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Thermal stress induced band gap variation of ZnO thin films
Current Applied Physics 14 (2014) 30e33
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Thermal stress induced band gap variation of ZnO thin films Y.-E. Jeong, S. Park* Department of Physics, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 September 2013 Accepted 2 October 2013 Available online 12 October 2013
The growth temperature and post annealing-dependent optical and structural effect of RF magnetron sputtered ZnO thin films were examined. As the growth temperature increased, the lattice constant increased and approached the bulk value, suggesting a decrease in interfacial strain between the substrate and thin film. For the post annealed samples, the interfacial strain decreased further and was close to the bulk value regardless of the post annealing environments (in air and O2). The optical properties of all ZnO thin films examined and revealed higher transparency (>90%). Furthermore, the optical band gap varied according to the growth temperature and post annealing environments due to a decrease in the interfacial strain effect. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: ZnO thin films Growth temperature Annealing environment Optical band gap
1. Introduction Oxide thin films with high optical transparency and proper electrical conduction can be used in a range of applications, such as flexible displays, solar cells, surface acoustic wave devices, gas sensors, piezoelectric devices, etc [1,2]. Among the many available transparent conducting oxide thin films, ZnO based thin films are the most commonly used materials in many those applications [2e 7]. ZnO, an IIeVI compound, has a hexagonal wurtzite structure and is an n-type semiconductor [8]. Conventional ZnO contains a large number of oxygen vacancies (Vo) and interstitials Zn (Zni) and has band gap of 3.37 eV [9e12]. Furthermore, ZnO has a larger exciton binding energy (w60 meV) at room temperature than of ZnSe (w20 meV) and GaN (w28 meV). Therefore, it has highly efficient luminescent properties [13,14]. In addition to its outstanding optical properties, ZnO thin films can be grown at low temperatures and has tunable mobility and resistivity depending on the amount of oxygen [10]. However, fluctuations of the physical properties of the ZnO thin films due to changes in the ratio between Zn and O when the films are exposed to air and the lack of thermal stability [15] at high temperature limit even further applications, such as replacing ITO-based applications. Many studies have characterized the variations in the physical properties of ZnO thin films depending on the growth environment, such as pressure, temperature, film thickness, substrates, etc [16e18]. For example, Kappertz et al., reported the changes in roughness and stress depending on
* Corresponding author. E-mail address: [email protected] (S. Park). 1567-1739/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.10.003
the total pressure and oxygen flow during the growth of polycrystalline ZnO and Al-doped ZnO thin films grown on Al2O3(0001) by DC magnetron sputtering [16]. Ievtushenko et al., also showed the film thickness-dependent strain effect of multilayerd ZnO thin films grown on a range of substrates, such as Si, glass, various orientations of Al2O3(0001) using RF magnetron sputtering [17]. This paper reports the structural and optical properties of ZnO thin films grown on Al2O3(0001) substrates at various growth temperatures and post annealing conditions to examine a correlation between thermal energy and physical properties. 2. Experiments ZnO thin films were deposited on Al2O3(0001) substrates by RF magnetron sputtering at various deposition temperatures, i.e. room temperature (RT), 200 C, 400 C. The deposition time was 10 min for all samples. The base and working pressure of the system were 5.0 108 Torr and 1.6 102 Torr, respectively. The sputtering power was 100 W. Post annealing was performed at 500 C for 1 min in a rate of 10 C/min in air and O2 by rapid thermal annealing (RTP-1200, NEXTRON). The structure of the thin films was examined by X-ray diffraction (XRD, Rigaku Dmax) and the optical properties of the films were measured using a UVeVis spectrometer (Monora-320i, Dongwoo Optron) in the range of 180 nme600 nm to obtain optical band gaps of the films. 3. Results and discussion Fig. 1 shows the room temperature XRD patterns of the ZnO thin films deposited at various deposition temperatures. All the
Y.-E. Jeong, S. Park / Current Applied Physics 14 (2014) 30e33
31
Fig. 2. Annealing effect of stress for the ZnO thin films grown at various temperature. The lines between the data points are a guide to the eyes. The horizontal dotted-line in the figure indicates zero stress.
