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Preparation and mechanical properties of aluminum-doped zinc oxide transparent conducting films
Surface & Coatings Technology 202 (2008) 5416–5420
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Preparation and mechanical properties of aluminum-doped zinc oxide transparent conducting films Shou-Yi Chang ⁎, Yen-Chih Hsiao, Yi-Chung Huang Department of Materials Science and Engineering, National Chung Hsing University, Taichung, 402, Taiwan
A R T I C L E
I N F O
Available online 7 June 2008 Keywords: Transparent conductive films Mechanical properties Interface adhesion
A B S T R A C T Aluminum-doped zinc oxide transparent conducting films were deposited in this study by magnetron sputtering under different sputtering powers and substrate temperatures. At low sputtering powers and substrate temperatures, the deposited films were constructed by spherical grains. With increasing power and temperature, the grains became facet with an obvious (002) preferred orientation. The crystallinity and grain size of the films increased as well, and consequently the electrical resistivity decreased. By nanoindentation tests, the hardness of the deposited films was measured and found to increase from 8 to 10 GPa with higher sputtering power and substrate temperature because of higher densification and crystallinity. During nanoindentation and nanoscratch tests, interface delamination occurred between the films and substrates, and the interface adhesion energy was accordingly obtained. From the measurement of nanoscratch tests, the adhesion energy was found to be improved from 0.49 to 0.86 or 0.79 J/m2, respectively, with higher sputtering power or substrate temperature because of the deeper penetration, higher densification and easier interface reaction of the deposited films. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Transparent conducting oxide (TCO) films with good optical and electrical properties (transparency N80%, electrical resistivity b10− 3 Ω·cm) have been widely applied to optoelectronic industry such as solar cells and displays [1]. Typical TCO films include SnO2, In2O3, and ZnO, etc. Minor elements are further doped to obtain better film properties, like the most popularly used indium-doped tin oxide (ITO). However due to the poor thermal stability and high cost of ITO films, cheap and non-toxic ZnO films have attracted much attention in these years. Especially, aluminum-doped zinc oxide (ZnO:Al, AZO) films exhibit comparable optical and electrical properties with ITO films, and have a high potential to replace conventional TCO films [2,3]. For the preparation of TCO films, radio-frequency (RF) magnetron sputtering, chemical vapor deposition, thermal evaporation, and sol–gel methods have been used. Among them, the RF magnetron sputtering by which the films are deposited at lower temperatures with high qualities is more generally adopted [1–3]. However, the mechanical damages of TCO films, such as film cracking and interface delamination, severely suppress the processing yield and application reliability of the films [4]. Especially for the TCO films applied on flexible substrates, repeated flexure stresses during application more easily result in film damages. A high resistance to the mechanical damages is thus strongly demanded for the TCO films
⁎ Corresponding author. Tel.: +886 4 22857517. E-mail address: [email protected] (S.-Y. Chang). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.024
besides good optical and electrical properties, and the mechanical properties need to be clarified before practical applications. However, conventional testing tools are not longer suitable for the evaluation of mechanical properties of thin TCO films. Alternately, nanoindentation and nanoscratch tests have been widely applied for the measurement of the mechanical properties of thin films [5–15]. Besides hardness and elastic modulus [5,6], more information such as yielding stress and fracture toughness can be extracted to reveal more representative mechanical properties of thin films [7]. Moreover, the nanoindentation and nanoscratch tests are also promising to determine interface adhesion strength through film delamination [8–15]. Thus in this study, AZO transparent conducting films have been prepared on glass substrates by magnetron sputtering. Their microstructures and basic properties including electrical resistivity and optical transparency are characterized. Moreover, the mechanical properties of the films and the interface adhesion energy between the films and substrates are investigated by nanoindentation and nanoscratch tests. The interface delamination behaviors are examined to evaluate the mechanical reliability of the AZO films. Sputtering power and substrate temperature are varied to investigate their effects on the properties of the films. 2. Experimental details The AZO films were deposited on Corning 7059 glass substrates by RF magnetron sputtering at a pressure of 8 mTorr using a 98 wt.% ZnO2 wt.% Al2O3 target. The sputtering power was varied from 50 to 200 W (at room temperature, RT), and the substrate temperature was
