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Thin film fabrication using nanoscale flat substrates
Thin film fabrication using nanoscale flat substrates
6
Takashi Nishida Fukuoka University, Fukuoka, Japan
6.1
Introduction
Ferroelectric thin films have been widely investigated for use in micromachining and memory devices such as ferroelectric random-access memory units. Interest in the energy harvesting applications of ferroelectric films has also expanded rapidly in recent years. The evaluation of nanoscale ferroelectric behavior and the preparation of nanosize ferroelectric crystals are both key issues because such materials could lead to improvements in the electrical characteristics of these devices. Consequently, there have been a number of reports concerning the fabrication of nanosize ferroelectrics by either top-down or bottom-up methods. In top-down methods, a nanostructure is fabricated by patterning a ferroelectric thin film using various techniques, including electron beam lithography [1] and focused ion beam milling [2]. Well-ordered nanocapacitors with lateral dimensions of several tens of nanometers can be produced in this manner, as well as arrays of nanostructures having specific sizes, shapes, and positions. However, the patterning process damages the nanocrystals, and so their ferroelectric properties may be degraded. For this reason bottom-up methods have also been used for fabrication, and nanocrystals based on initial growth nucleation on substrate materials have been prepared by sputtering [3, 4], metal organic chemical vapor deposition [5], and various other methods [6]. However, it is not possible to control the position of growth nucleation, and so well-ordered nanostructures cannot be produced as easily by bottom-up as by top-down techniques. It is also difficult to use the resulting nanocrystals to investigate nanoscale phenomena because they have a range of sizes and orientations, and so experimental results usually show poor repeatability. This chapter describes the deposition of PbTiO3 (PTO) and platinum (Pt) ultra-thin films on atomically flat sapphire substrates. During these processes, there was an attempt to control the nucleation position by reducing the growth rate as well as by optimizing the migration of sputtered particles on the substrate surface. As a result, highly uniform surfaces with step-terrace structures were developed on atomically flat substrates by self-organization. Using these substrates in conjunction with the optimized deposition conditions should allow the fabrication of well-ordered, highly crystalline ferroelectric nanocrystals or thin films that are expected to improve the performance of energy harvesting devices. Nanoscale Ferroelectric-Multiferroic Materials for Energy Harvesting Applications. https://doi.org/10.1016/B978-0-12-814499-2.00006-2
© 2019 Elsevier Inc. All rights reserved.
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6.2
Nanoscale Ferroelectric-Multiferroic Materials for Energy Harvesting Applications
Experimental
Sapphire (α-Al2O3) single-crystal substrates (Kyocera Corp.) with optically flat polished surfaces were used for film deposition. The orientation of the substrate surface was (0001), although the substrates had a slight miscut angle of <0.3°. In order to form atomically flat surfaces, the substrates were annealed in air at 1000–1200°C for 3 h in a tubular furnace (Yamada Electric Co., Ltd.: TF-630-P) prior to use [3]. PTO films were deposited on the atomically flat sapphire surfaces by radio frequency (RF) magnetron sputtering (SPF-210H, Anelva Co.), applying the conditions summarized in Table 6.1. A sapphire shield was placed in front of the substrate to reduce the deposition rate, as shown in Fig. 6.1. During this process, some sputtered particles on the shield surface migrated to the back of the shield and grew into nanocrystals. Thus continuous thin PTO films and nanocrystals formed in the center of the exposed area and at the edges of this area that were covered by the shield, respectively. Pt thin films were also prepared by RF magnetron sputtering (RFS-200, Ulvac Co.) on atomically flat sapphire substrates. In each case the substrate was heated to 600°C to promote epitaxial growth. The sputtering conditions are summarized in Table 6.2. The atomically flat surfaces, thin films, and nanocrystals were all observed by atomic force microscopy (AFM: SPA400, Seiko Instruments Inc.). The PTO specimens were also evaluated by X-ray diffraction (XRD: X’Pert MRD, Panalytical), X-ray photoelectron spectroscopy (XPS: AXIS-165, Shimadzu Corp.) and laser micro-Raman spectrometry (μ-LR: NRS-2100, Jasco Corp.).
Table 6.1 Sputtering conditions for preparing PbTiO3 nanocrystals Target
PbTiO3 powder
RF power Substrate Temperature Sputtering gas Gas pressure Presputter time Deposition time
100 W Sapphire 600°C Ar:O2 ¼ 9:1 1.5 Pa 10 min 30 min
Fig. 6.1 A schematic showing nanocrystal deposition at low growth rates by sputtering using a shield.
