| 1. |
- Berg, Alexander, et al.
(författare)
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In situ etching for control over axial and radial III-V nanowire growth rates using HBr.
- 2014
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 25:50
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Tidskriftsartikel (refereegranskat)abstract
- We report on the influence of hydrogen bromide (HBr) in situ etching on the growth of InP, GaP and GaAs nanowires. We find that HBr can be used to impede undesired radial growth during axial growth for all three material systems. The use of HBr opens a window for optimizing the growth parameters with respect to the materials' quality rather than only their morphology. Transmission electron microscopy (TEM) characterization reveals a partial transition from a wurtzite crystal structure to a zincblende upon the use of HBr during growth. For InP, defect-related luminescence due to parasitic radial growth is removed by use of HBr. For GaP, the etching with HBr reduced the defect-related luminescence, but no change in peak emission energy was observed. For GaAs, the HBr etching resulted in a shift to lower photon emission energies due to a shift in the crystal structure, which reduced the wurtzite segments.
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| 2. |
- Bolinsson, Jessica, et al.
(författare)
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Diffusion length measurements in axial and radial heterostructured nanowires using cathodoluminescence
- 2011
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248. ; 315:1, s. 138-142
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Konferensbidrag (refereegranskat)abstract
- We have measured the ambipolar diffusion lengths in nanowires with GaAs and AlGaAs core material using cathodoluminescence imaging. This was done by combining III-V semiconductor materials with different bandgaps in single nanowires. We show that it is possible to record intensity profiles of the emission from segments of lower bandgap material positioned along the nanowire length and in this way gain an insight on important carrier transport properties of the nanowire core material. We present diffusion data for GaAs and AlGaAs nanowire core material in different radially and axially heterostructured nanowires and show that the diffusion of carriers is greatly increased by capping the nanowires with a higher-bandgap material. In addition, we show how a decoupling of the radial and axial growth during particle-seeded growth is necessary in order to reach long diffusion lengths along the core of axially heterostructured nanowires. In addition, for ternary compounds (InGaAs and AlGaAs), we observe compositional differences for radial and axial nanowire growth. (C) 2010 Elsevier B.V. All rights reserved.
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| 3. |
- Bolinsson, Jessica, et al.
(författare)
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GaAs/AlGaAs heterostructure nanowires studied by cathodoluminescence
- 2014
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Ingår i: Nano Reseach. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 7:4, s. 473-490
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Tidskriftsartikel (refereegranskat)abstract
- In this report we explore the structural and optical properties of GaAs/AlGaAs heterostructure nanowires grown by metalorganic vapour phase epitaxy using gold seed-particles. The optical studies were done by low-temperature cathodoluminescence (CL) in a scanning electron microscope (SEM). We perform a systematic investigation of how the nanowire growth-temperature affects the total photon emission, and variations in the emission energy and intensity along the length of the nanowires. The morphology and crystal structures of the nanowires were investigated using SEM and transmission electron microscopy (TEM). In order to correlate specific photon emission characteristics with variations in the nanowire crystal structure directly, TEM and spatially resolved CL measurements were performed on the same individual nanowires. We found that the main emission energy was located at around 1.48 eV, and that the emission intensity was greatly enhanced when increasing the GaAs nanowire core growth temperature. The data strongly suggests that this emission energy is related to rotational twins in the GaAs nanowire core. Our measurements also show that radial overgrowth by GaAs on the GaAs nanowire core can have a deteriorating effect on the optical quality of the nanowires. Finally, we conclude that an in situ pre-growth annealing step at a sufficiently high temperature significantly improves the optical quality of the nanowires.
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| 4. |
- Borgström, Magnus, et al.
(författare)
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Fabrication and characterization of AlP-GaP core-shell nanowires
- 2011
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248. ; 324:1, s. 290-295
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Tidskriftsartikel (refereegranskat)abstract
- We report on the particle assisted synthesis of core-shell AlP-GaP nanowires by use of metal-organic vapor phase epitaxy. The core-shell approach is chosen such as to stabilize the AlP which is highly sensitive to water. The nanowires were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. These nanowires have an indirect band-gap and form a type II staggered heterojunction. By designed capping of the AlP cores by GaP, we find the nanowires to be stable for more than a year. (C) 2011 Elsevier B.V. All rights reserved.
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| 5. |
- 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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| 6. |
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| 7. |
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| 8. |
- Johansson, Jonas, et al.
