Semiconductor nanowires (NWs) have promising properties for optoelectronic devices such as solar cells and light emitting diodes. For the development of such NW-based devices, the correlation between structural features, composition, and optoelectronic properties of the NWs must be well understood. This task can be challenging as the growth can induce large NW-to-NW variations. As a statistical meaningful sampling at the required spatial resolution by multiple techniques would be very time consuming, correlated studies where the exact same NWs are characterized optically and structurally by different techniques are an alternative [1]. In this study, the same single self-catalyzed GaAs/AlGaAs core-shell NW is studied using micro-photoluminescence (µ-PL) and transmission electron microscopy (TEM). After conventional TEM, cross-sections of two regions of the same NW were made using focused ion beam (FIB) to obtain a 3D impression of the NW. To study variations in the shell, a cross-section was made perpendicular to the growth direction from the lower half of the NW, and to study variations in the tip a section was made perpendicular to the -1-12-direction from the top of the NW (Fig. 1(a)). The cross-sections were studied using high-angle annular dark-field scanning TEM (HAADF STEM) and quantitative electron-dispersive x-ray spectroscopy (EDS) to reveal compositional variations in the different directions of the NW.

The NWs in the growth batch are mostly defect free zinc blende (ZB), with stacking faults and a wurtzite (WZ) region towards the tip area (Fig. 1(b-c)). µ-PL of 17 studied NWs shows a signal at the ZB GaAs free exciton energy at 12 K. About half of the NWs also have an additional PL signal at higher energy, as can be seen in Fig. 1(e). Compositional variations in the AlGaAs shell of the NWs could possibly explain this high-energy PL emission in the 1.6-1.8 eV energy range [2]. In both cross-sections (Fig. 1(d) and Fig. 2) this type of structure, with narrow Al-rich and Al-deficient bands parallel to the facets of the NW, is visible. Quantitative EDS maps based on the zeta-method [3] (Fig. 3(a)) shows that the Al concentration in the Al deficient bands for the observed widths is too high to explain the sharp PL emission in the range 1.6-1.8 eV. In addition to the shell, the tip region also depicts compositional variations. These features were only visible in the cross-section normal to the -1-12 – direction (Fig. 1(d) and 2(b)) and not apparent by conventional TEM imaging (Fig. 1(c)). Quantitative EDS (Fig. 3(b)) shows that the Al concentration is varying within the tip. Correlated studies on the very same NW including µ-PL, conventional TEM, FIB preparation in different directions and quantitative EDS are required to visualize and explain self-induced compositional variations and peculiar optical characteristics within these GaAs/AlGaAs core-shell NWs.

[1] J. Todorovic et al., Nanotechnology 22.32 (2011), 325707.         

[2] J. S. Nilsen et al., Journal of Physics: Conference Series 644 (2015), 012007

[3] M. Watanabe and D. B. Williams, Journal of Microscopy 221 (2006), 89-109.

Acknowledgements: The Research Council of Norway for the support to the NorFab (197411) and the NORTEM (197405) facilities, as well funding from the NANO2021 (239206) and FRINATEK (214235) programs.

Figures:

Figure 1: (a) Schematic of the NW showing the three directions characterized. (b) Diffraction pattern of the NW, indicating from which the dark-field (DF) images in (c) are acquired. (c) Stacked, false-colored DF image. (d) HAADF STEM image of the [-1-12] cross-section. (e) Power dependent µ-PL spectra of the NW acquired at 12 K.

Figure 2: HAADF STEM images of the FIB cross-section from (a) the lower half of the NW normal to the 111-direction, (b) from the top part of the NW normal to the -1-12-direction and (c) from the middle of the NW normal to the -1-12-direction.

Figure 3: Quantitative EDS maps of Al (100 % equals pure AlAs) of the FIB cross-sections from (a) the lower half of the NW normal to the 111-direction and (b) from the top part of the NW normal to the -1-12-direction (Same areas as in Fig. 2(a-b)). The quantification was done with the zeta-method implemented in Python.

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