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Structural analysis of NiO ultra-thin films epitaxially grown on ultra-smooth sapphire substrates by synchrotron X-ray diffraction measurements
Applied Surface Science 221 (2004) 450–454
Structural analysis of NiO ultra-thin films epitaxially grown on ultra-smooth sapphire substrates by synchrotron X-ray diffraction measurements O. Sakataa,*, Min-Su Yia,1, A. Matsudab, J. Liub, S. Satob, S. Akibab, A. Sasakib, M. Yoshimotob a
Materials Science, Japan Synchrotron Radiation Research Institute, SPring-8, Kouto, Mikazuki, Sayo, Hyogo 679-5198, Japan b Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan Received 20 June 2003; received in revised form 18 July 2003; accepted 25 July 2003
Abstract Crystallographic structures of nickel oxide (NiO) ultra-thin films epitaxially grown on ultra-smooth sapphire (0 0 0 1) substrates have been analyzed using synchrotron X-ray diffraction. Growth behaviors of a NiO crystal domain along both an inplane and an out-of-plane directions were able to be explained at nano-scale resolution. They were drastically changed around 10–15-nm thick film range. Thermodynamic factors on the nucleation and growth was dominant in an ultra-thin film range. On the other hand, the step edges or terrace width of the substrate limited the growth speed in a thicker film range. # 2003 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 61.10.Kw; 68.47.Gh Keywords: NiO ultra-thin film; Ultra-smooth sapphire; Synchrotron X-ray diffraction; Crystalline domain size
1. Introduction Nickel oxide (NiO) with the NaCl-type structure is an antiferromagnetic (TN ¼ 523 K) and wide band-gap (Egap ¼ 4:0 eV ) p-type semiconductors [1]. So far, NiO thin films have been widely studied as a promising material for the possible applications to electrochromic display devices [2], gas sensors [3], *
Corresponding author. Tel.: þ81-791582750; fax: þ81-791580830. E-mail address: [email protected] (O. Sakata). 1 Present address: Department of Materials Science and Engineering, Sangju National University, Sang-Ju 742-711, Korea.
p-type transparent conducting electrodes [4], thermoelectric devices [5], and magnetoresistance sensors [6]. From the viewpoint of developing the nano-scale electronic devices using NiO, quantitative structural analysis of ultra-thin epitaxial films is of importance in understanding the physical properties of nanostructured materials. On the other hand, the artificial sapphire of single crystalline a-Al2 O3 with the corundum-type structure has been widely used as representative insulating substrates for epitaxial growth of semiconductors such as blue-light emitting GaN [7,8] or Si (silicon-on-sapphire; SOS) [9]. Previously, we reported that sapphire wafers annealed in air at high temperatures (about 1270–1670 K) had ultra-smooth
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-4332(03)00961-9
O. Sakata et al. / Applied Surface Science 221 (2004) 450–454
surfaces with flat terraces and atomic steps [10]. The ultra-smooth sapphire substrates with straight atomic steps were verified to be useful for growth of very flat ultra-thin films [11] as well as step-decoration epitaxy resulting in the oxide nanostructures [12]. In order to examine the nano-structure of NiO ultra-thin films epitaxially grown on the ultra-smooth sapphire substrates, we conducted the synchrotron radiation X-ray diffraction measurements. The ultra-thin films were grown by laser molecular beam epitaxy (laser-MBE), i.e. pulsed laser deposition in ultra-high vacuum [13]. The synchrotron radiation provides the requisite number of photons for the experiments being performed here. By using the synchrotron X-ray technique, we examined a surface and interface structure of semiconductors on an atomic scale [14,15]. In this work, we report the relationship between an X-ray crystalline domain size and thickness of the NiO ultra-thin films.
