JOURNAL OF APPLIED PHYSICS VOLUME 85,NUMBER JUNE 1999 A study of low temperature crystallization of amorphous thin film indium-tin-oxide 2011/05/ /05/09 指導教授：林克默 博士 學 生：邱巧緣

Outline Introduction Experiment Results Discussion Conclusion 2PV Materials & Modules Lab

Introduction Indium–oxide has the bixbyite crystal structure which is a c-type rare earth, vacancy defect oxide. The bixbyite structure is similar except that the MO 8 coordination units (oxygen position on the corners of a cube and M, a metal atom located at the center of the cube) are replaced with units that have oxygen missing from either the body or the face diagonal. The removal of two oxygen ions from the MO 8 to form the MO 6 coordination units forces the displacement of the cation from the center of the cube. 3PV Materials & Modules Lab

Amorphous indium–oxide is likely formed during physical vapor deposition processing when MO 6 coordination units, that evolve from the source or are formed while chemisorbed on the growth surface, are incorrectly oriented when they are incorporated into the growing film. Remarkably, the crystallization of amorphous ITO occurs rapidly at very low homologous temperatures (T/T m ＜ 0.19) with 150 °C being commonly cited as the temperature at which crystallization occurs rapidly. The significance of this temperature is not known; however, it is noted that it is close to the melting point of In metal (157 °C). 4PV Materials & Modules Lab

Amorphous ITO films were deposited by electron beam evaporation from a sintered indium–tin oxide pellet containing 9.9 wt.% SnO 2. The base pressure of the deposition chamber was 3×10 -6 Torr and no gases were introduced during deposition. The source material was evaporated for 5 min imme- diately prior to deposition of 180 nm thick films on both oxidized Si and glass substrates at a deposition rate of approximately 0.8 nm/s. In situ measurements of resistivity were made during annealing in air at temperatures that ranged between 120 and 165 °C. Experiment 5PV Materials & Modules Lab

Results 6PV Materials & Modules Lab

These TEM observations suggest that the ITO in the as-deposited samples is fully amorphous. Exposure of the amorphous ITO to the electron beam changes the appearance of the material from a uniform amorphous contrast to a light–dark mottled contrast suggesting that the amorphous material is susceptible to electron-beam damage. After annealing in air for 1 h at 162 °C, the sample becomes fully crystalline and, as shown in the cross-sectional image,consists of large block- like grains that are, on average, approximately 100 nm in size. 7PV Materials & Modules Lab

Figure 2 shows that the rate of the transformation (change in resistivity) increases with increasing temperature, which indicates that it is thermally activated. During the course of the crystallization, the resistivity gradually decreases to be- low 1×10 -3 Ω cm. 8PV Materials & Modules Lab

A distinct crystalline peak corresponding to bixbyite In 2 O 3 [222] at a 2θ of 30.7 is apparent. Further anneal- ing sharpens the crystalline In 2 O 3 peaks and, while the diffuse amorphous peak integrated intensity drops to zero, the crystalline [222] peak reaches a maximum intensity. After crystallizat- ion, a weak peak can be resolved at a 2θ of about 33°, which is consistent with metallic indium. 9PV Materials & Modules Lab

Elementary electrostatics are then used following the procedure of Landauer* to derive the conductivity as a function of the volume fraction of the second phase: 10PV Materials & Modules Lab

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A set of reflectivity versus time data were collected over the temperature range 110–200 °C. The reflectivity changes rapidly initially then gradually approaches a constant value as shown in Fig. 5. Unlike the resistivity data which clearly shows two regimes, the reflectivity data appear to show only one. This can be seen in Fig. 5 which shows both resistivity and reflectivity data that were obtained at 135 °C plotted together. This plot shows that 80% of the reflectivity change occurs during regime I while, during the same interval, the resistivity changes by only 20% of the total. 12PV Materials & Modules Lab

ITO is a degenerate n-type semiconductor with a band gap that varies with carrier density between 3.5 and 4.0 eV due to the Burstein–Moss effect. It is well established that free carriers in ITO are contributed by two principal donors: four valent tin substituting in the crystalline lattice for In and doubly charged oxygen vacancies (conventionally represented by [Sn ] and V, respectively). In our experiments electronic transport measurements before and after crystallization revealed that the change in resistivity seen in Fig.2 is due primarily to an increase in carrier concentration rather than to an increase in carrier mobility. 13PV Materials & Modules Lab Discussion

Inspection of the resistivity versus time data Fig. 2 suggests that there processes which occur during the annealing of amorphous indium–oxide both of which affect the carrier density in the oxide. We speculate that the first regime is associated with the relaxation of the amorphous structure; the realignment of In–O bonds to generate a locally ordered structure that is smaller than a single bixbyite unit cell (1 nm) but has sufficiently organized InO 6 structural units to allow the creation of oxygen vacancies which contribute carriers and results in a sharp drop in the film resistivity. PV Materials & Modules Lab14

This analysis reveals that the activation energy for processes is approximately eV. Based on this, and the TEM observation that shows grain size and hence nucleation rate to be independent of the annealing temperature, it is likely that the microstructure is controlled by the same atomistic process—short range atomic rearrangement during the relaxation of the amorphous structure and similar short range movements across the amorphous / crystalline interface during crystallization. 15PV Materials & Modules Lab Avrami–Johnson–Mehl Equation

1. 結果表明在不同的時間常數中，電阻率的變化會 隨著材料在熱激活反應而改變，類似如活化能的 現象。 2. 由實驗得到的 ITO 薄膜，經 Avrami–Johnson–Mehl 方程式計算可得知其具有二 - 三維成長方向的特 徵。 3. 由此研究的這些過程，更完整了解 ITO 特性，有 利於今後沉積低電阻、高穿透性之 ITO 導電膜的 發展，使用於低動能及低基板溫度。 PV Materials & Modules Lab16 Conclusion

PV Materials & Modules Lab17 Thanks for your attention!!