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Growth of the 2-in-size bulk ZnO single crystals by the hydrothermal method
ARTICLE IN PRESS
Journal of Crystal Growth 260 (2004) 166–170
Growth of the 2-in-size bulk ZnO single crystals by the hydrothermal method Eriko Ohshimaa,*, Hiraku Oginoa, Ikuo Niikurab, Katsumi Maedab, Mitsuru Satob, Masumi Itoc, Tsuguo Fukudaa a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b Tokyo Denpa Co. Ltd., 5-6-11 Chuo, Ohta-ku, Tokyo 143-0024, Japan c Mitsubishi Chemical Corporation, 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan Received 18 July 2003; accepted 19 August 2003 Communicated by M. Schieber
Abstract The hydrothermal method combined with a platinum inner container was applied to grow zinc oxide (ZnO) single crystals. High-purity and transparent single crystals with a large size of 50 50 15 mm3 were successfully grown. The higher crystallinity of the hydrothermally grown ZnO crystal compared with the commercial ZnO substrate grown by the chemical vapor transport method was confirmed by X-ray diffraction. The +c region of the crystal is colorless and the c region of it is pale green, relating to the unique residual impurity distribution. In the photoluminescence spectrum, a strong emission from the band edge at 3.3 eV was observed. On the other hand, the emission from the deep level at 2.2 eV was not observed. These results clearly indicate that this study assures the commercialization of the 2-insize ZnO single crystal for future wide band gap device applications. r 2003 Elsevier B.V. All rights reserved. PACS: 81.05.Dz; 81.10.Dn; 61.10.Nz; 81.70.Jb; 78.55.Et Keywords: A2. Hydrothermal crystal growth; Single crystal growth; B1. Oxides; Zinc compounds; B2. Semiconducting II–VI materials
1. Introduction Zinc oxide (ZnO) has been well known as a transparent, dielectric, piezoelectric and wide band gap material. The potential capabilities have been demonstrated for a wide range of applications such as piezoelectric transducer, gas sensor, optical waveguides, transparent electrode, varistor and *Corresponding author. Tel.: +81-22-217-5167; fax: +8122-217-5102. E-mail address: [email protected] (E. Ohshima).
SAW filter [1]. Recently, ZnO has attracted strong interest as an ultra violet (UV) and blue lightemitting device, due to its direct wide band gap of 3.37 eV, and large exciton-binding energy of 60 meV [2,3]. Efficient monochromatical excitonic stimulated emission at room temperature is expected due to the large exciton binding energy. The crystal structure of ZnO is wurtzite type, the same as that of gallium nitride (GaN). The aim of this study is to grow high-quality and large-sized ZnO single crystals for substrates of the UV and blue light-emitting devices. Optical
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.08.019
ARTICLE IN PRESS E. Ohshima et al. / Journal of Crystal Growth 260 (2004) 166–170
device materials of our targets are not only the ZnO system, but also the GaN system. Although sapphire has been conventionally used for the substrates of these devices [2–7], dense dislocations exist in the devices, caused by the large lattice mismatch between the sapphire substrate and device materials. ZnO single crystal substrates are promising for homo-epitaxy of ZnO active layers, and could be useful for hetero-epitaxy of GaN-based active layers. The stacking order of ZnO substrates is the same as GaN, with a lattice mismatch of approximately 1.8%. ZnO single crystal growth has been mainly carried out by three methods such as the flux method [8–10], the chemical vapor transport (CVT) method [11–14] and the hydrothermal method [15–17]. The hydrothermal method comparatively demonstrated the high-quality crystals grown under the lower temperature. Moreover, its growth rate of B0.2 mm/day is reasonably high compared with the value of B0.5 mm/day for the industrialized quartz crystal growth. The hydrothermal growth of the quartz crystal has been well established; therefore, the application of this method for the mass production of the large-sized single crystals is quite suitable. For example, large quartz SAW wafers with 6-in diameter have been produced by the hydrothermal method. However, no study has been reported for the large-sized ZnO single crystals grown by the hydrothermal method, and the crystal size was smaller than that grown by the CVT method. One of the reasons of preventing large-sized ZnO single crystal growth by the hydrothermal method is the requirement to prevent impurity incorporation from the inner surface of the autoclave. Pt inner container is a solution for this requirement [16,17]; however, it is difficult to increase crystal size in the Pt inner container. In this study, we have investigated the high-quality and large-sized ZnO single crystal growth by the hydrothermal method with Pt inner container.