2. Sample preparation Ultra-smooth sapphire (0 0 0 1) substrates with 0.2nm high atomic steps used in the present experiments were obtained by annealing the mirror polished ones at 1270 K for several hours in air [10]. Fig. 1 shows an image (1 mm 1 mm) by atomic force microscopy (AFM) (SII Co. Ltd., SPI-3700); apparently, the terrace was about 100 nm in width. After the ultrasonic cleaning treatment, the substrates were introduced into
451
Fig. 2. AFM image (1 mm 1 mm) taken from an ultra-smooth surface after 13-nm thin film growth.
the ultra-high vacuum chamber (a base pressure of 6:7 107 Pa). A pulsed KrF excimer laser beam (248 nm, 20 ns) was impinged through a quartz window onto the sintered NiO target with an energy density of about 3 J/cm2 . The ablated plume particles proceeded onto the substrate heated at 940 K to deposit the film in the oxygen atmosphere of 1:3 103 Pa. The surface morphology of the films was observed by AFM in air (Fig. 2). The figure has an enough resolution that we can imagine step edges and terraces. It was found from reflection high energy electron diffraction (RHEED) that NiO ultra-thin films were grown epitaxially on sapphire (0 0 0 1) substrates. NiO films of four samples, A, B, C, and D grown on the substrates were 6.4, 13, 35, and 78 nm in thickness, respectively. The values were obtained from periods of intensity oscillations appeared in X-ray reflectivity curves under grazing incidence conditions. The substrates had a similar miscut angle, of which direction corresponded to the crystallographic orientation. The step edges were almost perpendicular to the [1 1 0 0] direction. The terrace width of ultra-smooth sapphire (0 0 0 1) substrates with 0.2-nm high atomic steps was solely determined by the miscut angle of pre-annealed substrates under the present annealing condition.
3. Synchrotron X-ray measurements
Fig. 1. AFM image (1 mm 1 mm) taken from an ultra-smooth surface before film growth.
Measurements were performed with a six-circle diffractometer at beamline BL13XU for surface and interface structures, SPring-8 [16]. The experimental
452
O. Sakata et al. / Applied Surface Science 221 (2004) 450–454
Intensity (arb. unit)
70 60
NiO ( )
50 40 30
sapphire ( )
20 10 0 -40 -20
0
20 40 60 80 100 120 Azimuth φ (deg)
Fig. 4. X-ray intensity as a function of a f angle. Fig. 3. Experimental layout of BL13XU. We borrowed this figure from [16].
setup included a beamline Si (1 1 1) double crystal monochromator and mirrors (shown in Fig. 3). A sample set on the diffractometer, which was operated using an o-fixed geometry controlled through the spec software. A wavelength used was 0.061 nm. We recorded X-ray intensities diffracted from the films in the reciprocal lattice space: scan along the q? (parallel to the substrate [0 0 0 1] direction), an azimuth scan (f scan) around the q? , qT and qR scans. qT and qR are parallel to the tangential and radius direction in a HK plane cross-sectioned at a constant L passing through a desired Bragg position in the reciprocal lattice space, respectively. jqj is defined as 2p=d. Here d is a lattice spacing in the real space. We obtained FWHM’s, 6 106 , 5:0 104 , and 9:1 104 around the sapphire (1 1 2 3) along the qT , qR , and q? , respectively. This indicates that experimental resolutions were better than these values. Measured angular widths of y–2y scan and a y rocking curve were 8 and 4 arc sec, respectively, around the (0 0 0 6) Bragg position.
We rotated the sample D along f to opt for one possibility from the two. The NiO (1 1 1) Bragg peak found in the f scan showed that the film was single crystal. Furthermore, we f-scanned the sample to record the sapphire (1 1 2 3) Bragg positions. By comparing the Bragg positions in the f scan for the NiO (1 1 1) and the sapphire (1 1 2 3), the cube-onhexagonal epitaxial relationship was verified (Fig. 4); the crystallographic relation was that the NiO [1 2 1jj the sapphire [1 1 2 0] and the NiO ½1 1 1jj the sapphire [0 0 0 1]. The other samples had also the same epitaxial structures. We note that the f scan of the nonspecular NiO (1 1 1) plane shows six-fold symmetry in Fig. 4, rather than three-fold symmetry that we expect from a bulk NiO crystal. Correspondingly, it is concluded that the NiO films had anti-domains. NiO on any adjacent terrace can have an anti-domain structure since the step height of the substrate is close to a distance between a Ni atomic plane and an O atomic plane in NiO (1 1 1) orientation. Measured FWHM values DqT , DqR , and Dq? around the NiO (1 1 1) positions are listed in Table 1. The averages and the standard deviations
4. Results and discussion The NiO (1 1 1) Bragg peaks of the four samples were located on the q? scan along the [0 0 0 1] direction. This indicates that there are two possibilities of the films’ being grown along the [1 1 1] preferred orientation and their having a single crystal with the [1 1 1] direction parallel to the substrate crystallographic [0 0 0 1] direction.
