263116-3
Dellas et al.
Appl. Phys. Lett. 97, 263116 ͑2010͒
FIG. 3. ͑a͒ TEM image of a nickel germanide segment
formed after annealing at 450 °C for 2 min showing the
break that forms in the nickel germanide segment. ͑b͒
Higher magnification image of the broken nickel ger-
manide segment showing the rounded surfaces that
form after the nanowire breaks.
from the Ge NW rather than right at the germanide/Ge NW
interface, which leads to some doubt about this explanation.
A less likely explanation is that the germanides segment be-
gins to agglomerate, akin to the agglomeration of NiGe thin
films on Ge wafers at temperatures as low as 500° ͑Ref. 21͒
However, the germanides nanowire segment in our study
does not break into beads anywhere else along its length,
making this explanation less likely. Another possible expla-
nation involves stress in the nanowire created by the thermal
expansion mismatch between the nickel germanide segment
and the GeNW. During rapid heating and cooling cycles in
the RTA, perhaps a crack initially forms in the nanowire to
relieve stress. It is also conceivable that more than one of
these factors is at play.
The break in the wires at annealing temperatures of
450 °C and higher does not eliminate the possibility of
forming longer nickel germanide segments. In order to form
longer nickel germanide segments without breaking the
NWs, lower annealing temperatures may be used for longer
times. For example, annealing at 400 °C for 5 min results in
forming nickel germanide segments with average lengths of
1.5 m. Under these annealing conditions, axial nickel ger-
those described by Sarpatwari et al.24 in order to study the
Schottky barrier height of axial contacts to semiconductor
NWs or to reduce the access resistance of GeNW transistors.
In summary, we have found that Ni contact pads react
with GeNWs to form axial nickel germanide segments after
annealing at 350 °C for 2 min. Fracture in the nickel ger-
manide segment occurs near the nickel germanide/GeNW in-
terface for annealing temperatures of 450 °C or higher, but
long segments may be grown if the annealing temperature is
limited to 400 °C and longer annealing times are used. The
ability to grow long nickel germanide segments microns in
length will be useful for incorporation of nickel germanide
contacts into GeNW field effect transistors and other nano-
electronic devices.
the use of the Penn State Nanofabrication Facility ͑Grant No.
NSF NNUN ECCS-0335765͒.
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The authors are grateful for financial support from NSF
through Grant No. ECS-0609282 and the Penn State Center
for Nanoscale Science ͑Grant No. DMR-0820404͒ as well as