| 1. |
- Isaksson, E, et al.
(författare)
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Ice cores from Svalbard :useful archives of past climate and pollution history.
- 2003
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Ingår i: Physics and chemistry of the earth. Part A. - : Elsevier BV. - 1464-1895 .- 1873-4642 .- 1474-7065. ; 28:28-32, s. 1217-1228
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Tidskriftsartikel (refereegranskat)abstract
- Ice cores from the relatively low-lying ice caps in Svalbard have not been widely exploited in climatic and environmental studies due to uncertainties about the effect of melt water percolation. However, results from two recent Svalbard ice cores, at Lomonosovfonna (1250 m asl) and Austfonna (750 m asl), have shown that with careful site selection, high-resolution sampling and multiple chemical analyses, it is possible to recover ice cores with partly preserved annual signals. These cores are estimated to cover at least the past 600 years and have been dated using a combination of known reference horizons and glacial modeling. The δ18O data from both Lomonosovfonna and Austfonna ice cores suggest that the 20th century was the warmest during the past 600 years. A comparison of the ice core and sea ice records from this period suggests that sea ice extent and Austfonna δ18O are linked over the past 400 years. This may reflect the position of the storm tracks and their direct influence on the relatively low altitude Austfonna. Lomonosovfonna may be less sensitive to such changes and primarily record atmospheric changes due to its higher elevation. The anthropogenic influence on Svalbard environment is illustrated by increased levels of non-sea-salt sulphate, nitrate, acidity, fly-ash and organic contaminants particularly during the second half of 1900s. Decreased concentrations of some components in recent decades most likely reflect emission and use restrictions. However, some current-use organic pesticide compounds show growing concentrations in near surface layers.
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| 2. |
- Jokubavicius, Valdas, et al.
(författare)
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Effects of source material on epitaxial growth of fluorescent SiC
- 2012
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Ingår i: Thin Solid Films. - : Elsevier. - 0040-6090 .- 1879-2731. ; 522, s. 7-10
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Tidskriftsartikel (refereegranskat)abstract
- The growth of fluorescent SiC using Fast Sublimation Growth Process was demonstrated using different types of SiC source materials. These were prepared by (i) high-temperature hot pressing, (ii) chemical vapor deposition and (iii) physical vapor transport. The optimized growth rates of 50 μm/h, 170 μm/h and 200 μm/h were achieved using the three types of sources, respectively. The best results in respect to growth rates are obtained using higher density sources. Fluorescent SiC layers with mirror-like morphology, very good crystal quality and yellowish or warm white light photoluminescence at room temperature were grown using all three types of the source materials.
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| 3. |
- Kamiyama, S., et al.
(författare)
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Extremely high quantum efficiency of donor-acceptor-pair emission in N-and-B-doped 6H-SiC
- 2006
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 99:9
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Tidskriftsartikel (refereegranskat)abstract
- High-efficiency visible light emission in N-and-B-doped 6H-SiC epilayers was observed in photoluminescence measurements at room temperature. The orange-yellow light emission due to the recombination of donor-acceptor pairs (DAPs) has a broad spectrum with a peak wavelength of 576 nm and a full width at half maximum of 110 nm at 250 K. The high B concentration of more than 1018 cm-3 improves the emission efficiency of the DAP recombination at a high temperature. Compared with the photoluminescence spectrum of GaN at 10 K, a high quantum efficiency of 95% was estimated for the highly B-doped sample. From time-resolved photoluminescence measurements, a DAP recombination time of 5.0 ms was obtained, which is in good agreement with the calculated value by the rate equation with the assumption of a 95% internal quantum efficiency. This is quite promising as a light-emitting medium by optical pumping, as well as monolithic light sources combined with nitride-based light-emitting diodes grown on the DA-doped SiC epilayer. © 2006 American Institute of Physics.
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| 4. |
- Monemar, Bo, 1942-, et al.
(författare)
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Dominant shallow acceptor related to oxygen and hydrogen in GaN
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 376-377, s. 440-443
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Tidskriftsartikel (refereegranskat)abstract
- We present new photoluminescence (PL) data of deliberately O-doped and Mg-doped GaN layers grown by MOCVD. The combination of these data with positron annihilation spectroscopy (PAS) and SIMS results obtained on the same samples shows a clear correlation of the PL intensity of the acceptor related emissions at 3.466 and 3.27 eV (at 2 K) with O doping. The acceptor is stable upon annealing in N-2 in our highly resistive samples, while it is known be unstable in p-GaN. Our tentative conclusion is that this very commonly occurring acceptor is either a V-Ga-O-H complex or a second configuration of the Mg acceptor containing H.
