Thermochemical Stability of IrO 2 Bottom Electrodes in Direct-Liquid-Injection Metalorganic Chemical Vapor Deposition of Pb(Zr,Ti)O 3 Films

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1 Japanese Journal of Applied Physics Vol. 43, No. 5A, 2004, pp #2004 The Japan Society of Applied Physics Thermochemical Stability of Bottom Electrodes in Direct-Liquid-Injection Metalorganic Chemical Vapor Deposition of Pb(Zr,Ti)O 3 Films Kyung-Mun BYUN and Won-Jong LEE Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Guseong-dong, Yuseoung-gu, Daejeon , R. O. Korea (Received July 29, 2003; accepted January 21, 2004; published May 11, 2004) The film has been regarded as a leading candidate bottom electrode of ferroelectric capacitors in ferroelectric random access memories (FRAMs). We have addressed a new issue on the thermochemical stability of bottom electrodes during the growth of Pb(Zr,Ti)O 3 () ferroelectric films on such electrode using direct liquid injection metalorganic chemical vapor deposition (DLI-MOCVD). The electrode thermally dissociated at elevated temperatures in vacuum ambient at a low oxygen pressure. It was also reduced by carbon and hydrogen dissociated from the solvent in liquid solution. The reduction of by the solvent was more pronounced at lower temperatures, which is attributed to the longer residence time of solvent molecules on the surface at lower temperatures. The reduction of was also induced by the metal elements Zr and Ti in metalorganic precursors because they have higher chemical affinities with oxygen than. The reduction by Zr and Ti metal elements was more pronounced at higher temperatures. The use of thin Pt interlayers is a promising solution for the prevention of the reduction of electrode during the growth of films. [DOI: /JJAP ] KEYWORDS:, FRAM,, MOCVD, direct liquid injection, film, electrode 1. Introduction idium oxide ( ) is a conductive metallic oxide with the highest bulk conductivity among transition metal oxides. When an film is used as the electrode of Pb(Zr,Ti)O 3 () capacitors in ferroelectric random access memories (FRAMs), it effectively alleviates the polarization fatigue problems of capacitors by compensating the charged defects of oxygen vacancies at /electrode interfaces. An film also acts as an excellent barrier layer against oxygen diffusion at high temperatures to suppress the oxidation of polycrystalline silicon plugs in capacitor-onplug (COP) memory cell structures. From these superior properties, the film has been regarded as a leading candidate for the bottom electrode of ferroelectric capacitors even though its material cost is relatively high. 1 5) The conformal deposition of films on a finely (subquarter-micron scale) patterned bottom electrode is essential in realizing the three-dimensional (3D) capacitor structure of high-density (128 Mb) FRAM devices. Low-temperature (450 C) metalorganic chemical vapor deposition (MOCVD) is considered to be the most suitable technique for fabricating films in 3D capacitors because it provides an excellent step coverage. 6 9) The key factors in the MOCVD process is the stable storage and reproducible delivery of metalorganic precursors. In conventional bubbler systems, metalorganic precursors oligomerize readily by heating at temperatures exceeding their melting points, resulting in changes in their vapor pressures with time. The direct-liquid-injection (DLI) system has emerged as an alternative to the bubbler system. In the DLI system, the liquid solution prepared by dissolving solid metalorganic precursors in organic solvents is stored and delivered at room temperature to prevent oligomerization thus minimizing the variation in vapor pressure. The liquid solution is directly injected into a heated vaporizer and flash-evaporated, and then the generated vapors are introduced into the reactor. It is now considered that DLI-MOCVD is the most address: wjlee@kaist.ac.kr suitable process for the mass production of high-density FRAM devices. 8 13) Our original objective was to fabricate ferroelectric films on electrodes at low temperatures by DLI- MOCVD. However, we encountered an undesirable problem associated with the reduction of bottom electrodes. Although this issue is of great importance in the fabrication of reliable capacitors, it has never been reported in the literature because the DLI-MOCVD technique as well as electrodes in the MOCVD system has only been adopted very recently. We systematically investigated the thermochemical stability of electrodes in the DLI- MOCVD of films and suggested a promising solution to the reduction problem of these electrodes Experimental 50-nm-thick bottom electrode films were deposited at 250 C by DC magnetron reactive sputtering on thermally oxidized silicon wafers. To investigate its thermal stability in vacuum ambient, was annealed at 420 C for 60 min in vacuum as a function of oxygen pressure (P O2 ). The effect of organic solvent on the reduction of was also assessed by annealing it at 420 C for 60 min as a function of (P O2 )in ambient containing solvent vapors. The solvents used were n-butylacetate (C 6 H 12 O 2 ) and tetrahydrofuran (C 4 H 8 O). The