CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 2 115-116
Communications Growth of CeO2 Films on Glass Substrates Using Electron-Beam-Assisted Evaporation Naomichi Sakamoto,*,† Tomoyasu Inoue,† and Kazuhiro Kato‡ Department of Electronics and Computer Science, Iwaki Meisei University, 5-5-1 Chuodai-Iino, Iwaki 970-8551, Japan, and Materials Development Department, Fukushima Technology Centre, 1-12 Machiikedai, Koriyama 963-0215, Japan Received October 24, 2002;
Revised Manuscript Received January 16, 2003
ABSTRACT: The growth of CeO2 films on glass substrates using electron-beam-assisted evaporation is investigated. Experiments with varying growth temperatures (room temperature to 750 °C) reveal that orientation controlled CeO2 films are obtained. Electron-beam-assisted evaporation enhances preferential orientation and enlarges the grain size of the films. To realize high-performance thin-film transistors (TFTs) for application to active-matrix liquid-crystal display devices, tremendous efforts have been made to grow highquality polycrystalline Si (poly-Si) films on glass substrates. In the present fabrication of TFT, poly-Si films are formed by the crystallization of amorphous Si films deposited by plasma-enhanced chemical vapor deposition, utilizing excimer laser annealing,1 solid-phase crystallization,2 or metal-induced lateral crystallization3,4 as the crystallization methods. These crystallization processes are required since it is difficult to grow poly-Si films directly on amorphous substrates. To realize high-quality poly-Si film on glass substrates without using these crystallization processes, we propose a novel method utilizing substrates with crystallinity. In this work, a CeO2/glass structure was investigated for the crystalline substrate. CeO2 films were grown on the glass substrates using electron-beam evaporation with simultaneous electron-beam irradiation toward the substrate surface (electron-beam-assisted evaporation). Inoue et al. reported that electron-beam-assisted evaporation enhanced the crystalline quality of the CeO2 films compared with conventional electron-beam evaporation.5-7 It was expected that electron-beam irradiation would be effective in the growth of CeO2 films on the glass substrates. In this paper, we describe the growth temperature dependence of the crystallographic orientation in the growth of CeO2 films, and the effect of electron-beamassisted evaporation. Silica glass plates with an optical flat finish were chosen as the substrates due to the viewpoint of microelectronic applications. The substrates were chemically cleaned by repeating the following procedure twice: dipping in a hot * To whom correspondence should be addressed. Iwaki Meisei University, Department of Electronics and Computer Science, 5-5-1 Chuodai-Iino, Iwaki 970-8551, Japan. Fax: +81-246-29-7029, Phone: +81-246-29-0577, Email:
[email protected]. † Iwaki Meisei University. ‡ Fukushima Technology Centre.
aqueous solution of HCl and H2O2, then diluted hydrofluoric acid, followed by rinsing in deionized water. The films were grown by both electron-beam-assisted evaporation and conventional evaporation. In the electron-beamassisted evaporation experiments, the accelerating energy of the electron beam and the electron beam current density were 360 eV and ∼6 µA/cm2, respectively, where these values were determined on the basis of previous research.5-7 The substrate temperature was varied from room temperature to 750 °C. The films were grown by the electron-beam evaporation of CeO2 tablets with a 99.999% purity. To control the stoichiometry of the CeO2 films, O2 gas was introduced into the chamber at a pressure of 8 × 10-6 Torr, so as to oxidize evaporating oxygen-deficient species such as CeO, Ce2O3, and Ce, which were generated from the dissociation of CeO2 at the electron-beam evaporation source. The film thickness and the growth rate were in the range of 100-210 nm and 0.10-0.22 nm/s, respectively. The crystallographic properties of the CeO2 films were investigated by X-ray diffraction (XRD) with Cu KR radiation. Figure 1 shows the XRD patterns of the CeO2 films grown using electron-beam-assisted evaporation. It was understood that the orientation of the films varied depending on the growth temperature. CeO2 films with (111) and (110) orientations grew in a low-temperature region, whereas those with (100) and (311) orientations grew in a high-temperature region. It was also found that CeO2 films grown using conventional evaporation have a similar orientation dependence. Figure 2a,b shows the integrated intensities of the (111) diffraction peak (I(111)) and (200) diffraction peak (I(200)) as a function of the growth temperature, respectively, where the peak intensity is normalized with the film thickness. From Figure 2a, I(111) is large below 200 °C and decreases rapidly with increasing growth temperatures. In contrast, Figure 2b indicates that I(200) is large above 600 °C, and decreases rapidly with decreasing growth temperatures, resulting in the disappearance of the
10.1021/cg025604o CCC: $25.00 © 2003 American Chemical Society Published on Web 02/05/2003
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Figure 3. Temperature dependence of grain size estimated from full widths at half-maximum values of the XRD peaks.
Figure 1. XRD patterns of CeO2 films grown using electron-beamassisted evaporation.
Figure 2. Normalized integrated intensity of the (111) diffraction peak (I(111)) (a) and (200) diffraction peak (I(200)) (b) as a function of the growth temperature.
peak. These results reveal that the orientation of CeO2 films can be controlled by varying the growth temperature. Preferential orientation of (111) or (100) can be selected by choosing the growth temperature below or above ∼400 °C, respectively. We think that this feature leads to the orientation controlled over growth of semiconductor thin films, which will be valuable for application to microelectronic devices. Here, we describe the effect of electron-beam irradiation on the crystallinity of CeO2 films. In Figure 2a,b, it was found that the normalized peak intensities of films grown using electron-beam-assisted evaporation were larger than those grown using conventional evaporation. These results indicate that assisting electron-beam irradiation enhances the preferential orientation of CeO2 films. Figure 3 shows the temperature dependence of the grain size estimated
from the full width at half-maximum values of XRD peaks. The grain size increased with increasing growth temperature over a range of growth temperature from room temperature to 750 °C in either growth method. From Figures 2b and 3, it was recognized that (111) intensity decreased with increasing the growth temperature and vanished at 750 °C, whereas the grain size increased. Comparing the values at the same growth temperature, the grain size of the films grown using electron-beamassisted evaporation were 1.1-1.4 times as large as those grown using conventional evaporation. It is thought that the assisting electron-beam gives sufficient kinetic energy to the surface atoms and molecules through Coulombic interaction.7 The above results prove the effectiveness of electron-beam-assisted evaporation on CeO2 film growth on glass substrates. In conclusion, we performed the growth of CeO2 films on glass substrates using electron-beam-assisted evaporation. It was found that polycrystalline CeO2 films were obtained at considerably low temperature. The crystallographic orientation of the films were controlled by varying the growth temperature. Preferential orientation of (111) or (100) can be selected by choosing the growth temperature below or above ∼400 °C, respectively. The grain size increased with increasing the growth temperature. Electronbeam-assisted evaporation enhanced the preferential orientation and enlarged the grain size of the films. It was proven that electron-beam-assisted evaporation was effective for CeO2 film growth on glass substrates compared with conventional evaporation. Acknowledgment. The authors are grateful to T. Suzuki, T. Takayama, T. Kotaki, and A. Kimiwada for their assistance in the experiments.
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