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Nanoindentation-induced interfacial fracture of ZnO thin films deposited on Si(1 1 1) substrates by atomic layer deposition
Journal of Alloys and Compounds 587 (2014) 313–317
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Nanoindentation-induced interfacial fracture of ZnO thin films deposited on Si(1 1 1) substrates by atomic layer deposition Sheng-Rui Jian ⇑, Ya-Hui Lee Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan
a r t i c l e
i n f o
Article history: Received 13 October 2013 Received in revised form 28 October 2013 Accepted 28 October 2013 Available online 4 November 2013 Keywords: ZnO thin films Atomic layer deposition Nanoindentation Interfacial fracture
a b s t r a c t In this study, the structural and nanomechanical characteristics of ZnO thin films are investigated by mans of X-ray diffraction (XRD), atomic force microscopy (AFM) and nanoindentation techniques. The ZnO thin films are deposited on Si(1 1 1) substrates by using atomic layer deposition (ALD). The interfacial fracture behaviors for ALD-derived ZnO thin films are characterized by Berkovich nanoindentation, and the morphologies of indentations are revealed by using scanning electron microscopy (SEM). During nanoindentation, the interfacial fracture of ZnO thin films is significantly observed as the indentation load reaches 50 mN and, the corresponding loading segment in the load–displacement curve displays an obvious discontinuity event. Based on the analysis of the energy release in cracking, the fracture toughness of ALD-derived ZnO thin films deposited on Si(1 1 1) substrates is calculated. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The II–VI wurtzite ZnO semiconductor has a wide direct band gap of 3.4 eV with an exciton binding energy of 60 meV, making it a competitive candidate for applications in transparent thin film transistors, UV/blue light-emitting diodes and lasers at high temperatures [1,2]. Recently, much attention has been paid to heteroepitaxially grown ZnO on Si substrates because of the unique possibility of integrating well-established Si electronics with ZnO-based optoelectronic devices. Nevertheless, the successful fabrication of devices based on ZnO thin films requires better understanding of the mechanical characteristics in addition to its optical and electrical performances, since the contact loading during processing or packaging can significantly degrade the device performance. Consequently, there is a growing demand of investigating the mechanical characteristics of these materials, in particular in the nanoscale regime thin films. Nanoindentation has been widely used for characterizing the mechanical properties (hardness and elastic modulus) of various nanomaterials [3–5] and thin films [6–10], due to its high sensitivity, good resolution and easy operation. In this technique, a diamond indenter is pressed into and withdrawn from a test sample and the load–displacement curve can be obtained. By analyzing the curve with the traditional Olive–Pharr method [11], the hardness and elastic modulus can be extracted. Previously, the mechanisms of plastic deformation in ZnO single crystals and thin films
⇑ Corresponding author. Tel.: +886 7 6577711x3130; fax: +886 7 6578444. E-mail address: [email protected] (S.-R. Jian). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.213
[12–15] have been extensively investigated by means of the spherical nanoindentation. The general conclusion drawn from these studies is that the contact-induced deformation behaviors are primarily governed by dislocation activities. Namely, when in 0Þ the deformation is mainly dented in the a-direction ð½1 1 2 accommodated by the extensive dislocation slip and the pile-up events on the basal plane under the indenter; whereas when indented in the c-direction, deformation is mostly due to slips on both basal and pyramid planes. Nevertheless, for the polycrystalline ZnO films, depending on the preparation methods, the hardness and modulus obtained can vary over a substantially wide range (the hardness ranged from 1.5 to 12 GPa, and the elastic modulus ranged from 40 to 160 GPa) [16,17]. Moreover, unlike the hardness and elastic modulus, the fracture toughness of ZnO, although represents yet another important mechanical property for quantifying the materials resistance to mechanical failure such as cracking event, has been largely ignored in the literature. For preparing ZnO thin films, several techniques, such as magnetron sputtering [7], molecular beam epitaxy (MBE) [18], metal–organic chemical vapor deposition (MOCVD) [19], pulsed laser deposition (PLD) [20] and, atomic layer deposition (ALD) [21,22] techniques have been developed. Among them, the ALD technique provides the unique features such as precise control of films thickness down to single atomic layer resolution, high uniformity, excellent conformity for high aspect ratio structures, as well as the low growth temperatures. Herein, in this study, we investigate the structural and nanomechanical characteristics of ALD-derived ZnO thin films deposited on Si(1 1 1) substrates (ZnO/Si(1 1 1)) at room-
314
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temperature by using X-ray diffraction (XRD), atomic force microscopy (AFM) and Berkovich nanoindentation techniques. Unlike the deformation behaviors reported previously [12–17], especially for the polycrystalline films prepared by ALD on Si(1 0 0) substrates [16], wherein no crack formation was observed, we observed a ring-like through-thickness crack behavior in these ZnO/Si(1 1 1) thin films. We believe this could be due to the differences in growth temperatures as well as in the lattice and thermal expansion coefficient mismatches between ZnO thin films and Si(1 1 1) substrates [23,24], which may have affected the thin film/substrate interface properties significantly. By combining the load–displacement curves during nanoindentation and the scanning electron microscopy (SEM) observations, the fracture toughness of the ZnO/Si(1 1 1) thin films was analyzed.
