Twinning
superlattices in indium phosphide nanowires
Rienk E. Algra, Marcel A. Verheijen, Magnus T. Borgström,
Lou-Fé Feiner, George Immink, Willem J. P. van Enckevort,
Elias Vlieg & Erik P. A. M. Bakkers
Nature 456, 369-372 (20 November 2008)
Abstract:
Semiconducting nanowires offer the possibility of
nearly unlimited complex bottom-up design1, 2, which allows for new
device concepts3, 4. However, essential parameters that determine the
electronic quality of the wires, and which have not been controlled yet
for the III–V compound semiconductors, are the wire crystal structure
and the stacking fault density5. In addition, a significant feature
would be to have a constant spacing between rotational twins in the
wires such that a twinning superlattice is formed, as this is predicted
to induce a direct bandgap in normally indirect bandgap
semiconductors6, 7, such as silicon and gallium phosphide. Optically
active versions of these technologically relevant semiconductors could
have a significant impact on the electronics8 and optics9 industry.
Here we show first that we can control the crystal structure of indium
phosphide (InP) nanowires by using impurity dopants. We have found that
zinc decreases the activation barrier for two-dimensional nucleation
growth of zinc-blende InP and therefore promotes crystallization of the
InP nanowires in the zinc-blende, instead of the commonly found
wurtzite, crystal structure10. More importantly, we then demonstrate
that we can, once we have enforced the zinc-blende crystal structure,
induce twinning superlattices with long-range order in InP nanowires.
We can tune the spacing of the superlattices by changing the wire
diameter and the zinc concentration, and we present a model based on
the distortion of the catalyst droplet in response to the evolution of
the cross-sectional shape of the nanowires to quantitatively explain
the formation of the periodic twinning.