Structural and elastic properties of InN nanowires

Article abstract of DOI:10.1002/pssa.201127530

Structural and elastic properties of InN nanowires (NWs) have been investigated. It was observed that the NWs bend spontaneously or upon meeting an obstacle in their growth path at angles that are multiples of 30°. Lithographically patterned trenches and barriers were found to influence the growth direction of the NWs, which depending on the angle of incidence, grew along the barrier or got deflected from it. Young's modulus of InN NWs, measured by three point bending method using a NW suspended across a trench, was found to be 266 GPa, which is in between the moduli of bulk and thin film InN. Overall, the InN NW properties were found to be very suitable for applications in nanoelectromechanical systems (NEMS) and sensors. Copyright

Full text of DOI:10.1002/pssa.201127530

Phys. Status Solidi A 209, No. 4, 718–723 (2012) / DOI 10.1002/pssa.201127530
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applications and materials science
Structural and elastic properties
of InN nanowires
,
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Ehtesham B. Quddus , Alina Wilson , Jie Liu , Zhihua Cai , Deepak Veereddy , Xinyong Tao ,
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Xiaodong Li , Richard A. Webb , and Goutam Koley
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Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208, USA
Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA
Department of Physics and Astronomy and USC Nanocenter, University of South Carolina, Columbia, SC 29208, USA
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Received 5 September 2011, revised 3 January 2012, accepted 6 January 2012
Published online 30 January 2012
Keywords guided growth, InN nanowires, Young’s modulus
*
Corresponding author: e-mail quddus@email.sc.edu, Phone: 8037775004, Fax: 8037778045
Structural and elastic properties of InN nanowires (NWs) have Young’s modulus of InN NWs, measured by three point
been investigated. It was observed that the NWs bend bending method using a NW suspended across a trench, was
spontaneously or upon meeting an obstacle in their growth found to be 266 GPa, which is in between the moduli of bulk
path at angles that are multiples of 308. Lithographically and thin film InN. Overall, the InN NW properties were found
patterned trenches and barriers were found to influence the to be very suitable for applications in nanoelectromechanical
growth direction of the NWs, which depending on the angle of systems (NEMS) and sensors.
incidence, grew along the barrier or got deflected from it.
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Introduction One-dimensional
nanostructures, uniform cross-sections, and poor material properties [8–10].
including nanowires (NWs) and nanotubes (NTs), have This has prevented full realization of their potential for
attracted significant research interest due to their novel electronic and optoelectronic applications. Recently, syn-
physical properties and potential for utilization as critical thesis of good quality and uniform diameter InN NWs have
components in a wide range of nanoscale device appli- been demonstrated by Cai et al., which also exhibited
cations. Semiconducting NWs, in particular, are interesting excellent electrical transport properties [11]. However, for
due to their predictable material properties, high surface-to- applications of these devices in robust and versatile
volume ratio, and excellent electronic transport properties. nanoscale devices, the material properties of these NWs
These properties are highly desirable for designing nanos- need to be further explored. For example, an understanding
cale devices including sensors, solar cells, electronic, of their elastic properties would be useful in designing
optoelectronic, and electromechanical devices. Exhibiting nanoscale resonators and nanoelectromechanical systems
unique material properties including direct bandgap span- (NEMS), while a better understanding of their material and
ning over a wide range, polarization, and chemical inertness, structural properties would enable us to find applications of
nanostructures of III–V nitride semiconductors (GaN, InN, these NWs and nanostructures in next generation multi-
AlN, or their alloys) have been utilized to fabricate field functional and hybrid devices, sensors, and systems. For gas
effect transistors (FETs) [1], light emitting diodes (LEDs) sensing applications, InN NWs have the advantage of being
[2], laser [3], and chemical sensors [4]. Among the III–V easily coated with a shell layer of In O , which is sensitive to
2 3
nitride nanostructures, InN NWs are particularly interesting a multitude of gases and vapors.
because of their narrow band gap, high electron mobility, and
surface electron accumulation [5–7].
