Anomalous strain relaxation in core-shell nanowire heterostructures via simultaneous coherent and incoherent growth

Heterostructures formed within nanowires offer the key advantage that misfit strain can relax at the nanowire sidewalls and partition throughout the entire nanostructure. As a result, using nanowires as nanoscale substrates enables the realization of coherent heterostructures well beyond the thickness and lattice mismatch ranges possible with conventional planar heterostructures. This design flexibility opens the door to novel device structures based on new material combinations, which would be highly defective in planar form. However, strain relaxation within these nanofaceted heterostructures is complex and still not fully understood.

Figure 1. Bright-field plan-view scanning transmission electron microscopy investigation (along the ⟨111⟩ zone axis) of nominally GaAs/In0.6Ga0.4As (140 nm/18 nm) nanowires. (a) Core-shell nanowires with InxGa1‑xAs mounds and {112} facets at the edges of {110} sidewalls. (b) High-resolution micrograph and (c) corresponding Fourier transform of an InxGa1‑xAs mound reveal near complete strain relaxation to the GaAs core. (d) High-resolution micrograph of the GaAs/InxGa1‑xAs core-shell interface near the center of a {110} facet and corresponding geometric phase analysis plot of in-plane (exx) strain, illustrating the coherent core-shell interface.

In this work we investigate strain relaxation in GaAs/(In,Ga)As core-shell nanowire heterostructures with systematically varying lattice mismatch as well as core and shell dimensions, and uncover a new form of strain relaxation that is not observed in planar heterostructures. Namely, we find that above a critical lattice mismatch which depends on the core diameter, (In,Ga)As shell growth proceeds partially through coherent shell growth and partially through the growth of plastically relaxed mounds. In direct contrast to Stranski-Krastanov growth, the coherent shell continues to grow after the mounds have formed and we do not observe a critical thickness for mound formation. Furthermore, the shell remains coherent with no evidence of misfit dislocations. For higher lattice mismatches, incoherent mound growth is increasingly favored over coherent shell growth. The mounds nucleate at the edges of the {110} nanowire sidewall facets and we explain this extraordinary strain relaxation process as a consequence of the nanofaceted nature of these heterostructures.

 

 

 

 

1 Autor R. B. Lewis , L. Nicolai , H. Küpers , M. Ramsteiner , A. Trampert , L. Geelhaar
Titel

Anomalous strain relaxation in core-shell nanowires via simultaneous coherent and incoherent growth

Source Nano Lett. , 17 , 136 ( 2017 )
DOI : 10.1021/acs.nanolett.6b03681 | 2824 Cite : Bibtex RIS
R. B. Lewis, L. Nicolai, H. Küpers, M. Ramsteiner, A. Trampert, and L. Geelhaar