While
reversibility is a fundamental concept in thermodynamics,
most reactions are not readily reversible, especially in solid-state
physics. For example, thermal diffusion is a widely known concept,
used among others to inject dopants into the substitutional positions
in the matrix and improve device properties. Typically, such a diffusion
process will create a concentration gradient extending over increasingly
large regions, without possibility to reverse this effect. On the
other hand, while the bottom-up growth of semiconducting nanowires
is interesting, it can still be difficult to fabricate axial heterostructures
with high control. In this paper, we report a thermally assisted partially
reversible thermal diffusion process occurring in the solid-state
reaction between an Al metal pad and a Si
x
Ge
1–
x
alloy nanowire observed
by in situ transmission electron microscopy. The thermally assisted
reaction results in the creation of a Si-rich region sandwiched between
the reacted Al and unreacted Si
x
Ge
1–
x
part, forming an axial Al/Si/Si
x
Ge
1–
x
heterostructure.
Upon heating or (slow) cooling, the Al metal can repeatably move in
and out of the Si
x
Ge
1–
x
alloy nanowire while maintaining the rodlike geometry
and crystallinity, allowing to fabricate and contact nanowire heterostructures
in a reversible way in a single process step, compatible with current
Si-based technology. This interesting system is promising for various
applications, such as phase change memories in an all crystalline
system with integrated contacts as well as Si/Si
x
Ge
1–
x
/Si heterostructures
for near-infrared sensing applications.