2011 Annual Report - Center for Integrated Nanotechnologies - Los ...

Nanoscale Electronics & Mechanics Thrust
Controlling the interface width in Si/Ge nanowire heterojunctions
Accomplishment: High quality semiconductor nanowire growth
requires the use of a liquid catalyst in a process called vaporliquid-solid
(VLS) growth. One limitation of the VLS growth
approach has been an inability to create sharp nanowire interfaces.
The reason for the lack of interface abruptness is that a
substantial amount of growth solute atoms are dissolve into the
liquid Au catalyst which
is typically used. Thus
when changing growth
species, for example
from Ge to Si, there is
a long transition region
as Ge atoms are slowly
depleted from the liquid
by growth and replaced
by Si atoms. The present
work reports for the
first time the systematic
increase in heterointerface
sharpness by using
an alloy catalyst. We
demonstrate this result
for Si/Ge axial nanowires
where the composition
of a AuGa alloy
catalyst is varied in situ
using the precursor trimethylgallium.
The key
Abrupt interfaces by AuGa catalyst alloying.
concept is that for VLS
nanowire growth the transition region width is dictated by the
rate of solute depletion from the catalyst, which in turn is proportional
to the solute solubility within the catalyst. Our approach
to controlling the Au(1-x)Ga(x) catalyst alloy composition in situ
provides a means to adjust the semiconductor solubility within
the catalyst alloy and thus tune the heterojunction width. Specifically,
we show a 4-times decrease in the heterointerface width
for 60 nm diameter Ge-Si nanowires grown from pure Au (44 nm
width) compared to interfaces grown from a Au(0.66)Ga(0.34)
catalyst alloy (11 nm width). We also developed a growth model
which accurately simulates heterointerface widths as a function
of solute solubility in alloy catalysts.
Significance: The importance of the control of interface sharpness
lies in the ability to precisely tune the electronic transport
properties by modulating the composition and/or dopant type
to create novel high-performance devices. Compared to compositionally
uniform nanowires, axial heterostructured and
modulation doped semiconductor nanowires have a much wider
relevance for electronic, photovoltaic, and thermoelectric device
applications. For high subthreshold slope nanowire devices such
as tunnel and avalanche field effect transistors, the performance
is directly related to the junction abruptness. However, for group
IV nanowires grown by the Au-catalyzed VLS mechanism, junction
abruptness is limited in magnitude to the order of the nanowire
diameter as described above. Thus, establishing a method to
create more abrupt axial heterojunctions, while maintaining VLS
growth, is important to further exploit the inherent benefits of
nanowires. Currently we are extending this concept to increase
the sharpness of axial dopant profiles in Si, Ge,
Si/Ge p-n junction nanowires using atom tomography
dopant profiling.
Reference:
Controlling Heterojunction Abruptness in VLS-
Grown Semiconductor Nanowires via in situ
Catalyst Alloying, Daniel Perea, Nan Li, Robert
M. Dickerson, A. Misra, and S. Tom Picraux,
Nano Lett. 11, 3117 (2011).
CINT Contact:Tom Picraux
A series of images showing a single nanowire grown out of a
micropost. Images from CINT User Danny Perea.
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30The Center for Integrated Nanotechnologies | 2011 Annual Report