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CaO buffer layer for the growth of ZnO thin film
Solid State Communications 150 (2010) 428–430
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
CaO buffer layer for the growth of ZnO thin film Y.S. Lim a,∗ , J.S. Jeong b , J. Bang c , J. Kim c a
Green Ceramics Division, Korea Institute of Ceramic Engineering and Technology, 233-5 Gasan-dong, Guemcheon-gu, Seoul 153-801, Republic of Korea
b
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan
c
LG Chem/Research Park, 104-1 Moonji-dong, Yuseong-gu, 305-380 Daejeon, Republic of Korea
article
info
Article history: Received 7 October 2009 Accepted 2 December 2009 by T. Kimura Available online 6 December 2009 Keywords: A. ZnO B. Sputtering C. Transmission electron microscopy D. Strain relaxation
abstract We report the effect of CaO buffer layers on the structural properties of sputter-grown ZnO thin films. X-ray diffraction patterns indicated that enhanced crystallinity and alleviated compressive strain in the ZnO thin film were achieved by inserting a very thin CaO buffer layer between ZnO and the sapphire substrate. The interface was investigated by high-resolution transmission electron microscopy, and the result showed that the growth of CaO on the sapphire substrate follows the Stranski–Kristanov mode. The mechanism for the control of crystallinity and strain in the ZnO thin film was discussed, and was found to be strongly related to the growth mode of the CaO buffer layer. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction ZnO has attracted much attention for optoelectronic applications due to its direct wide band gap (3.37 eV) and large exciton binding energy (60 meV) [1–3]. Because the electrical properties of ZnO is directly related to the structural properties, growth of highquality ZnO thin film is of great importance [4]. For the growth of ZnO thin films, sapphire has been commonly used as a substrate due to its wide availability and ease of cleaning process. However, there is a large lattice mismatch between the sapphire substrate and ZnO (∼18%), so that lack of a suitable substrate is one of the problems for ZnO thin film growth. To grow high-quality ZnO thin films using sapphire substrates, researchers have focused on finding an effective buffer layer, which is inserted between the ZnO thin film and the sapphire substrate. Some research groups have reported high quality ZnO thin films grown on MgO buffered sapphire substrate by molecular beam epitaxy at high temperature [4–7]. The crystal structure of MgO is rocksalt, and the growth of MgO on the sapphire substrate follows the Stranski–Kristanov (S–K) mode [5]. Due to the S–K growth mode, MgO islands on sapphire substrate provide proper sites for lateral epitaxial growth of ZnO [6,7]. Furthermore, because of its effective lattice constant (2.981 Å), the MgO buffer layer partially accommodates the lattice mismatch [5]. Therefore, the dislocation
∗
Corresponding author. Tel.: +82 2 3282 7834; fax: +82 2 3282 2470. E-mail address: [email protected] (Y.S. Lim).
0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.12.001
density could be dramatically reduced and high-quality ZnO thin film could be obtained. As well as MgO buffer layers, low-temperature grown ZnO homo-buffers, ZnO/MgO double buffers and GaN templates also have been applied to the various substrate [5,8–10]. However, to the best of our knowledge, other oxide buffer layers have not yet been reported. Moreover, most of the studies on buffer layers have been performed by molecular beam epitaxy. Compared to molecular beam epitaxy, sputtering is a suitable method for low-cost and large-area process, despite its relatively poor crystallinity. Hence, improving the crystallinity of sputter-grown ZnO thin films is of importance for numerous applications of ZnO. In this work, we report the effect of CaO buffer layers on the structural properties of ZnO thin films grown by radio-frequency (rf) magnetron sputtering. Without the CaO buffer layer, a strong compressive strain was exerted on the ZnO thin film due to the lattice mismatch between ZnO and the sapphire substrate. However, the compressive strain could be alleviated by inserting a CaO buffer layer at the ZnO/sapphire interface, and a remarkable improvement of the ZnO crystallinity was also observed. This structural improvement was closely related to the growth mode of the CaO buffer layer on sapphire, and this was confirmed by high-resolution transmission electron microscopy (HRTEM) at the ZnO/CaO/sapphire interface. 2. Experimental details The CaO buffer layer and ZnO thin film were grown on a c-plane sapphire (α -Al2 O3 ) by rf magnetron sputtering of CaO (99.9%) and ZnO (99.999%) targets. The sapphire substrate was cleaned
Y.S. Lim et al. / Solid State Communications 150 (2010) 428–430
429
a
Fig. 1. A schematic illustration of the atomic arrangement at the ZnO/CaO epitaxial interface. The in-plane lattice mismatch between them is 4.5%.
