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Sökning: WFRF:(Bergman Peder 1961 )

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1.
  • Monemar, Bo, 1942-, et al. (författare)
  • Defects in Gallium Nitride
  • 1999
  • Ingår i: International Workshop on Materials Science,1999. - Proc. of the International Workshop on Materials Science 99, ed. by F. F. Bekker et al., Vol. 1 : Hanoi National University Publishing House. ; , s. 28-
  • Konferensbidrag (refereegranskat)
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2.
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3.
  • Monemar, Bo, 1942-, et al. (författare)
  • The 3.466 eV Bound Exciton in GaN
  • 2001
  • Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 228:2, s. 489-492
  • Tidskriftsartikel (refereegranskat)abstract
    •  We discuss the available optical data for the 3.466 eV bound exciton in GaN, which has been a controversial issue in the recent literature. We conclude that the experimental results are only consistent with the identification as an exciton bound at a neutral acceptor with a spin-like bound hole. The chemical identity is still not clear.
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4.
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5.
  • Ayedh, H. M., et al. (författare)
  • Controlling the carbon vacancy in 4H-SiC by thermal processing
  • 2018
  • Ingår i: ECS Transactions. - : Electrochemical Society Inc.. - 1938-6737 .- 1938-5862. ; , s. 91-97
  • Konferensbidrag (refereegranskat)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. 
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6.
  • Bergman, Peder, 1961-, et al. (författare)
  • Improved SiC Epitaxial Material for Bipolar Applications
  • 2008
  • Ingår i: Proc. of MRS Spring Meeting 2008. ; , s. D05-
  • Konferensbidrag (refereegranskat)abstract
    • Epitaxial growth on Si-face nominally on-axis 4H-SiC substrates has been performed using horizontal Hot-wall chemical vapor deposition system. The formation of 3C inclusions is one of the main problem with growth on on-axis Si-face substrates. In situ surface preparation, starting growth parameters and growth temperature are found to play a vital role in the epilayer polytype stability. High quality epilayers with 100% 4H-SiC were obtained on full 2″ substrates. Different optical and structural techniques were used to characterize the material and to understand the growth mechanisms. It was found that the replication of the basal plane dislocation from the substrate into the epilayer can be eliminated through growth on on-axis substrates. Also, no other kind of structural defects were found in the grown epilayers. These layers have also been processed for simple PiN structures to observe any bipolar degradation. More than 70% of the diodes showed no forward voltage drift during 30 min operation at 100 A/cm2.
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7.
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8.
  • Brosselard, P., et al. (författare)
  • 3.3 kV-10A 4H-SiC PiN diodes
  • 2009
  • Ingår i: Materials Science Forum, Vols. 600-603. - : Trans Tech Publ.. ; , s. 991-994
  • Konferensbidrag (refereegranskat)abstract
    • An innovative process has been developed by Linköping University to prepare the 4HSiC substrate surface before epitaxial growth. The processed PiN diodes have been characterized in forward and reverse mode at different temperature. The larger diodes (2.56 mm2) have a very low leakage current around 20 nA @ 500V for temperatures up to 300°C. A performant yield (68%) was obtained on these larger diodes have a breakdown voltage superior to 500V. Electroluminescence characteristics have been done on these devices and they show that there is no generation of Stacking Faults during the bipolar conduction.
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9.
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10.
  • Buyanova, Irina, 1960-, et al. (författare)
  • Mechanism for low-temperature photoluminescence in GaNAs/GaAs structures grown by molecular-beam epitaxy
  • 1999
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 75:4, s. 501-
  • Tidskriftsartikel (refereegranskat)abstract
    •  The mechanism for low-temperature photoluminescence (PL) emissions in GaNAs epilayers and GaAs/GaNxAs1 - x quantum well (QW) structures grown by molecular-beam epitaxy is studied in detail, employing PL, PL excitation, and time-resolved PL spectroscopies. It is shown that even though quantum confinement causes a strong blueshift of the GaNAs PL emission, its major characteristic properties are identical in both QW structures and epilayers. Based on the analysis of the PL line shape, its dependence on the excitation power and measurement temperature, as well as transient data, the PL emission is concluded to be caused by a recombination of excitons trapped by potential fluctuations in GaNAs.
