Semiconductor Nanowires


Si Nanowires by CVD

S. Christiansen, M. Becker, Th. Stelzner, B. Eisenhawer, and V. Sivakov


Nanowires have received increased attention as a result of their properties which are often superior to their bulk counterparts. To integrate silicon nanowires (SiNWs) into device architectures, alignment and positioning of the NWs are essential. One method could be the use of prepositioned catalysts and epitaxial vapor-liquid-solid (VLS) growth. Our approach makes use of ion implantation for the deposition of the catalyst. This process has the potential for a grow-in-place method by introducing the ions into the substrate through masks or using focused ion beam techniques (FIB). We studied the annealing behavior of Au implanted substrates and the subsequent growth of SiNWs by chemical vapor deposition (CVD). A comparison of the growth results on silicon (111) and (100) wafers shows that the orientation of SiNW arrays can be controlled by the crystal orientation of the substrate. Si(111) and (100) substrates were implanted with 30 keV Au+ ions to a dose between 1 x 1015 and 5 x 1016 cm-2. The growth temperature of 500C and a mixture of 20 sccm Ar and 5 sccm SiH4 were used to grow NWs at a pressure of 5 mbar.


Fig. 1: Schematic drawing of the implantation and annealing processes to obtain metal nanoparticles for subsequent growth of semiconductor NWs by the VLS process. Structural appearance (a) after room temperature implantation of Au, (b) after annealing at temperatures < 400 C, and (c) after annealing at temperatures > 600 C for several 10 minutes.


Fig. 2: Secondary-electron images of Si NWs grown on Au-implanted and annealed Si(111) substrates. (a) It is visible that only in implanted areas NW growth occurs (implantation through a mask, area on the left is unimplanted); (b), (c) blow-ups showing straight Si NWs with a gold droplet at their tips. The NW orientations are partly perpendicular, partly oblique to the substrate surface. The NW diameters are determined by the Au droplet size and show a comparably narrow distribution with sizes well below 100 nm.



Si nanowires by EBE


The vertical and epitaxial growth of long (up to a few microns) silicon nanowires on Si(111) substrates by electron beam evaporation (EBE) (10-6-10-7 mbar) is demonstrated at temperatures between 600 and 700 C following the vapor-liquid-solid growth mechanism from gold nanoparticles. The silicon atoms are provided by evaporating silicon at varying evaporation currents (IE) between 35 mA and 80 mA, which result in growth rates between 1 nm/min and 100 nm/min. The growth peculiarities in the interaction triangle, evaporation current (IE), growth temperature (TS) and gold layer thickness (dAu) are investigated. Kinetics and energetics contribute to the morphology of silicon nanowires.



Fig. 3: HRSEM micrographs of Si NWs grown by EBE on Si(111) at 700C for 1h at emission current 80 mA and Au thickness of 1.2 nm. The faceting along the side facets of Si NWs is clearly visible. The growth direction is <111>.


Fig. 4: TEM and SEM micrographs of Si NWs grown by EBE on Si(111). The faceting along the side facets of Si NWs is clearly visible as well as the dark gold cap and gold particles at the side facets. The inset of  the NW that starts to grow on the substrate shows epitaxy without interfacial defects. The growth direction is <111>.

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