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While reversibility is a fundamental concept in thermodynamics, mostreactions are not readily reversible, especially in solid state physics. Forexample, thermal diffusion is a widely known concept, used among others toinject dopant atoms into the substitutional positions in the matrix and improvethe device properties. Typically, such a diffusion process will create aconcentration gradient extending over increasingly large regions, withoutpossibility to reverse this effect. On the other hand, while the bottom upgrowth of semiconducting nanowires is interesting, it can still be difficult tofabricate axial heterostructures with high control. In this paper, we report areversible thermal diffusion process occurring in the solid-state exchangereaction between an Al metal pad and a Si$_x$Ge$_{1-x}$ alloy nanowire observedby in-situ transmission electron microscopy. The thermally assisted reactionresults in the creation of a Si-rich region sandwiched between the reacted Aland unreacted SixGe1-x part, forming an axial Al/Si/Si$_x$Ge$_{1-x}$heterostructure. Upon heating or (slow) cooling, the Al metal can repeatablymove in and out of the Si$_x$Ge$_{1-x}$ alloy nanowire while maintaining therod-like geometry and crystallinity, allowing to fabricate and contact nanowireheterostructures in a reversible way in a single process step, compatible withcurrent Si based technology. This interesting system is promising for variousapplications, such as phase change memories in an all crystalline system withintegrated contacts, as well as Si/Si$_x$Ge$_{1-x}$/Si heterostructures fornear-infrared sensing applications.

Reversible Al Propagation in Si$_x$Ge$_{1-x}$ Nanowires