Precise strain mapping of nano-twinned axial ZnTe/CdTe hetero-nanowires by scanning nanobeam electron diffraction

Abstract

The occurrence of strain is inevitable for the growth of lattice mismatched heterostructures. It affects greatly the mechanical, electrical and optical properties of nano-objects. It is also the case for nanowires which are characterized by a high surface to volume ratio. Thus, the knowledge of the strain distribution in nano-objects is critically important for their implementation into devices. This paper presents an experimental data for II-VI semiconductor system. Scanning nanobeam electron diffraction strain mapping technique for hetero-nanowires characterized by a large lattice mismatch (>6% in the case of CdTe/ZnTe) and containing segments with nano-twins has been described. The spatial resolution of about 2 nm is 10 times better than obtained in synchrotron nanobeam systems. The proposed approach allows us to overcome the difficulties related to nanowire thickness variations during the acquisition of the nano-beam electron diffraction data. In addition, the choice of optimal parameters used for the acquisition of nano-beam diffraction data for strain mapping has been discussed. The knowledge of the strain distribution enables, in our particular case, the improvement of the growth model of extremely strained axial nanowires synthetized by vapor-liquid solid growth mechanism. However, our method can be applied for the strain mapping in nanowire heterostructures grown by any other method.

Reconstruction of three-dimensional strain field in an asymmetrical curved core–shell hetero-nanowire

“…NBED mapping of (Cd,Zn)Te/ZnTe hetero-NW was performed in STEM mode using the Gatan Ultrascan 1000 camera with binning 4 to increase the dynamic range and to reduce a blooming effect. DPs were acquired with a convergence semi-angle of 1 mrad and a probe size of 1.3 nm [37]. The integration time for NBED acquisition was chosen to be 100 ms per frame.…”

“…The analysis of NBED data was performed by a scripting procedure using DigitalMicrograph software. To obtain precise strain maps with subpixel accuracy the circular Hough transform was implemented in the script [37,45]. The pattern matching technique for correlation of experimental DPs and simulated data was used to obtain local orientation maps of bent NW.…”

“…The model consists of ZnTe NW-core surrounded by an asymmetric (Cd,Zn)Te shell. Crystallographic orientation and anisotropy of Young modulus [37] were also taken into account in the model. The simulated NW has the shape of a hexagonal frustum with the height of 235 nm and the length of the bottom face edge of 30 nm and that of the top face of 25 nm.…”

“…Among different experimental techniques, only x-ray synchrotron and transmission electron microscopy (TEM) based methods provide an opportunity for strain mapping with a sufficiently high spatial resolution. A previous report has shown that using scanning nanobeam electron diffraction (NBED) in TEM allows us to overcome the difficulties related to NW thickness variations and twinning of a crystal structure [37] and leads to the realization of precise 2D strain maps of axial NW heterostructures. Our approach does not employ the precession of electron beam, but it ensures the conservation of dynamical effects on diffraction pattern (DP) during the acquisition of experimental data.…”