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J Sol-Gel Sci Technol (2010) 55:369–376
DOI 10.1007/s10971-010-2252-3
Influences of preferred orientation
growth on electrical properties
of ZnO:Al films by sol–gel method
Keh-moh Lin • Hsin-Cheng Chen • Yu-Yu Chen •Keng-yu Chou
ORIGINAL PAPER
教授:林克默博士
學生:董祐成
日期:2010/10/18
K. Lin H.-C. Chen Y.-Y. Chen
Department of Mechanical Engineering,
Southern Taiwan University, Tainan, Taiwan
K. Lin (&) K. Chou
Institute of Nanotechnology, Southern Taiwan University,
No.1, Nantai Str., Yung-Kang City, Tainan 710, Taiwan, ROC
e-mail: [email protected]
Received: 18 February 2010 / Accepted: 14 May 2010 /
Published online: 27 May 2010
Springer Science+Business Media, LLC 2010
Outline
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Introduction
Experimental
Result and Discussion
Conclusion
Introduction
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In recent years, wide band-gap transparent
conducting oxides have received great attention
because they can be applied on UV Laser diodes,
transparent electrodes of LCDs, touch panels.
Among them, zinc oxide arouses great expectations,
not only because it has large exciton binding energy,
high breakdown strength, good resistance to radiative
damage, but also because the techniques for large
area single crystal and epitaxial growth of zinc oxide
have been well developed.
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In this study, our investigations will focus mainly on
two points: (1) the effects of different heat treatments,
MEA amount and initial layers on the preferred
orientation of the AZO films, (2) the influences of the
micro-structure, different nucleation behaviors, and
film growth behaviors on the films’ conductivity.
Furthermore, in order to enhance the accuracy of film
thickness measurement, we intend to use a multiangle spectroscopic ellipsometry (SE) combined with
a multi-layer model.
Experimental
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In our experiments, zinc acetate dihydrate (ZnAc)
was dissolved in methanol. Aluminum nitrate as well
as gallium trichloride were served as dopant sources.
Based on our earlier study , the best conductivity was
achieved with an Al/Zn ratio of 1.0 at 88%. The
solution concentration was 0.3 mol/L. Symbol A
represents the sol solution without any MEA as
stabilizer.
Symbol B means that the molar ratio of ZnAc:MEA
was 1:1 in the sol solution.
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The molar ratio of sol solution C was 1:4. The pH
values were 6.3, 7.4, and 10.5, respectively.
Several fabricating procedures were carried out to
obtain AZO films: Using corning glass (corning 1737,
hereafter referred as ‘‘P2-G’’ to differentiate from the
process in our earlier works) and silicon wafers
(hereafter referred as ‘‘P2-S’’) as substrates, the films
were deposited by spin-coating and were then preheated in a rapid thermal annealing (RTA) furnace at
600 ℃ for 10 min in air.
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In the third procedure (P3-G), we used a 3-inch AZO
(ZnO with 2wt.% Al2O3) target in the sputtering
process to create an initial layer on the corning glass.
The distance between the target and the corning glass
was 5 cm.
Ar was introduced at a flow rate of 40sccm.
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The working pressure was set at 8 × 10-3 torr.
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The power for sputtering was set at 70 W; the
substrate temperature was 270 ℃
The thicknesses of the initial layers were 50 and 343
nm.
Finally, all the films of these three deposition
processes were annealed in vacuum (~150 mtorr) at
600 ℃ for 1 h.
Result and Discussion
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For this reason, the microstructure was rather loose,
indicating that the growth of the films began inside
the sol-layer. However, the pores were smaller.
The reason could be that we prepared the films layer
by layer. Thus, every time was a new beginning and
the growth process was not continuous. As a result,
the effect of the initial layer was limited.
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The reason can be that MEA retarded the
condensation of the Zn2+ ions and promoted the
formation of zinc monoacetate as well as of ZnO later
on.
That means, the combination effects of adding MEA
and the strong heat flux additionally enabled the
crystallites to grow towards the direction of the
normal lines of the substrate, i.e., to enhance the (103)
reflection .
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At the same time, the sputtering layer was not only
favorable for the growth of AZO films in the (002)
direction, but also reduced the (103) intensity.
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Table 2 that the strong heat flux made the crystallite
size of the P2-S samples become the biggest.
i(002) also increased as the MEA amount increased.
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one can see that the sputtering layer could enhance
the i(002). The thicker the sputtering layers were, the
stronger the i(002) became.
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The conductivities of the P2-S and P2-G samples
reached their highest values when the MEA:ZnAc
ratio was 1:1.
Adding a proper amount of MEA made the zinc
monoacetate to evenly distribute in the sol solution,
which was favorable for the growth of the ZnO phase
in direction (002) and thus improved the crystallinity
as well as the carrier mobility. However, the sol
would become base when too much MEA was added.
Under such situation, the film density would become
loose causing the carrier mobility to drop.
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P3-G-D that the sputtering process was favorable for
the doping effect, i.e., the carrier concentration of this
sample was almost the highest in this study. However,
when the number of the sol–gel layers was small,
their relative densities were low. Consequently, the
carrier mobilities were also low.
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Shows that the compared SE data and SEM fitting
results are in good accordance. The deviation caused
by their errors in the Hall measurement is also within
the acceptable range.
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Shows that all the P2-G and P3-G samples are highly
transparent within the visible light range (the
transmittance reaches 90% at wavelength 550 nm).
As the wavelength of the incident light moved to the
short wavelength range, the transmittance dropped
dramatically. This is the so-called optical absorption
edge (wavelength at about 380 nm). As the film
layers increased, the absorption edge tended to move
toward the long wavelength region.
Conclusion
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結果發現，MEA 影響溶膠溶液裡的離子複合結構
而促進了（002）方向增長。由於結晶度提高，載
流子遷移和導電率也增強。
應用高品質的初始層雖能提高晶體性質、載子遷
移率、 AZO薄膜的電導率，但並不多。
另一方面，均相成核所造成的直接紅外加熱，增
強優選取向增長，這是由於摻雜增強了活化率而
使載子濃度上升。因此，最明顯的是導電率的改
善。
提高導電性對溶膠凝膠 AZO薄膜的關鍵因素就是
摻雜使活化率提高。
THANKS FOR YOUR ATTENTION