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In situ surface characterization of SrTiO3(100) substrates and homoepitaxial SrTiO3 thin films grown by molecular beam epitaxy and pulsed laser deposition
Applied Surface Science 130–132 Ž1998. 576–581
In situ surface characterization of SrTiO 3 ž100 / substrates and homoepitaxial SrTiO 3 thin films grown by molecular beam epitaxy and pulsed laser deposition T. Nakamura ) , H. Inada, M. Iiyama Basic High-Technology Laboratories, Sumitomo Electric Industries, 1-1-3, Shimaya, Konohana-ku, Osaka 554, Japan Received 16 September 1997; accepted 10 November 1997
Abstract The topmost atomic layer of SrTiO 3 ŽSTO. substrates was investigated by in situ low-energy ion scattering spectroscopy ŽLEISS.. After mechanochemical polishing, the topmost layer of the STO substrate consisted of SrO and TiO 2 planes. It was dominantly stabilized with TiO 2 planes after the STO substrate was treated with a pH-controlled NH 4 F–HFŽBHF. solution. STO thin films were deposited on the BHF-treated STO substrates by the molecular beam epitaxy ŽMBE. and pulsed laser deposition ŽPLD. methods. STO films were confirmed to have topmost layers with TiO 2 planes by the MBE method, and SrO planes by the PLD method. We also investigated the effects of the deposition conditions and surface treatments, and confirmed that the TiO 2 plane was more stable on the STO homoepitaxial film surface. Furthermore, the BHF-treated STO substrates greatly improved the thickness dependence of Tc of heteroepitaxial YBa 2 Cu 3 O x ŽYBCO. ultra-thin films. q 1998 Elsevier Science B.V. All rights reserved. PACS: 61.18.y j; 68.35.Bs; 74.25.Fy; 77.85.y s Keywords: YBa 2 Cu 3 O 7yx ; SrTiO 3 ; Molecular beam epitaxy; Pulsed laser deposition; LEISS
1. Introduction High-temperature superconductor ŽHTS. thin films have been extensively studied with enormous interest from both the scientific and technological viewpoints w1–3x. These films are generally grown on substrates, such as MgO, SrTiO 3 , LaAlO 3 and NdGaO 3 . Among them, SrTiO 3 ŽSTO. is widely used because STO has the similar crystal structure to HTS and is chemically and compositionally stable. A well-defined STO sub)
Corresponding author. Tel.: q81-6-466-6502; fax: q81-6464-3564; e-mail: [email protected].
strate surface is essential for fabrication of high-quality HTS thin films. The crystal structure of STO is of the perovskite type and has a stacked structure with alternatively two kinds of nonpolar atomic planes, i.e., SrO and TiO 2 , along the w001x direction. The topmost surface should be terminated by either one of these atomic planes. However, commercially available STO substrates are polished with an alkaline solution containing colloidal silica particles Žmechanochemical polishing. w4x. The surface has a corrugation of 0.2 to 0.8 nm in height and is contaminated with carbon-containing impurities. These surfaces are not applicable to both the atomic layer
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 1 1 8 - 4
T. Nakamura et al.r Applied Surface Science 130–132 (1998) 576–581
epitaxy and ultra-thin film growth because the surface imperfections affect the manner in which the epitaxial growth takes place. In these applications, the STO surfaces should be clean and atomically smooth. In general, in situ cleaning methods, such as heating in ultra-high vacuum ŽUHV. or O 2 flow, and Bi depositionrdesorption were reported to be effective to remove carbon-containing impurities from the surface w5–7x. However, these processes could not improve the smoothness. Recently, Kawasaki et al. obtained the atomically smooth STO surface with one-unit-cell steps in height by treating the crystal surface with a pH-controlled NH 4 F–HFŽBHF. solution w4x. Furthermore, they elucidated the topmost atomic layer of these STO substrates through lowenergy ion scattering spectroscopy ŽLEISS.. They reported that the topmost layer was a TiO 2 plane. Its coverage was 100% for the BHF-treated substrates and 75% to 95% for commercially available substrates. Thus, a BHF solution selectively dissolved SrO, and a smooth surface, stabilized with the TiO 2 atomic plane, was obtained. They also reported that the topmost surface of the homoepitaxial STO film was stabilized with the SrO planes by in situ X-ray photoelectron spectroscopy ŽXPS. and ex situ LEISS w4,8,9x. In general, the topmost layer of clean surfaces is definitively determined by the relation of atomic bonding energies, which results in potential minimum. However, their results suggested that the topmost surface of homoepitaxial STO film is stabilized with a different plane from the STO substrate. In this letter, we discuss the dependence of the topmost layers of STO substrates and homoepitaxial STO films on surface treatments and deposition methods through in situ LEISS analysis for high quality film growth. Furthermore, YBa 2 Cu 3 O x
ŽYBCO. ultra-thin films were deposited on these substrates to evaluate the relation between the T c of YBCO films and the topmost layers of STO surfaces.