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controlled at RT, 100 °C, and 200 °C (at 100 W). The deposition rate increased from 6.7 to 28.3 nm/min with sputtering power, while it remained at about 14 nm/min under different substrate temperatures. The thickness of the deposited films was controlled at about 500 nm by varying deposition time. The crystal structure of the AZO films was analyzed by an X-ray diffractometer (XRD, MAC Science MXP3). A scanning electron microscope (SEM, JEOL JSM-6700F) was used to observe the surface morphologies and microstructures of the films. Atomic force microscopy (AFM, SEIKO Instrument SPI3800N) was applied to characterize surface roughness. A four-point probe method was applied to measure electrical resistivity, and UV/visible spectroscopy (Hitachi U-3010) was used to measure optical transparency. The mechanical properties of AZO films were measured by using a UMIS nanoindenter (Based Model, CSIRO) with a Berkovich diamond indenter (tip radius ~ 100 nm, edge angle 130.6 ) and a nanoscratch test module. During nanoindentation tests, the load was applied to a maximum value of 1 mN, held for 30 s, and then released under a loading/unloading rate of 0.033 mN/s. The indentation depth was controlled at 1/15 to 1/10 of film thickness to avoid a substrate effect. For the measurement of interface adhesion energy between the AZO films and glass substrates by nanoindentation tests, a series of indentations with varied maximum loads from 110 to 170 mN were performed. Under nanoscratch tests, the load was ramped from 0 to 50 mN in 30 s, and the scratch length was set as 1500 μm at a moving velocity of 50 μm/s. An SEM was used to observe interface delamination around indent marks and scratch tracks after the nanoindentation and nanoscratch tests. 3. Results and discussion 3.1. Microstructures and basic characterizations of AZO films Fig. 1 shows the XRD patterns of AZO films deposited under different sputtering powers and substrate temperatures. At low sputtering powers and substrate temperatures, three main diffraction peaks of ZnO (100), (002) and (101) crystal planes were detected, indicating no preferred crystalline orientation in the films. With increasing sputtering power to 150 W or substrate temperature to 100 °C, higher energy for the regular arrangement of incident atoms promoted the growth of the AZO films in a low-energy (002) direction, then enhancing the formation of more obvious (002) preferred orientation [16]. By using the full widths at half maximum of the (002) diffraction peaks and the Scherrer equation [17], the grain sizes of the AZO films can be calculated. With higher sputtering powers, the grain size increased from 11.0 to 14.1 nm since the higher energy of incident atoms promoted grain growth. At higher substrate temperatures, the grain size even increased from 11.2 to 17.5 nm because of a rapid grain growth. Fig. 2 shows the SEM surface morphologies and cross-sectional microstructure of AZO films deposited under different sputtering powers and substrate temperatures. From Fig. 2(a), it was observed that the AZO films deposited under low sputtering powers and substrate temperatures were constructed by spherical particles of about 10 nm in size without specific preferred orientation. With increasing sputtering power and substrate temperature, the particles grew and became a facet shape as shown in Fig. 2(b) and (c) with an obvious preferred orientation, in accordance with the XRD analyses shown in Fig. 1. From Fig. 2(d), it was observed that the deposited AZO films were continuous, dense, and smooth with a uniform thickness of about 500 nm. The films possessed a columnar structure. Moreover from the AFM analyses of the AZO films, very small surface roughness was measured for most of the films as only about few nanometers. The electrical resistivity of AZO films was lowered from 0.9 to 0.09 and 0.03 Ω·cm as the sputtering power increased to 200 W and the substrate temperature to 200 °C, respectively. As aforementioned, the crystallinity of the deposited films was enhanced with an obvious
Fig. 1. XRD patterns of AZO films deposited under (a) different sputtering powers at RT and (b) different substrate temperatures at a sputtering power of 100 W.
(002) preferred orientation when the sputtering power and substrate temperature increased. Moreover, grain growth and film densification occurred, reducing defects (grain boundaries, etc.) and thus electrical resistivity. Furthermore from the measurement of optical transparency at different wavelengths, it was observed that all of the AZO films deposited under different sputtering powers and substrate temperatures possessed an optical transparency of about 90% at the film thickness of 500 nm. The bandgap energy Eg of the AZO films decreased from 3.35 to 3.28 eV with increasing sputtering power, while it remained at a constant value of about 3.33 eV with varied substrate temperatures. 3.2. Nanomechanical properties of AZO films From the load-penetration depth curves of nanoindentation tests of AZO films, it was found that, under small indentation depths below 15 nm, the loading curves matched to the elastic unloading curves in accordance with a “Hertzian elastic relation” [18], indicating the elastic deformation of the AZO films. Beyond the depth, the curves began to deviate from the “Hertzian response”, and the permanently plastic deformation of the AZO films was expected to occur. From the Oliver–Pharr relation [5], the hardness and elastic modulus of the AZO films deposited under different experimental conditions were obtained as plotted in Fig. 3. It was found that, although the deposition
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Fig. 2. SEM surface morphologies of AZO films deposited under different sputtering powers and substrate temperatures, (a) 100 W at RT, (b) 200 W at RT, and (c) 100 W at 200 °C. (d) SEM cross-sectional microstructure of AZO film deposited under 100 W at RT.
