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| 2. |
- Burke, Adam, et al.
(författare)
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InAs Nanowire Transistors with Multiple, Independent Wrap-Gate Segments.
- 2015
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 15:5, s. 2836-2843
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Tidskriftsartikel (refereegranskat)abstract
- We report a method for making horizontal wrap-gate nanowire transistors with up to four independently controllable wrap-gated segments. While the step up to two independent wrap-gates requires a major change in fabrication methodology, a key advantage to this new approach, and the horizontal orientation more generally, is that achieving more than two wrap-gate segments then requires no extra fabrication steps. This is in contrast to the vertical orientation, where a significant subset of the fabrication steps needs to be repeated for each additional gate. We show that cross-talk between adjacent wrap-gate segments is negligible despite separations less than 200 nm. We also demonstrate the ability to make multiple wrap-gate transistors on a single nanowire using the exact same process. The excellent scalability potential of horizontal wrap-gate nanowire transistors makes them highly favorable for the development of advanced nanowire devices and possible integration with vertical wrap-gate nanowire transistors in 3D nanowire network architectures.
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| 3. |
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| 4. |
- Dorsch, Sven, et al.
(författare)
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Characterization of electrostatically defined bottom-heated InAs nanowire quantum dot systems
- 2021
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 23:12
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Tidskriftsartikel (refereegranskat)abstract
- Conversion of temperature gradients to charge currents in quantum dot systems enables probing various concepts from highly efficient energy harvesting and fundamental thermodynamics to spectroscopic possibilities complementary to conventional bias device characterization. In this work, we present a proof-of-concept study of a device architecture where bottom-gates are capacitively coupled to an InAs nanowire and double function as local joule heaters. The device design combines the ability to heat locally at different locations on the device with the electrostatic definition of various quantum dot and barrier configurations. We demonstrate the versatility of this combined gating- and heating approach by studying, as a function of the heater location and bias, the Seebeck effect across the barrier-free nanowire, fit thermocurrents through quantum dots for thermometry and detect the phonon energy using a serial double quantum dot. The results indicate symmetric heating effects when the device is heated with different gates and we present detection schemes for the electronic and phononic heat transfer contribution across the nanowire. Based on this proof-of-principle work, we propose a variety of future experiments.
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| 5. |
- Fahlvik Svensson, Sofia, et al.
(författare)
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Control and understanding of kink formation in InAs-InP heterostructure nanowires.
- 2013
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 24:34
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Tidskriftsartikel (refereegranskat)abstract
- Nanowire heterostructures are of special interest for band structure engineering due to an expanded range of defect-free material combinations. However, the higher degree of freedom in nanowire heterostructure growth comes at the expense of challenges related to nanowire-seed particle interactions, such as undesired composition, grading and kink formation. To better understand the mechanisms of kink formation in nanowires, we here present a detailed study of the dependence of heterostructure nanowire morphology on indium pressure, nanowire diameter, and nanowire density. We investigate InAs-InP-InAs heterostructure nanowires grown with chemical beam epitaxy, which is a material system that allows for very abrupt heterointerfaces. Our observations indicate that the critical parameter for kink formation is the availability of indium, and that the resulting morphology is also highly dependent on the length of the InP segment. It is shown that kinking is associated with the formation of an inclined facet at the interface between InP and InAs, which destabilizes the growth and leads to a change in growth direction. By careful tuning of the growth parameters, it is possible to entirely suppress the formation of this inclined facet and thereby kinking at the heterointerface. Our results also indicate the possibility of producing controllably kinked nanowires with a high yield.
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| 6. |
- Fahlvik Svensson, Sofia, et al.
(författare)
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Lineshape of the thermopower of quantum dots
- 2012
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 14
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Tidskriftsartikel (refereegranskat)abstract
- Quantum dots are an important model system for thermoelectric phenomena, and may be used to enhance the thermal-to-electric energy conversion efficiency in functional materials, by tuning the Fermi energy relative to the dots' transmission resonances. It is therefore important to obtain a detailed understanding of a quantum dot's thermopower as a function of the Fermi energy. However, so far it has proven difficult to take the effects of interactions into account in the interpretation of experimental data. In this paper, we present detailed measurements of the thermopower of quantum dots defined in heterostructure nanowires. We show that the thermopower lineshape is described well by a Landauer-type transport model that uses as its input experimental values of the dot conductance, which contains information about interaction effects.
