低溫電熱機制對奈米線 ZnO/Al 結構之界面機制探討
曾譯葦
洪飛義* 呂傳盛
陳冠仁
蕭人瑄
蕭彤宣
國立成功大學材料科學及工程學系與奈米科技暨微系統工程研究所
Both solution nano wire ZnO and sputtered Al thin film on SiO2 as the wire-film structure, and the
Al film was a conductive channel for electrical induced crystallization (EIC). Direct current (DC)
raised the temperature of the Al film and improved the crystallization of the nano-structure. The effects
of EIC not only induced Al atomic interface diffusion, but also doped Al on the roots of ZnO wires to
form AZO/ZnO wires. The Al doping concentration and the distance of the ZnO wire increased with
increasing the electrical duration. Also, the electrical current induced temperature was ~2110C
(solid-state doped process) and so could be applied to low temperature optoelectronic devices.
Keywords: ZnO, nano wire, solution method, electrical current induced crystallization (EIC), Al doped.
Email: fyhung@mail.ncku.edu.tw
1.
for applications [7,12]. The solution method is a
Introduction
Zinc
oxide
(ZnO)
compound semiconductor
quartzite
with
a
a
II-VI
low temperature process, but doping metal atoms
hexagonal
and concentration control are difficult. Also, the
ZnO
effects of Al atom doping using the solid-state
light
method on 1D ZnO nanowires have still not
extraction efficiency and different light-emitting
been reported, and in particular, the electric
mechanisms (UV emission, Green emission), so
current induced crystallization (EIC) process [13]
they
is a solid-state method at room temperature and
nanowires
structure.
is
have
have
the
One-dimensional
advantages
extensive
nano-optoelectronic
devices
of
applications
[1-2].
in
Much
is worthy of further investigation.
research shows that high quality ZnO nanowires
In this study, we used the low temperature
are a very important factor in nano-devices [3-4].
solution method to synthesize uniform and
Heat treatment [5] and doped-metal atoms [6-8]
ordered ZnO nanowires onto ZnO/Al film on
are effective and convenient methods to improve
silicon substrate. The nanowires were grown
the physical properties (e.g. structure, magnetics
preferentially in the c-axis direction (002) using
and n-p junction)
a textured ZnO seeding layer [14]. The Al
Synthetic methods for large area 1D ZnO
conductive layer was subjected to a constant
nanostructure, include hydrothermal synthesis
voltage and current using electric current
[9], the Volid-Liquid-Solid method [10] and low
induced crystallization (EIC). Then, EIC doped
temperature solution method [11]. In addition,
Al on the ZnO nanowire roots through
many researchers have shown that Al doped ZnO
thermoelectric effects (including joule heating
(AZO) nanostructures have excellent potential
and electro-migration). Also, the structural
characteristics of Al doped ZnO nanowires used
(FIB). The characteristics of the cross-section
solid-state EIC testing were studied.
and the mechanism of Al atom migration with
2.
joule heating were investigated by Transmission
Experimental Procedures
The low temperature aqueous solution
method was used to obtain zinc oxide (ZnO)
Electron
Microscopy
(TEM)
and
X-ray
photoelectron spectroscopy (XPS).
nanowires which were grown onto ZnO/Al film
on silicon substrate. This was followed by
aluminum doping using an electrical current
3.
Results and Discussion
A
surface
image
cleaned using the Radio Corporation of America
structures are shown in Fig. 3. The ZnO nano
(RCA) method of cleaning and drying. Al film
wires had grown on the ZnO/Al film uniformly
(800 nm) was deposited on the silicon substrate
(Fig. 3(a)). From observations of the interfaces
by thermal evaporation. After deposition of the
(Fig. 3(b)), the ZnO nano wires had an excellent
Al film, there was a 100 nm thick layer of ZnO
bonding interface with the seed layer and the
(as seed layer) grown by RF magnetron
growth direction was perpendicular to the
sputtering.
surface.
of
Zinc
nano-wire
section
characteristics
solution
the
cross
(EIC). Fig. 1 shows that the silicon substrate was
Aqueous
of
and
ZnO/Al
In addition, the average length of the
nitrate
nano wires was 910 nm and the average
[(NO3)2 .6H2O] (99.5%, J. T. Baker) and
diameter was 89 nm. The Al layer was a
Hexamethylenetetramine
conductive layer and facilitated the electrical
[HMT:
C6H12N4]
(99.9%, Alfa Aersar) were mixed with an equal
current induced crystallization (EIC).
molar concentration. Then, the ZnO/Al silicon
In fact, the EIC structure would break if the
substrate was put in the aqueous solution
input voltage or power was too large [13,15]. So
(105mM) at a temperature of 90°C for 1 hour.