parameters varied, the elastic modulus of the AZO films basically remained at the level of about 110 GPa because the elastic modulus was an intrinsic property of materials. In comparison, the hardness increased from 8 to 10 GPa with higher sputtering powers and substrate temperatures. As aforementioned, the stronger bombardments under higher powers and the higher energy for crystallization at higher temperatures induced higher densification (less porosity) and crystallinity of the deposited films, thus enhancing the hardness [19]. 3.3. Interface adhesion measured by nanoindentation test Nanoindentation introduced localized mechanical deformation to AZO films, and shear stresses began to accumulate at the interface between the AZO films and glass substrates due to strain mismatches. Once the accumulated stresses exceeded interface adhesion strength, the interface would then delaminate. Fig. 4(a) shows the SEM surface morphology of the AZO film deposited under a sputtering power of 100 W and a substrate temperature of 200 °C after nanoindentation test at a maximum applied load of 130 mN. When the load of 120 mN was applied, unclear film blister and buckling just formed around the indented region, indicating the occurrence of interface decohesion. Under higher applied loads than 130 mN, film blister and buckling became much more obvious, and the buckled film was further pressed by the indenter tip and then slightly cracked. By using the following equation [8–13], the fracture energy release rate Gc, i.e. the adhesion energy per unit area, for interface delamination between AZO films and substrates can be then obtained. 2 1− 2f σ 2rx t ð1Þ Gc ¼ h i2 Ef 1 þ f þ 1− f ða=xÞ2
where σrx is the stress acting on the AZO films, equal to P/A (P: applied load, A: contact area). The thickness t of the films is about 500 nm, and the elastic modulus Ef is 110 GPa. The νf denotes the Poisson's ratio, approximate 0.3. The crack length of interface delamination a was measured under SEM observations, and the contact length of the indenter tip x was determined by the instrument. After calculation, the interfacial adhesion energy between the substrate and AZO film deposited at 100 W and 200 °C was obtained as plotted in Fig. 4(b) with different maximum applied loads from 110 to 170 mN. It was found that the interface adhesion energy drastically decreased from 25.4 J/m2 and then stabilized at about 2.4 J/m2 with increasing applied loads. When low indentation loads (110 and 120 mN) were applied, the adhesion energy was inaccurately estimated as larger because film blister and buckling was unclear to identify; whereas with higher applied loads, the blister and buckling became much obvious, and the adhesion energy was more accurately determined as a stable value. 3.4. Interface adhesion measured by nanoscratch test From the load-distance and indentation depth-distance curves of nanoscratch tests of AZO films, a variation was found as the applied load was raised to about 20–30 mN (scratch distance ~ 900–1100 μm). Afterwards, a more drastic fluctuation in the curves occurred. By comparing the surface morphologies of the films along the scratch track as shown in Fig. 5, the fracture behaviors of the films at different scratch stages were realized. Firstly at the early stage of the scratch tests, the AZO films were just pressed, and only a slight scratch trace was observed as shown at the left side of Fig. 5(a). When the load was applied to a critical value Pc, the interfaces between the AZO films and substrates began to delaminate, where clear film buckling and peeling were observed as shown at the right side of Fig. 5(a). During the scratch
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penetration as well as high densification [19]. With higher substrate temperatures, the interface adhesion energy increased as well from 0.49 to 0.79 J/m2 as seen in Fig. 6(b) since it would be easier for the deposited AZO films to react with or to interdiffuse into the substrates to form chemical bonding at the interface at higher temperatures [19]. However, the interface adhesion energy measured by a nanoindentation test was higher than that obtained by a nanoscratch test. The most important difference between these two tests was that the scratch test, besides a normal compressive stress, also provided lateral shear and even tensile stresses moving forward with the indenter tip, thus causing earlier interface delamination. Therefore consisting with the “mode mixity (phase angle)” effect reported by Volinsky [8], the required energy for interface delamination measured under the scratch test was lower and much closer to the theoretical work of adhesion. 4. Conclusions In this study, dense and smooth AZO transparent conducting films with a columnar structure were deposited by magnetron sputtering. The AZO films deposited at low sputtering powers and substrate temperatures were constructed by spherical grains. As the power and temperature increased, the grains grew and became facet with a (002) preferred orientation. The electrical resistivity of the AZO films decreased to 0.03 Ω·cm with higher sputtering powers and substrate temperatures. At a thickness of 500 nm, the AZO films possessed an optical transparency of about 90% and bandgap energy of 3.33 eV. By using nanoindentation tests, the hardness and elastic modulus of the AZO films were measured as about 8 and 110 GPa, respectively, and the
Fig. 3. Hardnesses and elastic moduli of AZO films deposited under (a) different sputtering powers at RT and (b) different substrate temperatures at a sputtering power of 100 W measured by nanoindentation tests.
