| 1. |
- Anttu, Nicklas, et al.
(författare)
-
Drastically increased absorption in vertical semiconductor nanowire arrays: A non-absorbing dielectric shell makes the difference
- 2012
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Ingår i: Nano Reseach. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 5:12, s. 863-874
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Tidskriftsartikel (refereegranskat)abstract
- Enhanced absorption of especially long wavelength light is needed to enable the full potential of semiconductor nanowire (NW) arrays for optoelectronic applications. We show both experimentally and theoretically that a transparent dielectric shell (Al2O3 coating) can drastically improve the absorption of light in InAs NW arrays. With an appropriate thickness of the Al2O3 shell, we achieve four times stronger absorption in the NWs compared to uncoated NWs and twice as good absorption as when the dielectric completely fills the space between the NWs. We provide detailed theoretical analysis from a combination of full electrodynamic modeling and intuitive electrostatic approximations. This reveals how the incident light penetrates better into the absorbing NW core with increasing thickness of the dielectric shell until a resonant shell thickness is reached. We provide a simple description of how to reach this strongly absorbing resonance condition, making our results easy to apply for a broad wavelength range and a multifold of semiconductor and dielectric coating material combinations.
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| 2. |
- Anttu, Nicklas, et al.
(författare)
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Excitations of surface plasmon polaritons in double layer metal grating structures
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:9
-
Tidskriftsartikel (refereegranskat)abstract
- We study the light scattering properties of double layer gratings (DLGs) made from Au on SiO2 substrates. It is found that surface plasmon polaritons (SPPs) can be excited in the DLGs for a separation of up to 150 nm between the two Au grating layers and the collective reflectance spectra exhibit a strong resonant peak and a closely lying dip as a result of the surface plasmon polariton excitations. It is also found that the angle-resolved specular reflectance spectra show a dip-peak pair structure, while the angle-resolved reflectance spectra of higher diffracted orders show a complementary peak-dip pair structure. Finally, operation of the DLGs for efficient wavelength demultiplexing is proposed and discussed in light of these results. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3690947]
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| 3. |
- Chen, Jianing, et al.
(författare)
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Probing Strain in Bent Semiconductor Nanowires with Raman Spectroscopy.
- 2010
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 10:4, s. 1280-1286
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Tidskriftsartikel (refereegranskat)abstract
- We present a noninvasive optical method to determine the local strain in individual semiconductor nanowires. InP nanowires were intentionally bent with an atomic force microscope and variations in the optical phonon frequency along the wires were mapped using Raman spectroscopy. Sections of the nanowires with a high curvature showed significantly broadened phonon lines. These observations together with deformation potential theory show that compressive and tensile strain inside the nanowires is the physical origin of the observed phonon energy variations.
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| 4. |
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| 5. |
- Chen, Jianing, et al.
(författare)
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Surface-enhanced Raman scattering of rhodamine 6G on nanowire arrays decorated with gold nanoparticles
- 2008
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 19:27, s. 5-275712
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the surface-enhanced Raman scattering (SERS) of rhodamine 6G (R6G) adsorbed on Au nanoparticles attached to InP nanowires. We find that nanowire arrays act as frameworks for effective SERS substrates with a significantly higher Raman signal sensitivity than a planar framework of Au nanoparticles adsorbed two-dimensionally on a flat surface. The SERS signal displays a clear polarization-dependent effect when the nanowires are arranged in a row. We also find that the SERS signal increases with time during continuous laser illumination. The plasmon-enhanced optical forces between Au nanoparticles may either move pairs of nanoparticles closer together or attract adsorbed molecules by moving them to the junctions of Au nanoparticle aggregates. Such effects by plasmon optical forces may cause the observed increase of the SERS signal with continuous laser illumination.
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| 6. |
- Chen, Jianing, et al.
(författare)
-
Tip-enhanced Raman scattering of p-thiocresol molecules on individual gold nanoparticles
- 2008
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 92:9
-
Tidskriftsartikel (refereegranskat)abstract
- We present a study of tip-enhanced Raman scattering on Au aerosol nanoparticles deposited on gold films. Under the tunneling current state, the Au tip and the Au aerosol nanoparticle form a narrow cavity, where large electromagnetic field enhancements are created to enhance Raman scattering enormously. Colorless p-thiocresol molecules are used as probe molecules. The estimated Raman enhancement is about nine orders of magnitude for the tip/particle configuration.
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| 7. |
- Ma, Liang, et al.
(författare)
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Room-Temperature Near-Infrared Photodetectors Based on Single Heterojunction Nanowires
- 2014
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 14:2, s. 694-698
-
Tidskriftsartikel (refereegranskat)abstract
- Nanoscale near-infrared photodetectors are attractive for their potential applications in integrated optoelectronic devices. Here we report the synthesis of GaSb/GaInSb p-n heterojunction semiconductor nanowires for the first time through a controllable chemical vapor deposition (CVD) route. Based on these nanowires, room-temperature, high-performance, near-infrared photodetectors were constructed. The fabricated devices show excellent light response in the infrared optical communication region (1.55 mu m), with an external quantum efficiency of 10(4), a responsivity of 10(3) A/W, and a short response time of 2 ms, which shows promising potential applications in integrated photonics and optoelectronics devices or systems.
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| 8. |
- Xu, Guangwei, et al.
