Space-Limited Crystal Growth Mechanism of TiO2 Films by Atomic

Mar 26, 2010 - Chiang , Y.-M. ; Birnie , D. P. ; Kingery , W. D. Physical Ceramics: Principles for Ceramic Science and Engineering; Wiley: New York, 1...
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J. Phys. Chem. C 2010, 114, 6917–6921

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Space-Limited Crystal Growth Mechanism of TiO2 Films by Atomic Layer Deposition Wen-Jen Lee* and Min-Hsiung Hon Department of Materials Science and Engineering, National Cheng Kung UniVersity, 1, Ta-Hsueh Road, Tainan, 701 Taiwan, ROC ReceiVed: NoVember 25, 2009; ReVised Manuscript ReceiVed: March 04, 2010

In this paper, a novel space-limited crystal growth mechanism of TiO2 films on a Si substrate by ALD is presented. The results show that two distinct grain-growth behaviors are observed at two different growth temperature ranges. At 300-500 °C, grain growth of the films has a conventional temperature dependence with an activation energy of about 12.39 kJ/mol. However, the grain growth has an unconventional temperature dependence, as the grain size decreases with increasing growth temperature for films grown at temperatures between 150 and 250 °C. This TiO2 growth is dominated by the surface nucleation of the films. It is suggested that this interesting phenomenon may also occur for other materials grown by ALD. In addition, the nucleating kinetics of ALD-TiO2 films has also been investigated, and the nucleating activation energy of films is approximately 48.25 kJ/mol. Introduction Titanium oxide (TiO2) is a nontoxic, low cost wide bandgap semiconducting material with strong chemical stability, high refractive index, visible-light transmittance, dielectric constant, and dielectric breakdown strength. It has been widely investigated as a key material for applications in solar cells,1 sensors,2 batteries,3 dielectric layers,4 or channels5 for electronic devices, photonic crystals,6 optical emissions,7 transparent conducting oxides (TCOs),8 and photocatalysts.9 Methods used for preparing TiO2 films include the sol-gel method,10 liquid phase deposition (LPD),11 pulsed laser deposition (PLD),12 sputtering,13 electron beam evaporation,14 molecular beam epitaxy (MBE),15 chemical vapor deposition (CVD),16 and atomic layer deposition (ALD).17 ALD is an excellent technique for preparing highly conformal thin films with accuracy at control of thickness on the surface of nanostructures,17 such as nanoporous materials, nanoparticles, nanosheets, nanofibers, nanorods, and nanotubes, making it a powerful tool for the fabrication of nanodevices.18 Therefore, it is important to understand the growth behaviors of films grown by ALD. In this study, TiO2 films are grown by ALD at different temperatures (100-500 °C) and deposition cycles. The results show that the crystal grain size decreases with growth temperature in the temperature range from 150 to 250 °C. Moreover, a novel space-limited crystal growth mechanism is proposed. Experimental Details TiO2 films used in this study were grown on n+-Si (100) substrates in a low-pressure cold-wall ALD reactor. The substrates were cleaned in an ultrasonic bath sequentially with acetone, methanol, and deionized water for 5 min each and then dried with N2 gas before introducing into the ALD reactor. TiCl4 and H2O were used as the precursors to grow the TiO2 films, and Ar was used as the purge gas. The growth temperatures were from 100 to 500 °C. The reservoirs of the TiCl4 and H2O precursors were kept at the temperatures of 30 and 28 °C, respectively, while the flow rates of the TiCl4 and H2O vapor determined by the reservoir temperature and vapor injection time * To whom correspondence should be addressed. Tel: +886-6-2380208. Fax: +886-6-2380208. E-mail: [email protected].

