The effect of excimer laser annealing on ZnO nanowires and their field effect transistors.

Abstract

We have investigated the effect of excimer laser annealing on the chemical bonding, electrical, and optical properties of ZnO nanowires. We demonstrate that after laser annealing on the ZnO nanowire field effect transistors, the on-current increases and the threshold voltage shifts in the negative gate bias direction. These electrical results are attributed to the increase of oxygen vacancies as n-type dopants after laser annealing, consistent with the shifts towards higher binding energies of Zn 2p and O 1s in the x-ray photoelectron spectroscopy analysis of as-grown nanowires and laser-annealed ZnO nanowires.

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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 20 (2009) 095203 (7pp)
The effect of excimer laser annealing on
ZnO nanowires and their field effect
transistors
doi:10.1088/0957-4484/20/9/095203
Jongsun Maeng, Sungho Heo1, Gunho Jo, Minhyeok Choe,
Seonghyun Kim, Hyunsang Hwang and Takhee Lee2
Department of Materials Science and Engineering, Gwangju Institute of Science and
Technology, Gwangju 500-712, Korea
E-mail: tlee@gist.ac.kr
Received 15 October 2008, in final form 2 January 2009
Published 6 February 2009
Online at stacks.iop.org/Nano/20/095203
Abstract
We have investigated the effect of excimer laser annealing on the chemical bonding, electrical,
and optical properties of ZnO nanowires. We demonstrate that after laser annealing on the ZnO
nanowire field effect transistors, the on-current increases and the threshold voltage shifts in the
negative gate bias direction. These electrical results are attributed to the increase of oxygen
vacancies as n-type dopants after laser annealing, consistent with the shifts towards higher
binding energies of Zn 2p and O 1s in the x-ray photoelectron spectroscopy analysis of
as-grown nanowires and laser-annealed ZnO nanowires.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
One-dimensional systems, such as nanowires and nanotubes,
have attracted interest due to their potential applications
as building blocks for nanoscale transistors, sensors, and
optoelectronic devices [1–4]. ZnO nanowires especially have
received considerable attention due to their unique properties,
such as a wide direct bandgap (∼3.37 eV), a large exciton
binding energy (∼60 meV), and piezoelectricity [3–7]. As a
post-fabrication process, thermal annealing or laser annealing
has been employed in order to improve the electrical properties
of ZnO materials [8, 9]. In particular, laser annealing is a
useful method because the nanosecond pulse duration of an
excimer laser can induce low thermal budget processing and
heating confinement near the surface region [10, 11].
example, Aoki et al studied ZnO p–n diode fabrication by
phosphorous doping as p-type dopants, using the KrF excimer
laser annealing of an Zn3P2 film on a single n-type ZnO
wafer [12]. Oh et al reported a high quality Pt Schottky contact
to an n-type ZnO wafer, improving the electrical conductivity
of the n-type ZnO wafer after KrF excimer laser annealing [9].
In addition, the effects of KrF excimer laser annealing on
For
1Present address: Samsung electronics, Nongseo-dong, Giheung-gu, Yongin,
Gyeonggi-do 449-711, Korea.
2Author to whom any correspondence should be addressed.
the optical and structural properties of ZnO films have also
been studied [13–15].However, compared with the many
studies about the laser annealing on ZnO bulk and films, little
research hasbeen doneontheeffects of laserannealingonZnO
nanowires.
In this study, we investigated the effects of excimer laser
annealing on ZnO nanowires and their field effect transistors
(FETs). The structural, chemical bonding, optical, and
electrical properties of ZnO nanowires and FET devices before
and after laser annealing were studied. With recrystallization
on the ZnO nanowire surface by excimer laser annealing, the
capability of generating oxygen vacancies as n-type dopants is
demonstrated.
2. Experimental details
The ZnO nanowires in this study were synthesized by the
vapour–liquid–solid (VLS) mechanism using a carbothermal
reduction process with hot-wall chemical vapour deposition.
A c-plane sapphire substrate was coated with a ∼2 nm thick
gold film using an electron-beam evaporator. The gold film
on the substrate was transformed into molten nanoparticles
during the process of heating, and these nanoparticles served
as catalysts for nanowire growth in the VLS mechanism. The
mixedsources of ZnO and graphite (4:1 ratio) and the substrate
0957-4484/09/095203+07$30.00
© 2009 IOP Publishing LtdPrinted in the UK
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Nanotechnology 20 (2009) 095203J Maeng et al
Figure 1. (a) FESEM image of as-grown ZnO nanowires on Au-coated sapphire substrate. ((b)–(d)) FESEM images of laser-annealed
ZnO nanowires at fluences of 50, 100, and 200 mJ cm−2, respectively.
on the alumina boat were inserted into the tube furnace. The
tube was sealed and pumped with a mechanical pump. The
mixture gas of N2(99%)/O2(1%), which serves as the source
of elemental oxygen for nanowire growth, was supplied at a
flow rate of 100 sccm, and the tube was filled with this gas at
700 Torr. Then, the temperature was increased to 920◦C and
held for 20–30 min during the nanowire growth. The grown
ZnO nanowires were examined with a field emission scanning
electron microscope (FESEM, HITACHI S-4700) and a high
resolution transmission electron microscope (HRTEM, Tecnai
F20).
