| 1. |
- Hultin, Olof, et al.
(författare)
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Simplifying Nanowire Hall Effect Characterization by Using a Three-Probe Device Design
- 2017
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Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 17:2, s. 1121-1126
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Tidskriftsartikel (refereegranskat)abstract
- Electrical characterization of nanowires is a time-consuming and challenging task due to the complexity of single nanowire device fabrication and the difficulty in interpreting the measurements. We present a method to measure Hall effect in nanowires using a three-probe device that is simpler to fabricate than previous four-probe nanowire Hall devices and allows characterization of nanowires with smaller diameter. Extraction of charge carrier concentration from the three-probe measurements using an analytical model is discussed and compared to simulations. The validity of the method is experimentally verified by a comparison between results obtained with the three-probe method and results obtained using four-probe nanowire Hall measurements. In addition, a nanowire with a diameter of only 65 nm is characterized to demonstrate the capabilities of the method. The three-probe Hall effect method offers a relatively fast and simple, yet accurate way to quantify the charge carrier concentration in nanowires and has the potential to become a standard characterization technique for nanowires.
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| 2. |
- Anttu, Nicklas, et al.
(författare)
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Crystal Phase-Dependent Nanophotonic Resonances in InAs Nanowire Arrays
- 2014
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 14:10, s. 5650-5655
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Tidskriftsartikel (refereegranskat)abstract
- Nanostructures have many material, electronic, and optical properties that are not found in bulk systems and that are relevant for technological applications. For example, nanowires realized from III-V semiconductors can be grown into wurtzite crystal structure. This crystal structure does not naturally exist in bulk where these materials form the zinc-blende counterpart. Being able to concomitantly grow these nanowires in the zinc-blende and/or wurtzite crystal structure prlovides an important degree of control for the design and optimization of optoelectronic applications based on these semiconductor nanostructures. However, the refractive indices of this new crystallographic phase have so far not been elucidated. This shortcoming makes it impossible to predict and utilize he full potential of these new nanostructured materials for optoelectronics applications a careful design and optimization of optical resonances by tuning the nanostrucuted geometry is needed to achieve optimal performance. Here, we report and analyze striking differeences in the optical response of nanophotonic resonances in wurtzite and zinc-blend InAs nanowire arrays. Specifically, through reflectance measurements we find that the resonance can be tuned down to lambda approximate to 380 nm in wurtzite nanowires by decreasing the nanowire diameter. In stark contrast, a similar tuning to below approximate to 500 nm is not possible in the zinc-blende nanowires. Furthermore, we find that the wurtzite nanowires can absorb twice as strongly as the zinc-blende nanowires. We attribute these strikingly large differences in resonant behavior to large differences between the refractive indices of the two crystallographic phases realized in these nanostructures. We anticipate our finding to be relevant for other III-B materials as well as for all material systems that manifest polytypism. Taken together, our results demonstrate crystal phase engineering as a potentially new design dimension for optoelectronics applications.
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| 3. |
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| 4. |
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| 5. |
- Astromskas, Gvidas, et al.
(författare)
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Doping Incorporation in InAs nanowires characterized by capacitance measurements
- 2010
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 108
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Tidskriftsartikel (refereegranskat)abstract
- Sn and Se doped InAs nanowires are characterized using a capacitance-voltage technique where the threshold voltages of nanowire capacitors with different diameter are determined and analyzed using an improved radial metal-insulator-semiconductor field-effect transistor model. This allows for a separation of doping in the core of the nanowire from the surface charge at the side facets of the nanowire. The data show that the doping level in the InAs nanowire can be controlled on the level between 2×1018 to 1×1019 cm−3, while the surface charge density exceeds 5×1012 cm−2 and is shown to increase with higher dopant precursor molar fraction.
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| 6. |
- Astromskas, Gvidas, et al.
(författare)
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Temperature and frequency characterization of InAs nanowire and HfO2 interface using capacitance-voltage method
- 2011
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Ingår i: Microelectronic Engineering. - : Elsevier BV. - 1873-5568 .- 0167-9317. ; 88:4, s. 444-447
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Konferensbidrag (refereegranskat)abstract
- InAs/HfO2 nanowire capacitors using capacitance-voltage (CV) measurements are investigated in the range of 10 kHz to 10 MHz. The capacitors are based on vertical nanowire arrays that are coated with an 8 nm-thick HfO2 layer by atomic layer deposition. CV characteristics are measured at temperatures in the range between -140 and 40 degrees C and the CV characteristics for nanowires with different Sn and Se n-type doping levels are compared. The comparison of the data at various doping levels points towards large number of traps for highly doped samples, caused by the preferential dopant precursor incorporation at the nanowire surface. We also evaluate the frequency dispersion of the accumulation capacitance and determine values below 2% with weak temperature dependence, indicating the existence of border traps in these nanowire capacitors. (C) 2010 Elsevier B.V. All rights reserved.