2. Experimental We have developed an ultra-high vacuum ŽUHV. system. Three chambers used for molecular beam epitaxy ŽMBE., pulsed laser deposition ŽPLD. and surface analysis are connected to an UHV transfer chamber. The details of the UHV system are reported elsewhere w10x. STO substrates were treated under two different conditions and were evaluated by LEISS analysis: Ž1. an as-supplied STOŽ100. substrate, and Ž2. a BHF-treated STOŽ100. substrate ŽShinkosha, Japan.. The surface roughness of these substrates was reported in Ref. w4x. We conducted O 3 cleaning treatment in the MBE chamber before LEISS measurements w11,12x. The substrate temperature was 5008C and the O 3 pressure was 3 = 10y5 Torr. The crystallinity and its orientations were confirmed by reflection high-energy electron diffraction ŽRHEED.. Subsequently, the specimens were transferred within 10 min to the analysis chamber without breaking the vacuum. The base pressure of the analysis chamber was 1 = 10y1 0 Torr. The double pass cylindricalmirror analyzer ŽCMA. was used as a detector for both XPS and LEISS measurements. For XPS measurements, an anode with a photon energy of MgK a Ž1253.6 eV. excitation was used. For LEISS measurements, the helium-ion beam was extracted from an electron-bombardment type source and had a primary energy of 0.5 keV. The ion source was operated with a typical current of 20 nA. By differential pumping from this ion source, the vacuum
Table 1 Deposition conditions of YBCO and STO thin films STO
Temperature Pressure Rate Thickness Source Laser
577
YBCO
PLD
MBE
MBE
6008C 0.03 Torr, O 2 30 nmrmin 20 nm sintered STO Ž3 N. KrF, 2 Jrcm2
6008C 5 = 10y5 Torr, O 3 1 nmrmin 20 nm Sr Ž3 N., Ti Ž5 N. y
6908C 3 = 10y5 Torr, O 3 0.3 nmrmin 3–10 nm Y Ž3 N., Ba Ž3 N., Cu Ž5 N. y
578
T. Nakamura et al.r Applied Surface Science 130–132 (1998) 576–581
pressure was kept at 5 = 10y9 Torr. The incident angle and the scattering angle were 37.78 and 102.38, respectively. After LEISS measurements, STO thin films were deposited on these substrates by the ozone-assisted MBE and PLD methods. The deposition conditions are listed in Table 1. The surface roughness of the STO films was reported elsewhere w13x. These STO films were also transferred to the analysis chamber within 10 min and were then evaluated. We also deposited YBCO films on the STO substrates to confirm the effect of the surface of STO substrates on electrical properties of heteroepitaxial YBCO film. The deposition conditions are also listed in Table 1. Electric properties of YBCO films were measured using Ag electrodes.
3. Results and discussion Prior to LEISS measurements of STO substrates, the effects of O 3 cleaning were confirmed by in situ XPS. We confirmed that the peak intensities corresponding to hydrocarbon and carbonate decreased below our detection limit. Fig. 1 shows the LEISS spectra of an as-supplied STO substrate Ža. and a BHF-treated STOŽ100. substrate Žb.. Two distinctive peaks were observed from the as-supplied STO substrate. In our experimental configuration, Heq ions scattered by Sr, Ti, and O had energy lines of 425 eV, 348 eV and 204 eV calculated from the two-body collision. Therefore, these two peaks were revealed to correspond to Sr and Ti atoms. We also evaluated the ion trajectories using the Thomas–Fermi–Moliere potential w11,12x. The Heq ion scattered by Sr atoms could not reach the detector in our experimental configuration when the topmost plane of STO was a TiO 2 plane. These results suggest that the topmost layer consists of SrO and TiO 2 planes. The intensity ratio of TirSr does not always show content ratio because the sensitivities are expressed as a complex function of the primary beam, orientations, and geometry. In our analysis chamber, we could not investigate the scattering angle dependence of the scattering intensities and could not evaluate the differential cross-section of Sr and Ti atoms. According to other reports, the TiO 2 plane should be dominant at the STO substrate surface, and the differential cross-section of Sr atoms should be relatively larger than that
Fig. 1. In situ LEISS spectra of Ža. an as-supplied STO substrate, Žb. a BHF-treated STO substrate, Žc. homoepitaxial STO thin film deposited on the as-supplied STO substrate, and Žd. homoepitaxial STO thin film deposited on the BHF-treated STO substrate. These STO films were deposited by ozone-assisted MBE method.