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| 7. |
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| 8. |
- Fahlvik Svensson, Sofia, et al.
(författare)
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Nonlinear thermovoltage and thermocurrent in quantum dots
- 2013
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 15
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Tidskriftsartikel (refereegranskat)abstract
- Quantum dots are model systems for quantum thermoelectric behavior because of their ability to control and measure the effects of electron-energy filtering and quantum confinement on thermoelectric properties. Interestingly, nonlinear thermoelectric properties of such small systems can modify the efficiency of thermoelectric power conversion. Using quantum dots embedded in semiconductor nanowires, we measure thermovoltage and thermocurrent that are strongly nonlinear in the applied thermal bias. We show that most of the observed nonlinear effects can be understood in terms of a renormalization of the quantum-dot energy levels as a function of applied thermal bias and provide a theoretical model of the nonlinear thermovoltage taking renormalization into account. Furthermore, we propose a theory that explains a possible source of the observed, pronounced renormalization effect by the melting of Kondo correlations in the mixed-valence regime. The ability to control nonlinear thermoelectric behavior expands the range in which quantum thermoelectric effects may be used for efficient energy conversion.
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| 9. |
- Fahlvik Svensson, Sofia
(författare)
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Thermoelectric Phenomena in Quantum Dots
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Thermoelectricity is being intensively researched as it is believed to hold great promise for applications in power generation and cooling. One way to quantify the electrical power output of a thermoelectric material is the power factor, a function of electrical conductivity and thermopower. There are relationships between these relevant material properties that make efficient thermoelectric materials challenging to produce. The development of methods for creating nanostructured materials has allowed such trade-offs in material properties to be circumvented. Quantum dots are useful as model systems in this context since they have tunable energy filtering effects that are straightforward to characterize. The work described in this thesis explores thermoelectric phenomena in quantum dots. The aim of this work was to gain a better understanding of the most basic thermoelectric behavior of quantum dots. This knowledge can provide deeper insight into which mechanisms may be of interest in increasing the efficiency of a thermoelectric material. A deeper understanding also allows the measurement method itself to be used as a tool for characterization. A thermoelectric measurement can complement the more commonly used electrical conductance measurements, by both confirming and supplementing data. This could be of great importance for the investigation of physical phenomena in nanostructures. The quantum dots used in this work were defined in semiconductor nanowires. They were formed either by heterostructure growth or afterwards during fabrication of devices. The thermoelectric properties of the quantum dots were thoroughly investigated in the Coulomb blockade regime, and both linear and nonlinear responses as a function of the applied thermal gradient were observed and explained. Thermoelectric measurements were also successfully used to characterize different InAs nanowire devices, either with the nanowire as is or covered by a polymer electrolyte. Closer investigations of these devices revealed physical properties of the nanowires that could be used to improve thermoelectric efficiency. In fact, this thesis presents the first measurements demonstrating an increase in thermoelectric power factor at low temperatures.
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| 10. |
- Fahlvik Svensson, Sofia, et al.
(författare)
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Using Polymer Electrolyte Gates to Set-and-Freeze Threshold Voltage and Local Potential in Nanowire-based Devices and Thermoelectrics
- 2015
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-3028 .- 1616-301X. ; 25:2, s. 255-262
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Tidskriftsartikel (refereegranskat)abstract
- The strongly temperature-dependent ionic mobility in polymer electrolytes is used to freeze in specific ionic charge environments around a nanowire using a local wrap-gate geometry. This makes it possible to set both the threshold voltage for a conventional doped substrate gate and the local disorder potential at temperatures below 220 K. These are characterized in detail by combining conductance and thermovoltage measurements with modeling. The results demonstrate that local polymer electrolyte gates are compatible with nanowire thermoelectrics, where they offer the advantage of a very low thermal conductivity, and hold great potential towards setting the optimal operating point for solid-state cooling applications.
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