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| 7. |
- Astromskas, Gvidas, et al.
(författare)
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Transient studies on InAs/HfO2 nanowire capacitors
- 2011
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 98:1
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Tidskriftsartikel (refereegranskat)abstract
- Single-shot transients and deep-level transient spectroscopy are used to investigate the origins of capacitance hysteresis in n-doped InAs nanowire/HfO2 capacitors. Capacitance transients with a characteristic time in the order of 100 mu s are attributed to emission from electron traps, located in the oxide film. The trap energy is determined to be in the range from 0.12 to 0.17 eV with capture cross-sections of about 1.7 x 10(-17) cm(-2). The capture is measured to be shorter than 100 ns with no sign of capture barrier. Under the reverse bias, the transients show a reduced emission rate indicating a minority carrier assisted complex dynamics. (C) 2011 American Institute of Physics. [doi:10.1063/1.3533379]
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| 8. |
- Berg, Alexander, et al.
(författare)
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Radial Nanowire Light-Emitting Diodes in the (AlxGa1-x)yIn1-yP Material System
- 2016
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Ingår i: Nano letters (Print). - Washington, DC : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 16:1, s. 656-662
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Tidskriftsartikel (refereegranskat)abstract
- Nanowires have the potential to play an important role for next-generation light-emitting diodes. In this work, we present a growth scheme for radial nanowire quantum-well structures in the AlGaInP material system using a GaInP nanowire core as a template for radial growth with GaInP as the active layer for emission and AlGaInP as charge carrier barriers. The different layers were analyzed by X-ray diffraction to ensure lattice-matched radial structures. Furthermore, we evaluated the material composition and heterojunction interface sharpness by scanning transmission electron microscopy energy dispersive X-ray spectroscopy. The electro-optical properties were investigated by injection luminescence measurements. The presented results can be a valuable track toward radial nanowire light-emitting diodes in the AlGaInP material system in the red/orange/yellow color spectrum. © 2015 American Chemical Society.
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| 9. |
- 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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| 10. |
- Borg, Mattias, et al.
(författare)
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Geometric model for metalorganic vapour phase epitaxy of dense nanowire arrays
- 2013
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248. ; 366, s. 15-19
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Tidskriftsartikel (refereegranskat)abstract
- We propose a geometric model to estimate the contribution of substrate surface diffusion to growth of dense nanowire arrays using metalorganic vapour phase epitaxy. It is shown that the nanowire mantle area becomes the most significant collector of growth material when the nanowire diameter is large and the inter-nanowire pitch is small. It is concluded that this growth regime is important for the array geometries typically used for optoelectronic and photovoltaic applications of nanowires. The growth rate under these conditions is also not constant with time, due to the growing nanowire mantle area. This result was confirmed by experiments in which dense arrays of InAs nanowires were grown and the nanowire growth rate was measured as a function of time. (C) 2013 Elsevier B.V. All rights reserved.
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| 11. |
- Boyd, Erin E., et al.
(författare)
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Scanning gate imaging of quantum dots in 1D ultra-thin InAs/InP nanowires
- 2011
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 22:18
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Tidskriftsartikel (refereegranskat)abstract
- We use a scanning gate microscope (SGM) to characterize one-dimensional ultra-thin (diameter approximate to 30 nm) InAs/InP heterostructure nanowires containing a nominally 300 nm long InAs quantum dot defined by two InP tunnel barriers. Measurements of Coulomb blockade conductance versus backgate voltage with no tip present are difficult to decipher. Using the SGM tip as a charged movable gate, we are able to identify three quantum dots along the nanowire: the grown-in quantum dot and an additional quantum dot near each metal lead. The SGM conductance images are used to disentangle information about individual quantum dots and then to characterize each quantum dot using spatially resolved energy-level spectroscopy. S Online supplementary data available from stacks.iop.org/Nano/22/185201/mmedia
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| 12. |
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| 13. |
- 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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| 14. |
- Carrad, Damon J., et al.
