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The effects of ZnO layer and annealing temperature on the structure, optical and film–substrate cohesion properties of SiGe thin films prepared by radio frequency magnetron sputtering
Applied Surface Science 259 (2012) 393–398
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The effects of ZnO layer and annealing temperature on the structure, optical and film–substrate cohesion properties of SiGe thin films prepared by radio frequency magnetron sputtering Jinsong Liu a,b,∗ , Ziquan Li a , Kongjun Zhu b , Mingxia He a , Mengqi Cong a , Shuo Zhang a , Jie Peng a , Yani Liu a a b
College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Jiangsu, Nanjing 210016, China State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
a r t i c l e
i n f o
Article history: Received 26 November 2011 Received in revised form 11 July 2012 Accepted 13 July 2012 Available online 20 July 2012 Keywords: ZnO/SiGe thin films Magnetron sputtering Optical absorption Film–substrate cohesion
a b s t r a c t ZnO/SiGe thin films were prepared by radio frequency magnetron sputtering. The effects of the ZnO layer and the annealing temperature on the structure, optical absorption and film–substrate cohesion properties of the films were investigated by XRD, SEM, UV–vis and coating adhesion automatic scratch tester. The results indicated that the additional ZnO layer and the annealing behavior could effectively improve the crystallinity of the SiGe films, and enhance the optical absorption intensity or range of the films. The film–substrate cohesion property test showed that critical loading Lc values of the ZnO/SiGe films were almost in accordance with those of the SiGe films when annealing temperature Tan is increased to 700 and 800 ◦ C. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As an important semiconductor with high carrier mobility, Si1−x Gex alloy could modulate its optical properties by changing the germanium concentration, annealed temperature and annealed time, and has become a material of major importance for MOS, HBT technologies and optoelectronics [1–3]. To date, a number of methods, such as molecular beam epitaxy (MBE) [4,5], low pressure chemical vapor deposition (LPCVD) [6,7], plasma enhanced chemical vapor deposition (PECVD) [8,9] have been taken to prepare the Si1−x Gex film. In contrast to other methods, magnetron sputtering method [10,11] exhibits many advantages such as low deposition temperature, and high deposition rate. Moreover, the method could decrease the sputtering pressure, and obtain more dense and better adherent films [12,13], and has been extensively used in the synthesis of the Si1−x Gex film [14–16]. ZnO with a wide band gap of 3.37 eV is a promising semiconductor for a wide variety of applications such as UV light-emitting diodes and lasers, surface acoustic wave devices, and window material for display and solar cells [17–20]. Relevant research
∗ Corresponding author at: College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Jiangsu, Nanjing 210016, China. Tel.: +86 25 84895871; fax: +86 25 84895871. E-mail address: [email protected] (J. Liu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.057
indicated that the property of the film could be improved by a co-dopant approach with ZnO. For example, Sorar and SayginHinczewski prepared the thin films of Si-doped ZnO by the sol–gel spin coating method, and found that the optical property could be affected by Si dopant concentration [21]. Clatot et al. deposited ZnO:Si thin films under oxygen pressure using the pulsed laser deposition technique, and the results confirmed better optical property for SZO thin films with Si content of 1.0–1.5% [22]. Zheng et al. obtained Ge/ZnO multilayer films by radio frequency magnetron sputtering, and the results suggested that the Zn2 GeO4 formed in the process of annealing could improve PL property of the films [23]. However, few attentions have embarked on the effect of ZnO layer on the structure and optical absorption property of Si1−x Gex alloy thin films. In addition, Lattice mismatch between SiGe and ZnO can introduce crystal defects, such as misfit dislocations and threading dislocations in the microstructure, that degrade the quality of the resulting film. How to improve optical property and film–substrate cohesion together is a challenge. In this study, ZnO/SiGe thin films were deposited on the quartz glass substrate by radio frequency magnetron sputtering method. As comparison, SiGe films without ZnO layer were also deposited under the same condition. All films were then annealed at different temperatures (500–800 ◦ C). The results suggested that the optical absorption properties of the films were improved by introducing the ZnO layer into the SiGe films, while the film–substrate cohesion kept stable for the films annealed at 700 and 800 ◦ C.
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Fig. 1. XRD patterns of the films in as-grown state and annealed at different temperatures (a), (b), the dependence of XRD peak intensities on temperature (c), and an enlarge image (d) of the ellipsoid part in (c) shows the tendency clearly.