solvents were directly injected at 0.05 cc/min into the flash vaporizer heated at 230 C. The evaporated solvent vapors were swept by the carrier gas (Ar) and carried into the reactor with the oxidant gas (O 2 ). The partial pressures of n-butylacetate (P n-ba ) and tetrahydrofuran (P THF ) in the reactor were 1: and 2: Torr, respectively. ngle oxide (PbO, ZrO 2 and TiO 2 ) films deposited on the electrodes were deposited by DLI-MOCVD to investigate the reduction effect of metal elements in metalorganic precursors. The metalorganic precursors used in our study were Pb(TMHD) 2 (TMHD = tetramethylheptanedionate, C 11 H 19 O 2 ), Zr(TMHD)(O-i-Pr) 3 (O-i-Pr = iso-propoxide, C 3 H 7 O) and Ti(TMHD) 2 (O-i-Pr) 2. Each solid metalorganic precursor was dissolved in n-butylacetate with a molar concentration of 0.05 M to prepare the liquid solution. The

2 2656 Jpn. J. Appl. Phys., Vol. 43, No. 5A (2004) K.-M. BYUN and W.-J. LEE liquid solution was injected at a rate of 0.05 cc/min and the partial pressure of the metalorganic precursor in the reactor was Torr. The deposition of the single oxide films was conducted at 420 CataP O2 of 1.2 Torr. The films were deposited using a cocktail solution prepared by mixing the three liquid solutions. Phase identification was performed by X-ray diffraction (XRD, Rigaku D/MAX-RC) with a Bragg Brentano ( 2) diffraction geometry using Cu Ka radiation. The surface and cross-sectional morphologies of the specimens were observed by scanning electron microscopy (SEM, Philips XL- 30s). 3. Results and Discussion The XRD pattern and cross-sectional SEM image of the as-prepared 50-nm-thick bottom electrode are shown in Fig. 1. A film was deposited on by DLI- MOCVD at 420 CataP O2 of 1.2 Torr. Figure 1 displays the XRD pattern and cross-sectional SEM image of the electrode after deposition of the film. The XRD pattern shows that peaks diminish in intensity and peaks appear, indicating the phase transformation of into. As shown in the SEM image in Fig. 1, the thickness of the bottom electrode decreases to 35 nm because of volume shrinkage by the phase transformation of into. Based on the densities and molecular weights of and, we can show that a layer of with a thickness of 50 nm is completely transformed to a layer of with a thickness of 22 nm. These results indicate that the electrode is partially reduced during the growth of a film at 420 C at a P O2 of 1.2 Torr. If the deposited film forms contact with the metal at the interface, polarization fatigue may readily occur in the absence of an oxygen vacancy sink. Moreover, the drastic volume change of the electrode could degrade oxygen diffusion barrier performance. It is therefore necessary that the bottom electrode remains stable without reduction during the growth of the film. Some researchers fabricated films on electrodes by chemical solution deposition (CSD) or plasma-enhanced MOCVD techniques. 3 5) Although the issue on reduction was not considered in their works, we found from their XRD patterns that electrodes were not reduced. It is thus thought that the serious reduction problem of electrodes is unique to the DLI-MOCVD process. We have investigated the causes of the reduction of during the deposition of films by DLI-MOCVD and attempted to find a solution to this reduction problem. 3.1 Thermal stability of electrode in vacuum ambient It has been reported that is reduced to at elevated temperatures in vacuum ambient ) We have confirmed this phenomenon and systematically investigated the thermal stability of films in vacuum ambient. Figure 2 illustrates the XRD patterns of films annealed at 420 C for 60 min in vacuum ambient at various oxygen pressures. The phase remains stable at a P O2 of Torr or higher. At Torr, however, is completely reduced to. This result suggests that the thermally stable phase is not but in ambient where the oxygen pressure is Torr at 420 C. The binary-phase diagram of the system with oxygen pressure and temperature as coordinates is shown in Fig. 3. The phases of and were identified by XRD. The boundary line, which represents the equilibrium oxygen pressure (P O 2 ) at which two phases coexist, is drawn as a solid line to separate two phases in the diagram. The region above the P O 2 line is where the film is thermochemically stable, whereas the region below the line is where the film is stable. P O 2 increases monotonically on the log scale at higher temperatures, which implies that thermally dissociates more actively at higher temperatures. The higher the oxygen pressure required to keep films stable, the higher the temperature. Standard free energy (G ) for the reduction of an (! + O 2 ) can be numerically obtained from its P O 2 line in Fig. 3. Assuming that the activities of both and solid films are unity and the activity of O 2 is equal to P O2, XRD Intensity (arb. units) + as-prepared (c) 4x10-3 Torr 20x10-3 Torr (d) 1x10-3 Torr Fig. 1. XRD patterns and cross-sectional SEM images: as-prepared bottom electrode, after deposition of film on by DLI- MOCVD at 420 CataP O2 of 1.2 Torr Fig. 2. XRD patterns of electrodes: as-prepared, (d) annealed at 420 C in vacuum ambient at various oxygen pressures.