Table 1 DqT , DqR , and Dq? measured Sample
DqT
DqR
Dq?
A B C D
0.0525(34) 0.0335(13) 0.0291(3) 0.0235(4)
0.034(2) 0.0238(8) 0.0168(4) 0.0126(3)
0.120(5) 0.056(1) 0.045(1) 0.0344(14)
Numbers in parentheses mean the standard deviation.
O. Sakata et al. / Applied Surface Science 221 (2004) 450–454
453
Domain size (nm)
50
Fig. 5. FWHM DqT and DqR in the HK plane in the reciprocal lattice space.
40 In-plane 30 20 10
Out-of-plane
0 0
10
20
30
40
50
60
70
80
Film thickness (nm)
Table 2 Estimated domain sizes Djj and D? in unit of nm Sample
Djj
D?
A B C D
18.7(10) 26.4(9) 37.5(9) 49.6(13)
5.3(2) 11.3(3) 14.0(4) 18.3(8)
Numbers in parentheses mean the standard deviation.
Fig. 6. Domain sizes Djj ð Þ and D? ð Þ vs. film thickness. The standard deviations are smaller than the mark’s diameter.
terrace around the thickness of 10–15 nm. It corresponds to suppression of in-plane growth as well as on enhancement of the nucleation along the out-of-plane direction. In addition, we can estimate crystal mosaic spread c of the thin films around the q? . The spread is derived from p deconvolution ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi of DqR out of DqT as follows: c ¼ Dq2T Dq2R =qfilm . Similarly, the spread is 1 1 0 drastically changed around the film thickness of 10–15 nm (Fig. 7). When the domain like a single crystal with almost no mosaic spreads becomes larger, an average of mosaic spreads over its area should be smaller. In other words, the mosaic spread could be inversely proportional to the dimension of the in-plane domain. In the NiO ultra-thin film range, a terrace width (ca. 100 nm) can be enough large compared with an in-plane dimension of a NiO coherent domain. Film growth ideally might occur on a ultra-smooth surface. Crystal mosaic spread ψ (deg)
include results of other crystallographic symmetry reflections. It is noted that the experimental resolution was negligibly small compared with these values. Fig. 5 illustrates schematically the results in the HK plane cross-sectioned passing through the NiO (1 1 1) Bragg position. The FWHM value along the radius direction is smaller than that along the tangential direction. The asymmetry between the DqT and DqR does not arise from the substrate terrace orientation since it is also seen for the other equivalent reflections. It can be attributed to mosaic spread of the thin films around the q? . An in-plane (Djj ) and an out-of-plane domain sizes (D? ) are inversely proportional to DqR and Dq? , respectively. The expressions are Djj ¼ 2p=DqR and D? ¼ 2p=Dq? . The estimated values are also summarized in Table 2. Fig. 6 shows Djj and D? as a function of thickness of those NiO films. In a NiO ultra-thin film range of 0 to about 10 nm, Djj and D? increased proportionally with the film thickness; Djj and D? are 2.7 t and 0.88 t, respectively, where t is an average film thickness obtained by using X-ray reflectivity measurements. The growth behavior could be changed around a film thickness of 10–15 nm. D?’s of 35- and 78-nm thick films are smaller than the average thickness’s. This means that the coherent domains started to coalesce with each other on the in-plane
1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0
10
20 30 40 50 60 Film thickness (nm)
70
80
Fig. 7. Crystal mosaic spread as a function of film thickness.
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O. Sakata et al. / Applied Surface Science 221 (2004) 450–454
On the other hand, in a range thicker than about 10 nm, the NiO domain faced a step edge; correspondingly, film growth rates looked slower than those of the ultrathin film. Two NiO coherent domains on neighbor terraces cannot be unified since they have the antidomain structure. We would plan similar measurements using a nonultra-smooth sapphire substrates for investigating the effect of a surface with an irregular roughness on film growth.
