| 521. |
- Stankevic, Tomas, et al.
(författare)
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Measurement of strain in InGaN/GaN nanowires and nanopyramids
- 2015
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Ingår i: Journal of Applied Crystallography. - 1600-5767. ; 48, s. 344-349
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Tidskriftsartikel (refereegranskat)abstract
- The growth and optoelectronic properties of core-shell nanostructures are influenced by the strain induced by the lattice mismatch between core and shell. In contrast with planar films, nanostructures contain multiple facets that act as independent substrates for shell growth, which enables different relaxation mechanisms. In this study, X-ray diffraction data are presented that show that InGa1-N shells grown on GaN cores are strained along each of the facets independently. Reciprocal space maps reveal multiple Bragg peaks, corresponding to different parts of the shell being strained along the individual facet planes. The strained lattice constants were found from the positions of the Bragg peaks. Vegard's law and Hooke's law for an anisotropic medium were applied in order to find the composition and strain in the InGaN shells. A range of nanowire samples with different InGaN shell thicknesses were measured and it is concluded that, with an In concentration of around 30%, major strain relaxation takes place when the thickness reaches 23nm. InGaN shells of 6 and 9nm thickness remain nearly fully strained biaxially along each of the facets of the nanowires and the facets of the nanopyramids.
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| 522. |
- Stankevic, Tomas, et al.
(författare)
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Nanofocused x-ray beams applied for mapping strain in core-shell nanowires
- 2015
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Ingår i: Proceedings of SPIE. - : SPIE. - 1996-756X .- 0277-786X. ; 9592, s. 95920-95920
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Konferensbidrag (refereegranskat)abstract
- The core-shell nanowires have the promise to become the future building blocks of light emitting diodes, solar cells and quantum computers. The high surface to volume ratio allows efficient elastic strain relaxation, making it possible to combine a wider range of materials into the heterostructures as compared to the traditional, planar geometry. As a result, the strain fields appear in both the core and the shell of the nanowires, which can affect the device properties. The hard x-ray nanoprobe is a tool that enables a nondestructive mapping of the strain and tilt distributions where other techniques cannot be applied. By measuring the positions of the Bragg peaks for each point on the sample we can evaluate the values of local tilt and strain. In this paper we demonstrate the detailed strain mapping of the strained InGaN/GaN core-shell nanowire. We observe an asymmetric strain distribution in the GaN core caused by an uneven shell relaxation. Additionally, we analyzed the local micro-tilt distribution, which shows the edge effects at the top and bottom of the nanowire. The measurements were compared to the finite element modelling and show a good agreement.
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| 523. |
- Stankevic, Tomas, et al.
(författare)
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Strain mapping in an InGaN/GaN nanowire using a nano-focused x-ray beam
- 2015
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 107:10
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Tidskriftsartikel (refereegranskat)abstract
- Strained InGaN/GaN core-shell nanowires (NWs) are promising candidates for solid state lighting applications due to their superior properties compared to planar films. NW based devices consist of multiple functional layers, which sum up to many hundred nanometers in thickness, that can uniquely be accessed in a non-destructive fashion by hard X-rays. Here, we present a detailed nanoscale strain mapping performed on a single, 400 nm thick and 2 lm long core-shell InGaN/GaN nanowire with an x-ray beam focused down to 100 nm. We observe an inhomogeneous strain distribution caused by the asymmetric strain relaxation in the shell. One side of the InGaN shell was fully strained, whereas the other side and the top part were relaxed. Additionally, tilt and strain gradients were determined at the interface with the substrate. (C) 2015 AIP Publishing LLC.
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| 524. |
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| 525. |
- Storm, Kristian, et al.