(författare)
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Electron-Beam Patterning of Polymer Electrolyte Films To Make Multiple Nanoscale Gates for Nanowire Transistors
- 2014
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 14:1, s. 94-100
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Tidskriftsartikel (refereegranskat)abstract
- We report an electron-beam based method for the nanoscale patterning of the poly(ethylene oxide)/LiClO4 polymer electrolyte. We use the patterned polymer electrolyte as a high capacitance gate dielectric in single nanowire transistors and obtain subthreshold swings comparable to conventional metal/oxide wrap-gated nanowire transistors. Patterning eliminates gate/contact overlap, which reduces parasitic effects and enables multiple, independently controllable gates. The method's simplicity broadens the scope for using polymer electrolyte gating in studies of nanowires and other nanoscale devices.
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| 15. |
- Dubrovskii, Vladimir G., et al.
(författare)
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Length Distributions of Nanowires Growing by Surface Diffusion
- 2016
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Ingår i: Crystal Growth & Design. - : American Chemical Society (ACS). - 1528-7483 .- 1528-7505. ; 16:4, s. 2167-2172
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Tidskriftsartikel (refereegranskat)abstract
- We present experimental data on the time and radius-dependent length distributions of Au-catalyzed InAs nanowires grown by metal organic vapor phase epitaxy. We show that these distributions are not as sharp as commonly believed. Rather, they appear to be much broader than Poissonian from the very beginning and spread quickly as the nanowires grow. We develop a model that attributes the observed broadening to the diffusion-induced character of growth. In the initial growth stage, the nanowires are fed from their entire length, leading to a Polya-like length distribution whose standard deviation is proportional to the mean length. After the nanowire length exceeds the adatom diffusion length, the growth acquires a Poissonian character in which the standard deviation scales as a square root of the mean length. We explain why wider nanowires have smaller length dispersion and speculate on the length distributions in Au-catalyzed versus self-catalyzed growth methods.
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| 16. |
- Gaulton, Kyle J, et al.
(författare)
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Genetic fine mapping and genomic annotation defines causal mechanisms at type 2 diabetes susceptibility loci.
- 2015
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Ingår i: Nature Genetics. - : Springer Science and Business Media LLC. - 1546-1718 .- 1061-4036. ; 47:12, s. 1415-1415
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Tidskriftsartikel (refereegranskat)abstract
- We performed fine mapping of 39 established type 2 diabetes (T2D) loci in 27,206 cases and 57,574 controls of European ancestry. We identified 49 distinct association signals at these loci, including five mapping in or near KCNQ1. 'Credible sets' of the variants most likely to drive each distinct signal mapped predominantly to noncoding sequence, implying that association with T2D is mediated through gene regulation. Credible set variants were enriched for overlap with FOXA2 chromatin immunoprecipitation binding sites in human islet and liver cells, including at MTNR1B, where fine mapping implicated rs10830963 as driving T2D association. We confirmed that the T2D risk allele for this SNP increases FOXA2-bound enhancer activity in islet- and liver-derived cells. We observed allele-specific differences in NEUROD1 binding in islet-derived cells, consistent with evidence that the T2D risk allele increases islet MTNR1B expression. Our study demonstrates how integration of genetic and genomic information can define molecular mechanisms through which variants underlying association signals exert their effects on disease.
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| 17. |
- Gustafsson, Anders, et al.
(författare)
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A cathodoluminescence study of the influence of the seed particle preparation method on the optical properties of GaAs nanowires.
- 2012
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 23:26
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Tidskriftsartikel (refereegranskat)abstract
- Cathodoluminescence at 8 K is used to compare the optical properties of AlGaAs-capped GaAs nanowires, grown by metal-organic vapour phase epitaxy and seeded by gold particles prepared by different methods. Six different methods were used to fabricate and deposit gold seed particles onto GaAs substrates: colloid particles, aerosol particles and particles defined by electron beam lithography. The nanowires were grown with and without an in situ annealing step prior to the nanowire growth. The morphology showed no significant differences between the nanowires. The emissions from ensembles of nanowires have the same peak position, irrespective of seed particle type. Without the in situ annealing step prior to the nanowire growth, there are significant differences in the emission intensity and emission patterns from nanowires grown from different seed particles. When an in situ annealing step is included, all the resulting nanowires show identical optical emission intensity and emission patterns. This shows the importance of using an in situ annealing step prior to growth. This study demonstrates that different preparation methods for gold seed particles can be used to produce GaAs nanowires with highly similar optical properties. The choice of particle preparation method to be used can therefore be based on availability and cost.
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| 18. |
- Heurlin, Magnus, et al.
(författare)
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Synthesis of Doped InP Core-Shell Nanowires Evaluated Using Hall Effect Measurements.