absorption coefficient was obtained from the Tauc relation [29] as follows: Fig. 1. XRD patterns of ZnO thin films grown on Al2O3(0001) at (a) RT, (b) 200 C and (c) 400 C.
samples exhibited a (002) orientation because the (002) surface has the lowest surface free energy in the wurtzite structure [8]. As the growth temperature increases, the ZnO(002) peak position moved toward the stress free bulk value, suggesting some relaxation of the lattice. In general, the physical origin of the interfacial stress can be considered thermal stress due to the different thermal expansion coefficients [19,20], lattice mismatch due to the different lattice constants of the adjacent materials, and imperfection in the thin films during growth [21e23]. ZnO thin films grown at room temperature exhibits compressive stress due to the intrinsic effect originating from the deposition temperature, pressure, power, gas mixture, etc [19,25e27]. Therefore, the lower ZnO(002) peak position for the sample grown at room temperature (Fig. 1(a)) with respect to the ZnO bulk value (2q ¼ 34.4 ) [24] might be associated with compressive stress. Furthermore, the lattice relaxation of the sample grown at higher temperatures might be due to the different thermal expansion coefficients of ZnO and the substrate. Fig. 2 shows the in-plane stress, s, obtained from the ZnO(002) peak XRD profiles of the as-grown and annealed (both in air and O2 environment) ZnO thin films. The positive and negative values of the in-plane stress represent the tensile and compressive stress, respectively [28]. The calculated in-plane stress showed a negative value for the as-grown samples indicating the compressive stress for the as-grown ZnO thin films, regardless of the growth temperature. Furthermore, the in-plane compressive stress decreased regardless of the annealing environment. The reduction of stress was most prominent in the samples, grown at room temperature. This reduction behavior is similar to the samples grown at higher temperatures, suggesting both annealing and high temperature growth reduce the in-plane stress and promote grain growth. Fig. 3 shows the absorption coefficient of the as-grown and annealed ZnO thin films at various growth temperatures. The
ðahnÞn z hn Eg :
(1)
where hn is the incident photon energy, a is the absorption coefficient, and n is either 2 for a direct transition or 1/2 for an indirect transition. For the as-grown samples, the absorption coefficients between 3.2 eV and 3.35 eV changed rapidly. Furthermore, the amount of the change (slope) became steeper with increasing growth temperature (see - in Fig. 3). The change in the slope is related to the film quality and surface roughness [30]. Therefore, as the growth temperature increases, the thin films have a smoother surface and/or better structural ordering. Fig. 4 shows growth and annealing-dependent vertical grain size (inverse of the FWHM) of the ZnO(002) peak. For the as-grown samples, the grain size became slightly smaller as the growth temperature increased, indicating the improvement of the crystallinity with growth temperatures. Furthermore, the reduced in-plane stress with increasing growth temperature, shown in Fig. 2 might also be due to the reduced interface roughness which can be reflected as a smoother surface roughness. For the annealed thin films, the slope became even steeper and the FWHM (the inverse of grain size) of the ZnO(002) peaks from XRD measurement became smaller, suggesting improved surface roughness and crystallization for both annealing environments. The optical energy gap of the ZnO thin films can be extracted from a linear extrapolation of the absorption coefficient with n ¼ 2, suggesting that the thin films have a direct energy gap. Fig. 5 shows the optical energy gap of the as-grown and annealed ZnO thin films. For the as-grown ZnO thin films deposited at RT, 200 C, and 400 C, the optical energy gap was 3.119 eV, 3.136 eV and 3.123 eV, respectively. After annealing in air, the optical energy gap of the ZnO thin films, deposited at RT, 200 C, and 400 C was 3.141 eV, 3.192 eV and 3.153 eV, respectively. For the thin films annealed in an O2 environment, the optical energy gap of the ZnO thin films deposited at RT, 200 C, and 400 C was 3.134 eV, 3.185 eV and 3.152 eV, respectively. Therefore, the measured band gap energies for all samples are smaller than that of bulk ZnO (3.37 eV [9,11,12]),
32
Y.-E. Jeong, S. Park / Current Applied Physics 14 (2014) 30e33
Fig. 5. Annealing effect of the band gap energy of ZnO thin films grown at (a) RT, (b) 200 C and (c) 400 C. The lines between data points are a guide to the eyes.
