Crystal face-dependent nanopiezotronics of an obliquely aligned InN nanorod array.

Abstract

This paper proposes an obliquely aligned InN nanorod array to maximize nanorod deformation in the application of nanopiezotronics. The surface-dependent piezotronic I-V characteristics of the InN nanorod array with exposed polar (0002) and semipolar ( ̅1102) planes were studied by conductive atomic force microscopy. The effects of the piezopotential, created in the InN under straining, and the surface quantum states on the transport behavior of charge carriers in different crystal planes of the InN nanorod were investigated. The crystal plane-dependent electron density in the electron surface accumulation layer and the strain-dependent piezopotential distribution modulate the interfacial contact of the Schottky characteristics for the (0002) plane and the quasi-ohmic behavior for the ( ̅1102) plane. Regarding the piezotronic properties under applied forces, the Schottky barrier height increases in conjunction with the deflection force with high current density at large biases because of tunneling. The strain-induced piezopotential can thus tune the transport process of the charge carriers inside the InN nanorod over a larger range than in ZnO. The quantized surface electron accumulation layer is demonstrated to modulate the piezopotential-dependent carrier transport at the metal/InN interfaces and become an important factor in the design of InN-based piezotronic devices and nanogenerators.

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Crystal Face-Dependent Nanopiezotronics of an Obliquely Aligned
InN Nanorod Array
Nai-Jen Ku,†Jun-Han Huang,†Chao-Hung Wang,†Hsin-Chiao Fang,†and Chuan-Pu Liu*,†,‡,§,∥
†Department of Materials Science and Engineering,‡Center for Micro/Nano Science and Technology,§Research Center for Energy
Technology and Strategy, and∥Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan
*
S Supporting Information
ABSTRACT: This paper proposes an obliquely aligned InN nanorod array to maximize nanorod deformation in the application
of nanopiezotronics. The surface-dependent piezotronic I−V characteristics of the InN nanorod array with exposed polar (0002)
and semipolar (1̅102) planes were studied by conductive atomic force microscopy. The effects of the piezopotential, created in
the InN under straining, and the surface quantum states on the transport behavior of charge carriers in different crystal planes of
the InN nanorod were investigated. The crystal plane-dependent electron density in the electron surface accumulation layer and
the strain-dependent piezopotential distribution modulate the interfacial contact of the Schottky characteristics for the (0002)
plane and the quasi-ohmic behavior for the (1̅102) plane. Regarding the piezotronic properties under applied forces, the Schottky
barrier height increases in conjunction with the deflection force with high current density at large biases because of tunneling.
The strain-induced piezopotential can thus tune the transport process of the charge carriers inside the InN nanorod over a larger
range than in ZnO. The quantized surface electron accumulation layer is demonstrated to modulate the piezopotential-
dependent carrier transport at the metal/InN interfaces and become an important factor in the design of InN-based piezotronic
devices and nanogenerators.
KEYWORDS: InN nanorods, molecular beam epitaxy, glancing angle deposition, surface accumulation, piezotronic effect,
surface quantum state
W
has become more attractive. Recently, based on the piezo-
electric and semiconductor properties, a new field of
piezotronics1,2has been created as basic building blocks for
fabricating innovative devices such as nanogenerators,3−6
piezopotential-gated field effect transistors,7,8piezoelectric
diodes,9piezoelectric logic nanodevices,10,11piezoelectric
chemical sensors,12and piezophototronic devices,13−15which
uses the effects of piezoelectric potential created in the crystal
for controlling or tuning the charge carrier transport character-
istics to fabricate mechanical electronic devices. The
fundamental principle of the piezotronic devices is to control
the carrier transport by creating a piezopotential within the
semiconductor through the application of a strain.2Therefore,
the polarity-dependent piezopotential distribution inside piezo-
electric semiconductor materials becomes an important factor
for the design of nanopiezotronic devices. Since 2005, when
ZnO nanomaterials matured, various unique piezotronic
ith natural resources being depleted at an ever fast rate,
it is not surprising that research into alternative energy
nanodevices based on ZnO micro/nanowires have been
realized.3To develop a higher output power nanogenerator,
materials with higher piezoelectric coefficients such as ZnS,16
CdS,17GaN,18,19InN,20and mechanisms for larger deforma-
tions are being investigated, but the research is only in its
infancy. Among all the promising piezoelectric nanomaterials
that have been studied, a single-InN-nanowire nanogenerator is
distinct with its largest output voltage of up to 1 V,20which is
more than 10 times higher than that of conventional ZnO
nanowires. Therefore, InN could be a candidate for high-output
power nanogenerator devices. Moreover, the current nano-
generators incorporating vertically aligned nanowires function
mainly by shear forces under which the total deformation is
restricted. To provide the maximum deformation under a
Received:
Revised:
August 11, 2011
December 16, 2011
Letter
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simple normal force for the largest piezoelectric potential, we
propose an InN nanorod (NR) array aligned obliquely with
deformation along the direction of the piezoelectric field.
