Carrier Localization at Atomic‐Scale Compositional Fluctuations in Single AlGaN Nanowires with Nano‐Cathodoluminescence
Authors: Woo Steffi Y , Tizei Luiz , Bugnet Matthieu , Zhao Songrui , Mi Zetian , Kociak Mathieu , Botton Gianluigi A 
Published: December 2016

Considerable interest has been generated to develop highly efficient deep ultraviolet (DUV) emitters using AlGaN‐based alloys with direct bandgaps between 3.4 – 6.1 eV for a broad range of applications. Conventional planar AlGaN DUV devices, such as electrically injected solid‐state lasers and light‐emitting diodes, experience limited efficiencies due to the high dislocation density and inefficient
p
‐doping with increasing Al‐content using Mg. Nanowire (NW) structures can be a promising alternative to enhance strain relaxation and
p
‐doping, both mediated by the additional free surfaces. Exceptionally high internal quantum efficiencies have been reported in high Al‐content AlGaN NWs [1, 2], suggesting strong charge carrier confinement at nanoscale kinetically‐driven alloy inhomogeneities [3] that behave optically as quantum dots [4]. Recently, atomic‐scale compositional modulations have been reported in high Al‐content AlGaN NWs and suggested to act as localization centers to enhance radiative recombination [1, 2]. Comprehensive understanding of emission characteristics in such spontaneously‐formed compositional fluctuations in AlGaN NWs, from directly correlating the localized optical response to structural/chemical properties at relevant lengthscales is still lacking.

High and low Al‐content Al

x

Ga

1‐x

N
p‐i‐n
homojunctions were grown on Si‐doped GaN NW templates on Si(111) substrates by plasma‐assisted molecular beam epitaxy. Characterization on single NWs using nanometer‐scale cathodoluminescence (CL) spectral imaging at 150 K was performed [5], subsequently correlated to structural information obtained with aberration‐corrected scanning transmission electron microscopy (STEM). The low‐Al AlGaN NWs (Sample A, nominally
x
= 0.11) exhibit an Al‐rich shell that can passivate the surface. A high degree of homogeneity within the AlGaN core region is observed from high‐angle annular dark‐field (HAADF) Z‐contrast imaging, and is further confirmed with electron energy‐loss spectroscopy (EELS) [2]. Individual NWs in Sample A show the presence of up to three emission peaks highly delocalized along the NW length (Fig. 1f–h), two peaks originate from the AlGaN region (sharp 336 nm band‐edge peak, broad and asymmetric peak with 360 nm maximum), and one from the
n
‐GaN base (~355 nm).

With increased Al concentration (Sample B, nominally
x
= 0.88), extensive atomic‐scale HAADF intensity fluctuations are present in the AlGaN regions, and these are indicative of strong Ga‐rich/Al‐rich compositional modulations, as validated using EELS elemental mapping at atomic‐resolution [2]. The nature of the laterally discontinuous compositional fluctuations varies between
p
‐,
i
‐,
n
‐doped AlGaN regions (Fig. 2a–e): single atomic layers occurring on
c
‐planes along the growth direction in
p
‐AlGaN and on semi‐polar {10‐13} planes in
i
‐ and
n
‐AlGaN. Other nm‐sized modulations and segregation are also observed. NWs in Sample B display a spectrally dense series of narrow lines blue‐shifted (230 – 300 nm), and are drastically more spatially localized within the AlGaN regions in the wavelength‐filtered CL images, relative to Sample A. Subsequent high‐resolution (HR)STEM on these NWs shows that the different spectral behaviors (emission energy) can be correlated to positions along the NW with different types of compositional fluctuations. Specifically, the shortest wavelength peaks (230 – 240 nm) originate from volumes with atomic‐scale Ga‐rich
c
‐planes in
p
‐AlGaN (Fig. 2c, ROI 1 circled in 2g), while high intensity sharp peaks (250 – 290 nm) can be assigned to regions with atomic‐scale modulations on inclined {10‐13} planes or other nm‐sized segregation (Fig. 2b,d). The presence/absence of extended defects and its role as localization centers will also be discussed [6].