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Artificial control of ZnO nanostructures grown by metalorganic chemical vapor deposition
ARTICLE IN PRESS
Journal of Crystal Growth 272 (2004) 138–142 www.elsevier.com/locate/jcrysgro
Artificial control of ZnO nanostructures grown by metalorganic chemical vapor deposition Shizuo Fujitaa,b,, Sang-Woo Kima,c, Masaya Uedac, Shigeo Fujitab,c a International Innovation Center, Kyoto University, Kyoto 606-8501, Japan Advanced Research Institute of Nanoscale Science and Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan c Department of Electronic Science and Engineering, Kyoto University, Kyoto 615-8510, Japan
b
Available online 19 October 2004
Abstract Advantage and role of focused ion beam (FIB) for nanopatterning of the substrates toward the artificial control of self-assembly processes of semiconductor nanostructures are shown and discussed. Two-dimensional ZnO nanodot arrays well-ordered in size and position of the nanodots are demonstrated, for example, the nanodots with diameter of 130710 and 1875 nm were two dimensionally arrayed with periods of 750 and 100 nm, respectively. Gallium implanted in the FIB-nanopatterning is suggested as a plausible reason for the selective nucleation of ZnO nanodots on nanopatterned substrates. r 2004 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 81.07. b; 81.16. c; 81.16.Dn; 81.16.Rf Keywords: A1. Nanostructures; A1. Nucleation; A2. Metalorganic chemical vapor deposition; B1. Oxides; B2. Semiconducting II–VI materials
1. Introduction Although self-assembly processes have contributed to the fabrication of novel nanostructures such as nanodots [1–3], the control of their size, Corresponding author. International Innovation Center, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 6068501, Japan. Tel.: +81 75 753 9148; fax: +81 75 753 9145. E-mail address: [email protected] (S. Fujita).
position, density, and shape has been the longlasting issue for which a novel solution is strongly requested so that the self-assembly processes can actually be developed as a basic and key tool for nanotechnology. For that purpose, attempts ever reported include X-ray [4], electron beam [5], ion beam [6], and proximal probe [7] lithography. We have proposed the focused ion beam (FIB) nanopattering technique, where the nanolines and nanoholes acted as nucleation centers or sinks
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.078
ARTICLE IN PRESS S. Fujita et al. / Journal of Crystal Growth 272 (2004) 138–142
for the metalorganic chemical vapor deposition (MOCVD) of zinc oxide (ZnO), and selective formation of the nanodots on nanopatterns has been accomplished [8–10]. The application of this technique to ZnO nanodots may lead to extract novel optical, electrical, and magnetic properties supported by multifunctional properties and strong excitonic properties of ZnO which are further enhanced by quantum and/or confinement effects in the nanodots. In this paper, following the above achievements, we investigate how uniformly in size and how closely (or densely) in position the nanodots can be accommodated, in the viewpoint of their application to optical devices and photonic materials. Further, the mechanism of arraying nanodots is discussed in terms of the effects of gallium (Ga) implanted by the FIB.
2. Growth of ZnO nanodots The growth technique of ZnO nanodots is essentially the same as has been reported previously [8–10]. The SiO2 layer formed by the thermal oxidation of a silicon (Si) substrate was nanopatterned by the FIB etching at the acceleration voltage of 30 keV and the beam diameter of 10–30 nm. Two-dimensional nanohole arrays were formed by the low magnification mode as described in the publication [10]. The growth of ZnO nanodots was done by MOCVD using diethylzinc (DEZn) and nitrous oxide (N2O) with nitrogen (N2) carrier gas at 700 1C. The pressure in the reactor was 200 Torr.
139
Fig. 1 shows the secondary electron microscope (SEM) images of the sample surface evolved with the growth time 40 s, 10 and 15 min. At the shortest growth time, nanodots with average diameter and height of 130 and 9 nm, respectively, are selectively found in the nanoholes one by one. With increasing the growth time to 10 min, the size of the dots increases and the dots seem to be surrounded by small facets. Further increase of the growth time to 15 min results in elongate rods directing to several different directions. From the above observation, it is speculated that the small nanodots possibly consist of small crystal (more detailed crystal structure has not been investigated at the present stage). Additional time promotes the growth of the small nanodots and small facets are formed on the surface. Then once such facets on whose plane the growth rate is remarkably enhanced appear on the surface, the growth to certain directions occurs faster than to other, resulting in rods structure.