a power loading test of the nano-wire ZnO/Al
After this, the ZnO nanowires formed. Fig. 2
structure was performed. The voltage (V)-
shows that the Al film was a conductive channel
current (I) curve is shown as Fig. 4(a). It can be
to facilitate the electrical induced crystallization
seen that the breakdown voltage of the Al film is
at room temperature. EIC had doped Al on the
4V (0.73A), and a voltage of 3.75V and current
ZnO thin film and ZnO nanowires. During the
of 0.68A was selected for the EIC test (Fig. 4(b)).
electrical process, a thermocouple wire was used
During 10mins of EIC testing (the structure was
to measure the sample surface temperature [13].
not damaged), the surface temperature was
Before and after the electrical current testing,
stable starting from 6 seconds and the average
we analyzed the crystalline structure and
temperature of the nano-wire ZnO/Al structure
orientation of the ZnO nanowires by X-ray, with
was about 211oC by joule heating.
angle 1.5 0, scan speed: 2 /min, and degree
In order to understand the influence of EIC
range from 30~90[6-8]. Furthermore, the
on the structural characteristics, the non-EIC
microstructure of the nanowires was investigated
specimens and the EIC specimens with 3.75 V
using a
field emission scanning electron
for 10 min and 1hr were subjected to XRD
microscope (FE-SEM) and focused ion beam
analysis as shown in Fig. 5. The pattern of Fig.
5(a) is the ZnO/Al structure (no nano wires).
compositions, but also underwent some ion
Even when nano-wires grew (Fig. 5(b)), the Al
diffusion at the root zones.
film still had significant peaks at (111) and (311).
EDX is a semi-quantitative analysis, so the
After EIC, the intensity of the Al phase
electrical current time of the nano-wire ZnO/Al
substantially decreased and the crystallization of
structure was increased from 10 min to 1 hour
ZnO increased (Fig. 5(c)(d), 002; 103) [7,16-17].
and then XPS was performed. For the un-EIC
Notably, this result increased significantly with
nano-wire ZnO/Al structure, only zinc and
an increased EIC duration. When the time of
oxygen ions were detected at the root of the wire
EIC was increased to 1hr, the ZnO nano wires
and seed layer. After 1 hour of EIC testing,
had
general,
aluminum ions were detected at the root zone of
as-deposited films are annealed (400~6000C for
the wire and seed layer (Fig. 7). Meanwhile the
1hour) to improve the crystalline characteristics.
zinc content gradually decreased which indicates
It is clear that the EIC method induced thermal
that Auger electrons had got into the interface
excellent
crystallization.
In
o
energy (~211 C) was insufficient to improve the
between the ZnO seed layer and the aluminum
crystallization of ZnO nano-wires [14]. The ion
layer. After that, the aluminum content increased
diffusion of EIC helps to account for this result.
and an aluminum layer was detected. To put it
Fig. 6 shows a bright filed image of the
more precisely, the atom distributions were
nano-wire ZnO/Al structure after EIC for 10min.
regular and were similar to those in Fig. 6.
The ZnO nano-wires had a crystalline structure
This figure clearly shows that the root of the
and grew with a (002) orientation (Fig. 5). In
ZnO nano wires not only contained zinc and
fact, the ZnO nanowires had already crystallized
oxygen ions, but also possessed some aluminum
during the as-grown state but the degree of
ions (the non-electrical current sample had no
crystallization was low (as amorphous) [8-11,14].
aluminum ions). In particular, the diffusion path
Notably, diffusion behavior of EIC was apparent
of the aluminum ions was about 480 nm and
in the interface zone between the ZnO nano
their average concentration ions was about at
wires (including the seed layer) and the Al layer.
0.757 at.% in the root zone of the ZnO nano
To understand the concentration of Al ions,
wires.
points A (bottom seed layer) to D (the root of
After EIC, the intensity and concentration of
ZnO wires) were examined by EDX as shown in
Al ions substantially increased in the root zone
Fig. 6. The bottom seed layer (point A, B) and
of the nano wires. The main reason is that the
root of the ZnO nano wires (point C, D)
EIC caused the micro Al ions embedded into
contained a higher zinc concentration and traces
ZnO structure to form an AZO structure which
of aluminum (0.198~0.236 at%). When the
improved the crystallization of ZnO nano wires
examined zone approached the ZnO nano wires
that combined ion migration with joule heating.
(points C and D), both zones contained mainly
This result is similar to the metal doping
zinc, oxygen and aluminum (concentration value
mechanism in our previous report [6-8,13].
was similar). The concentration data proves that
Based on the above results, it is confirmed that
the nano-wires not only had identical chemical
the improvement in crystallization can be
attributed to the EIC and ion diffusion.
From the EIC data, aluminum migration was
6.
References
the main crystallization mechanism which
1. H. Q. Liang, L. Z. Pan, Z. J. Liu, Mater. Lett.
improved the opto-electronic properties of the
62 (2008) 1797-1800.
nano-wire ZnO/Al structure. A relevant report
2. R. Hejazi, H. R. M. Hosseini, M. S. Ghamsari,
[5,18] showed that metal ions would gradually
J. Alloy. Compound. 455 (2008) 353-357.
diffuse under a higher annealing temperature
3. Q. X. Zhao, P. Klason, M. Willander, Appl.
leading to an improvement in film conductivity.