2. Experimental details The ZnO thin films used in this study were deposited on Si(1 1 1) substrates at room temperature by using an atomic layer deposition (ALD) system operated with flow-rate interruption method. The detailed growth procedures can be found elsewhere [22]. For a typical run of 1000 ALD cycles, the thickness of ZnO thin films was about 200 nm. The crystal structure of ZnO thin films were analyzed by X-ray diffraction [Panalytical X’Pert XRD (PANalytical, Almelo, The Netherlands) Cu Ka, k = 1.5406 Å]. The surface profiles of ZnO thin films were examined by atomic force microscopy (AFM; Topometrix-Accures-II, Topometrix Corporation, Santa Clara, CA, USA). For the AFM operation, a constant scan speed of 1 lm/s with a constant load of 30 nN applied to the cantilever was practiced. In addition, a scanning electron microscopy (SEM, Hitachi S-4700, Tokyo, Japan) is used to analyze the Berkovich nanoindentation-induced fracture patterns of ZnO thin films. The nanoindentation experiments were performed on a MTS Nano IndenterÒ XP system (MTS Cooperation, Nano Instruments Innovation Center, Oak Ridge, TN, USA) with a diamond pyramid-shaped Berkovich-type indenter tip, whose radius of curvature is 50 nm. Prior to applying loading on ZnO/Si(1 1 1) thin films, nanoindentation was conducted on the standard fused silica sample to obtain the reasonable range (Young’s modulus of fused silica is 68–72 GPa). The thermal drift was kept below ±0.05 nm/s for all indentations considered in this work. The same loading/unloading rate of 10 mN/s was used. The cyclic nanoindentation tests were performed in a sequence described as followings. Firstly, the indentation loading was applied to the set maximum load and unloaded by 90%. Then, reload the indenter to the maximum load and unload it by 90% again. At this stage, the indenter was hold for 10 s at 10% of the maximum load for thermal drift correction. After that, the indenter was completely unloaded to complete a cyclic nanoindentation test for a certain targeted maximum load. At least 20 indents were performed on each sample. The nanoindentations were sufficiently spaced to prevent each other from mutual interactions. The hardness and Young’s modulus of ZnO/Si(1 1 1) thin films were measured by nanoindentation with a continuous stiffness measurements (CSM) technique [25] and, the indentations were made using a constant nominal strain rate of 0.05 s1. Consequently, the hardness and Young’s modulus of ZnO/ Si(1 1 1) thin films, obtained from the load–displacement curves by using the analytic method developed by Oliver and Pharr [11], are 10.3 GPa and 168.6 GPa, respectively.
3. Results and discussion The typical XRD data of 200 nm-thick ZnO/Si(1 1 1) thin films, indicating that the films are polycrystalline with moderate c-axis-oriented preferred growth, as shown in Fig. 1a. The grain size is estimated to be 25 nm from the XRD measurements by using Scherrer’s formula [26]. In addition, the AFM scan of a 1 lm 1 lm area displayed in Fig. 1b reveals a surface roughness (RMS) [27] value of 2.3 nm, which is much smaller than the typical indentation depth thus should not have noticeable effects on the indentation tests. It is noted that the surface roughness and grain size of the similar ALD-derived ZnO thin films grown on Si(1 0 0) at 150 °C by Tapily et al. [16] were 4 nm and 28 nm, respectively, and showed much stronger c-axis-oriented growth nature. Moreover, as will be compared below, those films also exhibited vastly different deformation behaviors from the present ZnO/Si(1 1 1) thin films. Typical cyclic nanoindentation load–displacement (P–h) curves for ZnO/Si(1 1 1) thin films at the indentation loads of (a) 20 mN, (b) 50 mN, (c) 80 mN and (d) 100 mN are displayed in Fig. 2, respectively. The P–h curve of indentation load made at 20 mN (Fig. 2a) is smooth whereas that made at 50 mN, 80 mN and 100 mN (Fig. 2b–d) exhibits a significantly discontinuity (pop-in) phenomenon in the loading curve. Such the characteristic pop-in even has been reported in numerous materials [8,13,28–32] and has been linked to abrupt plastic flow generated by high dislocation nucleation and propagation rates [8,29], crack formation [28,30,31] or phase transformation [32,33]. For single-crystal ZnO, the single pop-in event (indented along the a-axis) or the multiple pop-in events (indented along the c-axis) have been observed in the loading range of 4–13 mN [12], while no popin was observed in epitaxial ZnO thin films grown on a- and c-axis sapphire substrates [15]. The apparent hardening of ZnO thin films over their single crystal counterparts has been attributed to the effects of strain compensation by the native defects (such as grown-in dislocations) within the epilayers, which may have inhibited the slip mechanisms giving rise to the low-loading pop-in events [15]. We believe that similar effects may also prevail in the present ZnO/Si(1 1 1) thin films. On the other hand, when the indentation load is above 50 mN, a single substantial pop-in-like step is observed near the indentation load of 20 mN, as shown in Fig. 2b–d. Since this event occurs at a relatively higher load in a rather dramatic fashion, it probably cannot be ascribed to conventional dislocation nucleation and propagation mechanism. Indeed, as shown in Fig. 3, in these cases
Fig. 1. (a) XRD data of ALD-derived ZnO thin film. The sample shows the preferential growth in the (0 0 2) plane and (b) AFM micrographs of 200 nm thick ALD-derived ZnO thin film.
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Fig. 2. Typical cyclic nanoindentation load–displacement curves for ZnO thin film at the indentation loads of (a) 20 mN, (b) 50 mN, (c) 80 mN and (d) 100 mN. In (b), the shaded area of ABC in the load–displacement curve is the energy released during the secondary ring-like through-thickness cracking.