2. Experimental procedure ZnO single crystals were grown by the hydrothermal method from the precursor of sintered ZnO poly crystals in an aqueous solution of the
167
mineralizers of LiOH (1 mol/l) and KOH (3 mol/l). Basicity of mineralizer solution for the ZnO crystal growth is higher than the artificial quartz growth. Therefore, a closed Pt inner container is essential for preventing impurity incorporation from the autoclave inner surface and growth of high-quality ZnO single crystals. Crystal growth zone and precursor dissolution zone were designed in the Pt inner container, and were separated by a Pt baffle. The precursor was put into the bottom of Pt inner container, and seed crystals were suspended in the growth zone by Pt wires. The Pt inner container was filled with the mineralizer aqueous solution and sealed up. The Pt inner container was put into a pressure-resistant autoclave. Suitable quantity of distilled water was supplied into the volume between the autoclave and the Pt inner container for pressure valancing. ZnO single crystals were grown under the temperature of 300–400 C and the pressure of 80–100 MPa. The crystallinity of the grown ZnO single crystals was characterized by X-ray diffraction (XRD) scans using Cu Ka radiation monochromatized by four crystals of Ge (2 2 0). PL exited by a He–Cd laser (l ¼ 325 nm) was measured at room temperature. The impurity concentration was determined by inductively coupled plasma mass spectroscopy (ICP-MS).
3. Results and discussion Fig. 1(a) shows ZnO single crystals grown by the hydrothermal method in the Pt inner container, with 50 mm inner diameter and 1 m length. They are transparent with pale yellow color, and size of the largest one is 25 15 12 mm3. Schematic drawing of growth sectors of the crystal is shown in Fig. 1(b). There are five sectors named +c, +p, m, p, and c, respectively. The face of +c is terminated with the zinc atom, and c is terminated with the oxygen atom. The ZnO crystals shown in Fig. 1(a) were grown from the seeds predominated by the m face. Growth rates of the +c and m directions were almost the same, but that of the c direction was low. Based on the crystal growth in the small Pt inner container, the modification and the scale-up of the growth system
ARTICLE IN PRESS E. Ohshima et al. / Journal of Crystal Growth 260 (2004) 166–170
168
50mm
(a)
(a)
+c +c +p
+c +p m
+p
+c
m
m
-p -c (b)
seed
-c
Fig. 1. (a) Photograph of ZnO single crystals grown by the hydrothermal method; (b) schematic drawing of growth sectors.
were performed for growing 2-in-size ZnO crystals. Fig. 2(a) shows the ZnO single crystal grown in a Pt inner container with 200 mm inner diameter and 3 m length. The crystal is transparent, and its size of 50 50 15 mm3 (2-in size) is large enough for the commercialization as a substrate material. Schematic drawing of the growth sectors of the crystal is shown in Fig. 2(b). Although the 2-in-size crystal seems pale green, this color is originated in the c region, and +c region of the crystal is colorless. In Fig. 3, a ZnO wafer sliced from the +c region of the 2-in-size crystal is shown. It is
(b)
-c
-p
-c
-p seed
Fig. 2. (a) Photograph of a 2-in-size ZnO single crystal grown by the hydrothermal method; (b) schematic drawing of growth sectors.