with an acid solution (deionized water:sulfuric acid:phosphoric acid = 2:1:1) and rinsed by acetone, methanol, and deionized water. Next, the substrate was placed in the sputtering chamber (base pressure < 1 × 10−6 Torr) and heated up to the growth temperature of 400 ◦ C. Sputtering of the CaO buffer layer and the ZnO thin film was done by a mixed gas (Ar:O2 = 1:3) and the working pressure was 9 mTorr. The growth time of the CaO buffer was varied at 0, 200, 400, 800, and 1600 s and that of the ZnO layer thin film was fixed at 7200 s. Regardless of the growth time of the CaO buffer layer, the thickness of ZnO thin film was 160 nm in all samples. XRD analysis was performed by a Bruker D4 Endeavor with Cu Kα radiation operating at 40 kV and 40 mA, and TEM characterization was performed with a Tecnai F30 SuperTwin transmission electron microscope operating at 300 keV.
b
3. Results and discussion CaO is a II–VI compound oxide, as are ZnO and MgO. CaO has a rocksalt structure, and its lattice constant is 4.805 Å [11]. It has hexagonal symmetry on the {111} plane which is composed of oxygen atoms, and these oxygen atoms are coincident with the Osurface (0002) plane of ZnO with a 4.5% lattice mismatch. Therefore, ZnO could be grown on CaO with the epitaxial relationship ¯ iZnO //h110iCaO . Fig. 1 shows a of h0002iZnO //h111iCaO and h21¯ 10 schematic diagram of the epitaxial relationship between CaO and ZnO. Due to the lattice mismatch between ZnO and CaO, a tensile strain will be exerted on the ZnO thin film. However, because this heteroepitaxial structure is grown on a sapphire substrate, the structural properties of the ZnO thin film with a CaO buffer layer on a sapphire substrate depend in a complex way on the lattice mismatch between all the materials. The effective lattice constant of sapphire is 2.747 Å, so that the lattice mismatch between the sapphire substrate and the CaO buffer layer is much larger than that between CaO and ZnO. Therefore, it is assumed that this large mismatch at the CaO/sapphire interface could confine misfit dislocations and defects to the CaO buffer layer, and hence that the ZnO thin film grown on the CaO buffer layer could have an improved crystallinity [5,6]. Firstly, the effect of CaO buffer layer on the structural property of the ZnO thin film was investigated from the XRD pattern. Fig. 2(a) and (b) show the 2-theta position of the (0002) ZnO XRD diffraction peak and its full-width at half-maximum (FWHM) as a function of the deposition time of the CaO buffer layer, respectively. Without the CaO buffer layer (0 s), a strong compressive strain was applied to the ZnO thin film due to the lattice mismatch between the ZnO and sapphire, as shown in Fig. 1(a). However, as the deposition time of the CaO buffer layer increased, the compressive strain was partially relaxed until a deposition time of 400 s. After 400 s, no further relaxation was observed. Furthermore, Fig. 2(b) clearly shows the improved crystallinity of ZnO after only 200 s of CaO deposition. From these results, it was confirmed that
Fig. 2. (a) 2-theta position of the (0002) XRD peak of the ZnO thin film and its (b) corresponding FWHM, as a function of the deposition time of the CaO buffer layer.