the radial cracking and irregular fracture pattern are observed along the indentation corners. Moreover, the film around the indenter appears to be bulged upwards, indicating that delamination and bulking might have occurred. The pop-in event occurring at 20 mN is believed to arise from the formation of the ring-like through-thickness cracks. With increasing the indentation load (see, for instance, Fig. 3c for the case of an indentation load of 100 mN), more radial cracks are initiated in outer periphery of
315
the indented region. As a consequence, the tearing of ZnO thin films outside the indented region becomes more severe. Nevertheless, the upward bulging of the other parts of thin film around the indenter appears to behave similarly in both 50 mN and 100 mN cases (respectively shown in Fig. 3b and c), presumably being resulted from the same mechanism of interfacial crack propagation. At the indentation loads of 50, 80 and 100 mN, in addition to the above-mentioned pop-in event during loading, a discontinuity phenomenon in the unloading curves (pop-out) is also observed in these cases, as shown in Fig. 2b–d. Since at these indentation loads, the part of thin film under or around the indenter is separated from the bulk film via the ring-like through-thickness cracking or spalling during indentation loading, the indentation is expected to penetrate into the supporting Si substrate. Therefore, the pop-out event may be attributed to the formation of lateral cracks on the base of radial and/or median cracks aided by a pressure-induced phase transformation in the underneath Si substrate [32]. For Si, it has been demonstrated by Pharr [33] that the popout event can be observed at an indentation load greater than about 15 mN during unloading. In the present cases, when the indentation load is greater than 20 mN, the corresponding indentation load on Si substrate could exceed the critical load and result in the formation of lateral cracking associated with the pop-out event seen in the unloading curves. It is evident from Fig. 3b–c that over the process of radial cracking, delamination and buckling, the ZnO thin film around the indenter still subjects to the pressure exerted by the indenter. Consequently, when the interfacial cracking develops, there exists a stress concentration at the end of the interfacial cracking, which is expected to increase with the increasing indentation load. For a thin film, there are two possible ways to relax the stress concentration. One is through delamination and the other is via the formation of the ring-like through-thickness cracking. As depicted schematically in Fig. 3a, at the early stages of interfacial cracking, the stress concentration might be relaxed by delamination. However, when the stress concentration cannot be fully relaxed by delamination alone, it will be relaxed by forming the through-thickness cracks. Therefore, the secondary ring-like through-thickness cracking can be considered as a separate event from the delamination. In order to understand the nature of fracture mechanisms of the ZnO/Si(1 1 1) thin films in more quantitative manner, the relationship between the cracking behaviors during nanoindentation and the corresponding characteristic pop-in event in the P–h curves is analyzed. The schematics displayed in Figs. 2b and 3a depict the correlation between the Berkovich nanoindentation-induced P–h curve and the proposed fracture pattern for the ZnO/Si(1 1 1) thin films at various stages of indentation. In 1997, Li et al. [34] proposed that the steps in the loading curve are resulting primarily from crack formation. Based on their model, the fracture processes progress in three stages. Stage (I): The ring-like through-thickness cracking form firstly around the Berkovich indenter by high stresses in the contact area; Stage (II): With increasing indentation load, the crack opening increases and the interface of thin film/substrate delaminates and buckles by the high lateral pressure; Stage (III): Secondary ring-like through-thickness cracks and spalling are formed by the high bending stresses accumulated at the edges of the bulked films, as depicted in Fig. 3a. In the following analysis, the interfacial fracture toughness of the ZnO/Si(1 1 1) thin films is calculated from the secondary ring-like through-thickness cracking generated in stage III. In this stage, the film around the indenter has bulged upwards and the stress concentration at the end of the interfacial cracking cannot be relaxed by propagation of the interfacial cracking. With the increasing indentation load, the height of the bulged film increases. When the height reaches a critical value, the bending stresses caused by the bulged film around the indenter will result in
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Fig. 3. (a) Schematic of secondary ring-like through-thickness cracking; the SEM images of indentations in ALD-derived ZnO thin film on Si(1 1 1) using a Berkovich indenter at an indentation load of (b) 50 mN and (c) 100 mN.
the formation of the secondary ring-like through-thickness cracking and spalling at the edge of the buckled film (Fig. 3b–c), which together with the possible pressure-induced phase transition in the underneath Si substrate could lead to the pop-out event seen in the unloading curve (Fig. 2b–d). Within this scenario, the area under the P–h curve can be regarded as the work performed by the indenter during elastic–plastic deformation of the thin film/ substrate system. The strain energy released in the secondary ring-like through-thickness cracking and spalling, thus, can be calculated from the corresponding steps in the loading curve. As indicated schematically in Fig. 2b, the secondary ring-like through-thickness cracking is supposed to take place at AB. The energy difference before and after the crack generation can then be approximated as the area of ABC; i.e., this energy stored in the area of ABC will be released as the strain energy dissipated in creating the ring-like through-thickness crack. According to the theoretical analysis by Li et al. [34], the fracture toughness of ZnO/Si(1 1 1) thin films can be estimated by the following expression:
