Sputter deposition of polycrystalline and epitaxial TiO 2 films with anatase and rutile structures Lei Miao, Ping Jin, Kenji Kaneko, Sakae Tanemura Presented at the 8th International Conference on Electronic Materials (IUMRS-ICEM 2002, Xi an, China, 10 14 June 2002) 943
Sputter deposition of polycrystalline and epitaxial TiO2 films with anatase and rutile structures Lei Miao 1, Ping Jin 2, Kenji Kaneko 3, Sakae Tanemura 1 1 Department of Environmental Technology & urban planning, Nagoya Institute of Technology, Gokisho-cho, Showa-ku, Nagoya, 466-855 Japan. 2 Institute of Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology (AIST), Moriyama-ku,Nagoya, 463-8560 Japan. 3 Department of Materials Science and Engineering HVEM lab. Kyushu University, Higasi-ku, Hakozaki 6-10-1,Fukuoka, 812-8581 Japan. E-mail: lmiao@system.nitech.ac.jp Abstract Polycrystalline and epitaxial TiO 2 films of rutile and anatase were formed on selected substrates by rf magnetron sputtering of TiO 2 target in Ar under a variety of sputtering parameters. The phase formation was confirmed by X-ray diffraction (XRD), and the in-plane epitaxial relationship between the film and substrate was studied by XRD pole plots. The microstructure of the films was observed by transmission electron microscopy (TEM). Optical properties were evaluated using spectral ellipsometry (SE). The films show higher values of refractive indices than the bulk. Optical band gaps were calculated using the extinction coefficient from SE measurement. The relationship between the optical properties and the film structure was clarified with the experiment results. 944
1. Introduction Hetero-epitaxy of oxide thin films provides a mean to create materials with diverse optical, magnetic, and electrical properties that cannot be accessed in equilibrium bulk crystals [1]. In recent years, titanium dioxide (TiO 2 ) thin films have attracted a lot of interests for its wide uses in photovoltaic devices [2-4], photocatalysts [5], waveguiding[6], semiconductor[7-8], storage capacitors in dynamic random access memories (DRAM) [9] and others. Anatase and rutile are the most common and widely used TiO 2 polymorphs. Although TiO 2 films have been fabricated by a lot of methods, there is still a lack of systematical studies on the sputter deposition and characterization of TiO 2 films with defined crystal structures, and the comparison of the film properties with those of the bulk, particularly on the differences in crystal structure and semiconductor properties. In this study, we reported the formation, crystal structure, film configuration and optical characterization of both the polycrystalline and the epitaxial TiO 2 thin films with anatase and rutile structure prepared by rf magnetron sputtering. Polycrystalline films of rutile and anatase were formed on Si substrate by optimizing the substrate temperature and oxygen additives, and epitaxial films of rutile and anatase were obtained by selecting proper substrates of single crystalline sapphire and SrTiO 3 under similar conditions of substrate temperature, total pressure and oxygen additives for polycrystalline ones, respectively. The deposited films were characterized by XRD and TEM, and the optical band gaps were calculated using the extinction coefficient obtained by SE. The structural and optical properties of the films were compared with those of the bulk using the characterization results, and reasons accounting for these property differences were inferred. 945
The detailed discussion about microstructure and dispersion of complex refractive index from 0.75 ev to 5 ev of epitaxial films will be reported elsewhere [10-11]. 2. Experiment Film depositions were performed in an rf magnetron sputtering system (ULVAC MPS-2000-HC3) composed of a cylindrical chamber (d = 0.34 m, V = 0.047 m 3 ) connected to a 0.21 m 3 /s turbo molecular pump. The system was equipped with 2-inch magnetron sputtering sources positioned at a 30 inclination to the substrate center with a target-to-substrate distance of 150 mm. Silicon (100) (without removal of the natural oxide) and pyrex substrates were used for depositing polycrystalline TiO 2 film while sapphire (110) and SrTiO 3 (100) were used for epitaxial film growth. The substrates were kept at a constant temperature (RT to 600 C) during the deposition using lamp heating on the backside of the alloy