transparent and colorless. The transmissivity of visible light was 80% or more in the +c region. The seed crystal of the 2-in-size crystal was predominated by the c face. Growth rate in the c direction was higher than that in the m direction, and that in the +c direction was slightly higher than that in the c direction. The growth mechanism of ZnO microcrystals grown from spontaneous nuclearization by the hydrothermal method under the various conditions was investigated by W.-J. Li et al. [18]. They reported that the crystal shapes depended on the precursor and the solution basicity. Our results show that the shapes of seed crystals influence those of ZnO crystals
ARTICLE IN PRESS E. Ohshima et al. / Journal of Crystal Growth 260 (2004) 166–170
169
mally grown crystal and 42 arcsec for the CVTgrown one. In Fig. 5, the X-ray rocking curves for (1 0 1) reflections were shown. The peak of hydrothermally grown ZnO single crystal is sharper than that of the CVT-grown crystal. This is the same tendency as in (0 0 2) reflections. The FWHM is 25 arcsec for the hydrothermally grown crystal and 370 arcsec for the CVT-grown one. These results show that crystallinity of the ZnO single crystal grown by the hydrothermal method is better than that of the CVT crystal. Fig. 6 shows the PL spectrum of the +c region ZnO crystal grown by the hydrothermal method measured at room temperature. Strong UV emission at 3.3 eV from the band edge was observed, but the visible region emission at 2.2 eV from deep Fig. 3. Photograph of a ZnO wafer cut away from the +c region.
Intensity (arb. units)
Hydrothermal CVT
Intensity (arb. units)
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
ω (degree) -0.02
-0.01
0
ω (degree)
0.01
0.02
0.03
Fig. 4. X-ray rocking curves for (0 0 2) reflection of ZnO single crystals.
grown from seeds, in spite of the same precursor and the solution basicity. The X-ray rocking curves for (0 0 2) reflections of ZnO single crystals are shown in Fig. 4. A sharp peak of the solid line is the reflection of a ZnO single crystal grown by the hydrothermal method, and a broad one of the dotted line is the reflection of a commercial substrate grown by the CVT method. It is clearly obvious that the peak of hydrothermally grown ZnO is sharper than that of the CVT-grown crystal. The full-width at halfmaximum (FWHM) is 8 arcsec for the hydrother-
Fig. 5. X-ray rocking curves for (1 0 1) reflection of ZnO single crystals.
PL Intensity (arb. unit)
-0.03
hydrothermal CVT
1.5
2.0
2.5
3.0
3.5
Photon Energy (eV) Fig. 6. PL spectrum at room temperature of a ZnO single crystal grown by the hydrothermal method.
ARTICLE IN PRESS E. Ohshima et al. / Journal of Crystal Growth 260 (2004) 166–170
170
Concentration (ppm)
12 10
+c
No+4 No+3 No+2 No+1 Seed No-1 No-2 No-3
8 6
-c
Acknowledgements
4 Al Fe
2 0
No-3
No-2
No-1
No+1
cial ZnO substrate grown by the CVT method. Hydrothermally grown ZnO crystals show strong UV emission and no visible emission at room temperature.
No+2
No+3
No+4
Position
The authors thank J.M. Ko (IMRAM, Tohoku university) for his basic experiments and A. Yoshikawa (IMRAM, Tohoku university) for his useful discussions.
Fig. 7. Concentration of Al and Fe in hydrothermally grown ZnO single crystal pieces.
References level was not observed. This result indicates the high optical quality of this crystal. The ZnO single crystal grown by the hydrothermal method was cut out at the plane perpendicular to the c-axis, and the impurity concentration was analyzed by ICP-MS. Incorporation of Li and K originated from LiOH and KOH was observed in all pieces, and their concentrations were 0.5–12 ppm for Li and 0.04– 0.2 ppm for K. In addition, incorporation of Fe and Al was also observed. The Fe and Al concentrations of each piece are shown in Fig. 7. The schematic drawing put into Fig. 7 shows the cutting of ZnO crystal. The concentrations of both Fe and Al were high in the c region of the crystal; on the other hand, they were low in the +c region. From this result, it is considered that the pale green color of c region crystal is related to Fe or Al impurities. The difference of the growth mechanism at +c and c should also be considered.