" K IC ¼
Ef U 2pC R ð1 m2f Þ t
#1=2 ð1Þ
where Ef, mf, 2pCR, U and t are denoted as the Young’s modulus, the Poisson’s ratio, the crack length in thin film plane, the strain energy difference before and after cracking and the film thickness, respectively. In the following analysis, we used only the data obtained from the case with the indentation load of 50 mN. The strain energy difference, U, of ZnO/Si(1 1 1) thin films is estimated to be 0.5 109 N m. The value of CR is taken to be 6 lm, which is the average radius value of the secondary ring-like throughthickness cracks measured from the SEM observations displayed in Fig. 3b. Consequently, by using Eq. (1), the fracture toughness of ZnO/Si(1 1 1) thin films is calculated to be about 3.1 MPa m1/2. 4. Conclusion In summary, a combination of Berkovich nanoindentation and SEM techniques was carried out to investigate the Berkovich nanoindentation-induced interfacial fracture behaviors in ALDderived in ZnO thin films grown on Si(1 1 1) substrates. The loading at which a characteristic ‘‘pop-in’’ event occurs in the loading curve corresponds consistently with the SEM observations revealing the appearance of radial and ring-like through-thickness cracking. The
S.-R. Jian, Y.-H. Lee / Journal of Alloys and Compounds 587 (2014) 313–317
fracture toughness of ZnO thin film was calculated from the discontinuity plateau in the loading curve in term of the energy release associated with cracking. And, the result gives that the fracture toughness of ALD-derived ZnO/Si(1 1 1) thin films is about 3.1 MPa m1/2. Acknowledgements This work was partially supported by the National Science Council of Taiwan, under Grant No.: NSC102-2221-E-214-016. Authors like to thank Prof. J.-Y. Juang, Prof. C.-M. Lin, Dr. H.-Y. Lee and Dr. P.-F. Yang for their technical supports. References [1] Y. Ryu, T.S. Lee, J.A. Lubguban, H.W. White, B.J. Kim, Y.S. Park, Appl. Phys. Lett. 88 (2006) 241108. [2] K.C. Aw, Z. Tsakadze, A. Lohani, S. Mhaisalkar, Scr. Mater. 60 (2009) 48. [3] X.Y. Tao, X.D. Li, Nano Lett. 8 (2008) 505. [4] L. Bao, Z.H. Xu, R. Li, X.D. Li, Nano Lett. 10 (2010) 255. [5] S.R. Jian, T.H. Sung, J.C. Huang, J.Y. Juang, Appl. Phys. Lett. 101 (2012) 151905. [6] S.R. Jian, J.Y. Juang, N.C. Chen, Jason S.C. Jang, J.C. Huang, Y.S. Lai, Nanosci. Nanotechnol. Lett. 2 (2010) 315. [7] S.K. Wang, T.C. Lin, S.R. Jian, J.Y. Juang, Jason S.C. Jang, J.Y. Tseng, Appl. Surf. Sci. 258 (2011) 1261. [8] S.R. Jian, Y.C. Tseng, I.J. Teng, J.Y. Juang, Materials 6 (2013) 4259. [9] S.R. Jian, G.J. Chen, W.M. Hsu, Materials 6 (2013) 4505. [10] S.R. Jian, J.Y. Juang, IEEE Trans. Nanotechnol. 12 (2013) 304. [11] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [12] S.O. Kucheyev, J.E. Bradby, J.S. Williams, C. Jagadish, M.V. Swain, Appl. Phys. Lett. 80 (2002) 956. [13] J.E. Bradby, S.O. Kucheyev, J.S. Williams, C. Jagadish, M.V. Swain, P. Munroe, M.R. Philips, Appl. Phys. Lett. 80 (2002) 4537.
317
[14] S. Basu, M.W. Barsoum, J. Mater. Res. 22 (2007) 2470. [15] V.A. Coleman, J.E. Bradby, C. Jagadish, P. Munroe, Y.W. Heo, S.J. Pearton, D.P. Norton, M. Inoue, M. Yano, Appl. Phys. Lett. 86 (2005) 203105. [16] K. Tapily, D. Gu, H. Baumgart, G. Namkoong, D. Stegall, A.A. Elmustafa, Semicond. Sci. Technol. 26 (2011) 115005. [17] R. Zabels, F. Muktepavela, L. Grigorjeva, Thin Solid Films 520 (2012) 4685. [18] S.P. Wang, C.X. Shan, B. Yao, B.H. Li, J.Y. Zhang, D.X. Zhao, D.Z. Shen, X.W. Fan, Appl. Surf. Sci. 255 (2009) 4913. [19] Q.W. Li, J.M. Bian, J.C. Sun, H.W. Liang, C.W. Zou, Y.L. Sun, Y.M. Luo, Appl. Surf. Sci. 257 (2010) 1634. [20] S.R. Jian, I.J. Teng, P.F. Yang, Y.S. Lai, J.M. Lu, J.G. Chang, S.P. Ju, Nanoscale Res. Lett. 3 (2008) 186. [21] S.J. Lim, S. Kwon, H. Kim, Thin Solid Films 516 (2008) 1523. [22] C.Y. Yen, S.R. Jian, G.J. Chen, C.M. Lin, H.Y. Lee, W.C. Ke, Y.Y. Liao, P.F. Yang, C.T. Wang, Y.S. Lai, Jason S.C. Jang, J.Y. Juang, Appl. Surf. Sci. 257 (2011) 7900. [23] L. Wang, Y. Pu, Y.F. Chen, C.L. Mo, W.Q. Fang, C.B. Xiong, J.N. Dai, F.Y. Jiang, J. Cryst. Growth 284 (2005) 459. [24] N. Gopalakrishnan, B.C. Shin, H.S. Lim, G.Y. Kim, Y.S. Yu, Physica B 376–377 (2006) 756. [25] X.D. Li, B. Bhushan, Mater. Charact. 48 (2002) 11. [26] B.D. Cullity, S.R. Stock, Element of X-ray Diffraction, Prentice Hall, New Jersey, 2001. p. 170. [27] K. Miyoshi, Y.W. Chung, Surface Diagnostics in Tribology: Fundamental Principles and Applications, World Scientific Publishing, Singapore, 1993. [28] X.D. Li, B. Bhushan, Thin Solid Films 355 (1999) 330. [29] S.R. Jian, Nanoscale Res. Lett. 3 (2008) 6. [30] S.R. Jian, Jason S.C. Jang, J. Alloys Comp. 482 (2009) 498. [31] C.Y. Yen, S.R. Jian, Y.S. Lai, P.F. Yang, Y.Y. Liao, Jason S.C. Jang, T.H. Lin, J.Y. Juang, J. Alloys Comp. 508 (2010) 523. [32] S.R. Jian, G.J. Chen, J.Y. Juang, Curr. Opin. Solid State Mater. Sci. 14 (2010) 69. [33] G.M. Pharr, The anomalous behavior of silicon during nanoindentation, in: W.D. Nix, J.C. Bravman, E. Arzt, L.B. Freund (Eds.), Thin Films: Stresses and Mechanical Properties III, Materials Research Society Symposium Proceedings, vol. 239, Materials Research Society, Pittsburgh, PA, 1992, pp. 301–312. [34] X.D. Li, D. Diao, B. Bhushan, Acta Mater. 45 (1997) 4453.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Nanoindentation-induced interfacial fracture of ZnO thin films deposited on Si(1 1 1) substrates by atomic layer deposition Sheng-Rui Jian ⇑, Ya-Hui Lee Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan
a r t i c l e
i n f o
Article history: Received 13 October 2013 Received in revised form 28 October 2013 Accepted 28 October 2013 Available online 4 November 2013 Keywords: ZnO thin films Atomic layer deposition Nanoindentation Interfacial fracture
a b s t r a c t In this study, the structural and nanomechanical characteristics of ZnO thin films are investigated by mans of X-ray diffraction (XRD), atomic force microscopy (AFM) and nanoindentation techniques. The ZnO thin films are deposited on Si(1 1 1) substrates by using atomic layer deposition (ALD). The interfacial fracture behaviors for ALD-derived ZnO thin films are characterized by Berkovich nanoindentation, and the morphologies of indentations are revealed by using scanning electron microscopy (SEM). During nanoindentation, the interfacial fracture of ZnO thin films is significantly observed as the indentation load reaches 50 mN and, the corresponding loading segment in the load–displacement curve displays an obvious discontinuity event. Based on the analysis of the energy release in cracking, the fracture toughness of ALD-derived ZnO thin films deposited on Si(1 1 1) substrates is calculated. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The II–VI wurtzite ZnO semiconductor has a wide direct band gap of 3.4 eV with an exciton binding energy of 60 meV, making it a competitive candidate for applications in transparent thin film transistors, UV/blue light-emitting diodes and lasers at high temperatures [1,2]. Recently, much attention has been paid to heteroepitaxially grown ZnO on Si substrates because of the unique possibility of integrating well-established Si electronics with ZnO-based optoelectronic devices. Nevertheless, the successful fabrication of devices based on ZnO thin films requires better understanding of the mechanical characteristics in addition to its optical and electrical performances, since the contact loading during processing or packaging can significantly degrade the device performance. Consequently, there is a growing demand of investigating the mechanical characteristics of these materials, in particular in the nanoscale regime thin films. Nanoindentation has been widely used for characterizing the mechanical properties (hardness and elastic modulus) of various nanomaterials [3–5] and thin films [6–10], due to its high sensitivity, good resolution and easy operation. In this technique, a diamond indenter is pressed into and withdrawn from a test sample and the load–displacement curve can be obtained. By analyzing the curve with the traditional Olive–Pharr method [11], the hardness and elastic modulus can be extracted. Previously, the mechanisms of plastic deformation in ZnO single crystals and thin films
⇑ Corresponding author. Tel.: +886 7 6577711x3130; fax: +886 7 6578444. E-mail address: [email protected] (S.-R. Jian). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.213
[12–15] have been extensively investigated by means of the spherical nanoindentation. The general conclusion drawn from these studies is that the contact-induced deformation behaviors are primarily governed by dislocation activities. Namely, when in 0Þ the deformation is mainly dented in the a-direction ð½1 1 2 accommodated by the extensive dislocation slip and the pile-up events on the basal plane under the indenter; whereas when indented in the c-direction, deformation is mostly due to slips on both basal and pyramid planes. Nevertheless, for the polycrystalline ZnO films, depending on the preparation methods, the hardness and modulus obtained can vary over a substantially wide range (the hardness ranged from 1.5 to 12 GPa, and the elastic modulus ranged from 40 to 160 GPa) [16,17]. Moreover, unlike the hardness and elastic modulus, the fracture toughness of ZnO, although represents yet another important mechanical property for quantifying the materials resistance to mechanical failure such as cracking event, has been largely ignored in the literature. For preparing ZnO thin films, several techniques, such as magnetron sputtering [7], molecular beam epitaxy (MBE) [18], metal–organic chemical vapor deposition (MOCVD) [19], pulsed laser deposition (PLD) [20] and, atomic layer deposition (ALD) [21,22] techniques have been developed. Among them, the ALD technique provides the unique features such as precise control of films thickness down to single atomic layer resolution, high uniformity, excellent conformity for high aspect ratio structures, as well as the low growth temperatures. Herein, in this study, we investigate the structural and nanomechanical characteristics of ALD-derived ZnO thin films deposited on Si(1 1 1) substrates (ZnO/Si(1 1 1)) at room-
314
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temperature by using X-ray diffraction (XRD), atomic force microscopy (AFM) and Berkovich nanoindentation techniques. Unlike the deformation behaviors reported previously [12–17], especially for the polycrystalline films prepared by ALD on Si(1 0 0) substrates [16], wherein no crack formation was observed, we observed a ring-like through-thickness crack behavior in these ZnO/Si(1 1 1) thin films. We believe this could be due to the differences in growth temperatures as well as in the lattice and thermal expansion coefficient mismatches between ZnO thin films and Si(1 1 1) substrates [23,24], which may have affected the thin film/substrate interface properties significantly. By combining the load–displacement curves during nanoindentation and the scanning electron microscopy (SEM) observations, the fracture toughness of the ZnO/Si(1 1 1) thin films was analyzed.
