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Growth of SrTiO3 thin films on LaAlO3(001) substrates; the influence of growth temperature on composition, orientation, and surface morphology
Thin Solid Films 360 (2000) 181±186 www.elsevier.com/locate/tsf
Growth of SrTiO3 thin ®lms on LaAlO3(001) substrates; the in¯uence of growth temperature on composition, orientation, and surface morphology Xin Wang 1, Sveinn Olafsson 2, Per SandstroÈm, Ulf Helmersson* Department of Physics, LinkoÈping University, SE-581 83 LinkoÈping, Sweden Received 21 July 1999; received in revised form 30 November 1999; accepted 3 December 1999
Abstract SrTiO3 ®lms have been grown on LaAlO3(001) single crystal substrates using rf-sputtering. The substrates were held at temperatures ranging from 100 to 8508C. For growth temperatures as low as 3508C epitaxial growth is observed. Below 3508C the ®lms are polycrystalline and three different orientations (100), (110), and (111) can be observed using X-ray diffraction. Atomic force microscopy shows that ®lms deposited at temperatures below 3508C and above 6508C are smooth while the surfaces of the ®lms made at intermediate temperatures are rough and faceted. As growth temperatures decrease below 2508C, the ®lms show decreasing amount of Sr. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Dielectrics; Oxides; Sputtering; Surface roughness
1. Introduction SrTiO3 is an interesting perovskite compound both from a fundamental and a practical application point of view. It is an incipient ferroelectric material with a relatively high dielectric constant with possible applications in, e.g. dynamic random access memories (DRAMs) [1,2] and in high value or tunable capacitors [3±5]. Low temperature growth of this compound ®lm is vital for many practical applications since it can hinder interdiffusion and interface reactions with substrate materials and make SrTiO3 ®lms compatible with existing technologies. This article describes results on growth of SrTiO3 on lattice matched LaAlO3(100) substrate using r.f. sputtering. The temperature of the substrate during growth was varied between 100 and 8508C. The lowest growth temperature where epitaxial growth of SrTiO3 has been reported is, to our knowledge, 3308C [6]. They utilized molecular beam epitaxy for growth on Nbdoped SrTiO3(100) substrates at a low growth rate of 1 nm/ min. Lesaicherre et al. [7] were using electron cyclotron resonance chemical vapor deposition and was able to * Corresponding author. Tel.: 1 46-1328-1685; fax: 146-1328-8918. E-mail address: [email protected] (U. Helmersson) 1 Present address: Acreo AB, Bredgatan 34, SE-602 21 NorrkoÈping, Sweden. 2 Present address: Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavik, Iceland.
grow (110) oriented ®lms at a temperature of 4008C on rcut sapphire (11Å02) substrates. For ®lms grown by laser ablation, the lowest reported growth temperature where the SrTiO3 phase is formed is 2508C, however, the ®lms are polycrystalline [8]. For r.f. magnetron sputtering the lowest reported growth temperature achieving epitaxial ®lms is 5408C, also using a low growth rate of 1±4 nm/ min [9]. In sputtering, the presence of energetic particles provides a possibility to increase the surface atom mobility during growth to promote epitaxial growth at lower temperatures. Too high a degree of bombardment, especially with high energies, will on the other hand lead to creation of lattice defects and sputter etching of the ®lm. In the present work, the degree of ion bombardment is controlled by varying the sputtering gas pressure. At low sputtering gas pressures the number of ionized atoms in the plasma is low (low plasma density) and the ionic ¯ux incident on the sample is usually low. With higher gas pressure the plasma density increases and the amount of ionic ¯ux increases. At still higher pressure, however, gas phase collisions will reduce the number of bombarding ions as well as their energy. Generally, for each source-sample con®guration an intermediate optimum pressure will give the highest ionic ¯ux to the sample [10,11]. In the present work the ionic ¯ux was optimized using a plasma probe. The as-deposited ®lms were then analyzed with respect to
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)01099-8
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X. Wang et al. / Thin Solid Films 360 (2000) 181±186
crystallinity, lattice orientation, composition, and surface roughness.
on top of the substrate table. The probe is described in detail elsewhere [14]. The ionic current was measured with the probe biased to 2100 V.