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Table 6.2 Sputtering conditions for preparing platinum films Target
Pt ϕ80 mm
RF power Substrate Temperature Sputtering gas Gas pressure Deposition time
40 W α-Al2O3 600°C Ar 3 sccm 4 Pa 3 min
6.3
Observations of atomically flat surfaces with atomic steps and terraces by atomic force microscopy
As noted, the sapphire wafers were subjected to a treatment to produce an atomically flat surface. As a result, a drastic change in the surface morphology was observed upon increasing the annealing temperature, as shown in Fig. 6.2, which presents AFM images of surfaces annealed at 1000°C, 1100°C, and 1200°C. The atomically flat terraces in Fig. 6.2A were 40–45 nm wide and homogeneous, and the degree of flatness of the terraces was consistent. The single atomic steps on this substrate were 0.22–0.44 nm in height. It should be noted that 0.22 nm corresponds to a single atomic step height in an α-Al2O3 (0001) crystal [7]. This surface structure is thought to be stable and would be expected to contribute to the formation of highquality thin films and nanocrystals. The atomically flat terraces in Fig. 6.2B were 40–150 nm wide and somewhat inhomogeneous, as the flatness of the terraces was inconsistent. The atomic steps were 0.22–1.10 nm high (equivalent to n 0.22 nm, where n is an integer with a value of 1–5). This surface structure is thought to result from step-bunching [7], which was just beginning in this sample and was unstable. Finally, the atomically flat terraces in Fig. 6.2C were 50–250 nm wide and very inhomogeneous but with consistent flatness. The atomic steps were 0.44–1.10 nm
Fig. 6.2 Atomic force microscopy images of atomically flat surfaces obtained at annealing temperatures of (A) 1000°C, (B) 1100°C, and (C) 1200°C.
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in height. This surface structure is attributed to step-bunching during atom migration at the higher temperature, which may contribute to nucleus formation on the steps. Each of these three substrates was used to deposit PTO thin films and nanocrystals.
6.4
Evaluation of PbTiO3 thin films by X-ray fluorescence spectroscopy and X-ray diffraction
PTO thin films deposited on the atomically flat substrates shown in Fig. 6.2 were evaluated by XRF and XRD. On the basis of the X-ray fluorescence spectroscopy (XRF) data, the Pb:Ti ratio in each of the films was found to be 1.2:1, indicating that they were Pb-rich by 20 at.%. The thickness of each film was 100 nm. The XRD patterns for the films are shown Fig. 6.3. These films were epitaxially oriented, and the results of a φ scan are presented in Fig. 6.4. An epitaxial PTO thin film oriented solely in the (111) direction was deposited only on the substrate annealed at 1000°C. In contrast, the films on the other two substrates generated either a PbO (111) or pyrochlore peak in addition to the
Fig. 6.3 X-ray diffraction patterns generated by PTO thin films on atomically-flat substrates prepared at various temperatures [3]. Fig. 6.4 The results of a φ scan of a PbTiO3 thin film on an atomically flat substrate together with data for α-Al2O3 (annealing temperature 1000°C) for comparison purposes [3].
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PTO (111) peak. These other phases could possibly have resulted from the substrate surface conditions. For this reason, it appears that single steps and flat terraces might contribute to the deposition of an epitaxial PTO thin film. In addition, because the number of peaks in the φ scan for the PTO was twice that for an α-Al2O3 reference (Fig. 6.4), the epitaxial PTO film evidently had two different in-plane orientations.
6.5
Observations of PbTiO3 nanocrystals by atomic force microscopy
PTO nanocrystals were observed by AFM. The sample annealed at 1000°C was investigated in more detail because it appeared to consist of an epitaxial PTO thin film. Fig. 6.5A and A0 , B and B0 , and C and C0 presents AFM images of PTO nanocrystals deposited on a nonatomically flat (conventional) α-Al2O3 (0001) substrate, and on atomically flat substrates with shields allowing sputtered particles to approach the descending atomic steps (Fig. 6.6A) or ascending atomic steps (Fig. 6.6B), Fig. 6.5 Atomic force microscopy images of PbTiO3 nanocrystals: (A), (A0 ) on a nonatomically flat α-Al2O3 (0001) substrate, and (B), (B0 ) and (C), (C0 ) on atomically flat substrates with the shield placed so as to allow sputtered particles to approach the atomic steps in the descending and ascending directions, respectively [3].
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Fig. 6.6 Diagrams showing the shield placements allowing sputtering particles to approach the atomic steps in the (A) descending and (B) ascending directions.
respectively. Each of these images show two triangular PTO nanocrystals of rotational symmetry of 180°. This result agrees with the XRD patterns and the φ scan. The lengths and heights of the crystals were in the ranges of 100–150 nm and 20–30 nm, respectively. On closer inspection of the triangular crystals in Fig. 6.5A, it can be seen that there are no grains in the vicinity of the crystals. It is thought that an initial nucleus was formed, followed by the agglomeration of migrating sputtered particles, resulting in the growth of large triangular crystals. The nucleation density was much higher near the center, as shown in Fig. 6.5A0 . Thus, controlling the nucleation location on a substrate without steps is evidently difficult. Fig. 6.5B demonstrates that the migration of the sputtered particles to the left was suppressed by the atomic steps. Therefore only a few grains were aligned along the steps while many rightward sputtered particles were deposited to grow large crystals. Once again, the nucleation density is higher near the center (Fig. 6.5B0 ). These results suggest that either migration suppression resulting from atomic steps or the optimization of sputtering conditions is necessary to precisely control the nucleation position. Fig. 6.5C confirms that the migration of the sputtered particles in the rightward direction was suppressed by the atomic steps and to a greater extent than that in Fig. 6.5B. This explains why some grains fell into alignment along the atomic steps. Additionally, many grains and nanocrystals were lined up along the atomic steps near the center, as can be seen in Fig. 6.5C0 . Further evidence for this phenomenon is provided by the two-dimensional fast Fourier transform (FFT) of Fig. 6.5C0 , shown in Fig. 6.7. The points at (18, 4) and (18, 4), indicated by arrows in Fig. 6.7, should be noted because they demonstrate nanocrystal alignment at approximately 50 nm intervals, tilted at about 12.5°. This 50 nm interval is equal to the width of the surface terraces on the atomically flat substrate annealed at 1000°C. However, nanocrystals and grains tend to overhang the atomic steps when these crystals are larger than the terrace width. To address this issue, it will be necessary to reduce the miscut angle by polishing the α-Al2O3 substrates to widen the terraces.
