- Email: [email protected]
Homoepitaxial YBa2Cu3Ox films grown on single-crystal YBa2Cu3Ox substrates by metalorganic chemical vapor deposition using β-diketonates
Journal of Crystal Growth 221 (2000) 440}443
Homoepitaxial YBa Cu O "lms grown on single-crystal V YBa Cu O substrates by metalorganic chemical vapor V deposition using b-diketonates Hideaki Zama*, Nobue Tanaka, Tadataka Morishita Superconductivity Research Laboratory, International Superconductivity Technology Center, 10-13 Shinonome 1-Chome, Koto-ku, Tokyo 135-0062, Japan
Abstract Homoepitaxial YBa Cu O (YBCO) "lms were grown on single-crystal (0 0 1)YBCO substrates by a metalorganic V chemical vapor deposition (MOCVD) method using metal b-diketonate complexes as metalorganic sources. The single-crystal substrate was fabricated from bulk single crystals. Its surface was polished mechanically, treated by dipping in a HCl/methanol solution and modi"ed by depositing one monolayer of CuO . High-quality (0 0 1)-oriented YBCO V homoepitaxial growth was carried out using the substrate surface and the reproducible MOCVD technique which has been developed for realizing the process. 2000 Elsevier Science B.V. All rights reserved. PACS: 61.16.Ch; 68.35.Bs; 68.55.!a; 74.76.Bz Keywords: Homoepitaxial growth; YBa Cu O (YBCO) "lm; Single-crystal YBCO substrate; Metalorganic chemical vapor deposition V (MOCVD); b-diketonate
1. Introduction A combination of metalorganic chemical vapor deposition (MOCVD) and homoepitaxial growth is a candidate for developing the device processing of high-¹ superconducting cuprates. New Ba b diketonate complexes have been developed as a Ba precursor for the growth of YBa Cu O (YBCO) V thin "lms, which are normal-chain-amine adduct materials, such as Ba(DPM) -2tetraen [1] (DPM" dipivaloylmethane, C H O ; tetraen"tetraethy lenepentamine, NH (CH CH NH) CH CH NH ) * Corresponding author. Tel.: #81-3-3536-5713; fax: #813-3536-5717. E-mail address: [email protected] (H. Zama).
and Ba(DPM) -2pentaen [2] (pentaen"pentaethyl enehexamine, NH (CH CH NH) CH CH NH ), which are more volatile and stable than a conventional Ba source, Ba(DPM) , having lower volatility and stability than other metal sources of Y(DPM) and Cu(DPM) . The Ba b-diketonate adduct complexes improved the reproducibility of the MOCVD process. When Ba(DPM) -2pentaen was used at a temperature of 1403C, the Ba supply rate was kept stable within a standard deviation of 1.6% for over 300 h [2]. We have adopted single-crystal YBCO substrates for the growth of YBCO thin "lms, which were fabricated from bulk single crystals grown by a modi"ed top-seeded pulling method [3]. Singlecrystal (0 0 1)-oriented substrates were polished
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 3 5 - 1
H. Zama et al. / Journal of Crystal Growth 221 (2000) 440}443
mechanically [4] and their surface was treated by a chemical etching prior to deposition [5]. In order to modify a topmost surface of the substrate suitable for homoepitaxial growth, two techniques of the chemical etching and metal oxide pre-deposition were developed. In this paper, we study the relationship between the surface modi"cation for single-crystal YBCO substrates and quality of YBCO homoepitaxial "lms grown on them by the MOCVD process.
