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Si(100) system
Superlattices and Microstructures 39 (2006) 75–82 www.elsevier.com/locate/superlattices
Brillouin light scattering characterization of the surface acoustic wave velocity in the ZnO/Si3 N4 /Si(100) system E. Céspedes∗, R.J. Jiménez-Riobóo, M. Vila, C. Prieto Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Cantoblanco, 28049-Madrid, Spain Available online 12 September 2005
Abstract The Surface Acoustic Wave (SAW) phase velocity of ZnO thin films has been determined by High Resolution Brillouin Light Scattering Spectroscopy in order to study the influence of an intermediate layer with a set of elastic constants higher than the corresponding ones for ZnO. The study of the system formed by ZnO/Si3 N4 /Si(100), deposited by reactive magnetron sputtering, shows a SAW phase velocity increment with respect to the ZnO/Si(100) single layer system. Results are discussed in terms of the elasticity theory, which predicts an increase of the acoustic wave phase velocity when an intermediate layer stiffer than the film at the surface is present in the system. © 2005 Elsevier Ltd. All rights reserved.
1. Introduction Surface Acoustic Wave (SAW) devices are electronic components widely used as frequency filters or resonators in a variety of systems such as mobile phones, television tuners and optical-communication systems. Due to their compactness, great stability, reproducibility and high performance of frequency characteristics, these are excellent devices for the recent communication systems [1]. SAW devices based on piezoelectric ∗ Corresponding author. Tel.: +34 913349000; fax: +34 913720623.
E-mail address: [email protected] (E. Céspedes). 0749-6036/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2005.08.032
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thin films and grown on silicon substrates present several advantages over the conventional ones (quartz, lithium tantalate and lithium niobate): low power consumption, reduction of cost and circuit miniaturization and integration. Among lately developed piezoelectric thin films, zinc oxide is a very promising material for developing SAW sensors on silicon due to its high piezoelectric coupling factor [2–4]. Zinc oxide thin films exhibit piezoelectric behaviour when the crystallites have the c-axis perpendicular to the substrate [2]. A variety of deposition techniques, such as the sol–gel process, spray pyrolysis, chemical vapour deposition (CVD), pulsed laser deposition (PLD) [5], evaporation and sputtering have been employed for the growth of ZnO films on different substrates [6–8]. Among them, the sputtering technique is considered to be the most favorable deposition method because it allows obtaining well-oriented and uniform ZnO films even on amorphous substrates at high deposition rates [8]. With new advances in high frequency and high bit-rate communication systems, the demand for higher frequency SAW devices is increasing, and great effort has been made in the development of higher frequency SAW devices. Since the signal processing frequency ( f 0 ) is determined by the ratio between the phase velocity of the surface acoustic wave V SAW and the spatial period λ of the interdigital transducers (IDT), there are two approaches to get high frequency SAW devices. One is to reduce the line width of electrodes by means of special lithography techniques. However, the reduction of the IDT size causes problems of less reliability, power durability and fabrication margin in the manufacturing process. Another approach is to find the high-velocity material for the SAW substrate, as the operating frequency is proportional to the SAW velocity [1]. Multilayers including piezoelectric thin films coupled with high phase velocity substrates, such as diamond, sapphire, and sometimes diamond-like carbon (DLC) have been reported to achieve high-frequency SAW devices without requiring submicron technologies [9,10]. Among the proposed multilayer configurations, the ZnO/diamond structure has exhibited the largest phase velocities (approx. 10 000 m/s) [1]. The possibility of V SAW up to 10 000 m/s has been also reported in the case of AlN/diamond/Si and LiNbO3 /diamond/Si structures [11], though, there have been just a few approaches to fabricate c-axis oriented films of these materials on diamond substrates [12,13]. However, although ZnO/diamond/Si is one of the most promising material components for gigahertzband SAW filters, there still exist several difficulties in employing diamond for SAW devices, such as complicated growth conditions for high-quality film, incompatibility with conventional semiconductor processes, and large surface roughness (typically, several hundred nanometers) hindering the propagation of surface waves [14,15]. There have also been some attempts to obtain experimentally high SAW velocity in a ZnO/dielectric/Si layered structure, by using SiO2 , Si3 N4 and TiO2 dielectric layers [16]. Acoustic investigations of the phase velocity of surface phonons propagating on ZnO films deposited on Si, Al2 O3 and quartz substrates by means of Brillouin scattering have been previously reported [17]. In this work, the propagation characteristics of SAW modes in ZnO/Si and ZnO/Si3 N4 /Si layered structures were examined theoretically and experimentally. The experimental behavior of V SAW was observed by means of high resolution Brillouin spectroscopy (HRBS).
