| 1. |
- Conache, Gabriela, 1977-, et al.
(författare)
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Nanowire friction with an applied bias
- 2009
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Konferensbidrag (refereegranskat)abstract
- Recently, we have shown how the friction acting on nanowires pushed across a surface by an AFM tip can be determined by measuring the radius of curvature of the bent wire aŸer manipulation. This technique allows us to study the friction properties of an extended mesoscale contact. Our main focus has been to determine whether such contacts behave like macroscopic objects, in which dišerences between the 'true' and 'apparent' contact areas play a key role and friction varies linearly with the applied normal force, or whether they are more like atomic-scale point contacts, wheremore fundamental processes dominate and friction oŸen is independent of the normal force. In this work we show how the friction between InAs nanowires and an insulating silicon nitride layer on a conductive silicon substrate varies when a DC voltage is applied to the AFM tip during manipulation. e tip charges the capacitor formed by the wire and the grounded silicon back contact, giving rise to attractive Coulomb forces and thus increasing the contact pressure between the wire and the silicon nitride. In this way we can vary the normal force on the sliding surfaces using a single wire, with a constant structure and contact geometry. Using nanowires of about 40-50 nm diameter and a few microns in length we have applied tip voltages in the range +12 to -12 V. Simplemodeling indicates that these voltages su›ce to give similar levels of band-lling and depletion to when the same wires are used in working wrap-gate or back-gate devices. A monotonic increase of the sliding friction with the voltage applied on the tip was observed. is implies that the friction increases with the normal force and that this mesoscopic system behaves more like a macroscopic contact, despite the nanometer size of the contact in the direction of motion.
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| 2. |
- 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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| 3. |
- Pettersson, Håkan, 1962-, et al.
(författare)
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Electrical and optical properties of InP nanowire ensemble p(+)-i-n(+) photodetectors
- 2012
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Ingår i: Nanotechnology. - Bristol, UK : Institute of Physics Publishing (IOPP). - 0957-4484 .- 1361-6528. ; 23:13
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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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| 4. |
- Pettersson, Håkan, 1962-, et al.
(författare)
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Nano-Schottky contacts realized by bottom-up technique
- 2010
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Ingår i: Bulletin of American Physical Society. - : American Physical Society.
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Konferensbidrag (refereegranskat)abstract
- Here we present a comprehensive study of a rectifying nano-Schottky contact formed at the interface between a gold catalytic particle and an epitaxially grown GaInAs/InAs nanowire. Selective electrical connections formed by electron beam lithography to the catalytic particle on one side, and to the InAs segment on the other side allowed electrical and optical characterization of the formed Schottky junction. From IV measurements taken at different temperatures we have deduced the Schottky barrier height and the height of the barrier formed in the graded GaInAs nanowire segment. The IV characteristics measured under laser stimulation showed that the device can be used as a unipolar photodetector with extremely small detection volume and potentially ultra fast response.
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| 5. |
- Pettersson, Håkan, 1962-, et al.
(författare)
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Nanowire friction with an applied bias
- 2010
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Ingår i: Bulletin of the American Physical Society. - : American Physical Society.
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Konferensbidrag (refereegranskat)abstract
- Recently, we have shown how the friction experienced by nanowires pushed by an AFM tip can be determined by measuring their radius of curvature after manipulation [1]. It is of fundamental interest to know whether the wires behave like macroscopic objects, or if they are more like true atomic-scale point contacts where friction becomes independent of the applied normal force. Here we study how the friction between InAs nanowires and a SiN layer on conductive silicon varies when a DC voltage is applied. The tip charges the capacitor formed by the wire and the silicon back contact, causing attractive Coulomb forces and so increasing the contact pressure. A monotonic increase of the sliding friction with voltage was observed. This implies that the friction increases with the normal force and that this mesoscopic system behaves more like a macroscopic contact, despite being only nanometers in size in the direction of motion.
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| 6. |
- Pettersson, Håkan, 1962-, et al.
(författare)
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Shear stress measurements on InAs nanowires by AFM manipulation
- 2007
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Ingår i: Bulletin of the American Physical Society. - New York : American Physical Society. - 0003-0503. ; 52:1
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we report on a novel approach to measure shear stress between elastic nanowires and a SiO2 surface. The method is based on the fact that the curvature of an elastically deformed nanowire pinned to a flat surface contains information about the maximal static friction force, i.e., the shear stress between the wire and the surface. At rest, the deformed wire is kept in equilibrium by counterbalancing static friction forces and restoring elastic forces. In the present work, InAs nanowires are bent in a controlled manner using the tip of an atomic force microscope (AFM). After the manipulation, the curvature of the most bent state can be determined from AFM micrographs. Assuming bulk values for the Young’s modulus, the shear stress can be obtained from straight- forward analyses according to standard theory of elasticity.
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| 7. |
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| 8. |
- Aghaeipour, Mahtab
(författare)
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Tailoring the Optical Response of III-V Nanowire Arrays
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semiconductor nanowires show a great deal of promise for applications in a wide range of important fields, including photovoltaics, biomedicine, and information technology. Developing these exciting applications is strongly dependent on understanding the fundamental properties of nanowires, such as their optical resonances and absorption spectra. In this thesis we explore optical absorption spectra of arrays of vertical III-V nanowires with a special emphasis on structures optimized to enhance absorption in the solar spectrum. First, we analyze experimentally determined absorption spectra of both indium phosphide (InP) and gallium phosphide (GaP) nanowire arrays. The study provides an intuitive understanding of how the observed absorption resonances in the nanowires may be tuned as a function of their geometrical parameters and crystal structure. As a consequence, the spectral position of absorption resonances can be precisely controlled through the nanowire diameter. However, the results highlight how the blue-shift in the optical absorption resonances as the diameter of the nanowires decreases comes to a halt at low diameters. The stop point is related to the behavior of the refractive indices of the nanowires. The wavelength of the stop is different for nanowire polytypes of similar dimensions due to differences in their refractive indices. We then present a theoretical argument that it is important to consider symmetry properties when tailoring the optical modes excited in the nanowires for enhanced absorption. We show that absorption spectra may be enhanced compared to vertical nanowires at normal incidence by tilting the nanowires with normal incidence light, or by using off-normal incidence with vertical nanowires. This is because additional optical modes inside the nanowires are excited when the symmetry is broken. Looking forward to omnidirectional applications, we consider branched nanowires as a way to enhance the absorption spectra at normal incidence by taking advantage of simultaneous excitation of the spectrally different optical modes in the branches and the stems. Third, we describe in theoretical terms how integrating distributed Bragg reflectors (DBRs) with the nanowires can improve absorption spectra compared to conventional nanowires. DBRs provide a way to employ light trapping mechanisms which increases the optical path length of the excited modes and thereby improves the absorption of the excited modes. At normal incidence, DBR-nanowires improve the absorption efficiency to 78%, compared to 72% for conventional nanowires. We show that the efficiency is increased to 85% for an off-normal incident angle of 50˚. Overall, our results show that studies of optical resonances in nanowires that take the light-matter interaction into account provide opportunities to develop novel optical and optoelectronic functionalities in nanoscience and nanotechnology.
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| 9. |
- 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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| 10. |
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