(författare)
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Calculation of hole concentrations in Zn doped GaAs nanowires
- 2020
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Ingår i: Nanomaterials. - : MDPI AG. - 2079-4991. ; 10:12
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Tidskriftsartikel (refereegranskat)abstract
- We have previously demonstrated that we can grow p-type GaAs nanowires using Zn doping during gold catalyzed growth with aerotaxy. In this investigation, we show how to calculate the hole concentrations in such nanowires. We base the calculations on the Zhang–Northrup defect formation energy. Using density functional theory, we calculate the energy of the defect, a Zn atom on a Ga site, using a supercell approach. The chemical potentials of Zn and Ga in the liquid catalyst particle are calculated from a thermodynamically assessed database including Au, Zn, Ga, and As. These quantities together with the chemical potential of the carriers enable us to calculate the hole concentration in the nanowires self-consistently. We validate our theoretical results against aerotaxy grown GaAs nanowires where we have varied the hole concentration by varying the Zn/Ga ratio in the aerotaxy growth.
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| 9. |
- Kailuweit, Peter, et al.
(författare)
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Numerical simulations of absorption properties of InP nanowires for solar cell applications
- 2012
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Ingår i: Progress in Photovoltaics. - : Wiley. - 1099-159X. ; 20:8, s. 945-953
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present results of optical simulations on the absorption of nanowires. The simulation was performed with rigorous coupled wave analysis to determine the potential of nanowires for use in a new kind of nanostructured solar cell. The overall absorption of the solar cell structure was calculated for four different diameters of the nanowires and also in dependence of light wavelength and nanowire period. Absorption modes were observed at distinct wavelengths, but not dependent on the wire period. To derive a local optical generation function for a nanowire solar cell, the spatially resolved absorption profile was modelled for two wire periods. The calculated generation profiles can be readily used as input for an electrical simulation of a nanowire solar cell to calculate its quantum efficiency and the currentvoltage characteristics. Copyright (C) 2011 John Wiley & Sons, Ltd.
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| 10. |
- Mergenthaler, Kilian, et al.
(författare)
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Large-energy-shift photon upconversion in degenerately doped InP nanowires by direct excitation into the electron gas
- 2013
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Ingår i: Nano Reseach. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 6:10, s. 752-757
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Tidskriftsartikel (refereegranskat)abstract
- Realizing photon upconversion in nanostructures is important for many next-generation applications such as biological labelling, infrared detectors and solar cells. In particular nanowires are attractive for optoelectronics because they can easily be electrically contacted. Here we demonstrate photon upconversion with a large energy shift in highly n-doped InP nanowires. Crucially, the mechanism responsible for the upconversion in our system does not rely on multi-photon absorption via intermediate states, thus eliminating the need for high photon fluxes to achieve upconversion. The demonstrated upconversion paves the way for utilizing nanowires-with their inherent flexibility such as electrical contactability and the ability to position individual nanowires-for photon upconversion devices also at low photon fluxes, possibly down to the single photon level in optimised structures.
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| 11. |
- Mergenthaler, Kilian, et al.
(författare)
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Photon upconversion in degenerately sulfur doped InP nanowires.
- 2015
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 7:48, s. 20503-20509
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Tidskriftsartikel (refereegranskat)abstract
- Radiative recombination in degenerately n-doped InP nanowires is studied for excitation above and below the Fermi energy of the electron gas, using photoluminescence. Laser-induced electron heating is observed, which allows absorption below the Fermi energy. We observe photon upconversion where photo-excited holes recombine with high |k| electrons. This can be attributed to hole scattering to high |k|-values, and the temperature dependence of this process is measured. We show that hole relaxation via phonon scattering can be observed in continuous wave excitation luminescence measurements.
