Si / Ge Nanowires by MBE
Abstract
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P. Werner, N.D. Zakharov, K. Nguyen-Duc, and L. Schubert
Si nanowires (NW) grow via the Au droplets. These droplets do not act as a catalyst for a precursor gas, but result in a liquid Si/Au eutectic phase formation considerably reducing the nucleation barrier for whisker growth. A supersaturation is supplied by the difference of the chemical potential of Si atoms located in the overgrown layer and on the top of the whisker. It appears due to relaxation of the elastic energy stored in overgrown layer caused by gold intrusions. The resulting size of the droplet determines the diameter of the growing NW.
However, the NW growth by MBE shows clear differences to the classical VLS-mechanism valid for CVD and MOCVD growth. First, in the case of MBE, the Au:Si droplets do not act as classical catalysts, but causes a difference of the chemical potentials between the upper growth front of the wire and the substrate surface.
Second, concerning kinetic aspects, the NWs are formed by two components of material flow. Beside the flux of ad-atoms collected by the droplet at the tip (I1), a significant amount of Si ad-atoms diffuse from the surrounding substrate surface to the {111} Si/Au interface – the place of ad-atom incorporation into the droplet. The Si lattice growth seems to proceed via the fast flow of {111} steps on the {111} growth surface; the ad-atoms have to diffuse to the corresponding steps where they are incorporated into the crystal lattice.
