In-situ TEM study of low dimensional structures at kinetically constrained and equilibrium growth regimes

The structural and morphological evolution of low dimensional Fe-Ge structures (i.e. 0D/1D) during the reactive deposition of Fe on Ge(001) has been comprehensively studied by using a ultra-high vacuum in-situ transmission electron microscope. The reactive deposition of Fe on Ge(001) lead to the formation of hexagonal Fe13Ge8 islands on the cubic Ge surface at all temperatures studied. It has been observed that the epitaxial relationship between the hex-Fe13Ge8 islands and underlying substrates keeps the same at all growth temperatures. However, the morphology of these islands shows remarkably different geometry with different deposition temperatures. It was observed that all islands could grow into Ge(001) substrate through endotaxial growth. At kinetically constrained regime, islands morphology evolved from small compact shape to wire-like one, mainly attributed to anisotropic ledge diffusion and corner barrier. However, at high temperature, NWs disappeared while dome-like islands formed with much larger size than those grown at low temperatures. By comparing the total energy of the islands formed below and above 510 °C, these islands formed at higher temperature have a lower total surface/interface energy per unit volume. The shape transition, i.e. from NW to domes observed in high temperature regime is therefore driven by minimization of total surface and interface energies of the growing islands, more akin to the growth in the equilibrium regime. On the basis of the observations on Fe growth on Ge(001), it has been demonstrated that self-assembly nanowire can be formed not only due to strain induced instability as predicted by conventional models, but also by the ‘endotaxy’ growth. It also illustrates that nanowires can be formed through kinetic constraints in terms of anisotropy in ledge diffusion and corner barriers. More significantly, this study shows that not all the compact islands or nanowires are necessary thermodynamically equilibrium structures. In this Fe13Ge8/Ge material system, large compact islands formed at high temperatures are equilibrium structures instead of the nanowire formation. In addition, this work suggests that temperature can provide an avenue to selectively control dimensionality (i.e. 0D and 1D) and configuration (i.e. facets evolution) of growing islands at both thermodynamically limited and kinetically constrained growth regimes.