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Fig. 6.7 Two-dimensional Fourier transform of Fig. 6.5C0 [3].
Therefore these results show that it should be possible to fabricate PTO nanocrystals on step structures via deposition on atomically flat substrates with sufficiently wide terraces.
6.6
Effects of growth rate on PbTiO3 nanocrystals
The PTO nanocrystals in the edge areas of the substrates, as indicated in Fig. 6.1, were observed by AFM at various distances from the shield edge, to assess the effects of growth rate on surface morphology. Fig. 6.8 presents the images obtained with the nonatomically flat (i.e., conventional) substrate. In this case nanocrystals were generated relatively slowly, as shown in Fig. 6.8A, while the two-dimensional growth of crystals was more rapid, as in Fig. 6.8B and C, demonstrating the formation of a
Fig. 6.8 The effect of growth rate on the surface morphology of PbTiO3 films prepared on a nonatomically flat substrate [8].
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Nanoscale Ferroelectric-Multiferroic Materials for Energy Harvesting Applications
Fig. 6.9 The effect of growth rate on the surface morphology of PbTiO3 films prepared on an atomically flat substrate [8].
continuous film. From these images, it is apparent that lateral crystal growth occurred during sputtering as a result of surface migration. However, the crystallites were not uniform in size or in terms of their growth positions. The surface morphologies of a specimen grown on the atomically flat sapphire surface in Fig. 6.2A are shown in Fig. 6.9. In this sample, a homogeneous step-terrace structure was formed by self-organization and the PTO was deposited on the surface. In this case, the crystallites formed a line because nanocrystal nucleation proceeded at the steps. The crystal sizes increased with increases in the deposition rate, and these crystals show more homogeneous sizes compared to those on the conventional substrate (Fig. 6.8). Thus a more uniform film was obtained via growth on an atomically flat substrate.
6.7
Evaluation of crystal structure of PbTiO3 nanocrystals
The shielded area in Fig. 6.1 was also observed by AFM, revealing that nanocrystals with uniform sizes in the range of 20–30 nm had grown on the surface (Fig. 6.10A). However, some larger particles were also observed, since the sputtering conditions had not been sufficiently optimized, which resulted in a second growth phase. The enlarged AFM image in Fig. 6.10B seems to show the presence of lines. A twodimensional FFT of the image in Fig. 6.10B is presented in Fig. 6.11. Spots are clearly observable at 14 mm1 in the FFT image, indicating that the nanocrystals were grown at intervals of 70 nm. Because the terrace width of the atomically flat surface seems to agree with the intervals between the crystals, a highly magnified image was produced to further investigate this phenomenon (Fig. 6.12). The enlarged view confirms that crystallites were grown at the atomic steps. The initial growth nucleation appears to have occurred at the steps because the sputtered particles were captured at the step edges and because migration on the surface was assisted by the low deposition rate resulting from the use of a shield. An attempt was made to evaluate the crystal structure of a continuous film based on the XRD pattern in Fig. 6.3. However, it was not possible to examine the structure with sufficient accuracy because the reflection intensity was too low. Thus both continuous
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Fig. 6.10 Atomic force microscopy images of the shielded area, showing (A) standard and (B) enlarged views [9].
Fig. 6.11 Two-dimensional Fourier transform of Fig. 6.10B [9].
films and nanocrystals specimens were evaluated by XPS and μ-LR, and the XPS profiles for the nanocrystals were compared with those for the films. The resulting XPS profiles are shown in Fig. 6.13, and Pb 4f and Ti 2p peaks from the PTO film are clearly visible in Fig. 6.13A and B, respectively. The chemical shifts of these peaks demonstrate that the Pb and Ti in the film were present as Pb2+ and Ti4+, respectively [10]. The intensities of the peaks obtained from the nanocrystals were weak, but the peak profiles and chemical shifts were almost the same as those for the film specimen. Raman measurements were also performed, and Fig. 6.14 shows the resulting spectra, from which the vibrational modes were identified based on a report by Friere et al. [11]. The spectrum of the nanocrystals was very weak, although similar to that of the film. Therefore both the XPS and μ-LR results demonstrate that the nanocrystals on the atomically flat surface had crystallized in a perovskite structure.
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Nanoscale Ferroelectric-Multiferroic Materials for Energy Harvesting Applications
Fig. 6.12 Atomic force microscopy image of nanocrystals on an atomically flat surface [9].
Fig. 6.13 X-ray photoelectron spectroscopy data acquired from a PbTiO3 film and from nanocrystals.
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Fig. 6.14 The Raman spectra of a PbTiO3 thin film and of nanocrystals.
