| 1. |
- Holmér, Jonatan, 1990, et al.
(författare)
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An STM – SEM setup for characterizing photon and electron induced effects in single photovoltaic nanowires
- 2018
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Ingår i: Nano Energy. - : Elsevier BV. - 2211-2855. ; 53, s. 175-181
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Tidskriftsartikel (refereegranskat)abstract
- Vertical arrays of semiconductor nanowires show great potential for material-efficient and high-performance solar cells. The characterization and correlation between material structure and properties of the individual nanowires are crucial for the continued performance improvement of such devices. In this work, we developed a method with a scanning tunneling microscope (STM) probe inside a scanning electron microscope (SEM) to enable the studies of single photovoltaic nanowires. The STM probe is used to contact individual nanowires in ensembles. We combine the STM-SEM with an in situ light emitting diode (LED) illumination source to study both the electrical and photovoltaic properties of vertical GaAs nanowires with radial p-i-n junctions. We also illustrate that the local charge separation ability within the nanowires can be studied by electron beam induced current (EBIC) measurements. The in situ SEM setup allows the correlation between properties and nanowire structure. The data show that the quality of the electrical contact to the semiconductor nanowire is crucial to be able to investigate the inherent properties of the nanowires. We have established a procedure to make high-quality ohmic contacts to the nanowires with the STM probe. We also show that the effect of mechanical strain on the electrical properties can be investigated by the STM-SEM setup.
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| 2. |
- Holmér, Jonatan, 1990, et al.
(författare)
-
Enhancing the NIR Photocurrent in Single GaAs Nanowires with Radial p-i-n Junctions by Uniaxial Strain
- 2021
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 2:21, s. 9038-9043
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Tidskriftsartikel (refereegranskat)abstract
- III-V compound nanowires have electrical and optical properties suitable for a wide range of applications, including photovoltaics and photodetectors. Furthermore, their elastic nature allows the use of strain engineering to enhance their performance. Here we have investigated the effect of mechanical strain on the photocurrent and the electrical properties of single GaAs nanowires with radial p-i-n junctions, using a nanoprobing setup. A uniaxial tensile strain of 3% resulted in an increase in photocurrent by more than a factor of 4 during NIR illumination. This effect is attributed to a decrease of 0.2 eV in nanowire bandgap energy, revealed by analysis of the current-voltage characteristics as a function of strain. This analysis also shows how other properties are affected by the strain, including the nanowire resistance. Furthermore, electron-beam-induced current maps show that the charge collection efficiency within the nanowire is unaffected by strain measured up to 0.9%.
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| 3. |
- Holmér, Jonatan, 1990
(författare)
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In situ electron microscopy of strain-induced effects on electrical and photovoltaic properties of GaAs nanowires - site specific and quantitative studies
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semiconductor nanowires have different physical properties than their bulk counterparts due to their small physical dimensions and high surface-to-volume ratio. The nanowire geometry entails enhanced optical absorption, widened possibilities to grow material heterostructures and ability to withstand high levels of strain. The strain may alter the physical properties further and can be used to tune them. Because of their unique properties, solar cells based on III-V compound nanowires hold promise of becoming both more efficient and less expensive than conventional solar cells. However, nanowire solar cell efficiencies are still far below their theoretical maximum and further optimization is needed. This demands versatile characterization techniques where the microstructure of single nanowires can be related to their properties, and strain-induced effects may be investigated. In this thesis, a nanoprobing in situ electron microscopy technique has been developed to study the electrical and photovoltaic properties of single GaAs nanowires. Furthermore, the quantitative effects of uniaxial strain on these properties were investigated. The results show that the nanowires function as solar cells, with a highest measured single nanowire efficiency of 10.8% during white light emitting diode illumination. Optimization of the electrical contact was found to be crucial for the photovoltaic performance of the wires. Furthermore, tensile strain was shown to increase the photocurrent in the near-infrared spectrum due to a reduction in bandgap energy. These findings provide insights for further optimization of nanowire solar cells.
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| 4. |
- Zeng, Lunjie, 1983, et al.
(författare)
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Tuning Hole Mobility of Individual p-Doped GaAs Nanowires by Uniaxial Tensile Stress
- 2021
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 21:9, s. 3894-3900
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Tidskriftsartikel (refereegranskat)abstract
- Strain engineering provides an effective way of tailoring the electronic and optoelectronic properties of semiconductor nanomaterials and nanodevices, giving rise to novel functionalities. Here, we present direct experimental evidence of strain-induced modifications of hole mobility in individual gallium arsenide (GaAs) nanowires, using in situ transmission electron microscopy (TEM). The conductivity of the nanowires varied with applied uniaxial tensile stress, showing an initial decrease of similar to 5-20% up to a stress of 1-2 GPa, subsequently increasing up to the elastic limit of the nanowires. This is attributed to a hole mobility variation due to changes in the valence band structure caused by stress and strain. The corresponding lattice strain in the nanowires was quantified by in situ four dimensional scanning TEM and showed a complex spatial distribution at all stress levels. Meanwhile, a significant red shift of the band gap induced by the stress and strain was unveiled by monochromated electron energy loss spectroscopy.
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