of Ti atoms w4,8,9x. In contrast to the as-supplied STO substrate, only the Ti peak was detected on the surface of the BHF-treated STO substrate. Our in situ LEISS results indicate that the topmost layer of the BHF-treated STO substrate is stabilized with TiO 2 planes, which are in agreement with previous reports w4x. Furthermore, the suitability of our ion trajectories simulation was confirmed. In order to clarify the effect of the substrate surface on homoepitaxial growth, we deposited 20-nm thick STO films on the STO substrates by the ozone-assisted MBE method. Fig. 1c shows the LEISS spectra of homoepitaxial STO films deposited on the as-supplied STO substrate. The Sr and Ti peaks were also observed at the topmost surface of the homoepitaxial STO film. The TirSr ratio for the STO film in-
T. Nakamura et al.r Applied Surface Science 130–132 (1998) 576–581
creased as compared to the STO substrate. Fig. 1d shows the LEISS spectra of STO homoepitaxial film grown on the BHF-treated STO substrate. Only the Ti peak was detected on the surface of the homoepitaxial film. Our in situ LEISS results indicate that the topmost layers of both the BHF-treated STO substrate and the homoepitaxial STO film deposited on it are TiO 2 planes, while the as-supplied STO substrate and homoepitaxial STO film deposited on it are a mixture of SrO and TiO 2 planes. These indicate that the surface structure of the STO substrates affects the topmost layer of the STO thin films deposited on the STO substrates, and that the BHFtreated STO substrate has a well-defined surface to identify the topmost layers of the deposited films. In contrast to our experimental results, Yoshimoto et al. w9x reported that the topmost layer of homoepitaxial STO film grown by the PLD method was stabilized with the SrO plane by ex situ LEISS. They concluded that the energetically stable plane at the topmost surface of STO was a SrO plane, and that the preferential etching of the SrO occurred in the wet etching process during the treatment of the STO substrate surface. Our experimental results suggested that it was necessary to perform in situ analysis and use STO substrates with a well-defined surface for the identification of the topmost layers. In order to define the topmost layer of STO films, we conducted in situ LEISS evaluation of STO films grown by the PLD method. Fig. 2a shows the LEISS spectra of a homoepitaxial STO film deposited on the BHFtreated STO substrate. Only the Sr peak was detected on the surface. Our in situ LEISS results indicate that the topmost layer of homoepitaxial STO film is stabilized with SrO planes, in contrast to the homoepitaxial STO film deposited by the MBE method. There are considerable differences between the MBE and PLD methods, such as excitation mechanism, energies of molecular species, deposition rate and pressure. It is very difficult to clarify the origin of this difference on the homoepitaxial STO film surface. Therefore, we changed one of the deposition conditions of the PLD method to investigate which plane is more stable on the STO film surface. We deposited STO films by the PLD method at a lower oxygen pressure than the conventional STO deposition condition, because the deposition pressure of the MBE method is much lower than that of the PLD
579
Fig. 2. In situ LEISS spectra of homoepitaxial STO thin films deposited on the BHF-treated STO substrates by PLD method. The deposition pressure was Ža. 0.03 Torr and Žb. 3=10y4 Torr. The STO film deposited by the PLD method was etched with 3 keV Arq and annealed at 6008C in O 3 ambient. Žc. In situ LEISS spectra of homoepitaxial STO thin film after this treatment.
method. We deposited STO film at the pressure of 3 = 10y4 Torr, which was one hundredth of that for SrO-terminated STO film deposition. The film surface was also investigated by RHEED and XPS. We confirmed the epitaxial growth by RHEED and the compositional deviation of about 10% Sr-rich by XPS. Fig. 2b shows LEISS spectra of the STO film deposited at the pressure of 3 = 10y4 Torr. Two distinctive peaks were detected, suggesting that these surfaces consisted of SrO and TiO 2 planes, in contrast with the results of STO film deposited at the pressure of 0.03 Torr O 2 . These results show that the terminating layer of STO film deposited by the PLD depends on the oxygen pressure during deposition,
580
T. Nakamura et al.r Applied Surface Science 130–132 (1998) 576–581
and that SrO planes are not always the topmost layer on the homoepitaxial STO films. Furthermore, we conducted surface treatments in order to confirm what planes stabilized the STO homoepitaxial films. First, we etched the SrO-terminated STO film by Ar-ion accelerated at 3 keV for 10 min. The ion current was 20 nA. After etching, the sample was annealed at 6008C in O 3 ambient of 5 = 10y5 Torr. Fig. 2c shows the LEISS spectra after annealing. Two distinctive peaks were detected, suggesting that this surface consisted of SrO and TiO 2 planes. The surface structure of STOŽ001. after Arq sputtering and high-temperature annealing has also been studied by Bickel et al. w14x by means of LEED. They concluded that the surface consisted of TiO 2 and SrO planes, consistent with our results. These two experimental results suggest that the TiO 2 plane is more stable on the STO surface than the SrO plane. However, SrO and TiO 2 are nonpolar atomic planes, and the SrO-terminated STO surface can be achieved by depositing only SrO w7x. Therefore, for heteroepitaxial growth on STO films, it is important to control the topmost layer, i.e., SrO or TiO 2 planes for improved heterointerface. The well-defined substrate was revealed to be applicable not only for homoepitaxial but for heteroepitaxial depositions, as well. In the case of heteroepitaxial growth, such as YBCO ultra-thin film on STO substrate, well-defined substrates are expected to improve the electric properties of YBCO films. We deposited YBCO ultra-thin films on STO substrates by ozone-assisted MBE and evaluated the thickness dependence of T c’s. Fig. 3 shows the thickness dependence of T c’s for YBCO films deposited on as-supplied and BHF-treated STO substrates. The Tc did not show any thickness dependence down to 10 nm and had almost the same values for both STO substrates. The 10-nm thick YBCO films had the T c of 77–80 K. The Tc drastically decreased with the YBCO film thickness below 10 nm. The T c’s of YBCO films deposited on the BHF-treated STO substrates dropped slowly compared with YBCO films deposited on the as-supplied STO substrates. The BHF-treated STO substrates had atomically smooth surfaces stabilized by the TiO 2 plane, which enhanced lateral spreading of the nucleated islands. As a result, continuous YBCO films were obtained. For example, the T c of 3-nm thick
Fig. 3. YBCO thickness dependence of T c’s. YBCO thin films deposited on the as-supplied and BHF-treated STO substrates.