plate using an automatic temperature control unit. The deposition system was evacuated to a background pressure down to 2 10-6 Pa, and pure gases of Ar (99.9995%) and O 2 (99.9995%) with required quantity were introduced through mass flow controllers to fill to required pressures for the deposition. Prior to deposition, Si and sapphire substrates were cleaned ultrasonically in ethanol rinse for 15 minutes, while the SrTiO 3 substrate was cleaned with 3 steps, i.e., ultrasonically cleaned in acetone twice (2 minutes for each), then cleaned three times in a 80 acetone vapor, and a final heating at 700 in vacuum in the reactive chamber before sputtering. Deposition was started after 10 minutes pre-sputtering for the cleaning of the target surface and to stabilize the sputtering process. Conditions for the formation of films with rutile and anatase single phase were 946
investigated by changing the deposition parameters, i.e., the substrate temperature, the total Ar pressure and the oxygen additives. First, the substrate temperature was varied from room temperature up to 600 at a fixed Ar pressure of 0.1Pa where single phase rutile appeared at 600. Second, Ar pressure was changed from 0.1 to 1 Pa at a fixed substrate temperature 325, where the anatase phase was the dominant at 1 Pa among an anatase-rutile mixture. Finally, oxygen additive was introduced at a flow rate from 0.1 to 0.7 sccm with fixed substrate temperature of 325 and Ar pressure of 0.25 Pa where the anatase phase, which is much more difficult to form in a single phase under the present deposition conditions, is the most dominant. The optimal conditions for the formation of polycrystalline and epitaxial TiO 2 film were listed in Table1 and 2. The deposited films were characterized by X-ray diffraction (XRD, CuK radiation) with a thin-film scan at a fixed angle of 2 degrees for polycrystalline films, and a normal -2 scan for epitaxial ones. The in-plane epitaxial relationship between the rutile, anatase and the substrates were studied with XRD pole plots by a Philips MRD system. The microstructure, particularly that at the film/substrate interface, was examined in cross section by high-resolution transmission electron microscope (TEM) (JEM-2010 electron microscope operated at 200KV with 1.94 Å resolution and Philips Tech. electron microscopy at 200 KV with the same resolution and fully digitalized image system). 3. Results and discussions 3.1 Characterization of polycrystalline films 947
3.1.1 XRD pattern Figure 1 shows typical XRD patterns of the TiO 2 films with a single phase of rutile or anatase deposited under optimal conditions summarized in Table 1. A rutile phase was relatively easier to obtain in most cases, probably because that the rutile phase is more thermodynamically stable at room temperature, as has been reported [12-13], while the formation of an anatase single phase seemed to be more complex, which requires a relatively higher Ar pressure, a proper substrate temperature and with suitable oxygen additive. 3.1.2 TEM images The microstructure was then observed by TEM. Figure 2 and 3 show cross-sectional HRTEM lattice images of the films on Si with the anatase and rutile structure, respectively. The films are identified as polycrystalline from the random orientation of several lattice planes. Some of the d spacings were measured as indexed in the figures. A native silicon oxide layer of approximate 2 nm in thickness and with smooth surface was also seen in the images. All the d spacing values of TiO 2 films obtained from TEM lattice images were compared with the bulk values [14] and the results were shown in Table 3. The results indicate that the lattice d spacing of the films is slightly smaller than that of the bulk, which implies that the fabricated films are contracted at some lattice directions compared with the bulk. This could be explained as follows: because the lattice constants mismatch between TiO 2 films and Si substrate, the lattice space of film was deformed which lead to the results that all d spacing values we observed are smaller than the bulk. 948