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| 5. |
- 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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| 6. |
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| 7. |
- Siefert, Andrew, et al.
(författare)
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A global meta-analysis of the relative extent of intraspecific trait variation in plant communities
- 2015
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Ingår i: Ecology Letters. - : Wiley. - 1461-023X .- 1461-0248. ; 18:12, s. 1406-1419
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Forskningsöversikt (refereegranskat)abstract
- Recent studies have shown that accounting for intraspecific trait variation (ITV) may better address major questions in community ecology. However, a general picture of the relative extent of ITV compared to interspecific trait variation in plant communities is still missing. Here, we conducted a meta-analysis of the relative extent of ITV within and among plant communities worldwide, using a data set encompassing 629 communities (plots) and 36 functional traits. Overall, ITV accounted for 25% of the total trait variation within communities and 32% of the total trait variation among communities on average. The relative extent of ITV tended to be greater for whole-plant (e.g. plant height) vs. organ-level traits and for leaf chemical (e.g. leaf N and P concentration) vs. leaf morphological (e.g. leaf area and thickness) traits. The relative amount of ITV decreased with increasing species richness and spatial extent, but did not vary with plant growth form or climate. These results highlight global patterns in the relative importance of ITV in plant communities, providing practical guidelines for when researchers should include ITV in trait-based community and ecosystem studies.
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| 8. |
- Sun, Jianwu, et al.
(författare)
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Room temperature luminescence properties of fluorescent SiC as white light emitting diode medium
- 2012
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Ingår i: Thin Solid Films. - : Elsevier. - 0040-6090 .- 1879-2731. ; 522, s. 33-35
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Tidskriftsartikel (refereegranskat)abstract
- The high quantum efficiency of donor–acceptor-pair emission in N and B co-doped 6H–SiC opens the way for SiC to constitute as an efficient light-emitting medium for white light-emitting diodes. In this work, we evidence room temperature luminescence in N and B co-doped 6H–SiC fluorescent material grown by the Fast Sublimation Growth Process. Three series of samples, with eight different N and B doping levels, were investigated. In most samples, from photoluminescence measurements a strong N–B donor–acceptor-pair emission band was observed at room temperature, with intensity dependent on the nitrogen pressure in the growth chamber and boron doping level in the source. Low temperature photoluminescence spectra showed that N bound exciton peaks exhibited a continuous broadening with increasing N2 pressure during the growth, unambiguously indicating an opportunity to control the N doping in the epilayer by conveniently changing the N2 pressure. Finally, the crystal quality of the N and B doped 6H–SiC was evaluated by X-ray diffraction measurements. The ω rocking curves of (0006) Bragg diffractions from the samples grown with lower and higher N2 pressure show almost the same value of the full width at half maximum as that collected from the substrate. This suggests that the N and B doping, which is expected to give rise to an efficient donor–acceptor-pair emission at room temperature, does not degrade the crystal quality.
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| 9. |
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| 10. |
- Bergman, Peder, et al.
(författare)
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Photoluminescence and electroluminescence characterization of InxGa1-x/InyGa1-yN multiple quantum well light emitting diodes
- 2002
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Ingår i: Materials Science Forum, Vols. 389-393. ; , s. 1493-1496
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Konferensbidrag (refereegranskat)abstract
- We report on a study of radiative recombination in In0.11Ga0.89N/In.0.01Ga0.99N multiple quantum wells (MQWs). The QWs were nominally undoped, while the InGaN barriers were Si doped. The MQW part is situated in the depletion field of a pn-junction structure with electrical contacts, so that both photoluminescence (PL) and electroluminescence (EL) can be studied as a function of bias. The PL and EL spectra are distinctly different, in particular at low temperatures. The spectral properties and related differences in PL decay times reflect different recombination conditions in the MQW region for the individual QWs.
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| 11. |
- Esmaeili, M, et al.
(författare)
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Experimental and theoretical investigations of optical properties of GaN/AlGaN MQW nanostructures. Impact of built-in polarization fields
- 2009
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Ingår i: OPTO-ELECTRONICS REVIEW. - : Polish Academy of Sciences Chancellery. - 1230-3402 .- 1896-3757. ; 17:4, s. 293-299
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Tidskriftsartikel (refereegranskat)abstract
- We report the results from detailed optical spectroscopy from MOCVD grown GaN/AlGaN multiple quantum wells (MQWs), as opposed to most previous studies where MBE was employed by means of photoluminescence (PL) technique. In this paper we will present theoretical and experimental results demonstrating how polarization induced electric fields and bound interface charges in GaN/AlGaN MQWs affect the emission peak energy, PL line shape, as well as the emission line width. Theoretically estimated fields in this work are consistent with experimental data. Transition energy of the heavy hole and electron ground state Ee-hh in GaN/AlGaN MQWs were calculated and it is found that it stays in good agreement with the experimental data.