InN is well-known as having the lowest electron effective
mass for applications in high-speed electronic devices. In
contrast to ZnO, InN uniquely possesses an inversion layer of
electrons at the surface, resulting in downward surface band
bending, which affects the optical properties greatly.21−23
According to first principle calculations, the microscopic origin
of the surface electron accumulation is the donor type surface
states caused by In−In bonding.24The surface electron
accumulation layer is universal and has been observed for c-
plane,25a-plane,25and m-plane InN26within a distance
typically of the order of 10 nm. The influence of the
quantization of the electron accumulation layer because of
the nanoscale at the surface or interface on the electron
transport of InN nanostructures has been recently re-
ported.27−30This quantized surface electron accumulation
layer (QSEAL) causes an interesting resistance anomaly in
which the InN nanowire resistance increases with the cross-
sectional area when the wire radius is small.27A three-region
model31has been developed to characterize the contribution of
InN conductivity from the bulk electron density, quantized
surface electron accumulation layer, and defect donors.
Increasing InN conductivity will further lessen internal
consumption by the inner resistances of the devices, such as
the output voltage of a nanogenerator. However, the
piezopotential is also reduced because of the screen-effect
caused by the excess electrons, but the excess electrons cannot
deplete the piezopotential completely.32Therefore, InN NR
with a conductivity below an optimum value generated the
largest output voltage among the nitride nanogenerators.33
Although the degree of surface band bending in InN is crystal
plane-dependent, few facet-dependent electrical transport
properties have been explored.28,30These unique physical
properties of InN differ drastically from ZnO and are expected
to increase the efficiency of nanopiezotronic devices
immensely.
This study investigates the contribution of the QSEAL at the
NR surface, together with the nanopiezotronic effect on the
electron transport of an orientation-controlled InN NR. This
obliquely aligned nanostructure provides a unique geometry
with two-faceted surfaces: a common c-(0002) plane on top
and six r-(1̅102) side planes. The local transport properties are
studied on the formation of the Schottky barrier (SB) relative
to the surface morphology and piezotronic properties of InN
nanostructures. The degree of surface band bending with the
QSEAL for the c-plane and r-plane in relation to the I−V
character and the piezotronics are also studied. The distribution
of the carrier density across a NR has a profound influence on
the output voltage of a nanogenerator.33The higher electron
density inside the NR ensures a reduced inner resistance but
the piezopotential is also reduced by screening the piezoelectric
field. Nevertheless, the overall effect of combining a high
piezoelectric effect with the electron distribution in this study
might help improve the efficiency of the InN nanogenerator.
The orientational control of the obliquely aligned epitaxial
InN NR array was achieved by glancing angle deposition with a
plasma-assisted molecular beam epitaxy (MBE) system. Si(111)
was employed as the substrate, which was precoated with a
ZnO buffer layer composed of a faceted ZnO nanopillar array
approximately 100 nm in height using RF magnetron
sputtering. During growth, the indium/nitrogen ratio was
maintained at 480 without using any catalysts, and the substrate
temperature was held at 400 °C. The incident molecular beam
subtended an angle of approximately 60° relative to the
substrate surface. The morphology and crystallography of the
InN NRs were characterized by field emission scanning
electron microscopy (FE-SEM, JEOL JSM-7000F) and high-
resolution transmission electron microscopy (HR-TEM, JEOL
JEM-2100F). The force-dependent nanopiezotronic measure-
ments on individual NRs were performed by conductive atomic
force microscopy (C-AFM, Seiko SPA400) in contact mode at
room temperature. The freshly cut Pt/Ir coated tip of
tetrahedral shape with a cone angle of 25° was used for the
C-AFM. A silver electrode was used to guarantee contact with
the sample and the C-AFM stage with the tip grounded. (For
more information, please see Supporting Information, Figure
1S).