an alleviated compressive strain and enhanced crystallinity of ZnO thin film could be achieved by using a CaO buffer layer. However, while the XRD result shows the phenomena it does not explain how it is possible. To clarify the mechanism of the effect of the CaO buffer layer on the structural properties of the ZnO thin film, the lattice arrangement of the interface was explored with HRTEM. Fig. 3 shows a HRTEM micrograph of the interfacial structure of a ZnO/CaO/ sapphire sample. In this sample, the CaO buffer layer was grown for 800 s. As our model in Fig. 1, the CaO buffer layer was grown along the h111i direction. Due to the lattice mismatch between CaO and sapphire, the buffer layer was very defective and severely distorted. The angle between the {111} and {001} planes of CaO (α in Fig. 3) was around 57◦ (α = 54.7◦ in the unstrained lattice) and it shows that a compressive stress is applied along the {111} plane of the CaO island. Therefore, the confinement of defects to the CaO buffer layer could be one reason for the improved crystallinity [5,6]. The other mechanism originates from the growth mode of CaO. As marked by the arrow in Fig. 3, the CaO buffer layer was grown with an island shape. The CaO island indicates that the growth of the CaO layer on the sapphire substrate follows the S–K mode. The height of the island is ∼5 nm and the width seems to be less than 10 nm, as shown in Fig. 3. Chen et al. reported that rocksalt MgO is grown in the S–K mode, and that this MgO island could provide a suitable site for the lateral epitaxial growth of ZnO [6]. In this experiment, the CaO buffer layer also was grown in the S–K mode. Moreover, the lattice misfit between CaO and ZnO is smaller than that between MgO and ZnO. Therefore, the CaO island could be much more effective for the lateral epitaxial growth of ZnO than
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Y.S. Lim et al. / Solid State Communications 150 (2010) 428–430
reaches a balanced state between the compressive strain from the wetting layer and the tensile strain from the CaO islands, such that the compressive strain of the ZnO is partially relaxed, as shown in Fig. 2(b). 4. Conclusions
Fig. 3. A HRTEM micrograph of the ZnO/CaO/sapphire interface.
We have demonstrated the effect of CaO buffer layers on the structural properties of ZnO thin films grown on a sapphire substrate. Although the ZnO thin film was grown by rf magnetron sputtering at relatively low temperature (400 ◦ C), a remarkable improvement of crystallinity in the ZnO thin film with a CaO buffer layer was observed by XRD. Moreover, a partial relaxation of the compressive strain in the ZnO thin film was also achieved by the buffer layer. The buffer mechanism was investigated with HRTEM, and a growth model of CaO-buffered ZnO thin film was proposed from this result. Acknowledgement This work was supported by a Grant-in-Aid for R&D Programs (No. 10029940) from the Korea Ministry of Knowledge Economy. References
Fig. 4. A schematic model for the effect of the CaO buffer layer on the growth of a ZnO thin film.
