| 1. |
- Zeng, Xulu, et al.
(author)
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Electrical and optical evaluation of n-type doping in InxGa(1-x)P nanowires
- 2018
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In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 29:25
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Journal article (peer-reviewed)abstract
- To harvest the benefits of III-V nanowires in optoelectronic devices, the development of ternary materials with controlled doping is needed. In this work, we performed a systematic study of n-type dopant incorporation in dense InxGa(1-x)P nanowire arrays using tetraethyl tin (TESn) and hydrogen sulfide (H2S) as dopant precursors. The morphology, crystal structure and material composition of the nanowires were characterized by use of scanning electron microscopy, transmission electron microscopy and energy dispersive x-ray analysis. To investigate the electrical properties, the nanowires were broken off from the substrate and mechanically transferred to thermally oxidized silicon substrates, after which electron beam lithography and metal evaporation were used to define electrical contacts to selected nanowires. Electrical characterization, including four-probe resistivity and Hall effect, as well as back-gated field effect measurements, is combined with photoluminescence spectroscopy to achieve a comprehensive evaluation of the carrier concentration in the doped nanowires. We measure a carrier concentration of ∼1 ×1016 cm-3 in nominally intrinsic nanowires, and the maximum doping level achieved by use of TESn and H2S as dopant precursors using our parameters is measured to be ∼2 ×1018 cm-3, and ∼1 ×1019 cm-3, respectively (by Hall effect measurements). Hence, both TESn and H2S are suitable precursors for a wide range of n-doping levels in InxGa(1-x)P nanowires needed for optoelectronic devices, grown via the vapor-liquid-solid mode.
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| 2. |
- Zeng, Xulu, et al.
(author)
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InP/GaInP nanowire tunnel diodes
- 2018
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In: Nano Research. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 11:5, s. 2523-2531
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Journal article (peer-reviewed)abstract
- Semiconductor nanowire (NW) solar cells with a single p-n junction have exhibited efficiency comparable to that of their planar counterparts with a substantial reduction in material consumption. Tandem geometry is a path toward the fabrication of devices with even higher efficiencies, for which a key step is the fabrication of tunnel (Esaki) diodes within NWs with the correct diameter, pitch, and material combination for maximized efficiency. InP/GaInP and GaInP/InP NW tunnel diodes with band gap combinations corresponding to high-efficiency solar energy harvesting were fabricated and their electrical characteristics and material properties were compared. Four different configurations, with respect to material composition and doping, were investigated. The NW arrays were grown with metal–organic vapor-phase epitaxy from Au particles by use of nano-imprint lithography, metal evaporation and lift-off. Electrical measurements showed that the NWs behave as tunnel diodes in both InP (bottom)/GaInP (top) and GaInP (bottom)/InP (top) configurations, exhibiting a maximum peak current density of 25 A/cm2, and maximum peak to valley current ratio of 2.5 at room temperature. The realization of NW tunnel diodes in both InP/GaInP and GaInP/InP configurations represent an opportunity for the use of NW tandem solar cells, whose efficiency is independent of the growth order of the different materials, increasing the flexibility regarding dopant incorporation polarity. [Figure not available: see fulltext.]
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| 3. |
- Berg, Alexander, et al.
(author)
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Growth of wurtzite AlxGa1-xP nanowire shells and characterization by Raman spectroscopy
- 2017
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In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 28:3
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Journal article (peer-reviewed)abstract
- The phonon energies of AlGaP in wurtzite crystal structure are generally not known, as opposed to their zincblende counterparts, because AlGaP crystallizes in zincblende phase in bulk and thin films structures. However, in nanowires AlGaP can be grown in wurtzite crystal structure. In this work we have grown wurtzite GaP/AlGaP/GaP core-shell nanowires by use of MOVPE. After developing suitable growth conditions, the Al composition was determined by STEM-EDX measurements and the wurtzite AlGaP phonon energies by Raman spectroscopy. Raman measurements show a peak shift with increasing Al composition in the AlGaP shell. We find that the phonon energies for wurtzite AlGaP are slightly lower than for zincblende AlGaP. Our results can be used to determine the Al composition in wurtzite AlGaP by Raman scattering.
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| 4. |
- Dagyte, Vilgaile, et al.