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2. Experimental The Si3 N4 dielectric layer was grown by reactive sputtering at room temperature on Si(100) by using a planar 2 inch magnetron source (Angstrom Science) operated by RFpower supply (Huttinger Gmbh.). The residual pressure provided by the vacuum system was 1 × 10−4 Pa. A pure silicon circular target with thickness of 3 mm was mechanically clamped to the water-cooled electrode. N2 was used as reactive sputtering gas with an RF power of 100 W and the deposition rate was 2nm/min. A Si3 N4 layer 0.5 µm thick was deposited on top of the Si(100) wafers. This thickness was selected because, after calculations, the effect of the silicon substrate becomes negligible for silicon nitride layers thicker than 0.3 µm. After Si3 N4 deposition, ZnO films were grown by reactive sputtering at room temperature. The films were deposited at the same time on Si(100) and on the previous Si3 N4 layer, in order to compare V SAW in ZnO films grown under identical conditions. The planar magnetron source was operated by a DC-power supply and the working chamber was pumped down to 1 × 10−4 Pa. DC reactive sputtering from a pure zinc metallic target (99.99% purity) was carried out in a mixed atmosphere of argon and oxygen (60% Ar–40% O2 ). By using a current intensity of 20 mA (∼6 W), a deposition rate of about 1 nm/min was obtained. The thickness of the ZnO thin film, measured by X-ray reflectivity, was 73 nm. Simulations of SAW velocity behavior in ZnO/Si3 N4 layered structure showed that in this range of ZnO thickness, the effects of the Si3 N4 layer were appreciable. The crystallinity and crystallographic orientation of the ZnO films were determined by X-ray diffraction. A D8Brucker-AXS diffractometer operated at the Kα emission of a Cu anode has been used. The degree of c-axis orientation normal to the substrate was evaluated from analysis of X-ray rocking curves. The experimental Brillouin Light Scattering set-up has already been described elsewhere [18]. It can be summarized as follows: the light source was a 2060 Beamlok Spectra Physics Ar+ ion laser provided with an intracavity temperature stabilized singlemode and single-frequency z-lok etalon (λ0 = 514.5 nm). The scattered light was analyzed using a Sandercock-type 3 + 3 tandem Fabry–Pérot interferometer [19]. The incident polarization direction was chosen to be in-plane (p-polarization) while no polarization analysis of the scattered light was made. The typical values for finesse and contrast were 150 and 109 , respectively. 3. Results and discussion Fig. 1(a) shows the typical diffraction diagram of the sputtered ZnO films. Samples exhibit the characteristic peak of the ZnO hexagonal wurtzite-type structure. Fig. 1(b) gives the X-ray rocking curves for the (002) peak to show a c-axis preferred orientation. In order to study the influence of buried interlayers on the SAW propagation velocity (from now on SAW velocity), angular dependent HRBS was the experimental technique chosen [20]. ZnO films are very transparent materials thus making it extremely difficult to obtain information on SAW velocity by means of HRBS. There are two main scattering mechanisms that couple light with acoustic phonons; the elasto-optic coupling mechanism, and the ripple mechanism [21]. In transparent materials the first one dominates and
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Fig. 1. (a) Room temperature θ –2θ X-ray diffraction spectra of ZnO films deposited on Si(100) and on Si3 N4 /Si(100). (b) Rocking curves of ZnO/Si(100) and ZnO/Si3 N4 /Si(100) samples. The detector angle is the position of the (002) peak of ZnO.
