| 1. |
- Aberg, Fredrik, et al.
(författare)
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A Dynamic Aspartate-to-Alanine Aminotransferase Ratio Provides Valid Predictions of Incident Severe Liver Disease
- 2021
-
Ingår i: HEPATOLOGY COMMUNICATIONS. - : John Wiley & Sons. - 2471-254X. ; 5:6, s. 1021-1035
-
Tidskriftsartikel (refereegranskat)abstract
- The aspartate-to-alanine aminotransferase ratio (AAR) is associated with liver fibrosis, but its predictive performance is suboptimal. We hypothesized that the association between AAR and liver disease depends on absolute transaminase levels and developed and validated a model to predict liver-related outcomes in the general population. A Cox regression model based on age, AAR, and alanine aminotransferase (ALT) level (dynamic AAR [dAAR]) using restricted cubic splines was developed in Finnish population-based health-examination surveys (FINRISK, 2002-2012; n = 18,067) with linked registry data for incident liver-related hospitalizations, hepatocellular carcinoma, or liver death. The model was externally validated for liver-related outcomes in a Swedish population cohort (Swedish Apolipoprotein Mortality Risk [AMORIS] subcohort; n = 126,941) and for predicting outcomes and/or prevalent fibrosis/cirrhosis in biopsied patients with nonalcoholic fatty liver disease (NAFLD), chronic hepatitis C, or alcohol-related liver disease (ALD). The dynamic AAR model predicted liver-related outcomes both overall (optimism-corrected C-statistic, 0.81) and in subgroup analyses of the FINRISK cohort and identified persons with >10% risk for liver-related outcomes within 10 years. In independent cohorts, the C-statistic for predicting liver-related outcomes up to a 10-year follow-up was 0.72 in the AMORIS cohort, 0.81 in NAFLD, and 0.75 in ALD. Area-under-the-curve (AUC) for detecting prevalent cirrhosis was 0.80-0.83 in NAFLD, 0.80 in hepatitis C, but only 0.71 in ALD. In ALD, model performance improved when using aspartate aminotransferase instead of ALT in the model (C-statistic, 0.84 for outcome; AUC, 0.82 for prevalent cirrhosis). Conclusion: A dAAR score provides prospective predictions for the risk of incident severe liver outcomes in the general population and helps detect advanced liver fibrosis/cirrhosis. The dAAR score could potentially be used for screening the unselected general population and as a trigger for further liver evaluations.
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| 2. |
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| 3. |
- Akram, Muhammad Nadeem, et al.
(författare)
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Influence of base-region thickness on the performance of Pnp transistor-VCSEL
- 2014
-
Ingår i: Optics Express. - 1094-4087. ; 22:22, s. 27398-27414
-
Tidskriftsartikel (refereegranskat)abstract
- We have recently reported a 980nm GaAs-based three terminal Pnp transistor-vertical-cavity surface-emitting laser (TVCSEL) operating at room temperature with optical power up to 1.8mW. However, the current gain beta = Delta I-c/Delta I-b was near zero just before lasing and became negative after the lasing threshold. The main cause of the negative current gain was found to be a gradual and position-dependent forward-biasing (saturation) of the base-collector junction with increasing bias even before lasing threshold. In this article, detailed multi-physics device simulations are performed to better understand the device physics, and find ways to avoid the premature saturation of the base-collector junction. We have optimized the thickness of the base region as well as its doping concentration and the location of the quantum wells to ensure that the T-VCSEL is in the active mode throughout its range of operation. That is, the emitter-base junction is forward biased and base-collector junction is reversed biased for sweeping the excess charges out of the base region.
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| 4. |
- Andersson, J. Y., et al.
(författare)
-
Quantum structure based infrared detector research and development within Acreo's centre of excellence IMAGIC
- 2010
-
Ingår i: Infrared physics & technology. - : Elsevier BV. - 1350-4495 .- 1879-0275. ; 53:4, s. 227-230
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Tidskriftsartikel (refereegranskat)abstract
- Acreo has a long tradition of working with quantum structure based infrared (IR) detectors and arrays. This includes QWIP (quantum well infrared photodetector), QDIP (quantum dot infrared photodetector), and InAs/GaInSb based photon detectors of different structure and composition. It also covers R&D on uncooled microbolometers. The integrated thermistor material of such detectors is advantageously based on quantum structures that are optimised for high temperature coefficient and low noise. Especially the SiGe material system is preferred due to the compatibility with silicon technology. The R&D work on IR detectors is a prominent part of Acreo's centre of excellence "IMAGIC" on imaging detectors and systems for non-visible wavelengths. IMAGIC is a collaboration between Acreo, several industry partners and universities like the Royal Institute of Technology (KTH) and Linkoping University. (C) 2010 Elsevier B.V. All rights reserved.
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| 5. |
- Andersson, Magnus, et al.
(författare)
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Fiskbestånd och miljö i hav och sötvatten : Resurs- och miljööversikt 2012
- 2012
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Detta är den nionde utgåvan av den samlade översikten över fisk- och kräftdjursbeståndens status i våra vatten. Kunskap om fiskbestånden och miljön är en förutsättning för att utnyttjandet av fiskresurserna skall bli bärkraftigt. För svenska vattenområden beskrivs miljöutvecklingen i ett ekosystemsperspektiv, dels för att tydliggöra fiskens ekologiska roll och beskriva yttre miljöfaktorer som påverkar fiskbestånden, dels för att belysa fiskets effekter på miljön.Fiskbestånd och miljö i hav och sötvatten är utarbetad av Sveriges lantbruksuniversitet (SLU), Institutionen för akvatiska resurser (SLU Aqua), på uppdrag av Havs- och vattenmyndigheten. Rapporten sammanfattar utveckling och beståndsstatus för de kommersiellt viktigaste fisk- och kräftdjursarterna i våra vatten. Bedömningar och förvaltningsråd är baserade på Internationella Havsforskningsrådets (ICES) rådgivning, SLU Aquas nationella och regionala provfiskedata, samt yrkesfiskets rapportering.
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| 6. |
- Asplund, Carl, et al.
(författare)
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1260 nm InGaAs vertical-cavity lasers
- 2002
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Ingår i: Electronics Letters. - : Institution of Engineering and Technology (IET). - 0013-5194 .- 1350-911X. ; 38:13, s. 635-636
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Tidskriftsartikel (refereegranskat)abstract
- The fabrication and performance of highly strained double-quantum well InGaAs/GaAs vertical-cavity lasers with record-long emission wavelength of 1260 nm at room temperature is reported. Depending on device diameter, the minimum threshold current is in the low mA-regime while the maximum output power exceeds 1 mW. The devices work continuous-wave over a wide temperature range of at least 10-120degrees C.
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| 7. |
- Asplund, C., et al.
(författare)
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Doping-induced losses in AlAs/GaAs distributed Bragg reflectors
- 2001
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 90:2, s. 794-800
-
Tidskriftsartikel (refereegranskat)abstract
- We have studied n- and p-type doping-induced performance degradation of AlAs/GaAs distributed Bragg reflectors (DBRs) for applications in vertical cavity lasers (VCLs). Based on high-accuracy optical reflectance and triple-axis x-ray diffraction measurements on a variety of differently doped DBR structures grown by metalorganic vapor-phase epitaxy, a fitting procedure was employed to extract the doping-dependent optical loss. A striking observation is that the reflectance of these DBRs is much more sensitive to n- than p-type doping incorporation. While in the latter case the loss can be well accounted for by intervalence-band and free-carrier absorption, additional loss mechanisms must be considered for n-type DBRs. We relate the losses to doping-enhanced interdiffusion effects resulting in increased interface scattering. These findings should have important consequences for the design of VCLs, demonstrating the importance of reduced n-type doping concentrations and/or growth temperatures, or the application of alternative device concepts, e.g., employing intracavity contacts.