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| 12. |
- Esmaeili, M., et al.
(författare)
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Optical properties of GaN/AlGaN QW nanostructures with different well and barrier widths
- 2007
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 19:35
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Tidskriftsartikel (refereegranskat)abstract
- Optical properties of wurtzite AlGaN/GaN quantum well (QW) structures grown by molecular-beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates have been investigated by means of photoluminescence (PL) and time-resolved photoluminescence (TRPL) at low temperature. The PL spectra exhibit a blue-shifted emission of GaN/AlGaN QW nanostructures by decreasing the barrier width, in contrast to the arsenide system (Pabla A S et al 1993 Appl. Phys. Lett. 63 752). This behavior is attributed to a redistribution across the samples of the huge built-in electric field (several hundreds of kV cm(-1)) induced by the polarization difference between wells and barriers. The trend of the barrier width dependence of the internal polarization field is reproduced by using simple electrostatic arguments. In addition, the effect of well width variation on the optical transition and decay time of GaN multiple quantum wells (MQWs) have been investigated, and it has been shown that the screening of the piezoelectric field and the electron-hole separation are strongly dependent on the well thickness and have a profound effect on the optical properties of the GaN/AlGaN MQWs. The time-resolved PL spectra of 3 nm well MQWs reveal that the spectral peak position shifts toward lower energies as the decay time increases and becomes red-shifted at longer decay times.
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| 13. |
- Esmaeili, M., et al.
(författare)
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Photoluminescence study of MOCVD-grown GaN/AlGaN MQW nanostructures : Influence of Al composition and Si doping
- 2007
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 18:2
-
Tidskriftsartikel (refereegranskat)abstract
- A detailed study of low-temperature photoluminescence (PL) in GaN/AlGaN multiple quantum well (MQW) nanostructures has been reported. We have investigated the effect of Si doping and Al content on PL spectra and PL decay time of these structures. The temperature dependence of radiative as well as non-radiative lifetimes have been evaluated between 2K and room temperature for different Si doping. We found that radiative recombination at higher temperatures even up to RT is stronger in the doped sample, compared to the undoped one. Hole localization in GaN/AlGaN MQWs with different compositions of Al is demonstrated via PL transient decay times and LO phonon coupling. It is found that there is an increasing of the decay time at the PL peak emission with increasing Al composition. For the undoped sample, a non-exponential PL decay behaviour at 2K is attributed to localized exciton recombination. A slight upshift in QWs PL peak with increasing Al composition is observed, which is counteracted by the expected rise of the internal QW electric field with increasing Al. The localization energies have been evaluated by studying the variation of the QW emission versus temperature and we found out that the localization energy increases with increasing Al composition. © IOP Publishing Ltd.
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| 14. |
- Grivickas, V., et al.
(författare)
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Carrier lifetimes and influence of in-grown defects in N-B Co-doped 6H-SiC
- 2014
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Ingår i: IOP Conference Series. - : Institute of Physics Publishing (IOPP). ; 56:1, s. 012004-
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Konferensbidrag (refereegranskat)abstract
- The thick N-B co-doped epilayers were grown by the fast sublimation growth method and the depth-resolved carrier lifetimes have been investigated by means of the free-carrier absorption (FCA) decay under perpendicular probe-pump measurement geometry. In some samples, we optically reveal in-grown carbon inclusions and polycrystalline defects of substantial concentration and show that these defects slow down excess carrier lifetime and prevent donor-acceptor pair photo luminescence (DAP PL). A pronounced electron lifetime reduction when injection level approaches the doping level was observed. It is caused by diffusion driven non-radiative recombination. However, the influence of surface recombination is small and insignificant at 300 K.
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| 15. |
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| 16. |
- Harati Zadeh, Hamid, 1964-, et al.