2. Experimental details The ZnO thin films used in this study were deposited on Si(1 1 1) substrates at room temperature by using an atomic layer deposition (ALD) system operated with flow-rate interruption method. The detailed growth procedures can be found elsewhere [22]. For a typical run of 1000 ALD cycles, the thickness of ZnO thin films was about 200 nm. The crystal structure of ZnO thin films were analyzed by X-ray diffraction [Panalytical X’Pert XRD (PANalytical, Almelo, The Netherlands) Cu Ka, k = 1.5406 Å]. The surface profiles of ZnO thin films were examined by atomic force microscopy (AFM; Topometrix-Accures-II, Topometrix Corporation, Santa Clara, CA, USA). For the AFM operation, a constant scan speed of 1 lm/s with a constant load of 30 nN applied to the cantilever was practiced. In addition, a scanning electron microscopy (SEM, Hitachi S-4700, Tokyo, Japan) is used to analyze the Berkovich nanoindentation-induced fracture patterns of ZnO thin films. The nanoindentation experiments were performed on a MTS Nano IndenterÒ XP system (MTS Cooperation, Nano Instruments Innovation Center, Oak Ridge, TN, USA) with a diamond pyramid-shaped Berkovich-type indenter tip, whose radius of curvature is 50 nm. Prior to applying loading on ZnO/Si(1 1 1) thin films, nanoindentation was conducted on the standard fused silica sample to obtain the reasonable range (Young’s modulus of fused silica is 68–72 GPa). The thermal drift was kept below ±0.05 nm/s for all indentations considered in this work. The same loading/unloading rate of 10 mN/s was used. The cyclic nanoindentation tests were performed in a sequence described as followings. Firstly, the indentation loading was applied to the set maximum load and unloaded by 90%. Then, reload the indenter to the maximum load and unload it by 90% again. At this stage, the indenter was hold for 10 s at 10% of the maximum load for thermal drift correction. After that, the indenter was completely unloaded to complete a cyclic nanoindentation test for a certain targeted maximum load. At least 20 indents were performed on each sample. The nanoindentations were sufficiently spaced to prevent each other from mutual interactions. The hardness and Young’s modulus of ZnO/Si(1 1 1) thin films were measured by nanoindentation with a continuous stiffness measurements (CSM) technique [25] and, the indentations were made using a constant nominal strain rate of 0.05 s1. Consequently, the hardness and Young’s modulus of ZnO/ Si(1 1 1) thin films, obtained from the load–displacement curves by using the analytic method developed by Oliver and Pharr [11], are 10.3 GPa and 168.6 GPa, respectively.
3. Results and discussion The typical XRD data of 200 nm-thick ZnO/Si(1 1 1) thin films, indicating that the films are polycrystalline with moderate c-axis-oriented preferred growth, as shown in Fig. 1a. The grain size is estimated to be 25 nm from the XRD measurements by using Scherrer’s formula [26]. In addition, the AFM scan of a 1 lm 1 lm area displayed in Fig. 1b reveals a surface roughness (RMS) [27] value of 2.3 nm, which is much smaller than the typical indentation depth thus should not have noticeable effects on the indentation tests. It is noted that the surface roughness and grain size of the similar ALD-derived ZnO thin films grown on Si(1 0 0) at 150 °C by Tapily et al. [16] were 4 nm and 28 nm, respectively, and showed much stronger c-axis-oriented growth nature. Moreover, as will be compared below, those films also exhibited vastly different deformation behaviors from the present ZnO/Si(1 1 1) thin films. Typical cyclic nanoindentation load–displacement (P–h) curves for ZnO/Si(1 1 1) thin films at the indentation loads of (a) 20 mN, (b) 50 mN, (c) 80 mN and (d) 100 mN are displayed in Fig. 2, respectively. The P–h curve of indentation load made at 20 mN (Fig. 2a) is smooth whereas that made at 50 mN, 80 mN and 100 mN (Fig. 2b–d) exhibits a significantly discontinuity (pop-in) phenomenon in the loading curve. Such the characteristic pop-in even has been reported in numerous materials [8,13,28–32] and has been linked to abrupt plastic flow generated by high dislocation nucleation and propagation rates [8,29], crack formation [28,30,31] or phase transformation [32,33]. For single-crystal ZnO, the single pop-in event (indented along the a-axis) or the multiple pop-in events (indented along the c-axis) have been observed in the loading range of 4–13 mN [12], while no popin was observed in epitaxial ZnO thin films grown on a- and c-axis sapphire substrates [15]. The apparent hardening of ZnO thin films over their single crystal counterparts has been attributed to the effects of strain compensation by the native defects (such as grown-in dislocations) within the epilayers, which may have inhibited the slip mechanisms giving rise to the low-loading pop-in events [15]. We believe that similar effects may also prevail in the present ZnO/Si(1 1 1) thin films. On the other hand, when the indentation load is above 50 mN, a single substantial pop-in-like step is observed near the indentation load of 20 mN, as shown in Fig. 2b–d. Since this event occurs at a relatively higher load in a rather dramatic fashion, it probably cannot be ascribed to conventional dislocation nucleation and propagation mechanism. Indeed, as shown in Fig. 3, in these cases
Fig. 1. (a) XRD data of ALD-derived ZnO thin film. The sample shows the preferential growth in the (0 0 2) plane and (b) AFM micrographs of 200 nm thick ALD-derived ZnO thin film.
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Fig. 2. Typical cyclic nanoindentation load–displacement curves for ZnO thin film at the indentation loads of (a) 20 mN, (b) 50 mN, (c) 80 mN and (d) 100 mN. In (b), the shaded area of ABC in the load–displacement curve is the energy released during the secondary ring-like through-thickness cracking.