YBCO deposited on the BHF-treated STO substrate was 43.7 K, which was the highest value for 3-nm thick YBCO films deposited on STO substrates w15– 17x. Mukaida et al. discussed T c dependence on the lattice mismatch between YBCO and various substrates, such as MgO, STO and NaGaO 3 w18x. They successfully deposited ultra-thin YBCO film on NdGaO 3 substrates, which had a lattice mismatch of 0.36% to YBCO w18x. They reported that 3-nm thick YBCO film had the T c of 63 K. Our results indicate that not only lattice matching, but also the surface structure deposited on them, affect the electrical properties of ultra-thin YBCO film. In general, a buffer layer is effective to obtain ultra-thin superconducting YBCO films w1x. However, it is very difficult to distinguish the influence of the buffer layer because of the existence of coupling between YBCO and the buffer layer w1x. Therefore, ultra-thin YBCO films deposited on insulating substrates, which have a limited number of Cu–O chains, enable us to define the properties of the Cu–O plane itself.
4. Conclusions The topmost atomic layer of STO substrates was investigated by in situ LEISS. After mechanochemical polishing, it consisted of SrO and TiO 2 planes. It was dominantly stabilized with a TiO 2 plane after the STO substrate was treated with a BHF solution. STO thin films were deposited on the BHF-treated STO substrates by the MBE and PLD methods. They
T. Nakamura et al.r Applied Surface Science 130–132 (1998) 576–581
were confirmed to have a topmost layer of TiO 2 planes by the MBE method, and SrO planes by the PLD method. We also investigated the effects of the deposition conditions and surface treatments, and confirmed that the TiO 2 plane is more stable on the STO homoepitaxial films. Furthermore, the BHFtreated STO substrates greatly improved the thickness dependence of the T c of heteroepitaxial YBCO ultra-thin films.
Acknowledgements The authors wish to thank Professor Y. Okabe of the University of Tokyo, and Professor T. Kobayashi and Prof. K. Oura of Osaka University, for their fruitful discussions. A part of this work was performed under the management of FED as a part of the MITI R & D of Industrial Science and Technology Frontier Program ŽSuperconducting Electron Devices. supported by NEDO.
References w1x T. Terashima, K. Shimizu, Y. Bando, Y. Matsuda, A. Fujiyama, S. Komiyama, Phys. Rev. Lett. 67 Ž1991. 1362. w2x T. Hasegawa, K. Kitazawa, Jpn. J. Appl. Phys. 29 Ž1990. L434.