4. Conclusions We could grow the 2-in-size ZnO single crystal by the hydrothermal method. The crystals of +c region are transparent and colorless, while those of c region are pale green. The shapes of the ZnO crystals depend on the shapes of seed crystals. Crystallinity of ZnO crystals grown by the hydrothermal method was higher than a commer-
[1] R. Triboulet, Proc. SPIE 4412 (2001) 1. [2] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [3] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, M.Y. Shen, T. Goto, Appl. Phys. Lett. 73 (1998) 1038. [4] A. Ohtomo, M. Kawasaki, I. Ohkubo, H. Koinuma, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 75 (1999) 980. [5] S. Nakamura, M. Senoh, N. Iwasa, I. Nagahama, Appl. Phys. Lett. 67 (1995) 1868. [6] W. Kim, O. Aktas, A. Botchkarev, A. Salvador, S.N. Mohammad, H. Morko@, J. Appl. Phys. 79 (1996) 7657. [7] X.H. Wu, L.M. Brown, D. Kapolnek, S. Keller, S.P. DenBaars, J.S. Speck, J. Appl. Phys. 80 (1996) 3228. [8] J.W. Nielsen, E.F. Dearborn, J. Phys. Chem. 64 (1960) 1762. [9] K. Oka, H. Shibata, S. Kashiwaya, J. Crystal Growth 237–239 (2002) 5. [10] N. Ohashi, T. Sekiguchi, K. Aoyama, T. Ohgaki, Y. Terada, I. Sakaguchi, T. Tsurumi, H. Haneda, J. Appl. Phys. 91 (2002) 3658. [11] D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jones, C.W. Litton, G. Cantwell, W.C. Harsch, Solid State Commun. 105 (1998) 399. [12] M. Shiloh, J. Gutman, J. Crystal Growth 11 (1971) 105. [13] K. Matsumoto, K. Noda, J. Crystal Growth 102 (1990) 137. [14] J.-M. Ntep, S.S. Hassani, A. Lusson, A. Tromson-Carli, D. Ballutaud, G. Didier, R. Triboulet, J. Crystal Growth 207 (1999) 30. [15] R.A. Laudise, E.D. Kolb, A.J. Caporaso, J. Am. Ceram. Soc. 47 (1964) 9. [16] T. Sekiguchi, S. Miyashita, K. Obara, T. Shishido, N. Sakagami, J. Crystal Growth 214/215 (2000) 72. [17] T. Sakagami, M. Yamashita, T. Sekiguchi, S. Miyashita, K. Obara, T. Shishido, J. Crystal Growth 229 (2001) 98. [18] W.-J. Li, E.-W. Shi, W.-Z. Zhong, Z.-W. Yin, J. Crystal Growth 203 (1999) 186.
Journal of Crystal Growth 260 (2004) 166–170
Growth of the 2-in-size bulk ZnO single crystals by the hydrothermal method Eriko Ohshimaa,*, Hiraku Oginoa, Ikuo Niikurab, Katsumi Maedab, Mitsuru Satob, Masumi Itoc, Tsuguo Fukudaa a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b Tokyo Denpa Co. Ltd., 5-6-11 Chuo, Ohta-ku, Tokyo 143-0024, Japan c Mitsubishi Chemical Corporation, 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan Received 18 July 2003; accepted 19 August 2003 Communicated by M. Schieber
Abstract The hydrothermal method combined with a platinum inner container was applied to grow zinc oxide (ZnO) single crystals. High-purity and transparent single crystals with a large size of 50 50 15 mm3 were successfully grown. The higher crystallinity of the hydrothermally grown ZnO crystal compared with the commercial ZnO substrate grown by the chemical vapor transport method was confirmed by X-ray diffraction. The +c region of the crystal is colorless and the c region of it is pale green, relating to the unique residual impurity distribution. In the photoluminescence spectrum, a strong emission from the band edge at 3.3 eV was observed. On the other hand, the emission from the deep level at 2.2 eV was not observed. These results clearly indicate that this study assures the commercialization of the 2-insize ZnO single crystal for future wide band gap device applications. r 2003 Elsevier B.V. All rights reserved. PACS: 81.05.Dz; 81.10.Dn; 61.10.Nz; 81.70.Jb; 78.55.Et Keywords: A2. Hydrothermal crystal growth; Single crystal growth; B1. Oxides; Zinc compounds; B2. Semiconducting II–VI materials
1. Introduction Zinc oxide (ZnO) has been well known as a transparent, dielectric, piezoelectric and wide band gap material. The potential capabilities have been demonstrated for a wide range of applications such as piezoelectric transducer, gas sensor, optical waveguides, transparent electrode, varistor and *Corresponding author. Tel.: +81-22-217-5167; fax: +8122-217-5102. E-mail address: [email protected] (E. Ohshima).
