Copyright © 2011 American Scientific Publishers
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Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 11, 1–4, 2011
Structural, Optical and Electrical Properties of
Nitrogen Ion Implanted ZnO Nanorods
C.-H. Kwak1 , Y.-B. Lee1 , S.-Y. Seo1 , S.-H. Kim1 , C.-I. Park2 , B.-H. Kim2 ,
Zhenlan Jin3 , and S.-W. Han3 ∗
1
Department of Materials Science and Engineering, Pohang University of Science and Technology,
Pohang 790-784, Korea
2
Department of Physics, Chonbuk National University, Jeonju 561-756, Korea
3
Division of Science Education, Institute of Fusion Science, and Institute of Science Education,
Chonbuk National University, Jeonju 561-756, Korea
This study examined the micro-structural and electrical properties of N+ -ion-implanted ZnO
nanorods. Nitrogen ions with energies of 10–90 keV and beam fluxes of 1013 –1016 ions/cm2 were
implanted on vertically-aligned ZnO nanorods. Energy dispersive X-ray spectroscopy measurements
showed that N+ ions were spread uniformly over the nanorods. Extended X-ray absorption fine
structure measurements revealed that the implanted N+ s had partially substituted for the oxygen
sites. Photoluminescence measurements showed a neutral-donor bound exciton peak at 3.36 eV
and a two-electron-satellite peak at 3.33 eV independent of the ion energy and flux. The I–V characteristic curves showed that the current density was not changed by the N+ ion energy and flux
much. These results strongly suggested that the N ions substituted for the oxygen sites were neutral.
1. INTRODUCTION
Vertically-aligned ZnO nanorods can be building blocks
for nanodevices, including light-emitting diodes, gas sensors, biosensors, and actuators.1 Vertically-aligned ZnO
nanorods can be synthesized relatively easily by various
growth techniques. ZnO nanorods are light n-type due to
Zn-rich. However, the charge carrier density of natural
ZnO nanorods is insufficient for practical device applications. Furthermore, p-type doping of ZnO nanorods are
considerably difficult. There are some attempts to dope
nanorods with small amount of impurity including hydrogen, Ga, In, or As.2–7 However, some research groups
observed n-type characteristics of doped ZnO nanororods
while the other groups reported p-type properties.
The discrepancy of results between research groups can
be attributed to the interstitial replacement of impurities
and structural disorder due to the impurity. In order to
achieve a higher charge carrier density in ZnO nanorods,
we used nitrogen ion (N+ ) implantation techniques
because the size of nitrogen is similar to oxygen and can
be N− or N0 in the oxygen site of ZnO.8 We employed
polarization-dependent extended X-ray absorption fine
structure (EXAFS) to clarify the location of implanted
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2011, Vol. 11, No. xx
N+ ions in ZnO nanorods. Polarization-dependent EXAFS
can describe the orientation-dependent structural properties, including coordination number, atomic species, bond
length, and bond length distribution.9 10
2. EXPERIMENTAL DETAILS
Vertically-well aligned ZnO nanorods were synthesized on
Al2 O3 (0001) substrates using a catalyst-free metal-organic
chemical vapor deposition procedure. The details of ZnO
nanorod growth are published elsewhere.1 ZnO nanorods
with mean diameter and length of 70 nm and 1 m, respectively, were vertically exposed to N+ ion beam with energies of 10–90 keV and ion fluxes of 1013 − 1016 ions/cm2
in a vacuum. Ti (150 nm)/Au (50 nm) bilayers for the
Ohmic contact were used for the I–V measurements.
Polarization-dependent EXAFS measurements at the Zn
K-edge (9659 eV) were performed in a fluorescence mode
by selecting the incident X-ray energy with a three-quarters
tuned Si(111) double monochromator at the 3C1 beamline
of the Pohang Light Source at room temperature.
3. RESULTS AND DISCUSSION
Figure 1 shows energy dispersive X-ray spectroscopy
(EDS) images of the N+ -ZnO nanorods, which
1533-4880/2011/11/001/004
doi:10.1166/jnn.2011.4835
1
RESEARCH ARTICLE
Keywords: ZnO, Nanorod, Ion Implantation, Structure, Photoluminescence, EXAFS, I–V .
Structural, Optical and Electrical Properties of Nitrogen Ion Implanted ZnO Nanorods
Kwak et al.
RESEARCH ARTICLE
Fig. 1. EDS images of Zn, O, and N from the N+ -ZnO nanorods with
different N+ energies of 50, 70, 90 keV and ion flux of 1016 ions/cm2 .
demonstrated that N+ s were distributed uniformly over
the entire nanorod without depending on the N+ energy.