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w3x X.X. Xi, C. Doughty, A. Walkenhorst, C. Kwon, Q. Li, T. Venkatesan, Phys. Rev. Lett. 68 Ž1992. 1240. w4x M. Kawasaki, K. Takahashi, T. Maeda, R. Tsuchiya, M. Shinohara, O. Ishiyama, T. Yonezawa, M. Yoshimoto, H. Koinuma, Science 266 Ž1994. 1540. w5x H. Tanaka, T. Matsumoto, T. Kawai, S. Kanai, Jpn. J. Appl. Phys. 32 Ž1993. 1495. w6x M. Yoshimoto, H. Ohkubo, N. Kanda, H. Koinuma, Jpn. J. Appl. Phys. 11 Ž1992. 3664. w7x T. Hikita, T. Hanada, M. Maki, J. Vac. Sci. Technol. A 11 Ž1993. 2649. w8x S. Gonda, H. Nagata, M. Kawasaki, M. Yoshimoto, H. Koinuma, Physica C 216 Ž1993. 160. w9x M. Yoshimoto, T. Maeda, K. Shimozono, H. Koinuma, S. Shinohara, O. Ishiyama, F. Ohtani, Appl. Phys. Lett. 65 Ž1994. 3197. w10x T. Nakamura, H. Inada, M. Iiyama, IEEE Trans. Appl. Supercond. 7 Ž1997. 3540. w11x S. Tanaka, T. Nakamura, H. Tokuda, M. Iiyama, Appl. Phys. Lett. 62 Ž1991. 3040. w12x T. Nakamura, S. Tanaka, M. Iiyama, Appl. Phys. Lett. 66 Ž1995. 3362. w13x T. Nakamura, H. Tokuda, S. Tanaka, M. Iiyama, Jpn. J. Appl. Phys. 34 Ž1995. 1906. w14x N. Bickel, G. Schmidt, K. Heinz, K. Muller, Phys. Rev. Lett. 62 Ž1989. 2009. w15x Y. Bando, T. Terashima, T. Iijima, K. Yamamoto, H. Mazaki, Physica C 153 Ž1988. 810. w16x T. Venkatesan, X.D. Wu, B. Dutta, A. Inam, M.S. Hedge, C. Chang, L. Nazar, B. Wilkens, Appl. Phys. Lett. 54 Ž1989. 581. w17x O. Michikami, M. Asano, H. Asano, Jpn. J. Appl. Phys. 29 Ž1990. L298. w18x M. Mukaida, S. Miyazawa, M. Sasaura, K. Kuroda, Jpn. J. Appl. Phys. 29 Ž1990. L936.
In situ surface characterization of SrTiO 3 ž100 / substrates and homoepitaxial SrTiO 3 thin films grown by molecular beam epitaxy and pulsed laser deposition T. Nakamura ) , H. Inada, M. Iiyama Basic High-Technology Laboratories, Sumitomo Electric Industries, 1-1-3, Shimaya, Konohana-ku, Osaka 554, Japan Received 16 September 1997; accepted 10 November 1997
Abstract The topmost atomic layer of SrTiO 3 ŽSTO. substrates was investigated by in situ low-energy ion scattering spectroscopy ŽLEISS.. After mechanochemical polishing, the topmost layer of the STO substrate consisted of SrO and TiO 2 planes. It was dominantly stabilized with TiO 2 planes after the STO substrate was treated with a pH-controlled NH 4 F–HFŽBHF. solution. STO thin films were deposited on the BHF-treated STO substrates by the molecular beam epitaxy ŽMBE. and pulsed laser deposition ŽPLD. methods. STO films were confirmed to have topmost layers with TiO 2 planes by the MBE method, and SrO planes by the PLD method. We also investigated the effects of the deposition conditions and surface treatments, and confirmed that the TiO 2 plane was more stable on the STO homoepitaxial film surface. Furthermore, the BHF-treated STO substrates greatly improved the thickness dependence of Tc of heteroepitaxial YBa 2 Cu 3 O x ŽYBCO. ultra-thin films. q 1998 Elsevier Science B.V. All rights reserved. PACS: 61.18.y j; 68.35.Bs; 74.25.Fy; 77.85.y s Keywords: YBa 2 Cu 3 O 7yx ; SrTiO 3 ; Molecular beam epitaxy; Pulsed laser deposition; LEISS
1. Introduction High-temperature superconductor ŽHTS. thin films have been extensively studied with enormous interest from both the scientific and technological viewpoints w1–3x. These films are generally grown on substrates, such as MgO, SrTiO 3 , LaAlO 3 and NdGaO 3 . Among them, SrTiO 3 ŽSTO. is widely used because STO has the similar crystal structure to HTS and is chemically and compositionally stable. A well-defined STO sub)
Corresponding author. Tel.: q81-6-466-6502; fax: q81-6464-3564; e-mail: [email protected].
