TOPICAL COLLECTION: 59TH ELECTRONIC MATERIALS CONFERENCE 2017 Shape Evolution of Highly Lattice-Mismatched InN/InGaN Nanowire Heterostructures LIFAN YAN , 1,3,4 ARNAB HAZARI, 1,2 PALLAB BHATTACHARYA, 2 and JOANNA M. MILLUNCHICK 1,5 1.—Center for Photonics and Multiscale Nanomaterials, University of Michigan, Ann Arbor, MI 48109, USA. 2.—Department of Material Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA. 3.—Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109, USA. 4.—e-mail: ylifan@umich.edu. 5.—e-mail: joannamm@umich.edu We have investigated the structure and shape of GaN-based nanowires grown on (001) Si substrates for optoelectronic device applications. The nanowire heterostructures contained InN disks and In 0.4 Ga 0.6 N barrier layers in the active region. The resulting nanowire array comprised two differently shaped nanowires: shorter pencil-like nanowires and longer bead-like nanowires. The two different nanowire shapes evolve due to a variation in the In incorporation rate, which was faster for the bead-like nanowires. Both types of nanowires exhibited evidence of significant migration of both Ga and In during growth. Ga tended to diffuse away and down along the sidewalls, resulting in a Ga-rich shell for all nanowires. Despite the complex structure and great variability in the In composition, the optical properties of the nanowire arrays were very good, with strong luminescence peaking at  1.63 lm. Key words: InGaN, InN, nanowires, infrared, photoluminescence, STEM INTRODUCTION InGaN nanowire heterostructures have been pro- posed for use in optoelectronic devices, such as light- emitting diodes, 1–3 lasers, 4,5 and sensors, 6 due to the high tunability of their bandgap, which can range from the deep-ultraviolet to infrared wave- length regime depending on the In composition. 5,7,8 These nanowire structures also provide structural advantages, as they can be grown directly on (001) Si wafer, making them suitable for potential inte- gration, and have been shown to effectively relax lattice mismatch strain at low In compositions. 5,9 The shape of such nanowires can be altered by tuning the growth conditions and In composition. For low In concentrations, nanowire heterostructure growth occurs axially on the basal plane, resulting in disc-in-nanowire heterostructures. 10 As the In con- centration increases, the lattice mismatch also increases such that the growth mode of the In-rich region may transition from layer-by-layer to three- dimensional island nucleation within the nanowire, 11 analogous to Stranski–Krastanov growth of quantum dots in planar epitaxial systems. 12 Decreasing the substrate temperature has been shown to promote In incorporation and lateral growth of nanowires. 13 Reducing the growth temperature while simultane- ously increasing the In composition shifts the primary growth directions away from the basal plane to pyramidal planes; 14,15 For example, growth of In 0.4 Ga 0.6 N on top of GaN nanowires results in such a drastic increase in diameter that the nanowires coalesce. 16 It has been shown that pure InN nanowires also increase in diameter during growth, resulting in inverted pyramids or pinhead structures. 17–19 Details about the structure of highly lattice- mismatched InGaN nanowire heterostructures, where there is variation in the In composition along the length of the nanowire, have not been reported. However, it is expected that the interaction of composition- and temperature-induced morphologi- cal changes and strain relaxation will have a strong (Received June 26, 2017; accepted November 20, 2017; published online December 1, 2017) Journal of ELECTRONIC MATERIALS, Vol. 47, No. 2, 2018 https://doi.org/10.1007/s11664-017-5986-7 Ó 2017 The Minerals, Metals & Materials Society 966