5. Summary A combination of an ultra-smooth-surface substrate and the synchrotron X-ray diffraction technique has revealed the nano-scale growth manner of a NiO coherent domain like a single crystal. Thermodynamic factors on the nucleation and growth was dominant in the ultra-thin film range. On the other hand, the step edges or terrace width of the substrate limited the growth speed in a thicker film range.
Acknowledgements The synchrotron radiation experiments were performed at SPring-8 with the approval of Japan Synchrotron Radiation Research Institute (JASRI) as Nanotechnology Support Project of The Ministry of Education, Culture, Sports, Science and Technology (Proposal Nos. 2002B0736-ND1-np and 2003A0126-ND1-np/BL-No. 13). The measurements were also supported by JASRI under proposal No. 2002B0197-ND1-np/BL-No. 13. Fig. 3 is modified marginally—like a mirror image—from a drawing
(for the 5ID-C experimental setup) being originally prepared by T.-L. Lee.
References [1] S. Hufner, P. Steinen, I. Sander, M. Neumann, S. Witzel, Z. Phys. B 83 (1991) 185. [2] M. Kitao, K. Izawa, K. Urabe, T. Komatsu, S. Kuwano, S. Yamada, Jpn. J. Appl. Phys. 33 (1994) 6656. [3] I. Hotovy, J. Huran, P. Siciliano, S. Capone, L. Spiess, V. Rehacek, Sensors Actuators B 78 (2001) 126. [4] I.-M. Chan, T.-Y. Hsu, F.C. Hong, Appl. Phys. Lett. 81 (2002) 1899. [5] W. Shin, N. Murayama, Mater. Lett. 45 (2000) 302. [6] M.J. Carey, A.E. Berkowitz, J. Appl. Phys. 73 (1993) 6892. [7] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu, N. Sawaki, J. Cryst. Growth 98 (1989) 209. [8] S. Nakamura, M. Senoh, T. Mukai, Appl. Phys. Lett. 62 (1993) 2390. [9] M. Ishida, Y. Lee, T. Higashino, H. Seo, T. Nakamura, Jpn. J. Appl. Phys. 34 (1995) 831. [10] M. Yoshimoto, T. Maeda, T. Ohnishi, H. Koinuma, O. Ishiyama, M. Shinohara, M. Kubo, R. Miura, A. Miyamoto, Appl. Phys. Lett. 67 (1995) 2615. [11] T. Maeda, M. Yoshimoto, T. Ohnishi, G.H. Lee, H. Koinuma, J. Cryst. Growth 177 (1997) 95. [12] M. Takakura, T. Miyahara, J. Tashiro, A. Sasaki, M. Furusawa, M. Yoshimoto, Mater. Res. Soc. Symp. 648 (2001) P6.5.1. [13] M. Yoshimoto, K. Shimozono, T. Maeda, T. Ohnishi, M. Kumagai, T. Chikyow, O. Ishiyama, M. Shinohara, H. Koinuma, Jpn. J. Appl. Phys. 34 (1995) L688. [14] W. Rodrigues, O. Sakata, T.-L. Lee, D.A. Walko, D.L. Marasco, M.J. Bedzyk, J. Appl. Phys. 88 (2000) 2391. [15] O. Sakata, P.F. Lyman, B.P. Tinkham, D.A. Walko, D.L. Marasco, T.-L. Lee, M.J. Bedzyk, Phys. Rev. B 61 (2000) 16692. [16] O. Sakata, Y. Furukawa, S. Goto, T. Mochizuki, T. Uruga, K. Takeshita, H. Ohashi, T. Ohata, T. Matsushita, S. Takahashi, H. Tajiri, T. Ishikawa, M. Nakamura, M. Ito, K. Sumitani, T. Takahashi, T. Shimura, A. Saito, M. Takahasi, Surf. Rev. Lett. 10 (2003) 543.