- 2014
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 14:2, s. 749-753
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Tidskriftsartikel (refereegranskat)abstract
- InP core-shell nanowire pn-junctions doped with Zn and Sn have been investigated in terms of growth morphology and shell carrier concentration. The carrier concentrations were evaluated using spatially resolved Hall effect measurements and show improved homogeneity compared to previous investigations, attributed to the use of Sn as the n-type dopant. Anisotropies in the growth rate of different facets are found for different doping levels that in turn affects the migration of Sn and In on the nanowire surface. A route for increasing the In migration length to obtain a more homogeneous shell thickness is presented.
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| 19. |
- Hultin, Olof, et al.
(författare)
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Comparing Hall Effect and Field Effect Measurements on the Same Single Nanowire
- 2016
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 16:1, s. 205-211
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Tidskriftsartikel (refereegranskat)abstract
- We compare and discuss the two most commonly used electrical characterization techniques for nanowires (NWs). In a novel single-NW device, we combine Hall effect and back-gated and top-gated field effect measurements and quantify the carrier concentrations in a series of sulfur-doped InP NWs. The carrier concentrations from Hall effect and field effect measurements are found to correlate well when using the analysis methods described in this work. This shows that NWs can be accurately characterized with available electrical methods, an important result toward better understanding of semiconductor NW doping.
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| 20. |
- Jain, Vishal, 1989-, et al.
(författare)
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A comparative study of nanowire based infrared p+-i-n+ photodetectors
- 2012
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Konferensbidrag (refereegranskat)abstract
- We present a comparative study of electrical and optical properties of two types of p+-i-n+ photodetectors based on self-assembled ensembles of vertical InP nanowires (NWs) monolithically grown on InP. The detectors differ in the type of p+ contact, one detector geometry has p+-i-n+ segments integrated into the NWs (type A) while the other detector has i-n+ NW segments grown directly on a p+ substrate(type B). The samples were prepared by first depositing 80 nm Au nanoparticles on a p+ InP substrate using an aerosol technique and subsequently growing NWs using MOVPE. The NWs have a polytypecrystal structure of alternating wurtzite and zincblende segments. The processing of the detectors include deposition of SiO2, followed by an etching step to remove the oxide from the tip of the NWs, and finally sputtering of ITO on 1x1 mm2 device areas. The two most prominent differences between the detectors concern the current-voltage (I-V) characteristics and the spatial location of generated photocurrent. From spectrally resolved photocurrent measurements, we conclude that the photocurrent in detector type A is primarily generated in the NWs, whereas the photocurrent in type B detectors mainly stems from the substrate. Photogenerated carriers in the substrate diffuse to the NWs where they are effectively funnelled into the NWs. The I-V characteristics of the type A detector displays a non-trivial transport behaviour for forward biases, whereas type B shows excellent rectifying behavior with an ideality factor of about 2.5. We will discuss detailed analysis of the spectral fingerprints of the two detector types revealing the mixed crystal phase of the polytype NWs and bandstructure effects, temperature dependence of the I-V characteristics and typical photodetector parameters.
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| 21. |
- Karlström, Olov, et al.
(författare)
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Analysing the capacitance–voltage measurements of vertical wrapped-gated nanowires
- 2008
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 19:43
-
Tidskriftsartikel (refereegranskat)abstract
- The capacitance of arrays of vertical wrapped-gate InAs nanowires is analysed. With the help of a Poisson–Schrödinger solver, information about the doping density can be obtained directly. Further features in the measured capacitance–voltage characteristics can be attributed to the presence of surface states as well as the coexistence of electrons and holes in the wire. For both scenarios, quantitative estimates are provided. It is furthermore shown that the difference between the actual capacitance and the geometrical limit is quite large, and depends strongly on the nanowire material.
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| 22. |
- Kivisaari, Pyry, et al.