2. Experimental details The SiGe and the ZnO/SiGe thin films were prepared on quartz substrates by sputtering a ZnO target (99.99%) at 80 W and a SiGe target (99.999%) at 100 W alternately using radio frequency magnetron sputtering system (JGP-500) at substrate temperature of 300 and 400 ◦ C. The distance between the target and the substrate is 80 and 50 mm, respectively. The films were deposited at working pressure of 1.0 Pa with O2 /Ar = 1 and pure Ar ambient for 1 h for the ZnO and the SiGe thin films. The base pressure of the chamber is about 4 × 10−4 Pa. The thicknesses for the ZnO and the SiGe layer obtained were 130 nm and 1.1 m, respectively, which have been confirmed by ABIOS profilometer (XP-1). After the deposition, the films were annealed at Ar ambient for 30 min at various annealing temperature from 500 to 800 ◦ C in a horizontal quartz furnace. The structural properties were characterized by X-ray diffraction (XRD) with Bruker D8 Advance X-ray diffractometer equipped with Cu K␣ ( = 0.15418 nm) as the radiation. Scanning electron microscopy (SEM) images were taken with LEO1530VP field emission scanning electron microscopy. UV-2550 spectrophotometer was used to study the optical absorption property. WS-2005 coating adhesion automatic scratch tester was used to study the film–substrate cohesion. The range of automatic continuous load is from 0 N to 50 (or 70) N, and scratching speed is 1 mm/min, and loading rate is 10 N/min, and the scratching length is 5–7 mm. 3. Results and discussion 3.1. Structural properties Fig. 1a shows the XRD patterns from the as-grown and annealed SiGe films. The as-grown film exhibited only an amorphous peak (2 ≈ 22◦ ) corresponding to the quartz glass SiO2 . There is no other distinct peak appearing when Tan is lower than 700 ◦ C. At Tan = 700 ◦ C, three obvious peaks can be observed at 2 = 27.8◦ ,
46.5◦ , and 55.1◦ , and they are located between those of the standard Ge (JCPDF No. 04-0545: 2 = 27.3◦ , 45.3◦ and 53.7◦ ) and Si (JCPDF No. 27-1402: 2 = 28.4◦ , 47.3◦ and 56.1◦ ), which confirms the formation of the SiGe alloy. The three distinct peaks correspond to the diffraction peaks of the SiGe (1 1 1), (2 2 0) and (3 1 1), respectively, and it is also an evident that amorphous SiGe layer transforms to polycrystalline. With Tan increasing continually, the diffraction intensity of the SiGe peaks becomes much higher. XRD patterns of the as-grown and annealed ZnO/SiGe films are shown in Fig. 1b, the changing tendency of the SiGe diffraction peaks is similar to that of the SiGe films. The strong peaks of ZnO (0 0 2) located at 2 ≈ 34◦ shift toward larger angle apparently with increasing of annealed temperature. This indicates that ZnO has a better crystallinity and some internal stress in the film has been released. Relatively, the diffraction peak’s intensity of the ZnO/SiGe film is always stronger than that of the SiGe film for the same crystal plane, which can be seen clearly in Fig. 1c and d. The fact reveals that ZnO layer with (0 0 2) preferred orientation can provide a suitable condition for crystallization of the SiGe during annealing process. The crystallite size (d) of the film could be estimated from the FWHM (full width at half maximum) of (1 1 1) XRD characteristic peaks using the Scherrer formula: d=
k B cos
where is the wavelength (0.154 nm) of the X ray used, is the angle satisfying Bragg’s law, and B is the corrected FWHM in radian. The crystallite diameters of the SiGe and the ZnO/SiGe films annealed are calculated to be 9.4, 15.7 nm at 700 ◦ C, and 10.4, 20.8 nm at 800 ◦ C, respectively. As expected, the addition of the ZnO layer leads to an increase of the crystallite size at the same annealing temperature. The crystallinity is estimated by the Jade 5.0 analyzing software (Table 1), and the values are 9.2%, 19.1%, 17.5% and 43.6% for the SiGe and the ZnO/SiGe films at 700 and
J. Liu et al. / Applied Surface Science 259 (2012) 393–398 Table 1 Crystallinity and the Si to Ge atomic ratio for the SiGe and the ZnO/SiGe films. Sample
SiGe film
Temperature (◦ C) Crystallinity (%) At% (Si/Ge)
700 9.2 51.6:48.4
ZnO/SiGe film 800 19.1 51.5:48.5
700 17.5 51.5:48.5
800 43.6 52.4:47.6
800 ◦ C, respectively, which shows that both the ZnO layer and the annealing temperature affect the crystallinity of the films. To obtain further information on the crystalline structure, the temperature–dh k l (interplanar spacing) curves of the ZnO and SiGe in the films were constructed (Fig. 2). It can be seen from Fig. 2a that d0 0 2 of the ZnO is 0.260 nm for the as-grown film by Bragg’s law, and the d0 0 2 value shows very little decreasing after annealed at different temperatures. Based on the d0 0 2 value and lactic distance formulas of hexagonal crystal structure (Fig. 2b), the lattice constants of the ZnO layer are calculated and shown in the figure. The ˛ and c values of the films show very little change, and they are almost the same in compared to the standard values of ZnO (˛ = 0.324 nm, c = 0.519 nm). The same phenomenon occurred with the lattice constant of the SiGe (shown in Fig. 2c and d). The d1 1 1 value seems to be not affected by the annealing temperature, and it slightly decreases with increasing temperature. The ˛ value calculated by the formulas of face-center cubic structure is also very little difference each other. This indicates that the films possess good structural stability. In addition, from the Tables it could be found that there is a large lattice mismatch between SiGe and ZnO, which would lead to the rearrangement of the interface atom, affecting the properties of the films. 3.2. Surface and cross section morphology The morphology of the films in as-grown state and annealed at 800 ◦ C has been characterized by FESEM, as shown in Fig. 3.