Structural analysis of NiO ultra-thin films epitaxially grown on ultra-smooth sapphire substrates by synchrotron X-ray diffraction measurements O. Sakataa,*, Min-Su Yia,1, A. Matsudab, J. Liub, S. Satob, S. Akibab, A. Sasakib, M. Yoshimotob a
Materials Science, Japan Synchrotron Radiation Research Institute, SPring-8, Kouto, Mikazuki, Sayo, Hyogo 679-5198, Japan b Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan Received 20 June 2003; received in revised form 18 July 2003; accepted 25 July 2003
Abstract Crystallographic structures of nickel oxide (NiO) ultra-thin films epitaxially grown on ultra-smooth sapphire (0 0 0 1) substrates have been analyzed using synchrotron X-ray diffraction. Growth behaviors of a NiO crystal domain along both an inplane and an out-of-plane directions were able to be explained at nano-scale resolution. They were drastically changed around 10–15-nm thick film range. Thermodynamic factors on the nucleation and growth was dominant in an ultra-thin film range. On the other hand, the step edges or terrace width of the substrate limited the growth speed in a thicker film range. # 2003 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 61.10.Kw; 68.47.Gh Keywords: NiO ultra-thin film; Ultra-smooth sapphire; Synchrotron X-ray diffraction; Crystalline domain size
1. Introduction Nickel oxide (NiO) with the NaCl-type structure is an antiferromagnetic (TN ¼ 523 K) and wide band-gap (Egap ¼ 4:0 eV ) p-type semiconductors [1]. So far, NiO thin films have been widely studied as a promising material for the possible applications to electrochromic display devices [2], gas sensors [3], *
Corresponding author. Tel.: þ81-791582750; fax: þ81-791580830. E-mail address: [email protected] (O. Sakata). 1 Present address: Department of Materials Science and Engineering, Sangju National University, Sang-Ju 742-711, Korea.
p-type transparent conducting electrodes [4], thermoelectric devices [5], and magnetoresistance sensors [6]. From the viewpoint of developing the nano-scale electronic devices using NiO, quantitative structural analysis of ultra-thin epitaxial films is of importance in understanding the physical properties of nanostructured materials. On the other hand, the artificial sapphire of single crystalline a-Al2 O3 with the corundum-type structure has been widely used as representative insulating substrates for epitaxial growth of semiconductors such as blue-light emitting GaN [7,8] or Si (silicon-on-sapphire; SOS) [9]. Previously, we reported that sapphire wafers annealed in air at high temperatures (about 1270–1670 K) had ultra-smooth
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-4332(03)00961-9
O. Sakata et al. / Applied Surface Science 221 (2004) 450–454
surfaces with flat terraces and atomic steps [10]. The ultra-smooth sapphire substrates with straight atomic steps were verified to be useful for growth of very flat ultra-thin films [11] as well as step-decoration epitaxy resulting in the oxide nanostructures [12]. In order to examine the nano-structure of NiO ultra-thin films epitaxially grown on the ultra-smooth sapphire substrates, we conducted the synchrotron radiation X-ray diffraction measurements. The ultra-thin films were grown by laser molecular beam epitaxy (laser-MBE), i.e. pulsed laser deposition in ultra-high vacuum [13]. The synchrotron radiation provides the requisite number of photons for the experiments being performed here. By using the synchrotron X-ray technique, we examined a surface and interface structure of semiconductors on an atomic scale [14,15]. In this work, we report the relationship between an X-ray crystalline domain size and thickness of the NiO ultra-thin films.
2. Sample preparation Ultra-smooth sapphire (0 0 0 1) substrates with 0.2nm high atomic steps used in the present experiments were obtained by annealing the mirror polished ones at 1270 K for several hours in air [10]. Fig. 1 shows an image (1 mm 1 mm) by atomic force microscopy (AFM) (SII Co. Ltd., SPI-3700); apparently, the terrace was about 100 nm in width. After the ultrasonic cleaning treatment, the substrates were introduced into
451
Fig. 2. AFM image (1 mm 1 mm) taken from an ultra-smooth surface after 13-nm thin film growth.
the ultra-high vacuum chamber (a base pressure of 6:7 107 Pa). A pulsed KrF excimer laser beam (248 nm, 20 ns) was impinged through a quartz window onto the sintered NiO target with an energy density of about 3 J/cm2 . The ablated plume particles proceeded onto the substrate heated at 940 K to deposit the film in the oxygen atmosphere of 1:3 103 Pa. The surface morphology of the films was observed by AFM in air (Fig. 2). The figure has an enough resolution that we can imagine step edges and terraces. It was found from reflection high energy electron diffraction (RHEED) that NiO ultra-thin films were grown epitaxially on sapphire (0 0 0 1) substrates. NiO films of four samples, A, B, C, and D grown on the substrates were 6.4, 13, 35, and 78 nm in thickness, respectively. The values were obtained from periods of intensity oscillations appeared in X-ray reflectivity curves under grazing incidence conditions. The substrates had a similar miscut angle, of which direction corresponded to the crystallographic orientation. The step edges were almost perpendicular to the [1 1 0 0] direction. The terrace width of ultra-smooth sapphire (0 0 0 1) substrates with 0.2-nm high atomic steps was solely determined by the miscut angle of pre-annealed substrates under the present annealing condition.