tests, shear stresses accumulated at the interfaces when the tip indented and scratched. Under sufficient accumulation of shear stresses higher than the interface adhesion strengths, the interfaces then delaminated. As the indenter tip moved forward, the delaminated and buckled film under the tip cracked as shown in Fig. 5(b). The repeated film delamination, buckling, and cracking resulted in stress accumulation and release, leading to the fluctuation of the curves. By using the following equation and introducing critical scratch track widths dc as the interface delaminated, the critical stresses σc for interface delamination (adhesion strength) between AZO films and substrates were obtained as plotted in Fig. 6 [11–15]. σc ¼
2Pc πd2c
ð4 þ f Þ3πμ −ð1−2 f Þ 8
ð2Þ
in which μ is the measured friction coefficient of indenter sliding given by the nanoscratch tester as about 0.035. Afterwards by using the following equation, with the thickness t and elastic modulus Ef of the AZO films, the fracture energy release rates Gc for the interface delamination (adhesion energy) between the films and the substrates was then obtained as plotted in Fig. 6 as well [11–15]. Gc ¼
σ 2c t 2Ef
ð3Þ
From Fig. 6(a), it was found that the interface adhesion energy increased from 0.49 to 0.86 J/m2 with sputtering power because the strong bombardments of deposited atoms under high powers induced deep
Fig. 4. (a) SEM morphology and (b) interface adhesion energy of AZO film deposited under a sputtering power of 100 W and a substrate temperature of 200 °C measured by nanoindentation tests.
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Fig. 5. SEM fractographies of AZO film deposited under a sputtering power of 100 W and a substrate temperature of RT at different stages of nanoscratch test, (a) early-stage interface delamination and (b) middle- and late-stage severe film cracking.
hardness increased to 10 GPa with higher sputtering powers and substrate temperatures because of higher densification and crystallinity. During nanoindentation and nanoscratch tests, the interfaces between the AZO films and substrates delaminated, and the interface adhesion energy was accordingly obtained. By the nanoscratch tests, the adhesion energy increased from 0.49 to 0.86 and 0.79 J/m2, respectively, with increasing sputtering power and substrate temperature because of the deeper penetration, higher densification and easier interface reaction of the deposited films. Acknowledgements The authors gratefully acknowledge the financial supports for this research by the National Science Council, Taiwan, under Grant No. NSC-96-2221-E-005-007, and in part by the Ministry of Education, Taiwan, under the ATU plan. References
Fig. 6. Critical stresses for interface delamination and interface adhesion energy AZO films deposited under (a) different sputtering powers at RT and (b) different substrate temperatures at a sputtering power of 100 W, measured by nanoscratch tests.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[1] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics, Philadelphia, PA, 1995.