the radial cracking and irregular fracture pattern are observed along the indentation corners. Moreover, the film around the indenter appears to be bulged upwards, indicating that delamination and bulking might have occurred. The pop-in event occurring at 20 mN is believed to arise from the formation of the ring-like through-thickness cracks. With increasing the indentation load (see, for instance, Fig. 3c for the case of an indentation load of 100 mN), more radial cracks are initiated in outer periphery of
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the indented region. As a consequence, the tearing of ZnO thin films outside the indented region becomes more severe. Nevertheless, the upward bulging of the other parts of thin film around the indenter appears to behave similarly in both 50 mN and 100 mN cases (respectively shown in Fig. 3b and c), presumably being resulted from the same mechanism of interfacial crack propagation. At the indentation loads of 50, 80 and 100 mN, in addition to the above-mentioned pop-in event during loading, a discontinuity phenomenon in the unloading curves (pop-out) is also observed in these cases, as shown in Fig. 2b–d. Since at these indentation loads, the part of thin film under or around the indenter is separated from the bulk film via the ring-like through-thickness cracking or spalling during indentation loading, the indentation is expected to penetrate into the supporting Si substrate. Therefore, the pop-out event may be attributed to the formation of lateral cracks on the base of radial and/or median cracks aided by a pressure-induced phase transformation in the underneath Si substrate [32]. For Si, it has been demonstrated by Pharr [33] that the popout event can be observed at an indentation load greater than about 15 mN during unloading. In the present cases, when the indentation load is greater than 20 mN, the corresponding indentation load on Si substrate could exceed the critical load and result in the formation of lateral cracking associated with the pop-out event seen in the unloading curves. It is evident from Fig. 3b–c that over the process of radial cracking, delamination and buckling, the ZnO thin film around the indenter still subjects to the pressure exerted by the indenter. Consequently, when the interfacial cracking develops, there exists a stress concentration at the end of the interfacial cracking, which is expected to increase with the increasing indentation load. For a thin film, there are two possible ways to relax the stress concentration. One is through delamination and the other is via the formation of the ring-like through-thickness cracking. As depicted schematically in Fig. 3a, at the early stages of interfacial cracking, the stress concentration might be relaxed by delamination. However, when the stress concentration cannot be fully relaxed by delamination alone, it will be relaxed by forming the through-thickness cracks. Therefore, the secondary ring-like through-thickness cracking can be considered as a separate event from the delamination. In order to understand the nature of fracture mechanisms of the ZnO/Si(1 1 1) thin films in more quantitative manner, the relationship between the cracking behaviors during nanoindentation and the corresponding characteristic pop-in event in the P–h curves is analyzed. The schematics displayed in Figs. 2b and 3a depict the correlation between the Berkovich nanoindentation-induced P–h curve and the proposed fracture pattern for the ZnO/Si(1 1 1) thin films at various stages of indentation. In 1997, Li et al. [34] proposed that the steps in the loading curve are resulting primarily from crack formation. Based on their model, the fracture processes progress in three stages. Stage (I): The ring-like through-thickness cracking form firstly around the Berkovich indenter by high stresses in the contact area; Stage (II): With increasing indentation load, the crack opening increases and the interface of thin film/substrate delaminates and buckles by the high lateral pressure; Stage (III): Secondary ring-like through-thickness cracks and spalling are formed by the high bending stresses accumulated at the edges of the bulked films, as depicted in Fig. 3a. In the following analysis, the interfacial fracture toughness of the ZnO/Si(1 1 1) thin films is calculated from the secondary ring-like through-thickness cracking generated in stage III. In this stage, the film around the indenter has bulged upwards and the stress concentration at the end of the interfacial cracking cannot be relaxed by propagation of the interfacial cracking. With the increasing indentation load, the height of the bulged film increases. When the height reaches a critical value, the bending stresses caused by the bulged film around the indenter will result in
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Fig. 3. (a) Schematic of secondary ring-like through-thickness cracking; the SEM images of indentations in ALD-derived ZnO thin film on Si(1 1 1) using a Berkovich indenter at an indentation load of (b) 50 mN and (c) 100 mN.
the formation of the secondary ring-like through-thickness cracking and spalling at the edge of the buckled film (Fig. 3b–c), which together with the possible pressure-induced phase transition in the underneath Si substrate could lead to the pop-out event seen in the unloading curve (Fig. 2b–d). Within this scenario, the area under the P–h curve can be regarded as the work performed by the indenter during elastic–plastic deformation of the thin film/ substrate system. The strain energy released in the secondary ring-like through-thickness cracking and spalling, thus, can be calculated from the corresponding steps in the loading curve. As indicated schematically in Fig. 2b, the secondary ring-like through-thickness cracking is supposed to take place at AB. The energy difference before and after the crack generation can then be approximated as the area of ABC; i.e., this energy stored in the area of ABC will be released as the strain energy dissipated in creating the ring-like through-thickness crack. According to the theoretical analysis by Li et al. [34], the fracture toughness of ZnO/Si(1 1 1) thin films can be estimated by the following expression:
