2004 Trans Tech Publications, Switzerland. temperatures .... has a lower CTE than ZnO, and yet the (002) peak of ZnO on Si was also shifted to smaller values of.
Key Engineering Materials Vols. 264-268 (2004) pp. 1225-1228 online at http://www.scientific.net © 2004 Trans Tech Publications, Switzerland
Self-induced Crystallinity in RF Magnetron Sputtered ZnO Thin Films I. Özen, M. A. Gülgün and Meriç Özcan Sabanci University, Faculty of Engineering and Natural Sciences, 34956, Orhanli, Tuzla, Istanbul, Turkey Keywords: ZnO, thin films, RF magnetron sputtering, crystallinity, epitaxy.
Abstract. ZnO films were coated on the order of micrometer thickness on various substrates using RF magnetron sputtering. Glass, mica and Si were used as amorphous and crystalline substrates to study film growth. X-ray diffraction measurements revealed a self-induced, (002) oriented texture on all substrates. Effects of residual stresses on growth behavior and possible mechanisms of textured crystallization on crystalline and amorphous substrates are discussed. Introduction ZnO is a large band-gap (3.20-3.43 eV [1]) semiconductor with the added advantage of being a low cost electronic ceramic with good optical quality, excellent piezoelectric properties and several others. Thin films of ZnO have already found many electronic and opto-electronic applications such as piezoelectric devices, optical modulators, and acoustic wave filters. Various coating methods had been successfully applied for good quality textured layers, not only on crystalline but also on amorphous substrates. Textured growth of ZnO on amorphous substrates deposited by RF magnetron sputtering [1,2], CVD [3], DC magnetron sputtering [4], and laser ablation [5] had been reported. Morphological uniformity, phase purity and crystallinity are important parameters that strongly affect the electrical, mechanical, and optical properties of a material. For ZnO, such examples are the electromechanical coupling for the piezoelectric devices [6], the elastic constant [7], and optical indices of thin films in the UV-VIS wavelength range [1,8]. Thus, understanding the influence of substrate structure and the coating parameters on film structure is critical. Experiment ZnO films were coated on various amorphous and crystalline substrates by a radio frequency magnetron sputtering equipment (Rescue-E, Teknoplazma, Ankara, Turkey). Microscope slides (IsoLab, Wertheim, Germany) with dimensions of 20x20x0.16 mm3 were used as glass substrates. High quality mica slides (Ted Pella Inc., CA, USA) and phosphor doped, (100) oriented silicon wafer pieces were used as crystalline substrates. Si wafer pieces were of various sizes all with a thickness of 508 ± 20 µm. Pure (99.999 %) ZnO sputtering targets were provided by Kurt J. Lesker (Clairton, PA, USA). Target diameter and thickness were 50.8 and 3.175 mm, respectively. Experimental parameters such as temperature, pressure and specimen rotation were controlled during deposition. Substrate-to-target distance was kept at 14 cm. Before coating the chamber was evacuated to 10-7 mbar and then the substrate was subjected to argon-ion plasma cleaning. Coating was held under pure argon atmosphere at 0.02 mbar. Two different initial temperatures were 50 and 200 °C. The system was heated to these
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temperatures until thermal equilibrium was attained. Temperature was monitored by irreversible temperature strips stuck to both the substrates and the holder. Once the coating started the heating stopped and the films grew under decreasing temperature. The corresponding final plasma temperatures were 39 and 63 °C. RF power was 100 W and the coated films were 0.5 µm thick on the average. Crystal phases were examined by x-ray diffractometry (XRD; D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany). The x-ray generator voltage and current were held at 40 kV and 40 mA, respectively. The reflection angle (2θ) was varied continuously from 10° to 90° in locked-couple mode with a rate of 0.02°/s. The diffraction data was analyzed by DIFFRAC PLUS Evaluation software. Film thickness and microstructure was monitored by JEOL JSM-840A Scanning Electron Microscope (Tokyo, Japan). Results and Discussion ZnO films tend to grow in columnar microstructure on glass (Fig. 1). XRD measurements revealed that these films have highly oriented hexagonal lattice (Fig. 2). Comparing the shifts, shapes, and intensities of the (002) peak for ZnO films deposited on different substrates at different temperatures, the following observations are made: The (002) peak appeared in a lower 2θ value than that for the powder specimen, in all of the experiments. The shift in the (002) peak and hence the strain in the ZnO films on glass substrates could be attributed among other things to thermal effects considering the differences in thermal expansion coefficients (CTEs) and the accompanying warping of the laminate (CTE for ZnO is given as 5-8 x 10–6/°C
Figure 1. SEM image of ZnO film on glass (side-view). Initial substrate temperature = 200 °C. Film thickness is approximately 600 nm. in the range of 25-400 °C [9]. CTEs for soda-lime glass and silicon are 8-9 x 10–6/°C and 2.5 x 10-6/°C, respectively. Mica has a CTE of 10 x 10-6/°C for parallel to cleavage plane and 10 x 10-5/°C for perpendicular to cleavage plane.). During cooling, ZnO resists shrinkage due to its lower thermal expansion coefficient than glass and the laminate warps towards the glass side, leaving the film under strong compressive stress. Mica and silicon substrates appear to be too thick for such a warping effect. However, similar peak shifts were also observed for ZnO on both of them. Mica has a CTE slightly higher than glass; hence the (002) peak would be expected to show a similar shift to the left of the powder peak and similar strains to films on glass. However, ZnO on mica had the lowest strain among
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all the coatings at both temperatures. Most critical example is the ZnO film deposited on Si wafer. Si has a lower CTE than ZnO, and yet the (002) peak of ZnO on Si was also shifted to smaller values of 2θ and hence revealed a positive strain along c-axis. In light of these observations, thermal strains cannot be the only reason for residual stresses in deposited ZnO films. One explanation for the observed behavior could be a possible epitaxial growth of ZnO films on Si, creating additional stresses.