(författare)
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Optimization of Current Injection in AlGaInP Core−Shell Nanowire Light-Emitting Diodes
- 2017
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 17:6, s. 3599-3606
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Tidskriftsartikel (refereegranskat)abstract
- Core–shell nanowires offer great potential to enhance the efficiency of light-emitting diodes (LEDs) and expand the attainable wavelength range of LEDs over the whole visible spectrum. Additionally, nanowire (NW) LEDs can offer both improved light extraction and emission enhancement if the diameter of the wires is not larger than half the emission wavelength (λ/2). However, AlGaInP nanowire LEDs have so far failed to match the high efficiencies of traditional planar technologies, and the parameters limiting the efficiency remain unidentified. In this work, we show by experimental and theoretical studies that the small nanowire dimensions required for efficient light extraction and emission enhancement facilitate significant loss currents, which result in a low efficiency in radial NW LEDs in particular. To this end, we fabricate AlGaInP core–shell nanowire LEDs where the nanowire diameter is roughly equal to λ/2, and we find that both a large loss current and a large contact resistance are present in the samples. To investigate the significant loss current observed in the experiments in more detail, we carry out device simulations accounting for the full 3D nanowire geometry. According to the simulations, the low efficiency of radial AlGaInP nanowire LEDs can be explained by a substantial hole leakage to the outer barrier layer due to the small layer thicknesses and the close proximity of the shell contact. Using further simulations, we propose modifications to the epitaxial structure to eliminate such leakage currents and to increase the efficiency to near unity without sacrificing the λ/2 upper limit of the nanowire diameter. To gain a better insight of the device physics, we introduce an optical output measurement technique to estimate an ideality factor that is only dependent on the quasi-Fermi level separation in the LED. The results show ideality factors in the range of 1–2 around the maximum LED efficiency even in the presence of a very large voltage loss, indicating that the technique is especially attractive for measuring nanowire LEDs at an early stage of development before electrical contacts have been optimized. The presented results and characterization techniques form a basis of how to simultaneously optimize the electrical and optical efficiency of core–shell nanowire LEDs, paving the way to nanowire light emitters that make true use of larger-than-unity Purcell factors and the consequently enhanced spontaneous emission.
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| 23. |
- Lindelöw, Fredrik, et al.
(författare)
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Doping evaluation of InP nanowires for tandem junction solar cells
- 2016
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 27:6
-
Tidskriftsartikel (refereegranskat)abstract
- In order to push the development of nanowire-based solar cells further using optimized nanowire diameter and pitch, a doping evaluation of the nanowire geometry is necessary. We report on a doping evaluation of n-type InP nanowires with diameters optimized for light absorption, grown by the use of metal-organic vapor phase epitaxy in particle-assisted growth mode using tetraethyltin (TESn) as the dopant precursor. The charge carrier concentration was evaluated using four-probe resistivity measurements and spatially resolved Hall measurements. In order to reach the highest possible nanowire doping level, we set the TESn molar fraction at a high constant value throughout growth and varied the trimethylindium (TMIn) molar fraction for different runs. Analysis shows that the charge carrier concentration in nanowires grown with the highest TMIn molar fraction (not leading to kinking nanowires) results in a low carrier concentration of approximately 10(16) cm(-3). By decreasing the molar fraction of TMIn, effectively increasing the IV/III ratio, the carrier concentration increases up to a level of about 10(19) cm(-3), where it seems to saturate. Axial carrier concentration gradients along the nanowires are found, which can be correlated to a combination of changes in the nanowire growth rate, measured in situ by optical reflectometry, and polytypism of the nanowires observed in transmission electron microscopy.
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| 24. |
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| 25. |
- Lindgren, David, et al.
(författare)
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Study of carrier concentration in single InP nanowires by luminescence and Hall measurements
- 2015
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 26:4
-
Tidskriftsartikel (refereegranskat)abstract
- The free electron carrier concentrations in single InP core-shell nanowires are determined by micro-photoluminescence, cathodoluminescence (CL) and Hall effect measurements. The results from luminescence measurements were obtained by solving the Fermi-Dirac integral, as well as by analyzing the peak full width at half maximum (FWHM). Furthermore, the platform used for Hall effect measurements, combined with spot mode CL spectroscopy, is used to determine the carrier concentrations at specific positions along single nanowires. The results obtained via luminescence measurements provide an accurate and rapid feedback technique for the epitaxial development of doping incorporation in nanowires. The technique has been employed on several series of samples in which growth parameters, such as V/III-ratio, temperature and dopant flows, were investigated in an optimization procedure. The correlation between the Hall effect and luminescence measurements for extracting the carrier concentration of different samples were in excellent agreement.
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| 26. |
- Messing, Maria, et al.
(författare)
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A comparative study of the effect of gold seed particle preparation method on nanowire growth
- 2010
-
Ingår i: Nano Reseach. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 3:7, s. 506-519
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Tidskriftsartikel (refereegranskat)abstract
- Highly controlled particle-assisted growth of semiconductor nanowires has been performed for many years, and a number of novel nanowire-based devices have been demonstrated. Full control of the epitaxial growth is required to optimize the performance of devices, and gold seed particles are known to provide the most controlled growth. Successful nanowire growth from gold particles generated and deposited by various different methods has been reported, but no investigation has yet been performed to compare the effects of gold particle generation and deposition methods on nanowire growth. In this article we present a direct comparative study of the effect of the gold particle creation and deposition methods on nanowire growth characteristics and nanowire crystal structure, and investigate the limitations of the different generation and deposition methods used.
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| 27. |
- Nowzari, Ali, et al.