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The images of the as-grown (Fig. 3a and b) depict amorphous nature of SiGe, whereas the films annealed at 800 ◦ C (Fig. 3c and d) illustrate the presence of SiGe nanocrystals, which appear as spherical clusters and their agglomerates. The crystallite sizes of SiGe and ZnO/SiGe films lie in the range of 8–14 nm and 16–25 nm, respectively, which is in agreement with the XRD results. The cross section images (Fig. 3c and d) show the interface between ZnO and SiGe, and the distinct interface atom diffusing could be observed at 800 ◦ C. For evaluating the quality of crystallinity in the film further, the content of Si and Ge was measured for the films annealed at 700 and 800 ◦ C using EDX at beam energy of 10 keV. The Si to Ge atomic ratio of SiGe and ZnO/SiGe films is about 51.6:48.4, 51.5:48.5 at 700 ◦ C, and 51.5:48.5, 52.4:47.6 at 800 ◦ C, respectively (Table 1). The ratios are very close to the ideal value (1:1) implying the films have a uniform element distribution.
3.3. Optical absorption properties Optical absorption, transmittance and reflectance have been widely used as an effective characterization method for display and solar cells window film materials. The characteristics of the UV–vis spectra for the SiGe and the ZnO/SiGe films annealed at various temperatures were recorded, as shown in Fig. 4. The curves can be separated roughly into three regions of interest: strong, exponent, and weak absorption region. Comparing the UV–vis spectra of the SiGe films in as-grown state and annealed at 600 ◦ C, the strong absorption range is enlarged from 600 to 660 nm, and the absorption intensity almost keeps no change (Fig. 4a and b), which suggests the SiGe atoms’ diffusing during the heating-treatment, resulting in the wider absorption range. When Tan up to 700 ◦ C, the intensity of the UV–vis curve becomes stronger whereas the absorption wavelength range reduces more or less comparing with those at Tan = 600 ◦ C. That may come from the transition of the film microstructure at 700 ◦ C, which agrees well with the result of XRD.
Fig. 2. Temperature–d0 0 2 curve and crystal structure of ZnO layer (a), (b) and temperature–d1 1 1 curve and crystal structure of SiGe alloy layer (c), (d).
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Fig. 3. FESEM images of the SiGe (a) and ZnO/SiGe (b) films in as-grown state, the SiGe (c) and ZnO/SiGe (d) films annealed at 800 ◦ C.
Both the intensity and absorption wavelength range increased with Tan increasing to 800 ◦ C. The strong UV–vis absorption range (to ∼690 nm) of the ZnO/SiGe film in as-grown state is almost same to that of the film annealed at 600 ◦ C whereas the absorption intensity is increased (Fig. 4c and d). Changing regularity of UV–vis spectra for the ZnO/SiGe films annealed at 700 and 800 ◦ C is similar to that for the SiGe films. Comparing the UV–vis spectra of the SiGe films and
the ZnO/SiGe films obtained under same conditions, after the ZnO layer is introduced into the film, both the absorption intensity and the absorption range are stronger except that only the absorption range is enlarged for the films in as-grown state. This is because the absorption intensity and the absorption range are affected by the degree of crystallinity, and both ZnO introducing into the film and different annealing temperatures could change the degree of crystallinity. The results give us a simple way to modulate optical
Fig. 4. UV–vis absorption spectra (a) and strong UV–vis absorption and strong absorbance distribution (b) of the films in as-grown state and annealed at different temperatures.
J. Liu et al. / Applied Surface Science 259 (2012) 393–398
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cohesion is improved with the increasing of the annealing temperature, coming from atoms realigning after annealed. For the ZnO/SiGe films, same phenomenon occurs (shown in Fig. 5b). The Lc values of the ZnO/SiGe films in as-grown state and annealed at 600 ◦ C are 1.75 and 8.05 N, respectively. Comparing the critical loading values of the films, it can be seen that the Lc values of the ZnO/SiGe films are always lower than those of the SiGe films under the conditions of as-grown state and annealed at 600 ◦ C due to more interface defects (Fig. 5c). When Tan is increased to 700 and 800 ◦ C, Lc values of the ZnO/SiGe films are 20.50 and 20.65 N, which is almost in accordance with those of the SiGe films, demonstrating the eliminating of the interface defects. The results show that introducing the ZnO layer into the SiGe film improved the optical absorption property in the condition of keeping good film–substrate cohesion after annealed at 700 and 800 ◦ C. 4. Conclusion The SiGe and ZnO/SiGe thin films were prepared by radio frequency magnetron sputtering method. XRD, SEM and UV–vis spectra characterizations indicated that the ZnO layer and the annealing behavior could effectively improve the crystallinity of the films, and enhance the optical absorption intensity or range of the films. The film–substrate cohesion test showed that Lc values of the ZnO/SiGe films were in accordance with those of the SiGe films when Tan is increased to 700 and 800 ◦ C. Acknowledgments The work is supported by Natural Science Foundation of Jiangsu Province (BK2009379), Introducing Talents Funds of Nanjing university of Aeronautics and Astronautics (1006-909308) and Nanjing university of Aeronautics and Astronautics Research Funding (1006-56XNA12069). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. apsusc.2012.07.057. References
Fig. 5. Loading curves (a), (b) and curve of Lc –Tan (c) of the films in as-grown state and annealed at different temperatures.