2. Experimental The ®lms were grown in a high vacuum system with a base pressure of 5 £ 10 25 Pa. A 50 mm in diameter magnetron sputtering source was used with the samples placed facing the target 5 cm from the source. The 5 mm thick target, with a nominal composition of Sr1.1Ti0.9O3, was fabricated from the appropriate amount of SrCO3 and TiO2 powders following a standard milling and sintering procedure for ceramic materials. This target composition has previously shown to give stoichiometric SrTiO3 ®lms [12]. The applied r.f.-sputtering power was in all cases 50 W at a frequency of 13.56 MHz. A ¯owing Ar:O2 gas mixture was used during the depositions. The volume ratio between the two gases was 3:1, and the total pressure was varied between 1 and 14 Pa. Before each deposition the target was presputtered for 30 min to achieve stable conditions. The polished LaAlO3(001) and MgO(001) substrates were ®xed to a steel block by silver paste and the block was heated from the backside using a Pt ®lament. The temperature was measured using a thermocouple welded to the surface of the block. After deposition the chamber was ®lled with 0.5 atm. of O2 and slowly cooled at 48C/min to 1008C after which the system was vented with N2. The sputter deposition duration was six hours excluding the presputtering period. The structure of the ®lms was investigated using conventional X-ray diffraction (XRD) u ±2u scans with a Philips PW 1820 powder diffractometer with an accuracy of 0.0158 in 2u . The thickness of the SrTiO3 ®lms was measured using a Dektak pro®lometer. The ®lms grown on the MgO substrates were used for composition analysis by Rutherford backscattering spectroscopy (RBS). A 2 MeV He 1 incident beam was used and backscattered particles were detected at a 1508 scattering angle. For the experimental data analysis the software program RUMP [13] was used to simulate the composition depth pro®les. The thickness estimations by RBS are very accurate (in terms of atoms/cm 2). However, when using these values to compare growth rates for different samples grown under different process conditions one also has to consider errors in applied sputtering power, gas pressure, and substrate position. We estimate that ®lm thicknesses has to differ more than 10% before it is possible to conclude that the growth rate is different. The surface morphology and the root mean square (RMS) surface roughness was measured with atomic force microscopy (AFM) using a Nanoscope IIIa from Digital Instruments running in tapping mode. Finally, the optimization of the ionic ¯ux was carried out using a large ¯at disc probe with a diameter of 1.5 cm placed
3. Results and discussion In Fig. 1 the results from the plasma probe measurements are shown. The measurements estimate the ionic currents received by the substrate surface as a function of sputtering gas pressure. A maximum current of 3.0 mA is found at a gas pressure of 5 Pa corresponding to an ionic ¯ux of 1:1 £ 1016 ions/cm 2 s. A typical deposition showed a deposition rate of 170 nm/h equal to 3:8 £ 1014 atoms/cm 2 s. This gives the ratio between the ions and the deposition atoms incorporated in the ®lm to be as large as thirty. To evaluate the in¯uence from the ion bombardment on the ®lm growth, a set of ®lms were grown with varying gas pressure but a constant substrate temperature of 8508C. The ®lms were all (001)-oriented with no traces of other orientations in the XRD scans. The intensity of the 002 XRD-peak shows a clear variation with gas pressure, with a maximum at 5 Pa as demonstrated in Fig. 2a. This maximum, coincides with the maximum in ion current. Interestingly, the ®lm grown at 5 Pa as well as the one grown at 6.5 Pa have a lattice parameter that is closest to the bulk value, as shown in Fig. 2b. However, it should be pointed out that a change in sputtering gas pressure not only changes the ion current, but also other parameters, such as deposition rate and the energy of the bombarding species [15], as well as the degree of resputtering from energetic particles originating from the target [16]. The composition of the ®lms grown at different sputtering gas pressures are shown in Table 1. It can be seen that the ®lms grown at the lowest and highest pressure show a loss of Sr as compared with the Ti, while the ®lms grown at intermediate pressures show stoichiometric composition. Based on the results presented above the pressure 5 Pa
Fig. 1. Probe current as a function of sputtering pressure. The ¯at probe, that was placed at the substrate position, has a surface area of 1.8 cm 2.
X. Wang et al. / Thin Solid Films 360 (2000) 181±186
183
Fig. 3. X-ray diffraction intensity for different diffraction peaks as a function of growth temperature for SrTiO3 thin ®lms grown at 5 Pa.