(författare)
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A Comparative Study of Absorption in Vertically and Laterally Oriented InP Core–Shell Nanowire Photovoltaic Devices
- 2015
-
Ingår i: Nano letters (Print). - Washington : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 15:3, s. 1809-1814
-
Tidskriftsartikel (refereegranskat)abstract
- We have compared the absorption in InP core-shell nanowire p-i-n junctions in lateral and vertical orientation. Arrays of vertical core-shell nanowires with 400 nm pitch and 280 nm diameter, as well as corresponding lateral single core-shell nanowires, were configured as photovoltaic devices. The photovoltaic characteristics of the samples, measured under 1 sun illumination, showed a higher absorption in lateral single nanowires compared to that in individual vertical nanowires, arranged in arrays with 400 nm pitch. Electromagnetic modeling of the structures confirmed the experimental observations and showed that the absorption in a vertical nanowire in an array depends strongly on the array pitch. The modeling demonstrated that, depending on the array pitch, absorption in a vertical nanowire can be lower or higher than that in a lateral nanowire with equal absorption predicted at a pitch of 510 nm for our nanowire geometry. The technology described in this Letter facilitates quantitative comparison of absorption in laterally and vertically oriented core-shell nanowire p-i-n junctions and can aid in the design, optimization, and performance evaluation of nanowire-based core-shell photovoltaic devices. © 2014 American Chemical Society.
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| 28. |
- Nylund, Gustav, et al.
(författare)
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Designed Quasi-1D Potential Structures Realized in Compositionally Graded InAs1-xPx Nanowires.
- 2016
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984.
-
Tidskriftsartikel (refereegranskat)abstract
- III-V semiconductor heterostructures are important components of many solid-state optoelectronic devices, but the ability to control and tune the electrical and optical properties of these structures in conventional device geometries is fundamentally limited by the bulk dimensionality and the inability to accommodate lattice-mismatched material combinations. Here we demonstrate how semiconductor nanowires may enable the creation of arbitrarily shaped one-dimensional potential structures for new types of designed device functionality. We describe the controlled growth of stepwise compositionally graded InAs1-xPx heterostructures defined along the axes of InAs nanowires, and we show that nanowires with sawtooth-shaped composition profiles behave as near-ideal unipolar diodes with ratchet-like rectification of the electron transport through the nanowires, in excellent agreement with simulations. This new type of designed quasi-1D potential structure represents a significant advance in band gap engineering and may enable fundamental studies of low-dimensional hot-carrier dynamics, in addition to constituting a platform for implementing novel electronic and optoelectronic device concepts.
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| 29. |
- Nylund, Gustav, et al.
(författare)
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Transparently Wrap-Gated Semiconductor Nanowire Arrays For Studies Of Gate-Controlled Photoluminescence
- 2013
-
Ingår i: Physics of Semiconductors. - : AIP. - 0094-243X .- 1551-7616. ; 1566, s. 427-428
-
Konferensbidrag (refereegranskat)abstract
- We present a technique to measure gate-controlled photoluminescence (PL) on arrays of semiconductor nanowire (NW) capacitors using a transparent film of Indium-Tin-Oxide (ITO) wrapping around the nanowires as the gate electrode. By tuning the wrap-gate voltage, it is possible to increase the PL peak intensity of an array of undoped InP NWs by more than an order of magnitude. The fine structure of the PL spectrum reveals three subpeaks whose relative peak intensities change with gate voltage. We interpret this as gate-controlled state-filling of luminescing quantum dot segments formed by zincblende stacking faults in the mainly wurtzite NW crystal structure.
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| 30. |
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| 31. |
- Pettersson, Håkan, 1962-, et al.
(författare)
-
Electrical and optical properties of InP nanowire ensemble p(+)-i-n(+) photodetectors
- 2012
-
Ingår i: Nanotechnology. - Bristol, UK : Institute of Physics Publishing (IOPP). - 0957-4484 .- 1361-6528. ; 23:13
-
Tidskriftsartikel (refereegranskat)abstract
- We report on a comprehensive study of electrical and optical properties of efficient near-infrared p(+)-i-n(+) photodetectors based on large ensembles of self-assembled, vertically aligned i-n(+) InP nanowires monolithically grown on a common p(+) InP substrate without any buffer layer. The nanowires have a polytype modulated crystal structure of wurtzite and zinc blende. The electrical data display excellent rectifying behavior with an ideality factor of about 2.5 at 300 K. The ideality factor scales with 1/T, which possibly reflects deviations from classical transport models due to the mixed crystal phase of the nanowires. The observed dark leakage current is of the order of merely similar to 100 fA/nanowire at 1 V reverse bias. The detectors display a linear increase of the photocurrent with reverse bias up to about 10 pA/nanowire at 5 V. From spectrally resolved measurements, we conclude that the photocurrent is primarily generated by funneling photogenerated carriers from the substrate into the NWs. Contributions from direct excitation of the NWs become increasingly important at low temperatures. The photocurrent decreases with temperature with an activation energy of about 50 meV, which we discuss in terms of a temperature-dependent diffusion length in the substrate and perturbed transport through the mixed-phase nanowires. © 2012 IOP Publishing Ltd.