absorption property of the SiGe and ZnO/SiGe films to satisfy some factual demands for different devices. 3.4. Film–substrate cohesion properties To explore the effect of lattice mismatch between SiGe and ZnO on the films, the film–substrate cohesion of the films in the process of annealing has been investigated, and the loading curves are shown in Fig. 5. Lc is the value of the critical load where the films begin to spall off the substrate. Fig. 5a shows that Lc of the SiGe films in as-grown state is ∼4.85 N, and it is quickly increased to 19.85 N after annealed at 600 ◦ C. Then Lc almost keeps no change with increasing temperature, and the value is 20.55 and 20.85 N at 700 and 800 ◦ C, respectively. This indicates that the film–substrate
[1] A. Mikami, T. Okazawa, K. Saito, Y. Kido, Applied Surface Science 253 (2006) 1620–1625. [2] K. Oda, M. Miura, H. Shimamoto, K. Washio, Applied Surface Science 254 (2008) 6017–6020. [3] M. Willander, Q.X. Zhao, O. Nur, Q.H. Hu, Journal of Electronic Materials 34 (2005) 515–521. [4] Y. Kamiura, T. Ishiyama, S. Yoneyama, Y. Fukui, Materials Science and Engineering B 146 (2008) 135–140. [5] W.X. Ni, G.V. Hansson, I.A. Buyanova, A. Henry, W.M. Chen, B. Momemar, Applied Physics Letters 68 (1996) 238–240. [6] M. Avella, A.C. Prieto, J. Jimenez, A. Rodriguez, J. Sangrador, T. Rodriguez, M.I. Qrtiz, C. Ballesteros, Materials Science and Engineering B 147 (2008) 200–204. [7] G.W. Wang, X.P. Qu, G.P. Ru, H.X. Zheng, B.Z. Li, Microelectronics 37 (2007) 168–172. [8] P. Chaudhuri, A. Bhaduri, D.L. Williamson, Solar Energy Materials and Solar Cells 93 (2009) 1016–1019. [9] Z.G. Tang, W.B. Wang, D.S. Wang, D.Q. Liu, Q.M. Liu, D.Y. He, Journal of Alloys and Compounds 504 (2010) 403–406. [10] B.K. Sarma, A.R. Pal, H. Bailung, J. Chutia, Applied Surface Science 258 (2012) 5659–5665. [11] A.R. Pal, B.K. Sarma, N.C. Adhikary, J. Chutia, H. Bailung, Applied Surface Science 258 (2011) 1199–1205. [12] G. Brauer, B. Szyszka, M. Vergöhl, R. Bandorf, Vacuum 84 (2010) 1354–1359. [13] A.P. Ehiasarian, A. Anders, I. Petrov, Journal of Vacuum Science & Technology A 25 (2007) 543–550. [14] C.Y. Tsao, Z.H. Liu, X.J. Hao, M.A. Green, Applied Surface Science 257 (2011) 4354–4359. [15] H. Wang, X.J. Yang, S.P. Liu, Infrared Laser Engineering 36 (2007) 464–466.
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[16] I. Nakamura, T. Ajiki, H. Abe, D. Hoshi, M. Isomura, Vacuum 80 (2006) 712–715. [17] Y. Harada, S. Hashimoto, Physical Review B 68 (2003) 045421–45424. [18] J.A. Sung, Applied Physics Letters 100 (2012) 223115–223125. [19] S.S. Kanmani, K. Ramachandran, Renewable Energy 43 (2012) 149–156. [20] S.K. Panda, C. Jacob, Solid-State Electronics 73 (2012) 44–50.