(författare)
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Optical observation of discrete well width fluctuations in wide band gap III-nitride quantum wells
- 2007
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 244:5, s. 1727-1734
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Tidskriftsartikel (refereegranskat)abstract
- A detailed observation of discrete well width fluctuations via localized excitons in the photoluminescence (PL) spectra of MOCVD-grown undoped GaN/Al0.07Ga0.93 N multiple quantum wells (MQWs) has been reported. Doublet excitonic features with a distance varying between 10 and 25 meV for different well widths (1.5 to 4.5 nm) are observed in the PL spectra. They are explained in terms of discrete well width variations by one c-lattice parameter, i.e. two GaN monolayers. By mapping the PL measurements across the samples with different excitation spot size, it is shown that the extension of areas with a constant well width is less than 1 μm2. TEM pictures give evidence of interface roughness, although the contrast is weak at this low Al composition. In addition we observe a long-range variation of the PL peak position across the sample, interpreted as a fluctuation in Al composition in the barriers. The residual broadening of an excitonic peak (apart from the splitting related to well width fluctuations) is about 10 meV, somewhat larger for larger well widths, and is mainly ascribed to hole localisation potentials in the QWs. Additional broadening occurs in the MQWs due to inequivalent properties of each QW within the excitation spot. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.
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| 17. |
- Harati Zadeh, Hamid, 1964-, et al.
(författare)
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Photoluminescence study of Si-doped GaN/Al0.07Ga0.93N multiple quantum wells with different dopant position
- 2004
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 84:25, s. 5071-5073
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Tidskriftsartikel (refereegranskat)abstract
- The Si-doped GaN/Al0.07Ga0.93N multiple quantum wells (MQW) were investigated, using photoluminescence (PL) and time-resolved (PL) measurements. The influence of Si doping on the emission energy and recombination dynamics of the MWQs were also investigated, with different dopant position in the wells. It was observed that the redshifted emission of the MQWs was attributed to the self-energy shift of the electron states due to the correlated motion of the electrons exposed to the fluctuating potential of the donor ions. It was also observed that the PL decay time of the sample was ∼760 ps, at low temperature.
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| 18. |
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| 19. |
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| 20. |
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| 21. |
- Haratizadeh, H., et al.
(författare)
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Time resolved photoluminescence study of Si modulation doped GaN/Al 0.07Ga0.93N multiple quantum wells
- 2004
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 241:5, s. 1124-1133
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Tidskriftsartikel (refereegranskat)abstract
- The effects of the Si doping level on the recombination dynamics and carrier (exciton) localization in modulation doped GaN/Al0.07Ga 0.93N multiple-quantum-well (MQW) structures were studied by means of photoluminescence (PL) and time-resolved PL measurements. All samples with different doping levels show a QW emission which is blue shifted with respect to the 3.48 eV PL peak from the GaN buffer layer. The decay time at the peak position remains nearly constant in the range of 320-420 ps at 2 K for all doping levels. For the undoped and low-doped samples (3 × 1018 cm-3), which have less free electrons in the QWs, a non-exponential PL decay behaviour at 2 K is attributed to localized exciton recombination. The more highly doped samples (5 × 1018 cm-3 to 10 20 cm-3) show almost exponential decay curves at 2 K, suggesting the recombination of free electrons and localized holes. This localization effect appears even at high electron concentrations to cancel the expected lowering of the radiative lifetime with doping at 2 K, such a lowering is clearly observed at elevated temperatures for the highly doped samples, however. The internal polarization-induced fields of the medium and highly-doped samples are partly screened by the electrons originating from the doping in the barriers. Only the PL peak of the undoped and low-doped samples shows a redshift with time delay, related to the photogenerated carriers. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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| 22. |
- Li, Yuanzhi, et al.
(författare)
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Habitat filtering determines the functional niche occupancy of plant communities worldwide
- 2018
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Ingår i: Journal of Ecology. - : Wiley. - 0022-0477 .- 1365-2745. ; 106:3, s. 1001-1009
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Tidskriftsartikel (refereegranskat)abstract
- How the patterns of niche occupancy vary from species-poor to species-rich communities is a fundamental question in ecology that has a central bearing on the processes that drive patterns of biodiversity. As species richness increases, habitat filtering should constrain the expansion of total niche volume, while limiting similarity should restrict the degree of niche overlap between species. Here, by explicitly incorporating intraspecific trait variability, we investigate the relationship between functional niche occupancy and species richness at the global scale. We assembled 21 datasets worldwide, spanning tropical to temperate biomes and consisting of 313 plant communities representing different growth forms. We quantified three key niche occupancy components (the total functional volume, the functional overlap between species and the average functional volume per species) for each community, related each component to species richness, and compared each component to the null expectations. As species richness increased, communities were more functionally diverse (an increase in total functional volume), and species overlapped more within the community (an increase in functional overlap) but did not more finely divide the functional space (no decline in average functional volume). Null model analyses provided evidence for habitat filtering (smaller total functional volume than expectation), but not for limiting similarity (larger functional overlap and larger average functional volume than expectation) as a process driving the pattern of functional niche occupancy. Synthesis. Habitat filtering is a widespread process driving the pattern of functional niche occupancy across plant communities and coexisting species tend to be more functionally similar rather than more functionally specialized. Our results indicate that including intraspecific trait variability will contribute to a better understanding of the processes driving patterns of functional niche occupancy.