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| 12. |
- Mergenthaler, Kilian
(författare)
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Photon Upconversion in Heavily Doped Semiconductors
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis the luminescence properties of highly doped semiconductors are studied with focus on degenerately n-doped InP. It is demonstrated how photoluminescence measurements on degenerately doped semiconductors allow an estimation of the doping concentration without need for electrical contacts. The degenerate doping can furthermore reveal the conduction band structure for energies higher than the bandgap, which is exploited to experimentally support the existence of a theoretically predicted second conduction band minimum in wurtzite InP. Excitation energy dependence measurements reveal band-to-band absorption for photon energies much lower than the Fermi energy. That absorption causes not only downconverted photoluminescence with photon energies lower than the excitation energy, but also upconverted photoluminescence with photon energies higher than the absorbed laser photon. From the results of the detailed study of this novel upconversion mechanism in degenerately n-doped InP nanowires and bulk InP we propose the following explanation: An elevated electron gas temperature in degenerately doped semiconductors allows absorption of photon with energies much lower than the Fermi energy. Band-to-band absorption of photons with energies lower than the Fermi energy excites holes with k-values lower than kF and scattering of the photexcited holes to higher k-values allows k-conserving radiative recombinations with photon energies higher than the energy of the absorbed photon. Similar upconversion luminescence is observed for degenerately n-doped bulk GaAs and degenerately p-doped GaAs nanowires, which suggest that similar photon upconversion could be observed in many degenerately doped direct band semiconductors. The three most important findings about degenerately doped direct band semiconductors are. There is significant photon upconversion for excitation energies between Eg and EF. The charge carrier recombination rate is higher than, or comparable to the scattering rate of the minority carriers. And, the radiative recombination is strongly dominated by k-conserving vertical transitions in contrast to the common assumption of relaxation of the k-selection rule in degenerately doped material.
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| 13. |
- Metaferia, Wondwosen, et al.
(författare)
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GaAsP Nanowires Grown by Aerotaxy
- 2016
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 16:9, s. 5701-5707
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Tidskriftsartikel (refereegranskat)abstract
- We have grown GaAsP nanowires with high optical and structural quality by Aerotaxy, a new continuous gas phase mass production process to grow III-V semiconductor based nanowires. By varying the PH3/AsH3 ratio and growth temperature, size selected GaAs1-xPx nanowires (80 nm diameter) with pure zinc-blende structure and with direct band gap energies ranging from 1.42 to 1.90 eV (at 300 K), (i.e., 0 ≤ x ≤ 0.43) were grown, which is the energy range needed for creating tandem III-V solar cells on silicon. The phosphorus content in the NWs is shown to be controlled by both growth temperature and input gas phase ratio. The distribution of P in the wires is uniform over the length of the wires and among the wires. This proves the feasibility of growing GaAsP nanowires by Aerotaxy and results indicate that it is a generic process that can be applied to the growth of other III-V semiconductor based ternary nanowires.
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| 14. |
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| 15. |
- Wallentin, Jesper, et al.
(författare)
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Probing the Wurtzite Conduction Band Structure Using State Filling in Highly Doped InP Nanowires.
- 2011
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 11:Online May 23, 2011, s. 2286-2290
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Tidskriftsartikel (refereegranskat)abstract
- We have grown InP nanowires doped with hydrogen sulfide, which exhibit sulfur concentrations of up to 1.4%. The highest doped nanowires show a pure wurtzite crystal structure, in contrast to bulk InP which has the zinc blende structure. The nanowires display photoluminescence which is strongly blue shifted compared with the band gap, well into the visible range. We find evidence of a second conduction band minimum at the gamma point about 0.23 eV above the band edge, in excellent agreement with recent theoretical predictions. Electrical measurements show high conductivity and breakdown currents of 10(7) A/cm(2).
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| 16. |
- Wallentin, Jesper, et al.
(författare)
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Semiconductor-Oxide Heterostructured Nanowires Using Postgrowth Oxidation.
- 2013
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 13:12, s. 5961-5966
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Tidskriftsartikel (refereegranskat)abstract
- Semiconductor-oxide heterointerfaces have several electron volts high-charge carrier potential barriers, which may enable devices utilizing quantum confinement at room temperature. While a single heterointerface is easily formed by oxide deposition on a crystalline semiconductor, as in MOS transistors, the amorphous structure of most oxides inhibits epitaxy of a second semiconductor layer. Here, we overcome this limitation by separating epitaxy from oxidation, using postgrowth oxidation of AlP segments to create axial and core-shell semiconductor-oxide heterostructured nanowires. Complete epitaxial AlP-InP nanowire structures were first grown in an oxygen-free environment. Subsequent exposure to air converted the AlP segments into amorphous aluminum oxide segments, leaving isolated InP segments in an oxide matrix. InP quantum dots formed on the nanowire sidewalls exhibit room temperature photoluminescence with small line widths (down to 15 meV) and high intensity. This optical performance, together with the control of heterostructure segment length, diameter, and position, opens up for optoelectrical applications at room temperature.