3.2 Characterization of epitaxial films 3.2.1 XRD patterns Figure 4 (a) and (b) show the XRD θ 2θ scan of the epitaxial films of anatase on SrTiO 3 and rutile on sapphire, respectively. Both films exhibit only one reflection peak which could be identified as those from the same lattice direction of the relevant compound, i.e. anatase (004) and rutile (101), in addition to the substrate double strong diffraction peaks [2θ = 37.8 and 2θ = 81 for sapphire (110), (220) plane, 2θ = 22.8 and 2θ = 46.5 for SrTiO 3 (001), (002) plane]. The values correspond well to those of (101) and (004) planes of rutile and anatase, respectively, as compared with the JCPDS cards [14]. From the XRD θ 2θ scan data, we can conclude that the films have strong orientation in the direction normal to the sample surface.the growth orientations between the film and the substrate normal was determined as rutile(101)//sapphire(110), anatase (001)//SrTiO 3 (001). In order to investigate the in plane orientation (the substrate parallel), XRD pole figure study was taken on the epitaxial films. The epitaxial relationship of films and substrates were determined as rutile (101) // sapphire (110), (010) f // (001) s, and anatase (001) // SrTiO 3 (001), (100) f // (100) s by the pole figure results, where f and s denote the film and the substrate, respectively. 3.3 Optical properties The optical properties of both polycrystalline and epitaxial films were evaluated by SE. The complex refractive index was analyzed by the application of two layers model for film configuration (surface thin layer and primary layer) and appropriate dispersion 949
relation of n and k. The detail of the optical model used here will be appeared on the other articles [11]. The obtained real part of refractive index (n) in the designated wavelength range shows higher values than the values cited in the recent articles [15-16]. The representative value at 500 nm wavelength, the maximum value from 400 nm to 800 nm and the maximum value from 250 nm to 1600 nm are given in Table 4. The higher refractive indices give an evidence for the fine crystallinity of the sputtered films. Comparing dispersion relation of refractive index n of epitaxial film with that of polycrystalline films, distinct two humps in epitaxial anatase case and steep peak in epitaxial rutile case are clearly observed respectively. The reasons can be described as densely fine crystalinity of epitaxial films. The optical band gap E g of the polycrystalline and epitaxial TiO 2 films was determined using the imaginary part of refractive index k from the following expression [17]: B( υ Ε g ) m =(4 κ/ λ) υ (1) where B: constant; m: constant depending on the optical transition mode of semiconductor (1/2: direct allowed, 3/2: direct forbidden, 2:indirect allowed, 3:indirect forbidden); υ: photon energy; 4 κ/ λ : absorption coefficient at wavelength λ, κ: imaginary part of refractive index. Since experimental results and theoretical calculation all suggest that TiO 2 has a direct forbidden gap, which is also degenerated with an indirect allowed transition, the indirect allowed transition dominates in the optical absorption just above the absorption edge due to the weak strength of the direct forbidden transition [18-19]: So m=2 was determined in Eq.(1). The extrapolated band gap values of anatase and rutile thin films are listed in Table 950
5. All values of the obtained optical band gap for the films are larger than the bulk [20-21] by about 10 % (reference, please). There are two possible reasons for a large band gap value of the film: first one is presumably due to an axial strain effect from lattice deformation as has been pointed out for ZnO films [22], second one being probably a change in the density of semiconductor carriers. Which of the above is the dominant requires further study. 4. Conclusions Polycrystalline films of rutile or anatase single phase were obtained through a precise process control over the substrate temperature, total pressure and oxygen addition. Epitaxial growth of rutile and anatase was achieved using substrates of sapphire and SrTiO 3 single crystals. The epitaxial relationships were determined as follows: rutile (101) // sapphire (110), (010) f // (001) s, Anatase (001) f // SrTiO 3 (001) s, (100) f // (100) s. TEM observation showed lattice distortion of the films to the bulk. SE measurement demonstrated higher values of refractive indices of the films than the bulk. Optical band gaps of the films were calculated using the imaginary part of refractive index, with values about 10% larger than the bulk. Reasons for a large band gap value of the film is presumably due to an axial strain effect and probably further to a change in the density of semiconductor carriers. 951