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| 23. |
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| 24. |
- Monemar, Bo, et al.
(författare)
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A hydrogen-related shallow donor in GaN?
- 2006
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Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 376, s. 460-463
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Tidskriftsartikel (refereegranskat)abstract
- We present photoluminescence (PL) data for deliberately O-doped, high-resistive GaN samples where a new shallow donor-bound exciton (DBE) peak at about 3.4746 eV (corrected for strain shift) at 2 K appears. This DBE is strongly enhanced upon annealing in the entire range 450-900 degrees C. The possible relation of this DBE to a metastable H donor state is discussed. (c) 2006 Elsevier B.V. All rights reserved.
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| 25. |
- Monemar, Bo, et al.
(författare)
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Influence of depletion fields on photoluminescence of n-doped InGaN/GaN multiple quantum well structures
- 2002
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Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 192:1, s. 21-26
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Tidskriftsartikel (refereegranskat)abstract
- We report on a detailed study of low temperature photoluminescence (PL) in InxGa1-xN multiple quantum wells (MQWs) with x in the range 0.1 and highly Si-doped barriers of In0.01Ga0.99N. The entire MQW structure was grown at 800 degreesC. One sample with 3 QWs of width 3.5 nm and barriers of width 10.5 nm had the MQW in the depletion region of the outer surface. Two PL peaks were observed, one QW exciton from the OW closest to the GaN buffer, one lower energy peak related to a 2DEG at the interface to the GaN buffer layer. In a second similar sample, 5 QWs of width 3 nm and with 6 nm highly Si-doped In0.01Ga0.99N barriers the MQW was placed in the n-side depletion region of a pn-junction, i.e. a complete LED structure with semitransparent top metallisation. Two PL peaks are observed also in this case, of similar origin as described above. With forward bias, this structure shows lower-energy PL emission, indicating the gradual activation of the other QWs closer to the pn-junction. At high forward bias the low-energy part of the PL spectrum becomes similar to the electroluminescence (EL) spectrum.
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| 26. |
- Monemar, Bo, 1942-, et al.
(författare)
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Influence of polarization fields and depletion fields on photoluminescence of AlGaN/GaN multiple quantum well structures
- 2003
-
Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 237:1, s. 353-364
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Tidskriftsartikel (refereegranskat)abstract
- We report on a detailed study of low temperature photoluminescence (PL) in Al0.07Ga0.93N/GaN multiple quantum wells (MQWs). The structures were grown on sapphire with the conventional low temperature AlN nucleation layer and thick GaN buffer layer. Several sets of 5 QW MQW samples were studied, one set with Si doping in the barriers up to or above the metallic limit. Nominally undoped MQW samples were also studied. The spectral behaviour of the doped samples was strongly affected by the near surface depletion field, causing overlap of different spectra from non-equivalent QWs. The QWs closest to the surface are presumably inactive in some samples, due to a very high depletion field. For the case of undoped samples, on the other hand, the near surface QWs are active and most prominent in the PL spectra. The structure from discrete well width variations is here resolved in the PL spectra. The results demonstrate that for structures with no additional capping layer both the depletion field and the polarisation fields need to be considered in the interpretation of experimental data. The theoretically estimated fields in this work are consistent with the experimental spectra. The presence of localisation even in the case of metallic samples, as observed by a constant PL decay time independent of doping, is discussed in terms of penetration of the hole wave functions into the AlGaN barriers. This localisation is also manifested in a sizeable LO phonon coupling strength in all samples studied.
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| 27. |
- Monemar, Bo, et al.
(författare)
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Optical investigation of AlGaN/GaN quantum wells and superlattices
- 2004
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Ingår i: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:10, s. 2251-2258
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Tidskriftsartikel (refereegranskat)abstract
- We report a detailed optical study of several sets of multiple quantum wells (MQWs) in the AlGaN/GaN system, as well as AlN/GaN superlattice (SL) structures. In this study all materials were grown by MOCVD, as opposed to most previous studies where MBE was employed. In undoped MQWs discrete photoluminescence (PL) peaks related to discrete well width fluctuations by one full c lattice parameter are clearly observed. In doped samples this effect appears to be screened. While the recombination process in undoped samples is excitonic, in MQWs doped with Si above about 5 x 10(18) cm(-3) free electrons (a 2DEG) are dominant, and the PL process is a free electrons-localized hole transition at low temperatures. The hole localization prevails up to very high n-doping, as was previously observed in bulk GaN. The hole localization is demonstrated via several experiments, including results on PL transient decay times and LO phonon coupling. Near surface band bending, due mainly to dopant depletion in doped structures or interaction with surface states in case of higher Al content in barriers, influences the distribution of electron filling among the QWs, making a detailed modeling of the spectral shape somewhat ambiguous. It is found that AlN barriers promote a strong room temperature PL signal from the QWs, as opposed to the case with AlGaN barriers.