(författare)
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Synthesis, properties, and top-gated metal-oxide-semiconductor field-effect transistors of p-type GaSb nanowires
- 2013
-
Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 3:43, s. 19834-19839
-
Tidskriftsartikel (refereegranskat)abstract
- High-quality GaSb nanowires (NWs) have been synthesized via chemical vapor deposition. The as-synthesized NWs have a zinc-blende structure with growth direction along a < 011 > direction. Raman spectrum of the GaSb NWs consists of two peaks, corresponding to the LO and TO phonon modes, respectively. The temperature dependence of the photoluminescence spectra shows a blue-shift as the temperature decreases from 300 to 13 K. The electrical properties of the GaSb NWs are investigated over a wide range of temperatures from 25 mK to 291 K. The results show that the GaSb NWs exhibit excellent p-type transistor performance at low temperatures (<40 K). The room-temperature hole density and mobility were found to be similar to 2.2 x 10(18) cm(-3) and similar to 14.2 cm(2) V-1 s(-1), respectively. The Schottky contact characteristics were observed and the barrier height was found to be similar to 14 meV. Our results show that the GaSb NWs could be used as building blocks for emerging p-type nanoelectronic devices in extremely low temperature environments.
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| 9. |
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| 10. |
- Xu, Hongxing, et al.
(författare)
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Unified treatment of fluorescence and Raman scattering processes near metal surfaces
- 2004
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Ingår i: Physical Review Letters. - 1079-7114. ; 93:24
-
Tidskriftsartikel (refereegranskat)abstract
- We present a general model study of surface-enhanced resonant Raman scattering and fluorescence focusing on the interplay between electromagnetic effects and the molecular dynamics. Our model molecule is placed close to two Ag nanoparticles and has two electronic levels. A Franck-Condon mechanism provides electron-vibration coupling. Using realistic parameter values for the molecule we find that an electromagnetic enhancement by 10 orders of magnitude can yield Raman cross sections sigma(R) of the order 10(-14) cm(2). We also discuss the dependence of sigma(R) on incident laser intensity.
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| 11. |
- ZhiQiang, Guan, et al.
(författare)
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Surface-enhanced Raman scattering on dual-layer metallic grating structures
- 2010
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Ingår i: Chinese Science Bulletin. - : Springer Science and Business Media LLC. - 1001-6538 .- 1861-9541. ; 55:24, s. 2643-2648
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Tidskriftsartikel (refereegranskat)abstract
- Dual-layer Metallic grating (DMG) structures as surface-enhanced Raman scattering (SERS) substrates are studied using benzenethiol as the probe analyte. The DMG structure consists of a SiO2 grating and 100-nm-thick gold coating layers. An enhancement factor of 10(5) is achieved by optimizing the SiO2 grating height within the range from 165 to 550 nm. The enhancement factor dependence on the SiO2 grating height is due to the surface plasmon excitation, which is dependent on the polarization of the incident light, and confirmed by finite difference time domain simulations. This study demonstrates the advantages of high uniformity, reproducibility and sensitivity in the DMG structures for SERS applications.
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| 12. |
- Abay, Simon, 1980, et al.
(författare)
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Charge transport in InAs nanowire Josephson junctions
- 2014
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969 .- 1098-0121. ; 89:21, s. 214508-
-
Tidskriftsartikel (refereegranskat)abstract
- We present an extensive experimental and theoretical study of the proximity effect in InAs nanowires connected to superconducting electrodes. We fabricate and investigate devices with suspended gate-controlled nanowires and nonsuspended nanowires, with a broad range of lengths and normal-state resistances. We analyze the main features of the current-voltage characteristics: the Josephson current, excess current, and subgap current as functions of length, temperature, magnetic field, and gate voltage, and compare them with theory. The Josephson critical current for a short-length device, L = 30 nm, exhibits a record high magnitude of 800 nA at low temperature that comes close to the theoretically expected value. The critical current in all other devices is typically reduced compared to the theoretical values. The excess current is consistent with the normal resistance data and agrees well with the theory. The subgap current shows a large number of structures; some of them are identified as subharmonic gap structures generated by multiple Andreev reflection. The other structures, detected in both suspended and nonsuspended devices, have the form of voltage steps at voltages that are independent of either the superconducting gap or length of the wire. By varying the gate voltage in suspended devices, we are able to observe a crossover from typical tunneling transport at large negative gate voltage, with suppressed subgap current and negative excess current, to pronounced proximity junction behavior at large positive gate voltage, with enhanced Josephson current and subgap conductance as well as a large positive excess current.
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| 13. |
- Abay, Simon, 1980, et al.
(författare)
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High Critical-Current Superconductor-InAs Nanowire-Superconductor Junctions
- 2012
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 12:11, s. 5622-5625
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Tidskriftsartikel (refereegranskat)abstract
- We report on the fabrication of InAs nanowires coupled to superconducting leads with high critical current and widely tunable conductance. We implemented a double lift off nanofabrication Method to get very short nanowire devices with Ohmic contacts. We observe very high critical. currents of up to 800 nA in a wire with a diameter of 80 nm. The current- voltage characteristics of longer and suspended nanowires display,either. Coulomb blockade or supercurrent depending on a local gate voltage, combining different regimes of transport in a single device.
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| 14. |
- Abay, Simon, 1980, et al.
(författare)
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Quantized Conductance and Its Correlation to the Supercurrent in a Nanowire Connected to Superconductors
- 2013
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 13:8, s. 3614-3617
-
Tidskriftsartikel (refereegranskat)abstract
- We report conductance and supercurrent of InAs nano wires coupled to Al-superconducting electrodes with short channel lengths and good Ohmic contacts. The nanowires are suspended 15 nm above a local gate electrode. The charge density in the nanowires can be controlled by a small change in the gate voltage. For large negative gate voltages, the number of conducting channels is reduced gradually, and we observe a stepwise decrease of both conductance and critical current before the conductance vanishes completely.
