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X-ray diffraction analysis of Zn0.85Co0.15O powder and thin films
Materials Research Bulletin 38 (2003) 1791–1796
X-ray diffraction analysis of Zn0.85Co0.15O powder and thin films Zeming Qia,*, Aixia Lib, Fenglian Sub, Shengming Zhoub, Yanmei Liub, Zongyan Zhaob a
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, PR China b Department of Physics, Anhui University, Hefei 230039, Anhui, PR China
Received 9 June 2003; received in revised form 3 September 2003; accepted 10 September 2003
Abstract Co-doped ZnO powder and thin films on Si(1 0 0) substrate were prepared by solid-phase reaction and reactive e-beam evaporation. All samples were characterized by X-ray diffraction. The structure of powder sample was determined using Rietveld full-profile analysis method. The study of the influence of substrate temperature on the structure of thin films samples showed that the quality of thin films depended largely on the substrate temperature. The film prepared at 400 8C had the highest quality with c-axis (0 0 2) preferred orientation. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; A. Thin films; C. X-ray diffraction; D. Crystal structure
1. Introduction Diluted magnetic semiconductors (DMSs), which were mostly composed of transition metal magnetic impurities in III–V and II–VI semiconductors, have attracted the attention of many researchers for the last few decades [1–5]. As a wide-gap II–VI oxide semiconductor, ZnO has attracted considerable attention since the possibility of ultraviolet light emitting devices has been reported [6–8]. Recently, Dietl [2] suggested Mn-doped ZnO as a candidate with a high Curie temperature and a large magnetization. Sato and Katavama-Yoshida [9] predicted by ab initio calculation that V, Cr, Fe, Co and Ni in ZnO should provide carriers, which owing to the double exchange mechanism should generate a ferromagnetic order. A few experimental result about ZnO–DMS were also reported [10–12]. In this paper, Co-doped ZnO powder was synthesized by solid-state chemical reaction and thin films on Si(1 0 0) substrate were prepared by reactive e-beam evaporation. One of the most important factors *
Corresponding author. Tel.: þ86-551-3602060; fax: þ86-551-5141078. E-mail address: [email protected] (Z. Qi). 0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2003.09.006
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Z. Qi et al. / Materials Research Bulletin 38 (2003) 1791–1796
that determine the physical properties of a material is its structure. Therefore, the structure analyses of all samples are important. In this work, Co-doped ZnO powder and thin films were characterized by Xray diffraction. The structure parameters were obtained and the most suitable prepared condition was studied.
2. Experimental Zn0.85Co0.15O powder was synthesized by traditional solid-state chemical reaction method. High purity ZnO and Co2O3 powders were mixed at a definite mol ratio. The starting mixture was sintered at 1000 8C in air for 10 h. Then the obtained powder was pressed into slice and sintered at 1100 8C for 10 h again. The Zn0.85Co0.15O thin film was prepared by reactive e-beam evaporation on Si(1 0 0) substrate at 250, 300, 350, 400, 450 8C using synthesized Zn0.85Co0.15O powder as target. Base pressure of the chamber was around 6 104 Pa. All samples were characterized by Mac Science MXP18AHF X-ray diffractometer using Cu Ka radiation operated at 40 kV and 100 mA. For powder sample, X-ray diffraction profile was collected with an angular range of 2y from 20 to 808 in steps of 0.028 with a counting time of 1 s. For thin film samples, the diffraction pattern were collected with a thin film attachment in continues mode.