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| 28. |
- Monemar, Bo, et al.
(författare)
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Optical properties of InGaN/GaN and AlGaN/GaN multiple quantum well structures
- 2003
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Ingår i: Proceedings of SPIE, the International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 5023, s. 63-67
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We report on low temperature photoluminescence (PL) in In xGa1-xN multiple quantum wells (MQWs) with x in the range 0.1 and highly Si doped barriers of In0.01Ga0.99N. One sample with 3 QWs of width 3.5 nm and barriers of width 10.5 nm had the MQW in the depletion region of the outer surface. Two PL peaks were observed, one QW exciton from the QW closest to the GaN buffer, one lower energy peak related to a 2DEG at the interface to the GaN buffer layer. In a second similar sample 5 QWs of width 3 nm and with 6 nm highly Si doped In0.01Ga 0.99N barriers the MQW was placed in the n-side depletion region of a pn-junction. At low temperatures the PL and electroluminescence (EL) spectra are quite different at no, low, or reverse bias, the PL appearing at higher energy. At high forward bias a spectral component at the EL position appears. This proves a strong influence of the depletion field on the optical spectra. Preliminary results are also reported for n-doped Al0.07Ga 0.93N/GaN structures, with near surface MQWs.
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| 29. |
- Monemar, Bo, 1942-, et al.
(författare)
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Oxygen related shallow acceptor in GaN
- 2005
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Ingår i: MRS Fall Meeting,2004. - : Materials Research Society. ; , s. E5.10.11-E5.10.11
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 30. |
- Monemar, Bo, et al.
(författare)
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Photoluminescence in n-doped In0.1Ga0.9N/In0.01Ga0.99N multiple quantum wells
- 2002
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Ingår i: MRS Internet Journal of Nitride Semiconductor Research. - 1092-5783. ; 7:7, s. 1-
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Tidskriftsartikel (refereegranskat)abstract
- In0.1Ga0.9N/In0.01Ga0.99N multiple quantum wells (MQWs) with heavily Si-doped barriers, grown with Metal Organic Vapor Phase Epitaxy (MOVPE) at about 800(0)C, have been studied in detail with optical spectroscopy. Such structures are shown to be very sensitive to a near surface depletion field, and if no additional layer is grown on top of the MQW structure the optical spectra from the individual QWs are expected to be drastically different. For a sample with 3 near surface QWs and Si-doped barriers, only the QW most distant from the surface is observed in photoluminescence (PL). The strong surface depletion field is suggested to explain these results, so that the QWs closer to the surface cannot hold the photo-excited carriers. A similar effect of the strong depletion field is found in an LED structure where the MQW is positioned at the highly doped n-side of the pn-junction. The internal polarization induced electric field in the QWs is also rather strong, and incompletely screened by carriers transferred from the doped barriers. The observed PL emission for this QW is of localized exciton character, consistent with the temperature dependence of peak position and PL decay time. The excitonic lineshape of 35-40 meV in the QW PL is explained as caused by a combination of random alloy fluctuations and interface roughness, the corresponding localization potentials are also responsible for the localization of the excitons in the low temperature range (
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| 31. |
- Monemar, Bo, et al.
(författare)
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Photoluminescence of excitons in InxGa1-xN/InyGa1-yN multiple quantum wells
- 2002
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Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 161-166
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Tidskriftsartikel (refereegranskat)abstract
- We report on a detailed study of excitons in InxGa1-xN/InyGa1-yN multiple quantum wells (MQWs) with an In composition x in the QWs of about 0.1, and a small In composition y in the barrier of 0.01-0.02. The MOVPE growth procedure was optimized to allow growth without In segregation. The InyGa1-yN barriers had a Si doping of about 5 x 10(18) cm(-3) . The low temperature photoluminescence spectra show two sets of exciton-like spectra with quite different properties. The lower energy emission has a small thermal activation energy (about 5 meV), and thus disappears at elevated temperatures, it is not observed at room temperature. The higher energy exciton state has a decay time of about 5 ns, while the lower energy process is much slower. We have also done preliminary studies on samples where the MQW region is situated in a p-n junction field, with semi-transparent contacts, to study the effects of varying the bias across the MQW structure. The combination of optical data can e interpreted in terms of a substantial potential gradient across the MQW region for both samples. The conclusion is that probably only one QW is emitting at low T (and no bias), and the second lower energy PL peak originates from a shallow notch in the conduction band at the interface between the thick GaN buffer layer and the first Ga(In)N barrier.