6.8
Fabrication of platinum nanocrystals for use as electrode nanolayer
In the areas of the substrate that were not shielded, such as at x ¼ 2 mm in Fig. 6.15, a continuous Pt film was obtained by sputtering using the conditions in Table 6.2 and the XRD pattern demonstrated that the film was grown epitaxially on the sapphire substrate. The surface morphology of the film was observed by AFM, as shown in Fig. 6.16, and it was determined that the film contained crystallites having various grain sizes and was not homogenous. Using this setup, the deposition of Pt was suppressed by the shield, and the growth rate varied with distance from the shield edge, x. The rate decreased with increasing x. Fig. 6.15 A schematic showing the fabrication of platinum nanocrystals using a shield.
Fig. 6.16 An atomic force microscopy image of a platinum film [12].
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Nanoscale Ferroelectric-Multiferroic Materials for Energy Harvesting Applications
Fig. 6.17 Atomic force microscopy images showing variations in surface morphology with growth position.
AFM observations were performed at various x values, and the resulting images are provided in Fig. 6.17. The surface at the shield edge was almost completely covered with small crystallites, while the crystallite density dropped significantly on moving even slightly away from the edge, such as at x ¼ 0.4 mm. However, the surface area that was covered by crystals was increased at a distance of 0.9 mm, although the growth rate decreased. The crystallite height was >1.0 nm at x ¼ 0.4 mm but decreased to below 0.8 nm at x ¼ 0.9. Thus three-dimensional growth evidently transitioned to two-dimensional growth at a low growth rate. The surface roughness of epitaxial Pt layers has been investigated in detail, revealing that the crystallite size can be greatly affected by the growth conditions [13], and the change in growth mode from
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three- to two-dimensional in this work appears to be caused by a similar mechanism. The Pt islands on the substrate were steadily enlarged at x ¼ 1.2 mm and appear to have spread over the sapphire surface at x ¼ 1.4 mm. These results demonstrate that the Pt layer had a greater degree of surface smoothness than high-quality Pt films previously reported in the literature [14, 15]. Conductive AFM (C-AFM) measurements were employed to confirm that the atomically flat surface was entirely covered with a Pt layer. The surface morphology of sapphire substrates with and without Pt deposition are shown in Fig. 6.18A and B, respectively, and are quite similar to one another. In contrast, the C-AFM images of surfaces with and without Pt in Fig. 6.18C and D are different. Although only a noise current is evident in the case of the sapphire surface in Fig. 6.18D, a stripe pattern similar to that in the AFM image can be observed in the C-AFM image of the Pt/sapphire specimen in Fig. 6.18C. Because the measured current was very low, a large DC offset current of approximately 35.4 pA was employed to acquire these C-AFM images, and so the current difference between Fig. 6.18C and D appears to be 1 pA smaller than it actually was. Because this low current was modulated by the contact conditions between the ultra-thin Pt layer and the AFM tip, the measured current was affected by the surface morphology, such as the step-terrace structure. From these results, it is apparent that a thin Pt layer can be deposited on an atomically flat surface by low-incident angle sputtering.
Fig. 6.18 Atomic force microscopy (AFM) and conductive AFM (C-AFM) images of sapphire substrates with and without platinum (Pt) deposition.
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6.9
Nanoscale Ferroelectric-Multiferroic Materials for Energy Harvesting Applications
Conclusion
In recent years there has been significant interest in nanosized ferroelectric materials because of the need to produce electronic devices with greater reliability. In the work described herein, ferroelectric PTO thin films and Pt electrode thin layers were prepared on atomically flat sapphire substrates by sputtering at a very low deposition rate. The deposition rate was reduced by placing shields in front of the substrates and, although the migration energy of the sputtered particles on the substrate surface was not reduced, nucleation growth appeared to improve. Furthermore, a well-ordered surface was formed on the atomically flat substrates due to self-organization, which resulted in the formation of uniform nanocrystal arrays. These nanocrystals were evaluated by XPS and μ-LR, and it was confirmed that nanosized crystals of the perovskite PbTiO3 were obtained. Therefore this technique allows tuning of the nanocrystal deposition location in conjunction with the growth of high-quality crystals and so should permit the optimization of deposition control in bottom-up fabrication methods. This approach to controlling the growth of nanosized single crystals while ensuring uniform grain sizes across the entire substrate is thought to have significant potential.