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J.F. Chang, C.C. Shen, M.H. Hon, Ceram. Int. 29 (2003) 245. W. Gao, Z. Li, Ceram. Intern. 30 (2004) 1155. K. Zeng, F. Zhu, J. Hu, L. Shen, K. Zhang, H. Gong, Thin Solid Films 443 (2003) 60. W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. A.C. Fischer-Cripps, Nanoindentation, Springer-Verlag, New York, NY, 2002. S.Y. Chang, T.Q. Chang, J. Appl. Phys. 101 (2007) 033507. A.A. Volinsky, N.R. Moody, W.W. Gerberich, Acta Mater. 50 (2002) 441. A.A. Volinsky, J.B. Vella, W.W. Gerberich, Thin Solid Films 429 (2003) 201. S.J. Bull, J. Phys. D: Appl. Phys. 38 (2005) R393. J. Malzbender, J.M.J. den Toonder, A.R. Balkenende, G. De With, Mater. Sci. Eng. R 36 (2002) 47. S.Y. Chang, Y.C. Huang, Microelectron. Eng. 84 (2007) 319. S.Y. Chang, Y.S. Lee, C.L. Lu, J. Electrochem. Soc. 154 (2007) D241. S.J. Bull, D.S. Rickerby, Surf. Coat. Technol. 42 (1990) 151. S.Y. Chang, H.L. Chang, Y.C. Lu, S.M. Jang, S.J. Lin, M.S. Liang, Thin Solid Films 460 (2004) 167. T. Radhouane, H. Bel, J. Eur. Ceram. Soc. 25 (2005) 3301. B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, Prentice-Hall, Inc., Upper Saddle River, NJ, 2001, pp. 169–171. K.L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, UK, 1985. J.W. Lee, S.K. Tien, Y.C. Kuo, C.M. Chen, Surf. Coat. Technol. 200 (2006) 3330.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Preparation and mechanical properties of aluminum-doped zinc oxide transparent conducting films Shou-Yi Chang ⁎, Yen-Chih Hsiao, Yi-Chung Huang Department of Materials Science and Engineering, National Chung Hsing University, Taichung, 402, Taiwan
A R T I C L E
I N F O
Available online 7 June 2008 Keywords: Transparent conductive films Mechanical properties Interface adhesion
A B S T R A C T Aluminum-doped zinc oxide transparent conducting films were deposited in this study by magnetron sputtering under different sputtering powers and substrate temperatures. At low sputtering powers and substrate temperatures, the deposited films were constructed by spherical grains. With increasing power and temperature, the grains became facet with an obvious (002) preferred orientation. The crystallinity and grain size of the films increased as well, and consequently the electrical resistivity decreased. By nanoindentation tests, the hardness of the deposited films was measured and found to increase from 8 to 10 GPa with higher sputtering power and substrate temperature because of higher densification and crystallinity. During nanoindentation and nanoscratch tests, interface delamination occurred between the films and substrates, and the interface adhesion energy was accordingly obtained. From the measurement of nanoscratch tests, the adhesion energy was found to be improved from 0.49 to 0.86 or 0.79 J/m2, respectively, with higher sputtering power or substrate temperature because of the deeper penetration, higher densification and easier interface reaction of the deposited films. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Transparent conducting oxide (TCO) films with good optical and electrical properties (transparency N80%, electrical resistivity b10− 3 Ω·cm) have been widely applied to optoelectronic industry such as solar cells and displays [1]. Typical TCO films include SnO2, In2O3, and ZnO, etc. Minor elements are further doped to obtain better film properties, like the most popularly used indium-doped tin oxide (ITO). However due to the poor thermal stability and high cost of ITO films, cheap and non-toxic ZnO films have attracted much attention in these years. Especially, aluminum-doped zinc oxide (ZnO:Al, AZO) films exhibit comparable optical and electrical properties with ITO films, and have a high potential to replace conventional TCO films [2,3]. For the preparation of TCO films, radio-frequency (RF) magnetron sputtering, chemical vapor deposition, thermal evaporation, and sol–gel methods have been used. Among them, the RF magnetron sputtering by which the films are deposited at lower temperatures with high qualities is more generally adopted [1–3]. However, the mechanical damages of TCO films, such as film cracking and interface delamination, severely suppress the processing yield and application reliability of the films [4]. Especially for the TCO films applied on flexible substrates, repeated flexure stresses during application more easily result in film damages. A high resistance to the mechanical damages is thus strongly demanded for the TCO films
⁎ Corresponding author. Tel.: +886 4 22857517. E-mail address: [email protected] (S.-Y. Chang). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.06.024