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| 32. |
- Monemar, Bo, et al.
(författare)
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Photoluminescence of InGaN/GaN and AlGaN/GaN multiple quantum well structures : Role of depletion fields and polarization fields
- 2003
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Ingår i: Physica status solidi. A, Applied research. - Weinheim, Germany : Wiley-VCH Verlagsgesellschaft. - 0031-8965 .- 1521-396X. ; 195:3, s. 523-527
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Tidskriftsartikel (refereegranskat)abstract
- We report on low temperature photoluminescence (PL) in InxGa1-xN multiple quantum wells (MQWs) with x in the range 0.1 and highly Si doped barriers of In0.01Ga0.99N. The MQW was placed either in the near surface depletion field or at the n-side depletion field of a pn-junction. At low temperatures the PL and electroluminescence (EL) spectra are quite different at no, low, or reverse bias, the PL appearing at higher energy. At high forward bias a spectral component at the EL position appears in PL. This proves a strong influence of the depletion field on the optical spectra. Preliminary results are also reported for Al0.07Ga0.93N/GaN structures, with near surface MQWs with 5 QWs, both Si-doped and nominally undoped. These structures show clear spectral features related to nonequivalent QWs in a graded depletion field.
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| 33. |
- Ou, H., et al.
(författare)
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Fluorescent SiC for white light-emitting diodes
- 2012
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Ingår i: Asia Commun. Photonics Conf.. - Washington, D.C. : OSA.
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Konferensbidrag (refereegranskat)abstract
- The strong photoluminescence from f-SiC was achieved after the optimization of the B and N concentrations. Surface nanostructures were successfully applied to enhance the extraction efficiency. f-SiC is a promising wavelength convertor for white LEDs.
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| 34. |
- Ou, Y., et al.
(författare)
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Photoluminescence and Raman spectroscopy characterization of boron-and nitrogen-doped 6H silicon carbide
- 2012
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Ingår i: Silicon Carbide and Related Materials 2011. - : Trans Tech Publications Inc.. - 9783037854198 ; , s. 233-236
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Konferensbidrag (refereegranskat)abstract
- Nitrogen-boron doped 6H-SiC epilayers grown on low off-axis 6H-SiC substrates have been characterized by photoluminescence and Raman spectroscopy. The photoluminescence results show that a doping larger than 10 18 cm -3 is favorable to observe the luminescence and addition of nitrogen leads to an increased luminescence. A dopant concentration difference larger than 4×10 18 cm -3 is proposed to achieve intense photoluminescence. Raman spectroscopy further confirmed the doping type and concentrations for the samples. The results indicate that N-B doped SiC can serve as a good wavelength converter in white LEDs applications.
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| 35. |
- Paskov, Plamen, 1959-, et al.
(författare)
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Nonpolar GaN layers grown by sidewall epitaxial lateral overgrowth : optical evidences for a reduced stacking fault density
- 2008
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Ingår i: Proc. of the 7th International Conference on Nitride Semiconductors,2007. - Linköping : Department of Physics, Chemistry and Biology, Linköping University. ; , s. 1768-1770
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Konferensbidrag (refereegranskat)abstract
- Nonpolar a -plane and m -plane GaN layers grown by MOCVD employing sidewall epitaxial lateral overgrowth (SELO) are studied by photoluminescence (PL) and spatially resolved micro-PL. The effects of the groove orientations and the groove/terrace width ratio on the emission spectra, particularly on the stacking fault (SF) related emission bands in the 3.29–3.42 eV spectral region, are examined. The PL spectra of both types of nonpolar layers reveal a significant reduction of the defect related emissions when the grooves are oriented perpendicular to the c-axis of GaN and the groove/terrace width ratio is smaller than one. The suppression of SF formation in the areas where a lateral overgrowth along the [0001] GaN direction occurs is confirmed by micro-PL measurements showing no SF related emissions over the terrace regions. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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| 36. |
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| 37. |
- Paskov, Plamen, 1959-, et al.
