| 1. |
- Allentoft, Morten E., et al.
(författare)
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Population genomics of post-glacial western Eurasia
- 2024
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Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 625:7994, s. 301-311
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Tidskriftsartikel (refereegranskat)abstract
- Western Eurasia witnessed several large-scale human migrations during the Holocene1–5. Here, to investigate the cross-continental effects of these migrations, we shotgun-sequenced 317 genomes—mainly from the Mesolithic and Neolithic periods—from across northern and western Eurasia. These were imputed alongside published data to obtain diploid genotypes from more than 1,600 ancient humans. Our analyses revealed a ‘great divide’ genomic boundary extending from the Black Sea to the Baltic. Mesolithic hunter-gatherers were highly genetically differentiated east and west of this zone, and the effect of the neolithization was equally disparate. Large-scale ancestry shifts occurred in the west as farming was introduced, including near-total replacement of hunter-gatherers in many areas, whereas no substantial ancestry shifts happened east of the zone during the same period. Similarly, relatedness decreased in the west from the Neolithic transition onwards, whereas, east of the Urals, relatedness remained high until around 4,000 bp, consistent with the persistence of localized groups of hunter-gatherers. The boundary dissolved when Yamnaya-related ancestry spread across western Eurasia around 5,000 bp, resulting in a second major turnover that reached most parts of Europe within a 1,000-year span. The genetic origin and fate of the Yamnaya have remained elusive, but we show that hunter-gatherers from the Middle Don region contributed ancestry to them. Yamnaya groups later admixed with individuals associated with the Globular Amphora culture before expanding into Europe. Similar turnovers occurred in western Siberia, where we report new genomic data from a ‘Neolithic steppe’ cline spanning the Siberian forest steppe to Lake Baikal. These prehistoric migrations had profound and lasting effects on the genetic diversity of Eurasian populations.
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| 2. |
- Papamichail, A., et al.
(författare)
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Compositionally graded channel HEMTs towards improved linearity for low-noise RF amplifiers
- 2022
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Ingår i: 2022 Compound Semiconductor Week, CSW 2022.
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Konferensbidrag (refereegranskat)abstract
- Although AlGaN/GaN HEMTs with high power and current gain have been demonstrated in RF device applications, at high signal operation they show an inherent non-linear behavior which leads to gain compression and signal distortion. Polarization-doped AlGaN/GaN HEMTs, with a compositionally graded channel enables a linear response improvement through formation of a 3-D electron gas. In this work, we develop the growth process for graded channel HEMTs in a hot-wall MOCVD reactor. Control of the grading profile is established through growth parameter tuning. Afterwards, analysis by EDS allows for precise determination of Al composition across the channel. Conventional and graded channel HEMT structures were fabricated and characterized. Furthermore, the sheet resistance, carrier density and mobility in HEMT structures with different grading profiles are compared and discussed. The conventional (non-graded) structure revealed the highest electron mobility of ~2350 cm2/V.s, which is among the highest values reported.
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| 3. |
- Chen, Ding-Yuan, 1991, et al.
(författare)
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Structural investigation of ultra-low resistance deeply recessed sidewall ohmic contacts for AlGaN/GaN HEMTs based on Ti/Al/Ti-metallization
- 2023
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Ingår i: Semiconductor Science and Technology. - : IOP Publishing Ltd. - 1361-6641 .- 0268-1242. ; 38:10
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Tidskriftsartikel (refereegranskat)abstract
- This study presents a novel approach to forming low-resistance ohmic contacts for AlGaN/GaN HEMTs. The optimized contacts exhibit an outstanding contact resistance of approximately 0.15 & omega;& BULL;mm. This is achieved by firstly recessing the barrier of the heterostructure to a depth beyond the channel. In this way, the channel region is exposed on the sidewall of the recess. The coverage of the Ti/Al/Ti ohmic metalization on the sidewall is ensured through tilting of the sample during evaporation. The annealing process is performed at a low temperature of 550 & DEG;C. The approach does not require precise control of the recess etching. Furthermore, the method is directly applicable to most barrier designs in terms of thickness and Al-concentration. The impact of recessed sidewall angle, thickness and ratio of Ti and Al layers, and the annealing procedure are investigated. Structural and chemical analyses of the interface between the ohmic contacts and epi-structure indicate the formation of ohmic contacts by the extraction of nitrogen from the epi-structure. The approach is demonstrated on HEMT-structures with two different barrier designs in terms of Al-concentration and barrier thickness. The study demonstrate large process window in regard to recess depth and duration of the annealing as well as high uniformity of the contact resistance across the samples, rendering the approach highly suitable for industrial production processes.