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| 32. |
- Roddaro, Stefano, et al.
(författare)
-
Growth of vertical InAs nanowires on heterostructured substrates
- 2009
-
Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 20:28
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the Au-assisted growth of InAs nanowires on two different kinds of heterostructured substrates: GaAs/AlGaAs structures capped by a 50 nm thick InAs layer grown by molecular beam epitaxy and a 2 mu m thick InAs buffer layer on Si(111) obtained by vapor phase epitaxy. Morphological and structural properties of substrates and nanowires are analyzed by atomic force and transmission electron microscopy. Our results indicate a promising direction for the integration of III-V nanostructures on Si-based electronics as well as for the development of novel micromechanical structures incorporating nanowires as their active elements.
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| 33. |
- Roddaro, Stefano, et al.
(författare)
-
InAs nanowire metal-oxide-semiconductor capacitors
- 2008
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 92:25
-
Tidskriftsartikel (refereegranskat)abstract
- We present a capacitance-voltage study for arrays of vertical InAs nanowires. Metal-oxide-semiconductor (MOS) capacitors are obtained by insulating the nanowires with a conformal 10 nm HfO2 layer and using a top Cr/Au metallization as one of the capacitor's electrodes. The described fabrication and characterization technique enables a systematic investigation of the carrier density in the nanowires as well as of the quality of the MOS interface.
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| 34. |
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| 35. |
- Schukfeh, Muhammed Ihab, et al.
(författare)
-
Conductance Enhancement of InAs/InP Heterostructure Nanowires by Surface Functionalization with Oligo(phenylene vinylene)s
- 2013
-
Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 7:5, s. 4111-4118
-
Tidskriftsartikel (refereegranskat)abstract
- We have investigated the electronic transport through 3 mu m long, 45 nm diameter InAs nanowires comprising a 5 nm long InP segment as electronic barrier. After assembly of 12 nm long oligo(phenylene vinylene) derivative molecules onto these InAs/InP nanowires, we observed a pronounced, nonlinear I-V characteristic with significantly increased currents of up to 1 mu A at 1 V bias, for a back-gate voltage of 3 V. As supported by our model calculations based on a nonequilibrium Green Function approach, we attribute this effect to charge transport through those surface-bound molecules, which electrically bridge both InAs regions across the embedded InP barrier.
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| 36. |
- Schukfeh, M. I., et al.
(författare)
-
Formation of nanogaps in InAs nanowires by selectively etching embedded InP segments
- 2014
-
Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 25:46
-
Tidskriftsartikel (refereegranskat)abstract
- We present a method to fabricate nanometer scale gaps within InAs nanowires by selectively etching InAs/InP heterostructure nanowires. We used vapor-liquid-solid grown InAs nanowires with embedded InP segments of 10-60 nm length and developed an etching recipe to selectively remove the InP segment. A photo-assisted wet etching process in a mixture of acetic acid and hydrobromic acid gave high selectivity, with accurate removal of InP segments down to 20 nm, leaving the InAs wire largely unattacked, as verified using scanning electron and transmission electron microscopy. The obtained nanogaps in InAs wires have potential as semiconducting electrodes to investigate electronic transport in nanoscale objects. We demonstrate this functionality by dielectrophoretically trapping 30 nm diameter gold nanoparticles into the gap.
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| 37. |
- Schukfeh, Muhammed Ihab, et al.
(författare)
-
Selective etching of InP in InAs/InP nanowires resulting in 11 nm nanogaps
- 2016
-
Ingår i: IEEE-NANO 2015 - 15th International Conference on Nanotechnology. - 9781467381550 ; , s. 1489-1492
-
Konferensbidrag (refereegranskat)abstract
- We demonstrate a wet chemical method to selectively etch InP segments within InAs/InP heterostructure nanowires based on a photo-assisted HAc/HBr solution etching process. We successfully etched InP segments ranging from 60 nm down to about 10 nm in size.
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| 38. |
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| 39. |
- Storm, Kristian, et al.