3. ZnO nanodot arrays on nanopatterned substrates Fig. 2 demonstrates the atomic force microscope (AFM) images of the ZnO nanodots formed on FIB-patterned nanoholes with the period of 750, 190, and 100 nm. For the period of 190 nm, nanodots with the average diameter and height of 40 and 5 nm, respectively, which are smaller than those for the period of 750 nm (130 and 9 nm, respectively, as shown in Fig. 1a), accommodate in nanoholes one by one almost completely. Reducing the period of FIB-patterned nanoholes to 100 nm,
Fig. 1. SEM images showing the growth processes of ZnO on FIB-nanopatterned SiO2 substrate surface. The results are at (a) 40 s, (b) 10 min, and (c) 15 min after starting the growth of ZnO.
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S. Fujita et al. / Journal of Crystal Growth 272 (2004) 138–142
Fig. 2. AFM images of the ZnO nanodots formed on FIB-patterned nanoholes with the period of (a) 750 nm, (b) 190 nm, and (c) 100 nm.
100
Yield [arb. units]
the size of ZnO nanodots further decreases, that is, the average diameter and height are 18 and 4 nm, respectively. However, only 64% of nanoholes are filled with ZnO nanodots and the rest of them are empty. This is probably limited by the present growth conditions and the further optimization will lead to the complete two-dimensional array of ZnO nanodots with period of smaller than 100 nm in the future. In this case, the small size of nanodots is effective for quantum confinement, the density of the nanodots calculated to be 1010 cm 2 is close to the value of self-assembled nanodots generally known, and the small separation of nanodots encourages further development toward near-field optical transmission or carrier tunneling. It is expected that this series of achievement for the artificial control of nanodots will open new application field of nanodevices. The histograms showing the distribution of diameter of nanodots are given in Fig. 3 for the arrays of different periods in comparison to the selfassembled nanodots on an unpatterned SiO2 substrate. It is clearly seen that the diameters of nanodots were 130710 and 1875 nm with arraying in 750- and 100-nm periods, respectively, while it is widely distributed from 10 to 120 nm in the simple self-assembled nucleation, that is, the size distribution is remarkably reduced on FIB-nanopatterned substrates by artificial arraying. This result further encourages the advantage of the artificially arrayed nanodots for the actual application.
period 100 nm period 750 nm
80 60
self-assembled nanodots on unpatterned SiO2
40 20 0 0
20
40
60
80
100
120
140
Diameter [nm] Fig. 3. Histograms showing the distribution of diameter of nanodots formed on the FIB-patterned nanoholes (periods of 750 and 100 nm) and on unpatterned SiO2 substrate surface. The graphs are normalized so that the total areas of the bars become constant for all samples.
Fig. 4 shows the energy dispersive X-ray (EDX) spectroscopy result for a single dot measured in an analytical SEM. Besides zinc (Zn) and oxygen (O), slight incorporation of Ga is suggested from the shoulder structure of the Zn-spectrum. From this result, we speculate the association of Ga for the selective formation of ZnO nanodots. The discussion will be given below. It should be emphasized that no Zn was detected between the nanodots, suggesting no wetting layer on the SiO2 surface.
ARTICLE IN PRESS S. Fujita et al. / Journal of Crystal Growth 272 (2004) 138–142
O Zn Ga
Si
1
2
141
Intensity [arb. units]
50 40 30 20 10 0 0
3
4
5
Energy [keV] Fig. 4. EDX spectrum for a single dot measured in an analytical SEM. Note that this spectrum is the overlap of those for the ZnO nanodot and for the underlying SiO2, resulting in Si and enhanced O peaks in this figure.
4. Discussions on mechanism for arraying It is of our great interest why a nanodot is selectively formed on a nanohole. There can be assumed two different causes as the mechanism, that is, as the possible nucleation centers or sinks. One is the hole itself on the surface and another is the Ga implanted in the hole. In order for the discussion, we fabricated a nanohole structure according to the process shown in Fig. 5b, different from the conventional process shown in Fig. 5a, where the Ga expected to be implanted by the FIB-nanopatterning is probably removed with the SiO2 layer formed by the successive thermal oxidation and Ga-free nanoholes may be formed on the surface. As a result of MOCVD growth of ZnO, nanodots are hardly seen on the Ga-free surface as is demonstrated by AFM images in Fig. 5. This result suggests us the crucial role of residual Ga implanted in the nanoholes for the preferable nucleation of ZnO.
Fig. 5. Two different techniques to form nanohole arrays on the SiO2 substate surface, that is, (a) conventional technique with FIB-nanopatterning and (b) alternative technique in order to remove the FIB-implanted Ga and/or FIB-damaged area by thermal oxidation and etching. AFM images of the sample surface after the growth of ZnO are also shown.
for forming nanostructures, and two dimensional ZnO nanodot arrays well-ordered in position and size of the nanodots are demonstrated. The period of the nanodot array can be as small as 100 nm by slightly modifying the growth conditions. It seems that Ga implanted in the nanoholes is playing a crucial role for the selective nucleation of ZnO nanodots in nanoholes.