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| 15. |
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| 16. |
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| 17. |
- Anttu, Nicklas, et al.
(författare)
-
Coupling of Light into Nanowire Arrays and Subsequent Absorption
- 2010
-
Ingår i: Journal of Nanoscience And Nanotechnology. - : American Scientific Publishers. - 1533-4899 .- 1533-4880. ; 10:11, s. 7183-7187
-
Konferensbidrag (refereegranskat)abstract
- We present a theoretical study of the absorption of light in periodic arrays of InP nanowires. The absorption in the array depends strongly on the diameter and the length of the nanowires, as well as the period of the array. Nanowires of a length of just 2 Am are able, after an appropriate choice for the other parameters, to absorb more than 90% of the incident energy of TE and TM polarized light, with photon energies almost all the way down to the band gap energy and an incidence angle up to 50 degree. This high total absorption arises from a good coupling of the incident light into the nanowire array at the top interface between air and the array and absorption inside the array before the light reaches the interface between the nanowires and the substrate. We find that for a given photon energy there exists a critical nanowire diameter above which a dramatic increase in the absorption occurs. The critical diameter decreases for increasing photon energies, and is directly related to the dispersion of waveguiding modes in single isolated nanowires. A characterization showed that the absorption characteristics of the nanowire arrays are very promising for photovoltaic applications.
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| 18. |
- Anttu, Nicklas, et al.
(författare)
-
Efficient light management in vertical nanowire arrays for photovoltaics
- 2013
-
Ingår i: Optics Express. - 1094-4087. ; 21:9, s. 558-575
-
Tidskriftsartikel (refereegranskat)abstract
- Vertical arrays of direct band gap III-V semiconductor nanowires (NWs) hold the prospect of cheap and efficient next-generation photovoltaics, and guidelines for successful light-management are needed. Here, we use InP NWs as a model system and find, through electrodynamic modeling, general design principles for efficient absorption of sun light in nanowire arrays by systematically varying the nanowire diameter, the nanowire length, and the array period. Most importantly, we discover the existence of specific band-gap dependent diameters, 170 nm and 410 nm for InP, for which the absorption of sun light in the array is optimal, irrespective of the nanowire length. At these diameters, the individual InP NWs of the array absorb light strongly for photon energies just above the band gap energy due to a diameter-tunable nanophotonic resonance, which shows up also for other semiconductor materials of the NWs. Furthermore, we find that for maximized absorption of sun light, the optimal period of the array increases with nanowire length, since this decreases the insertion reflection losses. (C) 2013 Optical Society of America
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| 19. |
- Anttu, Nicklas, et al.
(författare)
-
Light scattering and plasmon resonances in a metal film with sub-wavelength nano-holes
- 2008
-
Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6596 .- 1742-6588. ; 100, s. 052037-052037
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Konferensbidrag (refereegranskat)abstract
- We report on a theoretical study of optical extinction in a metal film of 15-230 nm in thickness patterned periodically with sub-wavelength nano-holes of 140 nm in diameter. The gold plate was on a thick SiO2 wafer and the nano-holes as well as the top side of the metal plate were filled with water or solvent. Light was sent in toward the plate from the SiO2 side. The simulations were performed by solving the Maxwell equations using the scattering matrix method. It was seen that the extinction can, depending on the periodicity of the hole array, show one or several peaks in the visible wavelength range. The positions of the peaks were redshifted when the thickness of the gold plate was decreased. It was found that the peak positions for a thick plate can be identified from a simple surface plasmon dispersion relation.
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| 20. |
- Anttu, Nicklas, et al.
(författare)
-
Optical Far-Field Method with Subwavelength Accuracy for the Determination of Nanostructure Dimensions in Large-Area Samples.
- 2013
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 13:6, s. 2662-2667
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Tidskriftsartikel (refereegranskat)abstract
- The physical, chemical, and biological properties of nanostructures depend strongly on their geometrical dimensions. Here we present a fast, noninvasive, simple-to-perform, purely optical method that is capable of characterizing nanostructure dimensions over large areas with an accuracy comparable to that of scanning electron microscopy. This far-field method is based on the analysis of unique fingerprints in experimentally measured reflectance spectra using full three-dimensional optical modeling. We demonstrate the strength of our method on large-area (millimeter-sized) arrays of vertical InP nanowires, for which we simultaneously determine the diameter and length as well as cross-sample morphological variations thereof. Explicitly, the diameter is determined with an accuracy better than 10 nm and the length with an accuracy better than 30 nm. The method is versatile and robust, and we believe that it will provide a powerful and standardized measurement technique for large-area nanostructure arrays suitable for both research and industrial applications.
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| 21. |
- Anttu, Nicklas, et al.