" K IC ¼
Ef U 2pC R ð1 m2f Þ t
#1=2 ð1Þ
where Ef, mf, 2pCR, U and t are denoted as the Young’s modulus, the Poisson’s ratio, the crack length in thin film plane, the strain energy difference before and after cracking and the film thickness, respectively. In the following analysis, we used only the data obtained from the case with the indentation load of 50 mN. The strain energy difference, U, of ZnO/Si(1 1 1) thin films is estimated to be 0.5 109 N m. The value of CR is taken to be 6 lm, which is the average radius value of the secondary ring-like throughthickness cracks measured from the SEM observations displayed in Fig. 3b. Consequently, by using Eq. (1), the fracture toughness of ZnO/Si(1 1 1) thin films is calculated to be about 3.1 MPa m1/2. 4. Conclusion In summary, a combination of Berkovich nanoindentation and SEM techniques was carried out to investigate the Berkovich nanoindentation-induced interfacial fracture behaviors in ALDderived in ZnO thin films grown on Si(1 1 1) substrates. The loading at which a characteristic ‘‘pop-in’’ event occurs in the loading curve corresponds consistently with the SEM observations revealing the appearance of radial and ring-like through-thickness cracking. The
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fracture toughness of ZnO thin film was calculated from the discontinuity plateau in the loading curve in term of the energy release associated with cracking. And, the result gives that the fracture toughness of ALD-derived ZnO/Si(1 1 1) thin films is about 3.1 MPa m1/2. Acknowledgements This work was partially supported by the National Science Council of Taiwan, under Grant No.: NSC102-2221-E-214-016. Authors like to thank Prof. J.-Y. Juang, Prof. C.-M. Lin, Dr. H.-Y. Lee and Dr. P.-F. Yang for their technical supports. References [1] Y. Ryu, T.S. Lee, J.A. Lubguban, H.W. White, B.J. Kim, Y.S. Park, Appl. Phys. Lett. 88 (2006) 241108. [2] K.C. Aw, Z. Tsakadze, A. Lohani, S. Mhaisalkar, Scr. Mater. 60 (2009) 48. [3] X.Y. Tao, X.D. Li, Nano Lett. 8 (2008) 505. [4] L. Bao, Z.H. Xu, R. Li, X.D. Li, Nano Lett. 10 (2010) 255. [5] S.R. Jian, T.H. Sung, J.C. Huang, J.Y. Juang, Appl. Phys. Lett. 101 (2012) 151905. [6] S.R. Jian, J.Y. Juang, N.C. Chen, Jason S.C. Jang, J.C. Huang, Y.S. Lai, Nanosci. Nanotechnol. Lett. 2 (2010) 315. [7] S.K. Wang, T.C. Lin, S.R. Jian, J.Y. Juang, Jason S.C. Jang, J.Y. Tseng, Appl. Surf. Sci. 258 (2011) 1261. [8] S.R. Jian, Y.C. Tseng, I.J. Teng, J.Y. Juang, Materials 6 (2013) 4259. [9] S.R. Jian, G.J. Chen, W.M. Hsu, Materials 6 (2013) 4505. [10] S.R. Jian, J.Y. Juang, IEEE Trans. Nanotechnol. 12 (2013) 304. [11] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [12] S.O. Kucheyev, J.E. Bradby, J.S. Williams, C. Jagadish, M.V. Swain, Appl. Phys. Lett. 80 (2002) 956. [13] J.E. Bradby, S.O. Kucheyev, J.S. Williams, C. Jagadish, M.V. Swain, P. Munroe, M.R. Philips, Appl. Phys. Lett. 80 (2002) 4537.
317
[14] S. Basu, M.W. Barsoum, J. Mater. Res. 22 (2007) 2470. [15] V.A. Coleman, J.E. Bradby, C. Jagadish, P. Munroe, Y.W. Heo, S.J. Pearton, D.P. Norton, M. Inoue, M. Yano, Appl. Phys. Lett. 86 (2005) 203105. [16] K. Tapily, D. Gu, H. Baumgart, G. Namkoong, D. Stegall, A.A. Elmustafa, Semicond. Sci. Technol. 26 (2011) 115005. [17] R. Zabels, F. Muktepavela, L. Grigorjeva, Thin Solid Films 520 (2012) 4685. [18] S.P. Wang, C.X. Shan, B. Yao, B.H. Li, J.Y. Zhang, D.X. Zhao, D.Z. Shen, X.W. Fan, Appl. Surf. Sci. 255 (2009) 4913. [19] Q.W. Li, J.M. Bian, J.C. Sun, H.W. Liang, C.W. Zou, Y.L. Sun, Y.M. Luo, Appl. Surf. Sci. 257 (2010) 1634. [20] S.R. Jian, I.J. Teng, P.F. Yang, Y.S. Lai, J.M. Lu, J.G. Chang, S.P. Ju, Nanoscale Res. Lett. 3 (2008) 186. [21] S.J. Lim, S. Kwon, H. Kim, Thin Solid Films 516 (2008) 1523. [22] C.Y. Yen, S.R. Jian, G.J. Chen, C.M. Lin, H.Y. Lee, W.C. Ke, Y.Y. Liao, P.F. Yang, C.T. Wang, Y.S. Lai, Jason S.C. Jang, J.Y. Juang, Appl. Surf. Sci. 257 (2011) 7900. [23] L. Wang, Y. Pu, Y.F. Chen, C.L. Mo, W.Q. Fang, C.B. Xiong, J.N. Dai, F.Y. Jiang, J. Cryst. Growth 284 (2005) 459. [24] N. Gopalakrishnan, B.C. Shin, H.S. Lim, G.Y. Kim, Y.S. Yu, Physica B 376–377 (2006) 756. [25] X.D. Li, B. Bhushan, Mater. Charact. 48 (2002) 11. [26] B.D. Cullity, S.R. Stock, Element of X-ray Diffraction, Prentice Hall, New Jersey, 2001. p. 170. [27] K. Miyoshi, Y.W. Chung, Surface Diagnostics in Tribology: Fundamental Principles and Applications, World Scientific Publishing, Singapore, 1993. [28] X.D. Li, B. Bhushan, Thin Solid Films 355 (1999) 330. [29] S.R. Jian, Nanoscale Res. Lett. 3 (2008) 6. [30] S.R. Jian, Jason S.C. Jang, J. Alloys Comp. 482 (2009) 498. [31] C.Y. Yen, S.R. Jian, Y.S. Lai, P.F. Yang, Y.Y. Liao, Jason S.C. Jang, T.H. Lin, J.Y. Juang, J. Alloys Comp. 508 (2010) 523. [32] S.R. Jian, G.J. Chen, J.Y. Juang, Curr. Opin. Solid State Mater. Sci. 14 (2010) 69. [33] G.M. Pharr, The anomalous behavior of silicon during nanoindentation, in: W.D. Nix, J.C. Bravman, E. Arzt, L.B. Freund (Eds.), Thin Films: Stresses and Mechanical Properties III, Materials Research Society Symposium Proceedings, vol. 239, Materials Research Society, Pittsburgh, PA, 1992, pp. 301–312. [34] X.D. Li, D. Diao, B. Bhushan, Acta Mater. 45 (1997) 4453.