Acknowledgements 5. Conclusions FIB-nanopatterning is shown to be a useful tool for the artificial control of self-assembly processes
This work was partly supported by a Grant-inAid for Scientific Research and also by Grants for Regional Science and Technology Promotion,
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S. Fujita et al. / Journal of Crystal Growth 272 (2004) 138–142
from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaas, P.M. Petroff, Appl. Phys. Lett. 63 (1993) 3203. [2] F. Widmann, B. Daudin, G. Feuillet, Y. Samson, J.L. Rouviere, N. Pelekanos, J. Appl. Phys. 83 (1998) 7618. [3] S.H. Xin, P.D. Wang, A. Yin, C. Kim, M. Dobrowolska, J.L. Merz, J.K. Furdyna, Appl. Phys. Lett. 69 (1996) 3884. [4] A.C. Warren, I. Plotnik, E.H. Anderson, M.L. Schattenburg, D.A. Antoniadis, H.I. Smith, J. Vac. Sci. Technol. B 4 (1986) 365.
[5] M. Illing, G. Bacher, T. Kummel, A. Forchel, D. Hommel, B. Jobst, G. Landwehr, J. Vac. Sci. Technol. B 13 (1995) 2792. [6] T. Aign, P. Meyer, S. Lemerle, J.P. Jamet, J. Ferre, V. Mathet, C. Chappert, J. Gierak, C. Vieu, F. Rousseaux, H. Launois, H. Bernas, Phys. Rev. Lett. 81 (1998) 5656. [7] D.M. Eigler, E.K. Schweizer, Nature 344 (1990) 524. [8] S.-W. Kim, M. Ueda, T. Kotani, Sz. Fujita,, Sg. Fujita, Jpn. J. Appl. Phys. 42 (2003) L568. [9] S.-W. Kim, T. Kotani, M. Ueda, Sz. Fujita, Sg. Fujita, Physica E 21 (2004) 601. [10] M. Ueda, S.-W. Kim, Sz. Fujita, Sg. Fujita, Jpn. J. Appl. Phys. 43 (2004) L652.
Journal of Crystal Growth 272 (2004) 138–142 www.elsevier.com/locate/jcrysgro
Artificial control of ZnO nanostructures grown by metalorganic chemical vapor deposition Shizuo Fujitaa,b,, Sang-Woo Kima,c, Masaya Uedac, Shigeo Fujitab,c a International Innovation Center, Kyoto University, Kyoto 606-8501, Japan Advanced Research Institute of Nanoscale Science and Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan c Department of Electronic Science and Engineering, Kyoto University, Kyoto 615-8510, Japan
b
Available online 19 October 2004
Abstract Advantage and role of focused ion beam (FIB) for nanopatterning of the substrates toward the artificial control of self-assembly processes of semiconductor nanostructures are shown and discussed. Two-dimensional ZnO nanodot arrays well-ordered in size and position of the nanodots are demonstrated, for example, the nanodots with diameter of 130710 and 1875 nm were two dimensionally arrayed with periods of 750 and 100 nm, respectively. Gallium implanted in the FIB-nanopatterning is suggested as a plausible reason for the selective nucleation of ZnO nanodots on nanopatterned substrates. r 2004 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 81.07. b; 81.16. c; 81.16.Dn; 81.16.Rf Keywords: A1. Nanostructures; A1. Nucleation; A2. Metalorganic chemical vapor deposition; B1. Oxides; B2. Semiconducting II–VI materials
1. Introduction Although self-assembly processes have contributed to the fabrication of novel nanostructures such as nanodots [1–3], the control of their size, Corresponding author. International Innovation Center, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 6068501, Japan. Tel.: +81 75 753 9148; fax: +81 75 753 9145. E-mail address: [email protected] (S. Fujita).
position, density, and shape has been the longlasting issue for which a novel solution is strongly requested so that the self-assembly processes can actually be developed as a basic and key tool for nanotechnology. For that purpose, attempts ever reported include X-ray [4], electron beam [5], ion beam [6], and proximal probe [7] lithography. We have proposed the focused ion beam (FIB) nanopattering technique, where the nanolines and nanoholes acted as nucleation centers or sinks
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.078
ARTICLE IN PRESS S. Fujita et al. / Journal of Crystal Growth 272 (2004) 138–142
for the metalorganic chemical vapor deposition (MOCVD) of zinc oxide (ZnO), and selective formation of the nanodots on nanopatterns has been accomplished [8–10]. The application of this technique to ZnO nanodots may lead to extract novel optical, electrical, and magnetic properties supported by multifunctional properties and strong excitonic properties of ZnO which are further enhanced by quantum and/or confinement effects in the nanodots. In this paper, following the above achievements, we investigate how uniformly in size and how closely (or densely) in position the nanodots can be accommodated, in the viewpoint of their application to optical devices and photonic materials. Further, the mechanism of arraying nanodots is discussed in terms of the effects of gallium (Ga) implanted by the FIB.