Fig. 2. (a) X-ray diffraction intensity and (b) lattice parameter as a function of sputtering pressures for SrTiO3 ®lms deposited at 8508C. The table value of the 002-peak is 0.19525.
was chosen for further studies and a series of samples were grown varying the substrate temperature. The intensities of the XRD-peaks, for all orientations found in the grown ®lms, as function of growth temperature, are shown in Fig. 3. From 3508C and above only the (001)orientation is observed (represented by the 002 peak). The intensity of the 002 peak increases with increasing temperature. For growth temperatures below 3508C a mixture of orientations appear. All ®lms growth at 3508C and above also show an in-plane orientation relation to the substrate, hence ®lms are epitaxial with substrate. This is demonstrated in Fig. 4 for the ®lm grown at 3508C where pole ®gures of the 110 ®lm peak (Fig. Table 1 Composition of SrTiO3 thin ®lms as measured by RBS. The substrate temperature was held constant at 8508C while the sputtering pressure was varied. The error in estimated atomic concentration is ^ 5 in the last digit Sample
P (Pa)
Sr
Ti
O
Thickness (nm)
Ge3_3 Ge3_8 Ge3_5 Ge3_2 Ge3_4
1.0 3.5 5.0 8.0 14.0
0.80 1.03 1.03 0.95 0.78
1.00 0.98 1.05 1.00 1.10
3.00 3.00 2.90 3.00 3.00
1350 1090 840 800 500
4a) and the 110 substrate peak (Fig. 4b) are shown. The pole ®gures show that that the four-fold symmetry of the ®lm matches the symmetry of the substrate, hence the ®lms are epitaxial with the substrate and 001Film i 001Substrate; [110]Filmi[110]Substrate. The same epitaxial relationship is obtained for all ®lms grown at temperatures from 3508C and above. In the low temperature region the ®lms grown are polycrystalline with broad peaks of low intensity. The peaks are, however, still well de®ned (111, 110, and 002 peaks) except for the lowest growth temperature investigated, as seen in Fig. 5. For this ®lm, grown at 1008C, only one broad peak is observed that could match several other oxides such as rutile (TiO2), brookite (TiO2), SrO (cubic), SrO (tetragonal), and SrO2. It is thus impossible to decide whether this peak corresponds to single phase SrTiO3 with an expanded unit cell, or a mixture of oxides. As seen in Fig. 6 the lattice parameter of all ®lms are expanded in the growth direction as compared to bulk. (The lattice direction probed by the u ±2u scans are the one perpendicular to the substrate surface.) For the highest growth temperature this expansion is only 0.31% but it increases with decreasing temperature. For the lowest growth temperature (1008C) lattice parameter drastically expanded. However, it is not clear whether this ®lm is of the SrTiO3 phase or not, as discussed above. RBS measurements of the composition is presented in Table 2. All ®lms, except for the two ®lms grown at the lowest temperatures are, within error bars, stoichiometric SrTiO3. For the two ®lms grown at the lowest temperatures, 100 and 2008C, however the Sr/Ti ratios are 0.67 and 0.72, respectively. All ®lms show the same thickness, within the experimental error. The reason for the reduced Sr/Ti ratio, at low growth temperatures, is not clear to us. Based on the thickness measurements it is not possible to draw any conclusion over a possible variation in growth rate with temperature. The change in Sr/Ti ratio could therefore be an effect from
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X. Wang et al. / Thin Solid Films 360 (2000) 181±186
Fig. 5. X-ray diffraction scans over the SrTiO3 110-peak from ®lms grown at low temperatures.
tions give values of 0.35 and 5.05 eV for Sr±O and Ti±O bonds, respectively at room temperature. Even though these bond strengths might be altered, on the growth surface, the large difference between the two bonds gives a hint to the reason for the selective loss of Sr. The increased Sr loss at low growth temperatures are, however, more dif®cult to understand. However, we have recently found using nuclear resonance analysis that these ®lms (grown on MgO substrates) contain hydrogen possible in the form of hydroxides [20]. This and the fact that the ®lm microstructure changes as growth temperature is decreased below 3508C may affect resputtering behaviour and thereby the net sticking probability. That sputter etching of the growing ®lm can be a reason for the loss of Sr is supported by the etched-like surface appearance of some of the deposited ®lm as seen in the AFM images presented below. In Fig. 7 collection of AFM images taken from samples grown at 200, 350, 550, and 6508C are shown. These results show that for ®lms grown at 2008C the structure is ®ne grained and relatively smooth with lateral dimension of Fig. 4. {110} pole ®gure plots for SrTiO3 thin ®lm grown at 3508C, ®lm (a) and substrate (b).
either an increased loss of Sr or an increase in Ti incorporation. However, since Ti incorporation is not further affected at higher temperature we tend to believe that the reduced Sr/ Ti ratio at low temperatures is due to an increased loss of Sr from energetic bombardment. We have previously shown that an excess of Sr is needed in the sputtering target due to loss of Sr from the growth surface for off-axis sputter deposited SrTiO3 ®lms [12]. The loss of Sr has also been demonstrated for thermal processes such as molecular beam epitaxy [17]. This can partly be understood if we compare the Sr±O and Ti±O bonds in SrTiO3. Using interatomic potentials for these bonds [18] and corresponding interatomic distances [19], the bond strengths can be estimated. The results from such estima-
Fig. 6. Film (002) lattice spacing as a function of growth temperature. The values were either measured directly or deduced from the other diffraction peaks.
