By using in situ electron microscopy (EM) we show that strain engineering is an effective way of tuning the electrical and opto-electronic properties of semiconducting nanostructures. Combining imaging, diffraction, spectroscopy and in situ EM methods enables correlative measurements of crystal and electronic structure, mechanical behaviour, charge transport and photovoltaic properties on individual nanostructures.

   A direct correlation between mechanical and charge transport properties was determined for InAs nanowires by using in situ TEM methods [1]. As uniaxial tensile stress was applied, strain mapping was performed by using 4D scanning TEM (4D STEM), while electrical measurements were carried out simultaneously on individual nanowires. An enhanced piezoresistive response of the nanowires, compared to bulk InAs, was observed with increasing strain.

   In individual p-GaAs nanowires, evidence for hole mobility modification by uniaxial strain was found, using the same in situ TEM approach [2]. The conductance of the nanowires shows an initial decrease of ~5-20% with applied stress up to 1~ 2 GPa, and a subsequent increase at higher stress. This anomalous change in conductance is attributed to a hole mobility variation due to changes in the valence band structure caused by strain. The lattice strain in the nanowires was quantified by in situ 4D STEM, showing a complex spatial distribution at all stress levels. A red shift of the band gap caused by the strain was revealed by in situ monochromated electron energy loss spectroscopy (EELS).

   For individual GaAs nanowires with build-in radial p-i-n junctions, the photovoltaic properties were investigated using an in situ scanning tunnelling microscope – scanning electron microscope (STM-SEM) setup [3,4]. This STM-SEM setup also enables the mechanical manipulation of the nanowires. Applying an uniaxial tensile strain of 3% resulted in an increase of photocurrent by more than a factor of 4 during near-infrared (NIR) illumination. This increase is attributed to a decrease of 0.2 eV in bandgap energy, thus reflecting the effect of tensile strain on light absorption.

References

[1] L. Zeng, C. Gammer, B. Ozdol, T. Nordqvist, J. Nygård, P. Krogstrup, A. M. Minor, W. Jäger, and E. Olsson, Nano Lett. 2018, 18, 4949.

[2] L. Zeng, J. Holmér, R. Dhall, C. Gammer, A. M. Minor, and E. Olsson, Nano Lett. 2021, 21, 3894.

[3] J. Holmér, L. Zeng, T. Kanne, P. Krogstrup, J. Nygård, L. de Knoop, and E. Olsson, Nano Energy 2018, 53, 175.

[4] J. Holmér, L. Zeng, T. Kanne, P. Krogstrup, J. Nygård, and E. Olsson, Nano Letters 2021, 21, 9038.