Fig. 3. Annealing effect of the absorption coefficient of ZnO thin films grown at (a) RT, (b) 200 C and (c) 400 C. The solid lines are a linear extrapolation of the band gap estimation.
and the band gap energy increases after annealing regardless of the annealing environment. In general, the origin of the variation in the optical energy gap can be explained by the Burstein-Moss effect [31,32], band gap narrowing effect [33,34], and stress effect [35]. However, the Burstein-Moss effect can be applied to the increased band gap, and both the Burstein-Moss and band gap narrowing effect can also be applied to a higher carrier concentration system [36]. Furthermore, all thin films are electrically insulating (not shown here). Therefore, the physical origin of the smaller band gap energy is associated
with the compressive stress effect. After annealing, the energy gap (Fig. 5) increased, which was associated with the relaxation of compressive stress. 4. Conclusion ZnO thin films were grown by RF magnetron sputtering, and the structural and optical properties were characterized. XRD showed that the as-grown thin films exhibited a ZnO(002) preferred orientation with in-plane compressive stress. After annealing, the FWHM of the ZnO(002) peak became sharper and the peak position moved to the bulk values, indicating an improvement in crystallization and a reduction of the in-plane stress. UVeVis spectroscopy showed that the slope of the thin films increased from 3.2 eV to 3.35 eV after annealing due to the improved crystallinity. The energy gap obtained from a linear extrapolation of the transmittance data showed the energy gap was smaller than that of the bulk value and increased after annealing. Therefore, the smaller energy gap for the as-grown and increased energy gap for the annealed thin films might be related to the in-plane stress effect. Acknowledgments This study was supported in part by NRF Korea (2012-005940, 2011-0031933, 2010-371-B00008, 2011-330-B00044). References
Fig. 4. Annealing effect of the vertical grain size of (002) orientation of ZnO thin films grown at (a) RT, (b) 200 C and (c) 400 C. The lines between data points are a guide to the eyes.
[1] A. Janotti, C.G. Van de Walle, Rep. Prog. Phys. 72 (2009) 126501. an, V. Avrutin, S.[2] Ü. Özgür, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [3] K. Tonooka, H. Bando, Y. Aiura, Thin Solid Films 445 (2) (2003) 327. [4] H. Hosono, Thin Solid Films 515 (15) (2007) 6000. [5] A. El Amrani, F. Hijazi, B. Lucas, J. Bouclé, M. Aldissi, Thin Solid Films 518 (16) (2010) 4582. [6] E. Fortunato, A. Gonçalves, A. Pimentel, P. Barquinha, G. Gonçalves, L. Pereira, I. Ferreira, R. Martins, Appl. Phys. A 96 (1) (2009) 197. [7] D.P. Norton, Y.W. Heo, M.P. Ivill, K. Ip, S.J. Pearton, M.F. Chisholm, T. Steiner, Mater. Today 7 (6) (2004) 34. [8] Y. Natsume, H. Sakata, Thin Solid Films 372 (2000) 30. [9] P.K. Nayak, J.H. Yang, J.W. Kim, S.J. Chung, J.W. Jeong, C.H. Lee, Y.T. Hong, J. Phys. D Appl. Phys. 42 (2009) 139801. [10] Z.L. Wang, J. Phys. Condens. Matter 16 (2004) R829. [11] H. Kim, A. Pique, J.S. Horwitz, H. Murata, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey, Thin Solid Films 377e378 (2000) 798. [12] B.K. Sharma, N. Khare, J. Phys. D Appl. Phys. 43 (2010) 465402.
