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Effect of evaporation on surface morphology of epitaxial ZnO films during postdeposition annealing
Applied Surface Science 241 (2005) 179–182 www.elsevier.com/locate/apsusc
Effect of evaporation on surface morphology of epitaxial ZnO films during postdeposition annealing I.W. Kima, S.J. Doha, C.C. Kima, Jung Ho Jea,*, J. Tashirob, M. Yoshimotob a
Synchrotron X-ray Laboratory, Department of Materials Science and Engineering, Center for Information Materials, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea b Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8502, Japan Available online 18 November 2004
Abstract We investigated the effect of evaporation on the surface morphology of c-oriented epitaxial ZnO (40 nm thick)/ Al2O3(0 0 0 1) films during postdeposition annealing using real time synchrotron X-ray scattering and atomic force microscopy (AFM). We find that evaporation as well as grain coalescence play crucial roles on the surface morphology of the ZnO/ Al2O3(0 0 0 1) films. Grain growth occurring in initial stage of annealing forms facets with higher surface energies than the (0 0 0 1) planes. By the preferential evaporation of the prism planes, the surface morphology of the ZnO film eventually evolves into 2D flat (0 0 0 1) surface at 800 8C, as confirmed by AFM. The real time measurement of the film thickness during annealing also supports the effect of the evaporation on the morphology. The evaporation rate is high in initial stage by the preferential evaporation from high energy facets but slows down after transition to the flat (0 0 0 1) surface. # 2004 Elsevier B.V. All rights reserved. PACS: 68.43.Vx; 68.35.Md; 68.35. p; 61.10.Nz; 68.55.Ac Keywords: ZnO; Evaporation; Annealing; Surface morphology
1. Introduction Zinc oxide (ZnO) with a wide band gap of 3.37 eV is a very attractive material for the development of optoelectronic devices, such as blue light emitting diodes and laser diodes [1]. In addition, ZnO has relatively high physical and chemical stability, so that * Corresponding author. Tel.: +82 54 279 2143; fax: +82 54 279 2399. E-mail address: [email protected] (J.H. Je).
it has many high temperature applications such as buffer for III–V nitrides [2] and solar cells [3]. To obtain maximum efficiency for various applications, ZnO films are basically required to have high structural quality. Many studies have been done on annealing process to get high structural quality [4,5], mainly because it relieves accumulated strain energy, diminishes defects, and enlarges grain size. The annealing process is also adopted to flatten the surface of lowtemperature buffer layers in fabricating high quality
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.09.087
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epitaxial films [6]. Interestingly, when annealing process is carried out at high temperatures (>600 8C), evaporation of lattice constituents takes place at ZnO surface together with grain growth [7,8]. However, the evaporation studies of the ZnO have been mostly on bulk single crystals. The effect of the evaporation on the surface morphology of the epitaxial ZnO films during annealing has yet to be studied. In this article, we study the evaporation effect on the surface morphology of the ZnO films during the postdeposition annealing using real-time synchrotron X-ray scattering measurements. Ex situ atomic force microscopic (AFM) images are measured as well. Our study reveals that the evaporation as well as grain growth play crucial roles on the surface morphology of the ZnO/Al2O3(0 0 0 1) films during annealing. By the preferential evaporation of high energy prism planes that are formed by grain growth, the surface morphology of the ZnO film eventually evolves into 2D flat (0 0 0 1) surface at 800 8C.
Fig. 1. AFM surface images of ZnO (40 nm thick)/sapphire(0 0 1) thin films as-deposited and annealed: (a) as-deposited at 300 8C, annealed at 800 8C for (b) 30 min, (c) 90 min, and (d) 200 min.