strate surface is essential for fabrication of high-quality HTS thin films. The crystal structure of STO is of the perovskite type and has a stacked structure with alternatively two kinds of nonpolar atomic planes, i.e., SrO and TiO 2 , along the w001x direction. The topmost surface should be terminated by either one of these atomic planes. However, commercially available STO substrates are polished with an alkaline solution containing colloidal silica particles Žmechanochemical polishing. w4x. The surface has a corrugation of 0.2 to 0.8 nm in height and is contaminated with carbon-containing impurities. These surfaces are not applicable to both the atomic layer
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 1 1 8 - 4
T. Nakamura et al.r Applied Surface Science 130–132 (1998) 576–581
epitaxy and ultra-thin film growth because the surface imperfections affect the manner in which the epitaxial growth takes place. In these applications, the STO surfaces should be clean and atomically smooth. In general, in situ cleaning methods, such as heating in ultra-high vacuum ŽUHV. or O 2 flow, and Bi depositionrdesorption were reported to be effective to remove carbon-containing impurities from the surface w5–7x. However, these processes could not improve the smoothness. Recently, Kawasaki et al. obtained the atomically smooth STO surface with one-unit-cell steps in height by treating the crystal surface with a pH-controlled NH 4 F–HFŽBHF. solution w4x. Furthermore, they elucidated the topmost atomic layer of these STO substrates through lowenergy ion scattering spectroscopy ŽLEISS.. They reported that the topmost layer was a TiO 2 plane. Its coverage was 100% for the BHF-treated substrates and 75% to 95% for commercially available substrates. Thus, a BHF solution selectively dissolved SrO, and a smooth surface, stabilized with the TiO 2 atomic plane, was obtained. They also reported that the topmost surface of the homoepitaxial STO film was stabilized with the SrO planes by in situ X-ray photoelectron spectroscopy ŽXPS. and ex situ LEISS w4,8,9x. In general, the topmost layer of clean surfaces is definitively determined by the relation of atomic bonding energies, which results in potential minimum. However, their results suggested that the topmost surface of homoepitaxial STO film is stabilized with a different plane from the STO substrate. In this letter, we discuss the dependence of the topmost layers of STO substrates and homoepitaxial STO films on surface treatments and deposition methods through in situ LEISS analysis for high quality film growth. Furthermore, YBa 2 Cu 3 O x
ŽYBCO. ultra-thin films were deposited on these substrates to evaluate the relation between the T c of YBCO films and the topmost layers of STO surfaces.
2. Experimental We have developed an ultra-high vacuum ŽUHV. system. Three chambers used for molecular beam epitaxy ŽMBE., pulsed laser deposition ŽPLD. and surface analysis are connected to an UHV transfer chamber. The details of the UHV system are reported elsewhere w10x. STO substrates were treated under two different conditions and were evaluated by LEISS analysis: Ž1. an as-supplied STOŽ100. substrate, and Ž2. a BHF-treated STOŽ100. substrate ŽShinkosha, Japan.. The surface roughness of these substrates was reported in Ref. w4x. We conducted O 3 cleaning treatment in the MBE chamber before LEISS measurements w11,12x. The substrate temperature was 5008C and the O 3 pressure was 3 = 10y5 Torr. The crystallinity and its orientations were confirmed by reflection high-energy electron diffraction ŽRHEED.. Subsequently, the specimens were transferred within 10 min to the analysis chamber without breaking the vacuum. The base pressure of the analysis chamber was 1 = 10y1 0 Torr. The double pass cylindricalmirror analyzer ŽCMA. was used as a detector for both XPS and LEISS measurements. For XPS measurements, an anode with a photon energy of MgK a Ž1253.6 eV. excitation was used. For LEISS measurements, the helium-ion beam was extracted from an electron-bombardment type source and had a primary energy of 0.5 keV. The ion source was operated with a typical current of 20 nA. By differential pumping from this ion source, the vacuum
Table 1 Deposition conditions of YBCO and STO thin films STO
Temperature Pressure Rate Thickness Source Laser
577
YBCO
PLD
MBE
MBE
6008C 0.03 Torr, O 2 30 nmrmin 20 nm sintered STO Ž3 N. KrF, 2 Jrcm2
6008C 5 = 10y5 Torr, O 3 1 nmrmin 20 nm Sr Ž3 N., Ti Ž5 N. y
6908C 3 = 10y5 Torr, O 3 0.3 nmrmin 3–10 nm Y Ž3 N., Ba Ž3 N., Cu Ž5 N. y
578
T. Nakamura et al.r Applied Surface Science 130–132 (1998) 576–581
pressure was kept at 5 = 10y9 Torr. The incident angle and the scattering angle were 37.78 and 102.38, respectively. After LEISS measurements, STO thin films were deposited on these substrates by the ozone-assisted MBE and PLD methods. The deposition conditions are listed in Table 1. The surface roughness of the STO films was reported elsewhere w13x. These STO films were also transferred to the analysis chamber within 10 min and were then evaluated. We also deposited YBCO films on the STO substrates to confirm the effect of the surface of STO substrates on electrical properties of heteroepitaxial YBCO film. The deposition conditions are also listed in Table 1. Electric properties of YBCO films were measured using Ag electrodes.