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| 8. |
- Asplund, Carl, et al.
(författare)
-
Optimization of MOVPE-grown GaInNAs/GaAs quantum wells for 1.3-μm laser applications
- 2002
-
Konferensbidrag (refereegranskat)abstract
- The effects of high group-V precursor flows during metal-organic vapor phase epitaxial growthof GaInNAs QWs are investigated. It is shown that photoluminescence line width, waferuniformity, and sensitivity to growth temperature can be improved using V/III ratios in excess of2000. Broad area GaInNAdGaAs SQW lasers with dimensions 100 x 820 pm2 grown under theseconditions have threshold current densities as low as 660 kA/cm2 at 1.26 pm emission wavelength.
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| 9. |
- Atwa, Mohamed M., et al.
(författare)
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Trilayer Graphene as a Candidate Material for Phase-Change Memory Applications
- 2016
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Ingår i: MRS Advances. - : Cambridge University Press. - 2059-8521. ; 1:20, s. 1487-1494
-
Tidskriftsartikel (refereegranskat)abstract
- There is pressing need in computation of a universal phase change memory consolidating the speed of RAM with the permanency of hard disk storage. A potentiated scanning tunneling microscope tip traversing the soliton separating a metallic, ABA-stacked phase and a semiconducting ABC-stacked phase in trilayer graphene has been shown to permanently transform ABA-stacked regions to ABC-stacked regions. In this study, we used density functional theory (DFT) calculations to assess the energetics of this phase-change and explore the possibility of organic functionalization using s-triazine to facilitate a reverse phase-change from rhombohedral back to Bernal in graphene trilayers. A significant deviation in the energy per simulated atom arises when s-triazine is adsorbed, favoring the transformation of the ABC phase to the ABA phase once more. A phase change memory device utilizing rapid, energy-efficient, reversible, field-induced phase-change in graphene trilayers could potentially revolutionize digital memory industry.
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| 10. |
- Baranowski, M., et al.
(författare)
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Nitrogen-related changes in exciton localization and dynamics in GaInNAs/GaAs quantum wells grown by metalorganic vapor phase epitaxy
- 2015
-
Ingår i: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 118:2, s. 479-486
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, we show the results of low-temperature photoluminescence (PL), time-resolved photoluminescence, and photoreflectance (PR) investigations, performed on a series of three Ga0.64In0.34As1-x N (x) /GaAs single quantum wells (SQW) grown by metalorganic vapor phase epitaxy with the nitrogen content of 0, 0.5, and 0.8 %. Comparing the PL and PR data, we show that at low excitation intensity and temperature, the radiative recombination occurs via localizing centers (LCs) in all samples. The excitation intensity-dependent PL measurements combined with theoretical modeling of hopping excitons in this system allow us to provide quantitative information on the disorder parameters describing population of LCs. It has been found that the average energy of LCs increases about two times and simultaneously the number of LCs increases about 10 and 20 times after the incorporation of 0.5 and 0.8 % of nitrogen, respectively. The value of average localization energy E > (0) determined for N-containing samples (similar to 6-7 meV) is in the range typical for dilute nitride QWs grown by molecular beam epitaxy (MBE). On the other hand, the "effective" concentration of LCs seems to be higher than for GaInNAs/GaAs QW grown by MBE. The dramatic increase in localizing centers also affects the PL dynamics. Observed PL decay time dispersion is much stronger in GaInNAs SQW than in nitrogen-free SQW. The change in PL dynamic is very well reproduced by model of hopping excitons.
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| 11. |
- Bergström, Ulf, et al.
(författare)
-
Long-term effects of no-take zones in Swedish waters
- 2022
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Marine protected areas (MPAs) are increasingly established worldwide to protect and restore degraded ecosystems. However, the level of protection varies among MPAs and has been found to affect the outcome of the closure. In no-take zones (NTZs), no fishing or extraction of marine organisms is allowed. The EU Commission recently committed to protect 30% of European waters by 2030 through the updated Biodiversity Strategy. Importantly, one third of these 30% should be of strict protection. Exactly what is meant by strict protection is not entirely clear, but fishing would likely have to be fully or largely prohibited in these areas. This new target for strictly protected areas highlights the need to evaluate the ecological effects of NTZs, particularly in regions like northern Europe where such evaluations are scarce. The Swedish NTZs made up approximately two thirds of the total areal extent of NTZs in Europe a decade ago. Given that these areas have been closed for at least 10 years and can provide insights into long-term effects of NTZs on fish and ecosystems, they are of broad interest in light of the new 10% strict protection by 2030 commitment by EU member states.In total, eight NTZs in Swedish coastal and offshore waters were evaluated in the current report, with respect to primarily the responses of focal species for the conservation measure, but in some of the areas also ecosystem responses. Five of the NTZs were established in 2009-2011, as part of a government commission, while the other three had been established earlier. The results of the evaluations are presented in a synthesis and also in separate, more detailed chapters for each of the eight NTZs. Overall, the results suggest that NTZs can increase abundances and biomasses of fish and decapod crustaceans, given that the closed areas are strategically placed and of an appropriate size in relation to the life cycle of the focal species. A meta-regression of the effects on focal species of the NTZs showed that CPUE was on average 2.6 times higher after three years of protection, and 3.8 times higher than in the fished reference areas after six years of protection. The proportion of old and large individuals increased in most NTZs, and thereby also the reproductive potential of populations. The increase in abundance of large predatory fish also likely contributed to restoring ecosystem functions, such as top-down control. These effects appeared after a 5-year period and in many cases remained and continued to increase in the longer term (>10 years). In the two areas where cod was the focal species of the NTZs, positive responses were weak, likely as an effect of long-term past, and in the Kattegat still present, recruitment overfishing. In the Baltic Sea, predation by grey seal and cormorant was in some cases so high that it likely counteracted the positive effects of removing fisheries and led to stock declines in the NTZs. In most cases, the introduction of the NTZs has likely decreased the total fishing effort rather than displacing it to adjacent areas. In the Kattegat NTZ, however, the purpose was explicitly to displace an unselective coastal mixed bottom-trawl fishery targeting Norway lobster and flatfish to areas where the bycatches of mature cod were smaller. In two areas that were reopened to fishing after 5 years, the positive effects of the NTZs on fish stocks eroded quickly to pre-closure levels despite that the areas remained closed during the spawning period, highlighting that permanent closures may be necessary to maintain positive effects.We conclude from the Swedish case studies that NTZs may well function as a complement to other fisheries management measures, such as catch, effort and gear regulations. The experiences from the current evaluation show that NTZs can be an important tool for fisheries management especially for local coastal fish populations and areas with mixed fisheries, as well as in cases where there is a need to counteract adverse ecosystem effects of fishing. NTZs are also needed as reference for marine environmental management, and for understanding the effects of fishing on fish populations and other ecosystem components in relation to other pressures. MPAs where the protection of both fish and their habitats is combined may be an important instrument for ecosystembased management, where the recovery of large predatory fish may lead to a restoration of important ecosystem functions and contribute to improving decayed habitats.With the new Biodiversity Strategy, EUs level of ambition for marine conservation increases significantly, with the goal of 30% of coastal and marine waters protected by 2030, and, importantly, one third of these areas being strictly protected. From a conservation perspective, rare, sensitive and/or charismatic species or habitats are often in focus when designating MPAs, and displacement of fisheries is then considered an unwanted side effect. However, if the establishment of strictly protected areas also aims to rebuild fish stocks, these MPAs should be placed in heavily fished areas and designed to protect depleted populations by accounting for their home ranges to generate positive outcomes. Thus, extensive displacement of fisheries is required to reach benefits for depleted populations, and need to be accounted for e.g. by specific regulations outside the strictly protected areas. These new extensive EU goals for MPA establishment pose a challenge for management, but at the same time offer an opportunity to bridge the current gap between conservation and fisheries management.