3. Results and discussion 2. Experimental Epitaxial ZnO films of 40 nm thickness were grown on single crystalline Al2O3(0 0 0 1) at substrate temperature of 300 8C by radio frequency (rf) sputtering with a stoichiometric sintered ZnO target (2 in. diameter). The synchrotron X-ray scattering experiments during postdeposition annealing in air were performed at beamline 5C2 (K-JIST) at Pohang Light Source in Korea. To get the exact scattering geometry, the heating cell with sample was placed on four-circle X-ray diffractometer. We measured the powder diffraction profiles (u–2u scan), rocking curves, and surface reflectivities along the surface normal in real time during annealing. The surface morphology of the annealed ZnO films was investigated ex situ by AFM. In evaporation study, the examination of a lattice polarity of a wurtzite structured ZnO films is very important [7]. To analyze the polarity of the as-deposited ZnO films, we employed coaxial impact collision ion scattering spectroscopy (CAICISS) analysis [9]. The CAICISS analysis showed that the ZnO has ‘ c’ polarity that is O-faced ZnO films.
We first discuss the surface morphology change with annealing time at 800 8C. Fig. 1 shows the AFM surface images of as-deposited at 300 8C (a), annealed at 800 8C for (b) 10 min, (c) 90 min, and (d) 210 min. In the as-deposited film, island-like grains, which completely cover the whole film surface as shown in Fig. 1(a), are due to the 3D columnar growth [10]. As the temperature increases to 800 8C, the surface morphology becomes very rough by grain growth (Fig. 1(b)). The grain growth (coarsening) is driven to reduce the boundaries between grains [11]. We note that the rough surface gradually transits to twodimensional (2D) flat surface with annealing time as shown in Fig. 1(b)–(d). Generally the majority of the grain growth rapidly occurs within the first several minutes during post-annealing [5]. Thus, we can easily infer that another factor besides grain growth affects structural change of 3D–2D morphological transition in later stage. During high temperature (>600 8C) annealing, the surface morphology of bulk ZnO single crystal is affected by the evaporation of lattice constituents [7,12]. So it is conceivable that evaporation also takes place on the ZnO films. To investigate evaporation in the epitaxial ZnO films, we carried out real time
I.W. Kim et al. / Applied Surface Science 241 (2005) 179–182
Fig. 2. The thickness change of the ZnO films during annealing at 800 8C.
measurements of thickness change during the annealing. The thickness was assessed from the X-ray reflectivities of the films. Fig. 2 shows the thickness change of the ZnO films at 800 8C as a function of the annealing time. We note that the thickness continuously decreases during the annealing. This clearly suggests that evaporation (or sublimation) occurs in the films during annealing. The rapid evaporation in early stage is presumably due to the very rough surface [Fig. 1(b)], which provides more evaporation sites than flat one. About half of the ˚ /min original film is evaporated with rate of 1 A during the annealing time (200 min). Interestingly, the evaporation of the ZnO little occurs until the annealing temperature reaches to the high temperature of 800 8C. This is attributed to the deposition of O-faced ZnO films as previously
181
mentioned. The O-faced crystal is more resistant to the evaporation than the Zn-faced during thermal annealing. Active evaporation starts below 600 8C in Zn-faced crystal [12]. To explain the surface morphology evolution of the ZnO/Al2O3(0 0 0 1) films during the annealing, we present a schematic diagram as illustrated in Fig. 3. In the as-deposited ZnO (40 nm thick) films, 3D columnar grain structures are developed as illustrated in Fig. 3(a) [10]. As the annealing temperature increases to 800 8C, grain growth and evaporation occur simultaneously. The driving force for the grain growth is the excess boundary energy of grains, as previously explained [11]. So, lateral growth dominantly occurs because of the columnar structured grains. Besides the grain growth can be also promoted by misfit dislocations [13] which inherently originate from the large lattice mismatch of 18.3% between ZnO and Al2O3(0 0 0 1). While the grain growth rapidly completes in early stage [5], the evaporation would continue. By the continual evaporation, the rough surface eventually becomes flat. Another factor contributing to the surface flattening is the enhanced grain growth which is promoted by misfit dislocations. The surface flattening mechanism by the evaporation can be understood in terms of the surface free energy of ZnO plane (face). ZnO has three lower surface energy planes of (0 0 0 1), (1 1 2¯ 0), and (1 0 1¯ 0), where their energy densities are 0.099, ˚ 2, respectively [14]. Interest0.123, and 0.209 eV/A ingly the lateral (prismatic) planes [(1 0 1¯ 0) and (1 1 2¯ 0)] of the ZnO unit cell have higher surface free
Fig. 3. (a) Schematic diagram of the grain growth and the evaporation behavior of the ZnO/Al2O3(0 0 0 1) films during annealing. (b) Detailed diagram of the preferential evaporation at lateral faces of the c-oriented ZnO single grain.
