Atomic Ordering in InGaN Alloys within Nanowire Heterostructures
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Abstract
Ternary
III-nitride based nanowires (NWs) are promising for optoelectronic
applications by offering advantageous design and control over composition,
structure, and strain. Atomic-level chemical ordering in wurtzite
InGaN alloys along the <i>c</i>-plane direction with a 1:1
periodicity within InGaN/GaN NW heterostructures was investigated
by scanning transmission electron microscopy. Atomic-number-sensitive
imaging contrast was used to simultaneously assign the In-rich and
Ga-rich planes and determine the crystal polarity to differentiate
unique sublattice sites. The nonrandom occupation of the <i>c</i>-planes in the InGaN alloys is confirmed by the occurrence of additional
superlattice spots in the diffraction pattern within the ternary alloy.
Compositional modulations in the ordered InGaN was further studied
using atomic-resolution elemental mapping, outlining the substantial
In-enrichment. Confirming the preferential site occupation of In-atoms
provides experimental validation for the previous theoretical model
of ordered InGaN alloys in bulk epilayers based on differences in
surface site energy. Therefore, this study strongly suggests that
atomic ordering in InGaN has a surface energetics-induced origin.
Optimization of atomic ordering, in particular in III-nitride NW heterostructures,
could be an alternative design tool toward desirable structural and
compositional properties for various device applications operating
at longer visible wavelengths