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| 12. |
- Bernabé, S., et al.
(författare)
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Highly integrated VCSEL-based 10Gb/s miniature optical sub-assembly
- 2005
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Ingår i: Proceedings - Electronic Components and Technology Conference. - Lake Buena Vista, FL : IEEE. ; , s. 1333-1338
-
Konferensbidrag (refereegranskat)abstract
- In order to fit with the present and future needs of the Datacom transceiver market, newly designed high rate transmitter optical subassemblies (TOSAs) have to be compact, low cost and compatible with mass production. We propose here an innovative design strategy that reaches all these targets by integrating a 10Gbps 850nmVCSEL laser diode and its laser driver with other functionalities (e.g. power monitoring and thermal monitoring) in a small form factor package. Taking advantages of the optical properties of the VCSEL and using flip-chip techniques, the transmitter exhibits excellent hyperfrequency performances and compatibility with mass production due to the use of collective manufacturing technologies and passive optical alignment. This versatile approach is also applicable to high rate receivers, parallel optics emitters, and singlemode low cost transmitter integrating long wavelength VCSELs.
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| 13. |
- Bowallius, Olof, et al.
(författare)
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Scanning Capacitance Microscopy for Two-Dimensional Doping Profiling in Si- and InP-Based Device Structures
- 1999
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Ingår i: Physica Scripta T. - 0281-1847. ; 79, s. 163-166
-
Tidskriftsartikel (refereegranskat)abstract
- We report on the application of cross-sectional Scanning Capacitance Microscopy (SCM) for studying two-dimensional doping variations in Si and InP device structures. Different sample preparation methods were evaluated and the response of the SCM signal from various test structures, including epitaxially grown layers with n- and p-doping concentrations ranging from 5 × 1014 to 2 × 1019 cm-3, were examined under different imaging conditions. The technique was further evaluated by imaging a Si bipolar transistor structure and an InP-based buried heterostructure diode laser. We conclude that valuable information can be gained also from complex device structures.
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| 14. |
- Bowallius, Olof, et al.
(författare)
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Scanning capacitance microscopy investigations of buried heterostructure laser structures
- 1999
-
Ingår i: Applied Surface Science. ; 144-145:0, s. 137-140
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Tidskriftsartikel (refereegranskat)abstract
- In this work, InP-based buried heterostucture lasers are used to demonstrate the utility of scanning capacitance microscopy (SCM) for characterising complex device structures. The lasers use p-n junctions formed by selective regrowth of p and n doped InP layers around a mesa for current confinement. For comparison, the regrowth was performed by liquid phase epitaxy (LPE) and metal organic vapour phase epitaxy (MOVPE). Our investigations show that scanning capacitance microscopy is capable of detecting the p-n junctions formed at different regions of the device and thereby allows visualisation of the current confinement regions. Variations in the imaged depletion regions are attributed to doping variations due to modification of the regrowth process by the mesa. The SCM data show significant differences between the devices regrown by LPE and MOVPE and the results are consistent with the different regrowth mechanisms. Finally, the implications of the SCM data on device performance are discussed. © 1999 Elsevier Science B.V. All rights reserved.
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| 15. |
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| 16. |
- Chacinski, Marek, et al.
(författare)
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Dynamic properties of electrically p-n confined, epitaxially regrown 1.27 μm InGaAs single-mode vertical-cavity surface-emitting lasers
- 2009
-
Ingår i: IET optoelectronics. - : Institution of Engineering and Technology (IET). - 1751-8768 .- 1751-8776. ; 3:3, s. 163-167
-
Tidskriftsartikel (refereegranskat)abstract
- The dynamic performance including chirp measurements of 1.27 mu m single-mode InGaAs/GaAs vertical-cavity surface-emitting lasers (VCSELs) with a large gain-cavity offset is presented. The VCSELs are based on a novel p-n confinement structure with selective area epitaxial regrowth. A resonance frequency of 9.11 GHz, a slope efficiency of 0.25 W/A and an alpha-factor of 5.7 were measured. The modulation bandwidth is limited by electrical parasitics. Eye diagrams at 5 Gb/s with 7 dB extinction ratio and Q-factor around 5 were obtained. The results are compared with the performance of oxide-confined VCSELs with similar active layer and negative gain-cavity detuning.
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| 17. |
- Chacinski, Marek, et al.
(författare)
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Single-mode 1.27 μm InGaAs vertical cavity surface-emitting lasers with temperature-tolerant modulation characteristics
- 2005
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 86:21, s. 211109-1-211109-3
-
Tidskriftsartikel (refereegranskat)abstract
- The dynamic performance of InGaAs/GaAs 1.27 μ m single-mode vertical cavity surface emitting lasers (VCSELs) is presented. In order to reach such a long wavelength, the devices utilize highly strained double-quantum wells and a large detuning between the material gain peak and cavity resonance. It is found that the large detuning improves the temperature stability of both static and modulation characteristics. A resonance frequency of 7.8-9.5 GHz and optical power of 0.30 mW in fiber was maintained throughout the investigated temperature range of 20-90 ° C. The intrinsic response of the device suggests that long-wavelength InGaAs/GaAs VCSELs have the potential to be used as low cost uncooled optical transmitters at 10 Gbit/s. © 2005 American Institute of Physics. © 2005 American Institute of Physics.
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| 18. |
- Chang, Tzu-Hsuan, et al.
(författare)
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Selective release of InP heterostructures from InP substrates
- 2016
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Ingår i: Journal of Vacuum Science & Technology B. - : American Institute of Physics (AIP). - 1071-1023 .- 1520-8567. ; 34:4
-
Tidskriftsartikel (refereegranskat)abstract
- The authors report here a method of protecting the sidewall for the selective release of InGaAsP quantum-well (QW) heterostructure from InP substrates. An intact sidewall secured by SiO2 was demonstrated during the sacrificial layer selective etching, resulting in the suspended InGaAsP QW membranes which were later transferred to the Si substrate with polydimethylsiloxane stamp. The quality of the transferred InGaAsP QW membranes has been validated through photoluminescence and EL measurements. This approach could extend to arbitrary targeting substrate in numerous photonics and electronics applications.
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| 19. |
- Fan, W., et al.
(författare)
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Electrically-pumped membrane-reflector surface-emitters on silicon
- 2013
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Ingår i: 2013 IEEE Photonics Society Summer Topical Meeting Series, PSSTMS 2013. - : IEEE. - 9781467350600 ; , s. 19-20
-
Konferensbidrag (refereegranskat)abstract
- We report here electrically-pumped membrane reflector surface-emitters on silicon based on transferred InGaAsP QW structures sandwiched in between two single-layer Fano resonance photonic crystal membrane reflectors on silicon substrate.