Figure 1. X-ray diffraction patterns of TiO2 films deposited with 1000 ALD cycles at various temperatures.

were 0.80 and 0.70 cc/pulse, respectively. The flow rate of Ar gas was 5 sccm, as controlled by a mass flow controller (MFC). A combination of four conventional gas injection steps and four additional pump-down steps was applied in the ALD process. The pump-down steps are useful to evacuate excess precursors and byproducts from the chamber,19 which can effectively improve the quality of the films and ensure the achievement of “true ALD mode” growth. The time used for each step in an ALD cycle was 0.15, 1, 0.5, 1, 0.1, 1, 0.5, and 1 s for TiCl4 vapor injection, pump-down, Ar purge, pump-down, H2O vapor injection, pump-down, Ar purge, and pump-down, respectively. The 1000 and 100 deposition cycles of ALD were used to grow TiO2 films for different thicknesses, of about 100 and 10 nm, respectively. The crystalline structures of the films were examined by X-ray diffraction (XRD). The surface morphologies of the films were observed with a Hitachi S-4800 highresolution scanning electron microscope (SEM). In addition, to obtain real surface morphologies of the films, the SEM analysis was performed without any conductive coating. Results and Discussion Figure 1 shows the XRD patterns of TiO2 films grown at different temperatures (100-500 °C) by ALD with 1000

10.1021/jp911210q  2010 American Chemical Society Published on Web 03/26/2010

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Figure 2. SEM micrographs of TiO2 films deposited with 1000 ALD cycles and various temperatures.

deposition cycles. According to the XRD patterns, the TiO2 film grown at 100 °C has an amorphous structure. When the growth temperature of the films was raised to 150 °C, the TiO2 film has a polycrystalline structure. Furthermore, only the anatase phase is obtained in the TiO2 films at the growth temperatures between 150 and 350 °C. However, the high-temperature stable rutile phase is obtained in TiO2 films when the growth temperature is above 400 °C. Thus, the TiO2 films grown at temperatures between 400 and 500 °C have a two-phase mixed crystalline structure, containing both anatase and little rutile phases. Figure 2 shows the SEM micrographs of the TiO2 films grown at the different temperatures (100-500 °C), with 1000 ALD cycles. The TiO2 film grown at 100 °C shows a smooth surface due to its amorphous structure, as seen in the XRD pattern. The TiO2 films grown at temperatures above 150 °C have polycrystalline structures, and their surfaces exhibit a lot of large grains. Interestingly, the film grown at 150 °C has a crystalline anatase grain area surrounded by amorphous smooth areas. The average grain sizes of the films, as calculated from the SEM results, are about 208, 154, 67, 59, 77, 81, 87, and 113 nm as deposited at temperatures of 150, 200, 250, 300, 350, 400, 450, and 500 °C, respectively. The relationship between the grain size and growth temperature of TiO2 films deposited with 1000 ALD cycles is shown in Figure 3a. The corresponding dependence of the grain growth rate on the growth temperatures is shown in Figure 3b, following the Arrhenius equation. The Arrhenius equation is a simple, but remarkably accurate, formula for the temperature dependence of the rate constant. The activation energy of grain growth can also be calculated by the Arrhenius equation, as shown below

Figure 3. (a) The relationship between grain size distribution and deposition temperature for TiO2 films deposited with 1000 ALD cycles. (b) Arrhenius plot of the grain size vs the reciprocal temperature (1/ T). The linear fitted lines were used to calculate the activation energy of grain growth.

k ) Ae-Q/RT

(1)

where k is the specific rate constant (in this work, k is the grain growth rate), A is the preexponential factor, Q is the activation

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Figure 4. SEM micrographs of TiO2 films deposited with 100 ALD cycles and various temperatures.

energy, T is the absolute temperature, and R is the gas constant. Taking the natural logarithm of the Arrhenius equation yields

ln(k) ) -Q/RT + ln(A)

(2)

