Template effect of the nanowire core on the growth of hexagonal Si/Ge shell:a first principles modeling

Silicon is still by far the most widely used semiconductor in electronics, but its Achilles heel, i.e. the indirect bandgap, limits the development of silicon-based optoelectronic devices. Recent advances in the growth of nano-structures have given access to new meta-stable phase of Si, not present in nature. In fact, Si is known to be stable in its diamond cubic crystal structure, but under extreme pressure conditions, transition to the hexagonal phase can be observed. Recently, Bakkers et al., succeeded in the growth of hexagonal Si and Ge, by exploiting GaP/Si and GaAs/Ge 1 core/shell nanowires (NWs) . Besides, recent theoretical studies have predicted a direct band-gap nature for hexagonal Ge, which is preserved by alloying Ge with to up 30% Si. This new hexagonal crystal semiconductor, made of Silicon and Germanium, can potentially be a breakthrough for Si- based photonics and optoelectronics. By first-principles simulations, the structural properties of GaAs/Ge interface have been studied, with the goal of understanding the mechanism responsible for the growth of the hexagonal Ge crystal. Firstly, we identify the <1100> orientation as the most stable for the NW side faces, then by modeling the core/shell interface in this direction, we show that it is not possible to create a defects- free interface between the hexagonal substrate and a cubic Ge layer. The structural evolution of the cubic Ge layer from 0 K to 1000 K, with a subsequent cooling down and equilibration at 300 K is illustrate in Fig.1, as obtained by performing ab-initio molecular dynamic (AIMD) simulations. Despite the higher stability of the cubic phase, we predict that even below 300K the cubic crystal structure of the Ge layer is lost, then the Ge layer tends to re-crystallize in the hexagonal phase, possibly due to a transfer of the hexagonal crystal structure from the GaAs core, along the [1100] direction. Thus, our simulations evidence a crucial role of the NWs core as template for the growth of the hexagonal crystals. Finally, the role of hydrogen in the growth surface process is elucidated.