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| 20. |
- Fan, W., et al.
(författare)
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Fabrication of electrically-pumped resonance-cavity membrane-reflector surface-emitters on silicon
- 2013
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Ingår i: 2013 IEEE Photonics Conference (IPC). - 9781457715075 ; , s. 643-644
-
Konferensbidrag (refereegranskat)abstract
- Various lasers and light sources on Si via heterogeneous integration of Si/III-V have been reported based on direct growth on Si [1] or wafer bonding technology [2-4]. We reported earlier optically-pumped Si membrane-reflector vertical-cavity surface-emitting lasers (MRVCSELs) fabricated by low-temperature membrane transfer printing processes [5, 6]. Here we report electrically-pumped devices based on an intra-cavity contact configuration.
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| 21. |
- Gholami, M., et al.
(författare)
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Alternation of band gap and localization of excitons in InGaNAs nanostructures with low nitrogen content
- 2008
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 19:31
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Tidskriftsartikel (refereegranskat)abstract
- Continuous wave photoluminescence (cw PL) spectroscopy has been used to study the optical properties of a set of InGaNAs epilayers and single quantum wells with nitrogen concentration less than a few per cent at different temperatures and different excitation powers. We found that nitrogen has a critical role on the emission light of InGaNAs nanostructures and the recombination mechanism. The incorporation of a few per cent of nitrogen leads to shrinkage of the InGaNAs band gap. The physical origin of such band gap reduction has been investigated both experimentally and theoretically by using a band anticrossing model. We have found that localization of excitons that have been caused by incorporation of a few per cent of nitrogen in these structures is the main explanation of such anomalous behavior observed in the low-temperature photoluminescence spectra of these nanostructures. The localization energies of carriers have been evaluated by studying the variation of the quantum well (QW) emission versus temperature, and it was found that the localization energy increases with increasing nitrogen composition. Our data also show that, with increasing excitation intensity, the PL peak position moves to higher energies (blue shift) due to the filling of localized states and capture centers for excitons by photo-generated carriers.
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| 22. |
- Gholami, M., et al.
(författare)
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Evaluation of optical quality and properties of Ga0.64In0.36N0.006As0.994 lattice matched to GaAs by using photoluminescence spectroscopy
- 2009
-
Ingår i: Opto-Electronics Review. - : Polish Academy of Sciences Chancellery. - 1230-3402 .- 1896-3757. ; 17:3, s. 260-264
-
Tidskriftsartikel (refereegranskat)abstract
- We have investigated optical properties of Ga0.64In0.36N0.006As0.994/GaAs single quantum-well structures using photoluminescence technique. We have found that nitrogen creates potential fluctuations in the InGaNAs structures, so it is the cause of trap centres in these structures and leads to localized excitons recombination dynamics. The near-band edge PL at 2 K exhibited a blueshift with an increase in excitation intensity of a sample but there is not such a shift in the PL peak position energy of same sample at 150 K. It has been found that PL spectra have a large full width at half maximum (FWHM) value at 2 K. These results are discussed in terms of carrier localization. Additionally, our results suggest decreasing PL integrated intensity in this structure, possibly due to non-radiative recombination. It has been shown that thermal annealing reduces the local strain created by nitrogen. By annealing process, a blue shifted emission can be observed.
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| 23. |
- Gilet, Ph., et al.
(författare)
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1.3 μm VCSELs : InGaAs/GaAs, GaInNAs/GaAs multiple quantum wells and InAs/GaAs quantum dots- Three candidates as active material
- 2007
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Ingår i: Vertical - Cavity Surface - Emitting Lasers XI. - San Jose, CA : SPIE. ; , s. F4840-F4840
-
Konferensbidrag (refereegranskat)abstract
- In this article, we report our results on 1.3ÎŒm VCSELs for optical interconnection applications. Room temperature continuous-wave lasing operation is demonstrated for top emitting oxide-confined devices with three different active materials, highly strained InGaAs/GaAs(A) and GalnNAs/GaAs (B) multiple quantum wells (MQW) or InAs/GaAs (C) quantum dots (QD). Conventional epitaxial structures grown respectively by Metal Organic Vapour Phase Epitaxy (MOVPE), Molecular Beam Epitaxy (MBE) and MBE, contain rully doped GaAs/AlGaAs DBRs. All three epilayers are processed in the same way. Current and optical confinement are realized by selective wet oxidation. Circular apertures from 2 ÎŒm to 16 ÎŒm diameters are defined. At room temperature and in continuous wave operation, all three systems exhibit lasing operation at wavelengths above 1 275nm and reached 1 300nm for material (A). Typical threshold currents are in the range [1-10]mA and are strongly dependent firstly on oxide diameter and secondly on temperature. Room temperature cw maximum output power corresponds respectively to 1.77mW, 0.5mW and 0.6mW. By increasing driving current, multimode operation occurs at different level depending on the oxide diameter. In case (A), non conventional modal behaviors will be presented and explained by the presence of specific oxide modes. Thermal behaviors of the different devices have been compared. In case (A) and (C) we obtain a negative T0. We will conclude on the different active materials in terms of performances with respect to 1300nm VCSEL applications.
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| 24. |
- Gustafsson, Oscar, et al.
(författare)
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A performance assessment of type-II interband In0.5Ga 0.5Sb QD photodetectors
- 2013
-
Ingår i: Infrared physics & technology. - : Elsevier BV. - 1350-4495 .- 1879-0275. ; 61, s. 319-324
-
Tidskriftsartikel (refereegranskat)abstract
- Self-assembled quantum-dot (QD) structures with type-II band alignment to the surrounding matrix material have been proposed as a III/V material approach to realize small-bandgap device structures suitable for photon detection and imaging in the long-wavelength infrared (LWIR) band. Here, we analyze the photoresponse of In0.5Ga0.5Sb/InAs QD photodiodes and estimate the system performance of type-II QD -based photodetectors. A review of alternative design approaches is presented and the choice of matrix material is discussed in terms of band alignment and its effect on the photoresponse. Photodiodes were fabricated consisting of 10 layers of In0.5Ga 0.5Sb QDs grown on InAs (0 0 1) substrates with metal-organic vapor-phase epitaxy (MOVPE). The photoresponse and dark current were measured in single pixel devices as a function of temperature in the range 20-230 K. The quantum efficiency shows an Arrhenius type behavior, which is attributed to hole trapping. This severely limits the detector performance at typical LWIR sensor operating temperatures (60-120 K). A device design with the matrix material InAs0.6Sb0.4 is proposed as a mean to improve the performance by reducing the barrier for hole transport. This can potentially allow type-II InGaSb QDs to be a competitive sensor material for LWIR detection.
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| 25. |
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| 26. |
- Gustafsson, Oscar, et al.