References [1] S. Okamura, Y. Yagi, S. Ando, T. Tsukamoto, K. Mori, Jpn. J. Appl. Phys. 35 (1996) 6579–6583. [2] A. Stanishevsky, S. Aggarwal, A.S. Prakash, J. Melngailis, R. Ramesh, J. Vac. Sci. Technol. B 16 (1998) 3899–3902. [3] K. Kubo, M. Echizen, T. Nishida, H. Takeda, K. Uchiyama, T. Shiosaki, Trans. Mater. Res. Soc. Jpn. 33 (1)(2008) 69–72. [4] K. Wasa, Y. Haneda, T. Sato, H. Adachi, I. Kanno, D.G. Schlom, S.T. McKinstry, Q. Gang, C. Beom, Int. Soc. Opt. Eng. 3481 (1998) 182–189. [5] H. Nonomura, H. Fujisawa, M. Shimizu, H. Niu, K. Honda, J. Cryst. Growth 275 (2005) e2433–e2438. [6] C.D. Theis, D.G. Schlom, Mater. Res. Soc. Symp. Proc. 401 (1996) 171–177. [7] M. Yoshimoto, T. Maeda, T. Ohnishi, H. Koinuma, O. Ishiyama, M. Shinohara, M. Kubo, R. Miura, A. Miyamoto, Appl. Phys. Lett. 67 (1995) 2615–2617. [8] T. Nishida, K. Kubo, M. Echizen, H. Takeda, K. Uchiyama, T. Shiosaki, Key Eng. Mater. 421–422 (2010) 502–505. [9] T. Nishida, K. Kubo, H. Takeda, K. Uchiyama, T. Shiosaki, Ferroelectrics 381 (2009) 74–79. [10] J.F. Moulder, et al., Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, 1995, pp. 73, 189. [11] J.D. Freire, R.S. Katiyal, Phys. Rev. B Condens. Matter 37 (1988 2074–2085. [12] T. Nishida, K. Asahi, Y. Yoneda, K. Tamura, D. Matsumura, H. Kimura, Y. Ishikawa, Y. Uraoka, Trans. Mater. Res. Soc. Jpn. 36 (1)(2011) 11–13. [13] M.L. Hildner, T.J. Minvielle, R.J. Wilson, Surf. Sci. 396 (1998) 16–23. [14] S. Schmidt, D.O. Klenov, S.P. Kaene, J. Lu, T.E. Mates, S. Stemmer, Appl. Phys. Lett. 87 (2006). [15] H. Zhou, P. Wochner, A. Schops, T. Wagner, J. Cryst. Growth 234 (2002) 561–568.
6
Takashi Nishida Fukuoka University, Fukuoka, Japan
6.1
Introduction
Ferroelectric thin films have been widely investigated for use in micromachining and memory devices such as ferroelectric random-access memory units. Interest in the energy harvesting applications of ferroelectric films has also expanded rapidly in recent years. The evaluation of nanoscale ferroelectric behavior and the preparation of nanosize ferroelectric crystals are both key issues because such materials could lead to improvements in the electrical characteristics of these devices. Consequently, there have been a number of reports concerning the fabrication of nanosize ferroelectrics by either top-down or bottom-up methods. In top-down methods, a nanostructure is fabricated by patterning a ferroelectric thin film using various techniques, including electron beam lithography [1] and focused ion beam milling [2]. Well-ordered nanocapacitors with lateral dimensions of several tens of nanometers can be produced in this manner, as well as arrays of nanostructures having specific sizes, shapes, and positions. However, the patterning process damages the nanocrystals, and so their ferroelectric properties may be degraded. For this reason bottom-up methods have also been used for fabrication, and nanocrystals based on initial growth nucleation on substrate materials have been prepared by sputtering [3, 4], metal organic chemical vapor deposition [5], and various other methods [6]. However, it is not possible to control the position of growth nucleation, and so well-ordered nanostructures cannot be produced as easily by bottom-up as by top-down techniques. It is also difficult to use the resulting nanocrystals to investigate nanoscale phenomena because they have a range of sizes and orientations, and so experimental results usually show poor repeatability. This chapter describes the deposition of PbTiO3 (PTO) and platinum (Pt) ultra-thin films on atomically flat sapphire substrates. During these processes, there was an attempt to control the nucleation position by reducing the growth rate as well as by optimizing the migration of sputtered particles on the substrate surface. As a result, highly uniform surfaces with step-terrace structures were developed on atomically flat substrates by self-organization. Using these substrates in conjunction with the optimized deposition conditions should allow the fabrication of well-ordered, highly crystalline ferroelectric nanocrystals or thin films that are expected to improve the performance of energy harvesting devices. Nanoscale Ferroelectric-Multiferroic Materials for Energy Harvesting Applications. https://doi.org/10.1016/B978-0-12-814499-2.00006-2
© 2019 Elsevier Inc. All rights reserved.
96
6.2
Nanoscale Ferroelectric-Multiferroic Materials for Energy Harvesting Applications
Experimental
Sapphire (α-Al2O3) single-crystal substrates (Kyocera Corp.) with optically flat polished surfaces were used for film deposition. The orientation of the substrate surface was (0001), although the substrates had a slight miscut angle of <0.3°. In order to form atomically flat surfaces, the substrates were annealed in air at 1000–1200°C for 3 h in a tubular furnace (Yamada Electric Co., Ltd.: TF-630-P) prior to use [3]. PTO films were deposited on the atomically flat sapphire surfaces by radio frequency (RF) magnetron sputtering (SPF-210H, Anelva Co.), applying the conditions summarized in Table 6.1. A sapphire shield was placed in front of the substrate to reduce the deposition rate, as shown in Fig. 6.1. During this process, some sputtered particles on the shield surface migrated to the back of the shield and grew into nanocrystals. Thus continuous thin PTO films and nanocrystals formed in the center of the exposed area and at the edges of this area that were covered by the shield, respectively. Pt thin films were also prepared by RF magnetron sputtering (RFS-200, Ulvac Co.) on atomically flat sapphire substrates. In each case the substrate was heated to 600°C to promote epitaxial growth. The sputtering conditions are summarized in Table 6.2. The atomically flat surfaces, thin films, and nanocrystals were all observed by atomic force microscopy (AFM: SPA400, Seiko Instruments Inc.). The PTO specimens were also evaluated by X-ray diffraction (XRD: X’Pert MRD, Panalytical), X-ray photoelectron spectroscopy (XPS: AXIS-165, Shimadzu Corp.) and laser micro-Raman spectrometry (μ-LR: NRS-2100, Jasco Corp.).