a MgO island, and that this is the other reason for the enhanced crystallinity of the ZnO thin film. The strain relaxation of the ZnO thin film in Fig. 2(a) could also be explained by the growth mode of CaO buffer layer. Because the CaO buffer layer grows on the sapphire substrate in the S–K mode, the CaO buffer layer is composed of (i) a CaO wetting layer on sapphire substrate and (ii) CaO islands on the wetting layer, as shown in Fig. 4 [12,13]. The former layer is ‘‘wetted’’ on the substrate, i.e., fully strained. The epitaxy of ZnO on the fully strained CaO wetting layer is strongly affected by the underlying sapphire substrate, so that compressive strain (C ) could be applied to the ZnO thin film from the substrate. On the other hand, the later CaO island, of which strain is partially relaxed, could exert a tensile strain (T ) on the ZnO thin film. Eventually, the ZnO thin film
[1] D.C. Look, Mater. Sci. Eng. B 80 (2001) 383. [2] D.-K Hwang, S.-H. Kang, J.-H. Lim, E.-J. Yang, J.-H. Oh, J.-H. Yang, S.-J. Park, Appl. Phys. Lett. 86 (2005) 222101. [3] Y.R. Ryu, T.S. Lee, J.A. Lubguban, H.W. White, Y.S. Park, C.J. Youn, Appl. Phys. Lett. 87 (2005) 153504. [4] K. Miyamoto, M. Sano, H. Kato, T. Yao, Japan J. Appl. Phys. 41 (2002) L1203. [5] S.K. Hong, Y. Chen, H.J. Ko, T. Yao, Phys. Status Solidi (b) 229 (2002) 803. [6] Y. Chen, S.K. Hong, H.J. Ko, V. Kirshner, H. Wenisch, T. Yao, K. Inaba, Y. Segawa, Appl. Phys. Lett. 78 (2001) 3352. [7] V. Vashaei, T. Minegishi, H. Suzuki, T. Hanada, M.W. Cho, T. Yao, A. Setiawan, J. Appl. Phys. 98 (2005) 054911. [8] M. Fujita, N. Kawamoto, M. Sasajima, Y. Horikoshi, J. Vac. Sci. Technol. B 22 (2004) 1484. [9] J.Y. Fan, Y.M. Lu, Y.C. Liu, H.W. Liang, B.H. Li, D.Z. Shen, J.Y. Yang, X.W. Fan, J. Cryst. Growth 266 (2004) 505. [10] Y. Deesirapipat, M. Fujita, M. Sasajima, R. Suzuki, C. Antarasena, Y. Horikoshi, Japan J. Appl. Phys. 44 (2005) 5150. [11] Inorganic Crystal Structure Database, Version 1.3.3, ICSD #75785, Fachinformationszentrum Karlsruhe, Germany & The National Institute of Standards and Technology, USA, 2005. [12] H. Kato, K. Miyamoto, M. Sano, T. Yao, Appl. Phys. Lett. 84 (2004) 4562. [13] M. Yan, H.T. Zhang, E.J. Widjaja, R.P.H. Chang, J. Appl. Phys. 94 (2003) 5240.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
CaO buffer layer for the growth of ZnO thin film Y.S. Lim a,∗ , J.S. Jeong b , J. Bang c , J. Kim c a
Green Ceramics Division, Korea Institute of Ceramic Engineering and Technology, 233-5 Gasan-dong, Guemcheon-gu, Seoul 153-801, Republic of Korea
b
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan
c
LG Chem/Research Park, 104-1 Moonji-dong, Yuseong-gu, 305-380 Daejeon, Republic of Korea
article
info
Article history: Received 7 October 2009 Accepted 2 December 2009 by T. Kimura Available online 6 December 2009 Keywords: A. ZnO B. Sputtering C. Transmission electron microscopy D. Strain relaxation
abstract We report the effect of CaO buffer layers on the structural properties of sputter-grown ZnO thin films. X-ray diffraction patterns indicated that enhanced crystallinity and alleviated compressive strain in the ZnO thin film were achieved by inserting a very thin CaO buffer layer between ZnO and the sapphire substrate. The interface was investigated by high-resolution transmission electron microscopy, and the result showed that the growth of CaO on the sapphire substrate follows the Stranski–Kristanov mode. The mechanism for the control of crystallinity and strain in the ZnO thin film was discussed, and was found to be strongly related to the growth mode of the CaO buffer layer. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction ZnO has attracted much attention for optoelectronic applications due to its direct wide band gap (3.37 eV) and large exciton binding energy (60 meV) [1–3]. Because the electrical properties of ZnO is directly related to the structural properties, growth of highquality ZnO thin film is of great importance [4]. For the growth of ZnO thin films, sapphire has been commonly used as a substrate due to its wide availability and ease of cleaning process. However, there is a large lattice mismatch between the sapphire substrate and ZnO (∼18%), so that lack of a suitable substrate is one of the problems for ZnO thin film growth. To grow high-quality ZnO thin films using sapphire substrates, researchers have focused on finding an effective buffer layer, which is inserted between the ZnO thin film and the sapphire substrate. Some research groups have reported high quality ZnO thin films grown on MgO buffered sapphire substrate by molecular beam epitaxy at high temperature [4–7]. The crystal structure of MgO is rocksalt, and the growth of MgO on the sapphire substrate follows the Stranski–Kristanov (S–K) mode [5]. Due to the S–K growth mode, MgO islands on sapphire substrate provide proper sites for lateral epitaxial growth of ZnO [6,7]. Furthermore, because of its effective lattice constant (2.981 Å), the MgO buffer layer partially accommodates the lattice mismatch [5]. Therefore, the dislocation
∗
Corresponding author. Tel.: +82 2 3282 7834; fax: +82 2 3282 2470. E-mail address: [email protected] (Y.S. Lim).