(författare)
-
Long-wavelength infrared photoluminescence from InGaSb/InAs quantum dots
- 2013
-
Ingår i: Infrared physics & technology. - : Elsevier BV. - 1350-4495 .- 1879-0275. ; 59, s. 89-92
-
Tidskriftsartikel (refereegranskat)abstract
- We study the growth of self-assembled InGaSb/InAs quantum dots (QDs) and investigate how gallium can be used to reduce the optical transition energy in the InSb QD system. InGaSb QDs were grown on InAs (0 0 1) substrates by metal-organic vapor-phase epitaxy (MOVPE) and the material was characterized by photoluminescence (PL) measurements. A PL peak wavelength is demonstrated beyond 8 μm at 77 K, which is significantly longer than what has been reported for InSb QDs. The results suggest that InGaSb QDs can be grown at a larger size than InSb QDs leading to reduced confinement in the QDs.
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| 27. |
- Gustafsson, Oscar, et al.
(författare)
-
Long-wavelength infrared quantum-dot based interband photodetectors
- 2011
-
Ingår i: Infrared physics & technology. - : Elsevier BV. - 1350-4495 .- 1879-0275. ; 54:3, s. 287-291
-
Tidskriftsartikel (refereegranskat)abstract
- We report on the design and fabrication of (Al)GaAs(Sb)/InAs tensile strained quantum-dot (QD) based detector material for thermal infrared imaging applications in the long-wavelength infrared (LWIR) regime. The detection is based on transitions between confined dot states and continuum states in a type-II band lineup, and we therefore refer to it as a dot-to-bulk (D2B) infrared photodetector with expected benefits including long carrier lifetime due to the type-II band alignment, suppressed Shockley-Read-Hall generation-recombination due to the relatively large-bandgap matrix material, inhibited Auger recombination processes due to the tensile strain and epitaxial simplicity. Metal-organic vapor-phase epitaxy was used to grow multiple (Al)GaAs(Sb) QD layers on InAs substrates at different QD nominal thicknesses, compositions, doping conditions and multilayer periods, and the material was characterized using atomic force and transmission electron microscopy, and Fourier-transform infrared absorption spectroscopy. Dot densities up to 1 x 10(11) cm(-2), 1 x 10(12) cm(-2) and 3 x 10(10) cm(-2) were measured for GaAs, AlGaAs and GaAsSb QDs, respectively. Strong absorption in GaAs, AlGaAs and GaAsSb multilayer QD samples was observed in the wavelength range 6-12 mu m. From the wavelength shift in the spectral absorption for samples with varying QD thickness and composition it is believed that the absorption is due to an intra- valance band transition. From this it is possible to estimate the type-II inter-band transition wavelength, thereby suggesting that (Al)GaAs(Sb) QD/InAs heterostructures are suitable candidates for LWIR detection and imaging.
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| 28. |
- Gustafsson, Oscar, et al.
(författare)
-
Photoluminescence photoresponse from InSb/InAs-based quantum dot structures
- 2012
-
Ingår i: Optics Express. - : Optical Society of America. - 1094-4087. ; 20:19, s. 21264-71
-
Tidskriftsartikel (refereegranskat)abstract
- InSb-based quantum dots grown by metal-organic vapor-phase epitaxy (MOVPE) on InAs substrates are studied for use as the active material in interband photon detectors. Long-wavelength infrared (LWIR) photoluminescence is demonstrated with peak emission at 8.5 μm and photoresponse, interpreted to originate from type-II interband transitions in a p-i-n photodiode, was measured up to 6 μm, both at 80 K. The possibilities and benefits of operation in the LWIR range (8-12 μm) are discussed and the results suggest that InSb-based quantum dot structures can be suitable candidates for photon detection in the LWIR regime.
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| 29. |
- Gustafsson, Oscar, 1983-
(författare)
-
Type-II interband quantum dot photodetectors
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Photon detectors based on single-crystalline materials are of great interest for high performance imaging applications due to their low noise and fast response. The major detector materials for sensing in the long-wavelength infrared (LWIR) band (8-14 µm) are currently HgCdTe (MCT) and AlGaAs/GaAs quantum wells (QW) used in intraband-based quantum-well infrared photodetectors (QWIPs). These either suffer from compositional variations that are detrimental to the system performance as in the case of MCT, or, have an efficient dark current generation mechanism that limits the operating temperature as for QWIPs. The need for increased on-wafer uniformity and elevated operating temperatures has resulted in the development of various alternative approaches, such as type-II strained-layer superlattice detectors (SLSs) and intraband quantum-dot infrared photodetectors (QDIPs).In this work, we mainly explore two self-assembled quantum-dot (QD) materials for use as the absorber material in photon detectors for the LWIR, with the aim to develop low-dark current devices that can allow for high operating temperatures and high manufacturability. The detection mechanism is here based on type-II interband transitions from bound hole states in the QDs to continuum states in the matrix material.Metal-organic vapor-phase epitaxy (MOVPE) was used to fabricate (Al)GaAs(Sb)/InAs and In(Ga)Sb/InAs QD structures for the development of an LWIR active material. A successive analysis of (Al)GaAs(Sb) QDs using absorption spectroscopy shows strong absorption in the range 6-12 µm interpreted to originate in intra-valence band transitions. Moreover, record-long photoluminescence (PL) wavelength up to 12 µm is demonstrated in InSb- and InGaSb QDs.Mesa-etched single-pixel photodiodes were fabricated in which photoresponse is demonstrated up to 8 µm at 230 K with 10 In0.5Ga0.5Sb QD layers as the active region. The photoresponse is observed to be strongly temperature-dependent which is explained by hole trapping in the QDs. In the current design, the photoresponse is thermally limited at typical LWIR sensor operating temperatures (60-120 K), which is detrimental to the imaging performance. This can potentially be resolved by selecting a matrix material with a smaller barrier for thermionic emission of photo-excited holes. If such an arrangement can be achieved, type-II interband InGaSb QD structures can turn out to be interesting as a high-operating-temperature sensor material for thermal imaging applications.
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| 30. |
- Gustafsson, Stefan, et al.
(författare)
-
Markers of imminent myocardial infarction
- 2024
-
Ingår i: Nature Cardiovascular Research. - : Springer Nature. - 2731-0590.
-
Tidskriftsartikel (refereegranskat)abstract
- Myocardial infarction is a leading cause of death globally but is notoriously difficult to predict. We aimed to identify biomarkers of an imminent first myocardial infarction and design relevant prediction models. Here, we constructed a new case–cohort consortium of 2,018 persons without prior cardiovascular disease from six European cohorts, among whom 420 developed a first myocardial infarction within 6 months after the baseline blood draw. We analyzed 817 proteins and 1,025 metabolites in biobanked blood and 16 clinical variables. Forty-eight proteins, 43 metabolites, age, sex and systolic blood pressure were associated with the risk of an imminent first myocardial infarction. Brain natriuretic peptide was most consistently associated with the risk of imminent myocardial infarction. Using clinically readily available variables, we devised a prediction model for an imminent first myocardial infarction for clinical use in the general population, with good discriminatory performance and potential for motivating primary prevention efforts.
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| 31. |
- Gyger, Samuel, et al.
(författare)
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Metropolitan single-photon distribution at 1550 nm for random number generation
- 2022
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 121:19, s. 194003-
-
Tidskriftsartikel (refereegranskat)abstract
- Quantum communication networks will connect future generations of quantum processors, enable metrological applications, and provide security through quantum key distribution. We present a testbed that is part of the municipal fiber network in the greater Stockholm metropolitan area for quantum resource distribution through a 20 km long fiber based on semiconductor quantum dots emitting in the telecom C-band. We utilize the service to generate random numbers passing the NIST test suite SP800-22 at a subscriber 8 km outside of the city with a bit rate of 23.4 kbit/s.