Thus, a plot of ln(k) versus 1/T gives a fitting straight line, as shown in Figure 3b, whose slope and intercept can be used to determine Q and A, respectively. From Figure 3, the conventional crystal growth trend is clearly observed at the temperatures between 300 and 500 °C, when the TiO2 grain size is enlarged with the increasing temperature, and the activation energy of grain growth calculated from the slope of Figure 3b is about 12.39 kJ/mol. This behavior is consistent with the principle of grain boundary migration and Ostwald ripening.20 However, an aberrant reverse trend at the temperatures between 150 and 250 °C is also observed in Figure 3, with the grain size decreasing with the increasing temperature, and the Arrhenius plot shows a positive slope, indicating that the grain growth is not controlled by temperature. Moreover, the grain sizes of films grown at lower temperatures (150 and 200 °C) are clearly larger than that grown at higher temperatures

(from 300 to 500 °C). Consequently, the results show that there are two different grain growth mechanisms for TiO2 films grown by ALD for the temperature ranges of 150-250 and 300500 °C, respectively. To further understand the crystal growth mechanisms, the TiO2 films were prepared with 100 ALD cycles to investigate the nucleation and grain growth in the initial period. Figure 4 shows the surface micrographs of these films grown at various temperatures from 150 to 500 °C. The grain density and grain size of the films’ surface as deposited with 100 ALD cycles were calculated from the SEM analysis and are shown in Figure 5a. Clearly, there are some crystalline grain areas and smooth amorphous areas distributed on the surfaces of the films. In other words, the amorphous layer is produced early in the film deposition process and the nucleation and grain growth of the films then take place later in the ALD process. The grain shapes of films grown at temperatures below 350 °C are spheroidal and isolated. However, the grains begin to coalesce as the growth temperature is raised to 500 °C, and thus, the irregular grain shapes are formed. This can also be explained by diffusion-controlled coarsening kinetics (i.e.,

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Figure 6. Schematic grain growth models of ALD TiO2 films: (a) low growth temperatures (150-250 °C) and (b) high growth temperatures (above 300 °C).

Figure 5. (a) The relationships among surface grain density, grain size distribution, and growth temperature for TiO2 films deposited with 100 ALD cycles. (b) Arrhenius plot of the ln(surface grain density) versus the reciprocal temperature (1/T) × 1000. The linear fitted lines were used to calculate the activation energy of nucleation of films.

Ostwald ripening). Because the diffusion coefficient of the materials is in a direct proportion to the temperature, so the diffusion rate of atoms is lower at the temperatures below 350 °C without noticeable Ostwald ripening. Consequently, isolated and spheroidal grains are shown on the surface of the films at temperatures below 350 °C. Because atoms have a higher diffusion rate at a temperature of 500 °C, the graincoalescent phenomenon leads to the irregular grain shapes and the increased grain size. The average grain size is about 15.9, 16.1, 20.7, 23.5, 24.2, and 53.5 nm for the growth temperatures of 150, 200, 250, 300, 350, and 500 °C, respectively. The grain size distribution of TiO2 films deposited with 100 ALD cycles above 300 °C is consistent with the conventional crystal growth trend in which the grain size enlarges with increasing growth temperature. However, the surface nucleated grains of the films grown at temperatures below 350 °C are very fine (about 20 ( 4 nm) and much smaller than those in the film grown at 500 °C. This result is also consistent with diffusion-controlled coarsening kinetics. Because the atoms at a lower temperature (below 350 °C) have a lower diffusion coefficient, it inhibits the Ostwald ripening process, resulting in smaller grains. However, the atoms have a higher diffusion coefficient at high temperatures (such as 500 °C), enhancing the Ostwald ripening process and increasing the size of the grains. Therefore, there are

enlarged grains for the films grown at high temperatures, but the surface grain density decreases 1 order of magnitude (to about 4.14 × 1010 grains/cm2). The surface grain density is about 4.51 × 109, 1.99 × 1010, 1.62 × 1011, 1.95 × 1011, 3.02 × 1011, and 4.14 × 1010 grains/ cm2 for the growth temperatures of 150, 200, 250, 300, 350, and 500 °C, respectively. Obviously, the surface grain density of the TiO2 films deposited at 100 ALD cycles changes from sparse to dense as the growth temperature is raised from 150 to 350 °C and reaches the highest level of 1011 quantitative order on a centimeter square at 250 °C. The nucleating kinetics of ALD-TiO2 films is analyzed by the Arrhenius plot, as shown in Figure 5b. The nucleating activation energy of films calculated from the slope of Figure 5b is approximately 48.25 kJ/mol, and the corresponding temperature dependence is