0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.12.001
density could be dramatically reduced and high-quality ZnO thin film could be obtained. As well as MgO buffer layers, low-temperature grown ZnO homo-buffers, ZnO/MgO double buffers and GaN templates also have been applied to the various substrate [5,8–10]. However, to the best of our knowledge, other oxide buffer layers have not yet been reported. Moreover, most of the studies on buffer layers have been performed by molecular beam epitaxy. Compared to molecular beam epitaxy, sputtering is a suitable method for low-cost and large-area process, despite its relatively poor crystallinity. Hence, improving the crystallinity of sputter-grown ZnO thin films is of importance for numerous applications of ZnO. In this work, we report the effect of CaO buffer layers on the structural properties of ZnO thin films grown by radio-frequency (rf) magnetron sputtering. Without the CaO buffer layer, a strong compressive strain was exerted on the ZnO thin film due to the lattice mismatch between ZnO and the sapphire substrate. However, the compressive strain could be alleviated by inserting a CaO buffer layer at the ZnO/sapphire interface, and a remarkable improvement of the ZnO crystallinity was also observed. This structural improvement was closely related to the growth mode of the CaO buffer layer on sapphire, and this was confirmed by high-resolution transmission electron microscopy (HRTEM) at the ZnO/CaO/sapphire interface. 2. Experimental details The CaO buffer layer and ZnO thin film were grown on a c-plane sapphire (α -Al2 O3 ) by rf magnetron sputtering of CaO (99.9%) and ZnO (99.999%) targets. The sapphire substrate was cleaned
Y.S. Lim et al. / Solid State Communications 150 (2010) 428–430
429
a
Fig. 1. A schematic illustration of the atomic arrangement at the ZnO/CaO epitaxial interface. The in-plane lattice mismatch between them is 4.5%.
with an acid solution (deionized water:sulfuric acid:phosphoric acid = 2:1:1) and rinsed by acetone, methanol, and deionized water. Next, the substrate was placed in the sputtering chamber (base pressure < 1 × 10−6 Torr) and heated up to the growth temperature of 400 ◦ C. Sputtering of the CaO buffer layer and the ZnO thin film was done by a mixed gas (Ar:O2 = 1:3) and the working pressure was 9 mTorr. The growth time of the CaO buffer was varied at 0, 200, 400, 800, and 1600 s and that of the ZnO layer thin film was fixed at 7200 s. Regardless of the growth time of the CaO buffer layer, the thickness of ZnO thin film was 160 nm in all samples. XRD analysis was performed by a Bruker D4 Endeavor with Cu Kα radiation operating at 40 kV and 40 mA, and TEM characterization was performed with a Tecnai F30 SuperTwin transmission electron microscope operating at 300 keV.