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| 32. |
- Gyger, Samuel, et al.
(författare)
-
Metropolitan Single-Photon Distribution at 1550 nm for Random Number Generation
- 2023
-
Ingår i: 2023 Conference on Lasers and Electro-Optics, CLEO 2023. - : Institute of Electrical and Electronics Engineers Inc..
-
Konferensbidrag (refereegranskat)abstract
- Quantum communication networks are used for QKD and metrological applications. We present research connecting two nodes ≈ 20 kilometers apart over the municipal fiber network using semiconductor quantum dots emitting at 1550 nm.
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| 33. |
- Gyger, Samuel, et al.
(författare)
-
Reconfigurable frequency coding of triggered single photons in the telecom C–band
- 2019
-
Ingår i: Optics Express. - : OSA. - 1094-4087. ; 27:10, s. 14400-14406
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, we demonstrate reconfigurable frequency manipulation of quantum states of light in the telecom C–band. Triggered single photons are encoded in a superposition state of three channels using sidebands up to 53 GHz created by an off-the-shelf phase modulator. The single photons are emitted by an InAs/GaAs quantum dot grown by metal-organic vapor-phase epitaxy within the transparency window of the backbone fiber optical network. A cross-correlation measurement of the sidebands demonstrates the preservation of the single photon nature; an important prerequisite for future quantum technology applications using the existing telecommunication fiber network.
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| 34. |
- Hagstrom, Hannes, et al.
(författare)
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Cardiovascular risk factors in non-alcoholic fatty liver disease
- 2019
-
Ingår i: Liver international (Print). - : WILEY. - 1478-3223 .- 1478-3231. ; 39:1, s. 197-204
-
Tidskriftsartikel (refereegranskat)abstract
- Background amp; Aims Patients with non-alcoholic fatty liver disease (NAFLD) are at an increased risk for cardiovascular disease (CVD). It is unclear whether histological variables may help predict CVD risk. We evaluated histology and traditional CV risk factors as predictors of CVD outcomes in a large NAFLD cohort. Methods We included 603 biopsy-proven NAFLD patients free of baseline CVD and matched these (1:10, by age, sex and municipality) to 6269 population controls. All individuals were cross-linked to national registries to ascertain incident CVD events, defined as acute ischaemic heart disease or stroke. The presence of CV risk factors and liver histology were available in NAFLD patients only. Cox regression models were used to estimate hazard ratios (HR) for incident CVD. Results During a mean follow-up of 18.6 years, 168 (28%) of NAFLD patients and 1325 (21%) of controls experienced a CVD event (HR 1.54, 95%CI 1.30-1.83). Within the NAFLD cohort, age, male sex, type 2 diabetes, smoking and triglycerides were associated with risk of CVD. Taking these CV risk factors into account, no histological parameter, including presence of NASH and fibrosis stage, were associated with incident CVD. Conclusions Patients with NAFLD are at an increased risk for CVD compared to matched controls, but histological parameters do not seem to independently predict this risk.
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| 35. |
- Hagström, Hannes, et al.
(författare)
-
Fibrosis stage but not NASH predicts mortality and time to development of severe liver disease in biopsy-proven NAFLD
- 2017
-
Ingår i: Journal of Hepatology. - : ELSEVIER SCIENCE BV. - 0168-8278 .- 1600-0641. ; 67:6, s. 1265-1273
-
Tidskriftsartikel (refereegranskat)abstract
- Background amp; Aims: Non-alcoholic fatty liver disease (NAFLD) is very common in the general population, but identifying patients with increased risk of mortality and liver-specific morbidity remains a challenge. Non-alcoholic steatohepatitis (NASH) is thought to enhance this risk; therefore, resolution of NASH is a major endpoint in current pharmacologic studies. Herein, we aim to investigate the long-term prognosis of a large cohort of NAFLD patients, and to study the specific effect of NASH and fibrosis stage on prognosis. Methods: We conducted a retrospective cohort study of 646 biopsy-proven NAFLD patients. Each case was matched for age, sex and municipality to ten controls. Outcomes on mortality and severe liver disease, defined as cirrhosis, liver decompensation/failure or hepatocellular carcinoma, were evaluated using population-based registers. Cox regression models adjusted for age, sex and type 2 diabetes were used to examine the long-term risk according to fibrosis stage. Likelihood ratio tests were used to assess whether adding NASH to these models increased the predictive capacity. Laplace regression was used to estimate the time to severe liver disease according to stage of fibrosis. Results: During a follow-up of mean 20 years (range 0-40) equivalent to 139,163 person-years, 12% of NAFLD patients and 2.2% of controls developed severe liver disease (p amp;lt; 0.001). Compared to controls, the risk of severe liver disease increased per stage of fibrosis (hazard ratio ranging from 1.9 in F0 to 104.9 in F4). Accounting for the presence of NASH did not change these estimates significantly (likelihood ratio test amp;gt; 0.05 for all stages of fibrosis). Similar results were seen for overall mortality. The lower end of the 95% confidence interval for the 10th percentile of time to development of severe liver disease was 22-26 years in F0-1, 9.3 years in F2, 2.3 years in F3, and 0.9 years to liver decompensation in F4. Conclusions: In this, the largest ever study of biopsy-proven NAFLD, the presence of NASH did not increase the risk of liver-specific morbidity or overall mortality. Knowledge of time to development of severe liver disease according to fibrosis stage can be used in individual patient counselling and for public health decisions. (C) 2017 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved.
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| 36. |
- Hagström, Hannes, et al.
(författare)
-
Health Care Costs of Patients With Biopsy-Confirmed Nonalcoholic Fatty Liver Disease Are Nearly Twice Those of Matched Controls
- 2020
-
Ingår i: Clinical Gastroenterology and Hepatology. - : ELSEVIER SCIENCE INC. - 1542-3565 .- 1542-7714. ; 18:7, s. 1592-
-
Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND & AIMS: Data on healthcare resource use and costs associated with nonalcoholic fatty liver disease (NAFLD) in clinical practice are lacking. We compared real-life healthcare costs of patients with NAFLD to matched controls. METHODS: We performed a retrospective study of 646 patients with biopsy-proven NAFLD in Sweden from 1971 through 2009. Each patient was matched for age, sex, and county of residence with 10 persons from the general population (controls). We retrieved all healthcare contacts through Dec 31, 2014 from national registers. Unit costs were assigned to arrive at a total healthcare cost (in USD [$]) per study subject. RESULTS: During a mean follow-up of 19.9 years, we recorded a mean of 0.27 hospitalizations per year for patients with NAFLD vs 0.16 for controls (P <.001). This corresponded to an incremental cost of $635 per year for patients with NAFLD. Patients with NAFLD had a higher mean use of outpatient care visits: 1.46 contacts per year compared with 0.86 per year in controls, corresponding to $255 in additional costs (P <.001). Total costs incurred by patients with stage 3-4 fibrosis were higher than by patients with fibrosis stage 0-2 (mean annual costs, $4397 vs $629). Cumulative costs were higher for all stages of fibrosis compared to controls. CONCLUSIONS: Healthcare costs are nearly twice as high in patients with NAFLD than in matched controls. This is mostly attributable to higher costs for hospitalizations, but also to more outpatient visits. Patients with advanced fibrosis had the highest costs.
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| 37. |
- Hagström, Hannes, et al.