besides good optical and electrical properties, and the mechanical properties need to be clarified before practical applications. However, conventional testing tools are not longer suitable for the evaluation of mechanical properties of thin TCO films. Alternately, nanoindentation and nanoscratch tests have been widely applied for the measurement of the mechanical properties of thin films [5–15]. Besides hardness and elastic modulus [5,6], more information such as yielding stress and fracture toughness can be extracted to reveal more representative mechanical properties of thin films [7]. Moreover, the nanoindentation and nanoscratch tests are also promising to determine interface adhesion strength through film delamination [8–15]. Thus in this study, AZO transparent conducting films have been prepared on glass substrates by magnetron sputtering. Their microstructures and basic properties including electrical resistivity and optical transparency are characterized. Moreover, the mechanical properties of the films and the interface adhesion energy between the films and substrates are investigated by nanoindentation and nanoscratch tests. The interface delamination behaviors are examined to evaluate the mechanical reliability of the AZO films. Sputtering power and substrate temperature are varied to investigate their effects on the properties of the films. 2. Experimental details The AZO films were deposited on Corning 7059 glass substrates by RF magnetron sputtering at a pressure of 8 mTorr using a 98 wt.% ZnO2 wt.% Al2O3 target. The sputtering power was varied from 50 to 200 W (at room temperature, RT), and the substrate temperature was
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controlled at RT, 100 °C, and 200 °C (at 100 W). The deposition rate increased from 6.7 to 28.3 nm/min with sputtering power, while it remained at about 14 nm/min under different substrate temperatures. The thickness of the deposited films was controlled at about 500 nm by varying deposition time. The crystal structure of the AZO films was analyzed by an X-ray diffractometer (XRD, MAC Science MXP3). A scanning electron microscope (SEM, JEOL JSM-6700F) was used to observe the surface morphologies and microstructures of the films. Atomic force microscopy (AFM, SEIKO Instrument SPI3800N) was applied to characterize surface roughness. A four-point probe method was applied to measure electrical resistivity, and UV/visible spectroscopy (Hitachi U-3010) was used to measure optical transparency. The mechanical properties of AZO films were measured by using a UMIS nanoindenter (Based Model, CSIRO) with a Berkovich diamond indenter (tip radius ~ 100 nm, edge angle 130.6 ) and a nanoscratch test module. During nanoindentation tests, the load was applied to a maximum value of 1 mN, held for 30 s, and then released under a loading/unloading rate of 0.033 mN/s. The indentation depth was controlled at 1/15 to 1/10 of film thickness to avoid a substrate effect. For the measurement of interface adhesion energy between the AZO films and glass substrates by nanoindentation tests, a series of indentations with varied maximum loads from 110 to 170 mN were performed. Under nanoscratch tests, the load was ramped from 0 to 50 mN in 30 s, and the scratch length was set as 1500 μm at a moving velocity of 50 μm/s. An SEM was used to observe interface delamination around indent marks and scratch tracks after the nanoindentation and nanoscratch tests. 3. Results and discussion 3.1. Microstructures and basic characterizations of AZO films Fig. 1 shows the XRD patterns of AZO films deposited under different sputtering powers and substrate temperatures. At low sputtering powers and substrate temperatures, three main diffraction peaks of ZnO (100), (002) and (101) crystal planes were detected, indicating no preferred crystalline orientation in the films. With increasing sputtering power to 150 W or substrate temperature to 100 °C, higher energy for the regular arrangement of incident atoms promoted the growth of the AZO films in a low-energy (002) direction, then enhancing the formation of more obvious (002) preferred orientation [16]. By using the full widths at half maximum of the (002) diffraction peaks and the Scherrer equation [17], the grain sizes of the AZO films can be calculated. With higher sputtering powers, the grain size increased from 11.0 to 14.1 nm since the higher energy of incident atoms promoted grain growth. At higher substrate temperatures, the grain size even increased from 11.2 to 17.5 nm because of a rapid grain growth. Fig. 2 shows the SEM surface morphologies and cross-sectional microstructure of AZO films deposited under different sputtering powers and substrate temperatures. From Fig. 2(a), it was observed that the AZO films deposited under low sputtering powers and substrate temperatures were constructed by spherical particles of about 10 nm in size without specific preferred orientation. With increasing sputtering power and substrate temperature, the particles grew and became a facet shape as shown in Fig. 2(b) and (c) with an obvious preferred orientation, in accordance with the XRD analyses shown in Fig. 1. From Fig. 2(d), it was observed that the deposited AZO films were continuous, dense, and smooth with a uniform thickness of about 500 nm. The films possessed a columnar structure. Moreover from the AFM analyses of the AZO films, very small surface roughness was measured for most of the films as only about few nanometers. The electrical resistivity of AZO films was lowered from 0.9 to 0.09 and 0.03 Ω·cm as the sputtering power increased to 200 W and the substrate temperature to 200 °C, respectively. As aforementioned, the crystallinity of the deposited films was enhanced with an obvious
Fig. 1. XRD patterns of AZO films deposited under (a) different sputtering powers at RT and (b) different substrate temperatures at a sputtering power of 100 W.