3. Result and discussion Fig. 1 shows the X-ray diffraction pattern of power sample. Only the diffraction lines of the wurtzite structure as pure ZnO are observed without any other peaks, which indicate that the Co has entered into ZnO lattice without changing the wurtzite structure. This is consistent with the result of Lee et al. [12] that the doping does not change the wurtzite of ZnO for doping concentrations below 25%. In order to obtain microstructure information of the sample in detail, structure refinement was performed by the Rietveld method [13,14] using RIETICA program [15]. In the refining process, wurtzite ZnO structure was selected as starting model structure. Co ions were assumed to incorporate into the ZnO lattice and occupy the Zn2þ cites. The refined instrumental and structural parameters were peak shape (using a pseudo-Voigt peak profile function), scale factor, background, unit cell parameters and position coordinates parameters. The difference of observed and calculated X-ray diffraction patterns is also shown in Fig. 1a. It is obvious that the agreement between the experimental data and the simulations is excellent. The structure parameters and R factor are given in Table 1. Compared with the structure Table 1 Crystallographic data of Zn0.85Co0.15O powder by Rietveld analysis ˚) Unit cell parameters (A ˚) a ¼ 3.2538(12) (A ˚ c ¼ 5.2044(7) (A) a ¼ b ¼ (908)a g ¼ (1208)a a
Zn(Co) coordiante a
x ¼ 0.3333 y ¼ 0.6667a z ¼ 0.0000a
The value is fixed during the fitting.
O coordinate
Rwp (%)
Rexp (%)
10.83
6.54
a
x ¼ 0.3333 y ¼ 0.6667a z ¼ 0.3815(16)
Z. Qi et al. / Materials Research Bulletin 38 (2003) 1791–1796
1793
Fig. 1. (a) Measured X-ray diffraction profiles (þ dots) and Rietveld refinement profiles (solid line) of Zn0.85Co0.15O. The difference curve is given at the bottom. (b) (1 0 0) and (0 0 2) peaks positions of Zn0.85Co0.15O powder compared with those of ZnO.
˚ ) of Zn0.85Co0.15O is larger than parameters of starting ZnO model, the unit cell parameter a (3.2538 A ˚ ) while the parameter c (5.2044 A ˚ ) is smaller that of ZnO (5.2066 A ˚ ). These that of ZnO (3.2498 A changes are consistent with the shift of (1 0 0) and (0 0 2) peaks of Zn0.85Co0.15O powder as shown in Fig. 1b, where the (1 0 0) peak shifts to lower angle that indicates a larger parameter a and the (0 0 2) peaks shifts to higher angle that indicates a smaller parameter c. The diffraction patterns of Zn0.85Co0.15O film at different substrate temperature are shown in Fig. 2. The diffraction pattern of Zn0.85Co0.15O powder is also shown for comparison. It is clear that the substrate temperature plays an important role in determining the structure of the films. The film grown under 300 8C has a polycrystalline structure, which has (0 0 2), (1 0 1), (1 0 2), (1 0 3) peaks. When the substrate temperature increases to 350 8C, more diffraction peaks appear. With the substrate temperature increasing more, the (0 0 2) peak intensity increases obviously and becomes dominant. When the substrate temperature reaches 400 8C, the film has a very good (0 0 2) orientation. However, a further temperature increases to 450 8C decreases the (0 0 2) peak intensity and the film becomes less oriented. Table 2 gives the (0 0 2) peak positions of thin film samples and powder sample as well as the lattice parameter c calculated from them. The (0 0 2) peak positions of thin film samples shift to smaller angle and c become larger than for the powder sample when the substrate temperature increases from
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Z. Qi et al. / Materials Research Bulletin 38 (2003) 1791–1796 002
300
(a )
101
200 100 0 300
002
200
(b )
101
100
Intensity(cps)
100
102
0 2000 1500 1000 500 0
110
103
002
(c ) 103
102
101
112
002
(d )
4000 2000 101
0 300
103
102
112
002
(e ) 101
200
102
103
100
112
0 4
1.5x10
(f )
4
1.0x10
3
5.0x10 0.0 20
30
40
50
60
70
80
2θ (degree)
Fig. 2. The X-ray diffraction profiles of Zn0.85Co0.15O thin films on Si(1 0 0) substrate at different substrate temperature and Zn0.85Co0.15O powder. (a) 250 8C, (b) 300 8C, (c) 350 8C, (d) 400 8C, (e) 450 8C, (f) powder sample.