3. Synchrotron X-ray measurements
Fig. 1. AFM image (1 mm 1 mm) taken from an ultra-smooth surface before film growth.
Measurements were performed with a six-circle diffractometer at beamline BL13XU for surface and interface structures, SPring-8 [16]. The experimental
452
O. Sakata et al. / Applied Surface Science 221 (2004) 450–454
Intensity (arb. unit)
70 60
NiO ( )
50 40 30
sapphire ( )
20 10 0 -40 -20
0
20 40 60 80 100 120 Azimuth φ (deg)
Fig. 4. X-ray intensity as a function of a f angle. Fig. 3. Experimental layout of BL13XU. We borrowed this figure from [16].
setup included a beamline Si (1 1 1) double crystal monochromator and mirrors (shown in Fig. 3). A sample set on the diffractometer, which was operated using an o-fixed geometry controlled through the spec software. A wavelength used was 0.061 nm. We recorded X-ray intensities diffracted from the films in the reciprocal lattice space: scan along the q? (parallel to the substrate [0 0 0 1] direction), an azimuth scan (f scan) around the q? , qT and qR scans. qT and qR are parallel to the tangential and radius direction in a HK plane cross-sectioned at a constant L passing through a desired Bragg position in the reciprocal lattice space, respectively. jqj is defined as 2p=d. Here d is a lattice spacing in the real space. We obtained FWHM’s, 6 106 , 5:0 104 , and 9:1 104 around the sapphire (1 1 2 3) along the qT , qR , and q? , respectively. This indicates that experimental resolutions were better than these values. Measured angular widths of y–2y scan and a y rocking curve were 8 and 4 arc sec, respectively, around the (0 0 0 6) Bragg position.
We rotated the sample D along f to opt for one possibility from the two. The NiO (1 1 1) Bragg peak found in the f scan showed that the film was single crystal. Furthermore, we f-scanned the sample to record the sapphire (1 1 2 3) Bragg positions. By comparing the Bragg positions in the f scan for the NiO (1 1 1) and the sapphire (1 1 2 3), the cube-onhexagonal epitaxial relationship was verified (Fig. 4); the crystallographic relation was that the NiO [1 2 1jj the sapphire [1 1 2 0] and the NiO ½1 1 1jj the sapphire [0 0 0 1]. The other samples had also the same epitaxial structures. We note that the f scan of the nonspecular NiO (1 1 1) plane shows six-fold symmetry in Fig. 4, rather than three-fold symmetry that we expect from a bulk NiO crystal. Correspondingly, it is concluded that the NiO films had anti-domains. NiO on any adjacent terrace can have an anti-domain structure since the step height of the substrate is close to a distance between a Ni atomic plane and an O atomic plane in NiO (1 1 1) orientation. Measured FWHM values DqT , DqR , and Dq? around the NiO (1 1 1) positions are listed in Table 1. The averages and the standard deviations
4. Results and discussion The NiO (1 1 1) Bragg peaks of the four samples were located on the q? scan along the [0 0 0 1] direction. This indicates that there are two possibilities of the films’ being grown along the [1 1 1] preferred orientation and their having a single crystal with the [1 1 1] direction parallel to the substrate crystallographic [0 0 0 1] direction.
Table 1 DqT , DqR , and Dq? measured Sample
DqT
DqR
Dq?
A B C D
0.0525(34) 0.0335(13) 0.0291(3) 0.0235(4)
0.034(2) 0.0238(8) 0.0168(4) 0.0126(3)
0.120(5) 0.056(1) 0.045(1) 0.0344(14)
Numbers in parentheses mean the standard deviation.
O. Sakata et al. / Applied Surface Science 221 (2004) 450–454
453
Domain size (nm)
50
Fig. 5. FWHM DqT and DqR in the HK plane in the reciprocal lattice space.
