Ternary InGaN and AlGaN alloys have been sought after for the application of various optoelectronic devices spanning a large spectral range between the deep ultraviolet (DUV) and infrared (IR), including light-emitting diodes, and laser diodes. Conventional planar devices suffer from a high density of dislocations due to the large lattice mismatch, which together with the non-ideal alloy mixing, are established as the cause for various phase separation, surface segregation, and chemical ordering processes commonly observed in nitride alloys. Growth in a nanowire (NW) geometry can overcome these processes by providing enhanced strain relaxation at the free surfaces. In both InGaN and AlGaN, their superior operational characteristics can be attributed to enhanced charge carrier localization at alloy inhomogeneities down to the atomic-scale. Atomic-level chemical ordering in wurtzite InGaN and AlGaN epilayers, describing preferential site occupancy of the cation sublattice by the group III atoms, has been reported mostly with a 1:1 periodicity along the [0001] growth direction [1]. Reports of atomic ordering in cubic ternary III-V alloys (including III-As and III-P) have remained limited to planar thin films; its prevalence within NWs had not been explored.

InGaN/GaN dot-in-a-wire nanostructures grown on Si(111) by molecular beam epitaxy (MBE) were recently developed to achieve more controlled light emission across the entire visible spectrum [2], and characterized using aberration-corrected scanning transmission electron microscopy (STEM) [3]. High-angle annular dark-field (HAADF) Z-contrast imaging shows the InGaN quantum dots (QDs) with atypical oscillating HAADF image intensity at the atomic-level along the c-axis growth direction, exhibiting alternating bright/dark atomic-planes within the QDs [3]. Electron diffraction patterns obtained from the QDs show the presence of otherwise forbidden superlattice reflections, unambiguously confirming the presence of 1:1 bilayer atomic ordering [1]. In addition, atomic-resolution elemental mapping using electron energy-loss spectroscopy (EELS) shows significant In-enrichment in alternating c-planes matching the maxima in the ADF signal collected concurrently, with a deviation from the local mean composition by >25%. Corresponding annular bright field imaging (ABF) enables the visualization of light elements like N, and was used to directly deduce the NWs as N-face polarity. It also indicates that the In-atoms have a preferential occupation at the lower-coordination site along a pyramidal surface facet, which is the first experimental evidence [3] validating the existing theoretical structure model for ordered InGaN layers [4].

Compositional inhomogeneities were also investigated in MBE-grown self-catalyzed AlGaN NWs, which exhibit high luminescence efficiency in the DUV range [5]. With increasing Al concentration, atomic-scale compositional modulations can be induced due to differences in Ga- and Al-adatom migration and incorporation at the growth front. The modulating HAADF intensities were confirmed as Ga-rich/Al-rich regions using EELS elemental mapping at atomic-resolution. Furthermore, their QD/quantum dash-like nature was determined based on multi-orientation views of the same atomic-scale Ga-rich regions. Such atomic-scale compositional modulations in AlGaN can provide energy band fluctuations leading to strong three-dimensional confinement of charge carriers [6].

[1] P. Ruterana, G. Nouet, W. Van der Stricht et al., Appl. Phys. Lett., 72, 1742-1744 (1998)

[2] H.P.T. Nguyen, K. Cui, S. Zhang et al., Nano Lett., 12, 1317-1323 (2012)

[3] S.Y. Woo, M. Bugnet, H.P.T. Nguyen et al., Nano Lett., 15(10), 6413–6418 (2015)

[4] J.E. Northrup, L. Romano and J. Neugebauer, Appl. Phys. Lett., 74, 2319–2321 (1999)

[5] S. Zhao, S.Y. Woo, M. Bugnet et al., Nano Lett., 15(12), 7801–7807 (2015)

[6] The authors are grateful to NSERC for supporting this research. The microscopy was carried out at the Canadian Centre for Electron Microscopy, a National facility supported by NSERC, CFI and McMaster University.

Figures:

(a) HAADF image showing the oscillatory intensity at the atomic-level within the InGaN QDs, (b) integrated intensity line profile from boxed region. (c) Line profiles of the ABF image in (d), including both projected columns (Total, black line from yellow box), and separating into Top (red box) and Bottom (blue box) columns. (e) Model of ordered InGaN from the preferential incorporation of In at the reduced N-coordination site (N=2).

(a) HAADF image of an AlGaN NW, (b) high-magnification view of the p-AlGaN region (red box in (a)), showing intensity modulations along the c-axis, indicative of alternating Ga-rich/Al-rich planes at the atomic-scale. (c) ADF image and (d) concurrent Ga-signal EELS map (displayed in temperature scale) from blue box in (b), showing a direct correspondence between local increases in Ga-signal at single atomic-planes.

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