| 1. |
- Borschel, Christian, et al.
(författare)
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A New Route toward Semiconductor Nanospintronics : Highly Mn-Doped GaAs Nanowires Realized by Ion-Implantation under Dynamic Annealing Conditions
- 2011
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Ingår i: Nano letters (Print). - Washington : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 11:9, s. 3935-3940
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Tidskriftsartikel (refereegranskat)abstract
- We report on highly Mn-doped GaAs nanowires (NWs) of high crystalline quality fabricated by ion beam implantation, a technique that allows doping concentrations beyond the equilibrium solubility limit. We studied two approaches for the preparation of Mn-doped GaAs NWs: First, ion implantation at room temperature with subsequent annealing resulted in polycrystalline NWs and phase segregation of MnAs and GaAs. The second approach was ion implantation at elevated temperatures. In this case, the single-crystallinity of the GaAs NWs was maintained, and crystalline, highly Mn-doped GaAs NWs were obtained. The electrical resistance of such NWs dropped with increasing temperature (activation energy about 70 meV). Corresponding magnetoresistance measurements showed a decrease at low temperatures, indicating paramagnetism. Our findings suggest possibilities for future applications where dense arrays of GaMnAs nanowires may be used as a new kind of magnetic material system.
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| 2. |
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| 3. |
- Corrêa, Gregório B., et al.
(författare)
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Raman characterization of single-crystalline Ga0.96Mn0.04As:Zn nanowires realized by ion-implantation
- 2019
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Ingår i: Nanotechnology. - Bristol : IOP Publishing. - 0957-4484 .- 1361-6528. ; 30:33
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Tidskriftsartikel (refereegranskat)abstract
- Recent progress in the realization of magnetic GaAs nanowires (NWs) doped with Mn has attracted a lot of attention due to their potential application in spintronics. In this work, we present a detailed Raman investigation of the structural properties of Zn doped GaAs (GaAs:Zn) and Mn-implanted GaAs:Zn (Ga0.96Mn0.04As:Zn) NWs. A significant broadening and redshift of the optical TO and LO phonon modes are observed for these NWs compared to as-grown undoped wires, which is attributed to strain induced by the Zn/Mn doping and to the presence of implantation-related defects. Moreover, the LO phonon modes are strongly damped, which is interpreted in terms of a strong LO phonon-plasmon coupling, induced by the free hole concentration. Moreover, we report on two new interesting Raman phonon modes (191 and 252 cm-1) observed in Mn ion-implanted NWs, which we attribute to Eg (TO) and A1g (LO) vibrational modes in a sheet layer of crystalline arsenic present on the surface of the NWs. This conclusion is supported by fitting the observed Raman shifts for the SO phonon modes to a theoretical dispersion function for a GaAs NW capped with a dielectric shell.
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| 4. |
- Harberts, Jann, et al.
(författare)
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Culturing and patch clamping of Jurkat T cells and neurons on Al 2 O 3 coated nanowire arrays of altered morphology
- 2019
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Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 9:20, s. 11194-11201
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Tidskriftsartikel (refereegranskat)abstract
- Nanowire substrates play an increasingly important role for cell cultures as an approach for hybrid bio-semiconductor junctions. We investigate Jurkat T cells and neurons from mice cultured on Al 2 O 3 coated ordered and randomly distributed nanowires. Cell viability was examined by life/membrane staining reporting comparable viability on planar and nanowire substrates. Imaging the hybrid interface reveals a wrapping of the cell membrane around the very nanowire tip. Patch clamp recordings show similar electrophysiological responses on each type of nanowires compared to planar control substrates. We demonstrate that the morphological characteristic of the nanowire substrate plays a subordinate role which opens up the arena for a large range of nanowire substrates in a functionalized application such as stimulation or sensing.
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| 5. |
- Kumar, Sandeep, et al.
(författare)
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Evaluation of carrier density and mobility in Mn ion-implanted GaAs:Zn nanowires by Raman spectroscopy
- 2020
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Ingår i: Nanotechnology. - Bristol : IOP Publishing. - 0957-4484 .- 1361-6528. ; 31:20
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Tidskriftsartikel (refereegranskat)abstract
- The fabrication of complex nanoscale electronics with reduced dimensions poses challenges on novel techniques to accurately determine fundamental electronic parameters. In this article, we present a universal contactless method based on Raman scattering for measuring the mobility and hole concentration independently in GaAs:Zn and Mn ion-implanted GaAs:Zn nanowires, potentially of great interest for spintronics applications. Clear coupled longitudinal optical phonon-plasmon modes were recorded and fitted with a dielectric function, based on the Drude model, which includes contributions from both plasmons and phonons. From the fitting, we extract accurate values of the plasma frequency and plasma damping constant from which we directly calculate the hole density and mobility, respectively. The extracted mobilities were also used as input data for analysis of complementary four-probe transport measurements, where the corresponding hole concentrations could be calculated and found to be in good agreement with those extracted directly from the Raman data. We also investigated the influence of annealing of the GaAs:Zn nanowires on the hole concentration and mobility and found strong indications of thermally activated defects related to a formed crystalline As/oxide shell around the nanowires. The method proposed here is extremely powerful for the characterization of nanoelectronics in general, and nanospintronics in particular for which Hall measurements are difficult to pursue due to problems related to contact formation, as well as to inherent magnetic properties of the devices.