(författare)
-
Dual-gate induced InP nanowire diode
- 2011
-
Ingår i: Physics of Semiconductors: 30th International Conference on the Physics of Semiconductors. - : AIP. - 1551-7616 .- 0094-243X. ; 1399, s. 279-280
-
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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| 40. |
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| 41. |
- Storm, Kristian, et al.
(författare)
-
Gate-Induced Fermi Level Tuning in InP Nanowires at Efficiency Close to the Thermal Limit.
- 2011
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 11, s. 1127-1130
-
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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| 42. |
- Storm, Kristian
(författare)
-
Novel Processing and Electrical Characterization of Nanowires
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates novel electrical nanowire characterization tools and devices. Conventional characterization methods, long available to bulk semiconductor samples, have been adapted and transferred to the nanowire geometry. The first part of the thesis describes the development of Hall effect measurements, an entirely new characterization tool for nanowires. It is shown that Hall effect measurements can be performed on InP core-shell nanowires using a self-aligned lifting layer. By combining experimental data with results from simulations of band diagrams and current distribution under the influence of a magnetic field, the carrier density in the n-type nanowire shell can be determined. We found that the nanowire shell exhibits a doping gradient along the length of the nanowire. This doping inhomogeneity is important to account for and engineer when making devices using InP nanowires. The second part of the thesis demonstrates how capacitance-voltage measurements can be performed on arrays of InAs nanowires. Using a novel device structure, the capacitance signal from the nanowires can be distinguished from parasitic capacitances. A model was developed to simulate the capacitance-voltage behavior of the nanowires and was fitted to the experimental data to extract the doping concentration. Furthermore, the hysteresis observed in the capacitance-voltage sweeps was used to calculate the trap density close to the InAs-HfO2 dielectric interface. Finally, studies of gate effects in nanowires are presented. We demonstrate how the gating efficiency is improved by means of wrapped or semiwrapped gates on nanowires. Field-effect transistors made from InP nanowires are described that exhibit both ambipolar behavior and gating efficiency only 13% lower than the theoretical limit. These properties were used to fabricate a field-effect diode, a device in which a p-n junction is formed without any doping incorporation in the active region. We also demonstrate how nanowires positioned laterally on a substrate can be equipped with a fully wrapped gate electrode. This device was developed as part of a platform to perform basic research in which a uniform gating effect is desired.
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| 43. |
- Storm, Kristian, et al.
(författare)
-
Realizing Lateral Wrap-Gated Nanowire FETs: Controlling Gate Length with Chemistry Rather than Lithography.
- 2012
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 12:1, s. 1-6
-
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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| 44. |
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| 45. |
- Storm, Kristian, et al.
(författare)
-
Spatially resolved Hall effect measurement in a single semiconductor nanowire
- 2012
-
Ingår i: Nature Nanotechnology. - : Springer Science and Business Media LLC. - 1748-3387 .- 1748-3395. ; 7:11, s. 718-722
-
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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| 46. |
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| 47. |
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| 48. |
- Storm, Ludvig, 1995, et al.
(författare)
-
Constraints on parameter choices for successful time-series prediction with echo-state networks
- 2022
-
Ingår i: Machine Learning: Science and Technology. - : IOP Publishing. - 2632-2153. ; 3:4
-
Tidskriftsartikel (refereegranskat)abstract
- Echo-state networks are simple models of discrete dynamical systems driven by a time series. By selecting network parameters such that the dynamics of the network is contractive, characterized by a negative maximal Lyapunov exponent, the network may synchronize with the driving signal. Exploiting this synchronization, the echo-state network may be trained to autonomously reproduce the input dynamics, enabling time-series prediction. However, while synchronization is a necessary condition for prediction, it is not sufficient. Here, we study what other conditions are necessary for successful time-series prediction. We identify two key parameters for prediction performance, and conduct a parameter sweep to find regions where prediction is successful. These regions differ significantly depending on whether full or partial phase space information about the input is provided to the network during training. We explain how these regions emerge.
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| 49. |
- Storm, Ludvig, 1995, et al.
(författare)
-
Finite-Time Lyapunov Exponents of Deep Neural Networks
- 2024
-
Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 132:5
-
Tidskriftsartikel (refereegranskat)abstract
- We compute how small input perturbations affect the output of deep neural networks, exploring an analogy between deep feed-forward networks and dynamical systems, where the growth or decay of local perturbations is characterized by finite-time Lyapunov exponents. We show that the maximal exponent forms geometrical structures in input space, akin to coherent structures in dynamical systems. Ridges of large positive exponents divide input space into different regions that the network associates with different classes. These ridges visualize the geometry that deep networks construct in input space, shedding light on the fundamental mechanisms underlying their learning capabilities.
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| 50. |
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