(författare)
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Phonon-assisted photoluminescence in InGaN/GaN multiple quantum wells
- 2002
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 234:3, s. 755-758
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Tidskriftsartikel (refereegranskat)abstract
- The LO-phonon sidebands of the photoluminescence in InGaN/GaN multiple quantum wells has been investigated in the temperature range from 20 to 300 K. Analysing the intensity distribution among the phonon replicas, the strength of the exciton-phonon interaction has been estimated. The Huang-Rhys factor was found to be ÿ0.3, much larger than in GaN. The enhancement has been attributed to the exciton localization on a length scale smaller than the exciton Bohr radius and to the large internal electric field, which increases the spatial separation of the electron and hole charge densities along the growth axis.
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| 38. |
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| 39. |
- Paskov, Plamen, 1959-, et al.
(författare)
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Photoluminescence study of near-surface GaN/AlN superlattices
- 2008
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. ; , s. 68940G1-
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Konferensbidrag (refereegranskat)abstract
- We report on the emission properties of GaN/AlN superlattices (SLs) grown by metalorganic chemical vapor deposition on a thick GaN layer. Nominally undoped and Si-doped SL structures with the well/barrier thickness ratio 3:1 and different SL periods are investigated. It is found that in these SLs without capping layer the energy position, intensity and linewidth of the emission are determined by the interplay of the built-in polarization field, the depletion field arising from the pinning of the Fermi level at the surface, and the screening of the electric field in the quantum well due to the both the polarization-induced two-dimension electron gas (2DEG) and the photo-generated carriers. A non-uniform equilibrium electron distribution and an electron accumulation at the bottom AlN/GaN interface are evidenced by the observed recombination of the 2DEG with the photo-excited holes occurring below the GaN bandgap.
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| 40. |
- Pozina, Galia, et al.
(författare)
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Optical study of AlGaN/GaN multiple quantum well structures grown on laterally overgrown GaN templates
- 2002
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Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 107-111
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Tidskriftsartikel (refereegranskat)abstract
- We report on optical studies of Al0.1Ga0.9N/GaN structures with five 30 (A) over circle thick GaN quantum wells (QWs) grown by metal organic vapor phase epitaxy using the lateral overgrowth technique. Overgrown regions demonstrate better structural and optical properties. The low temperature photoluminescence (PL) is dominated by the multiple quantum well (MQW) emission at 3.53 eV with the linewidth of similar to50 meV. The PL decay time for this line was measured to be similar to600 ps. Comparison with an AlGaN/GaN MQW light-emitting diode (LED) structure is done. The LED structure was grown with a p-type doped AlGaN top layer and a p-GaN contact layer. The PL spectrum of the LED structure shows besides the donor-acceptor pair recombination from the top layer an additional 60 meV wide line at 3.64 eV. The presence of two MQW peaks may be related to the potential gradient present across the MOW structure.
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| 41. |
- Sun, Jianwu, et al.
(författare)
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Fluorescent silicon carbide as an ultraviolet-to-visible light converter by control of donor to acceptor recombinations
- 2012
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Ingår i: Journal of Physics D. - : Institute of Physics (IOP). - 0022-3727 .- 1361-6463. ; 45:23, s. 235107-
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Tidskriftsartikel (refereegranskat)abstract
- As an alternative to the conventional phosphors in white LEDs, a donor and acceptor co-doped fluorescent 6H-SiC can be used as an ultraviolet-to-visible light converter without any need of rare-earth metals. From experimental data we provide an explanation to how light can be obtained at room temperature by a balance of the donors and acceptors. A steady-state recombination rate model is used to demonstrate that the luminescence in fluorescent SiC can be enhanced by controlling the donor and acceptor doping levels. A doping criterion for optimization of this luminescence is thus proposed.
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| 42. |
- Syväjärvi, Mikael, et al.
(författare)
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Fluorescent SiC as a new material for white LEDs
- 2012
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Ingår i: Physica scripta. T. - 0281-1847 .- 0031-8949 .- 1402-4896. ; T148, s. 014002-
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Tidskriftsartikel (refereegranskat)abstract
- Current III–V-based white light-emitting diodes (LEDs) are available. However, their yellow phosphor converter is not efficient at high currents and includes rare-earth metals, which are becoming scarce. In this paper, we present the growth of a fluorescent silicon carbide material that is obtained by nitrogen and boron doping and that acts as a converter using a semiconductor. The luminescence is obtained at room temperature, and shows a broad luminescence band characteristic of donor-to-acceptor pair recombination. Photoluminescence intensities and carrier lifetimes reflect a sensitivity to nitrogen and boron concentrations. For an LED device, the growth needs to apply low-off-axis substrates. We show by ultra-high-resolution analytical transmission electron microscopy using aberration-corrected electrons that the growth mechanism can be stable and that there is a perfect epitaxial relation from the low-off-axis substrate and the doped layer even when there is step-bunching.
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