Table 6.1 Sputtering conditions for preparing PbTiO3 nanocrystals Target
PbTiO3 powder
RF power Substrate Temperature Sputtering gas Gas pressure Presputter time Deposition time
100 W Sapphire 600°C Ar:O2 ¼ 9:1 1.5 Pa 10 min 30 min
Fig. 6.1 A schematic showing nanocrystal deposition at low growth rates by sputtering using a shield.
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Table 6.2 Sputtering conditions for preparing platinum films Target
Pt ϕ80 mm
RF power Substrate Temperature Sputtering gas Gas pressure Deposition time
40 W α-Al2O3 600°C Ar 3 sccm 4 Pa 3 min
6.3
Observations of atomically flat surfaces with atomic steps and terraces by atomic force microscopy
As noted, the sapphire wafers were subjected to a treatment to produce an atomically flat surface. As a result, a drastic change in the surface morphology was observed upon increasing the annealing temperature, as shown in Fig. 6.2, which presents AFM images of surfaces annealed at 1000°C, 1100°C, and 1200°C. The atomically flat terraces in Fig. 6.2A were 40–45 nm wide and homogeneous, and the degree of flatness of the terraces was consistent. The single atomic steps on this substrate were 0.22–0.44 nm in height. It should be noted that 0.22 nm corresponds to a single atomic step height in an α-Al2O3 (0001) crystal [7]. This surface structure is thought to be stable and would be expected to contribute to the formation of highquality thin films and nanocrystals. The atomically flat terraces in Fig. 6.2B were 40–150 nm wide and somewhat inhomogeneous, as the flatness of the terraces was inconsistent. The atomic steps were 0.22–1.10 nm high (equivalent to n 0.22 nm, where n is an integer with a value of 1–5). This surface structure is thought to result from step-bunching [7], which was just beginning in this sample and was unstable. Finally, the atomically flat terraces in Fig. 6.2C were 50–250 nm wide and very inhomogeneous but with consistent flatness. The atomic steps were 0.44–1.10 nm
Fig. 6.2 Atomic force microscopy images of atomically flat surfaces obtained at annealing temperatures of (A) 1000°C, (B) 1100°C, and (C) 1200°C.
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in height. This surface structure is attributed to step-bunching during atom migration at the higher temperature, which may contribute to nucleus formation on the steps. Each of these three substrates was used to deposit PTO thin films and nanocrystals.
6.4
Evaluation of PbTiO3 thin films by X-ray fluorescence spectroscopy and X-ray diffraction
PTO thin films deposited on the atomically flat substrates shown in Fig. 6.2 were evaluated by XRF and XRD. On the basis of the X-ray fluorescence spectroscopy (XRF) data, the Pb:Ti ratio in each of the films was found to be 1.2:1, indicating that they were Pb-rich by 20 at.%. The thickness of each film was 100 nm. The XRD patterns for the films are shown Fig. 6.3. These films were epitaxially oriented, and the results of a φ scan are presented in Fig. 6.4. An epitaxial PTO thin film oriented solely in the (111) direction was deposited only on the substrate annealed at 1000°C. In contrast, the films on the other two substrates generated either a PbO (111) or pyrochlore peak in addition to the
Fig. 6.3 X-ray diffraction patterns generated by PTO thin films on atomically-flat substrates prepared at various temperatures [3]. Fig. 6.4 The results of a φ scan of a PbTiO3 thin film on an atomically flat substrate together with data for α-Al2O3 (annealing temperature 1000°C) for comparison purposes [3].
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PTO (111) peak. These other phases could possibly have resulted from the substrate surface conditions. For this reason, it appears that single steps and flat terraces might contribute to the deposition of an epitaxial PTO thin film. In addition, because the number of peaks in the φ scan for the PTO was twice that for an α-Al2O3 reference (Fig. 6.4), the epitaxial PTO film evidently had two different in-plane orientations.
6.5
Observations of PbTiO3 nanocrystals by atomic force microscopy
PTO nanocrystals were observed by AFM. The sample annealed at 1000°C was investigated in more detail because it appeared to consist of an epitaxial PTO thin film. Fig. 6.5A and A0 , B and B0 , and C and C0 presents AFM images of PTO nanocrystals deposited on a nonatomically flat (conventional) α-Al2O3 (0001) substrate, and on atomically flat substrates with shields allowing sputtered particles to approach the descending atomic steps (Fig. 6.6A) or ascending atomic steps (Fig. 6.6B), Fig. 6.5 Atomic force microscopy images of PbTiO3 nanocrystals: (A), (A0 ) on a nonatomically flat α-Al2O3 (0001) substrate, and (B), (B0 ) and (C), (C0 ) on atomically flat substrates with the shield placed so as to allow sputtered particles to approach the atomic steps in the descending and ascending directions, respectively [3].