Phys. A 88 (2007) 27-30.
In fact, the present structure with electrical
4. J. S. Lee, M. S. Islam, S. Kim, Sens. Actuators
current crystallization only needed relatively
B 126 (2007) 73-77.
little energy to make the metal ions migrate and
5. T. Li, C. S. Ong, T. S. Herng, J. B. Yi, N. N.
improve the quality of the structure (Fig. 8). In
Bao, J. M. Xue, Y. P. Feng, J. Ding, Appl. Phys.
short, the ZnO nano-wires can be doped by EIC.
Lett. 98 (2011)152505.
The
6. K. J. Chen, F. Y. Hung, S. J. Chang, Applied
crystallization
mechanism
using
the
electrical current method is a solid-state method.
Surface Science. 255 (2009) 6308-6312.
Therefore, the stability of upper (ZnO) and
7. K. J. Chen, T. H. Fang, F. Y. Hung, L. W. Ji, S.
lower (AZO) nano-wires can lead to excellent
J. Chang, S. J. Young, Y. J. Hsiao, Applied
1-D structural properties.
Surface Science. 254 (2008) 5791-5795.
8. K. J. Chen, F. Y. Hung, Y. T. Chen, S. J.
4.
Conclusion
The electrical induced crystallization (EIC)
Chang, Z. S. Hu, Materials Transactions. 51
(2010)1340-1345.
possessed excellent save energy and could be
9. Y. J. Kim, Hadiyawarman, A. Yoon, M. Kim,
applied to low temperature applications. The
G. C. Yi, C. Liu, Nanotechnology. 22 (2011)
crystallization of the nano-wire ZnO/Al structure
245603.
on a Si substrate was enhanced using EIC.
10. M. Ahmad, J. Zhu, J. Mater. Chem. 21
Owing to the thermoelectric effect, the Al ions
(2011) 599-614.
diffused into both the ZnO seed layer and the
11. T. Jun, K. Song, Y. Jeong, K. Woo, D. Kim,
root zone of the nano wires. The upper (ZnO)
C. Bae, J. Moon. J. Mater. Chem. 21
and lower (AZO) nano-wire was a composite
( 2011) 1102-1108.
wire that had unique optoelectronic properties
12. J. Kim, J. H. Yun, S. W. Jee, Y. C. Park, M.
and was confirmed as having contributed to the
Ju, S. Han, Y. Kim, J. H. Kim, W. A. Anderson, J.
ZnO/Al structure.
H. Lee, J. Yi, Materials Letters. 65 (2011)
786-789.
5.
Acknowledgements
13. F. Y. Hung, Materials Transactions. 52 (2011)
The authors are grateful to National Cheng Kung
1138-1141.
University, the Center for Micro / Nano Science
14. K. J. Chen, F. Y. Hung, S. J. Chang, Z. S. Hu,
and Technology (D99-2700) and NSC NSC
Journal of The Electrochemical Society (JES),
99-2221-E-006 -132 for the financial support.
157(3) (2010) H241-H245.
15. C. C. Chou, F. Y. Hung, T. S. Lui, Scripta
Materialia. 56 (2007) 1107-1110.
16. H. C. Huang, T. E. Hsieh, Nanotechnology
21 (2010) 295707.
17. H. S. Kang, J. S. Kang, S. S. Pang, E. S.
Shim, S. Y. Lee, Mater. Sci. Eng. B102 (2003)
313-316.
18. C. Periasamy, P. Chakrabarti, Science of
(a)
Advanced Materials. 3 (2011) 73-79.
(b)
Fig. 3 Observations of Nano-Wire ZnO/Al
structures:
(a)
Surface
characteristics
(b)
interface structure of cross-section.
Fig. 1 Schematic diagram of growth of ZnO
nanowires.
4V/0.75A
(a)
211℃
211℃
(b)
Fig. 4 (a) Voltage (V)-current (I) curve of
Fig. 2 Schematic diagram of nano-structure and
structures (b) surface temperature curve of
electrical current equipment.
thermal induced at 3.75V for 10mins.
Fig.
5 XRD patterns of specimens.
ZnO/Al/SiO2/Si
(un-nanowires),
(b)
(a)
ZnO
Fig. 7 XPS depth profiles of nano-wire ZnO/Al
structure after electrical current testing.
NWs-ZnO/Al/SiO2/Si (un-EIC), (c) EIC process
testing 10 mins , ZnO NWs-ZnO /Al/SiO2/Si
(EIC 10min ), (d) ZnO NWs-ZnO /Al/SiO2/Si
(EIC for 1hour).
Fig. 8 Schematic diagram of electrical current
induced crystallization and diffusion.
Fig. 6 Ion diffusion of nano-wire ZnO/Al
structure: cross-section of structure and atom
content from point A to point D.