2. Experimental procedure We used a horizontal and cold-wall-type MOCVD system for the growth of Y, Ba and Cu oxides and YBCO "lms. Ba source was a b-diketonate adduct complex of Ba(DPM) -2pentaen. Y and Cu sources were b-diketonate complexes of Y(DPM) and Cu(DPM) , respectively. The typical vaporizing temperatures of Y(DPM) , Ba(DPM) -2pentaen and Cu(DPM) were 107, 136 and 1123C, respectively. The reaction was made in 10 Torr Ar mixed with 0.6 Torr O . The temper
441
ature of the susceptor was 8253C. The temperature at the substrate surface was estimated to be approximately 1003C lower than the susceptor temperature. The deposition rate was 20 nm/h under these conditions. Single crystals were sliced into (0 0 1) platelets with slightly tilting (0.2}0.33) for the (1 0 0) face and then mechanically polished with colloidal silica. The substrate was subjected to etching by immersing in a solution of 0.003 wt%-HCl/methanol for a duration of 60 s at room temperature. The HCl/methanol solution was prepared by dissolving commercial HCl/methanol in dehydrated methanol and its HCl concentration was evaluated by ion chromatography. We used three varieties of substrates for growing homoepitaxial "lms: as-polished, etched and metal-oxide-deposited substrates. The surface morphology of substrates and "lms grown was observed by an atomic force microscope (AFM) in air at room temperature using a contact mode. The crystallinity of homoepitaxial layers was evaluated by Rutherford backscattering spectrometry (RBS) using He> beams of 950 keV.
Fig. 1. AFM images of (a) as-polished, (b) etched, and (c) etched and a molecular CuO -deposited YBCO(0 0 1) substrates, and (d)}(f) V YBCO homoepitaxial "lms grown on the substrates of (a)}(c).
442
H. Zama et al. / Journal of Crystal Growth 221 (2000) 440}443
Fig. 2. AFM images of a molecular layer of (a) YO , (b) BaO and (c) CuO deposited on the etched YBCO(0 0 1) substrates. V V V
3. Results and discussion The surface of as-polished YBCO substrate, shown in Fig. 1(a), was degraded by stress during mechanical polishing and by exposure to humidity after polishing. The step height of features increases by 0.5}0.8 nm compared with the c lattice constant of YBCO, 1.2 nm. Chemically etched surface, shown in Fig. 1(b), removed a degraded layer, resulting in an appearance of sharp steps and smooth terraces, which were still stable after the annealing under the same conditions as YBCO "lms were deposited [5]. That the etched surface had a wellde"ned topmost structure terminated by only CuO }Y}CuO subunit in the YBCO structure was con"rmed by coaxial impact-collision ion scattering spectroscopy using 4 keV-Ne> ions [6] and by the angle-resolved X-ray photoelectron spectroscopy (ARXPS) [6,7]. The HCl/methanol solution selectively etched Ba and Cu ions on the surface of as-polished substrate. As heated the etched substrate up to 400}5003C in 1;10\ Torr O atmo sphere, Ba appeared on the surface as examined by ARXPS [6,7]. A molecular layer of YO , BaO or V V CuO was deposited on the etched substrates at V 8253C (the susceptor temperature). Both YO and V BaO formed granulars at terraces on the substraV tes (see Figs. 2(a) and (b)), while CuO remained the V step-and-terrace feature, indicating that CuO V grew laterally on the substrate surface (see Figs. 1(c) and 2(c)). These results suggest that the substrate surface was terminated by Ba after annealing. Homoepitaxial "lms were grown on as-polished, etched and CuO -deposited YBCO(0 0 1) V
Fig. 3. Random and channeling spectra of YBCO homoepitaxial "lms measured by RBS using He> beams of 950 keV: (a) random spectrum, (b) channeling spectrum of the "lm grown on etched YBCO(0 0 1) substrate and (c) channeling spectrum of the "lm grown on etched and a molecular CuO V deposited YBCO(0 0 1) substrate.
substrates. All substrates show the unit-cell-high step and terrace feature in Figs. 1(a)}(c), but having di!erent surface structures. Figs. 1(d)}(f) show corresponding AFM images of 20 nm-thick YBCO homoepitaxial "lms on the substrates shown in Figs. 1(a)}(c). The "lms were deposited in the stoichiometric metal concentration by precisely controlling vaporizing temperatures of the metal sources in the MOCVD process. In Fig. 1(d), randomly nucleated YBCO islands and steps with dendrite-like edges appear on the as-polished substrate, Fig. 1(a). A similar morphological image is present in Fig. 1(e) for the etched substrate, Fig. 1(b). The "lm in Fig. 1(e) showed a s of 10% for
a channeling measurement.