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energies than the basal plane. Based on the surface energies, we can deduce that preferential evaporation occurs at lateral faces (planes) of the ZnO unit cell during the annealing, while passive evaporation at the normal face (basal plane). In the c-oriented epitaxial ZnO films, the (1 0 1¯ 0) and (1 1 2¯ 0) planes would be located at the lateral side of protruded (dome or hut shaped) grains as shown in Fig. 3(b). High evaporation rate in early stage of the annealing is due to the large lateral surface area (as shown in Fig. 1(b)). From the preferential evaporation at lateral faces, the ratio of the lateral face to the normal one gradually decreases as illustrated in Fig. 3(b), consistent with the slow down of the evaporation rate in later stage [Fig. 2]. In the final stage, the energetically stable normal face [(0 0 0 1)] remains. Therefore the rough (protruded) surface in early stage is able to be transformed eventually to flat one. Concurrently, the evaporation rate slows down. These results exhibit that evaporation as well as grain growth play crucial roles on the surface morphology of the ZnO/Al2O3(0 0 0 1) films during post-annealing.
4. Conclusion The effect of evaporation on the surface morphology of the c-oriented epitaxial ZnO (40 nm thick)/ Al2O3(0 0 0 1) films during the post-annealing was investigated using real-time synchrotron X-ray scattering and AFM. Our study reveals that evaporation as well as grain growth play crucial roles on the surface morphology of the ZnO films. By the preferential evaporation of the prism planes that are generated by grain growth in initial stage, the surface morphology of the ZnO film eventually evolves into 2D flat
(0 0 0 1) surface at 800 8C, as confirmed by AFM. The real time measurement of the film thickness during annealing also supports the effect of the evaporation on the morphology. The evaporation rate is high in initial stage by the preferential evaporation from high energy facets but slows down after transition to the flat (0 0 0 1) surface.
Acknowledgements This work was supported by BK 21 project and by the KISTEP through NRL program and by KOTEF.
Reference [1] D.C. Reynolds, D.C. Look, B. Jogai, Solid State Commun. 99 (1996) 873. [2] J. Narayan, K. Dovidenko, A.K. Sharma, S. Oktyabrsky, J. Appl. Phys. 84 (1998) 2597. [3] B. Sang, K. Dairiki, A. Yamada, M. Konagai, Jpn. J. Appl. Phys., Part 1 38 (1999) 4983. [4] V. Gupta, A. Mansingh, J. Appl. Phys. 80 (1996) 1063. [5] M.K. Puchert, P.Y. Timbrell, R.N. Lamb, J. Vac. Sci. Technol. A 14 (1996) 2220. [6] Y. Chen, H.J. Ko, S.K. Hong, T. Yao, Appl. Phys. Lett. 76 (2000) 559. [7] D. Kohl, M. Henzler, G. Heiland, Surf. Sci. 41 (1974) 403. [8] S. Zhu, C.H. Su, S.L. Lehoczky, M.T. Harris, M.J. Callahan, P. McCarty, M.A. George, J. Cryst. Growth 219 (2000) 361. [9] T. Ohnishi, A. Ohtomo, I. Ohkuboa, M. Kawasaki, M. Yoshimoto, H. Koinuma, Mater. Sci. Eng. B 56 (1998) 256. [10] I. Petrov, P.B. Barna, L. Hultman, J.E. Greene, J. Vac. Sci. Technol. A 21 (2003) S117. [11] C.V. Thompson, Annu. Rev. Mater. Sci. 30 (2000) 159. [12] M. Grunze, W. Hirschwald, D. Hofmann, J. Cryst. Growth 52 (1981) 241. [13] E. Bauser, H. Strunk, Thin Solid Films 93 (1982) 185. [14] N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, J. Cryst. Growth 130 (1993) 269.