bulk acoustic waves can be studied. In opaque materials (for instance metals) the ripple mechanism is the one to be observed allowing the study of surface acoustic waves. In the case of ZnO films, it is clear that the dominant mechanism is the elasto-optic coupling and, in order to obtain information about the SAWs, it is necessary to find a way to enhance the ripple scattering mechanism. A successful way to do this is to deposit a very thin metallic film on the transparent sample [22–25]. It has been shown that the thin metallic film will reproduce the main features of the SAWs of the transparent material. This technique was already employed for AlN and synthetic diamond films [22,25]. After that reported knowledge, an Al thin film was deposited on each of the different ZnO samples via DC magnetron sputtering. The Al film thickness was determined to be 50 nm by X-ray Reflectivity. The azimuthal angle dependent Brillouin spectroscopy was performed for all the samples at backscattering with a sagittal angle of 55◦. The sample rotates about the normal to the film surface. Brillouin spectra have been recorded under a geometric arrangement that makes k × h = 1 (k is the scattering wave vector and h is the thickness of the Al thin film).
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Fig. 2. (a) Brillouin spectra of Al/ZnO/Si(100) and Al/ZnO/Si3 N4 /Si(100) samples taken at scattering angle of α = 55◦ . (b) Brillouin frequency shift of surface acoustic phonons of Al/ZnO/Si(100) and Al/ZnO/Si3 N4 /Si(100) samples, as a function of the azimuth angle.
All the recorded HRBS spectra are of similar quality independently of the substrate (either Si or Si3 N4 ). Fig. 2(a) shows typical spectra, where the Rayleigh mode (SRM) and a higher order Sezawa mode (SSM) can be observed. Fig. 2(b) gives the azimuthal dependence of the Rayleigh mode for both studied samples. The Surface Acoustic Wave propagation velocity can be obtained straightforwardly from the Brillouin frequency shift ( f ): V SAW =
2π f f λ0 = 2 sin(α) q SAW
(1)
λ0 is the laser wavelength in vacuum, q SAW the scattering wave vector and α is the scattering angle (in this case 55◦ ). In order to evaluate the influence of the different substrates, we have proceeded to simulate the HRBS spectrum of the system Al thin film on ZnO layer on Si and on Si3 N4 substrates. The simulation procedure is based on the Green function algorithm developed by Every et al. [26]. Fig. 3 shows the results of the simulation for Al(50 nm)/ZnO(73 nm) on top of both substrates (Si and Si3 N4 ). The difference in the simulated spectra introduced only by the change of substrate becomes obvious, the one with the Si3 N4
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Fig. 3. (a) Numerical simulation of a Brillouin spectrum of Al/ZnO/Si(100). (b) Numerical simulation of a Brillouin spectrum of Al/ZnO/Si3 N4 . Peaks in the 10–12 GHz range correspond to the surface Rayleigh mode (SRW). These results have been obtained by using the following density and elastic constants of the different materials. Si: (ρ = 2329 kg/m3 ; c11 = 165.6 GPa; c12 = 63.9 GPa; c44 = 79.5 GPa); Si3 N4 : (ρ = 3300 kg/m3 ; c11 = 387.39 GPa; c12 = 143.30 GPa; c44 = 122.05 GPa); ZnO: (ρ = 5676 kg/m3 ; c11 = 209.7 GPa; c33 = 210.9 GPa; c13 = 105.1 GPa; c44 = 42.5 GPa; c66 = 44.3 GPa); Al: (ρ = 2702 kg/m3 ; c11 = 110.57 GPa; c12 = 56.47 GPa; c44 = 27.05 GPa).
