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指導老師：林克默
學
生：吳仕賢
老師
From：V. Khranovskyy, U. Grossner, V. Lazorenko, G. Lashkarev,
B.G. Svensson, R. Yakimova, “Conductivity increase of
ZnO:Ga films by rapid thermal annealing”, Superlattices
and Microstructures 42 (2007) 379–386.
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Ga was proposed as the best dopant for ZnO due to similar
atomic radius of Ga3+ compared to Zn2+ and its lower reactivity
with oxygen.
Rapid thermal annealing (RTA) was applied to increase the
conductivity of ZnO:Ga (1wt%) films and the optimal regime
was determined to be 800℃ in oxygen media for 35 s.
The resistivity reduction (ρbefore/ρafter≈80) was observed after
annealing at optimal regime and the final film resistivity was
approximately 4 × 10−4 Ω cm.
The route mean square roughness (Rq ) of the films was found
to decrease with increasing annealing time and the grain size
has been found to increase slightly for all annealed samples.
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Assuming that Ga atoms only partially replace Zn, we now
apply a rapid post-growth thermal treatment which could
provide a short migration of Ga atoms from interstitials to
Zn lattice sites.
we have investigated the effect of the annealing temperature on
the electrical properties of ZnO:Ga (1 wt%) and found an
abrupt drop of the resistivity after annealing for 1 h at 800℃
in both air and argon atmosphere.It has been concluded that 800
℃ is an appropriate temperature for Ga activation.
RTA as an alternative thermal processing provides a shorter
cycle time and larger flexibility compared to batch-type
furnaces.
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The ZnO:Ga (1 wt%) (ZGO) films were prepared by PEMOCVD
on silicon dioxide-coated Si wafers.
The substrate temperature was 250 ℃, zinc acetylacetonate
(Zn(AA)2), oxygen, and Ga acetylacetonate (Ga2(AA)3) were
used as precursors.
The thickness of the films was measured using a Dektak Stylus
Profiler and found to range from 200 to 250 nm.
The samples were annealed for 10, 20, 25, 28, 30, 35, 45 and 60 s
in oxygen ambient at 800 ℃,respectively. The heating speed was
set to 80 ℃/sec.
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The dependency of Z on the
annealing time is shown in Fig. 1.
As one can see, starting from an
annealing time of 10 s, Z
constantly increases and has a
maximum value (Z≈80) for an
annealing time of 35 s.
Fig. 1. The dependency of the dopant activation coefficient on
the RTA time. The annealing was carried out in oxygen
ambient at a temperature of 800 ℃.
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In order to check whether a crystallinity
change has occurred and, thus, has
affected the electrical properties, we
investigated the XRD spectra of the
annealed films (Fig. 2).
Fig. 2. XRD curves (fitted by a Gauss curve) of the ZnO:Ga (1% wt)
films annealed by RTA for 10 and 35 s. A shift of the (002)
peak position from 2 = 34.5876 ° for the samples annealed for 10
s to 2 = 34.5957 ° for samples annealed at 35 s is shown.
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The c-parameter starts to decrease
with the beginning of the annealing
and has a tendency to saturate within
the region 35–45 s. These results
indicate a correlation between the
structural and electrical properties:
the c-lattice parameter is small in the
case of high dopant activation and
vice versa.
Fig. 3. The change of the c-lattice parameter with RTA time.
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It should be also noted, that the surface
flattening begins at an annealing time of
approximately 20 s and is most intense in
the range from 35 to 60 s of annealing.
Fig. 4. The dependency of the smoothing coefficient
S on the annealing time. The points located
above the dashed line indicate a surface
smoothing, the ones below a surface roughening.
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Fig. 5. The change of the coefficient GC with
different annealing times. The data points
below the dash line indicate
grain growth.
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It was found that the RTA process in oxygen at an optimized
temperature of 800 ℃ can significantly increase the conductivity in
ZGO films due to effective dopant activation; the time of 35 s is the most
favorable annealing duration.
A shift of the (002) peak was noted and can be explained by Ga dopant
substitution of the Zn atoms on their lattice sites.
The ZGO films undergo a surface flattening from Rq = 7.5 nm to Rq =
5.8 nm.
The grain diameters have not significantly changed.
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