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11.
  • Buyanova, Irina, 1960-, et al. (författare)
  • Mechanism for radiative recombination in ZnCdO alloys
  • 2007
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 90:26
  • Tidskriftsartikel (refereegranskat)abstract
    • Temperature dependent cw- and time-resolved photoluminescence combined with absorption measurements are employed to evaluate the origin of radiative recombination in ZnCdO alloys grown by molecular-beam epitaxy. The near-band-edge emission is attributed to recombination of excitons localized within band tail states likely caused by nonuniformity in Cd distribution. Energy transfer between the tail states is argued to occur via tunneling of localized excitons. The transfer is shown to be facilitated by increasing Cd content due to a reduction of the exciton binding energy and, therefore, an increase of the exciton Bohr radius in the alloys with a high Cd content. © 2007 American Institute of Physics.
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12.
  • Buyanova, Irina, 1960-, et al. (författare)
  • Mechanism for rapid thermal annealing improvements in undoped GaNxAs1-x/GaAs structures grown by molecular beam epitaxy
  • 2000
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 77:15, s. 2325-
  • Tidskriftsartikel (refereegranskat)abstract
    •  A systematic investigation of the effect of rapid thermal annealing (RTA) on optical properties of undoped GaNAs/GaAs structures is reported. Two effects are suggested to account for the observed dramatic improvement in the quality of the GaNxAs1-x/GaAs quantum structures after RTA: (i) improved composition uniformity of the GaNxAs1-x alloy, deduced from the photoluminescence (PL), PL excitation and time-resolved measurements; and (ii) significant reduction in the concentration of competing nonradiative defects, revealed by the optically detected magnetic resonance studies.
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13.
  • Buyanova, Irina, 1960-, et al. (författare)
  • Optical study of spin injection dynamics in InGaN/GaN quantum wells with GaMnN injection layers
  • 2004
  • Ingår i: Journal of Vacuum Science & Technology B. - : American Vacuum Society. - 1071-1023 .- 1520-8567. ; 22:6, s. 2668-2672
  • Tidskriftsartikel (refereegranskat)abstract
    •  The spin injection dynamics of GaMnN/InGaN multiquantum well (MQW) light emitting diodes (LEDs) grown by molecular beam epitaxy were examined using picosecond-transient and circularly polarized photoluminescence (PL) measurements. Even with the presence of a room temperature ferromagnetic GaMnN spin injector, the LEDs are shown to exhibit very low efficiency of spin injection. Based on resonant optical orientation spectroscopy, the spin loss in the structures is shown to be largely due to fast spin relaxation within the InGaN MQW, which itself destroys any spin polarization generated by optical spin orientation or electrical spin injection. Typical photoluminescence decay times were 20-40 ns in both commercial GaN MQW LEDs with emission wavelengths between 420-470 nm and in the GaMnN/InGaN multi-quantum well MQW LEDs. In the wurtzite InGaN/GaN system, biaxial strain at the interfaces give rise to large piezoelectric fields directed along the growth axis. This built-in piezofield breaks the reflection symmetry of confining potential leading to the presence of a large Rashba term in the conduction band Hamiltonian which is responsible for the short spin relaxation times.
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14.
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15.
  • Buyanova, Irina, 1960-, et al. (författare)
  • Role of nitrogen in photoluminescence up-conversion in GaInNP/GaAs heterostructures
  • 2007
  • Ingår i: AIP Conference Proceedings / Volume 893. - : American Institute of Physics (AIP). ; , s. 381-382
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • Alloying of disordered GaInP with nitrogen is shown to lead to very efficient PLU in GaInNP/GaAs heterostructures grown by gas source molecular beam epitaxy (GS‐MBE). This is attributed to the N‐induced changes in the band alignment at the GaInNP/GaAs heterointerface from the type I for the N‐free structure to the type II in the samples with N compositions exceeding 0.5%. Based on the performed excitation power dependent measurements, a possible mechanism for the energy upconversion is suggested as being due to the two‐step two‐photon absorption. The photon recycling effect is shown to be important for the structures with N=1%, from time‐resolved PL measurements. © 2007 American Institute of Physics
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16.