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Fig. 6.6 Diagrams showing the shield placements allowing sputtering particles to approach the atomic steps in the (A) descending and (B) ascending directions.
respectively. Each of these images show two triangular PTO nanocrystals of rotational symmetry of 180°. This result agrees with the XRD patterns and the φ scan. The lengths and heights of the crystals were in the ranges of 100–150 nm and 20–30 nm, respectively. On closer inspection of the triangular crystals in Fig. 6.5A, it can be seen that there are no grains in the vicinity of the crystals. It is thought that an initial nucleus was formed, followed by the agglomeration of migrating sputtered particles, resulting in the growth of large triangular crystals. The nucleation density was much higher near the center, as shown in Fig. 6.5A0 . Thus, controlling the nucleation location on a substrate without steps is evidently difficult. Fig. 6.5B demonstrates that the migration of the sputtered particles to the left was suppressed by the atomic steps. Therefore only a few grains were aligned along the steps while many rightward sputtered particles were deposited to grow large crystals. Once again, the nucleation density is higher near the center (Fig. 6.5B0 ). These results suggest that either migration suppression resulting from atomic steps or the optimization of sputtering conditions is necessary to precisely control the nucleation position. Fig. 6.5C confirms that the migration of the sputtered particles in the rightward direction was suppressed by the atomic steps and to a greater extent than that in Fig. 6.5B. This explains why some grains fell into alignment along the atomic steps. Additionally, many grains and nanocrystals were lined up along the atomic steps near the center, as can be seen in Fig. 6.5C0 . Further evidence for this phenomenon is provided by the two-dimensional fast Fourier transform (FFT) of Fig. 6.5C0 , shown in Fig. 6.7. The points at (18, 4) and (18, 4), indicated by arrows in Fig. 6.7, should be noted because they demonstrate nanocrystal alignment at approximately 50 nm intervals, tilted at about 12.5°. This 50 nm interval is equal to the width of the surface terraces on the atomically flat substrate annealed at 1000°C. However, nanocrystals and grains tend to overhang the atomic steps when these crystals are larger than the terrace width. To address this issue, it will be necessary to reduce the miscut angle by polishing the α-Al2O3 substrates to widen the terraces.
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Fig. 6.7 Two-dimensional Fourier transform of Fig. 6.5C0 [3].
Therefore these results show that it should be possible to fabricate PTO nanocrystals on step structures via deposition on atomically flat substrates with sufficiently wide terraces.
6.6
Effects of growth rate on PbTiO3 nanocrystals
The PTO nanocrystals in the edge areas of the substrates, as indicated in Fig. 6.1, were observed by AFM at various distances from the shield edge, to assess the effects of growth rate on surface morphology. Fig. 6.8 presents the images obtained with the nonatomically flat (i.e., conventional) substrate. In this case nanocrystals were generated relatively slowly, as shown in Fig. 6.8A, while the two-dimensional growth of crystals was more rapid, as in Fig. 6.8B and C, demonstrating the formation of a
Fig. 6.8 The effect of growth rate on the surface morphology of PbTiO3 films prepared on a nonatomically flat substrate [8].
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Fig. 6.9 The effect of growth rate on the surface morphology of PbTiO3 films prepared on an atomically flat substrate [8].
continuous film. From these images, it is apparent that lateral crystal growth occurred during sputtering as a result of surface migration. However, the crystallites were not uniform in size or in terms of their growth positions. The surface morphologies of a specimen grown on the atomically flat sapphire surface in Fig. 6.2A are shown in Fig. 6.9. In this sample, a homogeneous step-terrace structure was formed by self-organization and the PTO was deposited on the surface. In this case, the crystallites formed a line because nanocrystal nucleation proceeded at the steps. The crystal sizes increased with increases in the deposition rate, and these crystals show more homogeneous sizes compared to those on the conventional substrate (Fig. 6.8). Thus a more uniform film was obtained via growth on an atomically flat substrate.
6.7
Evaluation of crystal structure of PbTiO3 nanocrystals
The shielded area in Fig. 6.1 was also observed by AFM, revealing that nanocrystals with uniform sizes in the range of 20–30 nm had grown on the surface (Fig. 6.10A). However, some larger particles were also observed, since the sputtering conditions had not been sufficiently optimized, which resulted in a second growth phase. The enlarged AFM image in Fig. 6.10B seems to show the presence of lines. A twodimensional FFT of the image in Fig. 6.10B is presented in Fig. 6.11. Spots are clearly observable at 14 mm1 in the FFT image, indicating that the nanocrystals were grown at intervals of 70 nm. Because the terrace width of the atomically flat surface seems to agree with the intervals between the crystals, a highly magnified image was produced to further investigate this phenomenon (Fig. 6.12). The enlarged view confirms that crystallites were grown at the atomic steps. The initial growth nucleation appears to have occurred at the steps because the sputtered particles were captured at the step edges and because migration on the surface was assisted by the low deposition rate resulting from the use of a shield. An attempt was made to evaluate the crystal structure of a continuous film based on the XRD pattern in Fig. 6.3. However, it was not possible to examine the structure with sufficient accuracy because the reflection intensity was too low. Thus both continuous
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Fig. 6.10 Atomic force microscopy images of the shielded area, showing (A) standard and (B) enlarged views [9].