Y.-E. Jeong, S. Park / Current Applied Physics 14 (2014) 30e33 [13] H.H. Shin, Y.H. Joung, S.J. Kang, J. Mater. Sci. Mater. Electron. 20 (2009) 704. [14] K.S. Jang, H.S. Park, S.W. Jung, N.V. Du, Y.K. Kim, J.H. Cho, H.W. Choi, T.Y. Kwon, W.B. Lee, D.Y. Gong, S.M. Park, J.S. Yi, D.Y. Kim, H.J. Kim, Thin Solid Films 518 (2010) 2808. [15] F.-J. Haug, Zs. Geller, H. Zogg, A.N. Tiwari, V. Vignali, J. Vac. Sci. Technol. A 19 (2001) 171. [16] O. Kappertz, R. Drese, M. Wuttig, J. Vac. Sci. Technol. A 20 (6) (2002) 2084. [17] A. Ievtushenko, V. Karpyna, G. Lashkarev, V. Lazorenko, V. Baturin, A. Karpenko, M. Lunika, A. Dan’ko, Acta Phys. Pol. A 114 (2008) 1131. [18] I. Ozen, M.A. Gulgun, Adv. Sci. Technol. 45 (2006) 1316. [19] W. Walter, S.Y. Chu, Mater. Lett. 55 (2002) 67. [20] W.M. Yim, R.J. Paff, Appl. Phys. Lett. 45 (1974) 1456. [21] L.P. Peng, L. Fang, X.F. Yang, Y.J. Li, Q.L. Huang, F. Wu, C.Y. Kong, J. Alloys Compd. 484 (2009) 575. [22] Z.B. Fang, Z.J. Yan, Y.S. Tan, X.Q. Liu, Y.Y. Wang, Appl. Surf. Sci. 241 (2005) 303. [23] M. Wang, J. Wang, W. Chen, Y. Cui, L. Wang, Mater. Chem. Phys. 97 (2006) 219. [24] ICSD #67849 or JCPDS #800075.
33
[25] V. Gupta, A. Mansingh, J. Appl. Phys. 80 (1996) 1063. [26] J.A. Thornton, D.W. Hoffman, Thin Solid Films 171 (1989) 5. [27] S.Y. Hu, Y.C. Lee, J.W. Lee, J.C. Huang, J.L. Shen, W. Water, Appl. Surf. Sci. 254 (2008) 1578. [28] Y.C. Lee, H.Y. Hu, W. Water, K.K. Tiong, Z.C. Feng, Y.T. Chen, J.C. Huang, J.W. Lee, C.C. Huang, J.L. Shen, M.H. Cheng, J. Lumin. 129 (2009) 148. [29] J. Tauc, A. Menth, J. Non-Cryst. Solids 8e10 (1972) 569. [30] S.G. Wang, Q. Zhang, S.F. Yoon, J. Ahn, Q. Wang, D.J. Yang, Q. Zhou, N.L. Yue, Opt. Mater. 24 (2003) 509. [31] E. Burstein, Phys. Rev. 93 (1954) 632. [32] T.S. Moss, Proc. Phys. Soc. 67 (1954) 775. [33] S.C. Jain, D.J. Roulston, Solid State Electron. 34 (1991) 453. [34] R.S. Ajimsha, A.K. Das, B.N. Singh, P. Misra, L.M. Kukreja, Phys. E 42 (2010) 1838. [35] T. Prasada Rao, M.C. Santhosh Kumar, S. Anbumozhi Angayarkanni, M. Ashok, J. Alloys Compd. 485 (2009) 413. [36] S. Gad, M.A. Rafea, Y. Badr, J. Alloys Compd. 515 (2012) 100.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Thermal stress induced band gap variation of ZnO thin films Y.-E. Jeong, S. Park* Department of Physics, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 September 2013 Accepted 2 October 2013 Available online 12 October 2013
The growth temperature and post annealing-dependent optical and structural effect of RF magnetron sputtered ZnO thin films were examined. As the growth temperature increased, the lattice constant increased and approached the bulk value, suggesting a decrease in interfacial strain between the substrate and thin film. For the post annealed samples, the interfacial strain decreased further and was close to the bulk value regardless of the post annealing environments (in air and O2). The optical properties of all ZnO thin films examined and revealed higher transparency (>90%). Furthermore, the optical band gap varied according to the growth temperature and post annealing environments due to a decrease in the interfacial strain effect. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: ZnO thin films Growth temperature Annealing environment Optical band gap