Figure 2. XRD scans of ZnO films on various substrates, the initial coating temperature being (a) 50 °C, (b) 200 °C. Vertical lines show the (002) plane of polycrystalline ZnO powder. From the shift in the peak positions the strain in the c- direction (ε1) and from ε1, the strain in the plane of the film (ε2) can be estimated (Eq. 1-3). Poisson’s ratio (ν) of ZnO is taken as 0.36. ε1 = (d1,film – d1,powder) / d1,powder x 100,
(1)
cfilm = 2 x d1, film,
(2)
ε2 = - ε1 / ν.
(3)
The results are given in Table 1 together with the XRD data. For powder ZnO the 2θ, d, and c values are 34.413°, 2.6033 Å, and 5.2066 Å, respectively. Table 1. Variation of lattice parameters and strain values on different substrates for films deposited at To of 50 and 200 °C. The XRD data belongs to the (002) peak. To (°°C) 50 200
substrate glass mica silicon glass mica silicon
2θ θ (°°) 34.117 34.171 34.124 33.974 34.063 33.710
d (Å) 2.6259 2.6219 2.6254 2.6366 2.6299 2.6567
c (Å) 5.2518 5.2438 5.2508 5.2732 5.2598 5.3134
ε1 (%) 0.87 0.72 0.85 1.28 1.02 2.05
ε2 (%) -2.41 -1.99 -2.36 -3.55 -2.84 -5.70
Films also showed an increasing intensity of the (002) peak with increasing temperature. For films deposited at 50 oC, the substrates make little difference. XRD peaks for all films are symmetric and the full-width-half-maximum (FWHM) values are very close to each other. However, when the deposition
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is carried out at 200 oC, the (002) peak of the film deposited on Si wafer is asymmetric and extremely broad. The films with narrowest and most symmetric peaks were the films deposited on amorphous substrates (i.e., glass). Broadening can be due to two effects: i) distortion introduced to the lattice creating a distribution in d values (asymmetric broadening) and ii) to a lesser degree due to a decrease in grain size (symmetric broadening), which is under investigation. In addition to this, the broadening in XRD peaks may be due to non-uniform strains in the films, caused by a possible substrate–induced in-plane texturing of films. Results of XRD experiments with fixed φ-angle (rotation of the film around its surface normal while taking θ−2θ measurements) supports this interpretation: only for films on Si, the peak intensity was a function of the rotation angle φ, indicating a possible in-plane texturing. Transmission electron microscopy (TEM) studies are currently underway to investigate the crystallography of the ZnO/Si interface to clarify this issue. To conclude it can be stated that c-axis textured ZnO films grew on both amorphous and crystalline substrates. The XRD peak shifts indicated residual compressive stress on the films where the stresses, hence the accompanying strains, increase with increasing substrate temperature. Thermal strains are believed to be dominant for amorphous glass. The next question to answer is whether there is an epitaxial relationship between ZnO films and crystalline substrates and if this could explain the observed strain states for the films. XRD examinations of annealed samples are necessary to understand the contribution of thermal strains. SEM and TEM analysis would reveal the changes in grain size and the effects of epitaxy. Acknowledgements We wish to thank Baybars Oral (Teknoplazma) and Bülent Köroğlu (Microelectronics – Sabancı University) for their help in the RF sputtering experiments. References [1] E. Dumont, B. Dugnoille and S. Bienfait: Thin Solid Films Vol. 353 (1999), p. 93. [2] E.M. Bachari, G. Baud, S. Ben Amor and M. Jacquet: Thin Solid Films Vol. 348 (1999), p. 165. [3] B.M. Ataev, I.K. Kamilov, A.M. Bagamadova, V.V. Mamedov, A.K. Omaev and M.Kh. Rabadanov: Mater. Sci. and Eng. B Vol. 68 (1999), p. 56. [4] L.-J. Meng and M. P. dos Santos: Thin Solid Films Vol. 250 (1994), p. 26. [5] Z.G. Liu, W.S. Hu, X.L. Guo, S.N. Zhu, D. Feng and C. Lin: Mater. Lett. Vol. 25 (1995), p. 5. [6] M. Akiyama, H.R. Kokabi, K. Nonaka, K. Shobu and T. Watanabe: J. Am. Ceram. Soc. Vol. 78 (1995), p. 3004. [7] W. Water and S.-Y. Chu: Mater. Lett. Vol. 55 (2002), p. 67. [8] P. Chindaudom and K. Vedam: Characterization of Inhomogeneous Transparent Films, Physics of Thin Films Vol. 19 (Academic Press, New York, 1994). [9] M.-Y. Han and J.-H. Jou: Thin Solid Films, Vol. 260 (1995), p. 58.