Effect of evaporation on surface morphology of epitaxial ZnO films during postdeposition annealing I.W. Kima, S.J. Doha, C.C. Kima, Jung Ho Jea,*, J. Tashirob, M. Yoshimotob a
Synchrotron X-ray Laboratory, Department of Materials Science and Engineering, Center for Information Materials, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea b Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8502, Japan Available online 18 November 2004
Abstract We investigated the effect of evaporation on the surface morphology of c-oriented epitaxial ZnO (40 nm thick)/ Al2O3(0 0 0 1) films during postdeposition annealing using real time synchrotron X-ray scattering and atomic force microscopy (AFM). We find that evaporation as well as grain coalescence play crucial roles on the surface morphology of the ZnO/ Al2O3(0 0 0 1) films. Grain growth occurring in initial stage of annealing forms facets with higher surface energies than the (0 0 0 1) planes. By the preferential evaporation of the prism planes, the surface morphology of the ZnO film eventually evolves into 2D flat (0 0 0 1) surface at 800 8C, as confirmed by AFM. The real time measurement of the film thickness during annealing also supports the effect of the evaporation on the morphology. The evaporation rate is high in initial stage by the preferential evaporation from high energy facets but slows down after transition to the flat (0 0 0 1) surface. # 2004 Elsevier B.V. All rights reserved. PACS: 68.43.Vx; 68.35.Md; 68.35. p; 61.10.Nz; 68.55.Ac Keywords: ZnO; Evaporation; Annealing; Surface morphology
1. Introduction Zinc oxide (ZnO) with a wide band gap of 3.37 eV is a very attractive material for the development of optoelectronic devices, such as blue light emitting diodes and laser diodes [1]. In addition, ZnO has relatively high physical and chemical stability, so that * Corresponding author. Tel.: +82 54 279 2143; fax: +82 54 279 2399. E-mail address: [email protected] (J.H. Je).
it has many high temperature applications such as buffer for III–V nitrides [2] and solar cells [3]. To obtain maximum efficiency for various applications, ZnO films are basically required to have high structural quality. Many studies have been done on annealing process to get high structural quality [4,5], mainly because it relieves accumulated strain energy, diminishes defects, and enlarges grain size. The annealing process is also adopted to flatten the surface of lowtemperature buffer layers in fabricating high quality
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.09.087
180
I.W. Kim et al. / Applied Surface Science 241 (2005) 179–182
epitaxial films [6]. Interestingly, when annealing process is carried out at high temperatures (>600 8C), evaporation of lattice constituents takes place at ZnO surface together with grain growth [7,8]. However, the evaporation studies of the ZnO have been mostly on bulk single crystals. The effect of the evaporation on the surface morphology of the epitaxial ZnO films during annealing has yet to be studied. In this article, we study the evaporation effect on the surface morphology of the ZnO films during the postdeposition annealing using real-time synchrotron X-ray scattering measurements. Ex situ atomic force microscopic (AFM) images are measured as well. Our study reveals that the evaporation as well as grain growth play crucial roles on the surface morphology of the ZnO/Al2O3(0 0 0 1) films during annealing. By the preferential evaporation of high energy prism planes that are formed by grain growth, the surface morphology of the ZnO film eventually evolves into 2D flat (0 0 0 1) surface at 800 8C.
Fig. 1. AFM surface images of ZnO (40 nm thick)/sapphire(0 0 1) thin films as-deposited and annealed: (a) as-deposited at 300 8C, annealed at 800 8C for (b) 30 min, (c) 90 min, and (d) 200 min.