3 Jpn. J. Appl. Phys., Vol. 43, No. 5A (2004) K.-M. BYUN and W.-J. LEE 2657 (Torr) Fig. 3. the equilibrium constant (K) for the reduction reaction becomes P O2. Standard free energy is thus given by G ¼ RT ln K ¼ RT ln P O2 ; ð1þ where R and T are the gas constant (1.987 cal/molk) and the absolute temperature (K), respectively. G, as a function of T, is generally expressed by the empirical formula: G ¼ a þ bt log T þ ct; ð2þ where a, b and c are the constants dependent on the reaction. Equations (1) and (2) are combined to give ln P O2 ¼ 1 a R T þ b log T þ c : ð3þ Using eq. (3), the line of P O 2 in Fig. 3 is fitted to ln P O 2 ¼ :11 log T þ 36:7: ð4þ T The constants a, b and c obtained from eqs. (3) and (4) are 33040, 18.1 and 72:9, respectively. Upon substituting these constants into eq. (2), we obtain G ¼ þ 18:1T log T 72:9Tðcal/molÞ ð5þ This G function of eq. (5) was used to estimate the free energies of various reactions which will be shown later. The P O 2 of the bulk can be calculated from its thermochemical data (a ¼ 52200, b ¼ 12:65 and c ¼ 77:25) 17) and is drawn as a dotted line in Fig. 3. The P O 2 of the film is higher than that of the bulk oxide, which implies that the film is likely to reduce more easily than the bulk oxide. This is probably because the film was sputter-grown at low temperatures to have higher defect densities than the bulk oxide. The P O2 adopted for the deposition of the film shown in Fig. 1 was 1.2 Torr. This P O2 value is much higher than the P O 2 of 1: Torr at 420 C from eq. (4), but it failed to prevent the reduction of the film. This indicates that thermal dissociation in vacuum is not the major factor for the reduction of during the deposition of film by DLI-MOCVD. film Bulk Temperature ( C) Binary-phase diagram of system. 3.2 Thermochemical stability of electrode in ambient containing solvent vapors Most metalorganic precursors for are solid powders at room temperature and organic solvents such as n- butylacetate and tetrahydrofuran are generally used to dissolve them; the liquid solutions obtained are used in DLI-MOCVD. 8 13) The effect of organic solvents on the reduction of films has not been reported yet. To examine this effect, films were annealed at 420 C for 60 min in ambient containing n-butylacetate vapors. The injection rate of n-butylacetate was 0.05 cc/min (P n-ba ¼ 1: Torr). Figure 4 shows the XRD patterns and planar SEM images of films annealed as functions of. The films remain stable at a P O 2 of 0.96 Torr or higher. At 0.8 Torr or below, the films are partially or completely reduced and exhibit large microvoids due to volume shrinkage induced by phase transformation. These microvoids degrade the diffusion barrier performance of electrodes, which is one of the reasons why should maintain its stability. The n-butylacetate vapors adsorbed on the surface 1.2Torr 0.96Torr 0.8Torr 0.48Torr 0.19Torr Fig. 4. XRD patterns and planar SEM images of electrodes annealed at 420 C at various oxygen partial pressures in the ambient containing n- butylacetate vapors (P n-ba ¼ 1: Torr).

