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Three-dimensional nanowire heterostructures

III-nitride nanowires grown by plasma assisted molecular beam epitaxy are attractive for many reasons.  They grow on a variety of substrates, including Si.  Strain accommodation in the nanowires allows for the use of materials with large lattice mismatch in a single heterostructure.  The geometry of nanowires allows for both vertical and coaxial heterostructures, while planar samples only allow for vertical heterostructures.  This last point is especially important since in III-nitrides growth direction largely determines the magnitude of internal electric fields due to polarization.

Our work uses the growth conditions of GaN nanowires to control how the nanowires nucleate and grow.  To do this, we first establish a growth phase diagram relating nanowire characteristics to growth conditions (i.e. substrate temperature and III/V ratio).  We can control density over arbitrarily long times.   Notice in Fig. 1 that while density is controlled, by changing growth kinetics we are able to tune whether nanowires grow vertically or coaxially.  This control allows formation of complex 3D nanowire heterostructures.  We have grown both vertical and coaxial AlN/GaN (~2nm /~2nm) nanowire superlattices (Fig. 2).  The latter of these represents a coaxial III-nitride nanowire heterostructure with multiple layers (a multiple quantum well) grown by molecular beam epitaxy.

Illustration showing the difference between (a) two-step (765 C for 22.5 min., 780 C for 67.5 min.) and (b)one-step growth (765 C for 90 min.). In the one-step growth, conditions are maintained for entire deposition. In two-step growth, conditions are adjusted after an initial nucleation stage to prevent the formation of further nanowires and grow the existing nanowires purely vertically.

(a) 50 period vertically-aligned AlN/GaN nanowire superlattice prepared using the two-step method. Bottom Inset: profile intensity scan measuring compositional modulation along the vertical c-axis direction. (b) 5 period coaxially-aligned AlN/GaN nanowire superlattice using the two-step procedure. Bottom Inset: section of coaxial layers imaged with atomic resolution. Dark (light) areas correspond to AlN (GaN) due to z-contrast. All scale bars correspond to 100 nm unless labeled otherwise.

For more information about this work, please see:
S. D. Carnevale, J. Yang, P. J. Phillips, M. J. Mills,  and R. C. Myers. Nano Lett. 11, 866-871 (2011) .