(002) preferred orientation when the sputtering power and substrate temperature increased. Moreover, grain growth and film densification occurred, reducing defects (grain boundaries, etc.) and thus electrical resistivity. Furthermore from the measurement of optical transparency at different wavelengths, it was observed that all of the AZO films deposited under different sputtering powers and substrate temperatures possessed an optical transparency of about 90% at the film thickness of 500 nm. The bandgap energy Eg of the AZO films decreased from 3.35 to 3.28 eV with increasing sputtering power, while it remained at a constant value of about 3.33 eV with varied substrate temperatures. 3.2. Nanomechanical properties of AZO films From the load-penetration depth curves of nanoindentation tests of AZO films, it was found that, under small indentation depths below 15 nm, the loading curves matched to the elastic unloading curves in accordance with a “Hertzian elastic relation” [18], indicating the elastic deformation of the AZO films. Beyond the depth, the curves began to deviate from the “Hertzian response”, and the permanently plastic deformation of the AZO films was expected to occur. From the Oliver–Pharr relation [5], the hardness and elastic modulus of the AZO films deposited under different experimental conditions were obtained as plotted in Fig. 3. It was found that, although the deposition
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Fig. 2. SEM surface morphologies of AZO films deposited under different sputtering powers and substrate temperatures, (a) 100 W at RT, (b) 200 W at RT, and (c) 100 W at 200 °C. (d) SEM cross-sectional microstructure of AZO film deposited under 100 W at RT.
parameters varied, the elastic modulus of the AZO films basically remained at the level of about 110 GPa because the elastic modulus was an intrinsic property of materials. In comparison, the hardness increased from 8 to 10 GPa with higher sputtering powers and substrate temperatures. As aforementioned, the stronger bombardments under higher powers and the higher energy for crystallization at higher temperatures induced higher densification (less porosity) and crystallinity of the deposited films, thus enhancing the hardness [19]. 3.3. Interface adhesion measured by nanoindentation test Nanoindentation introduced localized mechanical deformation to AZO films, and shear stresses began to accumulate at the interface between the AZO films and glass substrates due to strain mismatches. Once the accumulated stresses exceeded interface adhesion strength, the interface would then delaminate. Fig. 4(a) shows the SEM surface morphology of the AZO film deposited under a sputtering power of 100 W and a substrate temperature of 200 °C after nanoindentation test at a maximum applied load of 130 mN. When the load of 120 mN was applied, unclear film blister and buckling just formed around the indented region, indicating the occurrence of interface decohesion. Under higher applied loads than 130 mN, film blister and buckling became much more obvious, and the buckled film was further pressed by the indenter tip and then slightly cracked. By using the following equation [8–13], the fracture energy release rate Gc, i.e. the adhesion energy per unit area, for interface delamination between AZO films and substrates can be then obtained. 2 1− 2f σ 2rx t ð1Þ Gc ¼ h i2 Ef 1 þ f þ 1− f ða=xÞ2
where σrx is the stress acting on the AZO films, equal to P/A (P: applied load, A: contact area). The thickness t of the films is about 500 nm, and the elastic modulus Ef is 110 GPa. The νf denotes the Poisson's ratio, approximate 0.3. The crack length of interface delamination a was measured under SEM observations, and the contact length of the indenter tip x was determined by the instrument. After calculation, the interfacial adhesion energy between the substrate and AZO film deposited at 100 W and 200 °C was obtained as plotted in Fig. 4(b) with different maximum applied loads from 110 to 170 mN. It was found that the interface adhesion energy drastically decreased from 25.4 J/m2 and then stabilized at about 2.4 J/m2 with increasing applied loads. When low indentation loads (110 and 120 mN) were applied, the adhesion energy was inaccurately estimated as larger because film blister and buckling was unclear to identify; whereas with higher applied loads, the blister and buckling became much obvious, and the adhesion energy was more accurately determined as a stable value. 3.4. Interface adhesion measured by nanoscratch test From the load-distance and indentation depth-distance curves of nanoscratch tests of AZO films, a variation was found as the applied load was raised to about 20–30 mN (scratch distance ~ 900–1100 μm). Afterwards, a more drastic fluctuation in the curves occurred. By comparing the surface morphologies of the films along the scratch track as shown in Fig. 5, the fracture behaviors of the films at different scratch stages were realized. Firstly at the early stage of the scratch tests, the AZO films were just pressed, and only a slight scratch trace was observed as shown at the left side of Fig. 5(a). When the load was applied to a critical value Pc, the interfaces between the AZO films and substrates began to delaminate, where clear film buckling and peeling were observed as shown at the right side of Fig. 5(a). During the scratch