250 to 400 8C. When the substrate temperature increases above 400 8C, the (0 0 2) peak position shifts ˚. to larger angle and c become smaller. Si has cubic diamond structure with lattice parameter a ¼ 5:43 A The stress between Zn0.85Co0.15O thin films and Si substrate changes the lattice of Zn0.85Co0.15O thin films. When the substrate temperature is close to the most suitable growing condition (near 400 8C), the decrease of lattice mismatch between Zn0.85Co0.15O thin films and Si substrate improves the thin film quality. The crystalline sizes of different samples are calculated by Scherrer equation. The relation between crystalline size and substrate temperature are shown in Fig. 3. It is clear from these diffraction patterns that as the substrate temperature is increased the film adopts a preferred orientation, with the caxis (0 0 2) perpendicular to the substrate which changes above 400 8C and becomes randomly oriented as the substrate temperature is increased to 450 8C. The same result was reported by van Heerden and
Table 2 (0 0 2) peak positions and lattice parameter c of film samples and powder sample Samples
250 8C
300 8C
350 8C
400 8C
450 8C
Powder
(0 0 2) peak positions (8) ˚) Lattice parameter c (A
34.54 5.1894
34.38 5.2128
34.34 5.2187
34.32 5.2217
34.38 5.2128
34.44 5.2040
Z. Qi et al. / Materials Research Bulletin 38 (2003) 1791–1796
1795
180
o
D(A)
160 140 120 100 250
300
350
400
450
o
T( C )
Fig. 3. The relation between crystalline size and substrate temperature.
Swanepoel [16] who studied undoped and aluminum-doped ZnO thin films prepared on glass substrate by spray pyrolysis.
4. Conclusion In conclusion, Zn0.85Co0.15O powder was synthesized by solid-state chemical reaction. Co ions enter into ZnO lattice and forms Zn1xCoxO solid solution. The lattice parameters and atomic coordinates of powder sample were determined by Rietveld full-profile analysis. Zn0.85Co0.15O thin films were prepared by reactive e-beam evaporation on Si(1 0 0) substrate. The relation between the structure of films and substrate temperature was studied. The most suitable substrate temperature for preparing Zn0.85Co0.15O thin films is about 400 8C.
Acknowledgements This research was supported by the Natural Science Foundation of Anhui Province of China under grant No. 00047208 and the foundation of University of Science and Technology of China for young researchers.
References [1] [2] [3] [4] [5] [6] [7] [8]
T. Dietl, H. Ohno, F. Matsulura et al., Science 287 (2000) 1019. T. Dietl, J. Appl. Phys. 89 (2001) 7437. J.K. Furdyna, J. Appl. Phys. 64 (1988) R29. R. Fledring, M. Keim, G. Reuscher et al., Nature 402 (1999) 787. Y. Ohno, D.K. Young et al., Nature 402 (1999) 790. R.F. Service, Science 276 (1997) 895. D.M. Bagnall, Y.F. Chen, Z. Zhu et al., Appl. Phys. Lett. 70 (1997) 2230. H. Cao, Y.G. Zhao, S.T. Ho et al., Phys. Rev. Lett. 82 (1999) 2278.
1796 [9] [10] [11] [12] [13] [14] [15] [16]
Z. Qi et al. / Materials Research Bulletin 38 (2003) 1791–1796
K. Sato, H. Katavama-Yoshida, Jpn. J. Appl. Phys. 40 (2001) L334. Z. Jin, T. Fukumura, M. Kawasaki, Appl. Phys. Lett. 78 (2001) 3824. K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988. H.-J. Lee, S.-Y. Jeong et al., Appl. Phys. Lett 81 (2002) 4020. H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. H.M. Rietveld, Acta Crystallogr. Acta Crystallogr. 22 (1967) 151. B.A. Hunter, IUCC Powder Diffraction 20 (1998) 21. J.L. van Heerden, R. Swanepoel, Thin Solid Films 299 (1997) 72.