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| 4. |
- Ding Yuan, Chen, 1991, et al.
(författare)
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Impact of in situ NH3 pre-treatment of LPCVD SiN passivation on GaN HEMT performance
- 2022
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Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 1361-6641 .- 0268-1242. ; 37:3
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Tidskriftsartikel (refereegranskat)abstract
- The impact on the performance of GaN high electron mobility transistors (HEMTs) of in situ ammonia (NH3) pre-treatment prior to the deposition of silicon nitride (SiN) passivation with low-pressure chemical vapor deposition (LPCVD ) is investigated. Three different NH3 pre-treatment durations (0, 3, and 10 min) were compared in terms of interface properties and device performance. A reduction of oxygen (O) at the interface between SiN and epi-structure is detected by scanning transmission electron microscopy (STEM )-electron energy loss spectroscopy (EELS) measurements in the sample subjected to 10 min of pre-treatment. The samples subjected to NH3 pre-treatment show a reduced surface-related current dispersion of 9% (compared to 16% for the untreated sample), which is attributed to the reduction of O at the SiN/epi interface. Furthermore, NH3 pre-treatment for 10 min significantly improves the current dispersion uniformity from 14.5% to 1.9%. The reduced trapping effects result in a high output power of 3.4 W mm(-1) at 3 GHz (compared to 2.6 W mm(-1) for the untreated sample). These results demonstrate that the in situ NH3 pre-treatment before LPCVD of SiN passivation is critical and can effectively improves the large-signal microwave performance of GaN HEMTs.
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| 5. |
- Persson, Axel, et al.
(författare)
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Correlating cathodoluminescence and scanning transmission electron microscopy for InGaN platelet nano-LEDs
- 2023
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 123:2
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Tidskriftsartikel (refereegranskat)abstract
- Structural defects are detrimental to the efficiency and quality of optoelectronic semiconductor devices. In this work, we study InGaN platelets with a quantum well structure intended for nano-LEDs emitting red light and how their optical properties, measured with cathodoluminescence, relate to the corresponding atomic structure. Through a method of spectroscopy-thinning-imaging, we demonstrate in plan-view how stacking mismatch boundaries intersect the quantum well in a pattern correlated with the observed diminished cathodoluminescence intensity. The results highlight the importance of avoiding stacking mismatch in small LED structures due to the relatively large region of non-radiative recombination caused by the mismatch boundaries.
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| 6. |
- Zhang, Hengfang, et al.
(författare)
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On the polarity determination and polarity inversion in nitrogen-polar group III-nitride layers grown on SiC
- 2022
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 131:5
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the interfaces and polarity domains at the atomic scale in epitaxial AlN and GaN/AlN grown by hot-wall metal organic chemical vapor epitaxy on the carbon face of SiC. X-ray diffraction, potassium hydroxide (KOH) wet chemical etching, and scanning transmission electron microscopy combined provide an in-depth understanding of polarity evolution with the film thickness, which is crucial to optimize growth. The AlN grown in a 3D mode is found to exhibit N-polar pyramid-type structures at the AlN-SiC interface. However, a mixed N-polar and Al-polar region with Al-polarity domination along with inverted pyramid-type structures evolve with increasing film thickness. We identify inclined inversion domain boundaries and propose that incorporation of oxygen on the & lang;40-41 & rang; facets of the N-polar pyramids causes the polarity inversion. We find that mixed-polar AlN is common and easily etched and remains undetected by solely relying on KOH etching. Atomic scale electron microscopy is, therefore, needed to accurately determine the polarity. The polarity of GaN grown on mixed-polar AlN is further shown to undergo complex evolution with the film thickness, which is discussed in the light of growth mechanisms and polarity determination methods.