Figure 1a shows a cross-sectional SEM image of an obliquely
aligned InN NR array, exhibiting a fairly uniform height of
approximately 1 μm and an average diameter of approximately
82 nm. The InN NRs preferentially subtend an angle of ∼58°
to the Si substrate. Figure 1b shows a TEM image of a single
oblique InN NR. The corresponding nanobeam electron
diffraction pattern in Figure 1c taken along the [112̅0] zone
axis indicates that the NR has a wurtzite structure grown along
the [11̅01] direction. The InN NR in this study was verified to
have In polarity by TEM convergent-beam electron diffraction
(not shown). Figure 1d is the HRTEM image of the boxed
region in Figure 1b. The InN NR is composed of a top (0002)
plane and six {1̅102} sidewalls. Figure 1c,d shows that the InN
NRs are single-crystalline and free of dislocations. The presence
of a QSEAL is confirmed by the coupled plasmon-LO-phonon
mode21,26at ∼435 cm−1in the μ-Raman spectrum (Supporting
Information, Figure 2S) with the electron concentration of the
order of 1019∼1020cm−3close to the NR surface.
Figure 1. Morphology of obliquely aligned InN NRs. (a) A cross-
section SEM image of an obliquely aligned InN NR array; (b) TEM
image of a single InN NR; (c) corresponding nanobeam electron
diffraction pattern of the single InN NR; and (d) high-resolution TEM
image of the InN NR at the marked region in (b). The lattice fringe
spacing of 0.564 nm is consistent with the d0001spacing of the
hexagonal InN structure.
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Figure 2a shows the I−V characteristics measured by the
probe of the C-AFM held stationary at a spot on the (0002)
and (1̅102) plane of an InN NR. The nonlinear and asymmetric
characteristics are typical of the Schottky barrier height (SBH)
between the metal electrode and InN. Wener et al.27showed
that the resistance of the shell and core of an InN NR are equal
at a critical diameter ∼55 ± 15 nm. Therefore, the electron
conduction in the InN NRs is dominated by the core rather
than the surface electron accumulation layer for their diameters
are larger than the critical diameter. For any given voltage, the
current is lower for the (0002) plane than the (1̅102) plane,
indicating the involvement of different SBH. From the I−V
characteristics, the SBH is changed from demonstrating
rectifying behavior for the (0002) plane to quasi-ohmic
characteristics for the (1̅102) plane at V > 0.3 V or V < −0.2
V because of the increased surface electron density.34This
behavior is analogous to turning an SB into an ohmic contact
through a highly doped surface layer. For the Schottky contact
formed at the interface between the metal and InN shown in
Figure 2c, a certain quantity of electrons could accumulate in a
region near the surface because of the downward surface band
bending and could tunnel through the SBH due to the tip-
induced high electrical field under an appropriate bias
voltage.34,35
The electron conduction through the Schottky diodes for the
(0002) and the (1̅102) plane is confirmed to be caused by the
classic thermionic emission-diffusion (TED) mechanism, as
evidenced by the perfect match with the logarithmic plot of the
current with V1/4, as shown in Supporting Information Figure
3S. Assuming the temperature (T) and the donor concentration
(ND+) are the same between these two planes over the entire
InN NR and neglecting the different contact area of the SB, the
ratio of the current density between these two planes can be
expressed by
φ
≈
** + Δ **
**
A
−Δ
̅
̅
⎛
⎜
⎜
⎜
⎜
⎜
⎝
⎞
⎟
⎟
⎟
⎟
⎟
⎠
I
I
AA
kT
exp
(0002)
(1102)
s
(0002)
(1102)
(1)
where I(0002)and I(1̅102)are the current through the InN NR for
the (0002) and (1̅102) plane, respectively, ΔA** is the change
in the effective Richardson constant and Δφsis the change in
SBH between the (0002) and (1̅102) planes. Since A** is only
a function of stress dependent effective mass, the change in the
effective Richardson constant ΔA** ≪ A**, φscan thus be
deduced as2,36,37
⎛
⎝
I
(1102)
Δφ
̅
≈ −
|
|
̅
⎜⎜
⎞
⎠
⎟⎟
kT
I
(0002)
ln
V
V
s
(0002)
(1102)
(2)
By calculating the SBH difference between the two planes from
eq 2, the SBH for the (1̅102) plane is determined to be lower
than that of the (0002) plane by roughly 58.94 ± 1.33 meV.