substrate being stiffer. The difference is about 13% between the corresponding Rayleigh modes (SRM). The obtained Brillouin shifts for Al(50 nm)/ZnO(73 nm)/Si(100) and Al(50 nm)/ZnO(73 nm)/Si3 N4 (500 nm) systems are10.43 and 11.83 GHz. These results agree with the experimentally determined SRM modes of Al/ZnO/Si and Al/ZnO/Si3 N4 /Si samples shown in Fig. 2(a). There are at least two facts that are interesting in these data; the lack of clear angular symmetry (Fig. 2(b)) and the confirmed difference in elastic properties of the SAWs of ZnO depending on the type of substrate. At least in the case of the Al/ZnO/Si sample, one could expect some influence of the cubic Si substrate (that gives in the simulation an azimuthal oscillation about 2%) but our experimental data give no evidence of this influence. The difference observed in SAW velocity between our two samples (10.16 and 10.43 GHz; that by using Eq. (1)) correspond to SAW velocity values of 3193 m/s and 3276 m/s, respectively) is about 2.5%, far from the difference obtained by the simulations. The reason for these discrepancies may have
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a structural origin. Observing the X-ray data of Fig. 1 there are clear differences between both samples. The information obtained from the diffraction peaks and the rocking curves for the ZnO/Si3 N4 /Si and ZnO/Si samples clearly indicates a much worse orientation of the ZnO crystalline c-axis in the direction perpendicular to the film plane in the case of ZnO/Si3 N4 /Si. Even though the rocking curve of ZnO/Si is clearly better, the misalignment of the ZnO crystalline c-axis with the perpendicular to the film plane is enough to mask the influence of the Si substrate symmetry as the angle dependent HRBS data show. The Si3 N4 layer clearly increases this effect. The reduction of the difference in SAW velocity between both samples is also due to the same reason. Comparing the simulation with the experimental values of the Al/ZnO/Si sample, the small spatial distribution of the ZnO crystalline c-axis induces a clear reduction of the SAW velocity and an approach in velocity of the different SAW modes is observed. The much bigger delocalisation of the ZnO crystalline c-axis in the Al/ZnO/Si3 N4 /Si sample implies a bigger reduction in the value of the SAW velocity as it is observed in the experimental data. However, the effect of the Si3 N4 layer is clear and increases the SAW velocity of the ZnO sample. 4. Conclusions The SAW phase velocity of ZnO thin films has been determined by High Resolution Brillouin Light Scattering spectroscopy in order to prove the effect of an intermediate layer of Si3 N4 . The system formed by ZnO/Si3 N4 /Si(100) presents a SAW phase velocity higher than that corresponding to ZnO/Si(100). That increase is due to the effect of bigger elastic constants of Si3 N4 layer than that of ZnO. The increase is in agreement with the Elastic Theory calculations. There is a small difference between the experimentally determined values and the calculated ones; it should be explained because calculations are made for (001) ZnO films and, on the other hand, the films studied here present a high but not full texture degree. Acknowledgements This work has been supported by the Spanish DGICYT under contract No. MAT200301880 and MAT2003-6147-C04-02 and by CAM-DGI 07N/0077/2002. References [1] H. Nakahata, S. Fujii, K. Higaki, A. Hachigo, H. Kitabayashi, S. Shikata, N. Fujimori, Semicond. Sci. Technol. 18 (2003) 96. [2] I. Sayago, M. Aleixandre, A. Martínez, M.J. Fernández, J.P. Santos, J. Gutiérrez, I. Gràcia, M.C. Hornillo, Synth. Met. 148 (2005) 37. [3] T. Xu, G. Wu, G. Zhang, Y. Hao, Sensors Actuators A 104 (2003) 61. [4] K.H. Yoon, J.W. Choi, D.H. Lee, Thin Solid Films 302 (1997) 116. [5] E. Vasco, J. Rubio-Zuazo, L. Vázquez, C. Prieto, C. Zaldo, J. Vac. Sci. Technol. B 19 (2001) 224. [6] W. Water, S.-Y. Chu, Mater. Lett. 55 (2002) 67. [7] N.W. Emanetoglu, C. Gorla, Y. Liu, S. Liang, Y. Lu, Mater. Sci. Semicond. Process. 2 (1999) 247. [8] M.S. Wu, W.C. Shih, W.H. Tsai, J. Phys. D: Appl. Phys. 31 (1998) 943. [9] M.H. Lee, S.M. Chang, J.B. Lee, J.S. Park, IEEE Freq. Contr. Symp. Proc. (2002) 70.