2. Growth of ZnO nanodots The growth technique of ZnO nanodots is essentially the same as has been reported previously [8–10]. The SiO2 layer formed by the thermal oxidation of a silicon (Si) substrate was nanopatterned by the FIB etching at the acceleration voltage of 30 keV and the beam diameter of 10–30 nm. Two-dimensional nanohole arrays were formed by the low magnification mode as described in the publication [10]. The growth of ZnO nanodots was done by MOCVD using diethylzinc (DEZn) and nitrous oxide (N2O) with nitrogen (N2) carrier gas at 700 1C. The pressure in the reactor was 200 Torr.
139
Fig. 1 shows the secondary electron microscope (SEM) images of the sample surface evolved with the growth time 40 s, 10 and 15 min. At the shortest growth time, nanodots with average diameter and height of 130 and 9 nm, respectively, are selectively found in the nanoholes one by one. With increasing the growth time to 10 min, the size of the dots increases and the dots seem to be surrounded by small facets. Further increase of the growth time to 15 min results in elongate rods directing to several different directions. From the above observation, it is speculated that the small nanodots possibly consist of small crystal (more detailed crystal structure has not been investigated at the present stage). Additional time promotes the growth of the small nanodots and small facets are formed on the surface. Then once such facets on whose plane the growth rate is remarkably enhanced appear on the surface, the growth to certain directions occurs faster than to other, resulting in rods structure.
3. ZnO nanodot arrays on nanopatterned substrates Fig. 2 demonstrates the atomic force microscope (AFM) images of the ZnO nanodots formed on FIB-patterned nanoholes with the period of 750, 190, and 100 nm. For the period of 190 nm, nanodots with the average diameter and height of 40 and 5 nm, respectively, which are smaller than those for the period of 750 nm (130 and 9 nm, respectively, as shown in Fig. 1a), accommodate in nanoholes one by one almost completely. Reducing the period of FIB-patterned nanoholes to 100 nm,
Fig. 1. SEM images showing the growth processes of ZnO on FIB-nanopatterned SiO2 substrate surface. The results are at (a) 40 s, (b) 10 min, and (c) 15 min after starting the growth of ZnO.
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Fig. 2. AFM images of the ZnO nanodots formed on FIB-patterned nanoholes with the period of (a) 750 nm, (b) 190 nm, and (c) 100 nm.
100
Yield [arb. units]
the size of ZnO nanodots further decreases, that is, the average diameter and height are 18 and 4 nm, respectively. However, only 64% of nanoholes are filled with ZnO nanodots and the rest of them are empty. This is probably limited by the present growth conditions and the further optimization will lead to the complete two-dimensional array of ZnO nanodots with period of smaller than 100 nm in the future. In this case, the small size of nanodots is effective for quantum confinement, the density of the nanodots calculated to be 1010 cm 2 is close to the value of self-assembled nanodots generally known, and the small separation of nanodots encourages further development toward near-field optical transmission or carrier tunneling. It is expected that this series of achievement for the artificial control of nanodots will open new application field of nanodevices. The histograms showing the distribution of diameter of nanodots are given in Fig. 3 for the arrays of different periods in comparison to the selfassembled nanodots on an unpatterned SiO2 substrate. It is clearly seen that the diameters of nanodots were 130710 and 1875 nm with arraying in 750- and 100-nm periods, respectively, while it is widely distributed from 10 to 120 nm in the simple self-assembled nucleation, that is, the size distribution is remarkably reduced on FIB-nanopatterned substrates by artificial arraying. This result further encourages the advantage of the artificially arrayed nanodots for the actual application.
period 100 nm period 750 nm
80 60
self-assembled nanodots on unpatterned SiO2
40 20 0 0
20
40
60
80
100
120
140
Diameter [nm] Fig. 3. Histograms showing the distribution of diameter of nanodots formed on the FIB-patterned nanoholes (periods of 750 and 100 nm) and on unpatterned SiO2 substrate surface. The graphs are normalized so that the total areas of the bars become constant for all samples.