X. Wang et al. / Thin Solid Films 360 (2000) 181±186 Table 2 Composition of SrTiO3 thin ®lms deposited at 5 Pa but different substrate temperatures. The error in estimated atomic concentration is ^ 5 in the last digit Sample
Ts (8C)
Sr
Ti
O
Thickness (nm)
Ge3_18 Ge3_17 Ge3_15 Ge3_19 Ge3_14 Ge3_13 Ge3_12 Ge3_9 Ge3_5
100 200 250 300 350 450 550 750 850
0.64 0.72 1.10 1.10 1.00 1.00 1.05 0.97 1.03
0.95 1.00 1.05 1.00 1.00 1.00 1.00 1.00 1.05
3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 2.90
1080 990 980 1010 1070 990 990 1050 840
,25 nm. With increasing temperature the size of the surface features increases up to ,200 nm at 5508C. At 6508C, however, the features has disappeared and the ®lm is smooth again. The surface roughness is summarized in Fig. 8 where
185
the RMS-values are plotted as a function of growth temperature. The values of the surface roughness are not surprising with a small surface roughness at low temperature where a ®ne grained structure is obtained and again a smooth surface when the temperature allow for a high adatom mobility and quite likely a step ¯ow growth mode. Noticeable is that in an intermediate temperature range the structure has a etched-like appearance. In conclusion, SrTiO3 ®lms have been deposited on LaAlO3(001) single crystal substrates using r.f. magnetron sputtering of a Sr-enriched target. The substrate temperatures ranged from 150 to 8508C. Epitaxial growth is observed for growth temperatures as low as 3508C. For lower temperatures the ®lms are polycrystalline and mixture of orientations appears ((100), (110), and (111)). The ®lms show the correct composition for SrTiO3 except for ®lms grown at 2008C and below, where the ®lms show a decreasing Sr content. The increased loss of Sr at low growth
Fig. 7. AFM images for ®lms grown at different temperatures.
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X. Wang et al. / Thin Solid Films 360 (2000) 181±186
References
Fig. 8. The development of the roughness (RMS values) of the ®lms as a function of growth temperature. The thickness of the ®lms are 1000 nm.
temperatures is believed to be due to a change in selective resputtering behaviour as ®lm surface chemistry and crystal orientation changes.
Acknowledgements This work was ®nancially supported by Swedish National Board for Industrial and Technical Development (NUTEK), Swedish Natural Science Research Council (NFR), and Swedish Strategic Foundation (SSF) through the Superconductor Consortium.