(författare)
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Scattering matrix method for optical excitation of surface plasmons in metal films with periodic arrays of subwavelength holes
- 2011
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 83:16
-
Tidskriftsartikel (refereegranskat)abstract
- We present the formulation of a scattering matrix method for the study of light-scattering properties of metal films. The method is employed for the study of the optical excitation of surface plasmons in a gold film of 15-230 nm thickness, patterned periodically with subwavelength nanoholes. The gold film is placed on a thick SiO2 wafer, and the nanoholes as well as the top side of the gold film are filled with H2O. Light is incident on the gold film from either the SiO2 or the H2O side. The extinction and reflectance spectra of the system, as well as the electromagnetic field distributions at certain characteristic wavelengths, are calculated. The extinction spectra show, depending on system parameters, one or several peaks in the visible wavelength range. The extinction peaks are found to be caused by surface plasmons. A simple model based on the dispersion relation for surface plasmons in an unperforated gold film is shown to predict the peak positions of the extinction for thick perforated films very well. Even for thin films, this simple model, which includes coupling of surface plasmons on both surfaces of the film, predicts peak positions of the extinction well if the hole diameter is small enough. As the hole diameter increases, the extinction peaks of thin films show redshifts. Extinction peaks caused by surface plasmons at the SiO2/Au interface in thick films exhibit strong redshifts when the film thickness is decreased. However, the extinction peaks caused by surface plasmons at the H2O/Au interface in thick films show a completely different behavior. In this case, the extinction peaks do not move noticeably when the film thickness is decreased. Instead, they are weakened and finally disappear. It is also found that each extinction peak is accompanied by an extinction dip and that a reflectance dip is located in the wavelength between the extinction peak and the dip. This arrangement of an extinction peak, a reflectance dip, and an extinction dip is a general property of the surface-plasmon excitation. The calculated electromagnetic field distributions in both thick and thin films show clearly the signature of the excitation of surface plasmons at the extinction peaks, the extinction dips, and the reflectance dips. In thick films with small holes, the electric-field strength in the vicinity of the holes is weak at wavelengths for which surface plasmons are excited. In contrast to this, for thin films at the surface-plasmon excitations, a much stronger electric field is seen in the vicinity of the holes.
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| 22. |
- Balocco, C, et al.
(författare)
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Microwave detection at 110 GHz by nanowires with broken symmetry
- 2005
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 5:7, s. 1423-1427
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Tidskriftsartikel (refereegranskat)abstract
- By using arrays of nanowires with intentionally broken symmetry, we were able to detect microwaves up to 110 GHz at room temperature. This is, to the best of our knowledge, the highest speed that has been demonstrated in different types of novel electronic nanostructures to date. Our experiments showed a rather stable detection sensitivity over a broad frequency range from 100 MHz to 110 GHz. The novel working principle enabled the nanowires to detect microwaves efficiently without a dc bias. In principle, the need for only one high-resolution lithography step and the planar architecture allow an arbitrary number of nanowires to be made by folding a linear array as many times as required over a large area, for example, a whole wafer. Our experiment on 18 parallel nanowires showed a sensitivity of approximately 75 mV dc output/mW of nominal input power of the 110 GHz signal, even though only about 0.4% of the rf power was effectively applied to the structure because of an impedance mismatch. Because this array of nanowires operates simultaneously, low detection noise was achieved, allowing us to detect -25 dBm 110 GHz microwaves at zero bias with a standard setup.
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| 23. |
- Beyer, Jan, 1980-
(författare)
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Spin Properties in InAs/GaAs Quantum Dot based Nanostructures
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semiconductor quantum dots (QDs) are a promising building block of future spin-functional devices for applications in spintronics and quantum information processing. Essential to the realization of such devices is our ability to create a desired spin orientation of charge carriers (electrons and holes), typically via injection of spin polarized carriers from other parts of the QD structures. In this thesis, the optical orientation technique has been used to characterize spin generation, relaxation and detection in self-assembled single and multi-QD structures in the InAs/GaAs system prepared by modern molecular beam epitaxy technique.Optical generation of spin-oriented carriers in the wetting layer (WL) and GaAs barrier was carried out via circularly polarized excitation of uncorrelated electron-hole pairs from band-to-band transitions or via resonant excitation of correlated electron-hole pairs, i.e. excitons. It was shown that the generation and injection of uncorrelated electron-hole pairs is advantageous for spin-preserving injection into the QDs. The lower spin injection efficiency of excitons was attributed to an enhanced spin relaxation caused by the mutual electron-hole Coulomb exchange interaction. This correlation affects the spin injection efficiency up to elevated temperatures of around 150 K.Optical orientation at the energy of the WL light-hole (lh) exciton (XL) is accompanied by simultaneous excitation from the heavy-hole (hh) valence band at high ~k-vectors. Quantum interference of the two excitation pathways in the spectral vicinity of the XL energy resulted in occurrence of an asymmetric absorption peak, a Fano resonance. Complete quenching of spin generation efficiency at the resonance was observed and attributed to enhanced spin scattering between the hh and lh valence bands in conjunction with the Coulomb exchange interaction in the XL. This mechanism remains effective up to temperatures exceeding 100 K.In longitudinal magnetic fields up to 2 T, the spin detection efficiency in the QD ensemble was observed to increase by a factor of up to 2.5 in the investigated structures. This is due to the suppression of two spin depolarization mechanisms of the QD electron: the hyperfine interaction with the randomly oriented nuclear spins and the anisotropic exchange interaction with the hole. At higher magnetic fields, when these spin depolarization processes are quenched, only anisotropic QD structures (such as double QDs, aligned along a specific crystallographic axis) still exhibit a rather strong field dependence of the QD electron spin polarization under non-resonant excitation. Here, an increased spin relaxation in the spin injector, i.e. the WL or GaAs barrier, is suggested to lead to more efficient thermalization of the spins to the lower Zeeman-split spin state before capture to the QD.Finally, the influence of elevated temperatures on the spin properties of the QD structures was studied. The temperature dependence of dynamic nuclear polarization (DNP) of the host lattice atoms in the QDs and its effect on the QD electron spin relaxation and dephasing were investigated for temperatures up to 85 K. An increase in DNP efficiency with temperature was found, accompanied by a decrease in the extent of spin dephasing. Both effects are attributed to an accelerating electron spin relaxation, suggested to be due to phonon-assisted electronnuclear spin flip-flops driven by the hyperfine interaction. At even higher temperatures, reaching up to room temperature, a surprising, sharp rise in the QD polarization degree has been found. Experiments in a transverse magnetic field showed a rather constant QD spin lifetime, which could be governed by the spin dephasing time T*2. The observed rising in QD spin polarization degree could be likely attributed to a combined effect of shortening of trion lifetime and increasing spin injection efficiency from the WL. The latter may be caused by thermal activation of non-radiative carrier relaxation channels.