The excimer laser annealing was performed in a chamber
filled with a N2(97%)/H2(3%) mixture gas at a pressure of
100 Torr with a KrF excimer laser. The excimer laser in our
experiment is a homogenized KrF laser with an ultraviolet
wavelength (248 nm) and a short pulse time (25 ns). The
fluence of the excimer laser used in our study was 50, 100, 200,
and 300 mJ cm−2, as measured by a power meter. Due to the
high laser energy density and short pulse, the ZnO surface first
melts and then cools down rapidly [8]. Therefore, the excimer
laser annealing is expected to be an effective structural and
electrical tuning method for ZnO nanowires due to the laser
irradiation with a short pulse time on the small dimension ZnO
nanomaterials.
The effects of the laser annealing on the structural,
chemical bonding, optical, and electrical properties of ZnO
nanowires were investigated with FESEM, HRTEM, photo-
luminescence (PL) spectra, x-ray photoelectron spectroscopy
(XPS), and current–voltage (I–V) characterizations, respec-
tively. In particular, the ZnO nanowires were fabricated into
FET device structures to study the influence of laser annealing
on their electrical properties (see figures 5(a) and (b)). To fab-
ricate ZnO nanowire FET devices, individual ZnO nanowires
suspended in isopropyl alcohol were dropped onto a 100 nm
thick thermally-grown oxide on silicon. The silicon substrate
was a highly-doped p-type substrate that could serve as a back
gate. Metal electrodes consisting of Ti (50 nm)/Au (50 nm)
were then deposited by an electron-beam evaporator and de-
fined as the source and drain electrodes by photolithography
and a lift-off process. The source and drain electrodes were
typically separated by ∼3 μm. The electrical properties, such
asdrain–sourcecurrent versusvoltagecharacteristics asa func-
tion of drain voltage and gate voltage, were measured using a
semiconductorparameter analyser(AgilentB-1500). The mea-
sured electrical properties of ZnO nanowire FETs were com-
pared before and after laser annealing.
3. Results and discussion
3.1. Surface morphology and structural properties
Figure 1(a) shows an FESEM image of ZnO nanowires on the
growth substrate before laser annealing, and figures 1(b)–(d)
are FESEM images of ZnO nanowires on the growth substrates
after laser annealing at fluences of 50, 100, and 200 mJ cm−2,
respectively. As the fluence increased, the vertically-grown
ZnO nanowires began to melt from the top of the nanowires.
This is because the photon energy of the KrF excimer laser
used in our experiments (479.6 kJ mol−1) is enough to break
Zn–O bonds (248 kJ mol−1) [9, 13]. The insets of figure 1
show optical images of ZnO nanowires transferred onto Si
substrates from the growth substrates without laser annealing
(figure 1(a) inset) and followed by laser annealing at the same
fluences of 50, 100, and 200 mJ cm−2(insets of figures 1(b)–
(d)). At a high fluence such as 100 and 200 mJ cm−2, the
ZnO nanowires started to melt and broke into pieces.
has been reported that laser annealing with a high fluence
It
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Nanotechnology 20 (2009) 095203 J Maeng et al
Figure 2. (a) HRTEM image of an as-grown ZnO nanowire. The inset is a low magnification TEM image showing a smooth and straight ZnO
nanowire. (b) TEM image of a laser-annealed ZnO nanowire at a fluence of 50 mJ cm−2. (c) EDX data obtained from the line scan indicated
in (b). (d) HRTEM image obtained for the area marked in (b).
(?150 mJ cm−2) can even transform the amorphous phase into
the crystalline phase of ZnO films [14].
Figure 2 shows HRTEM images of as-grown ZnO
nanowires and ZnO nanowires detached from the growth
substrate after laser annealing at a fluence of 50 mJ cm−2.