[1] C.S. Hwang, S.O. Park, C.S. Kang, et al., Jpn. J. Appl. Phys. 34 (1995) 5178. [2] P.C. Joshi, S.B. Krupanidhi, J. Appl. Phys. 73 (1993) 7627. [3] O.G. Vendik, E. Kollberg, S.S. Gevorgian, A.B. Kozyrev, O.I. Soldatenkov, Electron. Lett. 31 (1995) 654. [4] A.T. Findikoglu, Q.X. Jia, I.H. Campbell, et al., Appl. Phys. Lett. 66 (1995) 3674. [5] A.T. Findokoglu, Q.X. Jia, X.D. Wu, G.J. Chen, T. Venkatesan, D.W. Reagor, Appl. Phys. Lett. 68 (1996) 1651. [6] T. Nakamura, H. Tokuda, S. Tanaka, M. Iiyama, Jpn. J. Appl. Phys. 34 (1995) 1906. [7] P.-Y. Lesaicherre, H. Yamaguchi, T. Sakuma, Y. Miyasaka, M. Yoshida, A. Ishitani, Ferroelectric Thin Films III, San Francisco, USA, April 13±16, in: E.R. Myers, B.A. Tuttle, S.B. Desu, P.K. Larsen (Eds.), Mater. Res. Soc. Symp. Proc., 1993, pp. 310±487. [8] F.A. Miranda, C.H. Mueller, R.E. Treece, et al., Integr. Ferroelectron. 14 (1997) 173. [9] M. Iwabuchi, T. Kobayashi, J. Appl. Phys. 75 (1994) 5295. [10] B. Window, G.L. Harding, J. Vac. Sci. Technol. A 8 (1990) 1277. [11] B. Window, G.L. Harding, J. Vac. Sci. Technol. A 10 (1992) 3300. [12] X. Wang, U. Helmersson, L.D. Madsen, et al., J. Vac. Sci. Technol. A 17 (1999) 564. [13] L.R. Doolittle, Nucl. Instrum. B 9 (1985) 344. [14] I. Petrov, I. Ivanov, V. Orlinov, J.-E. Sungren, J. Vac. Sci. Technol. A 11 (1993) 2733. [15] B. Chapman, Glow Discharge Processes, Sputtering and Plasma Etching, Wiley, New York, 1980. [16] T.I. Selinder, G. Larsson, U. Helmersson, J. Appl. Phys. 69 (1991) 390. [17] K. Koguchi, T. Matsumoto, T. Kawai, Science 267 (1995) 71. [18] M.J. Akhtar, Z.-U.-N. Akhtar, R.A. Jackson, C.R.A. Catlow, J. Am. Ceram. Soc. 78 (1995) 421. [19] International Centre for Diffraction Data, JCPDS-ICDD 35-734. [20] J.M. Schneider, B. HjoÈrvarsson, X. Wang, L. Hultman, Appl. Phys. Lett. 75 (1999) 3476.
Growth of SrTiO3 thin ®lms on LaAlO3(001) substrates; the in¯uence of growth temperature on composition, orientation, and surface morphology Xin Wang 1, Sveinn Olafsson 2, Per SandstroÈm, Ulf Helmersson* Department of Physics, LinkoÈping University, SE-581 83 LinkoÈping, Sweden Received 21 July 1999; received in revised form 30 November 1999; accepted 3 December 1999
Abstract SrTiO3 ®lms have been grown on LaAlO3(001) single crystal substrates using rf-sputtering. The substrates were held at temperatures ranging from 100 to 8508C. For growth temperatures as low as 3508C epitaxial growth is observed. Below 3508C the ®lms are polycrystalline and three different orientations (100), (110), and (111) can be observed using X-ray diffraction. Atomic force microscopy shows that ®lms deposited at temperatures below 3508C and above 6508C are smooth while the surfaces of the ®lms made at intermediate temperatures are rough and faceted. As growth temperatures decrease below 2508C, the ®lms show decreasing amount of Sr. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Dielectrics; Oxides; Sputtering; Surface roughness
1. Introduction SrTiO3 is an interesting perovskite compound both from a fundamental and a practical application point of view. It is an incipient ferroelectric material with a relatively high dielectric constant with possible applications in, e.g. dynamic random access memories (DRAMs) [1,2] and in high value or tunable capacitors [3±5]. Low temperature growth of this compound ®lm is vital for many practical applications since it can hinder interdiffusion and interface reactions with substrate materials and make SrTiO3 ®lms compatible with existing technologies. This article describes results on growth of SrTiO3 on lattice matched LaAlO3(100) substrate using r.f. sputtering. The temperature of the substrate during growth was varied between 100 and 8508C. The lowest growth temperature where epitaxial growth of SrTiO3 has been reported is, to our knowledge, 3308C [6]. They utilized molecular beam epitaxy for growth on Nbdoped SrTiO3(100) substrates at a low growth rate of 1 nm/ min. Lesaicherre et al. [7] were using electron cyclotron resonance chemical vapor deposition and was able to * Corresponding author. Tel.: 1 46-1328-1685; fax: 146-1328-8918. E-mail address: [email protected] (U. Helmersson) 1 Present address: Acreo AB, Bredgatan 34, SE-602 21 NorrkoÈping, Sweden. 2 Present address: Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavik, Iceland.