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E. Céspedes et al. / Superlattices and Microstructures 39 (2006) 75–82 H. Nakahata, K. Higaki, S. Fujii, K. Tanabe, Y. Seki, S. Shikata, IEEE Ultrason. Symp. Proc. (1996) 285. K. Yamanouchi, N. Sakurai, T. Satoh, IEEE Ultrason. Symp. Proc. (1989) 351. A.F. Belyanin, A.N. Blachev, L.L. Bouilov, B.V. Spitsyn, J. Chem. Vapor Depos. 5 (1997) 267. M. Ishihara, T. Manabe, T. Kumagai, T. Nakamura, S. Fujiwara, Y. Ebata, S. Shikata, H. Nakahata, A. Hachigo, Y. Koga, Japan. J. Appl. Phys. 40 (2001) 5065. J.P. Jung, J.B. Lee, J.S. Kim, J.S. Park, Thin Solid Films 447 (2004) 605. S.H. Seo, W.C. Shin, J.S. Park, Thin Solid Films 416 (2002) 190. K.H. Choi, J.Y. Kim, H.J. Kim, IEEE Ultrason. Symp. Proc. (2000) 353. G. Carlotti, G. Socino, A. Petri, E. Verona, Appl. Phys. Lett. 51 (1987) 1889; G. Carlotti, G. Socino, E. Verona, J. Appl. Phys. 65 (1989) 1370; G. Carlotti, D. Fioretto, G. Socino, E. Verona, J. Phys.: Condens. Matter 7 (1995) 9147. R.J. Jiménez Riobóo, M. García Hernández, C. Prieto, J.J. Fuentes-Gallego, E. Blanco, M. Ramírez del Solar, J. Appl. Phys. 81 (1997) 7739. J.R. Sandercock, in: M. Cardona, G. Guntherodt (Eds.), Light Scattering in Solids, vol. III, Springer-Verlag, Berlin, 1982, p. 173. J.K. Krüger, in: A. Bässler (Ed.), Optical Techniques to Characterize Polymer Systems, Elsevier, Amsterdam, 1989, p. 429. P. Mutti, C.E. Bottani, G. Ghislotti, M. Beghi, G.A.D. Briggs, J.R. Sandercock, in: A. Briggs (Ed.), Advanced Acoustic Microscopy, vol. 1, Plenum Press, New York, 1995, p. 249. R.J. Jiménez-Riobóo, E. Rodríguez-Cañas, M. Vila, C. Prieto, F. Calle, T. Palacios, M.A. Sánchez, F. Omnes, O. Ambacher, B. Assouar, O. Elmazria, J. Appl. Phys. 92 (2002) 6868. H. Sussner, J. Pelous, M. Schmidt, R. Vacher, Solid State Commun. 36 (1980) 123. B. Mröz, S. Mielcarek, J. Phys. D: Appl. Phys. 34 (2001) 395. R.J. Jiménez Riobóo, M. Belmahi, J. Appl. Phys. 97 (2005) 072509. A.G. Every, Private communication; X. Zhang, J.D. Comins, A.G. Every, P.R. Stoddart, W. Pang, T.E. Derry, Phys. Rev. B 58 (1998) 13677.
Brillouin light scattering characterization of the surface acoustic wave velocity in the ZnO/Si3 N4 /Si(100) system E. Céspedes∗, R.J. Jiménez-Riobóo, M. Vila, C. Prieto Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Cantoblanco, 28049-Madrid, Spain Available online 12 September 2005
Abstract The Surface Acoustic Wave (SAW) phase velocity of ZnO thin films has been determined by High Resolution Brillouin Light Scattering Spectroscopy in order to study the influence of an intermediate layer with a set of elastic constants higher than the corresponding ones for ZnO. The study of the system formed by ZnO/Si3 N4 /Si(100), deposited by reactive magnetron sputtering, shows a SAW phase velocity increment with respect to the ZnO/Si(100) single layer system. Results are discussed in terms of the elasticity theory, which predicts an increase of the acoustic wave phase velocity when an intermediate layer stiffer than the film at the surface is present in the system. © 2005 Elsevier Ltd. All rights reserved.