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penetration as well as high densification [19]. With higher substrate temperatures, the interface adhesion energy increased as well from 0.49 to 0.79 J/m2 as seen in Fig. 6(b) since it would be easier for the deposited AZO films to react with or to interdiffuse into the substrates to form chemical bonding at the interface at higher temperatures [19]. However, the interface adhesion energy measured by a nanoindentation test was higher than that obtained by a nanoscratch test. The most important difference between these two tests was that the scratch test, besides a normal compressive stress, also provided lateral shear and even tensile stresses moving forward with the indenter tip, thus causing earlier interface delamination. Therefore consisting with the “mode mixity (phase angle)” effect reported by Volinsky [8], the required energy for interface delamination measured under the scratch test was lower and much closer to the theoretical work of adhesion. 4. Conclusions In this study, dense and smooth AZO transparent conducting films with a columnar structure were deposited by magnetron sputtering. The AZO films deposited at low sputtering powers and substrate temperatures were constructed by spherical grains. As the power and temperature increased, the grains grew and became facet with a (002) preferred orientation. The electrical resistivity of the AZO films decreased to 0.03 Ω·cm with higher sputtering powers and substrate temperatures. At a thickness of 500 nm, the AZO films possessed an optical transparency of about 90% and bandgap energy of 3.33 eV. By using nanoindentation tests, the hardness and elastic modulus of the AZO films were measured as about 8 and 110 GPa, respectively, and the
Fig. 3. Hardnesses and elastic moduli of AZO films deposited under (a) different sputtering powers at RT and (b) different substrate temperatures at a sputtering power of 100 W measured by nanoindentation tests.
tests, shear stresses accumulated at the interfaces when the tip indented and scratched. Under sufficient accumulation of shear stresses higher than the interface adhesion strengths, the interfaces then delaminated. As the indenter tip moved forward, the delaminated and buckled film under the tip cracked as shown in Fig. 5(b). The repeated film delamination, buckling, and cracking resulted in stress accumulation and release, leading to the fluctuation of the curves. By using the following equation and introducing critical scratch track widths dc as the interface delaminated, the critical stresses σc for interface delamination (adhesion strength) between AZO films and substrates were obtained as plotted in Fig. 6 [11–15]. σc ¼
2Pc πd2c
ð4 þ f Þ3πμ −ð1−2 f Þ 8
ð2Þ
in which μ is the measured friction coefficient of indenter sliding given by the nanoscratch tester as about 0.035. Afterwards by using the following equation, with the thickness t and elastic modulus Ef of the AZO films, the fracture energy release rates Gc for the interface delamination (adhesion energy) between the films and the substrates was then obtained as plotted in Fig. 6 as well [11–15]. Gc ¼
σ 2c t 2Ef
ð3Þ
From Fig. 6(a), it was found that the interface adhesion energy increased from 0.49 to 0.86 J/m2 with sputtering power because the strong bombardments of deposited atoms under high powers induced deep
Fig. 4. (a) SEM morphology and (b) interface adhesion energy of AZO film deposited under a sputtering power of 100 W and a substrate temperature of 200 °C measured by nanoindentation tests.
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Fig. 5. SEM fractographies of AZO film deposited under a sputtering power of 100 W and a substrate temperature of RT at different stages of nanoscratch test, (a) early-stage interface delamination and (b) middle- and late-stage severe film cracking.
hardness increased to 10 GPa with higher sputtering powers and substrate temperatures because of higher densification and crystallinity. During nanoindentation and nanoscratch tests, the interfaces between the AZO films and substrates delaminated, and the interface adhesion energy was accordingly obtained. By the nanoscratch tests, the adhesion energy increased from 0.49 to 0.86 and 0.79 J/m2, respectively, with increasing sputtering power and substrate temperature because of the deeper penetration, higher densification and easier interface reaction of the deposited films. Acknowledgements The authors gratefully acknowledge the financial supports for this research by the National Science Council, Taiwan, under Grant No. NSC-96-2221-E-005-007, and in part by the Ministry of Education, Taiwan, under the ATU plan. References
Fig. 6. Critical stresses for interface delamination and interface adhesion energy AZO films deposited under (a) different sputtering powers at RT and (b) different substrate temperatures at a sputtering power of 100 W, measured by nanoscratch tests.
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