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| 24. |
- Boxberg, Fredrik, et al.
(författare)
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Elastic and Piezoelectric Properties of Zincblende and Wurtzite Crystalline Nanowire Heterostructures.
- 2012
-
Ingår i: Advanced Materials. - : Wiley. - 1521-4095 .- 0935-9648. ; 24:34, s. 4692-4706
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Tidskriftsartikel (refereegranskat)abstract
- The elastic and piezoelectric properties of zincblende and wurtzite crystalline InAs/InP nanowire heterostructures have been studied using electro-elastically coupled continuum elasticity theory. A comprehensive comparison of strains, piezoelectric potentials and piezoelectric fields in the two crystal types of nanowire heterostructures is presented. For each crystal type, three different forms of heterostructures-core-shell, axial superlattice, and quantum dot nanowire heterostructures-are considered. In the studied nanowire heterostructures, the principal strains are found to be insensitive to the change in the crystal structure. However, the shear strains in the zincblende and wurtzite nanowire heterostructures can be very different. All the studied nanowire heterostructures are found to exhibit a piezoelectric field along the nanowire axis. The piezoelectric field is in general much stronger in a wurtzite nanowire heterostructure than in its corresponding zincblende heterostructure. Our results are expected to be particularly important for analyzing and understanding the properties of epitaxially grown nanowire heterostructures and for applications in nanowire electronics, optoelectronics, and biochemical sensing.
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| 25. |
- Boxberg, Fredrik, et al.
(författare)
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Photovoltaics with Piezoelectric Core-Shell Nanowires
- 2010
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 10:4, s. 1108-1112
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Tidskriftsartikel (refereegranskat)abstract
- We report on a theoretical discovery of a generic piezoelectric held in strained core shell compound semiconductor nanowires. We show, using both an analytical model and numerical simulations based on fully electroelastically coupled continuum elasticity theory, that lattice-mismatch-induced strain in an epiraxial core shell nanowire gives rise to an internal electric held along the axis of the nanowire. This piezoelectric field results predominantly from atomic layer displacements along the nanowire axis within both the core and shell materials and can appear in both zinc blende and wurtzite crystalline core-shell nanowires. The effect can be employed to separate photon-generated electron hole pairs in the core shell nanowires and thus offers a new device concept for solar energy conversion.
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| 26. |
- Boxberg, Fredrik, et al.
(författare)
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Photovoltaics with piezoelectric core-shell nanowires
- 2011
-
Ingår i: Physics of Semiconductors: 30th International Conference on the Physics of Semiconductors. - : AIP. - 1551-7616 .- 0094-243X. ; 1399, s. 469-470
-
Konferensbidrag (refereegranskat)abstract
- We report on the discovery of a generic piezoelectric field in strained core-shell compound semiconductor nanowires. We present the lattice-mismatch induced strain in epitaxial core-shell nanowires and show that it gives, very generally, rise to an internal piezoelectric field along the axis of the nanowire. This field results predominantly from atomic displacements in polar bonds and microscopic polarizations along the nanowire axis within both the core and shell materials. The mechanism and internal piezoelectric field is particularly strong in < 111 > orientated zinc blende and [0001] orientated wurtzite crystalline core-shell nanowires. The effect can be employed to generate the potovoltaic effect in core-shell nanowires free of doping and thus offers a new device concept for solar cells and photodetectors.
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| 27. |
- Brusheim, Patrik, et al.
(författare)
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Field-driven geometrical phases in a time-periodic quantum system
- 2009
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 79:20
-
Tidskriftsartikel (refereegranskat)abstract
- We apply Floquet theory to explore the geometry of the Hilbert space under the influence of a time-periodic field. The geometrical phase is found to be induced by field-driven hybridizations when the photon energy of the driving field is close to the transition energies of the states of a quantum system. The phases of two hybridized states are phase locked to each other. We show that the geometrical phase is in general related to the Rabi frequency of the hybrid states. We also show that when the photon energy is equal to the transition energy of two states the geometrical phase acquired by each state is given exactly by an integer multiple of pi, independent of the strength of the driving field. We illustrate the derived generic properties of the geometric phase with an experimentally realizable quantum-wire system. It is shown that the interference between conductance channels in the wire presents a way to identify the geometrical phase.
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| 28. |
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| 29. |
- Brusheim, Patrik, et al.
(författare)
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Spin filtering devices based on ferromagnetic stripe modulated double quantum-dot structures
- 2006
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 73:4
-
Tidskriftsartikel (refereegranskat)abstract
- We report on a theoretical study of spin-dependent electron transport in double quantum dot structures, made from a two-dimensional electron gas, with a local magnetic field modulation. Spin-dependent conductance and probability density of electrons in the structures are calculated and the underlying physics of the results is discussed. We include in the study not only the magnetic field component perpendicular to the two-dimensional electron gas plane, but also a consideration of the in-plane component of the magnetic field. It is shown that giant spin polarization (similar to 100%) of the conductance, with tunable spin polarity, can be achieved with the double-dot structures. It is also shown that the structures can be used as efficient spin filtering devices at temperatures well above that defined by the spin splitting energy.