In this paper, we report on the synthesis and material
characterization of InN NWs grown by vapor–liquid–solid
To explore their applications in nanoscale devices, (VLS) method in a chemical vapor deposition chamber. The
significant efforts have been directed toward synthesizing NWs were found to exhibit properties of spontaneous
high quality InN NWs. However, most of these efforts bending, at multiples of 308, which have been investigated
yielded inhomogeneous NWs with low growth rates, non- in detail through transmission electron microscopy (TEM)
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Phys. Status Solidi A 209, No. 4 (2012)
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imaging. It has been observed for the first time that these
deflections can be initiated and controlled by lithographi-
cally patterned barriers. Young’s modulus of a NW
positioned across a patterned trench has also been measured
for the first time by three point bending method using an
atomic force microscope (AFM) tip.
2 Experimental Synthesis of InN NWs was carried
out by VLS mechanism in a high temperature horizontal
quartz-tube furnace at 650–700 8C through the direct
reaction between metallic In and NH using Au as catalyst.
A 2 nm thick Au catalyst patterned (by E-beam lithography)
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n Si (100) substrate with a 100 or 500 nm thick SiO layer
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was used for growth using pure NH and metallic In as
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precursors. NWs with high crystalline quality and uniform
diameters were grown at a fast rate of ꢀ30 mm/h, using
nanoscale catalyst patterns. Further details of the growth
conditions are reported elsewhere [11]. InN NW FET
devices were fabricated using coplanar NWs on Si/SiO2
surface in a back-gated configuration. The fabrication
process included wafer cleaning using organic solvents,
polymethylmethacrylate (PMMA)photoresist deposition, E-
beam exposure, pattern generation, metal deposition, and
lift-off. Ohmic contacts on the NWs were established by
depositing a 4 layer metal stack of Ti/Al/Ti/Au (20/50/20/
50 nm) which gave contact resistance of a few kV, which are
typically much less than the NW resistance of a few hundred
kV [13].
Figure 1 (a) SEM image of InN NWs growing from a catalyst strip
pattern(insetshowsanAucatalystparticleattheedgeoftheNW)and
For TEM imaging and elastic modulus determination, (b) statistical distribution of InN NWs plotted against their diame-
InN NWs were removed from the Si/SiO substrate by ters.
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ultrasonication in acetone solution for ꢀ30 min. Acetone
drops containing NWs were subsequently deposited on
SiO /Si samples with uniform trenches (ꢀ1 mm wide and Figure 2 shows examples of spontaneous bending of NWs
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ꢀ
200 nm deep) for elastic property measurements, or on Cu with growth deviation angles of 308 and 608. Of course, the
grids for TEM measurements, and allowed to dry. Before NWs also suffered growth redirection when obstructed by
performing the three point bending experiments for elastic barriers, at angles that are approximate multiples of 308 as
modulus determination, the ends of NWs were fixed by clearly seen in Fig. 1(a). Although the phenomenon of
depositing carbon thin film clamps by the electron beam spontaneous and barrier induced deflections of NWs has
induced deposition (EBID) technique. For depositing the
clamps, the sample was loaded inside an scanning electron
microscope (SEM) chamber with a few pieces of n-Docosane
(
99%, Alfa Aesar), sublimating to gaseous molecules.
Carbon thin films were deposited at the desired spots by
scanning by the electron beam in high speed mode for 5 min.
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Results and discussion
.1 NW deflections The InN NWs were observed to
grow outwardfrom catalyst patterns along the plane of the Si/
SiO substrate. Figure 1(a) is an SEM image showing typical
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NWs, which can either grow straight, or deflect from other
NWs to form K-junctions. The inset is a magnified SEM
image showing the Au catalyst tip at the apex of a NW.
The diameter and length of the NWs varied in the range 10–
80 nm and 5–20 mm (for half hour growth), respectively.
Figure 1(b) shows the distribution of the NW diameter
Figure 2 SEMimageofNWsbendingspontaneouslyatanglesthat
measured over a set of 1000 NWs. The modal (occurring are of multiples of 308. The growth deviation angles for some of the
with highest probability) diameter can be seen as ꢀ20 nm. NWs are shown.
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E. B. Quddus et al.: Structural and elastic properties of InN nanowires
Figure 3 Lattice resolved TEM images of the NWs shows atomic
arrangementsasthegrowthdirectionischanged(a)fromone[110]to
another[110]direction, and(c) from[110]to ½110ꢁdirection. (b)and
(
(
d) show atomic configurations for 608 (similar to 1208), and 908
similar to 308 or 1508) deflections, respectively.