(författare)
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Risk for development of severe liver disease in lean patients with nonalcoholic fatty liver disease : A long-term follow-up study.
- 2018
-
Ingår i: Hepatology communications. - : John Wiley & Sons. - 2471-254X. ; 2:1, s. 48-57
-
Tidskriftsartikel (refereegranskat)abstract
- Most patients with nonalcoholic fatty liver disease (NAFLD) are overweight or obese. However, a significant proportion of patients have a normal body mass index (BMI), denoted as lean NAFLD. The long-term prognosis of lean NAFLD is unclear. We conducted a cohort study of 646 patients with biopsy-proven NAFLD. Patients were defined as lean (BMI < 25.0), overweight (BMI 25.0-29.9), or obese (BMI ≥ 30.0) at the time of biopsy. Each case was matched for age, sex, and municipality to 10 controls. Overall mortality and development of severe liver disease were evaluated using population-based registers. Cox regression models adjusted for age, sex, type 2 diabetes, and fibrosis stage were used to examine the long-term risk of mortality and liver-related events in lean and nonlean NAFLD. Lean NAFLD was seen in 19% of patients, while 52% were overweight and 29% were obese. Patients with lean NAFLD were older, had lower transaminases, lower stages of fibrosis, and lower prevalence of nonalcoholic steatohepatitis at baseline compared to patients with a higher BMI. During a mean follow-up of 19.9 years (range 0.4-40 years) representing 12,631 person years and compared to patients who were overweight, patients with lean NAFLD had no increased risk for overall mortality (hazard ratio 1.06; P = 0.73) while an increased risk for development of severe liver disease was found (hazard ratio 2.69; P = 0.007). Conclusion: Although patients with lean NAFLD have lower stages of fibrosis, they are at higher risk for development of severe liver disease compared to patients with NAFLD and a higher BMI, independent of available confounders. (Hepatology Communications 2018;2:48-57).
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| 38. |
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| 39. |
- Hammar, Mattias, et al.
(författare)
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1.3-mu m InGaAs vertical-cavity surface-emitting lasers
- 2005
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Ingår i: 2005 IEEE LEOS Annual Meeting Conference Proceedings (LEOS). - 0780392175 ; , s. 396-397
-
Konferensbidrag (refereegranskat)abstract
- We report on the fabrication and performance of N-free InGaAs/GaAs 1.3-mu m range vertical-cavity surface-emitting lasers (VCSELs). Using optimized quantum-well (QW) growth conditions in combination with negative gain-cavity tuning, high-performance VCSELs with emission wavelength up to 1300 nm are realized. The performance figures include mA-range threshold currents, mW-range singlemode output power, continuous-wave operation up to 140 degrees C and 10 Gbit/s data transmission.
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| 40. |
- Hammar, Mattias, 1961-, et al.
(författare)
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Compound Semiconductors
- 2021
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 258:2
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Tidskriftsartikel (refereegranskat)
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| 41. |
- Hammar, Mattias, 1961-, et al.
(författare)
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Compound Semiconductors
- 2022
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 219:4
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 42. |
- Hammar, Mattias, 1961-, et al.
(författare)
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Impact-collision ion-scattering spectrometry studies of the VC0.8(111)-(8 × 1) surface
- 1994
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Ingår i: Nuclear Inst. and Methods in Physics Research, B. - 0168-583X. ; 85:1-4, s. 429-434
-
Tidskriftsartikel (refereegranskat)abstract
- We use impact-collision ion-scattering spectrometry (ICISS) to verify that the (8 × 1) reconstructed surface of VC0.8(111) arises from the superposition of a rectangular surface vanadium layer on top of the hexagonal bulk, in agreement with a surface structure proposed earlier in a scanning tunneling microscopy (STM) study. However, a major discrepancy with the STM results is the absence of any pronounced buckling within the surface unit cell. The ICISS results suggest that the vanadium layer on the surface is perfectly planar. © 1994.
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| 43. |
- Hammar, Mattias, et al.
(författare)
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Room-temperature operation of 980-nm transistor-vertical-cavity surface-emitting lasers
- 2013
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Ingår i: 2013 IEEE 6th International Conference on Advanced Infocomm Technology, ICAIT 2013. - : IEEE. - 9781479904655 ; , s. 141-142
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Konferensbidrag (refereegranskat)abstract
- We report on the design, fabrication and characterization of pnp-type 980-nm transistor-vertical-cavity surface-emitting lasers (T-VCSELs). Using an epitaxial regrowth process and a triple-intracavity current injection scheme we demonstrate static performance levels quite comparable to those of conventional VCSELs, including sub-mA threshold base current, mW-range output power and continuous-wave operation at least up to 50°C.
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| 44. |
- Hammar, Mattias, 1961-, et al.
(författare)
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Scanning tunnelling microscopy studies of Pt80Fe 20(110)
- 1993
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Ingår i: Journal of Physics: Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 5:18, s. 2837-2842
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Tidskriftsartikel (refereegranskat)abstract
- Results of scanning tunnelling microscopy (STM) show that two different kinds of surface structures coexist on the (110) surface of Pt 80Fe20. The predominant structure corresponds well to the missing row type (1*2) reconstruction previously observed on, for example Pt(110), whereas the minority domains show a faintly buckled structure with approximately (1*1) geometry. Based on the STM images, the authors propose that the two domains have different surface chemical compositions and correspond to two different metallurgical phases known for the PtFe system at this composition.
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| 45. |
- Höglund, Linda, 1974-
(författare)
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Growth and characterisation of InGaAs-based quantum dots-in-a-well infrared photodetectors
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents results from the development of quantum dot (QD) based infrared photodetectors (IPs). The studies include epitaxial growth of QDs, investigations of the structural, optical and electronic properties of QD-based material as well as characterisation of the resulting components.Metal-organic vapour phase epitaxy is used for growth of self-assembled indium arsenide (InAs) QDs on gallium arsenide (GaAs) substrates. Through characterisation by atomic force microscopy, the correlation between size distribution and density of quantum dots and different growth parameters, such as temperature, InAs deposition time and V/III-ratio (ratio between group V and group III species) is achieved. The V/III-ratio is identified as the most important parameter in finding the right growth conditions for QDs. A route towards optimisation of the dot size distribution through successive variations of the growth parameters is presented.The QD layers are inserted in In0.15Ga0.85As/GaAs quantum wells (QWs), forming so-called dots-in-a-well (DWELL) structures. These structures are used to fabricate IPs, primarily for detection in the long wavelength infrared region (LWIR, 8-14 μm).The electron energy level schemes of the DWELL structures are revealed by a combination of different experimental techniques. From Fourier transform photoluminescence (FTPL) and FTPL excitation (FTPLE) measurements the energy level schemes of the DWELL structures are deduced. Additional information on the energy level schemes is obtained from tunneling capacitance measurements and the polarization dependence studies of the interband transitions. From tunneling capacitance measurements, the QD electron energy level separation is confirmed to be 40-50 meV and from the polarization dependence measurements, the heavy hole character of the upper hole states are revealed.Further characterisation of the IPs, by interband and intersubband photocurrent measurements as well as dark current measurements, is performed. By comparing the deduced energy level scheme of the DWELL structure and the results of the intersubband photocurrent measurements, the origin of the photocurrent is determined. The main intersubband transition contributing to the photocurrent is identified as the QD ground state to a QW excited state transition. Optical pumping is employed to gain information on the origin of an additional photocurrent peak observed only at temperatures below 60 K. By pumping resonantly with transitions associated with certain quantum dot energy levels, this photocurrent peak is identified as an intersubband transition emanating from the quantum dot excited state. Furthermore, the detector response is increased by a factor of 10, when using simultaneous optical pumping into the quantum dots states, due to the increasing electron population created by the pumping. In this way, the potentially achievable responsivity of the detector is predicted to be 250 mA/W.Significant variations of photocurrent and dark currents are observed, when bias and temperature are used as variable parameters. The strong bias and temperature dependence of the photocurrent is attributed to the escape route from the final state in the QW, which is limited by tunneling through the triangular barrier. Also the significant bias and temperature dependence of the dark current could be explained in terms of the strong variation of the escape probability from different energy states in the DWELL structure, as revealed by interband photocurrent measurements. These results are important for the future optimisation of the DWELL IP.Tuning of the detection wavelength within the LWIR region is achieved by means of a varying bias across the DWELL structure. By positioning the InAs quantum dot layer asymmetrically in a 8 nm wide In0.15Ga0.85As/GaAs quantum well, a step-wise shift in the detection wavelength from 8.4 to 10.3 μm could be achieved by varying the magnitude and polarity of the applied bias. These tuning properties could be essential for applications such as odulators and dual-colour infrared detection.