4 2658 Jpn. J. Appl. Phys., Vol. 43, No. 5A (2004) K.-M. BYUN and W.-J. LEE will dissociate to produce various species including carbon and hydrogen. nce they have higher chemical affinities with oxygen than, they can reduce films through þ C! þ CO 2 ; ð6þ þ 2H 2! þ 2H 2 O: ð7þ The free energies G of the reactions (6) and (7) at 420 C are 76:2 and 81:8 kcal/mol, respectively. films were reported to be reduced when annealed in ambient containing H 2 or D 2 gas, 18,19) which supports the feasibility of reaction (7). When sufficient P O2 is maintained, the reduction can be effectively suppressed by the oxidation of carbon and hydrogen and by the reoxidation of. Figure 5 shows the phase diagram of the system in ambient containing n-butylacetate vapors (P n-ba ¼ 1: Torr). The P O 2 =P n-ba (and the corresponding P O 2 at 1: Torr P n-ba ) above which is thermochemically stable is drawn as a solid line in the diagram. Note that P O 2 =P n-ba ratio increases exponentially at lower temperatures. That is, the reduction of by n-butylacetate becomes more pronounced at lower temperatures, in contrast to the result in Fig. 3. The G values of the reactions (6) and (7) change little with temperature in the range from 340 to 480 C: the free energies for reaction (6) are 75:1 kcal/ mol at 340 C and 77:0 kcal/mol at 480 C, and those for reaction (7) are 82:6 kcal/mol at 340 C and 81:1 kcal/ mol at 480 C. This suggests that the thermodynamic driving force of reduction by n-butylacetate is nearly constant in the observed temperature range. Therefore, the pronounced / Pn-BA / PTHF Temperature ( C) 10 1 (Torr) at 1.4x10-2 Torr Pn-BA (Torr) at 2.2x10-2 Torr PTHF Fig. 5. Binary-phase diagram of system in the ambient containing solvent vapors: n-butylacetate (P n-ba ¼ 1: Torr) and tetrahydrofuran (P THF ¼ 2: Torr). reduction at lower temperatures is attributed to the prolongation of the residence time of n-butylacetate molecules adsorbed on the surface at lower temperatures. The mean residence time () of the molecules adsorbed on the solid surface at the substrate temperature T is expressed as 20) ¼ 1 exp E D ð8þ RT where is the vibrational frequency of the molecules on the surface (typically Hz) and E D is the energy required to redesorb the molecules. nce increases exponentially as T decreases, the prolonged residence time of n-butylacetate molecules on the surface would promote the reduction of. The mean residence time of oxygen molecules on is much higher and much less temperature-dependent than that of n-butylacetate molecules because oxygen molecules have a smaller desorption energy than n-butylacetate molecules. A tendency similar to that for n-butylacetate is also observed for tetrahydrofuran (P THF ¼ 2: Torr) as shown in Fig. 5. P O 2 =P THF ratio also increases exponentially at lower temperatures, but the degree of increase is slightly lower than that for n-butylacetate probably because the desorption energy of tetrahydrofuran molecules is somewhat smaller than that of n-butylacetate molecules. 3.3 Reduction of electrode by metalorganic precursors As shown in Fig. 1, the electrode was reduced during the deposition of at 420 CataP O2 of 1.2 Torr. This P O2 is higher than the P O 2 of about 1 Torr at 420 Cin the ambient containing n-butylacetate vapors as shown in Fig. 5. Therefore, the reduction of during the growth of in Fig. 1 cannot be fully explained by thermal dissociation in vacuum and by the chemical attack of solvent molecules. Another possible reducing agent could be metal elements in their corresponding metalorganic precursors. To verify this possibility, individual single oxide films of PbO, ZrO 2 and TiO 2 were deposited on electrodes at 420 C at a P O2 of 1.2 Torr. The partial pressure of each metalorganic precursor in the reactor was Torr. The XRD patterns and cross-sectional SEM images of the three single oxide films deposited on electrodes are shown in Fig. 6. The electrodes remain intact during the deposition of the PbO film. The reduction of