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| 30. |
- Brusheim, Patrik, et al.
(författare)
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Spin Hall effect and zitterbewegung in an electron waveguide
- 2006
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 74:20
-
Tidskriftsartikel (refereegranskat)abstract
- We study spin-resolved probability distributions for electrons in a multichannel waveguide in the presence of a spin-orbit interaction. For a spin-polarized electron injection, a zitterbewegung pattern is predicted in the probability distribution of electrons in the waveguide. For a spin-unpolarized injection, the spin-resolved electron probability in the waveguide shows spin accumulations. In addition to the spin Hall phenomenon-namely, accumulations of opposite spins at the lateral edges of the waveguide-we predict the existence of a regular stripe pattern of spin accumulations in the internal region of the waveguide. We show that the predicted zitterbewegung and spin Hall effect stem from the same mechanism and are formed from coherent states of electrons in the waveguide.
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| 31. |
- Brusheim, Patrik, et al.
(författare)
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Spin transport and spin Hall effect in an electron waveguide in the presence of an in-plane magnetic field and spin-orbit interaction
- 2007
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 75:19
-
Tidskriftsartikel (refereegranskat)abstract
- We study electron spin transport and the spin Hall effect in a quantum waveguide in the presence of Rashba spin-orbit interaction (SOI) and a homogeneous in-plane magnetic field, as well as the energy dispersion relation and spin polarization properties of the electron states in the corresponding infinite system. A general expression for the spin polarization of the outgoing flux, resulting from an arbitrarily polarized injection source, is derived for the quantum waveguide without the assumption of spin-degenerate leads. We further derive constraints on the spin-resolved transmission coefficients by analyzing the symmetries of the system. These constraints will impose restrictions on the spin-dependent conductances as well as the spin polarization of the outgoing flux. For an applied in-plane magnetic field parallel to the waveguide, conductance oscillations as a function of the SOI strength are diminished when the Zeeman energy exceeds the Rashba energy, due to spin alignment with the applied magnetic field. Furthermore, the calculated spin and charge probability distributions in the SOI waveguide region show spin Hall patterns and zitterbewegung patterns, respectively. For an applied transverse in-plane magnetic field, the SOI induced effective magnetic field is parallel or antiparallel with the applied field. The conductance of the system is then dominantly determined by spin-conserved transport processes, and spin-flipped transport processes can occur only through interchannel scattering and are therefore greatly suppressed. Furthermore, no zitterbewegung pattern in the charge density distribution can be found in the SOI region when the electrons are injected from a lead with spin polarization along the applied in-plane transverse field direction. However, the spin polarization probability distribution can still show a spin Hall pattern in the SOI region and the sign of the spin polarization in the spin Hall pattern can be reversed by tuning the applied magnetic field strength. For the corresponding infinite waveguide systems with uniform SOI and in-plane magnetic field, we show that the interplay between the applied in-plane magnetic field and the SOI induced effective magnetic field, together with subband hybridizations, will create a wave-vector-dependent spin polarization of the Bloch states with rich features and that these features cannot be captured by employing a simple one-dimensional model.
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| 32. |
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| 33. |
- Brusheim, Patrik, et al.
(författare)
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Symmetry of hole spin transport in a two-terminal quantum system
- 2006
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 74:23
-
Tidskriftsartikel (refereegranskat)abstract
- We study two-terminal hole spin transport in a p-type, light-hole-heavy-hole coupled, quantum-well system under the influence of Rashba spin-orbit interaction. An expression for the spin polarization of the outgoing hole flux is derived. It is shown that the in-plane spin polarization vanishes identically whenever the outgoing lead supports only heavy-hole channels. This result is independent of the details of the system. By analyzing the symmetries of the system, constraints on the spin-dependent transmission amplitudes are derived. It is shown that under time-reversal symmetry, the total hole spin polarization vanishes whenever the outgoing lead supports only one spin-degenerate hole channel, which is also independent of the details of the system. It is further shown that generally it is difficult to achieve spin polarized currents from a laterally defined Rashba hole device without breaking its transverse reflection symmetry.
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| 34. |
- Cai, Baoping, et al.
(författare)
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Application of Bayesian Networks in Reliability Evaluation
- 2019
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Ingår i: IEEE Transactions on Industrial Informatics. - : IEEE. - 1551-3203 .- 1941-0050. ; 15:4, s. 2146-2157
-
Tidskriftsartikel (refereegranskat)abstract
- The Bayesian network (BN) is a powerful model for probabilistic knowledge representation and inference and is increasingly used in the field of reliability evaluation. This paper presents a bibliographic review of BNs that have been proposed for reliability evaluation in the last decades. Studies are classified from the perspective of the objects of reliability evaluation, i.e., hardware, structures, software, and humans. For each classification, the construction and validation of a BN-based reliability model are emphasized. The general procedural steps for BN-based reliability evaluation, including BN structure modeling, BN parameter modeling, BN inference, and model verification and validation, are investigated. Current gaps and challenges in reliability evaluation with BNs are explored, and a few upcoming research directions that are of interest to reliability researchers are identified.
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| 35. |
- Churchill, H. O. H., et al.