  • Buyanova, Irina, 1960-, et al. (författare)
  • Time-resolved studies of photoluminescence in GaNxP1-x alloys : Evidence for indirect-direct band gap crossover
  • 2002
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 81:1, s. 52-
  • Tidskriftsartikel (refereegranskat)abstract
    •  Time resolved photoluminescence spectroscopy is employed to monitor the effect of N incorporation on the band structure of GaNP alloys. Abrupt shortening in radiative lifetime of near-band gap emissions, arising from excitonic radiative recombination within N-related centers, is found to occur at very low N compositions of around 0.5%, i.e., within the same range as the appearance of the direct-band gap-like transitions in the photomodulated transmission spectra of GaNP reported previously. The effect has been attributed to an enhancement in oscillator strength of optical transitions due to band crossover from indirect to direct-band gap of the alloy.
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17.
  • Choubina, Tatiana, 1950-, et al. (författare)
  • Light diffusion in GaN epilayers
  • 2007
  • Ingår i: 3rd International Conference on Spontaneous Coherence in Excitonic System,2007.
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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18.
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19.
  • Choubina, Tatiana, 1950-, et al. (författare)
  • Slow light in GaN
  • 2008
  • Ingår i: 16th Int. Symp. ¿Nanostructures: Physics and Technology,2008. ; , s. 257-
  • Konferensbidrag (refereegranskat)
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20.
  • Choubina, Tatiana, 1950-, et al. (författare)
  • The slow light in GaN
  • 2008
  • Ingår i: ICPS2008,2008. ; , s. 647-
  • Konferensbidrag (refereegranskat)
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21.
  • Esmaeili, M., et al. (författare)
  • Photoluminescence study of MOCVD-grown GaN/AlGaN MQW nanostructures : Influence of Al composition and Si doping
  • 2007
  • 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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22.
  • Ghezellou, Misagh, 1988-, et al. (författare)
  • The role of boron related defects in limiting charge carrier lifetime in 4H–SiC epitaxial layers
  • 2023
  • Ingår i: APL Materials. - : American Institute of Physics (AIP). - 2166-532X. ; 11:3
  • Tidskriftsartikel (refereegranskat)abstract
    • One of the main challenges in realizing 4H–SiC (silicon carbide)-based bipolar devices is the improvement of minority carrier lifetime in as-grown epitaxial layers. Although Z1/2 has been identified as the dominant carrier lifetime limiting defect, we report on B-related centers being another dominant source of recombination and acting as lifetime limiting defects in 4H–SiC epitaxial layers. Combining time-resolved photoluminescence (TRPL) measurement in near band edge emission and 530 nm, deep level transient spectroscopy, and minority carrier transient spectroscopy (MCTS), it was found that B related deep levels in the lower half of the bandgap are responsible for killing the minority carriers in n-type, 4H–SiC epitaxial layers when the concentration of Z1/2 is already low. The impact of these centers on the charge carrier dynamics is investigated by correlating the MCTS results with temperature-dependent TRPL decay measurements. It is shown that the influence of shallow B acceptors on the minority carrier lifetime becomes neutralized at temperatures above ∼422 K. Instead, the deep B related acceptor level, known as the D-center, remains active until temperatures above ∼570 K. Moreover, a correlation between the deep level concentrations, minority carrier lifetimes, and growth parameters indicates that intentional nitrogen doping hinders the formation of deep B acceptor levels. Furthermore, tuning growth parameters, including growth temperature and C/Si ratio, is shown to be crucial for improving the minority carrier lifetime in as-grown 4H–SiC epitaxial layers.
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23.
