy of 25 meV. The PL transient of the A FE is very short (about 100 ps) for the no-phonon (NP) line interpreted as dominated by nonradiative surface recombination. The longitudinal-optical (LO) phonon replicas of the A FE exhibit a longer decay of about 1.4 ns at 2 K, suggested to represent the bulk lifetime of the FE. The corresponding decay time at 290 K is 9 ns in our samples, a value that might be affected by nonradiative recombination. The Si and O donor bound exciton (DBE) spectra with sharp NP lines at 3.4723 eV and 3.4714 eV respectively, are well resolved together with the so-called two-electron transitions (TETs) and several optical phonon replicas. The electronic structure of the DBE states including excited rotational states is discussed and compared with experiment. The well-resolved TET lines allow an accurate determination of the ground state binding energy of the Si donor as 30.4 meV and 33.2 meV for the O donor. The PL transients of the DBEs reveal a non-exponential decay for the NP lines. The DBE NP transient lineshape is assumed to be influenced by optical dispersion and scattering in the vicinity of exciton resonances, as well as by surface effects. The DBE decay time can most properly be deduced from the PL decay of the respective TETs and LO replicas, leading to values in the range of 1.1-1.8 ns. These values differ significantly from previous theoretical predictions, where values about two orders of magnitude shorter were obtained. A tentative discussion of the main observed features of acceptor bound excitons (ABEs), which are much less studied in GaN, is given. A decay time of about 0.9 ns for the shallowest 3.466 eV ABE is estimated, i.e. shorter than that for the shallow donor BEs. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.
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| 6. |
- Monemar, Bo, et al.
(författare)
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Transient photoluminescence of shallow donor bound excitons in GaN
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 82, s. 235202-
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed study of photoluminescence transients for neutral donor bound excitons (DBEs) in GaN, notably the ON donor DBE at 3.4714 eV and the SiGa DBE at 3.4723 eV. The studied samples are thick strain free nominally undoped bulk GaN samples, with a spectroscopic linewidth <0.5 meV at 2 K. The photoluminescence (PL) decay curves for these no-phonon (NP) lines are strongly nonexponential, and do not allow a proper assessment of the characteristic BE decay time. The decay of the LO-phonon replicas as well as the so-called two-electron transitions (TETs) at lower energies show a nicely exponential behavior, and allow extraction of DBE decay times of about 1.1 ns for the Si DBE and 1.8 ns for the O DBE, respectively. The initial nonexponential decay behavior of the NP lines has been studied in both the common front surface excitation-detection mode and with detection in transmission through the sample. This initial decay is explained as related to scattering processes in the near surface region, involving the DBEs and free excitons (FEs). Light scattering processes may also contribute to this complex decay shape. The DBE-LO-phonon decay does not discriminate between the O and Si DBEs because of spectral overlap involving different LO modes. The TET decays at 2 K are very different for transitions related to the DBE ground state and DBE excited states (going to p-like donor final states), for T>10 K thermalization between the DBE ground state and DBE excited states produces a common decay time. Thermalization between free and bound excitons appears to occur above about 20 K, when the DBE decay follows the FE decay. A simple two-level modeling of exciton capture and recombination for the PL decay curves of the FE and the DBEs, as commonly used in the literature, is shown to be generally inadequate. A broad PL background in the TET spectral region is suggested to be related to a radiative Auger process, where the DBEs recombine while leaving the donors ionized.
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| 7. |
- Nguyen, Tien Son, et al.
(författare)
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Identification of the gallium vacancy-oxygen pair defect in GaN
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 80:15, s. 153202-
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Tidskriftsartikel (refereegranskat)abstract
- Cation vacancies like VGa, VAl and their complexes with oxygen are predicted to be abundant in III-nitrides and to play an important role in nonradiative recombination. Appearing in triple or double negatively charged states, they are not paramagnetic and have not so far been detected by magnetic resonance even under illumination. In this Brief Report, we demonstrate an efficient way to make cation vacancy defects in GaN detectable by electron paramagnetic resonance and present our identification of the VGaON pair in GaN which is the model material for the III-nitrides and their alloys.
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| 8. |
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| 9. |
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| 10. |
- Zeichner, Sarah S., et al.
(författare)
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Polycyclic aromatic hydrocarbons in samples of Ryugu formed in the interstellar medium
- 2023
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Ingår i: Science. - : AMER ASSOC ADVANCEMENT SCIENCE. - 0036-8075 .- 1095-9203. ; 382:6677, s. 1411-1415
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Tidskriftsartikel (refereegranskat)abstract
- Polycyclic aromatic hydrocarbons (PAHs) contain less than or similar to 20% of the carbon in the interstellar medium. They are potentially produced in circumstellar environments (at temperatures greater than or similar to 1000 kelvin), by (similar to 10 kelvin) interstellar clouds, or by processing of carbon-rich dust grains. We report isotopic properties of PAHs extracted from samples of the asteroid Ryugu and the meteorite Murchison. The doubly-C-13 substituted compositions (Delta 2x(13)C values) of the PAHs naphthalene, fluoranthene, and pyrene are 9 to 51 parts per thousand higher than values expected for a stochastic distribution of isotopes. The Delta 2x(13)C values are higher than expected if the PAHs formed in a circumstellar environment, but consistent with formation in the interstellar medium. By contrast, the PAHs phenanthrene and anthracene in Ryugu samples have Delta 2x(13)C values consistent with formation by higher-temperature reactions.
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