Studying the Formation Dynamics of
VLS
Silicon Nanowire Devices using in situ
TEM
Authors: Alam Sardar B. , Panciera Federico , Hansen Ole , Ross Frances M , Mølhave Kristian 
Published: December 2016

Control of the electrical properties of Si nanowires, and in particular their connection to the macroscale environment, is important when developing nanowire applications. We therefore use in situ TEM to create suspended Si nanowire devices so that we can correlate the structure and transport properties of the nanowires and their contacts. In an ultra high vacuum TEM, we grow Si nanowires by the vapor‐liquid‐solid process using AuSi eutectic droplet catalysts and disilane gas. The nanowires grow from one microfabricated heater [1,2] across to a second heater 2‐3 micrometers away (Figure 1). Temperature can be controlled in the VLS growth range 450–600°C, and we can control the voltage across the nanowire at the moment of contact, and perform IV measurements on the final nanowire device [3] at room temperature. We have shown that novel nanowire contact geometries such as necked or bulged contacts can be formed [4] by tuning the balance between the Si growth rate and the migration of Au from the contact region. This is achieved by controlling the growth conditions during contact formation.
Here we examine an additional parameter that is even more effective in controlling the contact geometry. This is electromigration, induced by flowing current through the nanowire during contact formation. In Figure 2 we show the effect of current flow (as well as disilane pressure) on the deposition of Si and the volume of AuSi. In Fig 2(a) a TEM image series shows the formation of a 10nm nano‐gap by a nanowire (Si NW) connecting to a Si cantilever side wall with an AuSi droplet, and removing the AuSi by using electromigration. In Fig 2 (b, c), the AuSi and deposited Si volumes is plotted along with (b) disilane pressure and (c) current through the wire. In (b) Si is incorporated only at high disilane pressure; when pressure is reduced, the morphology becomes constant. In (c), once a current is flowed through the nanowire, the AuSi starts to shrink at 5400nm3/s due to Au electromigration; as Au moves away, the Si is deposited at 1400nm3/s. The net decrease in volume creates the 10nm gap. Hence the current flow can cause rapid loss of Au from the contact site, forcing a rapid segregation of Si from the AuSi droplet. This we show can controls the contact formation dynamics to create bulged, straight, necked or nanogap contacts [4].
Once contact has been established, the nanowire device can be electrically characterized and further modified, for example by oxidation of the Si surface. We find that nanowires can sustain tens of volts before disconnecting, and exhibit fairly consistent IV characteristics at room temperature, Figure 2(d).
The ability to control the contact structure, and measure its transport properties directly after formation, is helpful in understanding the behaviour of nanowires in processed devices. Electromigration appears to be a useful parameter that allows novel nanowire contact geometries to be created and hence greater flexibility in nanowire device design.