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| 6. |
- Kumar, Sandeep, et al.
(författare)
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Magnetic polarons and large negative magnetoresistance in GaAs nanowires implanted with Mn ions
- 2013
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Ingår i: Nano letters (Print). - Washington, United States : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 13:11, s. 5079-5084
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Tidskriftsartikel (refereegranskat)abstract
- We report on low-temperature magnetotransport and SQUID measurements on heavily doped Mn-implanted GaAs nanowires. SQUID data recorded at low magnetic fields exhibit clear signs of the onset of a spin-glass phase with a transition temperature of about 16 K. Magnetotransport experiments reveal a corresponding peak in resistance at 16 K and a remarkably large negative magnetoresistance, reaching 40 % at 1.6 K and 8 T. The negative magnetoresistance decreases at elevated temperatures and vanishes at about 100 K. We interpret our transport data in terms of spin-dependent hopping in a complex magnetic nanowire landscape of magnetic polarons forming a paramagnetic/spin-glass phase.
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| 7. |
- Paschoal Jr., Waldomiro, 1977-, et al.
(författare)
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Electron transport in Mn+ implanted GaAs nanowires
- 2012
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Konferensbidrag (refereegranskat)abstract
- Mn-doped GaAs semiconductors have generated great interest in current research regarding the evolution from a paramagnetic insulator to a ferromagnetic metal governed by a carrier mediated exchange interaction. The interplay between the charge carriers in a semiconductor and the electron spin of incorporated ferromagnetic metals can be utilized for novel spin-sensitive spintronic devices. We have fabricated highly Mn-doped, single-crystalline GaAs nanowires (NWs) by ion implantation at elevated temperatures to facilitate in-situ dynamic annealing. To exploit these nanowires in spintronic applications, a detailed understanding of fundamental charge transport mechanisms is however necessary. It is generally expected that new features, different from any bulk counterparts, will emerge in systems with reduced dimensionality e.g. quasi-1D NWs. Here we report on a detailed study of different charge transport mechanisms and localization-related effects in single Mn-doped GaAs NWs in the temperature range from 300K to 1.6K, and with magnetic fields ranging from 0T to 8T. In general, the resistance of the nanowires increases strongly from a few M* at 300K to several G* at 1.6 K. More specially, the temperature dependence displays several different interesting regimes described by distinctly different models. Furthermore, the current-voltage (I-V) characteristics becomes strongly non-linear as the temperature decreases and shows apparent power-law behavior at low temperatures. In particular, we interpret our transport data in the temperature range from 80K to 275K in terms of a variable range hopping process influenced by Mn-induced disorder in the NWs. Below 50K the magnetotransport data reveals a large negative magnetoresistance (MR) under both paralleland perpendicular magnetic fields. We are presently developing models to explain this large MR signal, including low-temperature transport mechanisms and possible magnetic interaction between Mn ions.
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| 8. |
- Paschoal Jr., Waldomiro, et al.
(författare)
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Hopping Conduction in Mn Ion-Implanted GaAs Nanowires
- 2012
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Ingår i: Nano letters (Print). - Washington : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 12:9, s. 4838-4842
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Tidskriftsartikel (refereegranskat)abstract
- We report on temperature-dependent charge transport in heavily doped Mn+-implanted GaAs nanowires. The results clearly demonstrate that the transport is governed by temperature-dependent hopping processes, with a crossover between nearest neighbor hopping and Mott variable range hopping at about 180 K. From detailed analysis, we have extracted characteristic hopping energies and corresponding hopping lengths. At low temperatures, a strongly nonlinear conductivity is observed which reflects a modified hopping process driven by the high electric field at large bias.
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| 9. |
- Paschoal Jr., Waldomiro, 1977-, et al.
(författare)
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Magnetoresistance in Mn ion-implanted GaAs:Zn nanowires
- 2014
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Ingår i: Applied Physics Letters. - New York : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 104:15
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the magnetoresistance (MR) in a series of Zn doped (p-type) GaAs nanowires implanted with different Mn concentrations. The nanowires with the lowest Mn concentration (~0.0001%) exhibit a low resistance of a few kΩ at 300K and a 4% positive MR at 1.6K, which can be well described by invoking a spin-split subband model. In contrast, nanowires with the highest Mn concentration (4%) display a large resistance of several MΩ at 300K and a large negative MR of 85% at 1.6K. The large negative MR is interpreted in terms of spin-dependent hopping in a complex magnetic nanowire landscape of magnetic polarons, separated by intermediate regions of Mn impurity spins. Sweeping the magnetic field back and forth for the 4% sample reveals a hysteresis that indicates the presence of a weak ferromagnetic phase. We propose co-doping with Zn to be a promising way to reach the goal of realizing ferromagnetic Ga1-xMnxAs nanowires for future nanospintronics. © 2014 AIP Publishing LLC.
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