The SB formation between an InN NR and a Pt/Ir tip can be
visualized in Figure 2b. The actual barrier height not only relies
on the difference between the work function (eφm) of the metal
tip and electron affinity (eχ) of the InN NR, but also on the
image force, electrical field penetration, and the existence of the
QSEAL layer at the surface.34Figure 2c shows the SBH formed
Figure 2. (a) I−V curves measured at (0002) vs (1̅102) plane fitting with thermionic emission-diffusion model. (b) Schematic energy band diagram
of metal to (0002) and (1̅102) plane before contact. φtipand φInNare the work function of the Pt/Ir tip and InN, respectively; χsInNis the electron
affinity at an InN surface; and u0is the potential of the surface band bending relative to the Fermi level. (c) Schematic energy band diagram of
Schottky contact at (0002) and (1̅102) plane. (d) Conductance vs bias curve measured at (0002) and (1̅102) plane.
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at both interfaces for the (0002) and (1̅102) planes. It has been
reported that the degree of surface band bending for nonpolar
plane is larger than for the polar c-(0002) plane by
approximately 0.11 eV.25Therefore, based on the crystallo-
graphic similarities, it is reasonable to assume that the 2D
surface electron density accumulating at the (1̅102) plane is
higher than at the (0002) plane, as indicated in Figure 2(b). As
the Fermi level lines up for the formation of the Schottky
contact, the different conduction/valence band edges (u0
and u0
between the (0002) and (1̅102) surface, leading to the
emergence of a potential well near the surface. The degree of
surface band bending agrees with the SBH value calculated
above using eq 2.
The minimum energy of the quantized two-dimensional
electron subbands of the QSEAL layer for the (0002) and
(1̅102) plane can be observed in the plot of conductance (dI/
dV) versus voltage shown in Figure 2d.38,39The electrons in a
given subband Fnare calculated to have the same bound-state
wave function. Therefore, the conductance variation is assumed
to arise from the step discontinuity of the density of state
function at the subband minimum of the surface accumulation
layer.38,39The quantized energy levels determined by the
second derivatives d2I/dV2for the (0002) plane are V1
mV and V2
these two subband minima is V1,2
well with that of the two confined subband minima (k∥= 0),
which are 0.80 and 0.51 eV below the Fermi level, as measured
(0002)
(1̅102)) of InN bend upward to yield different SBH values
(0002)= 97
(0002)= 304 mV. Therefore, the energy difference of
(0002)= 207 mV, which agrees
by the angle resolved photoemission spectrum results.39,40A
further increase in surface state density would lead to a deeper
potential well at the surface, increasing the number and depth
of the subbands at which electrons are bound.39For the (1̅102)
plane with higher surface electron density, three subband
minima are observed in Figure 2d, V1(1̅102)= 172 mV, V2
274 mV, and V3
that the potential well for the (1̅102) plane is deeper than for
the (0002) plane. More quantitative comparisons of electron
subband minima and surface electron density for both planes
are made and discussed in Supporting Information and Figure
4S. Quantum effects become more apparent as the width of the
QSEAL layer is reduced from the (0002) to (1̅102) plane.
The C-AFM topographic and current images41,42of opposite
scanning directions shown in Figure 3a−c are expected to
provide deeper insights into the localized electrical properties.
The substrate bias was maintained at 0.5 V relative to the probe
(ground). Although the contact area differs between the top
and side surfaces, highly conductive paths are formed at the
(1̅102) plane over the (0002) plane, irrespective of the
scanning directions, as shown in Figure 3c, revealing an
asymmetric current distribution around the InN NR top
surface. Figure 3d shows the dual cross sections of the height
and current profiles along the line marked in Figure 3a,b.