[21] I. Sorar, D. Saygin-Hinczewski, M. Hinczewski, F.Z. Tepehan, Applied Surface Science 257 (2011) 7343–7349. [22] J. Clatot, G. Campet, A. Zeinert, C. Labrugere, M. Nistor, A. Rougier, Solar Energy Materials and Solar Cells 95 (2011) 2357–2362. [23] T.H. Zheng, Z.Q. Li, J.K. Chen, K. Shen, K.F. Sun, Applied Surface Science 252 (2006) 8482–8486.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The effects of ZnO layer and annealing temperature on the structure, optical and film–substrate cohesion properties of SiGe thin films prepared by radio frequency magnetron sputtering Jinsong Liu a,b,∗ , Ziquan Li a , Kongjun Zhu b , Mingxia He a , Mengqi Cong a , Shuo Zhang a , Jie Peng a , Yani Liu a a b
College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Jiangsu, Nanjing 210016, China State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
a r t i c l e
i n f o
Article history: Received 26 November 2011 Received in revised form 11 July 2012 Accepted 13 July 2012 Available online 20 July 2012 Keywords: ZnO/SiGe thin films Magnetron sputtering Optical absorption Film–substrate cohesion
a b s t r a c t ZnO/SiGe thin films were prepared by radio frequency magnetron sputtering. The effects of the ZnO layer and the annealing temperature on the structure, optical absorption and film–substrate cohesion properties of the films were investigated by XRD, SEM, UV–vis and coating adhesion automatic scratch tester. The results indicated that the additional ZnO layer and the annealing behavior could effectively improve the crystallinity of the SiGe films, and enhance the optical absorption intensity or range of the films. The film–substrate cohesion property test showed that critical loading Lc values of the ZnO/SiGe films were almost in accordance with those of the SiGe films when annealing temperature Tan is increased to 700 and 800 ◦ C. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As an important semiconductor with high carrier mobility, Si1−x Gex alloy could modulate its optical properties by changing the germanium concentration, annealed temperature and annealed time, and has become a material of major importance for MOS, HBT technologies and optoelectronics [1–3]. To date, a number of methods, such as molecular beam epitaxy (MBE) [4,5], low pressure chemical vapor deposition (LPCVD) [6,7], plasma enhanced chemical vapor deposition (PECVD) [8,9] have been taken to prepare the Si1−x Gex film. In contrast to other methods, magnetron sputtering method [10,11] exhibits many advantages such as low deposition temperature, and high deposition rate. Moreover, the method could decrease the sputtering pressure, and obtain more dense and better adherent films [12,13], and has been extensively used in the synthesis of the Si1−x Gex film [14–16]. ZnO with a wide band gap of 3.37 eV is a promising semiconductor for a wide variety of applications such as UV light-emitting diodes and lasers, surface acoustic wave devices, and window material for display and solar cells [17–20]. Relevant research
∗ Corresponding author at: College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Jiangsu, Nanjing 210016, China. Tel.: +86 25 84895871; fax: +86 25 84895871. E-mail address: [email protected] (J. Liu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.057
indicated that the property of the film could be improved by a co-dopant approach with ZnO. For example, Sorar and SayginHinczewski prepared the thin films of Si-doped ZnO by the sol–gel spin coating method, and found that the optical property could be affected by Si dopant concentration [21]. Clatot et al. deposited ZnO:Si thin films under oxygen pressure using the pulsed laser deposition technique, and the results confirmed better optical property for SZO thin films with Si content of 1.0–1.5% [22]. Zheng et al. obtained Ge/ZnO multilayer films by radio frequency magnetron sputtering, and the results suggested that the Zn2 GeO4 formed in the process of annealing could improve PL property of the films [23]. However, few attentions have embarked on the effect of ZnO layer on the structure and optical absorption property of Si1−x Gex alloy thin films. In addition, Lattice mismatch between SiGe and ZnO can introduce crystal defects, such as misfit dislocations and threading dislocations in the microstructure, that degrade the quality of the resulting film. How to improve optical property and film–substrate cohesion together is a challenge. In this study, ZnO/SiGe thin films were deposited on the quartz glass substrate by radio frequency magnetron sputtering method. As comparison, SiGe films without ZnO layer were also deposited under the same condition. All films were then annealed at different temperatures (500–800 ◦ C). The results suggested that the optical absorption properties of the films were improved by introducing the ZnO layer into the SiGe films, while the film–substrate cohesion kept stable for the films annealed at 700 and 800 ◦ C.
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Fig. 1. XRD patterns of the films in as-grown state and annealed at different temperatures (a), (b), the dependence of XRD peak intensities on temperature (c), and an enlarge image (d) of the ellipsoid part in (c) shows the tendency clearly.