Figure 2(a) shows an HRTEM image of as-grown ZnO
nanowires before laser annealing. The average atomic spacing
along the growth direction was ∼0.26 nm, and the lattice
spacing of the (0001) planes of wurtzite ZnO corresponds to
∼0.52 nm [16, 17]. The low resolution TEM image (inset
of figure 2(a)) indicates the straight and smooth surface of an
as-grown ZnO nanowire with a uniform diameter along the
nanowire. Figure 2(b) is a TEM image of a ZnO nanowire
detached from the growth substrate after laser annealing, and it
showsthat the end of the nanowire has changed intoa spherical
tip shape.Energy dispersive x-ray (EDX) analysis from a
line scan on the ZnO nanowire in figure 2(b) is displayed in
figure 2(c). The EDX results indicate that there is an excess
of Zn in the spherical tip region of the ZnO nanowire relative
to the other nanowire region that was not influenced by laser
annealing. Theoxygenvapourpressure ismuchhigherthanthe
vapour pressure of zinc in ZnO [18, 19], and the ZnO results
in a stoichiometric excess of Zn [19]. Therefore, it is expected
that the surface melting of ZnO nanowires by laser annealing
in a vacuum chamber that lacks oxygen induces Zn-rich ZnO
nanowires due to increased oxygen vacancies [20]. Figure 2(d)
shows lattice images near the boundary of the spherical tip
region (laser-annealed region) and the nanowire region that
was not influenced by laser annealing. The atomic spacing of
the ZnO nanowire in both regions was identical as ∼0.26 nm.
This result suggests that single crystals of ZnO nanowires were
kept from melting and recrystallization by laser annealing at
50 mJ cm−2.
3.2. Optical properties
In order to investigatethe optical properties of ZnO nanowires,
PL measurements were carried out in the range of 2.2 and
3.5 eV. Deep level luminescence such as the green band was
not obviously observed before and after laser annealing, as
shown in figure 3. Figure 3 shows PL spectra obtained at
room temperature for as-grown nanowires and laser-annealed
nanowires with fluences of 50, 100, 200, and 300 mJ cm−2.
Both the as-grown nanowires and laser-annealed nanowires
showed near-band emission (ultraviolet (UV) emission) at
∼3.26 eV, which is attributed to free exciton emission
and its first longitudinal optical phonon replicas of free
exciton [21, 22].
The intensity of the UV emission peaks was reduced
after laser annealing, except in the case of laser annealing
with a fluence of 50 mJ cm−2. The PL intensity is closely
related to the crystallinity and roughness of ZnO materials.
For example, Aoki et al reported that the decrease in the UV
emission intensity resulted from a deterioration of the surface
crystallinityof singlecrystalZnO wafers due tolaser annealing
with a high fluence such as 150 mJ cm−2[12]. On the other
hand, Zhao et al observed that polycrystalline ZnO thin films
showed more intense UV emission after laser annealing, as
compared with as-grown ZnO thin films [13]. They reported
that an optimum fluence of ∼450 mJ cm−2could lead to the
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Nanotechnology 20 (2009) 095203 J Maeng et al
2.73.02.4 3.3
Photon energy (eV)
Intensity (a. u.)
Figure 3. PL spectra of as-grown nanowires and laser-annealed
nanowires at the fluences of 50, 100, 200, and 300 mJ cm−2. The
inset shows XRD patterns in the (0002) plane of the as-grown and
laser-annealed ZnO nanowires at the fluences of 50 and
300 mJ cm−2.
most intense UV emission after laser annealing and a higher
fluence of 650 mJ cm−2showed lower photoluminescence
intensity accompanied with increased surface roughness and
decreased crystallinity while maintaining a good epitaxial
orientation and the wurtzite crystal structure of the ZnO
film [13].
The trend of the change in UV emission intensity in
figure 3 is due to the results that the ZnO nanowires after laser
annealing at 50 mJ cm−2maintained nanowire morphology,
whereas the morphology and crystallinity of ZnO nanowires
after laser annealing at higher energies changed considerably.
The inset of figure 3 shows x-ray diffraction (XRD) patterns
(theta–2 theta scan) of the as-grown and laser-annealed ZnO
nanowires at the fluences of 50 and 300 mJ cm−2.
XRD pattern of as-grown ZnO nanowires was measured at
34.5◦of the intense (0002) plane peak, which indicates single
phase with the preferred (0002) orientation.
diffraction intensityof the laser-annealed ZnO nanowires at the
fluence of 50 mJ cm−2was comparable with that of as-grown
The
The (0002)
nanowires. However, the (0002) diffraction intensity of the
laser-annealed ZnO nanowires at the fluence of 300 mJ cm−2
significantly decreased, which implies a poor crystallinity of
ZnO nanowires. Laser annealing at a high fluence above
50 mJ cm−2caused deterioration in the morphology and
crystallinity of ZnO nanowires.
broke into pieces at higher fluences (see figure 1). Therefore,
the ZnO nanowires at 50 mJ cm−2exhibited similar UV
emission as the as-grown nanowires, but the UV emission
decreased for the laser-annealed ZnO nanowires at energies
above 50 mJ cm−2.
The ZnO nanowires even
3.3. Chemical bonding by XPS analysis
XPS analysis was performed to study the chemical bonding of
ZnO nanowires before and after laser annealing. We expected
that the chemical bonding of ZnO nanowires could influence
the electrical properties of ZnO nanowires, mainly due to
an increase of oxygen vacancies that act as n-type dopants.