H. Zama et al. / Journal of Crystal Growth 221 (2000) 440}443
The surface morphology and crystallinity of homoepitaxial "lms were improved by depositing a molecular layer of CuO prior to YBCO growth. V The +1 0 0, facets and sharp step edges are seen in Fig. 1(f). The surface crystallinity of the "lm was improved from a s of 10% for the etched sub strate to a s of 4.3% for the CuO -deposited
V substrate as shown in Fig. 3. The channeling yield at the "lm-substrate interface was smaller for the CuO -deposited substrate than the etched substraV te as indicated by an arrow in Fig. 3.
443
measurements. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications under the New Sunshine Program administered by the Agency of Industrial Science and Technology (AIST) of the Ministry of International Trade and Industry (MITI) of Japan.
References 4. Conclusions The high-quality YBCO(0 0 1) homoepitaxial "lms were grown by MOCVD on the single-crystal YBCO(0 0 1) substrates which were applied by the chemical etching using a 0.003 wt%-HCl/methanol solution followed by one molecular CuO deposition. V Acknowledgements The authors wish to thank S. Koyama for providing YBCO single crystals and F. Wang for RBS
[1] H. Zama, T. Morishita, Jpn. J. Appl. Phys. 35 (1996) L770. [2] H. Zama, T. Morishita, Jpn. J. Appl. Phys., in press. [3] Y. Yamada, Y. Shiohara, Physica C 217 (1993) 182. [4] H. Zama, F. Wang, S. Koyama, Y. Shiohara, T. Morishita, Jpn. J. Appl. Phys. 35 (1996) L421. [5] N. Tanaka, H. Zama, T. Morishita, Jpn. J. Appl. Phys. 38 (1999) L731. [6] H. Zama, N. Tanaka, H. Uchiyama, T. Morishita, T. Nishihara, M. Shinohara, Extended Abstracts of 2000 International Workshop on Superconductivity (2000) 44. [7] H. Uchiyama, N. Tanaka, H. Zama, S. Tajima, T. Morishita, K. Saiki, A. Koma, Jpn. J. Appl. Phys. 39 (2000) 1320.
Homoepitaxial YBa Cu O "lms grown on single-crystal V YBa Cu O substrates by metalorganic chemical vapor V deposition using b-diketonates Hideaki Zama*, Nobue Tanaka, Tadataka Morishita Superconductivity Research Laboratory, International Superconductivity Technology Center, 10-13 Shinonome 1-Chome, Koto-ku, Tokyo 135-0062, Japan
Abstract Homoepitaxial YBa Cu O (YBCO) "lms were grown on single-crystal (0 0 1)YBCO substrates by a metalorganic V chemical vapor deposition (MOCVD) method using metal b-diketonate complexes as metalorganic sources. The single-crystal substrate was fabricated from bulk single crystals. Its surface was polished mechanically, treated by dipping in a HCl/methanol solution and modi"ed by depositing one monolayer of CuO . High-quality (0 0 1)-oriented YBCO V homoepitaxial growth was carried out using the substrate surface and the reproducible MOCVD technique which has been developed for realizing the process. 2000 Elsevier Science B.V. All rights reserved. PACS: 61.16.Ch; 68.35.Bs; 68.55.!a; 74.76.Bz Keywords: Homoepitaxial growth; YBa Cu O (YBCO) "lm; Single-crystal YBCO substrate; Metalorganic chemical vapor deposition V (MOCVD); b-diketonate
1. Introduction A combination of metalorganic chemical vapor deposition (MOCVD) and homoepitaxial growth is a candidate for developing the device processing of high-¹ superconducting cuprates. New Ba b diketonate complexes have been developed as a Ba precursor for the growth of YBa Cu O (YBCO) V thin "lms, which are normal-chain-amine adduct materials, such as Ba(DPM) -2tetraen [1] (DPM" dipivaloylmethane, C H O ; tetraen"tetraethy lenepentamine, NH (CH CH NH) CH CH NH ) * Corresponding author. Tel.: #81-3-3536-5713; fax: #813-3536-5717. E-mail address: [email protected] (H. Zama).
