Nanoscale 3D Strain Mapping and Structural Features of GaAs/In(Al,Ga)As Core-Shell Nanowires

Elastic accommodation of the high lattice mismatch in GaAs/In(Al,Ga)As core-shell nanowires holds great potential for applications in high-frequency electronics and optoelectronics. The unique core-shell geometry offers several advantages over planar systems. The reduced dimensionality of the core enables strain relaxation along the lateral dimension and strain partitioning, ultimately reducing the overall strain energy and expanding the limits of coherency [1]. Consequently, the GaAs core can undergo extreme elastic stretching, particularly along the nanowire axis, without the onset of plastic relaxation.

Growing thick Inx(Al,Ga)1-xAs shells with high In-contents on narrow [111]-oriented GaAs cores leads to extreme elastic dilatation of the core that promotes a 40% bandgap reduction [2] as well as a 30-50% boost in electron mobility [3]. In order to elucidate the intricate 3D strain fields of such nanowires, we have employed transmission and scanning-transmission electron microscopy ((S)TEM) methods to study a series of nanowires featuring narrow cores (25 nm diameter) and thick shells (80 nm thickness) with composition x ranging from 0.20 up to 1. To enable the nanoscale investigation of all strain components, we developed elaborate sample preparation methods to facilitate observation along three zone axes, i.e. [111], <1-10> and <11-2>, as shown in Fig.1. For (S)TEM and high-resolution TEM (HRTEM) observations, a 200 kV JEOL JEM F200 CFEG microscope was used. High resolution STEM (HRSTEM) was performed in a 300 kV probe-corrected Thermo Fisher Scientific Titan Themis 60/300 microscope. In an integrated approach, experimental strain fields were compared with finite element (FE) calculations, performed using thermal expansivity to model the lattice mismatch, and with energetic calculations by molecular dynamics (MD) using the Tersoff interatomic potential [4].

(S)TEM observations revealed that for indium contents up to x≈50%, the shells were coherent and dislocation-free (Fig.1), exhibiting only occasional (111) ortho-twins and stacking faults. These are typically associated with similar faults originating in the core and are unrelated to strain relaxation. The experimental strain fields were in good agreement with the FE and MD simulations. In the core, all strain components were tensile. The axial (ezz) strain was uniform, whereas the radial (err) and azimuthal (eθθ) strains were position-dependent and saturated at the same value, in the region where the shell was completely relaxed. Upon reaching a critical composition, threading dislocations emerged in the shell, spreading radially from the core towards the {1-10} free surfaces of the shell. Their Burgers vectors were analysed using weak-beam dark-field TEM and were found to lie on the (111) plane. In the defected nanowires, Moiré fringes observed in the core region normal to [111], revealed axial relaxation, attributed to the introduction of misfit dislocations. By measuring the spacing of the Moiré fringes, the residual elastic strain in the cores was calculated.

Figure 1. (a) Bright field STEM image showing an unrelaxed, coherent GaAs/In0.56Al0.44As core-shell NW observed along <11-2>. (b) Z-contrast STEM image showing a NW cross-section, observed along [111] (top) and the corresponding strain map from the GaAs core (bottom). The nominal In-composition in b is again ≈50%.

Acknowledgements
This work was supported by the program for the promotion of the exchange and scientific cooperation between Greece and Germany (IKYDA 2020), project title: Strain tuning of iii-v semiconductor nanowires (TUNE).

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