grow (110) oriented ®lms at a temperature of 4008C on rcut sapphire (11Å02) substrates. For ®lms grown by laser ablation, the lowest reported growth temperature where the SrTiO3 phase is formed is 2508C, however, the ®lms are polycrystalline [8]. For r.f. magnetron sputtering the lowest reported growth temperature achieving epitaxial ®lms is 5408C, also using a low growth rate of 1±4 nm/ min [9]. In sputtering, the presence of energetic particles provides a possibility to increase the surface atom mobility during growth to promote epitaxial growth at lower temperatures. Too high a degree of bombardment, especially with high energies, will on the other hand lead to creation of lattice defects and sputter etching of the ®lm. In the present work, the degree of ion bombardment is controlled by varying the sputtering gas pressure. At low sputtering gas pressures the number of ionized atoms in the plasma is low (low plasma density) and the ionic ¯ux incident on the sample is usually low. With higher gas pressure the plasma density increases and the amount of ionic ¯ux increases. At still higher pressure, however, gas phase collisions will reduce the number of bombarding ions as well as their energy. Generally, for each source-sample con®guration an intermediate optimum pressure will give the highest ionic ¯ux to the sample [10,11]. In the present work the ionic ¯ux was optimized using a plasma probe. The as-deposited ®lms were then analyzed with respect to
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)01099-8
182
X. Wang et al. / Thin Solid Films 360 (2000) 181±186
crystallinity, lattice orientation, composition, and surface roughness.
on top of the substrate table. The probe is described in detail elsewhere [14]. The ionic current was measured with the probe biased to 2100 V.
2. Experimental The ®lms were grown in a high vacuum system with a base pressure of 5 £ 10 25 Pa. A 50 mm in diameter magnetron sputtering source was used with the samples placed facing the target 5 cm from the source. The 5 mm thick target, with a nominal composition of Sr1.1Ti0.9O3, was fabricated from the appropriate amount of SrCO3 and TiO2 powders following a standard milling and sintering procedure for ceramic materials. This target composition has previously shown to give stoichiometric SrTiO3 ®lms [12]. The applied r.f.-sputtering power was in all cases 50 W at a frequency of 13.56 MHz. A ¯owing Ar:O2 gas mixture was used during the depositions. The volume ratio between the two gases was 3:1, and the total pressure was varied between 1 and 14 Pa. Before each deposition the target was presputtered for 30 min to achieve stable conditions. The polished LaAlO3(001) and MgO(001) substrates were ®xed to a steel block by silver paste and the block was heated from the backside using a Pt ®lament. The temperature was measured using a thermocouple welded to the surface of the block. After deposition the chamber was ®lled with 0.5 atm. of O2 and slowly cooled at 48C/min to 1008C after which the system was vented with N2. The sputter deposition duration was six hours excluding the presputtering period. The structure of the ®lms was investigated using conventional X-ray diffraction (XRD) u ±2u scans with a Philips PW 1820 powder diffractometer with an accuracy of 0.0158 in 2u . The thickness of the SrTiO3 ®lms was measured using a Dektak pro®lometer. The ®lms grown on the MgO substrates were used for composition analysis by Rutherford backscattering spectroscopy (RBS). A 2 MeV He 1 incident beam was used and backscattered particles were detected at a 1508 scattering angle. For the experimental data analysis the software program RUMP [13] was used to simulate the composition depth pro®les. The thickness estimations by RBS are very accurate (in terms of atoms/cm 2). However, when using these values to compare growth rates for different samples grown under different process conditions one also has to consider errors in applied sputtering power, gas pressure, and substrate position. We estimate that ®lm thicknesses has to differ more than 10% before it is possible to conclude that the growth rate is different. The surface morphology and the root mean square (RMS) surface roughness was measured with atomic force microscopy (AFM) using a Nanoscope IIIa from Digital Instruments running in tapping mode. Finally, the optimization of the ionic ¯ux was carried out using a large ¯at disc probe with a diameter of 1.5 cm placed
3. Results and discussion In Fig. 1 the results from the plasma probe measurements are shown. The measurements estimate the ionic currents received by the substrate surface as a function of sputtering gas pressure. A maximum current of 3.0 mA is found at a gas pressure of 5 Pa corresponding to an ionic ¯ux of 1:1 £ 1016 ions/cm 2 s. A typical deposition showed a deposition rate of 170 nm/h equal to 3:8 £ 1014 atoms/cm 2 s. This gives the ratio between the ions and the deposition atoms incorporated in the ®lm to be as large as thirty. To evaluate the in¯uence from the ion bombardment on the ®lm growth, a set of ®lms were grown with varying gas pressure but a constant substrate temperature of 8508C. The ®lms were all (001)-oriented with no traces of other orientations in the XRD scans. The intensity of the 002 XRD-peak shows a clear variation with gas pressure, with a maximum at 5 Pa as demonstrated in Fig. 2a. This maximum, coincides with the maximum in ion current. Interestingly, the ®lm grown at 5 Pa as well as the one grown at 6.5 Pa have a lattice parameter that is closest to the bulk value, as shown in Fig. 2b. However, it should be pointed out that a change in sputtering gas pressure not only changes the ion current, but also other parameters, such as deposition rate and the energy of the bombarding species [15], as well as the degree of resputtering from energetic particles originating from the target [16]. The composition of the ®lms grown at different sputtering gas pressures are shown in Table 1. It can be seen that the ®lms grown at the lowest and highest pressure show a loss of Sr as compared with the Ti, while the ®lms grown at intermediate pressures show stoichiometric composition. Based on the results presented above the pressure 5 Pa
Fig. 1. Probe current as a function of sputtering pressure. The ¯at probe, that was placed at the substrate position, has a surface area of 1.8 cm 2.
