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- Steiner, Ann, et al.
(författare)
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Introduktion
- 2017
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Ingår i: Litterära värdepraktiker : aktörer, rum, platser - aktörer, rum, platser. - 9789170612541 ; , s. 9-26
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 5. |
- Bi, Zhaoxia, et al.
(författare)
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InGaN Platelets : Synthesis and Applications toward Green and Red Light-Emitting Diodes
- 2019
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Ingår i: Nano Letters. - : American Chemical Society. - 1530-6984 .- 1530-6992. ; 19:5, s. 2832-2839
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we present a method to synthesize arrays of hexagonal InGaN submicrometer platelets with a top c-plane area having an extension of a few hundred nanometers by selective area metal-organic vapor-phase epitaxy. The InGaN platelets were made by in situ annealing of InGaN pyramids, whereby InGaN from the pyramid apex was thermally etched away, leaving a c-plane surface, while the inclined {101Ì1} planes of the pyramids were intact. The as-formed c-planes, which are rough with islands of a few tens of nanometers, can be flattened with InGaN regrowth, showing single bilayer steps and high-quality optical properties (full width at half-maximum of photoluminescence at room temperature: 107 meV for In 0.09 Ga 0.91 N and 151 meV for In 0.18 Ga 0.82 N). Such platelets offer surfaces having relaxed lattice constants, thus enabling shifting the quantum well emission from blue (as when grown on GaN) to green and red. For single InGaN quantum wells grown on the c-plane of such InGaN platelets, a sharp interface between the quantum well and the barriers was observed. The emission energy from the quantum well, grown under the same conditions, was shifted from 2.17 eV on In 0.09 Ga 0.91 N platelets to 1.95 eV on In 0.18 Ga 0.82 N platelets as a result of a thicker quantum well and a reduced indium pulling effect on In 0.18 Ga 0.82 N platelets. On the basis of this method, prototype light-emitting diodes were demonstrated with green emission on In 0.09 Ga 0.91 N platelets and red emission on In 0.18 Ga 0.82 N platelets.
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- Bi, Zhaoxia, et al.
(författare)
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Self-assembled InN quantum dots on side facets of GaN nanowires
- 2018
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 123:16
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Tidskriftsartikel (refereegranskat)abstract
- Self-assembled, atomic diffusion controlled growth of InN quantum dots was realized on the side facets of dislocation-free and c-oriented GaN nanowires having a hexagonal cross-section. The nanowires were synthesized by selective area metal organic vapor phase epitaxy. A 3 Å thick InN wetting layer was observed after growth, on top of which the InN quantum dots formed, indicating self-assembly in the Stranski-Krastanow growth mode. We found that the InN quantum dots can be tuned to nucleate either preferentially at the edges between GaN nanowire side facets, or directly on the side facets by tuning the adatom migration by controlling the precursor supersaturation and growth temperature. Structural characterization by transmission electron microscopy and reciprocal space mapping show that the InN quantum dots are close to be fully relaxed (residual strain below 1%) and that the c-planes of the InN quantum dots are tilted with respect to the GaN core. The strain relaxes mainly by the formation of misfit dislocations, observed with a periodicity of 3.2 nm at the InN and GaN hetero-interface. The misfit dislocations introduce I1 type stacking faults (...ABABCBC...) in the InN quantum dots. Photoluminescence investigations of the InN quantum dots show that the emissions shift to higher energy with reduced quantum dot size, which we attribute to increased quantum confinement.
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- Colvin, Jovana, et al.
(författare)
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Surface and dislocation investigation of planar GaN formed by crystal reformation of nanowire arrays
- 2019
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Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 3:9
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we present a process of forming monolithic GaN surface from an ordered nanowire array by means of material redistribution. This process, referred to as reformation, is performed in a conventional MOVPE crystal growth system with the gallium supply turned off and allows a crystal nanostructure to change shape according to differences in surface energies between its facets. Using reformation, coalescence may proceed closer to thermodynamic equilibrium, which is required for fabrication of high-quality substrate material. Scanning probe techniques are utilized, complemented by cathodoluminescence and electron microscopy, to investigate structural and electrical properties of the surface after reformation, as well as to assess densities, location, and formation of different types of defects in the GaN film. Spatial variations in material properties such as intrinsic majority-carrier types can be attributed to the radical changes in growth conditions required for sequential transition between nanowire growth, selective shell growth, and reformation. These properties enable us to assess the impact of the process on densities, locations, and formation of different types of dislocations in the GaN film. We find a fraction of the nanowires to comprise of a single electrically neutral edge dislocation, propagating from the GaN buffer, while electrically active dislocations are found at coalesced interfaces between nanowires. By decreasing the mask aperture size and changing the nucleation conditions the prevalence of nanowires comprising edge dislocation was significantly reduced from 6% to 3%, while the density of interface dislocations was reduced from 6×108 to 4×107cm-2. Using a sequential reformation process was found to create inversion domains with low surface potential N-polar regions in an otherwise Ga-polar GaN film. The inversion domains were associated with pinned dislocation pairs, and were further confirmed by selective wet etching in NaOH. This lateral polarity inversion was thoroughly eliminated in samples formed by a continuous reformation process. These results reveal a path and challenges for growing GaN substrates of superior crystal quality through nanowire reformation.
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- Eklund, Anders, et al.
(författare)
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A Brain Computer Interface for Communication Using Real-Time fMRI
- 2010
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Ingår i: Proceedings of the 20th International Conference on Pattern Recognition. - Los Alamitos, CA, USA : IEEE Computer Society. - 9781424475421 ; , s. 3665-3669
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Konferensbidrag (refereegranskat)abstract
- We present the first step towards a brain computer interface (BCI) for communication using real-time functional magnetic resonance imaging (fMRI). The subject in the MR scanner sees a virtual keyboard and steers a cursor to select different letters that can be combined to create words. The cursor is moved to the left by activating the left hand, to the right by activating the right hand, down by activating the left toes and up by activating the right toes. To select a letter, the subject simply rests for a number of seconds. We can thus communicate with the subject in the scanner by for example showing questions that the subject can answer. Similar BCI for communication have been made with electroencephalography (EEG). The subject then focuses on a letter while different rows and columns of the virtual keyboard are flashing and the system tries to detect if the correct letter is flashing or not. In our setup we instead classify the brain activity. Our system is neither limited to a communication interface, but can be used for any interface where five degrees of freedom is necessary.
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- Eklund, Anders, et al.
(författare)
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Using Real-Time fMRI to Control a Dynamical System by Brain Activity Classification
- 2010
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We present a method for controlling a dynamical system using real-time fMRI. The objective for the subject in the MR scanner is to balance an inverted pendulum by activating the left or right hand or resting. The brain activity is classified each second by a neural network and the classification is sent to a pendulum simulator to change the force applied to the pendulum. The state of the inverted pendulum is shown to the subject in a pair of VR goggles. The subject was able to balance the inverted pendulum during several minutes, both with real activity and imagined activity. In each classification 9000 brain voxels were used and the response time for the system to detect a change of activity was on average 2-4 seconds. The developments here have a potential to aid people with communication disabilities, such as locked in people. Another future potential application can be to serve as a tool for stroke and Parkinson patients to be able to train the damaged brain area and get real-time feedback for more efficient training.
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