1. Introduction Oxide thin films with high optical transparency and proper electrical conduction can be used in a range of applications, such as flexible displays, solar cells, surface acoustic wave devices, gas sensors, piezoelectric devices, etc [1,2]. Among the many available transparent conducting oxide thin films, ZnO based thin films are the most commonly used materials in many those applications [2e 7]. ZnO, an IIeVI compound, has a hexagonal wurtzite structure and is an n-type semiconductor [8]. Conventional ZnO contains a large number of oxygen vacancies (Vo) and interstitials Zn (Zni) and has band gap of 3.37 eV [9e12]. Furthermore, ZnO has a larger exciton binding energy (w60 meV) at room temperature than of ZnSe (w20 meV) and GaN (w28 meV). Therefore, it has highly efficient luminescent properties [13,14]. In addition to its outstanding optical properties, ZnO thin films can be grown at low temperatures and has tunable mobility and resistivity depending on the amount of oxygen [10]. However, fluctuations of the physical properties of the ZnO thin films due to changes in the ratio between Zn and O when the films are exposed to air and the lack of thermal stability [15] at high temperature limit even further applications, such as replacing ITO-based applications. Many studies have characterized the variations in the physical properties of ZnO thin films depending on the growth environment, such as pressure, temperature, film thickness, substrates, etc [16e18]. For example, Kappertz et al., reported the changes in roughness and stress depending on
* Corresponding author. E-mail address: [email protected] (S. Park). 1567-1739/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.10.003
the total pressure and oxygen flow during the growth of polycrystalline ZnO and Al-doped ZnO thin films grown on Al2O3(0001) by DC magnetron sputtering [16]. Ievtushenko et al., also showed the film thickness-dependent strain effect of multilayerd ZnO thin films grown on a range of substrates, such as Si, glass, various orientations of Al2O3(0001) using RF magnetron sputtering [17]. This paper reports the structural and optical properties of ZnO thin films grown on Al2O3(0001) substrates at various growth temperatures and post annealing conditions to examine a correlation between thermal energy and physical properties. 2. Experiments ZnO thin films were deposited on Al2O3(0001) substrates by RF magnetron sputtering at various deposition temperatures, i.e. room temperature (RT), 200 C, 400 C. The deposition time was 10 min for all samples. The base and working pressure of the system were 5.0 108 Torr and 1.6 102 Torr, respectively. The sputtering power was 100 W. Post annealing was performed at 500 C for 1 min in a rate of 10 C/min in air and O2 by rapid thermal annealing (RTP-1200, NEXTRON). The structure of the thin films was examined by X-ray diffraction (XRD, Rigaku Dmax) and the optical properties of the films were measured using a UVeVis spectrometer (Monora-320i, Dongwoo Optron) in the range of 180 nme600 nm to obtain optical band gaps of the films. 3. Results and discussion Fig. 1 shows the room temperature XRD patterns of the ZnO thin films deposited at various deposition temperatures. All the
Y.-E. Jeong, S. Park / Current Applied Physics 14 (2014) 30e33
31
Fig. 2. Annealing effect of stress for the ZnO thin films grown at various temperature. The lines between the data points are a guide to the eyes. The horizontal dotted-line in the figure indicates zero stress.