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| 46. |
- Iezekiel, S., et al.
(författare)
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Transistor lasers and their expected applications in microwave photonics
- 2015
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Ingår i: 2015 17th International Conference on Transparent Optical Networks (ICTON). - : IEEE Computer Society. - 9781467378802
-
Konferensbidrag (refereegranskat)abstract
- The transistor laser is a relatively new device that is based on the homogeneous integration of a heterojunction bipolar transistor with a laser diode. As such it offers very unique functionality, and the different device dynamics (compared to conventional lasers) offer a number of benefits, included increased modulation bandwidth, reduced chirp, reduced relative intensity noise and the option to both reduce intermodulation distortion and to eliminate the need for monitor photodiodes thorough the use of collector current feedback. Here we review recent progress in both the fabrication and modelling of this device, and point to its potential applications in a number of areas of microwave photonics, including links and optoelectronic mixing.
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| 47. |
- Jayasinghe, R. C., et al.
(författare)
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Plasma frequency and dielectric function dependence on doping and temperature for p-type indium phosphide epitaxial films
- 2012
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 24:43, s. 435803-
-
Tidskriftsartikel (refereegranskat)abstract
- The optical properties of p-type InP epitaxial films with different doping concentrations are investigated by infrared absorption measurements accompanied by reflection and transmission spectra taken from 25 to 300 K. A complete dielectric function (DF) model, including intervalence band (IVB) transitions, free-carrier and lattice absorption, is used to determine the optical constants with improved accuracy in the spectral range from 2 to 35 mu m. The IVB transitions by free holes among the split-off, light-hole, and heavy-hole bands are studied using the DF model under the parabolic-band approximation. A good understanding of IVB transitions and the absorption coefficient is useful for designing high operating temperature and high detectivity infrared detectors and other optoelectronic devices. In addition, refractive index values reported here are useful for optoelectronic device designing, such as implementing p-InP waveguides in semiconductor quantum cascade lasers. The temperature dependence of hole effective mass and plasma frequency is also reported.
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| 48. |
- Karim, Amir, et al.
(författare)
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Characterization of InSb QDs grown on InAs (100) substrate by MBE and MOVPE
- 2012
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Ingår i: Proc SPIE Int Soc Opt Eng. - : SPIE. - 9780819491312
-
Konferensbidrag (refereegranskat)abstract
- We report on the optical and structural characterization of InSb QDs in InAs matrix, grown on InAs (100) substrates, for infrared photodetection. InSb has 7% lattice mismatch with InAs forming strained QDs, which are promising for longwave IR applications, due to their type-II band alignment. This report contains material development results of InSb QDs for increasing their emission wavelength towards long-wave IR region. Samples were grown by two techniques of MBE and MOVPE, with different InSb coverage on InAs (100) substrates. Structures grown by MBE reveal QD related photoluminescence at 4 μm. AFM investigations of the MBE grown structures showed uncapped dots of ∼ 35 nm in size and ∼ 3 nm in height, with a density of about 2 × 1010 cm -2. Cross-section TEM investigations of buried InSb layers grown by MBE showed coherently strained QDs for nominal InSb coverage in the range of 1.6 - 2 monolayers (MLs). Layers with InSb coverage more than 2MLs contain relaxed QDs with structural defects due to large amount of strain between InSb and InAs. Samples with such large dots did not show any InSb related luminescence. The MOVPE grown InSb samples exhibit a strong QD related emission between 3.8 to 7.5 μm, depending on the amount of InSb coverage and other growth parameters. We report the longest wavelength observed so far in this material system.
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| 49. |
- Karim, Amir, et al.
(författare)
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In(Ga)Sb/InAs quantum dot based IR photodetectors with thermally activated photoresponse
- 2013
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Ingår i: Proceedings of SPIE. - : SPIE. - 9780819494955
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Konferensbidrag (refereegranskat)abstract
- We report on the device characterization of In(Ga)Sb/InAs quantum dots (QDs) based photodetectors for long wave IR detectors. The detection principle of these quantum-dot infrared photodetectors (QDIPs) is based on the spatially indirect transition between the In(Ga)Sb QDs and the InAs matrix, as a result of the type-II band alignment. Such photodetectors are expected to have lower dark currents and higher operating temperatures compared to the current state of the art InSb and mercury cadmium telluride (MCT) technology. The In(Ga)Sb QD structures were grown using metal-organic vapour-phase epitaxy and explored using structural, electrical and optical characterization techniques. Material development resulted in obtaining photoluminescence up to 10 μm, which is the longest wavelength reported in this material system. We have fabricated different photovoltaic IR detectors from the developed material that show absorption up to 8 μm. Photoresponse spectra, showing In(Ga)Sb QD related absorption edge, were obtained up to 200 K. Detectors with different In(Ga)Sb QDs showing different cut-off wavelengths were investigated for photoresponse. Photoresponse in these detectors is thermally activated with different activation energies for devices with different cut-off wavelengths. Devices with longer cut-off wavelength exhibit higher activation energies. We can interpret this using the energy band diagram of the dots/matrix system for different QD sizes.
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- Khartsev, Sergey, et al.
(författare)
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High‐Quality Si‐Doped β‐Ga 2 O 3 Films on Sapphire Fabricated by Pulsed Laser Deposition
- 2020
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 258:2, s. 2000362-2000362
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Tidskriftsartikel (refereegranskat)abstract
- Pulsed laser ablation is used to form high-quality silicon-doped β-Ga2O3 films on sapphire by alternatively depositing Ga2O3 and Si from two separate sources. X-ray analysis reveals a single crystallinity with a full width at half maximum for the rocking curve around the (−201) reflection peak of 1.6°. Silicon doping concentration is determined by elastic recoil detection analysis (ERDA), and the best electrical performance is reached at a Si concentration of about 1 × 1020 cm−3, using optimized deposition parameters. It is found that a high crystalline quality and a mobility of about 2.9 cm2 (V s)−1 can be achieved by depositing Si and Ga2O3 from two separate sources. Two types of Schottky contacts are fabricated: one with a pure Pt and one with a PtOx composition. Electrical results from these structures are also presented.
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