(author)
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Time-resolved photoluminescence characterization of GaAs nanowire arrays on native substrate
- 2017
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In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 28:50
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Journal article (peer-reviewed)abstract
- Time-resolved photoluminescence (TRPL) measurements of nanowires (NWs) are often carried out on broken-off NWs in order to avoid the ensemble effects as well as substrate contribution. However, the development of NW-array solar cells could benefit from non-destructive optical characterization to allow faster feedback and further device processing. With this work, we show that different NW array and substrate spectral behaviors with delay time and excitation power can be used to determine which part of the sample dominates the detected spectrum. Here, we evaluate TRPL characterization of dense periodic as-grown GaAs NW arrays on a p-type GaAs substrate, including a sample with uncapped GaAs NWs and several samples passivated with AlGaAs radial shell of varied composition and thickness. We observe a strong spectral overlap of substrate and NW signals and find that the NWs can absorb part of the substrate luminescence signal, thus resulting in a modified substrate signal. The level of absorption depends on the NW-array geometry, making a deconvolution of the NW signal very difficult. By studying TRPL of substrate-only and as-grown NWs at 770 and 400 nm excitation wavelengths, we find a difference in spectral behavior with delay time and excitation power that can be used to assess whether the signal is dominated by the NWs. We find that the NW signal dominates with 400 nm excitation wavelength, where we observe two different types of excitation power dependence for the NWs capped with high and low Al composition shells. Finally, from the excitation power dependence of the peak TRPL signal, we extract an estimate of background carrier concentration in the NWs.
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| 5. |
- Jain, Vishal, et al.
(author)
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Bias-dependent spectral tuning in InP nanowire-based photodetectors
- 2017
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In: Nanotechnology. - Bristol : IOP Publishing. - 0957-4484 .- 1361-6528. ; 28:11
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Journal article (peer-reviewed)abstract
- Nanowire array ensembles contacted in a vertical geometry are extensively studied and considered strong candidates for next generations of industrial scale optoelectronics. Key challenges in this development deal with optimization of the doping profile of the nanowires and the interface between nanowires and transparent top contact. Here we report on photodetection characteristics associated with doping profile variations in InP nanowire array photodetectors. Bias-dependent tuning of the spectral shape of the responsivity is observed which is attributed to a Schottky-like contact at the nanowire-ITO interface. Angular dependent responsivity measurements, compared with simulated absorption spectra, support this conclusion. Furthermore, electrical simulations unravel the role of possible self-gating effects in the nanowires induced by the ITO/SiO x wrap-gate geometry. Finally, we discuss possible reasons for the observed low saturation current at large forward biases.
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| 6. |
- Jain, Vishal, et al.
(author)
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InP/InAsP Nanowire-Based Spatially Separate Absorption and Multiplication Avalanche Photodetectors
- 2017
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In: ACS Photonics. - Washington : American Chemical Society (ACS). - 2330-4022. ; 4:11, s. 2693-2698
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Journal article (peer-reviewed)abstract
- Avalanche photodetectors (APDs) are key components in optical communication systems due to their increased photocurrent gain and short response time as compared to conventional photodetectors. A detector design where the multiplication region is implemented in a large band gap material is desired to avoid detrimental Zener tunneling leakage currents, a concern otherwise in smaller band gap materials required for absorption at 1.3/1.55 μm. Self-assembled III-V semiconductor nanowires offer key advantages such as enhanced absorption due to optical resonance effects, strain-relaxed heterostructures, and compatibility with mainstream silicon technology. Here, we present electrical and optical characteristics of single InP and InP/InAsP nanowire APD structures. Temperature-dependent breakdown characteristics of p+-n-n+ InP nanowire devices were investigated first. A clear trap-induced shift in breakdown voltage was inferred from I-V measurements. An improved contact formation to the p+-InP segment was observed upon annealing, and its effect on breakdown characteristics was investigated. The band gap in the absorption region was subsequently varied from pure InP to InAsP to realize spatially separate absorption and multiplication APDs in heterostructure nanowires. In contrast to the homojunction APDs, no trap-induced shifts were observed for the heterostructure APDs. A gain of 12 was demonstrated for selective optical excitation of the InAsP segment. Additional electron-beam-induced current measurements were carried out to investigate the effect of local excitation along the nanowire on the I-V characteristics. Simulated band profiles and electric field distributions support our interpretation of the experiments. Our results provide important insight for optimization of avalanche photodetector devices based on III-V nanowires.
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| 7. |
- Karimi, Mohammad, 1988-, et al.
(author)
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Infrared Photodetectors Based on Nanowire Arrays – Towards Far Infrared Region
- 2017
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Conference paper (peer-reviewed)abstract
- Nanowire semiconductors are promising candidates for optoelectronic applications such as solar cells, photodetectors and lasers due to their quasi-1D geometry and large surface to volume ratio. The functional wavelength range of NW-based detectors is typically limited to the visible/near-infrared region. In this work, we present electrical and optical properties of novel IR photodetectors based on large square millimeter ensembles (>1million) of vertically processed semiconductor heterostructure nanowires (NWs) grown on InP substrates which operates in longer wavelengths. InP NWs comprising single or multiple (20) InAs/InAsP QDics axially embedded in an n-i-n geometry, have been grown on InP substrates using MOVPE. The NWs are contacted in vertical direction by ALD deposition of 50 nm SiO2 as an insulating layer followed by sputtering of ITO and evaporation of Ti and Au as top contact layer. In order to extend the sensitivity range to the mid-wavelength and long-wavelength regions, the intersubband transition within conduction band of InAsP QDisc is suggested. We present first experimental indications of intersubband photocurrent in NW geometry and discuss important design parameters for realization of intersubband detectors. Key advantages with the proposed design include large degree of freedom in choice of materials compositions, possible enhanced optical resonance effects due to periodically ordered NW arrays and the compatibility with silicon substrates. We believe that our novel detector design offers the route towards monolithic integration of compact and sensitive III-V NW long wavelength detectors with Si technology.