References [1]. D.K.Fork, J.M.Phillips, R.Ramesh, and R.M. Wolf, Epitaxial Oxide and Heterostructures, MRS Symposia Proceedings NO.341.(Materials Research S0ociety, Pittsburgh,1996) [2]. K.Kalayanasundaram, M. Grazel, E.Pelizzetti, Coord. 1969 Chem.Rev. 69 57 [3]. B.A.Perkinson, M.T.Spitlet, 1992 Electrochem.Acta. 37 943 [4]. A.Haggfeldt, U. Bjorksten, S.-E. Lindquist, 1992 Solar Energy Mater. Solar Cells 27 293 [5]. S.tokita, N.Tanaka, and H.Saitoh, Jpn.J.Appl. Phys., Part 2 2000 39, L169 [6]. A.Bahtat, M.Bouderbala, M.Bahtat, M.Bouazaoui, J.Mugnier, and M.Druetta, 1998 Thin Solid Films 323,59 [7]. H.Fukuda, S.Namioka, M.Miura, Y. Ishikawa, M.Yoshino, and S. Nomura, 1999 Jpn.J.Appl. Phys., Part 1 38, 6034 [8]. M.B.Lee, M.Kawasaki, M.Yoshimoto, and B.K. Moon, 1995 Jpn.J.Appl. Phys. Part 1 34, 808 [9]. J.Y.Gan,Y.C.Chang, and T.B.Wu, 1998 Appl. Phys. Lett. 72,332 [10]. L. Miao, P. Jin, K.Kaneko, A.Terai, N.Nabatova-Gabain and S. Tanemura. Appl. Phys. Lett. (to be submitted) [11]. S. Tanemura, L. Miao, P. Jin, K. Kaneko, A. Terai and N. Nabotova-Gabin Appl.Surf. Sci. (to be submitted) [12] G.Thorwarth, S.mandl and B. Rauschenbach: 2001 Surface and coatings technology 136 236 [13]. Kunio Okimura: 2001 Surface and Coatings Technology 135 286 [14]. Powder Diffraction File (ICDD, Pennsylvania, U.S.A.1971) 21-1276,21-1272 952
[15]. C,C. Ting and S.Y.Chen, 2000 J.Appl.Phys. 88 (8), 4682. [16]. M. H.Suhail, G.Mohan Rao and S.Mohan, 1992 J. Appl. Phys. 71 (3) 1421. [17]. J.Tauc, Optical Properties of Solids (ed. by F.Abeles ),North-Holland, Amsterdam, pp303 [18]. N.Daude, C.Gout and C.Jouanin: 1977 Phys. Rev.B. 15 3229 [19]. K.M.Glassford, J.R.Chelikowsky: 1992 Phys. Rev.B. 46 1284 [20] D.C.Cronemeyer 1952 Phys. Rev. B 87 876 [21] H.Tang, F.Levy, H.Berger, and P.E. Schmid 1995 Phys. Rev.B. 52 7771 [22].H.C.Ong and A.X.E.Zhu 2002 Appl. Phys. Lett. 80 941 953
Figure caption Fig.1. XRD patterns of polycrystalline films of anatase and rutile on Si. Fig.2. Cross-sectional TEM image and the corresponding diffraction pattern of polycrystalline anatase film on Si (100). Fig.3. Cross-sectional TEM image and the corresponding diffraction pattern of polycrystalline rutile film on Si (100). Fig.4. XRD patterns of θ 2θ scan for epitaxial films of anatase on SrTiO 3 (001) (a)and rutile on sapphire (110) (b). 954
Table 1. The optimal deposition conditions of polycrystalline TiO 2 thin films Phase Substrate Power (W) T sub ( ) P (Pa) Ar (sccm) O 2 (sccm) Rutile Si, glass 160 600 0.1 5.8 0 Anatase Si, glass 160 325 0.25 13.5 0.1 955
Table 2. The optimal deposition conditions of epitaxial TiO 2 thin films Phase Substrates Power (W) Tsub ( ) P (Pa) Ar (sccm) Rutile sapphire 160 600 0.1 5.8 Anatase SrTiO 3 160 600 0.1 5.8 956
Table 3. Values of d spacing from TEM for polycrystalline TiO 2 films in comparison to the bulk. Lattice plane a b (a-b) / b Film d (Å) Bulk d (Å) % Rutile (101) 2.40 2.487-3.50 Rutile (110) 3.21 3.25-1.23 Anatase (101) 3.46 3.52-1.70 Anatase (004) 2.35 2.378-1.18 957
Table 4. Values of refractive index n at the designated wavelength. Rutile Anatase Polycrystalline Epitaxial Polycrystalline Epitaxial n at 500 nm 2.849 2.843 2.657 2.637 Maximum n at 3.182(400) 3.173(400) 2.898(400) 2.892(400) 400-800 nm Maximum n at 3.952(330) 4.142(320) 3.605(320) 3.79(330) 250-1600 nm 958
Table 5. Optical band gap of the anatase and rutile films in comparison to the bulk. Structure Polycrystalline rutile film Polycrystalline anatase film Epitaxial rutile film Epitaxial anatase film Bulk rutile Bulk anatase Band gap 3.34 3.39 3.37 3.51 3.03 3.20 Eg (ev) 959
R (101) R (200) R (111) R (210) R (211) R (220) Intensity (a.u.) R (110) A (101) A (112) A (220) A (211) 20 30 40 2θ (degree) 50 60 Fig.1. L.Miao et al 960
d 004 (anatase)=2.35 Å TiO 2 film Si substrate d 101 (anatase)=3.46 Å Amorphous layer d 111 =3.13 Å Fig.2 L.Miao et al. 961
d 110 (rutile) d 110 (rutile)=3.21 Å d 101 (rutile)=2.40 Å TiO 2 film Amorphous layer Si substrate d (111) =3.13 Å Fig.3.L.Miao et al. 962
120x10 3 100 (a) SrTiO3 (600 Intensity (CPS) 80 60 40 20 SrTiO3 (001) Anatase (004) SrTiO3 (002) 0 20 30 40 2θ (degree) 50 60 4000 (b) Sapphire (600 3000 Intensity (CPS) 2000 1000 (101) Rutile AL2O 3 (110) AL2O 3 (220) 0 30 40 50 60 2θ (degree) 70 80 90 Fig.4. L. Miao et al. 963