Si / Ge Nanowires by MBE


Nanowires and reactive ion etching (RIE)

A. Milenin, P. Werner, and N.D. Zakharov


The RIE is a top-down approach, which is an alternative to the bottom-up technique used for nanowire (NW) growth. Though RIE can separately be used to produce Si NWs (see image below), it can be used in combination with the growth technique allowing to study the details of a growth phenomena on plasma structured surfaces. Several examples of such an approach are presented below.



An array of nanowires etched in Si.

Alexey Milenin (unpublished)



Ordering of NWs grown on structured Si(111)


Based on pore-structured sample, it was demonstrated that the limitation of the diffusion area via plasma structuring of the Si substrate could help controlling the NW diameters and their growth localization in some particular cases. From the tilted view (a) and from the image showing superposition of the initial structure directly after plasma etching and the same region after NW growth (b), it is seen that Au droplets do not form inside of pores but they are regularly localized in between of the pores.
Since surface diffusion plays one of the main roles in the material transfer by MBE growth, designing the geometrical patterns one can obtain a partial separation of the areas for the material collection that will lead to the equalization of the material amount available for the collection in a single area and will finally sharpen the diameter distribution of NWs. The statistical representations of the diameter distribution for Au droplets inside the middle-sized structure array and outside of the structured region show the narrowing of a diameter distribution for the middle-sized array (c,d).



For the more details please see:
A.P. Milenin, P. Werner, N.D. Zakharov, and U. Goesele
Growth of Si nanowires and  nanowire arrays by molecular beam epitaxy, MRS Fall Meeting, 2006



NW growth on structured Si(100)


Two types of NWs were grown on Si(100). It was shown that formation of “typical” NWs on the structured Si(100) occurs only in the places where a direct possibility to grow in <111> directions exists. Another type of NWs was associated with the influence of residuals, which remain after plasma etching.  It was demonstrated that “atypical” NWs prefer to grow in <110> directions, they grow much faster, and they can have diameters below 50 nm, which is unlikely for the “typical” NWs grown by MBE.



For the more details please see:
A.P. Milenin, P. Werner, N.D. Zakharov, and U. Goesele
Growth of Si nanowires and  nanowire arrays by molecular beam epitaxy. MRS Fall Meeting, 2006



Bridging of NWs


Among a separate application to the NW formation by etching (such kind of NWs are shown in the image below with a PtIr probe approaching one of them), the RIE can be used to design a structure helpful to measure characteristics of the grown NWs. A simple example is shown below, where a trench was formed in a SOI wafer. After the growth by MBE, one can get bridges of NWs. The interconnections were grown under UHV conditions. It allows making ex-situ investigation of the contact properties of NWs, which were fabricated in-situ, and compare with the etched NWs.


back  |  print  |  to top