X-ray diffraction analysis of Zn0.85Co0.15O powder and thin films Zeming Qia,*, Aixia Lib, Fenglian Sub, Shengming Zhoub, Yanmei Liub, Zongyan Zhaob a
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, PR China b Department of Physics, Anhui University, Hefei 230039, Anhui, PR China
Received 9 June 2003; received in revised form 3 September 2003; accepted 10 September 2003
Abstract Co-doped ZnO powder and thin films on Si(1 0 0) substrate were prepared by solid-phase reaction and reactive e-beam evaporation. All samples were characterized by X-ray diffraction. The structure of powder sample was determined using Rietveld full-profile analysis method. The study of the influence of substrate temperature on the structure of thin films samples showed that the quality of thin films depended largely on the substrate temperature. The film prepared at 400 8C had the highest quality with c-axis (0 0 2) preferred orientation. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; A. Thin films; C. X-ray diffraction; D. Crystal structure
1. Introduction Diluted magnetic semiconductors (DMSs), which were mostly composed of transition metal magnetic impurities in III–V and II–VI semiconductors, have attracted the attention of many researchers for the last few decades [1–5]. As a wide-gap II–VI oxide semiconductor, ZnO has attracted considerable attention since the possibility of ultraviolet light emitting devices has been reported [6–8]. Recently, Dietl [2] suggested Mn-doped ZnO as a candidate with a high Curie temperature and a large magnetization. Sato and Katavama-Yoshida [9] predicted by ab initio calculation that V, Cr, Fe, Co and Ni in ZnO should provide carriers, which owing to the double exchange mechanism should generate a ferromagnetic order. A few experimental result about ZnO–DMS were also reported [10–12]. In this paper, Co-doped ZnO powder was synthesized by solid-state chemical reaction and thin films on Si(1 0 0) substrate were prepared by reactive e-beam evaporation. One of the most important factors *
Corresponding author. Tel.: þ86-551-3602060; fax: þ86-551-5141078. E-mail address: [email protected] (Z. Qi). 0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2003.09.006
1792
Z. Qi et al. / Materials Research Bulletin 38 (2003) 1791–1796
that determine the physical properties of a material is its structure. Therefore, the structure analyses of all samples are important. In this work, Co-doped ZnO powder and thin films were characterized by Xray diffraction. The structure parameters were obtained and the most suitable prepared condition was studied.
2. Experimental Zn0.85Co0.15O powder was synthesized by traditional solid-state chemical reaction method. High purity ZnO and Co2O3 powders were mixed at a definite mol ratio. The starting mixture was sintered at 1000 8C in air for 10 h. Then the obtained powder was pressed into slice and sintered at 1100 8C for 10 h again. The Zn0.85Co0.15O thin film was prepared by reactive e-beam evaporation on Si(1 0 0) substrate at 250, 300, 350, 400, 450 8C using synthesized Zn0.85Co0.15O powder as target. Base pressure of the chamber was around 6 104 Pa. All samples were characterized by Mac Science MXP18AHF X-ray diffractometer using Cu Ka radiation operated at 40 kV and 100 mA. For powder sample, X-ray diffraction profile was collected with an angular range of 2y from 20 to 808 in steps of 0.028 with a counting time of 1 s. For thin film samples, the diffraction pattern were collected with a thin film attachment in continues mode.