3. Results and discussion Prior to LEISS measurements of STO substrates, the effects of O 3 cleaning were confirmed by in situ XPS. We confirmed that the peak intensities corresponding to hydrocarbon and carbonate decreased below our detection limit. Fig. 1 shows the LEISS spectra of an as-supplied STO substrate Ža. and a BHF-treated STOŽ100. substrate Žb.. Two distinctive peaks were observed from the as-supplied STO substrate. In our experimental configuration, Heq ions scattered by Sr, Ti, and O had energy lines of 425 eV, 348 eV and 204 eV calculated from the two-body collision. Therefore, these two peaks were revealed to correspond to Sr and Ti atoms. We also evaluated the ion trajectories using the Thomas–Fermi–Moliere potential w11,12x. The Heq ion scattered by Sr atoms could not reach the detector in our experimental configuration when the topmost plane of STO was a TiO 2 plane. These results suggest that the topmost layer consists of SrO and TiO 2 planes. The intensity ratio of TirSr does not always show content ratio because the sensitivities are expressed as a complex function of the primary beam, orientations, and geometry. In our analysis chamber, we could not investigate the scattering angle dependence of the scattering intensities and could not evaluate the differential cross-section of Sr and Ti atoms. According to other reports, the TiO 2 plane should be dominant at the STO substrate surface, and the differential cross-section of Sr atoms should be relatively larger than that
Fig. 1. In situ LEISS spectra of Ža. an as-supplied STO substrate, Žb. a BHF-treated STO substrate, Žc. homoepitaxial STO thin film deposited on the as-supplied STO substrate, and Žd. homoepitaxial STO thin film deposited on the BHF-treated STO substrate. These STO films were deposited by ozone-assisted MBE method.
of Ti atoms w4,8,9x. In contrast to the as-supplied STO substrate, only the Ti peak was detected on the surface of the BHF-treated STO substrate. Our in situ LEISS results indicate that the topmost layer of the BHF-treated STO substrate is stabilized with TiO 2 planes, which are in agreement with previous reports w4x. Furthermore, the suitability of our ion trajectories simulation was confirmed. In order to clarify the effect of the substrate surface on homoepitaxial growth, we deposited 20-nm thick STO films on the STO substrates by the ozone-assisted MBE method. Fig. 1c shows the LEISS spectra of homoepitaxial STO films deposited on the as-supplied STO substrate. The Sr and Ti peaks were also observed at the topmost surface of the homoepitaxial STO film. The TirSr ratio for the STO film in-
T. Nakamura et al.r Applied Surface Science 130–132 (1998) 576–581
creased as compared to the STO substrate. Fig. 1d shows the LEISS spectra of STO homoepitaxial film grown on the BHF-treated STO substrate. Only the Ti peak was detected on the surface of the homoepitaxial film. Our in situ LEISS results indicate that the topmost layers of both the BHF-treated STO substrate and the homoepitaxial STO film deposited on it are TiO 2 planes, while the as-supplied STO substrate and homoepitaxial STO film deposited on it are a mixture of SrO and TiO 2 planes. These indicate that the surface structure of the STO substrates affects the topmost layer of the STO thin films deposited on the STO substrates, and that the BHFtreated STO substrate has a well-defined surface to identify the topmost layers of the deposited films. In contrast to our experimental results, Yoshimoto et al. w9x reported that the topmost layer of homoepitaxial STO film grown by the PLD method was stabilized with the SrO plane by ex situ LEISS. They concluded that the energetically stable plane at the topmost surface of STO was a SrO plane, and that the preferential etching of the SrO occurred in the wet etching process during the treatment of the STO substrate surface. Our experimental results suggested that it was necessary to perform in situ analysis and use STO substrates with a well-defined surface for the identification of the topmost layers. In order to define the topmost layer of STO films, we conducted in situ LEISS evaluation of STO films grown by the PLD method. Fig. 2a shows the LEISS spectra of a homoepitaxial STO film deposited on the BHFtreated STO substrate. Only the Sr peak was detected on the surface. Our in situ LEISS results indicate that the topmost layer of homoepitaxial STO film is stabilized with SrO planes, in contrast to the homoepitaxial STO film deposited by the MBE method. There are considerable differences between the MBE and PLD methods, such as excitation mechanism, energies of molecular species, deposition rate and pressure. It is very difficult to clarify the origin of this difference on the homoepitaxial STO film surface. Therefore, we changed one of the deposition conditions of the PLD method to investigate which plane is more stable on the STO film surface. We deposited STO films by the PLD method at a lower oxygen pressure than the conventional STO deposition condition, because the deposition pressure of the MBE method is much lower than that of the PLD
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Fig. 2. In situ LEISS spectra of homoepitaxial STO thin films deposited on the BHF-treated STO substrates by PLD method. The deposition pressure was Ža. 0.03 Torr and Žb. 3=10y4 Torr. The STO film deposited by the PLD method was etched with 3 keV Arq and annealed at 6008C in O 3 ambient. Žc. In situ LEISS spectra of homoepitaxial STO thin film after this treatment.
