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指導老師：林克默 老師 學 生：吳仕賢

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.

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.℃ 4

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 .℃

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.

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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.

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).

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Fig. 3. The change of the c-lattice parameter with RTA time.

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.

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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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