and Ba(DPM) -2pentaen [2] (pentaen"pentaethyl enehexamine, NH (CH CH NH) CH CH NH ), which are more volatile and stable than a conventional Ba source, Ba(DPM) , having lower volatility and stability than other metal sources of Y(DPM) and Cu(DPM) . The Ba b-diketonate adduct complexes improved the reproducibility of the MOCVD process. When Ba(DPM) -2pentaen was used at a temperature of 1403C, the Ba supply rate was kept stable within a standard deviation of 1.6% for over 300 h [2]. We have adopted single-crystal YBCO substrates for the growth of YBCO thin "lms, which were fabricated from bulk single crystals grown by a modi"ed top-seeded pulling method [3]. Singlecrystal (0 0 1)-oriented substrates were polished
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 3 5 - 1
H. Zama et al. / Journal of Crystal Growth 221 (2000) 440}443
mechanically [4] and their surface was treated by a chemical etching prior to deposition [5]. In order to modify a topmost surface of the substrate suitable for homoepitaxial growth, two techniques of the chemical etching and metal oxide pre-deposition were developed. In this paper, we study the relationship between the surface modi"cation for single-crystal YBCO substrates and quality of YBCO homoepitaxial "lms grown on them by the MOCVD process.
2. Experimental procedure We used a horizontal and cold-wall-type MOCVD system for the growth of Y, Ba and Cu oxides and YBCO "lms. Ba source was a b-diketonate adduct complex of Ba(DPM) -2pentaen. Y and Cu sources were b-diketonate complexes of Y(DPM) and Cu(DPM) , respectively. The typical vaporizing temperatures of Y(DPM) , Ba(DPM) -2pentaen and Cu(DPM) were 107, 136 and 1123C, respectively. The reaction was made in 10 Torr Ar mixed with 0.6 Torr O . The temper
441
ature of the susceptor was 8253C. The temperature at the substrate surface was estimated to be approximately 1003C lower than the susceptor temperature. The deposition rate was 20 nm/h under these conditions. Single crystals were sliced into (0 0 1) platelets with slightly tilting (0.2}0.33) for the (1 0 0) face and then mechanically polished with colloidal silica. The substrate was subjected to etching by immersing in a solution of 0.003 wt%-HCl/methanol for a duration of 60 s at room temperature. The HCl/methanol solution was prepared by dissolving commercial HCl/methanol in dehydrated methanol and its HCl concentration was evaluated by ion chromatography. We used three varieties of substrates for growing homoepitaxial "lms: as-polished, etched and metal-oxide-deposited substrates. The surface morphology of substrates and "lms grown was observed by an atomic force microscope (AFM) in air at room temperature using a contact mode. The crystallinity of homoepitaxial layers was evaluated by Rutherford backscattering spectrometry (RBS) using He> beams of 950 keV.
Fig. 1. AFM images of (a) as-polished, (b) etched, and (c) etched and a molecular CuO -deposited YBCO(0 0 1) substrates, and (d)}(f) V YBCO homoepitaxial "lms grown on the substrates of (a)}(c).