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| 8. |
- Lindgren, David, et al.
(author)
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Study of carrier concentration in single InP nanowires by luminescence and Hall measurements
- 2015
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In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 26:4
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Journal article (peer-reviewed)abstract
- The free electron carrier concentrations in single InP core-shell nanowires are determined by micro-photoluminescence, cathodoluminescence (CL) and Hall effect measurements. The results from luminescence measurements were obtained by solving the Fermi-Dirac integral, as well as by analyzing the peak full width at half maximum (FWHM). Furthermore, the platform used for Hall effect measurements, combined with spot mode CL spectroscopy, is used to determine the carrier concentrations at specific positions along single nanowires. The results obtained via luminescence measurements provide an accurate and rapid feedback technique for the epitaxial development of doping incorporation in nanowires. The technique has been employed on several series of samples in which growth parameters, such as V/III-ratio, temperature and dopant flows, were investigated in an optimization procedure. The correlation between the Hall effect and luminescence measurements for extracting the carrier concentration of different samples were in excellent agreement.
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| 9. |
- Mante, Pierre Adrien, et al.
(author)
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Confinement effects on Brillouin scattering in semiconductor nanowire photonic crystal
- 2016
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In: Physical Review B. - 1098-0121. ; 94:2
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Journal article (peer-reviewed)abstract
- Scattering of photons by phonons, or Brillouin scattering, enables manipulation and control of light and has led to revolutionary applications, from slow light to saser and cooling of micromechanical resonators. Recently, enhanced light and sound interaction has been demonstrated in waveguides. However, the design of the waveguide geometry tunes and alters the phonon and photon dispersion simultaneously. Here we investigate, through femtosecond pump-probe spectroscopy and theoretical modeling, the light and sound interaction in a bottom-up fabricated vertical nanowire photonic crystal. In such a system, the phonon dispersion can be tuned by varying the geometry of the constituent nanowires. In contrast, the placement of the nanowires in the photonic crystal can be used for tuning optical array modes, without altering the phonon dispersion. We demonstrate the forward and backward scattering, by acoustic phonons in the nanowires, of (1) such optical array modes and (2) guided modes of the constituent nanowires. Furthermore, our results reveal an enhanced interaction of array modes with phonons that we attribute to the specific scattering mechanism. Our results enable the design of a photonic crystal with separately tailored photon and phonon dispersion for Brillouin scattering. We anticipate these advances to be a starting point for enhanced control of light at the nanoscale.
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| 10. |
- Mattsson, Karin, et al.
(author)
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Translocation of 40 nm diameter nanowires through the intestinal epithelium of Daphnia magna
- 2016
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In: Nanotoxicology. - : Informa UK Limited. - 1743-5390 .- 1743-5404. ; 10:8, s. 1160-1167
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Journal article (peer-reviewed)abstract
- Nanowires (NWs) have unique electrical and optical properties of value for many applications including lighting, sensing, and energy harnessing. Consumer products containing NWs increase the risk of NWs being released in the environment, especially into aquatic ecosystems through sewage systems. Daphnia magna is a common, cosmopolitan freshwater organism sensitive to toxicity tests and represents a likely entry point for nanoparticles into food webs of aquatic ecosystems. Here we have evaluated the effect of NW diameter on the gut penetrance of NWs in Daphnia magna. The animals were exposed to NWs of two diameters (40 and 80 nm) and similar length (3.6 and 3.8 μm, respectively) suspended in water. In order to locate the NWs in Daphnia, the NWs were designed to comprise one inherently fluorescent segment of gallium indium phosphide (GaInP) flanked by a gallium phosphide (GaP) segment. Daphnia mortality was assessed directly after 24 h of exposure and 7 days after exposure. Translocation of NWs across the intestinal epithelium was investigated using confocal fluorescence microscopy directly after 24 h of exposure and was observed in 89% of Daphnia exposed to 40 nm NWs and in 11% of Daphnia exposed to 80 nm NWs. A high degree of fragmentation was observed for NWs of both diameters after ingestion by the Daphnia, although 40 nm NWs were fragmented to a greater extent, which could possibly facilitate translocation across the intestinal epithelium. Our results show that the feeding behavior of animals may enhance the ability of NWs to penetrate biological barriers and that penetrance is governed by the NW diameter.
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