Figure 4 (a) SEM image of a coplanar NW network. Type I and II
reflections are shown by the dotted circles. (b) SEM image showing
guidedgrowthofanInNNWcausedbymultipledeflectionsfromtwo
NW guides.
been reported in our earlier publications [11, 12], the details
on the atomic arrangements at bending points are reported
here for the first time.
To obtain information on the changes in atomic
arrangements as the NW bends, TEM imaging were
performed on NWs. Figure 3(a) shows a lattice resolved junction of the two NWGs, where it then grows outward from
TEM image of a 608 spontaneous bending. The atomic the plane of the substrate. The observation of this type of
arrangements in Fig. 3(b) clearly show that the NW changes guided growth of NWs motivated us to study if the NW
its growth direction from [110] to another [110] direction, growth can be similarly controlled by lithographically
similarly in Fig. 3(c), the lattice resolved TEM image of 908 patterned barriers.
spontaneous bending is illustrated along with atomic
arrangements in Fig. 3(d). The NW direction is changed
3.2 Lithographic trench and barrier guided NW
from [110] to ½110ꢁ in this case. While getting deflected by growth To investigate if lithographically patterned bar-
interactions between many different NWs, maintaining the riers are capable of deflecting NWs, trenches were patterned
co-planarity of horizontal surface growth, they give rise to on the surface of a Si/SiO substrate (etched by buffered
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the formation of nano-networks as shown in Fig. 4(a). There oxide etch). Figure 5(a) and (b) show surface morphology
are basically two different types of deflections: (i) a NW images of a deeper and a shallower trench etched on the
getting deflected by another NW with only point of contact surface of the Si/SiO substrate using AFM in non-contact
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(
shown by dotted circles marked ‘‘I’’) and (ii) a NW getting mode. The deeper trench was etched for 1.5 min with depth
deflected by another NW, but after deflection growing ꢀ120 nm [Fig. 5(c)], and the shallower trench was etched for
parallel to the deflecting NW (shown by dotted circles 1.0 min, depth ꢀ60 nm [Fig. 5(d)]. Figures 5(e) shows an InN
marked ‘‘II’’). This case is favored if the incident NW makes NW getting deflected from the sharper wall of the trench in
an angle close to multiple of 308. Since the NWs exhibit Fig. 5(a), during the growth (marked by a dashed eclipse),
good crystalline properties at the end points they are very while Fig. 5(f) shows a NW climbing along the wall of the
attractive for realizing NEMS devices and resonators.
shallower trench (marked by a dashed square). Clearly, the
An interesting growth case was observed when a NW slope of the barrier walls influence whether the NW would
alternately got deflected back and forth between two NWs get deflected from it or climb along it. Thus, by controlling
serving as guides (NWGs). Figure 4(b) shows a NW getting the slope of a patterned wall, it should be possible to control
deflected alternately by two NWGs (at a small angle the growth direction and shapes of the NWs. To further
with each other) at 1208 (in agreement with our previous explore this concept of barrier induced deflection of NWs,
discussion on [110] growth direction) until it reaches the growth of NWs was performed in a sample with parallel
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Phys. Status Solidi A 209, No. 4 (2012)
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Figure 5 (online color at: www.pss-a.com) Surface morphology
imagesof(a)adeepertrenchand(b)ashallowertrenchonthesurface
of the Si/SiO substrate etched by BOE for 1.5 and 1.0 min, respec-
2
Figure 6 (a) Magnified SEM image of SiO
image of the guided growth by using SiO stripe barriers.
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barriers. (b) SEM
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tively. (c) and (d) show line scans of the height profile across the
trenches corresponding to the blue and red lines in (a) and (b),
respectively. (e) SEM image showing an InN NW being deflected
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.3 Measurement of elastic modulus An InN
NW was suspended across a 200 nm deep and ꢀ1 mm wide
patterned SiO wall. (f)SEMimageshowinganInNNWisclimbing
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trench on a SiO /Si sample, following procedures described
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along a BOE-etched SiO wall.
above, for measuring the elastic modulus. Figure 7(a) and (b)
show the AFM and SEM images of the NW, respectively.