SAW filter [1]. Recently, ZnO has attracted strong interest as an ultra violet (UV) and blue lightemitting device, due to its direct wide band gap of 3.37 eV, and large exciton-binding energy of 60 meV [2,3]. Efficient monochromatical excitonic stimulated emission at room temperature is expected due to the large exciton binding energy. The crystal structure of ZnO is wurtzite type, the same as that of gallium nitride (GaN). The aim of this study is to grow high-quality and large-sized ZnO single crystals for substrates of the UV and blue light-emitting devices. Optical
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.08.019
ARTICLE IN PRESS E. Ohshima et al. / Journal of Crystal Growth 260 (2004) 166–170
device materials of our targets are not only the ZnO system, but also the GaN system. Although sapphire has been conventionally used for the substrates of these devices [2–7], dense dislocations exist in the devices, caused by the large lattice mismatch between the sapphire substrate and device materials. ZnO single crystal substrates are promising for homo-epitaxy of ZnO active layers, and could be useful for hetero-epitaxy of GaN-based active layers. The stacking order of ZnO substrates is the same as GaN, with a lattice mismatch of approximately 1.8%. ZnO single crystal growth has been mainly carried out by three methods such as the flux method [8–10], the chemical vapor transport (CVT) method [11–14] and the hydrothermal method [15–17]. The hydrothermal method comparatively demonstrated the high-quality crystals grown under the lower temperature. Moreover, its growth rate of B0.2 mm/day is reasonably high compared with the value of B0.5 mm/day for the industrialized quartz crystal growth. The hydrothermal growth of the quartz crystal has been well established; therefore, the application of this method for the mass production of the large-sized single crystals is quite suitable. For example, large quartz SAW wafers with 6-in diameter have been produced by the hydrothermal method. However, no study has been reported for the large-sized ZnO single crystals grown by the hydrothermal method, and the crystal size was smaller than that grown by the CVT method. One of the reasons of preventing large-sized ZnO single crystal growth by the hydrothermal method is the requirement to prevent impurity incorporation from the inner surface of the autoclave. Pt inner container is a solution for this requirement [16,17]; however, it is difficult to increase crystal size in the Pt inner container. In this study, we have investigated the high-quality and large-sized ZnO single crystal growth by the hydrothermal method with Pt inner container.
2. Experimental procedure ZnO single crystals were grown by the hydrothermal method from the precursor of sintered ZnO poly crystals in an aqueous solution of the
167
mineralizers of LiOH (1 mol/l) and KOH (3 mol/l). Basicity of mineralizer solution for the ZnO crystal growth is higher than the artificial quartz growth. Therefore, a closed Pt inner container is essential for preventing impurity incorporation from the autoclave inner surface and growth of high-quality ZnO single crystals. Crystal growth zone and precursor dissolution zone were designed in the Pt inner container, and were separated by a Pt baffle. The precursor was put into the bottom of Pt inner container, and seed crystals were suspended in the growth zone by Pt wires. The Pt inner container was filled with the mineralizer aqueous solution and sealed up. The Pt inner container was put into a pressure-resistant autoclave. Suitable quantity of distilled water was supplied into the volume between the autoclave and the Pt inner container for pressure valancing. ZnO single crystals were grown under the temperature of 300–400 C and the pressure of 80–100 MPa. The crystallinity of the grown ZnO single crystals was characterized by X-ray diffraction (XRD) scans using Cu Ka radiation monochromatized by four crystals of Ge (2 2 0). PL exited by a He–Cd laser (l ¼ 325 nm) was measured at room temperature. The impurity concentration was determined by inductively coupled plasma mass spectroscopy (ICP-MS).