2. Experimental details The SiGe and the ZnO/SiGe thin films were prepared on quartz substrates by sputtering a ZnO target (99.99%) at 80 W and a SiGe target (99.999%) at 100 W alternately using radio frequency magnetron sputtering system (JGP-500) at substrate temperature of 300 and 400 ◦ C. The distance between the target and the substrate is 80 and 50 mm, respectively. The films were deposited at working pressure of 1.0 Pa with O2 /Ar = 1 and pure Ar ambient for 1 h for the ZnO and the SiGe thin films. The base pressure of the chamber is about 4 × 10−4 Pa. The thicknesses for the ZnO and the SiGe layer obtained were 130 nm and 1.1 m, respectively, which have been confirmed by ABIOS profilometer (XP-1). After the deposition, the films were annealed at Ar ambient for 30 min at various annealing temperature from 500 to 800 ◦ C in a horizontal quartz furnace. The structural properties were characterized by X-ray diffraction (XRD) with Bruker D8 Advance X-ray diffractometer equipped with Cu K␣ ( = 0.15418 nm) as the radiation. Scanning electron microscopy (SEM) images were taken with LEO1530VP field emission scanning electron microscopy. UV-2550 spectrophotometer was used to study the optical absorption property. WS-2005 coating adhesion automatic scratch tester was used to study the film–substrate cohesion. The range of automatic continuous load is from 0 N to 50 (or 70) N, and scratching speed is 1 mm/min, and loading rate is 10 N/min, and the scratching length is 5–7 mm. 3. Results and discussion 3.1. Structural properties Fig. 1a shows the XRD patterns from the as-grown and annealed SiGe films. The as-grown film exhibited only an amorphous peak (2 ≈ 22◦ ) corresponding to the quartz glass SiO2 . There is no other distinct peak appearing when Tan is lower than 700 ◦ C. At Tan = 700 ◦ C, three obvious peaks can be observed at 2 = 27.8◦ ,
46.5◦ , and 55.1◦ , and they are located between those of the standard Ge (JCPDF No. 04-0545: 2 = 27.3◦ , 45.3◦ and 53.7◦ ) and Si (JCPDF No. 27-1402: 2 = 28.4◦ , 47.3◦ and 56.1◦ ), which confirms the formation of the SiGe alloy. The three distinct peaks correspond to the diffraction peaks of the SiGe (1 1 1), (2 2 0) and (3 1 1), respectively, and it is also an evident that amorphous SiGe layer transforms to polycrystalline. With Tan increasing continually, the diffraction intensity of the SiGe peaks becomes much higher. XRD patterns of the as-grown and annealed ZnO/SiGe films are shown in Fig. 1b, the changing tendency of the SiGe diffraction peaks is similar to that of the SiGe films. The strong peaks of ZnO (0 0 2) located at 2 ≈ 34◦ shift toward larger angle apparently with increasing of annealed temperature. This indicates that ZnO has a better crystallinity and some internal stress in the film has been released. Relatively, the diffraction peak’s intensity of the ZnO/SiGe film is always stronger than that of the SiGe film for the same crystal plane, which can be seen clearly in Fig. 1c and d. The fact reveals that ZnO layer with (0 0 2) preferred orientation can provide a suitable condition for crystallization of the SiGe during annealing process. The crystallite size (d) of the film could be estimated from the FWHM (full width at half maximum) of (1 1 1) XRD characteristic peaks using the Scherrer formula: d=
k B cos
where is the wavelength (0.154 nm) of the X ray used, is the angle satisfying Bragg’s law, and B is the corrected FWHM in radian. The crystallite diameters of the SiGe and the ZnO/SiGe films annealed are calculated to be 9.4, 15.7 nm at 700 ◦ C, and 10.4, 20.8 nm at 800 ◦ C, respectively. As expected, the addition of the ZnO layer leads to an increase of the crystallite size at the same annealing temperature. The crystallinity is estimated by the Jade 5.0 analyzing software (Table 1), and the values are 9.2%, 19.1%, 17.5% and 43.6% for the SiGe and the ZnO/SiGe films at 700 and
J. Liu et al. / Applied Surface Science 259 (2012) 393–398 Table 1 Crystallinity and the Si to Ge atomic ratio for the SiGe and the ZnO/SiGe films. Sample
SiGe film
Temperature (◦ C) Crystallinity (%) At% (Si/Ge)
700 9.2 51.6:48.4
ZnO/SiGe film 800 19.1 51.5:48.5
700 17.5 51.5:48.5
800 43.6 52.4:47.6
800 ◦ C, respectively, which shows that both the ZnO layer and the annealing temperature affect the crystallinity of the films. To obtain further information on the crystalline structure, the temperature–dh k l (interplanar spacing) curves of the ZnO and SiGe in the films were constructed (Fig. 2). It can be seen from Fig. 2a that d0 0 2 of the ZnO is 0.260 nm for the as-grown film by Bragg’s law, and the d0 0 2 value shows very little decreasing after annealed at different temperatures. Based on the d0 0 2 value and lactic distance formulas of hexagonal crystal structure (Fig. 2b), the lattice constants of the ZnO layer are calculated and shown in the figure. The ˛ and c values of the films show very little change, and they are almost the same in compared to the standard values of ZnO (˛ = 0.324 nm, c = 0.519 nm). The same phenomenon occurred with the lattice constant of the SiGe (shown in Fig. 2c and d). The d1 1 1 value seems to be not affected by the annealing temperature, and it slightly decreases with increasing temperature. The ˛ value calculated by the formulas of face-center cubic structure is also very little difference each other. This indicates that the films possess good structural stability. In addition, from the Tables it could be found that there is a large lattice mismatch between SiGe and ZnO, which would lead to the rearrangement of the interface atom, affecting the properties of the films. 3.2. Surface and cross section morphology The morphology of the films in as-grown state and annealed at 800 ◦ C has been characterized by FESEM, as shown in Fig. 3.