absorption coefficient was obtained from the Tauc relation [29] as follows: Fig. 1. XRD patterns of ZnO thin films grown on Al2O3(0001) at (a) RT, (b) 200 C and (c) 400 C.
samples exhibited a (002) orientation because the (002) surface has the lowest surface free energy in the wurtzite structure [8]. As the growth temperature increases, the ZnO(002) peak position moved toward the stress free bulk value, suggesting some relaxation of the lattice. In general, the physical origin of the interfacial stress can be considered thermal stress due to the different thermal expansion coefficients [19,20], lattice mismatch due to the different lattice constants of the adjacent materials, and imperfection in the thin films during growth [21e23]. ZnO thin films grown at room temperature exhibits compressive stress due to the intrinsic effect originating from the deposition temperature, pressure, power, gas mixture, etc [19,25e27]. Therefore, the lower ZnO(002) peak position for the sample grown at room temperature (Fig. 1(a)) with respect to the ZnO bulk value (2q ¼ 34.4 ) [24] might be associated with compressive stress. Furthermore, the lattice relaxation of the sample grown at higher temperatures might be due to the different thermal expansion coefficients of ZnO and the substrate. Fig. 2 shows the in-plane stress, s, obtained from the ZnO(002) peak XRD profiles of the as-grown and annealed (both in air and O2 environment) ZnO thin films. The positive and negative values of the in-plane stress represent the tensile and compressive stress, respectively [28]. The calculated in-plane stress showed a negative value for the as-grown samples indicating the compressive stress for the as-grown ZnO thin films, regardless of the growth temperature. Furthermore, the in-plane compressive stress decreased regardless of the annealing environment. The reduction of stress was most prominent in the samples, grown at room temperature. This reduction behavior is similar to the samples grown at higher temperatures, suggesting both annealing and high temperature growth reduce the in-plane stress and promote grain growth. Fig. 3 shows the absorption coefficient of the as-grown and annealed ZnO thin films at various growth temperatures. The
ðahnÞn z hn Eg :
(1)
where hn is the incident photon energy, a is the absorption coefficient, and n is either 2 for a direct transition or 1/2 for an indirect transition. For the as-grown samples, the absorption coefficients between 3.2 eV and 3.35 eV changed rapidly. Furthermore, the amount of the change (slope) became steeper with increasing growth temperature (see - in Fig. 3). The change in the slope is related to the film quality and surface roughness [30]. Therefore, as the growth temperature increases, the thin films have a smoother surface and/or better structural ordering. Fig. 4 shows growth and annealing-dependent vertical grain size (inverse of the FWHM) of the ZnO(002) peak. For the as-grown samples, the grain size became slightly smaller as the growth temperature increased, indicating the improvement of the crystallinity with growth temperatures. Furthermore, the reduced in-plane stress with increasing growth temperature, shown in Fig. 2 might also be due to the reduced interface roughness which can be reflected as a smoother surface roughness. For the annealed thin films, the slope became even steeper and the FWHM (the inverse of grain size) of the ZnO(002) peaks from XRD measurement became smaller, suggesting improved surface roughness and crystallization for both annealing environments. The optical energy gap of the ZnO thin films can be extracted from a linear extrapolation of the absorption coefficient with n ¼ 2, suggesting that the thin films have a direct energy gap. Fig. 5 shows the optical energy gap of the as-grown and annealed ZnO thin films. For the as-grown ZnO thin films deposited at RT, 200 C, and 400 C, the optical energy gap was 3.119 eV, 3.136 eV and 3.123 eV, respectively. After annealing in air, the optical energy gap of the ZnO thin films, deposited at RT, 200 C, and 400 C was 3.141 eV, 3.192 eV and 3.153 eV, respectively. For the thin films annealed in an O2 environment, the optical energy gap of the ZnO thin films deposited at RT, 200 C, and 400 C was 3.134 eV, 3.185 eV and 3.152 eV, respectively. Therefore, the measured band gap energies for all samples are smaller than that of bulk ZnO (3.37 eV [9,11,12]),
32
Y.-E. Jeong, S. Park / Current Applied Physics 14 (2014) 30e33
Fig. 5. Annealing effect of the band gap energy of ZnO thin films grown at (a) RT, (b) 200 C and (c) 400 C. The lines between data points are a guide to the eyes.