3. Results and discussion Fig. 1(a) shows ZnO single crystals grown by the hydrothermal method in the Pt inner container, with 50 mm inner diameter and 1 m length. They are transparent with pale yellow color, and size of the largest one is 25 15 12 mm3. Schematic drawing of growth sectors of the crystal is shown in Fig. 1(b). There are five sectors named +c, +p, m, p, and c, respectively. The face of +c is terminated with the zinc atom, and c is terminated with the oxygen atom. The ZnO crystals shown in Fig. 1(a) were grown from the seeds predominated by the m face. Growth rates of the +c and m directions were almost the same, but that of the c direction was low. Based on the crystal growth in the small Pt inner container, the modification and the scale-up of the growth system
ARTICLE IN PRESS E. Ohshima et al. / Journal of Crystal Growth 260 (2004) 166–170
168
50mm
(a)
(a)
+c +c +p
+c +p m
+p
+c
m
m
-p -c (b)
seed
-c
Fig. 1. (a) Photograph of ZnO single crystals grown by the hydrothermal method; (b) schematic drawing of growth sectors.
were performed for growing 2-in-size ZnO crystals. Fig. 2(a) shows the ZnO single crystal grown in a Pt inner container with 200 mm inner diameter and 3 m length. The crystal is transparent, and its size of 50 50 15 mm3 (2-in size) is large enough for the commercialization as a substrate material. Schematic drawing of the growth sectors of the crystal is shown in Fig. 2(b). Although the 2-in-size crystal seems pale green, this color is originated in the c region, and +c region of the crystal is colorless. In Fig. 3, a ZnO wafer sliced from the +c region of the 2-in-size crystal is shown. It is
(b)
-c
-p
-c
-p seed
Fig. 2. (a) Photograph of a 2-in-size ZnO single crystal grown by the hydrothermal method; (b) schematic drawing of growth sectors.
transparent and colorless. The transmissivity of visible light was 80% or more in the +c region. The seed crystal of the 2-in-size crystal was predominated by the c face. Growth rate in the c direction was higher than that in the m direction, and that in the +c direction was slightly higher than that in the c direction. The growth mechanism of ZnO microcrystals grown from spontaneous nuclearization by the hydrothermal method under the various conditions was investigated by W.-J. Li et al. [18]. They reported that the crystal shapes depended on the precursor and the solution basicity. Our results show that the shapes of seed crystals influence those of ZnO crystals
ARTICLE IN PRESS E. Ohshima et al. / Journal of Crystal Growth 260 (2004) 166–170
169
mally grown crystal and 42 arcsec for the CVTgrown one. In Fig. 5, the X-ray rocking curves for (1 0 1) reflections were shown. The peak of hydrothermally grown ZnO single crystal is sharper than that of the CVT-grown crystal. This is the same tendency as in (0 0 2) reflections. The FWHM is 25 arcsec for the hydrothermally grown crystal and 370 arcsec for the CVT-grown one. These results show that crystallinity of the ZnO single crystal grown by the hydrothermal method is better than that of the CVT crystal. Fig. 6 shows the PL spectrum of the +c region ZnO crystal grown by the hydrothermal method measured at room temperature. Strong UV emission at 3.3 eV from the band edge was observed, but the visible region emission at 2.2 eV from deep Fig. 3. Photograph of a ZnO wafer cut away from the +c region.
Intensity (arb. units)
Hydrothermal CVT
Intensity (arb. units)
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
ω (degree) -0.02
-0.01
0
ω (degree)
0.01
0.02
0.03
Fig. 4. X-ray rocking curves for (0 0 2) reflection of ZnO single crystals.
grown from seeds, in spite of the same precursor and the solution basicity. The X-ray rocking curves for (0 0 2) reflections of ZnO single crystals are shown in Fig. 4. A sharp peak of the solid line is the reflection of a ZnO single crystal grown by the hydrothermal method, and a broad one of the dotted line is the reflection of a commercial substrate grown by the CVT method. It is clearly obvious that the peak of hydrothermally grown ZnO is sharper than that of the CVT-grown crystal. The full-width at halfmaximum (FWHM) is 8 arcsec for the hydrother-
Fig. 5. X-ray rocking curves for (1 0 1) reflection of ZnO single crystals.
PL Intensity (arb. unit)
-0.03
hydrothermal CVT
1.5
2.0
2.5
3.0
3.5
Photon Energy (eV) Fig. 6. PL spectrum at room temperature of a ZnO single crystal grown by the hydrothermal method.