Fig. 6.11 Two-dimensional Fourier transform of Fig. 6.10B [9].
films and nanocrystals specimens were evaluated by XPS and μ-LR, and the XPS profiles for the nanocrystals were compared with those for the films. The resulting XPS profiles are shown in Fig. 6.13, and Pb 4f and Ti 2p peaks from the PTO film are clearly visible in Fig. 6.13A and B, respectively. The chemical shifts of these peaks demonstrate that the Pb and Ti in the film were present as Pb2+ and Ti4+, respectively [10]. The intensities of the peaks obtained from the nanocrystals were weak, but the peak profiles and chemical shifts were almost the same as those for the film specimen. Raman measurements were also performed, and Fig. 6.14 shows the resulting spectra, from which the vibrational modes were identified based on a report by Friere et al. [11]. The spectrum of the nanocrystals was very weak, although similar to that of the film. Therefore both the XPS and μ-LR results demonstrate that the nanocrystals on the atomically flat surface had crystallized in a perovskite structure.
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Fig. 6.12 Atomic force microscopy image of nanocrystals on an atomically flat surface [9].
Fig. 6.13 X-ray photoelectron spectroscopy data acquired from a PbTiO3 film and from nanocrystals.
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Fig. 6.14 The Raman spectra of a PbTiO3 thin film and of nanocrystals.
6.8
Fabrication of platinum nanocrystals for use as electrode nanolayer
In the areas of the substrate that were not shielded, such as at x ¼ 2 mm in Fig. 6.15, a continuous Pt film was obtained by sputtering using the conditions in Table 6.2 and the XRD pattern demonstrated that the film was grown epitaxially on the sapphire substrate. The surface morphology of the film was observed by AFM, as shown in Fig. 6.16, and it was determined that the film contained crystallites having various grain sizes and was not homogenous. Using this setup, the deposition of Pt was suppressed by the shield, and the growth rate varied with distance from the shield edge, x. The rate decreased with increasing x. Fig. 6.15 A schematic showing the fabrication of platinum nanocrystals using a shield.
Fig. 6.16 An atomic force microscopy image of a platinum film [12].
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Fig. 6.17 Atomic force microscopy images showing variations in surface morphology with growth position.
AFM observations were performed at various x values, and the resulting images are provided in Fig. 6.17. The surface at the shield edge was almost completely covered with small crystallites, while the crystallite density dropped significantly on moving even slightly away from the edge, such as at x ¼ 0.4 mm. However, the surface area that was covered by crystals was increased at a distance of 0.9 mm, although the growth rate decreased. The crystallite height was >1.0 nm at x ¼ 0.4 mm but decreased to below 0.8 nm at x ¼ 0.9. Thus three-dimensional growth evidently transitioned to two-dimensional growth at a low growth rate. The surface roughness of epitaxial Pt layers has been investigated in detail, revealing that the crystallite size can be greatly affected by the growth conditions [13], and the change in growth mode from
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three- to two-dimensional in this work appears to be caused by a similar mechanism. The Pt islands on the substrate were steadily enlarged at x ¼ 1.2 mm and appear to have spread over the sapphire surface at x ¼ 1.4 mm. These results demonstrate that the Pt layer had a greater degree of surface smoothness than high-quality Pt films previously reported in the literature [14, 15]. Conductive AFM (C-AFM) measurements were employed to confirm that the atomically flat surface was entirely covered with a Pt layer. The surface morphology of sapphire substrates with and without Pt deposition are shown in Fig. 6.18A and B, respectively, and are quite similar to one another. In contrast, the C-AFM images of surfaces with and without Pt in Fig. 6.18C and D are different. Although only a noise current is evident in the case of the sapphire surface in Fig. 6.18D, a stripe pattern similar to that in the AFM image can be observed in the C-AFM image of the Pt/sapphire specimen in Fig. 6.18C. Because the measured current was very low, a large DC offset current of approximately 35.4 pA was employed to acquire these C-AFM images, and so the current difference between Fig. 6.18C and D appears to be 1 pA smaller than it actually was. Because this low current was modulated by the contact conditions between the ultra-thin Pt layer and the AFM tip, the measured current was affected by the surface morphology, such as the step-terrace structure. From these results, it is apparent that a thin Pt layer can be deposited on an atomically flat surface by low-incident angle sputtering.
Fig. 6.18 Atomic force microscopy (AFM) and conductive AFM (C-AFM) images of sapphire substrates with and without platinum (Pt) deposition.
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6.9
Nanoscale Ferroelectric-Multiferroic Materials for Energy Harvesting Applications
Conclusion
In recent years there has been significant interest in nanosized ferroelectric materials because of the need to produce electronic devices with greater reliability. In the work described herein, ferroelectric PTO thin films and Pt electrode thin layers were prepared on atomically flat sapphire substrates by sputtering at a very low deposition rate. The deposition rate was reduced by placing shields in front of the substrates and, although the migration energy of the sputtered particles on the substrate surface was not reduced, nucleation growth appeared to improve. Furthermore, a well-ordered surface was formed on the atomically flat substrates due to self-organization, which resulted in the formation of uniform nanocrystal arrays. These nanocrystals were evaluated by XPS and μ-LR, and it was confirmed that nanosized crystals of the perovskite PbTiO3 were obtained. Therefore this technique allows tuning of the nanocrystal deposition location in conjunction with the growth of high-quality crystals and so should permit the optimization of deposition control in bottom-up fabrication methods. This approach to controlling the growth of nanosized single crystals while ensuring uniform grain sizes across the entire substrate is thought to have significant potential.
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