Surface energy and modes of catalytic growth of semiconductor nanowhiskers

A thermodynamic theory of the growth of semiconductor nanowhisker (NW) crystals according to the vapor-liquid-solid (VLS) mechanism has been developed. An expression is proposed for the effective surface energy of the system, which is considered as a function of the NW radius and the contact angle of a liquid catalyst drop. Minimization of the surface energy leads to two possible modes of the VLS growth. In a standard mode that is realized when the Nebolsin-Shchetinin-Glas (NSC) condition is valid, the drop is not wetting the side surface of the NW. In the opposite case, the growth proceeds in a wetting mode, whereby the drop spreads about the NW top. It is shown for the first time that, even when the NSC condition is valid, the effective surface energy has two minima separated by a barrier and the minimum corresponding to the wetting mode is lower than that for the non-wetting mode. The results are applied to an analysis of the polytypism observed for GaAs nanowhiskers grown with Au and Ga catalysts.