This was unexpected because a model calculation with the
SRIM code11 indicated that the mean penetration depth
of N+ ions with energy of 90 keV to ZnO was approximately 0.2 m only. It should be noted that the average
length of the nanorods were over 1 m. The N+ s likely
reached to the nanorod bottom through the space between
nanorods.6 Density of nitrogen atoms in the N+ -ZnO
was approximately 1/3 of oxygen density, corresponding
to 14 × 1022 atoms/cm3 . The nitrogen densities of the
N+ -ZnO nanorods with different ion energies of 50, 70,
and 90 keV were nearly identical. This is further evidence
that the nitrogen ions with the 50–90 keV fully penetrated
through the nanorods.
The orientation-dependent local structures around the
Zn atoms of the nanorods were examined by polarizationdependent EXAFS. Figure 2 show the Fourier-transformed
EXAFS from ZnO nanorods with different conditions
in r-space. IFEFFIT software12 and standard analysis
procedures9 10 were used for EXAFS data analysis.
In wurtzite structured ZnO, a Zn atom has four oxygen
atoms (one O(1) in the c-axis and three O(2)s off by 19
from the ab-plane) and 12 zinc atoms (six Zn(1)s placed
at 55 from the ab-plane and six Zn(2)s in the ab-plane),
as the first and second neighboring atoms, respectively.10
The first and second peaks in Figure 2 correspond to
the neighboring atoms of a zinc atom. The peak position was shifted from the actual bond length by approximately 0.3 Å because the phase shift of the back-scattered
X-rays was not counted. The two sets of EXAFS data
in r-space were fitted simultaneously to the theoretical
EXAFS calculation,13 as shown in Figures 2(c) and (d).
The details of polarization-dependent EXAFS data fits are
published elsewhere.9 10
2
Fig. 2. Magnitude of the Fourier-transformed polarization-dependent
EXAFS from N+ -ZnO nanorods with different N+ energies and ion flux
of 1016 ions/cm2 as functions of distance from a Zn atom. (a) and (b)
were measured from N+ -ZnO nanorods at the incident X-ray electric field
parallel and perpendicular to the nanorod length, respectively. (c) and (d)
are polarization-dependent EXAFS from N+ -ZnO nanorods with the N+
energy of 90 keV and flux of 1016 ions/cm2 . The dotted and solid lines
are the data and best fits. For the Fourier transform, the Hanning window
with the windowsill width of 1.0 Å−1 was used.
The fit revealed negligible N+ substitution for the zinc
sites with an uncertainty of 5%. The EXAFS fit results
are summarized in Table I. EXAFS analysis revealed that
the atomic pairs of Zn–O(2) and Zn–Zn(1) in the N+ -ZnO
nanorods were slightly shrunken whereas the Zn–Zn(2)
pairs were expended, compared to the as-grown nanorods.
The angle between the ab-plane and O(2) is approximately
147 for the N+ -ZnO nanorods whereas it was 187 for the
as-grown ZnO nanorods. This demonstrated that the O(2)s
moved toward the middle of the Zn–Zn triangles, causing the expansion of Zn–Zn(2) bond length. EXAFS barely
distinguish between oxygen and nitrogen due to its resolution limit. Theoretical calculation suggested that the bond
length of Zn–N pairs in N-added ZnO was approximately
0.12 Å shorter than that of Zn–O pairs.14 From the EXAFS
results, the local structural distortion strongly suggested
that the oxygen sites were partially substituted by nitrogen atoms because the bond lengths of atomic pairs would
change equally in all directions when the implanted N+ s
were placed interstitially. The displacement ( 2 , including
thermal vibration and static disorder) of the Zn–O pairs in
the N+ -ZnO nanorods was unchanged, whereas that of the
Zn–Zn pairs was increased substantially. The zinc atoms in
the nanorods were more affected by the N+ -ion implantation because the cross section of Zn ions was approximately
3 times larger than that of O ions.
J. Nanosci. Nanotechnol. 11, 1–4, 2011
Kwak et al.