3. Result and discussion Fig. 1 shows the X-ray diffraction pattern of power sample. Only the diffraction lines of the wurtzite structure as pure ZnO are observed without any other peaks, which indicate that the Co has entered into ZnO lattice without changing the wurtzite structure. This is consistent with the result of Lee et al. [12] that the doping does not change the wurtzite of ZnO for doping concentrations below 25%. In order to obtain microstructure information of the sample in detail, structure refinement was performed by the Rietveld method [13,14] using RIETICA program [15]. In the refining process, wurtzite ZnO structure was selected as starting model structure. Co ions were assumed to incorporate into the ZnO lattice and occupy the Zn2þ cites. The refined instrumental and structural parameters were peak shape (using a pseudo-Voigt peak profile function), scale factor, background, unit cell parameters and position coordinates parameters. The difference of observed and calculated X-ray diffraction patterns is also shown in Fig. 1a. It is obvious that the agreement between the experimental data and the simulations is excellent. The structure parameters and R factor are given in Table 1. Compared with the structure Table 1 Crystallographic data of Zn0.85Co0.15O powder by Rietveld analysis ˚) Unit cell parameters (A ˚) a ¼ 3.2538(12) (A ˚ c ¼ 5.2044(7) (A) a ¼ b ¼ (908)a g ¼ (1208)a a
Zn(Co) coordiante a
x ¼ 0.3333 y ¼ 0.6667a z ¼ 0.0000a
The value is fixed during the fitting.
O coordinate
Rwp (%)
Rexp (%)
10.83
6.54
a
x ¼ 0.3333 y ¼ 0.6667a z ¼ 0.3815(16)
Z. Qi et al. / Materials Research Bulletin 38 (2003) 1791–1796
1793
Fig. 1. (a) Measured X-ray diffraction profiles (þ dots) and Rietveld refinement profiles (solid line) of Zn0.85Co0.15O. The difference curve is given at the bottom. (b) (1 0 0) and (0 0 2) peaks positions of Zn0.85Co0.15O powder compared with those of ZnO.
˚ ) of Zn0.85Co0.15O is larger than parameters of starting ZnO model, the unit cell parameter a (3.2538 A ˚ ) while the parameter c (5.2044 A ˚ ) is smaller that of ZnO (5.2066 A ˚ ). These that of ZnO (3.2498 A changes are consistent with the shift of (1 0 0) and (0 0 2) peaks of Zn0.85Co0.15O powder as shown in Fig. 1b, where the (1 0 0) peak shifts to lower angle that indicates a larger parameter a and the (0 0 2) peaks shifts to higher angle that indicates a smaller parameter c. The diffraction patterns of Zn0.85Co0.15O film at different substrate temperature are shown in Fig. 2. The diffraction pattern of Zn0.85Co0.15O powder is also shown for comparison. It is clear that the substrate temperature plays an important role in determining the structure of the films. The film grown under 300 8C has a polycrystalline structure, which has (0 0 2), (1 0 1), (1 0 2), (1 0 3) peaks. When the substrate temperature increases to 350 8C, more diffraction peaks appear. With the substrate temperature increasing more, the (0 0 2) peak intensity increases obviously and becomes dominant. When the substrate temperature reaches 400 8C, the film has a very good (0 0 2) orientation. However, a further temperature increases to 450 8C decreases the (0 0 2) peak intensity and the film becomes less oriented. Table 2 gives the (0 0 2) peak positions of thin film samples and powder sample as well as the lattice parameter c calculated from them. The (0 0 2) peak positions of thin film samples shift to smaller angle and c become larger than for the powder sample when the substrate temperature increases from
1794
Z. Qi et al. / Materials Research Bulletin 38 (2003) 1791–1796 002
300
(a )
101
200 100 0 300
002
200
(b )
101
100
Intensity(cps)
100
102
0 2000 1500 1000 500 0
110
103
002
(c ) 103
102
101
112
002
(d )
4000 2000 101
0 300
103
102
112
002
(e ) 101
200
102
103
100
112
0 4
1.5x10
(f )
4
1.0x10
3
5.0x10 0.0 20
30
40
50
60
70
80
2θ (degree)
Fig. 2. The X-ray diffraction profiles of Zn0.85Co0.15O thin films on Si(1 0 0) substrate at different substrate temperature and Zn0.85Co0.15O powder. (a) 250 8C, (b) 300 8C, (c) 350 8C, (d) 400 8C, (e) 450 8C, (f) powder sample.