442
H. Zama et al. / Journal of Crystal Growth 221 (2000) 440}443
Fig. 2. AFM images of a molecular layer of (a) YO , (b) BaO and (c) CuO deposited on the etched YBCO(0 0 1) substrates. V V V
3. Results and discussion The surface of as-polished YBCO substrate, shown in Fig. 1(a), was degraded by stress during mechanical polishing and by exposure to humidity after polishing. The step height of features increases by 0.5}0.8 nm compared with the c lattice constant of YBCO, 1.2 nm. Chemically etched surface, shown in Fig. 1(b), removed a degraded layer, resulting in an appearance of sharp steps and smooth terraces, which were still stable after the annealing under the same conditions as YBCO "lms were deposited [5]. That the etched surface had a wellde"ned topmost structure terminated by only CuO }Y}CuO subunit in the YBCO structure was con"rmed by coaxial impact-collision ion scattering spectroscopy using 4 keV-Ne> ions [6] and by the angle-resolved X-ray photoelectron spectroscopy (ARXPS) [6,7]. The HCl/methanol solution selectively etched Ba and Cu ions on the surface of as-polished substrate. As heated the etched substrate up to 400}5003C in 1;10\ Torr O atmo sphere, Ba appeared on the surface as examined by ARXPS [6,7]. A molecular layer of YO , BaO or V V CuO was deposited on the etched substrates at V 8253C (the susceptor temperature). Both YO and V BaO formed granulars at terraces on the substraV tes (see Figs. 2(a) and (b)), while CuO remained the V step-and-terrace feature, indicating that CuO V grew laterally on the substrate surface (see Figs. 1(c) and 2(c)). These results suggest that the substrate surface was terminated by Ba after annealing. Homoepitaxial "lms were grown on as-polished, etched and CuO -deposited YBCO(0 0 1) V
Fig. 3. Random and channeling spectra of YBCO homoepitaxial "lms measured by RBS using He> beams of 950 keV: (a) random spectrum, (b) channeling spectrum of the "lm grown on etched YBCO(0 0 1) substrate and (c) channeling spectrum of the "lm grown on etched and a molecular CuO V deposited YBCO(0 0 1) substrate.
substrates. All substrates show the unit-cell-high step and terrace feature in Figs. 1(a)}(c), but having di!erent surface structures. Figs. 1(d)}(f) show corresponding AFM images of 20 nm-thick YBCO homoepitaxial "lms on the substrates shown in Figs. 1(a)}(c). The "lms were deposited in the stoichiometric metal concentration by precisely controlling vaporizing temperatures of the metal sources in the MOCVD process. In Fig. 1(d), randomly nucleated YBCO islands and steps with dendrite-like edges appear on the as-polished substrate, Fig. 1(a). A similar morphological image is present in Fig. 1(e) for the etched substrate, Fig. 1(b). The "lm in Fig. 1(e) showed a s of 10% for
a channeling measurement.
H. Zama et al. / Journal of Crystal Growth 221 (2000) 440}443
The surface morphology and crystallinity of homoepitaxial "lms were improved by depositing a molecular layer of CuO prior to YBCO growth. V The +1 0 0, facets and sharp step edges are seen in Fig. 1(f). The surface crystallinity of the "lm was improved from a s of 10% for the etched sub strate to a s of 4.3% for the CuO -deposited
V substrate as shown in Fig. 3. The channeling yield at the "lm-substrate interface was smaller for the CuO -deposited substrate than the etched substraV te as indicated by an arrow in Fig. 3.
443
measurements. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications under the New Sunshine Program administered by the Agency of Industrial Science and Technology (AIST) of the Ministry of International Trade and Industry (MITI) of Japan.
References 4. Conclusions The high-quality YBCO(0 0 1) homoepitaxial "lms were grown by MOCVD on the single-crystal YBCO(0 0 1) substrates which were applied by the chemical etching using a 0.003 wt%-HCl/methanol solution followed by one molecular CuO deposition. V Acknowledgements The authors wish to thank S. Koyama for providing YBCO single crystals and F. Wang for RBS
[1] H. Zama, T. Morishita, Jpn. J. Appl. Phys. 35 (1996) L770. [2] H. Zama, T. Morishita, Jpn. J. Appl. Phys., in press. [3] Y. Yamada, Y. Shiohara, Physica C 217 (1993) 182. [4] H. Zama, F. Wang, S. Koyama, Y. Shiohara, T. Morishita, Jpn. J. Appl. Phys. 35 (1996) L421. [5] N. Tanaka, H. Zama, T. Morishita, Jpn. J. Appl. Phys. 38 (1999) L731. [6] H. Zama, N. Tanaka, H. Uchiyama, T. Morishita, T. Nishihara, M. Shinohara, Extended Abstracts of 2000 International Workshop on Superconductivity (2000) 44. [7] H. Uchiyama, N. Tanaka, H. Zama, S. Tajima, T. Morishita, K. Saiki, A. Koma, Jpn. J. Appl. Phys. 39 (2000) 1320.