Structural, Optical and Electrical Properties of Nitrogen Ion Implanted ZnO Nanorods
Table I. Results of EXAFS data fits. d and 2 are the bond length and displacement of atomic pairs in the N+ -ZnO nanorods, respectively. S0 2 of
0.90(5) was used.10
As-grown
Pair
Zn–O(1)
Zn–O(2)
Zn–Zn(1)
Zn–Zn(2)
50 keV
2
2
70 keV
2
90 keV
d(Å)
(Å )
d(Å)
(Å )
d(Å)
(Å )
d(Å)
2 (Å2 )
19774
19833
32795
32567
00031
00061
00101
00101
19746
19598
32319
32796
00041
00041
00121
00101
19755
19675
32346
32916
00041
00051
00131
00111
19745
19505
32276
32696
00031
00051
00121
00101
2
2
in the nanorods.15 This agrees well with the EXAFS
results.
The electrical properties of the N+ -ZnO nanorods with
various N+ energies and fluxes were examined by I–V
measurements. The current density was not changed much
with the increase of N+ ion energy and flux. This result
suggests that the carrier charge density does not correspond linearly to the amount of implanted N+ ions, and
that the only N+ s in the oxygen sites contribute the current density. The I–V results demonstrated that the number of nitrogen atoms substituted for the oxygen sites of
N+ -ZnO nanorods was not changed much with the N+ ion energy and flux ranges of of 10–90 keV and 1013 –
1016 ions/cm2 . The current density is much higher than
previous reports.17 18 This strongly suggests that the nitrogen atoms in the oxygen sites are neutral and that the
N+ -ZnO nanorods are n-type. A theoretical study reported
that stable nitrogen in ZnO can be a N0 or N− .8 If the
implanted N+ ions are in the −1 charge state, N+ -ZnO
will have a very low charge carrier density because N+
ions require 2 electrons to have the −1 charge state.
4. CONCLUSION
A N+ -implantation technique was used to dope vertically
aligned ZnO nanorods. EDS demonstrated that N+ ions
spread randomly over entire nanorods. EXAFS revealed
that the implanted N+ ions were partially substituted for
the oxygen sites and that a vacancy or an extra atom
did not exist. PL measurements showed neither oxygen
vacancy nor N+ substitution for the zinc sites. The I–V
measurements revealed that the N+ -ZnO nanorods were
n-type and that the nitrogen in the ZnO nanorods was neutral. This study demonstrates that ion-implantation techniques can be widely used to dope nanostructures with no
significant structural deformation.
Fig. 3. PL from the N+ -ZnO nanorods (a) at the same energy of 50 keV
and different fluxes and (b) at the same flux of 1016 ions/cm2 and different
energies at 15 K, respectively. In (a) the ion flux of 1013 , 1014 , 1015 , and
1016 ions/cm2 from the top. In (b) 10, 30, and 50 keV from the top.
(c) and (d) show I–V characteristic curves from the N+ -ZnO nanorods
at the same energy and different flux, and at the same flux and different
energies, respectively.
J. Nanosci. Nanotechnol. 11, 1–4, 2011
Acknowledgment: The work was conducted under the
auspices of the Basic Science Research Program through
the National Research Foundation of Korea (NRF) grant
funded by the Korea government (MEST) (no. KRF-2007313-C00262), Mid-career Researcher Program through
NRF grant funded by the MEST (no. 2009-0085915), and
the MEST through the PEFP User Program.
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RESEARCH ARTICLE
Photoluminescence (PL) measurements of the N+ ZnO nanorods with different conditions are shown in
Figures 3(a) and (b). We observed clear two peaks from
N+ -ZnO nanorods with N+ energy of 50 keV and a
flux of 1013 ions/cm2 . The two peaks correspond to a
neutral-donor bound exciton peak (D0 X) and two-electronsatellite (TES) peak, respectively, compared with previous studies of as-grown ZnO nanorods and ZnO films.15 16
The peak positions of N+ -ZnO with different fluxes were
not changed much. However, the peak intensities were
decreased dramatically, as the N+ -ion flux was increased.
The PL peak positions were also not changed much with
the different N+ -ion energies, as shown in Figure 3(b),
while the PL peak intensity was decreased, as the N+ -ion
energy was increased. The PL peak intensity change with
the ion energy and flux were caused by local point defects
due to N+ -ion implantation. Previous studies demonstrated
that the structural distortion of ZnO nanorods was more
serious at higher energy and more flux of N+ ions.6 7
No deep level emission was observed, suggesting negligible replacement of N+ s in the Zn sites and little vacancy
2
Structural, Optical and Electrical Properties of Nitrogen Ion Implanted ZnO Nanorods
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RESEARCH ARTICLE
Received: 1 November 2010. Accepted: 16 February 2011.
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J. Nanosci. Nanotechnol. 11, 1–4, 2011