X. Wang et al. / Thin Solid Films 360 (2000) 181±186
183
Fig. 3. X-ray diffraction intensity for different diffraction peaks as a function of growth temperature for SrTiO3 thin ®lms grown at 5 Pa.
Fig. 2. (a) X-ray diffraction intensity and (b) lattice parameter as a function of sputtering pressures for SrTiO3 ®lms deposited at 8508C. The table value of the 002-peak is 0.19525.
was chosen for further studies and a series of samples were grown varying the substrate temperature. The intensities of the XRD-peaks, for all orientations found in the grown ®lms, as function of growth temperature, are shown in Fig. 3. From 3508C and above only the (001)orientation is observed (represented by the 002 peak). The intensity of the 002 peak increases with increasing temperature. For growth temperatures below 3508C a mixture of orientations appear. All ®lms growth at 3508C and above also show an in-plane orientation relation to the substrate, hence ®lms are epitaxial with substrate. This is demonstrated in Fig. 4 for the ®lm grown at 3508C where pole ®gures of the 110 ®lm peak (Fig. Table 1 Composition of SrTiO3 thin ®lms as measured by RBS. The substrate temperature was held constant at 8508C while the sputtering pressure was varied. The error in estimated atomic concentration is ^ 5 in the last digit Sample
P (Pa)
Sr
Ti
O
Thickness (nm)
Ge3_3 Ge3_8 Ge3_5 Ge3_2 Ge3_4
1.0 3.5 5.0 8.0 14.0
0.80 1.03 1.03 0.95 0.78
1.00 0.98 1.05 1.00 1.10
3.00 3.00 2.90 3.00 3.00
1350 1090 840 800 500
4a) and the 110 substrate peak (Fig. 4b) are shown. The pole ®gures show that that the four-fold symmetry of the ®lm matches the symmetry of the substrate, hence the ®lms are epitaxial with the substrate and 001Film i 001Substrate; [110]Filmi[110]Substrate. The same epitaxial relationship is obtained for all ®lms grown at temperatures from 3508C and above. In the low temperature region the ®lms grown are polycrystalline with broad peaks of low intensity. The peaks are, however, still well de®ned (111, 110, and 002 peaks) except for the lowest growth temperature investigated, as seen in Fig. 5. For this ®lm, grown at 1008C, only one broad peak is observed that could match several other oxides such as rutile (TiO2), brookite (TiO2), SrO (cubic), SrO (tetragonal), and SrO2. It is thus impossible to decide whether this peak corresponds to single phase SrTiO3 with an expanded unit cell, or a mixture of oxides. As seen in Fig. 6 the lattice parameter of all ®lms are expanded in the growth direction as compared to bulk. (The lattice direction probed by the u ±2u scans are the one perpendicular to the substrate surface.) For the highest growth temperature this expansion is only 0.31% but it increases with decreasing temperature. For the lowest growth temperature (1008C) lattice parameter drastically expanded. However, it is not clear whether this ®lm is of the SrTiO3 phase or not, as discussed above. RBS measurements of the composition is presented in Table 2. All ®lms, except for the two ®lms grown at the lowest temperatures are, within error bars, stoichiometric SrTiO3. For the two ®lms grown at the lowest temperatures, 100 and 2008C, however the Sr/Ti ratios are 0.67 and 0.72, respectively. All ®lms show the same thickness, within the experimental error. The reason for the reduced Sr/Ti ratio, at low growth temperatures, is not clear to us. Based on the thickness measurements it is not possible to draw any conclusion over a possible variation in growth rate with temperature. The change in Sr/Ti ratio could therefore be an effect from
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Fig. 5. X-ray diffraction scans over the SrTiO3 110-peak from ®lms grown at low temperatures.