40 In-plane 30 20 10
Out-of-plane
0 0
10
20
30
40
50
60
70
80
Film thickness (nm)
Table 2 Estimated domain sizes Djj and D? in unit of nm Sample
Djj
D?
A B C D
18.7(10) 26.4(9) 37.5(9) 49.6(13)
5.3(2) 11.3(3) 14.0(4) 18.3(8)
Numbers in parentheses mean the standard deviation.
Fig. 6. Domain sizes Djj ð Þ and D? ð Þ vs. film thickness. The standard deviations are smaller than the mark’s diameter.
terrace around the thickness of 10–15 nm. It corresponds to suppression of in-plane growth as well as on enhancement of the nucleation along the out-of-plane direction. In addition, we can estimate crystal mosaic spread c of the thin films around the q? . The spread is derived from p deconvolution ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi of DqR out of DqT as follows: c ¼ Dq2T Dq2R =qfilm . Similarly, the spread is 1 1 0 drastically changed around the film thickness of 10–15 nm (Fig. 7). When the domain like a single crystal with almost no mosaic spreads becomes larger, an average of mosaic spreads over its area should be smaller. In other words, the mosaic spread could be inversely proportional to the dimension of the in-plane domain. In the NiO ultra-thin film range, a terrace width (ca. 100 nm) can be enough large compared with an in-plane dimension of a NiO coherent domain. Film growth ideally might occur on a ultra-smooth surface. Crystal mosaic spread ψ (deg)
include results of other crystallographic symmetry reflections. It is noted that the experimental resolution was negligibly small compared with these values. Fig. 5 illustrates schematically the results in the HK plane cross-sectioned passing through the NiO (1 1 1) Bragg position. The FWHM value along the radius direction is smaller than that along the tangential direction. The asymmetry between the DqT and DqR does not arise from the substrate terrace orientation since it is also seen for the other equivalent reflections. It can be attributed to mosaic spread of the thin films around the q? . An in-plane (Djj ) and an out-of-plane domain sizes (D? ) are inversely proportional to DqR and Dq? , respectively. The expressions are Djj ¼ 2p=DqR and D? ¼ 2p=Dq? . The estimated values are also summarized in Table 2. Fig. 6 shows Djj and D? as a function of thickness of those NiO films. In a NiO ultra-thin film range of 0 to about 10 nm, Djj and D? increased proportionally with the film thickness; Djj and D? are 2.7 t and 0.88 t, respectively, where t is an average film thickness obtained by using X-ray reflectivity measurements. The growth behavior could be changed around a film thickness of 10–15 nm. D?’s of 35- and 78-nm thick films are smaller than the average thickness’s. This means that the coherent domains started to coalesce with each other on the in-plane
1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0
10
20 30 40 50 60 Film thickness (nm)
70
80
Fig. 7. Crystal mosaic spread as a function of film thickness.
454
O. Sakata et al. / Applied Surface Science 221 (2004) 450–454
On the other hand, in a range thicker than about 10 nm, the NiO domain faced a step edge; correspondingly, film growth rates looked slower than those of the ultrathin film. Two NiO coherent domains on neighbor terraces cannot be unified since they have the antidomain structure. We would plan similar measurements using a nonultra-smooth sapphire substrates for investigating the effect of a surface with an irregular roughness on film growth.
5. Summary A combination of an ultra-smooth-surface substrate and the synchrotron X-ray diffraction technique has revealed the nano-scale growth manner of a NiO coherent domain like a single crystal. Thermodynamic factors on the nucleation and growth was dominant in the ultra-thin film range. On the other hand, the step edges or terrace width of the substrate limited the growth speed in a thicker film range.
Acknowledgements The synchrotron radiation experiments were performed at SPring-8 with the approval of Japan Synchrotron Radiation Research Institute (JASRI) as Nanotechnology Support Project of The Ministry of Education, Culture, Sports, Science and Technology (Proposal Nos. 2002B0736-ND1-np and 2003A0126-ND1-np/BL-No. 13). The measurements were also supported by JASRI under proposal No. 2002B0197-ND1-np/BL-No. 13. Fig. 3 is modified marginally—like a mirror image—from a drawing
(for the 5ID-C experimental setup) being originally prepared by T.-L. Lee.
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