occurs partially for the ZrO 2 film and quite remarkably for the TiO 2 film. In the XRD pattern shown in Fig. 6(c), the peaks disappear and distinctive peaks are observed, indicating the complete reduction of. This is confirmed from the cross-sectional SEM image in which the electrode underneath the TiO 2 film decreases in thickness by almost 60%. From these results, the significant reducing agents based on Fig. 1 are Zr and Ti metal elements. The plausible reduction reactions of films by Pb, Zr and Ti are þ 2Pb! þ 2PbO; ð9þ þ Zr! þ ZrO 2 ; ð10þ þ Ti! þ TiO 2 : ð11þ The G values for reactions (9), (10) and (11) at 420 C are 53, 209 and 171 kcal/mol, respectively. These G

5 Jpn. J. Appl. Phys., Vol. 43, No. 5A (2004) K.-M. BYUN and W.-J. LEE 2659 PbO ZrO 2 TiO 2 (c) PbO ZrO 2 + TiO 2 Fig. 6. XRD patterns and cross-sectional SEM images of single oxide films deposited on : PbO film, ZrO 2 film and (c) TiO 2 film. The substrate temperature was 420 C and the P O2 was 1.2 Torr. values were calculated from the thermochemical data of PbO, ZrO 2 and TiO 2 bulk oxides 17) because the thermochemical data of films of these oxide were not available. All the values are negative, suggesting that the above reactions, especially the reduction reactions by Zr and Ti elements, are thermochemically favored. nce Ti was found to be the strongest reducing agent, the reduction of electrode during TiO 2 film deposition was investigated in detail. Figure 7 shows the binary-phase diagram of the system in the environment in which TiO 2 films are deposited. The vapor pressure of the Ti precursor (P Ti-precursor ) was Torr. All data points in the diagram were obtained above oxygen partial pressures high enough to prevent reduction by n-butylacetate. The P O 2 =P Ti-precursor ratio (and corresponding P O 2 at Torr P Ti-precursor ) above which is thermochemically stable is drawn as a solid line in the diagram. The reduction of becomes more pronounced at higher temperatures, which is ascribed to the increase in the reaction rate of Ti elements with temperature. To grow TiO 2 films at 420 C without the reduction of, an oxygen partial pressure higher than about 15 Torr is required. At such a high P O2, however, the TiO 2 film exhibited a very low growth rate and poor crystallinity. It is therefore concluded that increasing the P O2 to too high a value is not a desirable solution to the reduction problem. Instead, we adopted a diffusion barrier layer on the electrode to suppress the reactions of solvent molecules and metal elements in metalorganic precursors with the electrode. The Pt film was chosen as the barrier layer because it is chemically stable and generally provides favorable nucleation sites for perovskite phases. 21,22) The 20-nm-thick Pt layer was sputter-grown on to construct a Pt/ hybrid electrode. Figure 8 shows the XRD pattern and cross-sectional SEM image of the asprepared Pt/ hybrid electrode. We firstly deposited ZrO 2 and TiO 2 single oxide films on the Pt/ hybrid electrodes at 420 C at a P O2 of 1.2 Torr, and found that the electrodes were not reduced. Then we deposited a film under conditions identical to those shown in Fig. 1 and again found that the bottom electrode was not reduced at all as shown in Fig. 8. This shows that the insertion of thin Pt interlayers effectively suppresses the reduction of by blocking the diffusion of reducing species. When the Pt layers were thinned below 20 nm at 420 C, the reduction of was observed. The reduction also occurred when the temperature was elevated to more than 420 C for the 20-nm-thick Pt layer. Our previous works 21 23) have revealed that capacitors having a Pt/oxide (e.g. Pt / PTi-precursor Temperature ( C) + 10 (Torr) at 9x10-5 Torr PTi-precursor Fig. 7. Binary-phase diagram of system in ambient containing Ti-precursor vapors (P Ti-precursor ¼ Torr). Pt Pt Fig. 8. XRD patterns and cross-sectional SEM images: as-prepared Pt/ hybrid electrode, after deposition of film on Pt/ by DLI-MOCVD at 420 CataP O2 of 1.2 Torr.