  • Godlewski, M., et al. (författare)
  • Mechanism of radiative recombination in acceptor-doped bulk GaN crystals
  • 1999
  • Ingår i: 20th International Conference on Defects in Semiconductors ICDS-20,1999. - Physica B, Vol. 273-274 : Elsevier. ; , s. 39-
  • Konferensbidrag (refereegranskat)abstract
    •  Optical and electrical properties of acceptor-doped bulk GaN crystals are discussed. Though introducing Zn and Ca to bulk GaN does not significantly change electron concentration, it results in the appearance of a blue photoluminescence band accompanying the relatively strong yellow band usually present. Highly resistive GaN : Mg crystals are obtained when high amount of Mg is introduced to the Ga melt during high-pressure synthesis. Change of electrical properties of Mg-doped bulk crystals is accompanied by the appearance of a strong blue emission of GaN similar to that in Ca- and Zn-doped crystals. Optically detected magnetic resonance investigations indicate a multi-band character of this blue emission and suggest possible mechanism of compensation in acceptor-doped bulk GaN.
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24.
  • Godlewski, M., et al. (författare)
  • Photoluminescence mechanisms in undoped and in Mg doped bulk GaN
  • 1999
  • Ingår i: 24th International Conference on the Physics of Semiconductors,1998. - Proc. of the 24th International Conference on the Physics of Semiconductors, ed. by D. Gershoni : World Scientific, Singapore. ; , s. IX B17-
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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25.
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26.
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27.
  • Harati Zadeh, Hamid, 1964-, et al. (författare)
  • 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
  • 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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28.
  • Henry, Anne, et al. (författare)
  • Epitaxial growth on on-axis substrates
  • 2012
  • Ingår i: Silicon Carbide Epitaxy. - Kerala, India : Research Signpost. - 9788130805009 ; , s. 97-119
  • Bokkapitel (refereegranskat)abstract
    • SiC epitaxial growth using the Chemical Vapour Deposition (CVD) technique on nominally on-axis substrate is presented. Both standard and chloride-based chemistry have been used with the aim to obtain high quality layers suitable for device fabrication. Both homoepitaxy (4H on 4H) and heteroepitaxy (3C on hexag onal substrate) are addressed.
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29.
  • Henry, Anne, 1959-, et al. (författare)
  • Thick epilayers for power devices
  • 2007
  • Ingår i: Materials Science Forum, vol. 556-557. - : Trans Tech Publications. ; , s. 47-
  • Konferensbidrag (refereegranskat)abstract
    • Growth of thick epitaxial SiC layers needed for high power devices is presented for horizontal hot-wall CVD (HWCVD) reactors. We demonstrate thickness of epilayer of 100 μm and more with good morphology, low-doping with no doping variation through the whole thick layer and reasonable carrier lifetime which mainly depends on the substrate quality. Typical epidefects are described and their density can dramatically be reduced when choosing correctly the growth conditions as well as the polishing of the surface prior to the growth. The control of the doping and thickness uniformities as well as the run-to-run reproducibility is also presented. Various characterization techniques such as optical microscopy, AFM, reflectance, CV, PL and minority carrier lifetime have been used. Results of high-voltage SiC Schottky power devices are presented.
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30.
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31.
  • Henry, Anne, 1959-, et al. (författare)
  • Titanium Related Luminescence in SiC
  • 2009
  • Ingår i: ICSCRM2007,2007. - Materials Science Forum, Vols. 600-603 : Trans Tech Publications. ; , s. 461-464
  • Konferensbidrag (refereegranskat)abstract
    • We report on the luminescence spectra related to Ti impurity in both 4H- and 6H-SiC polytypes. The spectrum depends strongly on the polarization. They are two families of lines in 4H and three in 6H. The main no-phonon line of each family is shown as a triplet and its phonon structure contains both sharp and broad replicas. The higher energy family has also extra lines at high energy appearing when the temperature increases. The spectra can be detected with excitation energy below the excitonic bandgap and even with excitation energy below the spectrum itself. Time-resolved photoluminescence reveals 0.1 ms long lifetime at low temperature.
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32.
