1 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 Ga 3+ compared to Zn 2+ 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 (ρ before /ρ after ≈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. 2

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. 3

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. 5

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 = ° for the samples annealed for 10 s to 2 = ° 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). 6

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. 7

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. 8

Fig. 5. The change of the coefficient G C with different annealing times. The data points below the dash line indicate grain growth. 9

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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. 11