tions give values of 0.35 and 5.05 eV for Sr±O and Ti±O bonds, respectively at room temperature. Even though these bond strengths might be altered, on the growth surface, the large difference between the two bonds gives a hint to the reason for the selective loss of Sr. The increased Sr loss at low growth temperatures are, however, more dif®cult to understand. However, we have recently found using nuclear resonance analysis that these ®lms (grown on MgO substrates) contain hydrogen possible in the form of hydroxides [20]. This and the fact that the ®lm microstructure changes as growth temperature is decreased below 3508C may affect resputtering behaviour and thereby the net sticking probability. That sputter etching of the growing ®lm can be a reason for the loss of Sr is supported by the etched-like surface appearance of some of the deposited ®lm as seen in the AFM images presented below. In Fig. 7 collection of AFM images taken from samples grown at 200, 350, 550, and 6508C are shown. These results show that for ®lms grown at 2008C the structure is ®ne grained and relatively smooth with lateral dimension of Fig. 4. {110} pole ®gure plots for SrTiO3 thin ®lm grown at 3508C, ®lm (a) and substrate (b).
either an increased loss of Sr or an increase in Ti incorporation. However, since Ti incorporation is not further affected at higher temperature we tend to believe that the reduced Sr/ Ti ratio at low temperatures is due to an increased loss of Sr from energetic bombardment. We have previously shown that an excess of Sr is needed in the sputtering target due to loss of Sr from the growth surface for off-axis sputter deposited SrTiO3 ®lms [12]. The loss of Sr has also been demonstrated for thermal processes such as molecular beam epitaxy [17]. This can partly be understood if we compare the Sr±O and Ti±O bonds in SrTiO3. Using interatomic potentials for these bonds [18] and corresponding interatomic distances [19], the bond strengths can be estimated. The results from such estima-
Fig. 6. Film (002) lattice spacing as a function of growth temperature. The values were either measured directly or deduced from the other diffraction peaks.
X. Wang et al. / Thin Solid Films 360 (2000) 181±186 Table 2 Composition of SrTiO3 thin ®lms deposited at 5 Pa but different substrate temperatures. The error in estimated atomic concentration is ^ 5 in the last digit Sample
Ts (8C)
Sr
Ti
O
Thickness (nm)
Ge3_18 Ge3_17 Ge3_15 Ge3_19 Ge3_14 Ge3_13 Ge3_12 Ge3_9 Ge3_5
100 200 250 300 350 450 550 750 850
0.64 0.72 1.10 1.10 1.00 1.00 1.05 0.97 1.03
0.95 1.00 1.05 1.00 1.00 1.00 1.00 1.00 1.05
3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 2.90
1080 990 980 1010 1070 990 990 1050 840
,25 nm. With increasing temperature the size of the surface features increases up to ,200 nm at 5508C. At 6508C, however, the features has disappeared and the ®lm is smooth again. The surface roughness is summarized in Fig. 8 where
185
the RMS-values are plotted as a function of growth temperature. The values of the surface roughness are not surprising with a small surface roughness at low temperature where a ®ne grained structure is obtained and again a smooth surface when the temperature allow for a high adatom mobility and quite likely a step ¯ow growth mode. Noticeable is that in an intermediate temperature range the structure has a etched-like appearance. In conclusion, SrTiO3 ®lms have been deposited on LaAlO3(001) single crystal substrates using r.f. magnetron sputtering of a Sr-enriched target. The substrate temperatures ranged from 150 to 8508C. Epitaxial growth is observed for growth temperatures as low as 3508C. For lower temperatures the ®lms are polycrystalline and mixture of orientations appears ((100), (110), and (111)). The ®lms show the correct composition for SrTiO3 except for ®lms grown at 2008C and below, where the ®lms show a decreasing Sr content. The increased loss of Sr at low growth
Fig. 7. AFM images for ®lms grown at different temperatures.
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X. Wang et al. / Thin Solid Films 360 (2000) 181±186
References
Fig. 8. The development of the roughness (RMS values) of the ®lms as a function of growth temperature. The thickness of the ®lms are 1000 nm.
temperatures is believed to be due to a change in selective resputtering behaviour as ®lm surface chemistry and crystal orientation changes.
Acknowledgements This work was ®nancially supported by Swedish National Board for Industrial and Technical Development (NUTEK), Swedish Natural Science Research Council (NFR), and Swedish Strategic Foundation (SSF) through the Superconductor Consortium.
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