ARTICLE IN PRESS E. Ohshima et al. / Journal of Crystal Growth 260 (2004) 166–170
170
Concentration (ppm)
12 10
+c
No+4 No+3 No+2 No+1 Seed No-1 No-2 No-3
8 6
-c
Acknowledgements
4 Al Fe
2 0
No-3
No-2
No-1
No+1
cial ZnO substrate grown by the CVT method. Hydrothermally grown ZnO crystals show strong UV emission and no visible emission at room temperature.
No+2
No+3
No+4
Position
The authors thank J.M. Ko (IMRAM, Tohoku university) for his basic experiments and A. Yoshikawa (IMRAM, Tohoku university) for his useful discussions.
Fig. 7. Concentration of Al and Fe in hydrothermally grown ZnO single crystal pieces.
References level was not observed. This result indicates the high optical quality of this crystal. The ZnO single crystal grown by the hydrothermal method was cut out at the plane perpendicular to the c-axis, and the impurity concentration was analyzed by ICP-MS. Incorporation of Li and K originated from LiOH and KOH was observed in all pieces, and their concentrations were 0.5–12 ppm for Li and 0.04– 0.2 ppm for K. In addition, incorporation of Fe and Al was also observed. The Fe and Al concentrations of each piece are shown in Fig. 7. The schematic drawing put into Fig. 7 shows the cutting of ZnO crystal. The concentrations of both Fe and Al were high in the c region of the crystal; on the other hand, they were low in the +c region. From this result, it is considered that the pale green color of c region crystal is related to Fe or Al impurities. The difference of the growth mechanism at +c and c should also be considered.
4. Conclusions We could grow the 2-in-size ZnO single crystal by the hydrothermal method. The crystals of +c region are transparent and colorless, while those of c region are pale green. The shapes of the ZnO crystals depend on the shapes of seed crystals. Crystallinity of ZnO crystals grown by the hydrothermal method was higher than a commer-
[1] R. Triboulet, Proc. SPIE 4412 (2001) 1. [2] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [3] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, M.Y. Shen, T. Goto, Appl. Phys. Lett. 73 (1998) 1038. [4] A. Ohtomo, M. Kawasaki, I. Ohkubo, H. Koinuma, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 75 (1999) 980. [5] S. Nakamura, M. Senoh, N. Iwasa, I. Nagahama, Appl. Phys. Lett. 67 (1995) 1868. [6] W. Kim, O. Aktas, A. Botchkarev, A. Salvador, S.N. Mohammad, H. Morko@, J. Appl. Phys. 79 (1996) 7657. [7] X.H. Wu, L.M. Brown, D. Kapolnek, S. Keller, S.P. DenBaars, J.S. Speck, J. Appl. Phys. 80 (1996) 3228. [8] J.W. Nielsen, E.F. Dearborn, J. Phys. Chem. 64 (1960) 1762. [9] K. Oka, H. Shibata, S. Kashiwaya, J. Crystal Growth 237–239 (2002) 5. [10] N. Ohashi, T. Sekiguchi, K. Aoyama, T. Ohgaki, Y. Terada, I. Sakaguchi, T. Tsurumi, H. Haneda, J. Appl. Phys. 91 (2002) 3658. [11] D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jones, C.W. Litton, G. Cantwell, W.C. Harsch, Solid State Commun. 105 (1998) 399. [12] M. Shiloh, J. Gutman, J. Crystal Growth 11 (1971) 105. [13] K. Matsumoto, K. Noda, J. Crystal Growth 102 (1990) 137. [14] J.-M. Ntep, S.S. Hassani, A. Lusson, A. Tromson-Carli, D. Ballutaud, G. Didier, R. Triboulet, J. Crystal Growth 207 (1999) 30. [15] R.A. Laudise, E.D. Kolb, A.J. Caporaso, J. Am. Ceram. Soc. 47 (1964) 9. [16] T. Sekiguchi, S. Miyashita, K. Obara, T. Shishido, N. Sakagami, J. Crystal Growth 214/215 (2000) 72. [17] T. Sakagami, M. Yamashita, T. Sekiguchi, S. Miyashita, K. Obara, T. Shishido, J. Crystal Growth 229 (2001) 98. [18] W.-J. Li, E.-W. Shi, W.-Z. Zhong, Z.-W. Yin, J. Crystal Growth 203 (1999) 186.