b
3. Results and discussion CaO is a II–VI compound oxide, as are ZnO and MgO. CaO has a rocksalt structure, and its lattice constant is 4.805 Å [11]. It has hexagonal symmetry on the {111} plane which is composed of oxygen atoms, and these oxygen atoms are coincident with the Osurface (0002) plane of ZnO with a 4.5% lattice mismatch. Therefore, ZnO could be grown on CaO with the epitaxial relationship ¯ iZnO //h110iCaO . Fig. 1 shows a of h0002iZnO //h111iCaO and h21¯ 10 schematic diagram of the epitaxial relationship between CaO and ZnO. Due to the lattice mismatch between ZnO and CaO, a tensile strain will be exerted on the ZnO thin film. However, because this heteroepitaxial structure is grown on a sapphire substrate, the structural properties of the ZnO thin film with a CaO buffer layer on a sapphire substrate depend in a complex way on the lattice mismatch between all the materials. The effective lattice constant of sapphire is 2.747 Å, so that the lattice mismatch between the sapphire substrate and the CaO buffer layer is much larger than that between CaO and ZnO. Therefore, it is assumed that this large mismatch at the CaO/sapphire interface could confine misfit dislocations and defects to the CaO buffer layer, and hence that the ZnO thin film grown on the CaO buffer layer could have an improved crystallinity [5,6]. Firstly, the effect of CaO buffer layer on the structural property of the ZnO thin film was investigated from the XRD pattern. Fig. 2(a) and (b) show the 2-theta position of the (0002) ZnO XRD diffraction peak and its full-width at half-maximum (FWHM) as a function of the deposition time of the CaO buffer layer, respectively. Without the CaO buffer layer (0 s), a strong compressive strain was applied to the ZnO thin film due to the lattice mismatch between the ZnO and sapphire, as shown in Fig. 1(a). However, as the deposition time of the CaO buffer layer increased, the compressive strain was partially relaxed until a deposition time of 400 s. After 400 s, no further relaxation was observed. Furthermore, Fig. 2(b) clearly shows the improved crystallinity of ZnO after only 200 s of CaO deposition. From these results, it was confirmed that
Fig. 2. (a) 2-theta position of the (0002) XRD peak of the ZnO thin film and its (b) corresponding FWHM, as a function of the deposition time of the CaO buffer layer.
an alleviated compressive strain and enhanced crystallinity of ZnO thin film could be achieved by using a CaO buffer layer. However, while the XRD result shows the phenomena it does not explain how it is possible. To clarify the mechanism of the effect of the CaO buffer layer on the structural properties of the ZnO thin film, the lattice arrangement of the interface was explored with HRTEM. Fig. 3 shows a HRTEM micrograph of the interfacial structure of a ZnO/CaO/ sapphire sample. In this sample, the CaO buffer layer was grown for 800 s. As our model in Fig. 1, the CaO buffer layer was grown along the h111i direction. Due to the lattice mismatch between CaO and sapphire, the buffer layer was very defective and severely distorted. The angle between the {111} and {001} planes of CaO (α in Fig. 3) was around 57◦ (α = 54.7◦ in the unstrained lattice) and it shows that a compressive stress is applied along the {111} plane of the CaO island. Therefore, the confinement of defects to the CaO buffer layer could be one reason for the improved crystallinity [5,6]. The other mechanism originates from the growth mode of CaO. As marked by the arrow in Fig. 3, the CaO buffer layer was grown with an island shape. The CaO island indicates that the growth of the CaO layer on the sapphire substrate follows the S–K mode. The height of the island is ∼5 nm and the width seems to be less than 10 nm, as shown in Fig. 3. Chen et al. reported that rocksalt MgO is grown in the S–K mode, and that this MgO island could provide a suitable site for the lateral epitaxial growth of ZnO [6]. In this experiment, the CaO buffer layer also was grown in the S–K mode. Moreover, the lattice misfit between CaO and ZnO is smaller than that between MgO and ZnO. Therefore, the CaO island could be much more effective for the lateral epitaxial growth of ZnO than
430
Y.S. Lim et al. / Solid State Communications 150 (2010) 428–430
reaches a balanced state between the compressive strain from the wetting layer and the tensile strain from the CaO islands, such that the compressive strain of the ZnO is partially relaxed, as shown in Fig. 2(b). 4. Conclusions
Fig. 3. A HRTEM micrograph of the ZnO/CaO/sapphire interface.