D ) 4.9 × 1015 exp[-48.25 ( 7.18 (kJ/mol)/RT] grains/cm2

(3)

where D is the surface nucleated grain density, R is the gas constant, and T is the absolute temperature. The foregoing results show that the surface nucleated grain densities of the films increase with increasing growth temperature between 150 and 350 °C. In other words, the noncrystalline surface spaces in the films decrease with increasing growth temperature. According to the results of the foregoing analysis, the crystal growth mechanism of the TiO2 films grown by ALD is dominated by two important factors, the surface nucleated grain density and the Ostwald ripening effect. In the growth temperature range of 150-250 °C, the grain growth exhibits a negative temperature dependence that could be attributed to the surface nucleated grain density. With the lower surface nucleated grain density, there is a sidelong growth process whereby the grain could be enlarged, without being suppressed by the boundaries of nearby grains. Consequently, although the low diffusion coefficient of atoms leads to little Ostwald ripening at the low temperature, the grain could still be enlarged by sidelong growth. In the growth temperature range of 300-500 °C, the grain growth is dominated by Ostwald ripening. In addition, the diffusion rate of the atoms increases along with the increasing temperature, and thus, the grains coalesce more and the grain size is also enlarged. On the basis of the foregoing discussion, it is suggested that the crystal growth mechanism is dominated by the surface nucleated grain density when the films are grown at lower

Space-Limited Crystal Growth of TiO2 Films by ALD temperatures (e250 °C) but is dominated by the Ostwald ripening at higher temperatures (g300 °C). Figure 6a,b schematically illustrates the crystal growth models of ALD-TiO2 films at low and high temperatures, respectively. In the first stage, (a1) and (b1), the early depositing process of the films, a very thin amorphous layer is deposited on the surface of the substrate. As the depositing process continues, fine nucleated grains are formed in/on the amorphous layer, with a low surface nucleated grain density at the lower temperature (a2), but a high grain density at the higher temperature (b2). As the film deposition continues, the nucleated grains grow into separate and small grains at the lower temperature (a3) but grow quickly into large and coalescent grains through enhanced Ostwald ripening and grain boundary migration at the higher temperature (b3). Finally, because the grains are in the amorphous matrix as obtained at the lower temperature, due to the low surface grain density, the grains can easily be enlarged through horizontal grain growth until the boundaries of the closed grains are in contact. Consequently, the size of the grains is dominated by the surface nucleated grain density, as shown in (a4). However, the high surface grain densities of films as grown at high temperature make horizontal grain growth difficult due to the closed grain boundaries, and the grain growth is controlled by Ostwald ripening at high temperature. Thus, the grains produced at higher temperatures exist as a columnar structure, as shown in (b4). Conclusions In this study, TiO2 films deposited with 1000 cycles show an amorphous structure at 100 °C, a polycrystalline structure of the anatase phase at 150-350 °C, and a polycrystalline structure of the anatase mixed with little rutile phase at 400-500 °C. In addition, two distinct grain-growth behaviors are observed at different growth temperature ranges of 150-250 and 300-500 °C, respectively. The grain growth of the films shows a conventional temperature dependence with an activation energy of about 12.39 kJ/mol, and the grain size increases with the growth temperature of the films from 300 to 500 °C due to Ostwald ripening. At 150-250 °C, the grain sizes of the films exhibit an unconventional temperature dependence, as the grain size decreases with increasing growth temperature. This could be due to the low surface nucleated grain density of the film, resulting in more spaces for the horizontal growth of grains. Thus, it is suggested that this process could be assigned to the space-limited crystal growth mechanism, and it is believed that this interesting mechanism may also occur in other materials grown by ALD.

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