  • Izadifard, Morteza, 1965-, et al. (författare)
  • Effects of rapid thermal annealing on optical properties of GaNxP1−x alloys grown by solid source molecular beam epitaxy
  • 2005
  • Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 0268-1242 .- 1361-6641. ; 20:5, s. 353-356
  • Tidskriftsartikel (refereegranskat)abstract
    • Temperature-dependent photoluminescence (PL), PL excitation and time-resolved PL measurements were employed to study the effects of rapid thermal annealing (RTA) on optical properties of GaNxP1−x alloys grown by solid source molecular beam epitaxy. A substantial increase in radiative efficiency of GaNP epilayers, which is especially pronounced for the high-energy PL component, was achieved after RTA and is attributed to annealing out of competing non-radiative centres. The latter is evident from reduced quenching of the PL intensity with increasing measurement temperature, which results in a strong increase (up to 18 times) in the PL intensity at room temperature (RT), as well as from a substantial increase in carrier lifetime at RT deduced from time-resolved PL measurements.
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33.
  • Izadifard, Morteza, 1965-, et al. (författare)
  • Effects of rapid thermal annealing on optical quality of GaNP alloys
  • 2004
  • Ingår i: EMRS-2004 Spring Meeting,2004. - : Institution of Engineering and Technology (IET). ; , s. 335-
  • Konferensbidrag (refereegranskat)abstract
    • Significant improvements in radiative efficiency of GaNP epilayers grown on GaP substrates by solid-source molecular beam epitaxy (MBE) are achieved by post-growth rapid thermal annealing (RTA). From temperature-dependent CW and time-resolved photoluminescence (PL) spectroscopies combined with PL excitation measurements, the observed improvements are attributed to annealing out of competing nonradiative centres. This conclusion is supported by the following experimental evidence: reduced thermal quenching of the PL intensity resulting in a substantial (up to 18 times) increase at room temperature (RT) after RTA, and simultaneous improvements in carrier lifetime at RT deduced from time-resolved PL measurements.
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34.
  • Izadifard, Morteza, 1965-, et al. (författare)
  • Evaluation of optical quality and defect properties of GaNxP1−x alloys lattice matched to Si
  • 2004
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 85, s. 6347-
  • Tidskriftsartikel (refereegranskat)abstract
    • By using a variety of optical characterization techniques, including cathodoluminescence, temperature-dependent cw- and time-resolved photoluminescence (PL), and PL excitation spectroscopies, high optical quality of the GaN0.018P0.982 epilayers lattice matched to Si substrates is demonstrated and is shown to be comparable to that of the “state-of-the-art” GaNP alloys grown on GaP substrates. The growth of GaNP on Si is, however, found to facilitate the formation of several point defects, including complexes involving Ga interstitial atoms (Gai).
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35.
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36.
  • Izadifard, Morteza, et al. (författare)
  • Radiative recombination of GaInNP alloys lattice matched to GaAs
  • 2006
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 88:1, s. 011919-
  • Tidskriftsartikel (refereegranskat)abstract
    • Cw- and time-resolved photoluminescence (PL) spectroscopy is employed to evaluate dominant mechanisms for light emission in GayIn1−yNxP1−x alloys grown by gas source molecular-beam epitaxy on GaAs substrates. Different from other direct band gap dilute nitrides, the low temperature PL emission was shown to be largely attributed to radiative transitions involving spatially separated localized electron-hole pairs. The observed charge separation is tentatively attributed to the long range CuPt ordering promoted by the presence of nitrogen.
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37.
  • Jacobson, H., et al. (författare)
  • Properties and origins of different stacking faults that cause degradation in SiC PiN diodes
  • 2004
  • Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 95:3, s. 1485-1488
  • Tidskriftsartikel (refereegranskat)abstract
    • Different properties of the reported stacking faults (SFs) and that both these types of SF are present in the material after electrical degradation of pin diodes are shown. One is caused by perfect dislocations, deflected or misfit dislocation that had dissociated into two partial dislocations. The partials are assumed to be close to each other with a separation below the detection limit of the SWBT measurements. Thus, enough energy is provided and the leading partial moves away from the other partial and forms the extended SF.
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38.
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39.