6 2660 Jpn. J. Appl. Phys., Vol. 43, No. 5A (2004) K.-M. BYUN and W.-J. LEE Pt/RuO 2 or Pt/LaNiO 3 ) hybrid electrode system exhibit the better fatigue resistance when a thinner Pt interlayer is adopted. Therefore, the thickness of the Pt interlayer should be appropriately compromised to minimize fatigue as well as to prevent the reduction of the electrode. 4. Conclusions We investigated the thermochemical stability of bottom electrodes during the growth of films using DLI-MOCVD. The equilibrium oxygen pressures above which electrodes remain stable were systematically evaluated in various reducing environments. The electrodes thermally dissociated at elevated temperatures in vacuum ambient at a low oxygen pressure. The solvent also served as a reducing agent, which is attributed to the carbon and hydrogen dissociated from the solvent molecules. The reduction due to solvent was more pronounced at lower temperatures because of the longer residence time of solvent molecules on the surface. The metal elements Zr and Ti in the metalorganic precursors also reduced since they have higher chemical affinities with oxygen than. The reduction due to the metal elements Zr and Ti was more pronounced at higher temperatures. The use of thin Pt interlayers was found to be effective in preventing the reduction of electrodes during the growth of films using DLI-MOCVD. Acknowledgments This research was supported by the Center for Electronic Packaging Materials (CEPM) of Korea Science and Engineering Foundation (KOSEF). 1) T. S. Chen, V. Balu, B. Jiang, S. H. Kuah, J. C. Lee, P. Chu, R. E. Jones, P. Zurcher, D. J. Taylor and S. Gillespie: Integrated Ferroelectrics 16 (1997) ) C. U. Pinnow, I. Kasko, C. Dehm, B. Jobst, M. Seibt and U. Geyer: J. Vac. Sci. Technol. B 19 (2001) ) T. Nakamura, Y. Nakao, A. Kamisawa and H. Takasu: Jpn. J. Appl. Phys. 33 (1994) ) H. C. Lee and W. J. Lee: J. Vac. Sci. Technol. A 20 (2002) ) K. Maki, B. T. Liu, H. Vu, V. Nagarajan, R. Ramesh, Y. Fujimori, T. Nakamura and H. Takasu: Appl. Phys. Lett. 82 (2003) ) H. Fujisawa, K. Kita, M. Shimizu and H. Niu: Jpn. J. Appl. Phys. 40 (2001) ) M. Aratani, T. Oikawa, T. Ozeki and H. Funakubo: Appl. Phys. Lett. 79 (2001) ) H. R. Kim, S. Jeong, C. B. Jeon, O. S. Kwon, C. S. Hwang, Y. K. Han, D. Y. Yang and K. Y. Oh: J. Mater. Res. 16 (2001) ) D. H. Kim, J. S. Na and S. W. Rhee: J. Electrochem. Soc. 148 (2001) C ) P. C. Van Buskirk, S. M. Bilodeau, J. F. Roeder and P. S. Kirlin: Jpn. J. Appl. Phys. 35 (1996) ) T. Kawahara, M. Yamamuka, A. Yuuki and K. Ono: Jpn. J. Appl. Phys. 35 (1996) ) M. Miyake, K. Lee, S. Kawasaki, Y. Ueda, S. Okamura and T. Shiosaki: Jpn. J. Appl. Phys. 41 (2002) ) M. Kadoshima, T. Nabatame, M. Hiratani, Y. Nakamura, I. Asano and T. Suzuki: Jpn. J. Appl. Phys. 41 (2002) L ) M. Peukert: Surf. Sci. 144 (1984) ) S. Y. Cha and H. C. Lee: Jpn. J. Appl. Phys. 38 (1999) L ) K. L. Saenger, P. C. Andricacos, S. D. Athavale, J. D. Baniecki, C. Cabral, Jr., G. Costrini, K. T. Kwietniak, R. B. Laibowitz, J. J. Lian, Y. Limb, D. A. Neumayor and M. L. Wise: Mater. Res. Soc. Symp. Proc. 655 (2001) CC ) O. Kubaschewski and C. B. Alcock: Metallurgical Thermochemistry (Pergamon Press, Oxford, 1979) 5th ed. 18) K. Kushida-Abdelghafar, M. Hiratani and Y. Fujisaki: J. Appl. Phys. 85 (1999) ) J. S. Cross, Y. Horii, N. Mizuta, S. Watanabe and T. Eshita: Jpn. J. Appl. Phys. 41 (2002) ) M. Ohring: The Materials Science of Thin Films (Academic Press, New York, 1992) Chap ) S. O. Chung: Ph. D. Dissertation, Department of Materials Science and Technology, Korea Advanced Institute of Science and Technology, Korea, ) H. C. Lee and W. J. Lee: Jpn. J. Appl. Phys. 40 (2001) ) D. C. Kim and W. J. Lee: Jpn. J. Appl. Phys. 41 (2002) 1470.