(författare)
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Dual-gate induced InP nanowire diode
- 2011
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Ingår i: Physics of Semiconductors: 30th International Conference on the Physics of Semiconductors. - : AIP. - 1551-7616 .- 0094-243X. ; 1399, s. 279-280
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Konferensbidrag (refereegranskat)abstract
- Semiconductor devices are heavily dependent on dopant incorporation in order to control the electrical properties. In this paper we investigate the possibility of using gates wrapped around a nanowire (NW) channel as a way of tuning the Fermi level position, in certain cases removing the need for dopants and providing a more dynamical way of setting the device properties. InP NW devices with omega gates are fabricated, and a p-n junction is formed in a nominally intrinsic NW channel. In order to further increase the electrostatic control of the channel and other device properties, vertical devices are discussed as a promising way of implementing this type of device.
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| 526. |
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| 527. |
- Storm, Kristian, et al.
(författare)
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Gate-Induced Fermi Level Tuning in InP Nanowires at Efficiency Close to the Thermal Limit.
- 2011
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 11, s. 1127-1130
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Tidskriftsartikel (refereegranskat)abstract
- As downscaling of semiconductor devices continues, one or a few randomly placed dopants may dominate the characteristics. Furthermore, due to the large surface-to-volume ratio of one-dimensional devices, the position of the Fermi level is often determined primarily by surface pinning, regardless of doping level. In this work, we investigate the possibility of tuning the Fermi level dynamically with wrap-around gates, instead of statically setting it using the impurity concentration. This is done using Ω-gated metal-oxide-semiconductor field-effect transistors with HfO(2)-capped InP nanowires as channel material. It is found that induced n-type devices exhibit an optimal inverse subthreshold slope of 68 mV/decade. By adjusting the growth and process parameters, it is possible to produce ambipolar devices, in which the Fermi level can be tuned across the entire band gap, making it possible to induce both n-type and p-type conduction.
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| 528. |
- Storm, Kristian, et al.
(författare)
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Realizing Lateral Wrap-Gated Nanowire FETs: Controlling Gate Length with Chemistry Rather than Lithography.
- 2012
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 12:1, s. 1-6
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Tidskriftsartikel (refereegranskat)abstract
- An important consideration in miniaturizing transistors is maximizing the coupling between the gate and the semiconductor channel. A nanowire with a coaxial metal gate provides optimal gate-channel coupling but has only been realized for vertically oriented nanowire transistors. We report a method for producing laterally oriented wrap-gated nanowire field-effect transistors that provides exquisite control over the gate length via a single wet etch step, eliminating the need for additional lithography beyond that required to define the source/drain contacts and gate lead. It allows the contacts and nanowire segments extending beyond the wrap-gate to be controlled independently by biasing the doped substrate, significantly improving the subthreshold electrical characteristics. Our devices provide stronger, more symmetric gating of the nanowire, operate at temperatures between 300 and 4 K, and offer new opportunities in applications ranging from studies of one-dimensional quantum transport through to chemical and biological sensing.
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| 529. |
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| 530. |
- Storm, Kristian, et al.
(författare)
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Spatially resolved Hall effect measurement in a single semiconductor nanowire
- 2012
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Ingår i: Nature Nanotechnology. - : Springer Science and Business Media LLC. - 1748-3387 .- 1748-3395. ; 7:11, s. 718-722
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Tidskriftsartikel (refereegranskat)abstract
- Efficient light-emitting diodes and photovoltaic energy-harvesting devices are expected to play an important role in the continued efforts towards sustainable global power consumption. Semiconductor nanowires are promising candidates as the active components of both light-emitting diodes1, 2, 3, 4, 5, 6 and photovoltaic cells7, 8, 9, 10, primarily due to the added freedom in device design offered by the nanowire geometry. However, for nanowire-based components to move past the proof-of-concept stage and be implemented in production-grade devices, it is necessary to precisely quantify and control fundamental material properties such as doping and carrier mobility. Unfortunately, the nanoscale geometry that makes nanowires interesting for applications also makes them inherently difficult to characterize. Here, we report a method to carry out Hall measurements on single core–shell nanowires. Our technique allows spatially resolved and quantitative determination of the carrier concentration and mobility of the nanowire shell. As Hall measurements have previously been completely unavailable for nanowires, the experimental platform presented here should facilitate the implementation of nanowires in advanced practical devices.
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