method. We deposited STO film at the pressure of 3 = 10y4 Torr, which was one hundredth of that for SrO-terminated STO film deposition. The film surface was also investigated by RHEED and XPS. We confirmed the epitaxial growth by RHEED and the compositional deviation of about 10% Sr-rich by XPS. Fig. 2b shows LEISS spectra of the STO film deposited at the pressure of 3 = 10y4 Torr. Two distinctive peaks were detected, suggesting that these surfaces consisted of SrO and TiO 2 planes, in contrast with the results of STO film deposited at the pressure of 0.03 Torr O 2 . These results show that the terminating layer of STO film deposited by the PLD depends on the oxygen pressure during deposition,
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and that SrO planes are not always the topmost layer on the homoepitaxial STO films. Furthermore, we conducted surface treatments in order to confirm what planes stabilized the STO homoepitaxial films. First, we etched the SrO-terminated STO film by Ar-ion accelerated at 3 keV for 10 min. The ion current was 20 nA. After etching, the sample was annealed at 6008C in O 3 ambient of 5 = 10y5 Torr. Fig. 2c shows the LEISS spectra after annealing. Two distinctive peaks were detected, suggesting that this surface consisted of SrO and TiO 2 planes. The surface structure of STOŽ001. after Arq sputtering and high-temperature annealing has also been studied by Bickel et al. w14x by means of LEED. They concluded that the surface consisted of TiO 2 and SrO planes, consistent with our results. These two experimental results suggest that the TiO 2 plane is more stable on the STO surface than the SrO plane. However, SrO and TiO 2 are nonpolar atomic planes, and the SrO-terminated STO surface can be achieved by depositing only SrO w7x. Therefore, for heteroepitaxial growth on STO films, it is important to control the topmost layer, i.e., SrO or TiO 2 planes for improved heterointerface. The well-defined substrate was revealed to be applicable not only for homoepitaxial but for heteroepitaxial depositions, as well. In the case of heteroepitaxial growth, such as YBCO ultra-thin film on STO substrate, well-defined substrates are expected to improve the electric properties of YBCO films. We deposited YBCO ultra-thin films on STO substrates by ozone-assisted MBE and evaluated the thickness dependence of T c’s. Fig. 3 shows the thickness dependence of T c’s for YBCO films deposited on as-supplied and BHF-treated STO substrates. The Tc did not show any thickness dependence down to 10 nm and had almost the same values for both STO substrates. The 10-nm thick YBCO films had the T c of 77–80 K. The Tc drastically decreased with the YBCO film thickness below 10 nm. The T c’s of YBCO films deposited on the BHF-treated STO substrates dropped slowly compared with YBCO films deposited on the as-supplied STO substrates. The BHF-treated STO substrates had atomically smooth surfaces stabilized by the TiO 2 plane, which enhanced lateral spreading of the nucleated islands. As a result, continuous YBCO films were obtained. For example, the T c of 3-nm thick
Fig. 3. YBCO thickness dependence of T c’s. YBCO thin films deposited on the as-supplied and BHF-treated STO substrates.
YBCO deposited on the BHF-treated STO substrate was 43.7 K, which was the highest value for 3-nm thick YBCO films deposited on STO substrates w15– 17x. Mukaida et al. discussed T c dependence on the lattice mismatch between YBCO and various substrates, such as MgO, STO and NaGaO 3 w18x. They successfully deposited ultra-thin YBCO film on NdGaO 3 substrates, which had a lattice mismatch of 0.36% to YBCO w18x. They reported that 3-nm thick YBCO film had the T c of 63 K. Our results indicate that not only lattice matching, but also the surface structure deposited on them, affect the electrical properties of ultra-thin YBCO film. In general, a buffer layer is effective to obtain ultra-thin superconducting YBCO films w1x. However, it is very difficult to distinguish the influence of the buffer layer because of the existence of coupling between YBCO and the buffer layer w1x. Therefore, ultra-thin YBCO films deposited on insulating substrates, which have a limited number of Cu–O chains, enable us to define the properties of the Cu–O plane itself.
4. Conclusions The topmost atomic layer of STO substrates was investigated by in situ LEISS. After mechanochemical polishing, it consisted of SrO and TiO 2 planes. It was dominantly stabilized with a TiO 2 plane after the STO substrate was treated with a BHF solution. STO thin films were deposited on the BHF-treated STO substrates by the MBE and PLD methods. They
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were confirmed to have a topmost layer of TiO 2 planes by the MBE method, and SrO planes by the PLD method. We also investigated the effects of the deposition conditions and surface treatments, and confirmed that the TiO 2 plane is more stable on the STO homoepitaxial films. Furthermore, the BHFtreated STO substrates greatly improved the thickness dependence of the T c of heteroepitaxial YBCO ultra-thin films.
Acknowledgements The authors wish to thank Professor Y. Okabe of the University of Tokyo, and Professor T. Kobayashi and Prof. K. Oura of Osaka University, for their fruitful discussions. A part of this work was performed under the management of FED as a part of the MITI R & D of Industrial Science and Technology Frontier Program ŽSuperconducting Electron Devices. supported by NEDO.
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