stripes of 100 nm high SiO barriers (as opposed to etching as While the AFM image verifies the suspended nature of the
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described above), which are expected to give rise to very NW (clear from the height difference), the SEM image
sharp, almost vertical barriers. The SiO barrier patterns provides the dimensions of the NW, i.e., 1.1 mm in length,
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were realized by using E-beam lithography patterning and 88 nm in diameter. Figure 8(a) shows the schematic
followed by plasma enhanced chemical vapor deposition diagram of the three point measurement setup. Figure 8(b)
(
PECVD) of SiO layers. Figure 6(a) shows a magnified shows the load–displacement curves for AFM tip loading on
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SEM image of parallel barriers with the separation of 2 and the suspended NW and on the wafer surface. Since, during
mm between them. The barriers had a width of 2 mm, length loading, the cantilever also bends along with the NW, to
3
of 20 mm, and height of 100 nm. Au catalyst spots (with eliminate the cantilever bending factor, loading on the wafer
diameter of afew hundrednm) were deposited in between the
barriers to facilitate growth of NWs from them. Figure 6(b)
shows growth of InN NWs with multiple deflections from the
SiO barriers and other NWs. With closer inspection, it can
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be seen that although in most cases NWs change their growth
direction when obstructed by a barrier or another NW
sometimes they also grow along the barrier. We believe this
is due to the varying approach angles of the NWs toward
the barriers. For approach angles that are close to the
spontaneous bending angles of the NWs (i.e., multiples of
3
08), they follow the contour of the barrier. For other angles
of incidence, they would deflect backward from the barriers.
Thus, manipulation of the approach angles would enable us (3D) AFM image and (b) SEM image of a suspended InN NW
Figure 7 (online color at: www.pss-a.com) (a) Three-dimensional
to control the growth direction of the NWs.
clamped on a Si substrate by EBID clamping method.
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E. B. Quddus et al.: Structural and elastic properties of InN nanowires
where I is the moment of inertia. For a round-shaped NW,
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I ¼ pr /4, where r is the radius of the NW. The elastic
modulus calculated from Eq. (2) using k , r, and L values
NW
is ꢀ266 GPa. The experimentally measured and theoreti-
cally calculated elastic modulus for InN thin film falls in the
range 296–396 GPa [16], which is quite close to the elastic
modulus of InN NWs determined in our experiments.
Reports on elastic modulus measurement of InN in any form
are rather scant, and to the best of our knowledge, we are the
first to report elastic modulus measurement on InN NWs.
Therefore, it is not very clear to us whether the deviation in
elastic modulus of the InN NW from the thin film value is
indeed due to nanoscale dimensions of the NWs. Of course,
many other factors may also contribute to the observed
deviation in elastic modulus, including crystalline defects
and presence of oxide shell layer around the InN core. In
addition, the NW is not exactly perpendicular, and there is
some approximation of the effective length in Eq. (2), which
might also lead to some small errors.
4
Conclusions In conclusion, high quality InN NWs
has been synthesized using a CVD furnace through VLS
method, at a high growth rate of ꢀ30 mm/h. The NWs
exhibited growth directions changes either spontaneously or
when induced by a barrier, which can be in the form of other
NWs, trench walls patterned in SiO by etching, or patterned
2
SiO walls deposited by chemical vapor deposition tech-
2
nique. TEM investigations indicate good crystalline quality
of the NWs at the bend points. The elastic modulus of an
InN NW measured by three point bending method using an
AFM tip was found to be ꢀ266 GPa, which is quite close to
the reported values for InN thin films. The structural and
elastic properties exhibited by these NWs make them very
suitable candidates for applications in nanoelectromechanical
sensors.
Figure 8 (online color at: www.pss-a.com) (a) Schematic diagram
of a cantilever resting on a suspended NW clamped to a silicon
substrate, by EBID clamping method, for 3-point measurement. (b)
Load displacement plots for loading on the suspended NW, on the
wafer surface, and the calibrated NW.
Acknowledgements Financial supports for this work
from National Science Foundation (Grant ECCS-0846898) and
Army Research Office (Grant W911NF-08-0299) are thankfully
acknowledged. We would also like to acknowledge Dr. Ghoshroy
and Douglas Blom of the USC Electron Microscopy Center for their
help with TEM measurements, and Dr. Samir Garzon for helpful
surface was performed separately using the same cantilever.
The spring constant of NWs is then given as [14]
1
kNW ¼
;
(1) discussions.
1
1
ꢂ
kMeasured
kWafer
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