(författare)
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Superconductor-nanowire devices from tunneling to the multichannel regime: Zero-bias oscillations and magnetoconductance crossover
- 2013
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 87:24
-
Tidskriftsartikel (refereegranskat)abstract
- We present transport measurements in superconductor-nanowire devices with a gated constriction forming a quantum point contact. Zero-bias features in tunneling spectroscopy appear at finite magnetic fields and oscillate in amplitude and split away from zero bias as a function of magnetic field and gate voltage. A crossover in magnetoconductance is observed: Magnetic fields above similar to 0.5 T enhance conductance in the low-conductance (tunneling) regime but suppress conductance in the high-conductance (multichannel) regime. We consider these results in the context of Majorana zero modes as well as alternatives, including the Kondo effect and analogs of 0.7 structure in a disordered nanowire.
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| 36. |
- Csontos, Dan, et al.
(författare)
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Characteristics of electron transport through vertical double-barrier quantum-dot structures: Effects of symmetric and asymmetric variations of the lateral confinement potentials
- 2002
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 66:20
-
Tidskriftsartikel (refereegranskat)abstract
- We report on a theoretical study of the electron transport through laterally-confined, vertical double-barrier resonant-tunneling (DBRT) structures, defined as one-dimensional (1D)-0D-1D systems, with a tunable lateral confinement. The current and the differential conductance of the systems are calculated and the influence caused by varying the lateral confinement on the device characteristics is investigated. Three representative systems are studied. First of all, a 1D-0D-1D device, symmetric with respect to the current flow, with a variable lateral confinement in the double-barrier quantum-well (DBQW) region, is investigated. This device would in an experimental setup correspond to the structure in which a thin, lateral metallic gate is placed in the DBQW region. Subsequently, calculations are performed for two asymmetric 1D-0D-1D devices, in which the strongest, but varying, lateral confinement is placed either in the collector or in the emitter region. In experiments, these two devices would correspond to the situations where a lateral metallic gate is positioned below or on top of the DBQW structure. The calculations predict several phenomena for the device characteristics. It is shown that as the lateral confinement increases, in addition to those normally observed current onsets and pinch-offs that move toward higher bias voltages, several current onsets and pinch-offs move towards lower bias voltages. These negative shifts of the current onsets and pinch-offs with increasing of the lateral confinement have so far not been expected for gated DBRT devices. It is also found that the threshold voltages, at which the current onsets and pinch-offs appear, depend strongly on the strength and position of the lateral confinement and on the Fermi levels in the collector and the emitter. The models that explain these predictions are presented and discussed.
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| 37. |
- Csontos, D., et al.
(författare)
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Lande-like formula for the g factors of hole-nanowire subband edges
- 2008
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 78:3
-
Tidskriftsartikel (refereegranskat)abstract
- We have analyzed theoretically the Zeeman splitting of hole-quantum-wire subband edges. As is typical for any bound state, their g factor depends on both an intrinsic g factor of the material and an additional contribution arising from a finite bound-state orbital angular momentum. We discuss the quantum-confinement-induced interplay between bulk-material and orbital effects, which is nontrivial due to the presence of strong spin-orbit coupling. A compact analytical formula is provided that elucidates this interplay and can be useful for predicting Zeeman splitting in generic hole-wire geometries.
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| 38. |
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| 39. |
- Csontos, Dan, et al.
(författare)
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Quantum interference effects in electron transport through Y-branches
- 2002
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Ingår i: 7th International Conference on Nanometer-Scale Science and Technology and 21st European Conference on Surface Science.
-
Konferensbidrag (refereegranskat)abstract
- Recently, nonlinear properties of three-terminal ballistic junctions (TBJs) were predicted by theoretical calculations done by Xu, and potential applications such as rectification and logic function proposed. In that analysis the TBJ was modeled by connecting three quantum point contacts via a ballistic cavity with adiabatic boundaries, thus neglecting any backscattering of electrons. In this paper we develop a realistic model based on the scattering-matrix method for the calculation of transport through TBJs with arbitrary potential profiles. Temperature-dependent linear and nonlinear transport has been calculated and analyzed for a number of Y-shaped devices with different geometries, sizes, impurity concentrations and various degrees of boundary roughness. Interference effects are shown to influence the characteristics of the calculated transport in a strong way at low temperature where new quantum phenomena are predicted
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| 40. |
- Csontos, Dan, et al.
(författare)
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Scattering-matrix formalism of electron transport through three-terminal quantum structures: formulation and application to Y-junction devices
- 2002
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Ingår i: Journal of Physics: Condensed Matter. - : IOP Publishing. - 1361-648X .- 0953-8984. ; 14:47, s. 12513-12528
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we present a formalism for the calculation of electron transport through three-terminal junction devices, which have received attention due to their recently demonstrated non-linear electrical properties. The formalism, which is based on the scattering-matrix method, takes quantum interference effects fully into account. Furthermore, the formalism provides numerical stability in the calculations as well as large flexibility in the modelling of arbitrary potential profiles due to the common basis approach used in the formulation. The method is used to calculate the transport properties for Y-shaped three-terminal ballistic junction (TBJ) structures with configurations typical of recently performed experiments. Quantum interference effects are shown to strongly influence the transport characteristics of TBJ structures due to complex scattering of the electrons in the cavity-like coupling window between the three arms of the device. The theoretical approach presented in this paper provides a flexible tool for the study of such quantum interference effects, which may play an important role in the design and functionality of future nanoscale devices based on three-terminal junctions.
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| 41. |
- Csontos, D., et al.