Fig. 4 shows the energy dispersive X-ray (EDX) spectroscopy result for a single dot measured in an analytical SEM. Besides zinc (Zn) and oxygen (O), slight incorporation of Ga is suggested from the shoulder structure of the Zn-spectrum. From this result, we speculate the association of Ga for the selective formation of ZnO nanodots. The discussion will be given below. It should be emphasized that no Zn was detected between the nanodots, suggesting no wetting layer on the SiO2 surface.
ARTICLE IN PRESS S. Fujita et al. / Journal of Crystal Growth 272 (2004) 138–142
O Zn Ga
Si
1
2
141
Intensity [arb. units]
50 40 30 20 10 0 0
3
4
5
Energy [keV] Fig. 4. EDX spectrum for a single dot measured in an analytical SEM. Note that this spectrum is the overlap of those for the ZnO nanodot and for the underlying SiO2, resulting in Si and enhanced O peaks in this figure.
4. Discussions on mechanism for arraying It is of our great interest why a nanodot is selectively formed on a nanohole. There can be assumed two different causes as the mechanism, that is, as the possible nucleation centers or sinks. One is the hole itself on the surface and another is the Ga implanted in the hole. In order for the discussion, we fabricated a nanohole structure according to the process shown in Fig. 5b, different from the conventional process shown in Fig. 5a, where the Ga expected to be implanted by the FIB-nanopatterning is probably removed with the SiO2 layer formed by the successive thermal oxidation and Ga-free nanoholes may be formed on the surface. As a result of MOCVD growth of ZnO, nanodots are hardly seen on the Ga-free surface as is demonstrated by AFM images in Fig. 5. This result suggests us the crucial role of residual Ga implanted in the nanoholes for the preferable nucleation of ZnO.
Fig. 5. Two different techniques to form nanohole arrays on the SiO2 substate surface, that is, (a) conventional technique with FIB-nanopatterning and (b) alternative technique in order to remove the FIB-implanted Ga and/or FIB-damaged area by thermal oxidation and etching. AFM images of the sample surface after the growth of ZnO are also shown.
for forming nanostructures, and two dimensional ZnO nanodot arrays well-ordered in position and size of the nanodots are demonstrated. The period of the nanodot array can be as small as 100 nm by slightly modifying the growth conditions. It seems that Ga implanted in the nanoholes is playing a crucial role for the selective nucleation of ZnO nanodots in nanoholes.
Acknowledgements 5. Conclusions FIB-nanopatterning is shown to be a useful tool for the artificial control of self-assembly processes
This work was partly supported by a Grant-inAid for Scientific Research and also by Grants for Regional Science and Technology Promotion,
ARTICLE IN PRESS 142
S. Fujita et al. / Journal of Crystal Growth 272 (2004) 138–142
from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaas, P.M. Petroff, Appl. Phys. Lett. 63 (1993) 3203. [2] F. Widmann, B. Daudin, G. Feuillet, Y. Samson, J.L. Rouviere, N. Pelekanos, J. Appl. Phys. 83 (1998) 7618. [3] S.H. Xin, P.D. Wang, A. Yin, C. Kim, M. Dobrowolska, J.L. Merz, J.K. Furdyna, Appl. Phys. Lett. 69 (1996) 3884. [4] A.C. Warren, I. Plotnik, E.H. Anderson, M.L. Schattenburg, D.A. Antoniadis, H.I. Smith, J. Vac. Sci. Technol. B 4 (1986) 365.
[5] M. Illing, G. Bacher, T. Kummel, A. Forchel, D. Hommel, B. Jobst, G. Landwehr, J. Vac. Sci. Technol. B 13 (1995) 2792. [6] T. Aign, P. Meyer, S. Lemerle, J.P. Jamet, J. Ferre, V. Mathet, C. Chappert, J. Gierak, C. Vieu, F. Rousseaux, H. Launois, H. Bernas, Phys. Rev. Lett. 81 (1998) 5656. [7] D.M. Eigler, E.K. Schweizer, Nature 344 (1990) 524. [8] S.-W. Kim, M. Ueda, T. Kotani, Sz. Fujita,, Sg. Fujita, Jpn. J. Appl. Phys. 42 (2003) L568. [9] S.-W. Kim, T. Kotani, M. Ueda, Sz. Fujita, Sg. Fujita, Physica E 21 (2004) 601. [10] M. Ueda, S.-W. Kim, Sz. Fujita, Sg. Fujita, Jpn. J. Appl. Phys. 43 (2004) L652.