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The images of the as-grown (Fig. 3a and b) depict amorphous nature of SiGe, whereas the films annealed at 800 ◦ C (Fig. 3c and d) illustrate the presence of SiGe nanocrystals, which appear as spherical clusters and their agglomerates. The crystallite sizes of SiGe and ZnO/SiGe films lie in the range of 8–14 nm and 16–25 nm, respectively, which is in agreement with the XRD results. The cross section images (Fig. 3c and d) show the interface between ZnO and SiGe, and the distinct interface atom diffusing could be observed at 800 ◦ C. For evaluating the quality of crystallinity in the film further, the content of Si and Ge was measured for the films annealed at 700 and 800 ◦ C using EDX at beam energy of 10 keV. The Si to Ge atomic ratio of SiGe and ZnO/SiGe films is about 51.6:48.4, 51.5:48.5 at 700 ◦ C, and 51.5:48.5, 52.4:47.6 at 800 ◦ C, respectively (Table 1). The ratios are very close to the ideal value (1:1) implying the films have a uniform element distribution.
3.3. Optical absorption properties Optical absorption, transmittance and reflectance have been widely used as an effective characterization method for display and solar cells window film materials. The characteristics of the UV–vis spectra for the SiGe and the ZnO/SiGe films annealed at various temperatures were recorded, as shown in Fig. 4. The curves can be separated roughly into three regions of interest: strong, exponent, and weak absorption region. Comparing the UV–vis spectra of the SiGe films in as-grown state and annealed at 600 ◦ C, the strong absorption range is enlarged from 600 to 660 nm, and the absorption intensity almost keeps no change (Fig. 4a and b), which suggests the SiGe atoms’ diffusing during the heating-treatment, resulting in the wider absorption range. When Tan up to 700 ◦ C, the intensity of the UV–vis curve becomes stronger whereas the absorption wavelength range reduces more or less comparing with those at Tan = 600 ◦ C. That may come from the transition of the film microstructure at 700 ◦ C, which agrees well with the result of XRD.
Fig. 2. Temperature–d0 0 2 curve and crystal structure of ZnO layer (a), (b) and temperature–d1 1 1 curve and crystal structure of SiGe alloy layer (c), (d).
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Fig. 3. FESEM images of the SiGe (a) and ZnO/SiGe (b) films in as-grown state, the SiGe (c) and ZnO/SiGe (d) films annealed at 800 ◦ C.
Both the intensity and absorption wavelength range increased with Tan increasing to 800 ◦ C. The strong UV–vis absorption range (to ∼690 nm) of the ZnO/SiGe film in as-grown state is almost same to that of the film annealed at 600 ◦ C whereas the absorption intensity is increased (Fig. 4c and d). Changing regularity of UV–vis spectra for the ZnO/SiGe films annealed at 700 and 800 ◦ C is similar to that for the SiGe films. Comparing the UV–vis spectra of the SiGe films and
the ZnO/SiGe films obtained under same conditions, after the ZnO layer is introduced into the film, both the absorption intensity and the absorption range are stronger except that only the absorption range is enlarged for the films in as-grown state. This is because the absorption intensity and the absorption range are affected by the degree of crystallinity, and both ZnO introducing into the film and different annealing temperatures could change the degree of crystallinity. The results give us a simple way to modulate optical
Fig. 4. UV–vis absorption spectra (a) and strong UV–vis absorption and strong absorbance distribution (b) of the films in as-grown state and annealed at different temperatures.
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cohesion is improved with the increasing of the annealing temperature, coming from atoms realigning after annealed. For the ZnO/SiGe films, same phenomenon occurs (shown in Fig. 5b). The Lc values of the ZnO/SiGe films in as-grown state and annealed at 600 ◦ C are 1.75 and 8.05 N, respectively. Comparing the critical loading values of the films, it can be seen that the Lc values of the ZnO/SiGe films are always lower than those of the SiGe films under the conditions of as-grown state and annealed at 600 ◦ C due to more interface defects (Fig. 5c). When Tan is increased to 700 and 800 ◦ C, Lc values of the ZnO/SiGe films are 20.50 and 20.65 N, which is almost in accordance with those of the SiGe films, demonstrating the eliminating of the interface defects. The results show that introducing the ZnO layer into the SiGe film improved the optical absorption property in the condition of keeping good film–substrate cohesion after annealed at 700 and 800 ◦ C. 4. Conclusion The SiGe and ZnO/SiGe thin films were prepared by radio frequency magnetron sputtering method. XRD, SEM and UV–vis spectra characterizations indicated that the ZnO layer and the annealing behavior could effectively improve the crystallinity of the films, and enhance the optical absorption intensity or range of the films. The film–substrate cohesion test showed that Lc values of the ZnO/SiGe films were in accordance with those of the SiGe films when Tan is increased to 700 and 800 ◦ C. Acknowledgments The work is supported by Natural Science Foundation of Jiangsu Province (BK2009379), Introducing Talents Funds of Nanjing university of Aeronautics and Astronautics (1006-909308) and Nanjing university of Aeronautics and Astronautics Research Funding (1006-56XNA12069). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. apsusc.2012.07.057. References
Fig. 5. Loading curves (a), (b) and curve of Lc –Tan (c) of the films in as-grown state and annealed at different temperatures.