We have demonstrated the effect of CaO buffer layers on the structural properties of ZnO thin films grown on a sapphire substrate. Although the ZnO thin film was grown by rf magnetron sputtering at relatively low temperature (400 ◦ C), a remarkable improvement of crystallinity in the ZnO thin film with a CaO buffer layer was observed by XRD. Moreover, a partial relaxation of the compressive strain in the ZnO thin film was also achieved by the buffer layer. The buffer mechanism was investigated with HRTEM, and a growth model of CaO-buffered ZnO thin film was proposed from this result. Acknowledgement This work was supported by a Grant-in-Aid for R&D Programs (No. 10029940) from the Korea Ministry of Knowledge Economy. References
Fig. 4. A schematic model for the effect of the CaO buffer layer on the growth of a ZnO thin film.
a MgO island, and that this is the other reason for the enhanced crystallinity of the ZnO thin film. The strain relaxation of the ZnO thin film in Fig. 2(a) could also be explained by the growth mode of CaO buffer layer. Because the CaO buffer layer grows on the sapphire substrate in the S–K mode, the CaO buffer layer is composed of (i) a CaO wetting layer on sapphire substrate and (ii) CaO islands on the wetting layer, as shown in Fig. 4 [12,13]. The former layer is ‘‘wetted’’ on the substrate, i.e., fully strained. The epitaxy of ZnO on the fully strained CaO wetting layer is strongly affected by the underlying sapphire substrate, so that compressive strain (C ) could be applied to the ZnO thin film from the substrate. On the other hand, the later CaO island, of which strain is partially relaxed, could exert a tensile strain (T ) on the ZnO thin film. Eventually, the ZnO thin film
[1] D.C. Look, Mater. Sci. Eng. B 80 (2001) 383. [2] D.-K Hwang, S.-H. Kang, J.-H. Lim, E.-J. Yang, J.-H. Oh, J.-H. Yang, S.-J. Park, Appl. Phys. Lett. 86 (2005) 222101. [3] Y.R. Ryu, T.S. Lee, J.A. Lubguban, H.W. White, Y.S. Park, C.J. Youn, Appl. Phys. Lett. 87 (2005) 153504. [4] K. Miyamoto, M. Sano, H. Kato, T. Yao, Japan J. Appl. Phys. 41 (2002) L1203. [5] S.K. Hong, Y. Chen, H.J. Ko, T. Yao, Phys. Status Solidi (b) 229 (2002) 803. [6] Y. Chen, S.K. Hong, H.J. Ko, V. Kirshner, H. Wenisch, T. Yao, K. Inaba, Y. Segawa, Appl. Phys. Lett. 78 (2001) 3352. [7] V. Vashaei, T. Minegishi, H. Suzuki, T. Hanada, M.W. Cho, T. Yao, A. Setiawan, J. Appl. Phys. 98 (2005) 054911. [8] M. Fujita, N. Kawamoto, M. Sasajima, Y. Horikoshi, J. Vac. Sci. Technol. B 22 (2004) 1484. [9] J.Y. Fan, Y.M. Lu, Y.C. Liu, H.W. Liang, B.H. Li, D.Z. Shen, J.Y. Yang, X.W. Fan, J. Cryst. Growth 266 (2004) 505. [10] Y. Deesirapipat, M. Fujita, M. Sasajima, R. Suzuki, C. Antarasena, Y. Horikoshi, Japan J. Appl. Phys. 44 (2005) 5150. [11] Inorganic Crystal Structure Database, Version 1.3.3, ICSD #75785, Fachinformationszentrum Karlsruhe, Germany & The National Institute of Standards and Technology, USA, 2005. [12] H. Kato, K. Miyamoto, M. Sano, T. Yao, Appl. Phys. Lett. 84 (2004) 4562. [13] M. Yan, H.T. Zhang, E.J. Widjaja, R.P.H. Chang, J. Appl. Phys. 94 (2003) 5240.