| 1. |
- Chayanun, Lert, et al.
(författare)
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Spectrally resolved x-ray beam induced current in a single InGaP nanowire
- 2018
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 29:45
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate x-ray absorption fine structure spectroscopy (XAFS) detected by x-ray beam induced current (XBIC) in single n + -i-n + doped nanowire devices. Spatial scans with the 65 nm diameter beam show a peak of the XBIC signal in the middle segment of the nanowire. The XBIC and the x-ray fluorescence signals were detected simultaneously as a function of the excitation energy near the Ga K absorption edge at 10.37 keV. The spectra show similar oscillations around the edge, which shows that the XBIC is limited by the primary absorption. Our results reveal the feasibility of the XBIC detection mode for the XAFS investigation in nanostructured devices.
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| 2. |
- Dagyte, Vilgaile
(författare)
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Growth and optical properties of III-V semiconductor nanowires: : Studies relevant for solar cells
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with epitaxial growth and optical properties of semiconductor nanowires with implications and insights about materials for solar cells. The chapters leading up to the papers give a broader background and an introduction to the field and include nanowires and their synthesis, semiconductor properties, solar cell operational principles, light interaction with a nanowire array, and optical characterization of such arrays.The nanowires were grown from gold seed particles using metal organic vapor phase epitaxy. Growth of ternary GaInP nanowires has been developed with a triethylgallium precursor that has not been commonly used before for this material structure in nanowires. We have achieved high yield and wide composition range nanowires with high control, which will be a crucial element for development of tandem solar cells where a high band-gap GaInP nanowire array could be the top cell.In terms of optical properties, lifetimes and carrier dynamics are important parameters for optoelectronic devices, including solar cells. We have investigated surface passivation of nanowires by capping GaAs nanowires with in situ grown shells, at the same time evaluating the possibility of measuring time-resolved photoluminescence signal of as-grown nanowires, even when the substrate is made of the same material. We have identified that depending on doping levels in the substrate and the nanowires, excitation wavelength can be chosen to separate nanowire signal from the substrate signal. Moreover, we have preliminary proposed a simple way to extract doping, which needs to be tested more extensively in the future. Such measurements of as-grown arrays could provide a fast and completely non-destructive characterization method for solar cell materials and allow further processing of the devices.Further, we have investigated reflectance and transmittance of flexible nanowire arrays embedded in a transparent polymer. Such flexible membranes could be interesting as flexible solar cells on their own, or could be incorporated on top of a lower band-gap material, for example silicon, to create a tandem solar cell. We have identified two potential issues with such structures. First of all, gold can absorb a significant fraction of the incoming light. The gold particles can be etched away, which recovers transmittance for the long wavelengths. However, a resonant reflectance peak is then observed. Through our work, we have identified that in-plane array modes arise in nanowire arrays embedded in a polymer that lead to resonant reflectance or absorptance in weakly absorbing materials. Such effect would be detrimental for transmitting long wavelengths to the bottom cell. Thus, we have investigated how these resonances depend on geometry in order to give guidelines for controlling this effect.
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| 3. |
- Dagyte, Vilgaile, et al.
(författare)
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Growth kinetics of GaxIn(1−x)P nanowires using triethylgallium as Ga precursor
- 2018
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 29:39
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Tidskriftsartikel (refereegranskat)abstract
- GaxIn(1−x)P nanowire arrays are promising for various optoelectronic applications with a tunable band-gap over a wide range. In particular, they are well suited as the top cell in tandem junction solar cell devices. So far, most GaxIn(1−x)P nanowires have been synthesized by the use of trimethylgallium (TMGa). However, particle assisted nanowire growth in metal organic vapor phase epitaxy is typically carried out at relatively low temperatures, where TMGa is not fully pyrolysed. In this work, we developed the growth of GaxIn(1−x)P nanowires using triethylgallium (TEGa) as the Ga precursor, which reduced Ga precursor consumption by about five times compared to TMGa due to the lower homogeneous pyrolysis temperature of TEGa. The versatility of TEGa is shown by synthesis of high yield GaxIn(1−x)P nanowire arrays, with a material composition tunable by the group III input flows, as verified by x-ray diffraction measurements and photoluminescence characterization. The growth dynamics of GaxIn(1−x)P nanowires was assessed by varying the input growth precursor molar fractions and growth temperature, using hydrogen-chloride as in situ etchant. We observed a complex interplay between the precursors. First, trimethylindium (TMIn) inhibits Ga incorporation into the nanowires, resulting in higher In composition in the grown nanowires than in the vapor. Second, the growth rate increases with temperature, indicating a kinetically limited growth, which from nanowire effective binary volume growth rates of InP and GaP can be attributed to the synthesis of GaP in GaxIn(1−x)P. We observed that phosphine has a strong effect on the nanowire growth rate with behavior expected for a unimolecular Langmuir–Hinshelwood mechanism of pyrolysis on a catalytic surface. However, growth rates increase strongly with both TEGa and TMIn precursors as well, indicating the complexity of vapor–liquid–solid growth for ternary materials. One precursor can affect the decomposition of another, and each precursor can affect the wetting properties and catalytic activity of the metal particle.
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| 4. |
- Troian, Andrea, et al.
(författare)
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Nanobeam X-ray Fluorescence Dopant Mapping Reveals Dynamics of in Situ Zn-Doping in Nanowires
- 2018
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; , s. 6461-6468
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Tidskriftsartikel (refereegranskat)abstract
- The properties of semiconductors can be controlled using doping, making it essential for electronic and optoelectronic devices. However, with shrinking device sizes it becomes increasingly difficult to quantify doping with sufficient sensitivity and spatial resolution. Here, we demonstrate how X-ray fluorescence mapping with a nanofocused beam, nano-XRF, can quantify Zn doping within in situ doped III-V nanowires, by using large area detectors and high-efficiency focusing optics. The spatial resolution is defined by the focus size to 50 nm. The detection limit of 7 ppm (2.8 × 1017 cm-3), corresponding to about 150 Zn atoms in the probed volume, is bound by a background signal. In solar cell InP nanowires with a p-i-n doping profile, we use nano-XRF to observe an unintentional Zn doping of 5 × 1017 cm-3 in the middle segment. We investigated the dynamics of in situ Zn doping in a dedicated multisegment nanowire, revealing significantly sharper gradients after turning the Zn source off than after turning the source on. Nano-XRF could be used for quantitative mapping of a wide range of dopants in many types of nanostructures.
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| 5. |
- Zeng, Xulu, et al.
(författare)
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Electrical and optical evaluation of n-type doping in InxGa(1-x)P nanowires
- 2018
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 29:25
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Tidskriftsartikel (refereegranskat)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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