absorption property of the SiGe and ZnO/SiGe films to satisfy some factual demands for different devices. 3.4. Film–substrate cohesion properties To explore the effect of lattice mismatch between SiGe and ZnO on the films, the film–substrate cohesion of the films in the process of annealing has been investigated, and the loading curves are shown in Fig. 5. Lc is the value of the critical load where the films begin to spall off the substrate. Fig. 5a shows that Lc of the SiGe films in as-grown state is ∼4.85 N, and it is quickly increased to 19.85 N after annealed at 600 ◦ C. Then Lc almost keeps no change with increasing temperature, and the value is 20.55 and 20.85 N at 700 and 800 ◦ C, respectively. This indicates that the film–substrate
[1] A. Mikami, T. Okazawa, K. Saito, Y. Kido, Applied Surface Science 253 (2006) 1620–1625. [2] K. Oda, M. Miura, H. Shimamoto, K. Washio, Applied Surface Science 254 (2008) 6017–6020. [3] M. Willander, Q.X. Zhao, O. Nur, Q.H. Hu, Journal of Electronic Materials 34 (2005) 515–521. [4] Y. Kamiura, T. Ishiyama, S. Yoneyama, Y. Fukui, Materials Science and Engineering B 146 (2008) 135–140. [5] W.X. Ni, G.V. Hansson, I.A. Buyanova, A. Henry, W.M. Chen, B. Momemar, Applied Physics Letters 68 (1996) 238–240. [6] M. Avella, A.C. Prieto, J. Jimenez, A. Rodriguez, J. Sangrador, T. Rodriguez, M.I. Qrtiz, C. Ballesteros, Materials Science and Engineering B 147 (2008) 200–204. [7] G.W. Wang, X.P. Qu, G.P. Ru, H.X. Zheng, B.Z. Li, Microelectronics 37 (2007) 168–172. [8] P. Chaudhuri, A. Bhaduri, D.L. Williamson, Solar Energy Materials and Solar Cells 93 (2009) 1016–1019. [9] Z.G. Tang, W.B. Wang, D.S. Wang, D.Q. Liu, Q.M. Liu, D.Y. He, Journal of Alloys and Compounds 504 (2010) 403–406. [10] B.K. Sarma, A.R. Pal, H. Bailung, J. Chutia, Applied Surface Science 258 (2012) 5659–5665. [11] A.R. Pal, B.K. Sarma, N.C. Adhikary, J. Chutia, H. Bailung, Applied Surface Science 258 (2011) 1199–1205. [12] G. Brauer, B. Szyszka, M. Vergöhl, R. Bandorf, Vacuum 84 (2010) 1354–1359. [13] A.P. Ehiasarian, A. Anders, I. Petrov, Journal of Vacuum Science & Technology A 25 (2007) 543–550. [14] C.Y. Tsao, Z.H. Liu, X.J. Hao, M.A. Green, Applied Surface Science 257 (2011) 4354–4359. [15] H. Wang, X.J. Yang, S.P. Liu, Infrared Laser Engineering 36 (2007) 464–466.
398
J. Liu et al. / Applied Surface Science 259 (2012) 393–398
[16] I. Nakamura, T. Ajiki, H. Abe, D. Hoshi, M. Isomura, Vacuum 80 (2006) 712–715. [17] Y. Harada, S. Hashimoto, Physical Review B 68 (2003) 045421–45424. [18] J.A. Sung, Applied Physics Letters 100 (2012) 223115–223125. [19] S.S. Kanmani, K. Ramachandran, Renewable Energy 43 (2012) 149–156. [20] S.K. Panda, C. Jacob, Solid-State Electronics 73 (2012) 44–50.
[21] I. Sorar, D. Saygin-Hinczewski, M. Hinczewski, F.Z. Tepehan, Applied Surface Science 257 (2011) 7343–7349. [22] J. Clatot, G. Campet, A. Zeinert, C. Labrugere, M. Nistor, A. Rougier, Solar Energy Materials and Solar Cells 95 (2011) 2357–2362. [23] T.H. Zheng, Z.Q. Li, J.K. Chen, K. Shen, K.F. Sun, Applied Surface Science 252 (2006) 8482–8486.