The XPS measurements shown in figure 4 were performed
for three sets of nanowires: as-grown nanowires and laser-
annealed nanowires at 50 and 200 mJ cm−2.
energies of O 1s and Zn 2p were calibrated by taking the
binding energy of the C 1s peak (285.0 eV) as the reference
in XPS. As compared with the as-grown nanowires, the laser-
annealed nanowires show the peak of the Zn 2p spectrum
shifted towards a higher binding energy by 0.1–0.7 eV, which
is ascribed to the zinc enrichment by recrystallization caused
by laser annealing [6].Figure 4(b) shows that the O 1s
peak has shoulders at higher binding energies.
laser-annealed nanowires at 200 mJ cm−2, the O 1s peak is
decomposed into three peaks. Among these three peaks, the
dominant one is located at 530.9 eV (O1), which is assigned
to the O2−ion in the wurtzite structure surrounded by the Zn
atoms with their full complement of nearest-neighbour O2−
ions [23]. The peak located at 532.5 eV (O2) is assigned
to the O2−ion in the zinc oxyhydroxide species, ZnO(OH),
and the peak at 533.5 eV (O3) is ascribed to loosely bound
oxygen, such as absorbed O2or adsorbed H2O on the ZnO
surface [8, 23, 24].
The peak of O 1s also shifted towards higher binding
energiesasthefluence wasincreased. Forexample, for thecase
The binding
For the
Intensity (a. u.)
Intensity (a. u.)
1022 1020 1024
Binding energy (eV)Binding energy (eV)
(a)(b)
528530532534
Figure 4. XPS spectra obtained from as-grown nanowires and laser-annealed nanowires with fluences of 50 and 200 mJ cm−2. (a) Zn 2p3/2
spectra. (b) O 1s spectra.
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Nanotechnology 20 (2009) 095203J Maeng et al
Figure 5. (a) Schematic diagram of a ZnO nanowire FET. (b) FESEM image of a ZnO nanowire across metal electrodes. The Ids–Vgcurves
((c) and (d)) measured at Vds= 1.0 V and Ids–Vdscurves ((e) and (f)) at different gate voltages with a step of 5 V for ZnO nanowire FETs. ((c)
and (e)) Before and after laser annealing at a fluence of 50 mJ cm−2. ((d) and (f)) Before and after laser annealing at a fluence of
100 mJ cm−2.
of a fluence of 50 mJ cm−2, the O1, O2, and O3 peaks shifted
by 0.2, 0.03, and 0.2 eV, respectively, to higher energies, as
compared with the case of as-grown nanowires. For the case
of the laser energy density of 200 mJ cm−2, the shift is more
significant; the O1, O2, and O3 peaks shifted by 0.6, 0.5, and
0.5 eV, respectively, as compared with the case of as-grown
nanowires.
The shift of Zn 2p and O 1s peaks towards higher
binding energies after laser annealing can be explained by
the increase of oxygen vacancies.
vacancies acting as n-type dopants in ZnO materials donate
two electrons to the conduction band.
electron density due to increased oxygen vacancies raises
the Fermi level closer to the conduction band, but not into
the conduction band [8].The increased oxygen vacancies
decrease the work function, and this is the reason why the
O 1s peak shifts towards higher binding energies [8], as
shown in figure 4(b). The integral intensity (i.e., area under
the peak) ratio of O1/O2 increased with increasing fluence
(figure 4(b)), which indicates that the O2−bound ZnO(OH)
species was reduced due to the recrystallization by laser
annealing.
The ionized oxygen
The increased
3.4. Electrical properties
The effect of laser annealing on the electrical properties of
ZnO nanowires was also investigated. For this purpose, ZnO
nanowires were fabricated into conventional nanowire FET
device structures in a bottom-gate configuration, in which
the gate electrode is located under the nanowires, as shown
in figure 5(a).Figure 5(b) shows the FESEM image of a
typicalZnOnanowires FET device witha singleZnO nanowire
connected between source and drain electrodes.
Figure 5 shows representative data of the drain–source
current versus drain voltage (Ids–Vds) curves measured at two
different gate voltages and the drain–source current versus
gate voltage (Ids–Vg) curves measured at a fixed drain voltage
of 1.0 V for ZnO nanowire FETs before and after laser
annealing at fluences of 50 mJ cm−2(figures 5(c) and (e))
and 100 mJ cm−2(figures 5(d) and (f)).
nanowires were transferred onto thermally-grown oxide on
silicon substrates from the growth substrates. We measured
the electrical characteristics of nanowire FETs on substrate
(figure 5(b)). After laser annealing of these nanowire FETs,
we measured again and compared their electrical properties.
Note that nanowires are often broken into pieces after laser
As-grown ZnO
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