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| 17. |
- Wallentin, Jesper, et al.
(författare)
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Single GaInP nanowire p-i-n junctions near the direct to indirect bandgap crossover point
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:25
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Tidskriftsartikel (refereegranskat)abstract
- Axially defined GaInP single nanowire (NW) p-i-n junctions are demonstrated, with photocurrent response and yellow-green electroluminescence near the indirect bandgap crossover point at 2.18 eV (569 nm). We use DEZn and H2S as p- and n-type dopants, and find that they both affect the material composition and the crystal structure. The photovoltaic efficiency is comparable to single NW devices from binary III-V materials. These results demonstrate the potential of GaInP nanowires as a high-bandgap material for multijunction solar cells and light-emitting devices in the visible regime. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729929]
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| 18. |
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| 19. |
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| 20. |
- Yang, Fangfang, et al.
(författare)
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Zn-doping of GaAs nanowires grown by Aerotaxy
- 2015
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248. ; 414, s. 181-186
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Tidskriftsartikel (refereegranskat)abstract
- Nanowires were grown by means of a novel aerosol-based method called Aerotaxy. Here an aerosol of Au catalyst nanoparticles in N-2 is mixed with MOVPE precursors in a flow-through reactor at atmospheric pressure, whereby nanowires are produced continuously in high concentrations. We demonstrate the possibility of in situ doping of the NWs and the realization of well-controlled p-type GaAs nanowires using this Aerotaxy method. By controlling the cracking and concentration of the precursors, p-doped GaAs nanowires could be grown exhibiting a wide range of Zn doping levels. DEZn was used as the dopant source and the injected DEZn/TMGa ratio was varied from 0.1% to 3.4%. The morphology, the crystalline structure and the composition of the nanowires were studied using SEM, TEM and XEDS. The nanowires were grown straight without any significant tapering and this ideal morphology could be maintained up to an injected DEZn/TMGa ratio of 3.4%. The nanowires typically grew in the [111] direction with a pure zincblende structure, but by increasing the DEZn flow the number of twinning defects increased which we ascribe to Zn incorporation. Elemental analysis shows a high Zn content in the catalyst particle and also a gradient in the Zn content along the nanowire. The samples were analyzed optically using photoluminescence (PL). From the result we estimated the free hole concentration induced by Zn acceptors to be 1 x 10(20) cm(-3) for DEZn/TMGa ratio of 34%. To our knowledge this is the first report on in situ doping of GaAs nanowires grown by Aerotaxy. (C) 2014 Elsevier B.V. All rights reserved.
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| 21. |
- Zhang, Wei, et al.
(författare)
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Carrier Recombination Dynamics in Sulfur-Doped InP Nanowires.
- 2015
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 15:11, s. 7238-7244
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Tidskriftsartikel (refereegranskat)abstract
- Measuring lifetime of photogenerated charges in semiconductor nanowires (NW) is important for understanding light-induced processes in these materials and is relevant for their photovoltaic and photodetector applications. In this paper, we investigate the dynamics of photogenerated charge carriers in a series of as-grown InP NW with different levels of sulfur (S) doping. We observe that photoluminescence (PL) decay time as well as integrated PL intensity decreases with increasing S doping. We attribute these observations to hole trapping with the trap density increased due to S-doping level followed by nonradiative recombination of trapped charges. This assignment is proven by observation of the trap saturation in three independent experiments: via excitation power and repetition rate PL lifetime dependencies and by PL pump-probe experiment.
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| 22. |
- Zhang, Wei, et al.
(författare)
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Recombination dynamics in aerotaxy-grown Zn-doped GaAs nanowires
- 2016
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 27:45
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we have investigated the dynamics of photo-generated charge carriers in a series of aerotaxy-grown GaAs nanowires (NWs) with different levels of Zn doping. Time-resolved photo-induced luminescence and transient absorption have been employed to investigate radiative (band edge transition) and non-radiative charge recombination processes, respectively. We find that the photo-luminescence (PL) lifetime of intrinsic GaAs NWs is significantly increased after growing an AlGaAs shell over them, indicating that an AlGaAs shell can effectively passivate the surface of aerotaxy-grown GaAs NWs. We observe that PL decay time as well as PL intensity decrease with increasing Zn doping, which can be attributed to thermally activated electron trapping with the trap density increased due to the Zn doping level.
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