Further examination of Figure 3d shows that the tip-moving
current values measured from the (1̅102) planes of individual
InN NRs exceed the current limit of the C-AFM as the tip
scanning along the [1̅100] direction and are significantly higher
(1̅102)=
(1̅102)= 435 mV, which is consistent with the fact
Figure 3. (a) Surface morphology and (b) probe current (the InN NR-to-tip bias is 0.5 V) images for the obliquely aligned InN NR array. (c) Probe
current images obtained along the scan direction [11̅00] (left) and [1̅100] (right). (d) Dual cross section of InN NRs with the topography and
current signals superimposed. An increased current signal is observed on one side of an InN NR (1̅102) plane.
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than the tip-stationary current value of 70 nA shown in Figure
2b. Conversely, once the tip scanning direction was reversed to
the [11̅00] shown in Figure 3c, the sidewall current signals
decreased to 50−70 nA. Nevertheless, no current is measured
at the (0002) surfaces for both scanning directions. During
scanning, the tip exerted a deflection force of 1 nN, which
creates a transverse strain field and, thus, a piezopotential.20
The current increase and decrease at the (1̅102) plane are
clearly caused by the inversion of the piezopotential-induced
barrier height change. Nevertheless, high current regions always
correspond to the side surfaces, irrespective of scanning
direction, confirming the lower barrier height between the
Pt/Ir tip and the InN (1̅102) plane for the higher surface
electron concentration compared to the (0002) plane.
Conversely, current suppression to zero at the (0002) plane
is caused by a combined gating effect with a piezopotential-
induced barrier height increase. For the gating effect, free
electrons trapped at the tensile bending side of the NR for the
positive piezopotential that form a charge depletion zone
around the compressive bending side of the NR. Consequently,
when the Pt/Ir tip was swept across a NR, the width of the
conducting channel was reduced under the influence of the
shear forces. Hence, the piezoelectric field established across
the NR restrains the current signal measured at the (0002)
plane. This effect would dominate the current suppression
phenomenon, as confirmed by reversing the tip scan direction.
For the other effect, Figure 4 shows that the tip deflection force
increases in conjunction with the piezopotential-induced barrier
height. Therefore, the modulation of the carrier transport could
be a combinatory effect of the controlled piezopotential and the
SBH change associated with the (0002) plane and the (1̅102)
plane.
The SB formation at the metal−semiconductor interfaces
represents the fundamental mechanism of the nanogenerators.
The nanopiezotronic effect of the oblique InN NR is studied by
applying a deflection (normal) force to the AFM Pt/Ir tip on a
NR. Figure 4a shows a set of I−V curves with various deflection
forces ranging from ∼0 to 4 nN, which exhibit the asymmetric
characteristics of ideal Schottky diodes with different SBH
confirmed by perfect fits with TED model, as shown in
Supporting Information Figure 5S. The current reduction effect
with increasing tip deflection forces is a combination of the
piezopotential gating effect that reduces NR conductance and
the force-dependent SBH change. When an oblique NR is
pressed downward by a normal force, a strain field is created
with the exposed side plane stretched and the other side plane
compressed. Therefore, a piezoelectric potential is created
inside the NR with a positive and negative piezopotential for
the stretched and compressed side, respectively.2,36When the
deflection force was increased, the current for a given bias
under both positive and negative bias dropped significantly.
These asymmetric I−V curves are attributed to the different
SBH formed between both metal/InN interfaces at the two
ends of a NR. The strain distribution within an oblique InN NR
in this case is by no means homogeneous because of the
complex shape and different degree of deformation involved at
different heights and will not be further analyzed. When an
oblique NR is depressed, an asymmetric strain distribution
Figure 4. (a) Typical I−V characteristics of an InN NR at different tip deflection forces. (b) Schematic diagram showing the mechanism of the
charge depletion region created in the compressive strain region. (c) The derived change in SBH at a tip−substrate bias of 1.0 V based on the TED
model and the ideality factor in eq 3 as a function of tip deflection force.
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