Fig. 3. Annealing effect of the absorption coefficient of ZnO thin films grown at (a) RT, (b) 200 C and (c) 400 C. The solid lines are a linear extrapolation of the band gap estimation.
and the band gap energy increases after annealing regardless of the annealing environment. In general, the origin of the variation in the optical energy gap can be explained by the Burstein-Moss effect [31,32], band gap narrowing effect [33,34], and stress effect [35]. However, the Burstein-Moss effect can be applied to the increased band gap, and both the Burstein-Moss and band gap narrowing effect can also be applied to a higher carrier concentration system [36]. Furthermore, all thin films are electrically insulating (not shown here). Therefore, the physical origin of the smaller band gap energy is associated
with the compressive stress effect. After annealing, the energy gap (Fig. 5) increased, which was associated with the relaxation of compressive stress. 4. Conclusion ZnO thin films were grown by RF magnetron sputtering, and the structural and optical properties were characterized. XRD showed that the as-grown thin films exhibited a ZnO(002) preferred orientation with in-plane compressive stress. After annealing, the FWHM of the ZnO(002) peak became sharper and the peak position moved to the bulk values, indicating an improvement in crystallization and a reduction of the in-plane stress. UVeVis spectroscopy showed that the slope of the thin films increased from 3.2 eV to 3.35 eV after annealing due to the improved crystallinity. The energy gap obtained from a linear extrapolation of the transmittance data showed the energy gap was smaller than that of the bulk value and increased after annealing. Therefore, the smaller energy gap for the as-grown and increased energy gap for the annealed thin films might be related to the in-plane stress effect. Acknowledgments This study was supported in part by NRF Korea (2012-005940, 2011-0031933, 2010-371-B00008, 2011-330-B00044). References
Fig. 4. Annealing effect of the vertical grain size of (002) orientation of ZnO thin films grown at (a) RT, (b) 200 C and (c) 400 C. The lines between data points are a guide to the eyes.
[1] A. Janotti, C.G. Van de Walle, Rep. Prog. Phys. 72 (2009) 126501. an, V. Avrutin, S.[2] Ü. Özgür, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [3] K. Tonooka, H. Bando, Y. Aiura, Thin Solid Films 445 (2) (2003) 327. [4] H. Hosono, Thin Solid Films 515 (15) (2007) 6000. [5] A. El Amrani, F. Hijazi, B. Lucas, J. Bouclé, M. Aldissi, Thin Solid Films 518 (16) (2010) 4582. [6] E. Fortunato, A. Gonçalves, A. Pimentel, P. Barquinha, G. Gonçalves, L. Pereira, I. Ferreira, R. Martins, Appl. Phys. A 96 (1) (2009) 197. [7] D.P. Norton, Y.W. Heo, M.P. Ivill, K. Ip, S.J. Pearton, M.F. Chisholm, T. Steiner, Mater. Today 7 (6) (2004) 34. [8] Y. Natsume, H. Sakata, Thin Solid Films 372 (2000) 30. [9] P.K. Nayak, J.H. Yang, J.W. Kim, S.J. Chung, J.W. Jeong, C.H. Lee, Y.T. Hong, J. Phys. D Appl. Phys. 42 (2009) 139801. [10] Z.L. Wang, J. Phys. Condens. Matter 16 (2004) R829. [11] H. Kim, A. Pique, J.S. Horwitz, H. Murata, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey, Thin Solid Films 377e378 (2000) 798. [12] B.K. Sharma, N. Khare, J. Phys. D Appl. Phys. 43 (2010) 465402.
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