  • Kamiyama, S., et al. (författare)
  • Extremely high quantum efficiency of donor-acceptor-pair emission in N-and-B-doped 6H-SiC
  • 2006
  • Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 99:9
  • 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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40.
  • Lilja, Louise, 1985- (författare)
  • 4H-SiC epitaxy investigating carrier lifetime and substrate off-axis dependence
  • 2018
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Silicon carbide (SiC) is a wide bandgap semiconductor with unique material properties making it useful for various device applications using high power, high frequency and high temperature. Compared to Si-based electronics, SiC based electronics have an improved energy efficiency. One of the most critical problems is to reduce this planets power consumption, where large improvements can be made enhancing the energy efficiency. Independent on how the electrical power is generated, power conversion is needed and about 10% of the electrical power is lost for every power conversion step using Si-based electronics. Since the efficiency is related to the performance of the semiconductor device, SiC can make contributions to the efficiency. Compared to Si, SiC has three times larger bandgap, about ten times higher breakdown electric field strength and about three times higher thermal conductivity. The wide bandgap together with the chemical stability of SiC makes it possible for SiC electronic devices to operate at much higher temperatures (>250°C) compared to Si-based devices and do not require large cooling units as with Si power converters.The current status for 4H-SiC devices regard unipolar devices (≤ 1700 V), such as metal-oxide-semiconductor field-effect transistors (MOSFETs) and Schottky barrier diodes (SBDs), are now on the market for mass production. The research focus is now on high-voltage (>10 kV) bipolar devices, such as, bipolar junction transistors (BJTs), p‑i‑n diodes and insulated-gate bipolar transistors (IGBTs).The focus of this thesis are material improvements relevant for the development of 4H-SiC high-voltage bipolar devices. A key parameter for such devices is the minority carrier lifetime, where long carrier lifetimes reduce the on-resistance through conductivity modulation. However, too long carrier lifetimes give long reverse recovery times leading to large switching losses. Thus, a tailored carrier lifetime is needed for the specific application. Carrier lifetimes of the epilayers can both be controlled by the CVD growth conditions and by post-growth processing, such as thermal oxidation and carbon implantation followed by thermal annealing. Emphasis in this thesis (Paper 1‑2) is to find optimal CVD growth conditions (growth temperature, C/Si ratio, growth rate, doping) improving the carrier lifetime. Since the main lifetime limiting defect has shown to be the Z1/2 center, identified as isolated carbon vacancies, growth conditions minimizing the Z1/2 concentration are strived for.To achieve high-voltage bipolar devices, thick epilayers of high quality is needed. An important factor is then the growth rate that needs to be relatively high in order to reduce the fabrication time, and thus the cost of the final device. In this thesis the growth process has been optimized for high growth rates (30 µm/h) using standard silane and propane chemistry (Paper 3), compared to other chemistries that includes chlorine, which results in corroded reactor parts and new defects in the epitaxial layers.Another important parameter for 4H-SiC bipolar devices is the basal plane dislocations (BPDs) in the substrate and epilayers, since the BPDs can act as source of nucleation and expansion of Shockley stacking faults (SSFs). The expanded SSFs give a lowered carrier lifetime and form a potential barrier for carrier transport leading to an increased forward voltage drop which in turn leads to bipolar degradation. The bipolar degradation is detrimental for 4H-SiC bipolar devices. Several strategies are developed to reduce the density of BPDs including buffer layers, growth interrupts and decreasing the substrates off-cut angle. Paper 4‑6 is focused on developing a CVD growth process for low substrate off-cut angles (1° and 2°) compared to the today’s standard off-cut angle of 4°. By reducing the substrate off-cut angle the number of BPDs intersecting the substrate surface is reduced. In addition, the conversion from BPDs to threading edge dislocations (TEDs) during epitaxial growth is increased with lower off-cut angles.
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41.