(författare)
-
Spin-3/2 physics of semiconductor hole nanowires: Valence-band mixing and tunable interplay between bulk-material and orbital bound-state spin splittings
- 2009
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 79:15
-
Tidskriftsartikel (refereegranskat)abstract
- We present a detailed theoretical study of the electronic spectrum and Zeeman splitting in hole quantum wires. The spin-3/2 character of the topmost bulk-valence-band states results in a strong variation in subband-edge g factors between different subbands. We elucidate the interplay between quantum confinement and heavy-hole-light-hole mixing and identify a certain robustness displayed by low-lying hole-wire subband edges with respect to changes in the shape or strength of the wire potential. The ability to address individual subband edges in, e.g., transport or optical experiments enables the study of hole states with nonstandard spin polarization, which do not exist in spin-3/2 systems. Changing the aspect ratio of hole wires with rectangular cross section turns out to strongly affect the g factor of subband edges, providing an opportunity for versatile in situ tuning of hole-spin properties with possible application in spintronics. The relative importance of cubic crystal symmetry is discussed, as well as the spin splitting away from zone-center subband edges.
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| 42. |
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| 43. |
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| 44. |
- Deng, Mingtang, et al.
(författare)
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Anomalous Zero-Bias Conductance Peak in a Nb-InSb Nanowire-Nb Hybrid Device.
- 2012
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 12:12, s. 6414-6419
-
Tidskriftsartikel (refereegranskat)abstract
- Semiconductor InSb nanowires are expected to provide an excellent material platform for the study of Majorana fermions in solid state systems. Here, we report on the realization of a Nb-InSb nanowire-Nb hybrid quantum device and the observation of a zero-bias conductance peak structure in the device. An InSb nanowire quantum dot is formed in the device between the two Nb contacts. Due to the proximity effect, the InSb nanowire segments covered by the superconductor Nb contacts turn to superconductors with a superconducting energy gap Δ(InSb) ∼ 0.25 meV. A tunable critical supercurrent is observed in the device in high back gate voltage regions in which the Fermi level in the InSb nanowire is located above the tunneling barriers of the quantum dot and the device is open to conduction. When a perpendicular magnetic field is applied to the devices, the critical supercurrent is seen to decrease as the magnetic field increases. However, at sufficiently low back gate voltages, the device shows the quasi-particle Coulomb blockade characteristics and the supercurrent is strongly suppressed even at zero magnetic field. This transport characteristic changes when a perpendicular magnetic field stronger than a critical value, at which the Zeeman energy in the InSb nanowire is E(z) ∼ Δ(InSb), is applied to the device. In this case, the transport measurements show a conductance peak at the zero bias voltage and the entire InSb nanowire in the device behaves as in a topological superconductor phase. We also show that this zero-bias conductance peak structure can persist over a large range of applied magnetic fields and could be interpreted as a transport signature of Majorana fermions in the InSb nanowire.
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| 45. |
- Deng, Mingtang, et al.
(författare)
-
Parity independence of the zero-bias conductance peak in a nanowire based topological superconductor-quantum dot hybrid device.
- 2014
-
Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 4
-
Tidskriftsartikel (refereegranskat)abstract
- We explore the signatures of Majorana fermions in a nanowire based topological superconductor-quantum dot-topological superconductor hybrid device by charge transport measurements. At zero magnetic field, well-defined Coulomb diamonds and the Kondo effect are observed. Under the application of a finite, sufficiently strong magnetic field, a zero-bias conductance peak structure is observed. It is found that the zero-bias conductance peak is present in many consecutive Coulomb diamonds, irrespective of the even-odd parity of the quasi-particle occupation number in the quantum dot. In addition, we find that the zero-bias conductance peak is in most cases accompanied by two differential conductance peaks, forming a triple-peak structure, and the separation between the two side peaks in bias voltage shows oscillations closely correlated to the background Coulomb conductance oscillations of the device. The observed zero-bias conductance peak and the associated triple-peak structure are in line with Majorana fermion physics in such a hybrid topological system.
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| 46. |
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| 47. |
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| 48. |
- Fahlvik Svensson, Sofia, et al.
(författare)
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Lineshape of the thermopower of quantum dots
- 2012
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 14
-
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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| 49. |
- Fahlvik Svensson, Sofia, et al.
(författare)
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Nonlinear thermovoltage and thermocurrent in quantum dots
- 2013
-
Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 15
-
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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| 50. |
- Fan, Dingxun, et al.
(författare)
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Formation of long single quantum dots in high quality InSb nanowires grown by molecular beam epitaxy
- 2015
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 7:36, s. 14822-14828
-
Tidskriftsartikel (refereegranskat)abstract
- We report on realization and transport spectroscopy study of single quantum dots (QDs) made from InSb nanowires grown by molecular beam epitaxy (MBE). The nanowires employed are 50-80 nm in diameter and the QDs are defined in the nanowires between the source and drain contacts on a Si/SiO2 substrate. We show that highly tunable QD devices can be realized with the MBE-grown InSb nanowires and the gate-to-dot capacitance extracted in the many-electron regimes is scaled linearly with the longitudinal dot size, demonstrating that the devices are of single InSb nanowire QDs even with a longitudinal size of similar to 700 nm. In the few-electron regime, the quantum levels in the QDs are resolved and the Lande g-factors extracted for the quantum levels from the magnetotransport measurements are found to be strongly level-dependent and fluctuated in a range of 18-48. A spin-orbit coupling strength is extracted from the magnetic field evolutions of a ground state and its neighboring excited state in an InSb nanowire QD and is on the order of similar to 300 mu eV. Our results establish that the MBE-grown InSb nanowires are of high crystal quality and are promising for the use in constructing novel quantum devices, such as entangled spin qubits, one-dimensional Wigner crystals and topological quantum computing devices.
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