3. Results and discussion 2. Experimental Epitaxial ZnO films of 40 nm thickness were grown on single crystalline Al2O3(0 0 0 1) at substrate temperature of 300 8C by radio frequency (rf) sputtering with a stoichiometric sintered ZnO target (2 in. diameter). The synchrotron X-ray scattering experiments during postdeposition annealing in air were performed at beamline 5C2 (K-JIST) at Pohang Light Source in Korea. To get the exact scattering geometry, the heating cell with sample was placed on four-circle X-ray diffractometer. We measured the powder diffraction profiles (u–2u scan), rocking curves, and surface reflectivities along the surface normal in real time during annealing. The surface morphology of the annealed ZnO films was investigated ex situ by AFM. In evaporation study, the examination of a lattice polarity of a wurtzite structured ZnO films is very important [7]. To analyze the polarity of the as-deposited ZnO films, we employed coaxial impact collision ion scattering spectroscopy (CAICISS) analysis [9]. The CAICISS analysis showed that the ZnO has ‘ c’ polarity that is O-faced ZnO films.
We first discuss the surface morphology change with annealing time at 800 8C. Fig. 1 shows the AFM surface images of as-deposited at 300 8C (a), annealed at 800 8C for (b) 10 min, (c) 90 min, and (d) 210 min. In the as-deposited film, island-like grains, which completely cover the whole film surface as shown in Fig. 1(a), are due to the 3D columnar growth [10]. As the temperature increases to 800 8C, the surface morphology becomes very rough by grain growth (Fig. 1(b)). The grain growth (coarsening) is driven to reduce the boundaries between grains [11]. We note that the rough surface gradually transits to twodimensional (2D) flat surface with annealing time as shown in Fig. 1(b)–(d). Generally the majority of the grain growth rapidly occurs within the first several minutes during post-annealing [5]. Thus, we can easily infer that another factor besides grain growth affects structural change of 3D–2D morphological transition in later stage. During high temperature (>600 8C) annealing, the surface morphology of bulk ZnO single crystal is affected by the evaporation of lattice constituents [7,12]. So it is conceivable that evaporation also takes place on the ZnO films. To investigate evaporation in the epitaxial ZnO films, we carried out real time
I.W. Kim et al. / Applied Surface Science 241 (2005) 179–182
Fig. 2. The thickness change of the ZnO films during annealing at 800 8C.
measurements of thickness change during the annealing. The thickness was assessed from the X-ray reflectivities of the films. Fig. 2 shows the thickness change of the ZnO films at 800 8C as a function of the annealing time. We note that the thickness continuously decreases during the annealing. This clearly suggests that evaporation (or sublimation) occurs in the films during annealing. The rapid evaporation in early stage is presumably due to the very rough surface [Fig. 1(b)], which provides more evaporation sites than flat one. About half of the ˚ /min original film is evaporated with rate of 1 A during the annealing time (200 min). Interestingly, the evaporation of the ZnO little occurs until the annealing temperature reaches to the high temperature of 800 8C. This is attributed to the deposition of O-faced ZnO films as previously
181
mentioned. The O-faced crystal is more resistant to the evaporation than the Zn-faced during thermal annealing. Active evaporation starts below 600 8C in Zn-faced crystal [12]. To explain the surface morphology evolution of the ZnO/Al2O3(0 0 0 1) films during the annealing, we present a schematic diagram as illustrated in Fig. 3. In the as-deposited ZnO (40 nm thick) films, 3D columnar grain structures are developed as illustrated in Fig. 3(a) [10]. As the annealing temperature increases to 800 8C, grain growth and evaporation occur simultaneously. The driving force for the grain growth is the excess boundary energy of grains, as previously explained [11]. So, lateral growth dominantly occurs because of the columnar structured grains. Besides the grain growth can be also promoted by misfit dislocations [13] which inherently originate from the large lattice mismatch of 18.3% between ZnO and Al2O3(0 0 0 1). While the grain growth rapidly completes in early stage [5], the evaporation would continue. By the continual evaporation, the rough surface eventually becomes flat. Another factor contributing to the surface flattening is the enhanced grain growth which is promoted by misfit dislocations. The surface flattening mechanism by the evaporation can be understood in terms of the surface free energy of ZnO plane (face). ZnO has three lower surface energy planes of (0 0 0 1), (1 1 2¯ 0), and (1 0 1¯ 0), where their energy densities are 0.099, ˚ 2, respectively [14]. Interest0.123, and 0.209 eV/A ingly the lateral (prismatic) planes [(1 0 1¯ 0) and (1 1 2¯ 0)] of the ZnO unit cell have higher surface free
Fig. 3. (a) Schematic diagram of the grain growth and the evaporation behavior of the ZnO/Al2O3(0 0 0 1) films during annealing. (b) Detailed diagram of the preferential evaporation at lateral faces of the c-oriented ZnO single grain.
