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J Sol-Gel Sci Technol (2009) 49:238–242
Solution derived Al-doped zinc oxide films:
doping effect, microstructure and electrical
property
Keh-moh Lin, Yu-Yu Chen, Keng-Yu Chou
K.-m. Lin Y.-Y. Chen
Department of Mechanical Engineering, Southern Taiwan
University, No.1, Nantai Str, Yung-Kang City, Tainan 710,
Taiwan, ROC
指導教授：林克默 博士
報告學生：郭俊廷
報告日期：99/9/27
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Outline
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Introduction
Experimental detail
Results and discussions
Conclusions
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Introduction
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Recently, zinc oxide films have been widely investigated for
new opto-electrical devices owning to their attractive
electrical and optical properties [1, 2].
Techniques used to deposit pure and doped ZnO films include
sputtering technique [6, 11–13], pulsed laser deposition [14],
thermal plasma [15], MOCVD [16], spray pyrolysis [17] and
sol–gel method [18–24].
Among them, the sol–gel method is not only a low-cost and
simple deposition procedure to coat large area high quality
TCO films
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Experimental detail
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In our experiments, zinc acetate dihydrate was dissolved in
isopropylalcohol, and aluminum nitrate was served as dopant
sources.
The Al/Zn ratio in the solution varied from 0.25% to 4%. The
solution concentration was 0.5 mol/L.
After being deposited on corning glass 1737 (some samples
on silicon wafers) by dip-coating, the films were first dried at
70℃ for 10 min. Then, the films were heated in a tube furnace
at 600℃ for 1 h in air (pre-heat treatment).
The procedures from coating, drying, to annealing were
repeated 2–5 times.
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Finally, these films were annealed in vacuum (1–10 mtorr) at
600℃ for 1 h (post-heat treatment).
The structural characteristics of the AZO films were studied
by a thin-film X-ray diffractometry (Rigaku D/MAX 2500)
with Cu Ka radiation.
The film resistivity, carrier concentration and mobility were
obtained by Hall measurements (Ecopia HMS-3000).
The transmittances of the AZO films were measured by using
a UV–Vis–NIR spectroscopy (Perkin Elmer Lambda 25).
The refractive indices of the AZO films were estimated by a
spectroscopic ellipsometry (Woollam M-2000U).
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The PL spectra were obtained by a fluorescence spectrometer
with a 150 W xenon lamp (Hitachi F-4500).
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Results and discussions
The XRD pattern of the AZO
films indicated that the
crystal structure of the AZO
films is wurtzite (Fig. 1).
 The (002) peak was
characterized by using the
relative intensity
i(002) = I(002) /[I(100) + I(002) + I(101)].
 With increasing aluminum
Fig. 1 X-ray diffraction patterns of AZO
concentration, it became
films with different aluminum concentration,
0.5 mol/L, five-layer films
slightly stronger (Fig. 2).

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At the same time, the
crystallite sizes d(002)
which were calculated by
using Scherrer’s equation
[27] became distinctly
smaller when Al
concentration rose (Fig. 2).
Fig. 2 The normalized I(002) and crystallite
size in dependence on aluminum
concentration, 0.5 mol/L, five-layer films
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The additional aluminum dopant material promoted the
nucleation behavior of the ZnO phase, which means, as the
aluminum concentration rose, more ZnO nuclei emerged on
the substrate and therefore the grown crystallites were smaller.
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Furthermore, the relative density of the coating films was
calculated by using the equation [19]:
n f2 - 1
Relative density(%)  2 100
n d -1
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An average refractive index (maximal values between 370–
380 nm) was applied to present the optical property of the
graded films. All the refractive index values were smaller than
that of the dense wurtzite-type ZnO (2.0) [19].
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It is found that the film
resistivity decreased as
the film became thicker
and denser.
This implies that the
film properties changed
obviously after the third
dip-coating process.
Fig. 3 Film relative density and resistivity in
dependence on layer number, 1.0 at.%, 0.5
mol/L, SiO2/Si substrate
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It can be clearly found in Fig. 4
that for all concentrations of
aluminum, the carrier
concentration n could only rise
to a certain value (<1020 cm-3)
while the film thickness was
increasing.
The carrier concentration did
not become higher along with
the increasing aluminum
concentration.
Fig. 4 Carrier concentration n in
dependence on layer number, 0.5 mol/L
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Figure 5 showed that for all
concentrations of aluminum,
carrier mobility μH was slowly
enhanced along with the
increase in film thickness,
with a few values of one-layer
and two-layer as exceptions.
Results of SE measurements
indicated this can be attributed
to the increasing relative
density as the film became
thicker.
Fig. 5 Carrier mobility l in dependence
on layer number, 0.5 mol/L
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
Furthermore, it can be found in Fig. 6 that generally, the
mobility became lower while aluminum concentration was
rising.
Fig. 6 n, ,  in dependence on aluminum
concentration, 0.5 mol/L, five-layer films
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Though the crystallite size reduced from 18.5 to 12.5 nm
between 0.25 and 4.0 at.% (cf. Fig. 2), but it was still at least
two times larger than the mean free path length Lf (0.5–3 nm),
indicating the impurity scattering or other scattering centers
inside the crystallite affected the film conductivity more than
that of grain boundary scattering.
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
All the samples are
transparent in the visible
region. Although the 0.25
at.% sample shows the
highest conductivity, its
transmittance is relatively
lower than other samples.
Fig. 7 Film transmittance index in
dependence on wavelength, five-layer,
0.5 mol/L
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PL measurement results (Fig. 8)
show that, beside near bandedge emission, there are
several emission centers
between 425 and 550 nm
(2.91–2.25 eV).
These are mainly caused by
glass substrates and by
interstitial zinc atoms or related
defects.
Fig. 8 Photoluminescence spectra in
dependence on aluminum
concentration, five-layer, 0.5 mol/L
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Because interstitial zinc atoms will provide free electrons, this
also indicates another source of charge carriers in the AZO
films.
High PL peak value implies the good quality of the AZO films.
This in turn means the numbers of potential barriers and
scattering centers in the AZO films are also small. Thus, the
carrier mobility increases.
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Conclusions
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在這研究中，發現摻雜的濃度會影響晶粒尺寸的大小，也
會增加在(002)方向的相對強度。
隨著塗佈層數的增加，載子濃度也會增加。但遷移率卻會
在一定程度的載子濃度下減少。
由此可知，過度的摻雜無法提高載子濃度。同時，過多的
載子濃度亦會形成載子的散射中心，提高電阻率。
另外，從PL的結果來看，過多的摻雜反而會惡化薄膜的
品質。
本研究所獲得的最佳試片，為鋁的摻雜濃度為1at.%，其
片電阻為182，在可見光範圍下穿透率大於80%。
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Thanks for your attention
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