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| 7. |
- Zhang, Hengfang, et al.
(författare)
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Polarity Control by Inversion Domain Suppression in N-Polar III-Nitride Heterostructures
- 2023
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Ingår i: Crystal Growth and Design. - : American Chemical Society (ACS). - 1528-7483 .- 1528-7505. ; 23:2, s. 1049-1056
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Tidskriftsartikel (refereegranskat)abstract
- Nitrogen-polar III-nitride heterostructures offer advantages over metal-polar structures in high frequency and high power applications. However, polarity control in III-nitrides is difficult to achieve as a result of unintentional polarity inversion domains (IDs). Herein, we present a comprehensive structural investigation with both atomic detail and thermodynamic analysis of the polarity evolution in low- and high-temperature AlN layers on on-axis and 4° off-axis carbon-face 4H-SiC (0001̅) grown by hot-wall metal organic chemical vapor deposition. A polarity control strategy has been developed by variation of thermodynamic Al supersaturation and substrate misorientation angle in order to achieve the desired growth mode and polarity. We demonstrate that IDs are completely suppressed for high-temperature AlN nucleation layers when a step-flow growth mode is achieved on the off-axis substrates. We employ this approach to demonstrate high quality N-polar epitaxial AlGaN/GaN/AlN heterostructures.
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| 8. |
- Nilvebrant, N.-O., et al.
(författare)
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Limits for alkaline detoxification of dilute-acid lignocellulose hydrolysates
- 2003
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Ingår i: Applied Biochemistry and Biotechnology. - 1559-0291. ; 107:1-3, s. 615-628
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Tidskriftsartikel (refereegranskat)abstract
- In addition to fermentable sugars, dilute-acid hydrolysates of lignocellulose contain compounds that inhibit fermenting microorganisms, such as Saccharomyces cerevisiae. Previous results show that phenolic compounds and furan aldehydes, and to some extent aliphatic acids, act as inhibitors during fermentation of dilute-acid hydrolysates of spruce. Treatment of lignocellulose hydrolysates with alkali, usually in the form of overliming to pH 10.0, has been frequently employed as a detoxification method to improve fermentability. A spruce dilute-acid hydrolysate was treated with NaOH in a factorial design experiment, in which the pH was varied between 9.0 and 12.0, the temperature between 5 and 80°C, and the time between 1 and 7 h. Already at pH 9.0, >25% of the glucose was lost when the hydrolysate was treated at 80°C for 1 h. Among the monosaccharides, xylose was degraded faster under alkaline conditions than the hexoses (glucose, mannose, and galactose), which, in turn, were degraded faster than arabinose. The results suggest that alkali treatment of hydrolysates can be performed at temperatures below 30°C at any pH between 9.0 and 12.0 without problems with sugar degradation or formation of inhibiting aliphatic acids. Treatment with Ca(OH)2 instead of NaOH resulted in more substantial degradation of sugars. Under the harsher conditions of the factorial design experiment, the concentrations of furfural and 5-hydroxymethylfurfural decreased while the total phenolic content increased. The latter phenomenon was tentatively attributed to fragmentation of soluble aromatic oligomers in the hydrolysate. Separate phenolic compounds were affected in different ways by the alkaline conditions with some compounds showing an increase in concentration while others decreased. In conclusion, the conditions used for detoxification with alkali should be carefully controlled to optimize the positive effects and minimize the degradation of fermentable sugars.
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