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energies than the basal plane. Based on the surface energies, we can deduce that preferential evaporation occurs at lateral faces (planes) of the ZnO unit cell during the annealing, while passive evaporation at the normal face (basal plane). In the c-oriented epitaxial ZnO films, the (1 0 1¯ 0) and (1 1 2¯ 0) planes would be located at the lateral side of protruded (dome or hut shaped) grains as shown in Fig. 3(b). High evaporation rate in early stage of the annealing is due to the large lateral surface area (as shown in Fig. 1(b)). From the preferential evaporation at lateral faces, the ratio of the lateral face to the normal one gradually decreases as illustrated in Fig. 3(b), consistent with the slow down of the evaporation rate in later stage [Fig. 2]. In the final stage, the energetically stable normal face [(0 0 0 1)] remains. Therefore the rough (protruded) surface in early stage is able to be transformed eventually to flat one. Concurrently, the evaporation rate slows down. These results exhibit that evaporation as well as grain growth play crucial roles on the surface morphology of the ZnO/Al2O3(0 0 0 1) films during post-annealing.
4. Conclusion The effect of evaporation on the surface morphology of the c-oriented epitaxial ZnO (40 nm thick)/ Al2O3(0 0 0 1) films during the post-annealing was investigated using real-time synchrotron X-ray scattering and AFM. Our study reveals that evaporation as well as grain growth play crucial roles on the surface morphology of the ZnO films. By the preferential evaporation of the prism planes that are generated by grain growth in initial stage, the surface morphology of the ZnO film eventually evolves into 2D flat
(0 0 0 1) surface at 800 8C, as confirmed by AFM. The real time measurement of the film thickness during annealing also supports the effect of the evaporation on the morphology. The evaporation rate is high in initial stage by the preferential evaporation from high energy facets but slows down after transition to the flat (0 0 0 1) surface.
Acknowledgements This work was supported by BK 21 project and by the KISTEP through NRL program and by KOTEF.
Reference [1] D.C. Reynolds, D.C. Look, B. Jogai, Solid State Commun. 99 (1996) 873. [2] J. Narayan, K. Dovidenko, A.K. Sharma, S. Oktyabrsky, J. Appl. Phys. 84 (1998) 2597. [3] B. Sang, K. Dairiki, A. Yamada, M. Konagai, Jpn. J. Appl. Phys., Part 1 38 (1999) 4983. [4] V. Gupta, A. Mansingh, J. Appl. Phys. 80 (1996) 1063. [5] M.K. Puchert, P.Y. Timbrell, R.N. Lamb, J. Vac. Sci. Technol. A 14 (1996) 2220. [6] Y. Chen, H.J. Ko, S.K. Hong, T. Yao, Appl. Phys. Lett. 76 (2000) 559. [7] D. Kohl, M. Henzler, G. Heiland, Surf. Sci. 41 (1974) 403. [8] S. Zhu, C.H. Su, S.L. Lehoczky, M.T. Harris, M.J. Callahan, P. McCarty, M.A. George, J. Cryst. Growth 219 (2000) 361. [9] T. Ohnishi, A. Ohtomo, I. Ohkuboa, M. Kawasaki, M. Yoshimoto, H. Koinuma, Mater. Sci. Eng. B 56 (1998) 256. [10] I. Petrov, P.B. Barna, L. Hultman, J.E. Greene, J. Vac. Sci. Technol. A 21 (2003) S117. [11] C.V. Thompson, Annu. Rev. Mater. Sci. 30 (2000) 159. [12] M. Grunze, W. Hirschwald, D. Hofmann, J. Cryst. Growth 52 (1981) 241. [13] E. Bauser, H. Strunk, Thin Solid Films 93 (1982) 185. [14] N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, J. Cryst. Growth 130 (1993) 269.