250 to 400 8C. When the substrate temperature increases above 400 8C, the (0 0 2) peak position shifts ˚. to larger angle and c become smaller. Si has cubic diamond structure with lattice parameter a ¼ 5:43 A The stress between Zn0.85Co0.15O thin films and Si substrate changes the lattice of Zn0.85Co0.15O thin films. When the substrate temperature is close to the most suitable growing condition (near 400 8C), the decrease of lattice mismatch between Zn0.85Co0.15O thin films and Si substrate improves the thin film quality. The crystalline sizes of different samples are calculated by Scherrer equation. The relation between crystalline size and substrate temperature are shown in Fig. 3. It is clear from these diffraction patterns that as the substrate temperature is increased the film adopts a preferred orientation, with the caxis (0 0 2) perpendicular to the substrate which changes above 400 8C and becomes randomly oriented as the substrate temperature is increased to 450 8C. The same result was reported by van Heerden and
Table 2 (0 0 2) peak positions and lattice parameter c of film samples and powder sample Samples
250 8C
300 8C
350 8C
400 8C
450 8C
Powder
(0 0 2) peak positions (8) ˚) Lattice parameter c (A
34.54 5.1894
34.38 5.2128
34.34 5.2187
34.32 5.2217
34.38 5.2128
34.44 5.2040
Z. Qi et al. / Materials Research Bulletin 38 (2003) 1791–1796
1795
180
o
D(A)
160 140 120 100 250
300
350
400
450
o
T( C )
Fig. 3. The relation between crystalline size and substrate temperature.
Swanepoel [16] who studied undoped and aluminum-doped ZnO thin films prepared on glass substrate by spray pyrolysis.
4. Conclusion In conclusion, Zn0.85Co0.15O powder was synthesized by solid-state chemical reaction. Co ions enter into ZnO lattice and forms Zn1xCoxO solid solution. The lattice parameters and atomic coordinates of powder sample were determined by Rietveld full-profile analysis. Zn0.85Co0.15O thin films were prepared by reactive e-beam evaporation on Si(1 0 0) substrate. The relation between the structure of films and substrate temperature was studied. The most suitable substrate temperature for preparing Zn0.85Co0.15O thin films is about 400 8C.
Acknowledgements This research was supported by the Natural Science Foundation of Anhui Province of China under grant No. 00047208 and the foundation of University of Science and Technology of China for young researchers.
References [1] [2] [3] [4] [5] [6] [7] [8]
T. Dietl, H. Ohno, F. Matsulura et al., Science 287 (2000) 1019. T. Dietl, J. Appl. Phys. 89 (2001) 7437. J.K. Furdyna, J. Appl. Phys. 64 (1988) R29. R. Fledring, M. Keim, G. Reuscher et al., Nature 402 (1999) 787. Y. Ohno, D.K. Young et al., Nature 402 (1999) 790. R.F. Service, Science 276 (1997) 895. D.M. Bagnall, Y.F. Chen, Z. Zhu et al., Appl. Phys. Lett. 70 (1997) 2230. H. Cao, Y.G. Zhao, S.T. Ho et al., Phys. Rev. Lett. 82 (1999) 2278.
1796 [9] [10] [11] [12] [13] [14] [15] [16]
Z. Qi et al. / Materials Research Bulletin 38 (2003) 1791–1796
K. Sato, H. Katavama-Yoshida, Jpn. J. Appl. Phys. 40 (2001) L334. Z. Jin, T. Fukumura, M. Kawasaki, Appl. Phys. Lett. 78 (2001) 3824. K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988. H.-J. Lee, S.-Y. Jeong et al., Appl. Phys. Lett 81 (2002) 4020. H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. H.M. Rietveld, Acta Crystallogr. Acta Crystallogr. 22 (1967) 151. B.A. Hunter, IUCC Powder Diffraction 20 (1998) 21. J.L. van Heerden, R. Swanepoel, Thin Solid Films 299 (1997) 72.