  • Lilja, Louise, 1985-, et al. (författare)
  • Influence of n-Type Doping Levels on Carrier Lifetime in 4H-SiC Epitaxial Layers
  • 2017
  • Ingår i: Silicon Carbide and Related Materials 2016. - : Trans Tech Publications Ltd. ; , s. 238-241
  • Konferensbidrag (refereegranskat)abstract
    • In this study we have grown thick 4H-SiC epitaxial layers with different n-type doping levels in the range 1E15 cm-3 to mid 1E18 cm-3, in order to investigate the influence on carrier lifetime. The epilayers were grown with identical growth conditions except the doping level on comparable substrates, in order to minimize the influence of other parameters than the n-type doping level. We have found a drastic decrease in carrier lifetime with increasing n-type doping level. Epilayers were further characterized with low temperature photoluminescence and deep level transient spectroscopy.
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42.
  • Lilja, Louise, 1985-, et al. (författare)
  • Smooth 4H-SiC epilayers grown with high growth rates with silane/propane chemistry using 4° off-cut substrates
  • 2016
  • Ingår i: Silicon Carbide and Related Materials 2015. - : Trans Tech Publications. ; , s. 209-212
  • Konferensbidrag (refereegranskat)abstract
    • 4H-SiC epilayers with very smooth surfaces were grown with high growth rates on 4° off-cut substrates using standard silane/propane chemistry. Specular surfaces with RMS values below 0.2 nm are presented for epilayers grown with growth rates up to 30 μm/h using horizontal hot-wall chemical vapor deposition, with up to 100 μm thickness. Optimization of in-situ etching conditions and C/Si ratio are presented.
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43.
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44.
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45.
  • Monemar, Bo, 1942-, et al. (författare)
  • 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
  • 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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46.
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47.
  • Monemar, Bo, 1942-, et al. (författare)
  • Optical signatures of dopants in GaN
  • 2006
  • Ingår i: Materials Science in Semiconductor Processing. - : Elsevier BV. - 1369-8001 .- 1873-4081. ; 9:1-3, s. 168-174
  • 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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48.
  • Monemar, Bo, 1942-, et al. (författare)
  • Recombination of free and bound excitons in GaN
  • 2008
  • Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 245:9, s. 1723-1740
  • 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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49.
  • Neimontas, K., et al. (författare)
  • The determination of high-density carrier plasma parameters in epitaxial layers, semi-insulating and heavily doped crystals of 4H-SiC by a picosecond four-wave mixing technique
  • 2006
  • Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 0268-1242 .- 1361-6641. ; 21:7, s. 952-958
  • Tidskriftsartikel (refereegranskat)abstract
    • We applied a picosecond four-wave mixing technique for measurements of carrier lifetimes and diffusion coefficients in highly excited epitaxial layers, semi-insulating and heavily doped 4H-SiC substrates. Optical carrier injection at two different wavelengths (266 and 355 nm) allowed us to vary the depth of the excited region from 1-2 νm to 50 νm, and thus determine photoelectric parameters of carrier plasma in the density range from 2 × 1016 to 1019 cm-3. Strong dependence of carrier lifetime and mobility on carrier density was found in the epitaxial layers. The origin of fast decay components, not resolved previously by photoluminescence and free-carrier absorption techniques in SiC, was attributed to nonlinear carrier recombination. Numerical modelling provided a value of bimolecular recombination coefficient equal to B ≤ (2-4) × 10-11 cm3 s-1 and verified a surface recombination velocity S ≤ 4 × 104 cm s-1. In heavily doped crystals, nonlinear carrier recombination reduced the carrier lifetime down to 1.1 ns, while in semi-insulating ones a lifetime of 1.5-2.5 ns was measured. Temperature dependences of four-wave mixing provided monopolar carrier mobility in heavily doped and bipolar one in semi-insulating crystals, and revealed the contribution of ionized impurity and phonon scattering mechanisms. © 2006 IOP Publishing Ltd.
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50.
  • Ochalski, T.J., et al. (författare)
  • Optical study of AlGaN/GaN based HEMT structures
  • 2005
  • Ingår i: Physica Status Solidi. C, Current topics in solid state physics. - 1610-1634 .- 1610-1642. ; 2, s. 2791-2794
  • Tidskriftsartikel (refereegranskat)
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