1. Introduction Surface Acoustic Wave (SAW) devices are electronic components widely used as frequency filters or resonators in a variety of systems such as mobile phones, television tuners and optical-communication systems. Due to their compactness, great stability, reproducibility and high performance of frequency characteristics, these are excellent devices for the recent communication systems [1]. SAW devices based on piezoelectric ∗ Corresponding author. Tel.: +34 913349000; fax: +34 913720623.
E-mail address: [email protected] (E. Céspedes). 0749-6036/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2005.08.032
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thin films and grown on silicon substrates present several advantages over the conventional ones (quartz, lithium tantalate and lithium niobate): low power consumption, reduction of cost and circuit miniaturization and integration. Among lately developed piezoelectric thin films, zinc oxide is a very promising material for developing SAW sensors on silicon due to its high piezoelectric coupling factor [2–4]. Zinc oxide thin films exhibit piezoelectric behaviour when the crystallites have the c-axis perpendicular to the substrate [2]. A variety of deposition techniques, such as the sol–gel process, spray pyrolysis, chemical vapour deposition (CVD), pulsed laser deposition (PLD) [5], evaporation and sputtering have been employed for the growth of ZnO films on different substrates [6–8]. Among them, the sputtering technique is considered to be the most favorable deposition method because it allows obtaining well-oriented and uniform ZnO films even on amorphous substrates at high deposition rates [8]. With new advances in high frequency and high bit-rate communication systems, the demand for higher frequency SAW devices is increasing, and great effort has been made in the development of higher frequency SAW devices. Since the signal processing frequency ( f 0 ) is determined by the ratio between the phase velocity of the surface acoustic wave V SAW and the spatial period λ of the interdigital transducers (IDT), there are two approaches to get high frequency SAW devices. One is to reduce the line width of electrodes by means of special lithography techniques. However, the reduction of the IDT size causes problems of less reliability, power durability and fabrication margin in the manufacturing process. Another approach is to find the high-velocity material for the SAW substrate, as the operating frequency is proportional to the SAW velocity [1]. Multilayers including piezoelectric thin films coupled with high phase velocity substrates, such as diamond, sapphire, and sometimes diamond-like carbon (DLC) have been reported to achieve high-frequency SAW devices without requiring submicron technologies [9,10]. Among the proposed multilayer configurations, the ZnO/diamond structure has exhibited the largest phase velocities (approx. 10 000 m/s) [1]. The possibility of V SAW up to 10 000 m/s has been also reported in the case of AlN/diamond/Si and LiNbO3 /diamond/Si structures [11], though, there have been just a few approaches to fabricate c-axis oriented films of these materials on diamond substrates [12,13]. However, although ZnO/diamond/Si is one of the most promising material components for gigahertzband SAW filters, there still exist several difficulties in employing diamond for SAW devices, such as complicated growth conditions for high-quality film, incompatibility with conventional semiconductor processes, and large surface roughness (typically, several hundred nanometers) hindering the propagation of surface waves [14,15]. There have also been some attempts to obtain experimentally high SAW velocity in a ZnO/dielectric/Si layered structure, by using SiO2 , Si3 N4 and TiO2 dielectric layers [16]. Acoustic investigations of the phase velocity of surface phonons propagating on ZnO films deposited on Si, Al2 O3 and quartz substrates by means of Brillouin scattering have been previously reported [17]. In this work, the propagation characteristics of SAW modes in ZnO/Si and ZnO/Si3 N4 /Si layered structures were examined theoretically and experimentally. The experimental behavior of V SAW was observed by means of high resolution Brillouin spectroscopy (HRBS).
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2. Experimental The Si3 N4 dielectric layer was grown by reactive sputtering at room temperature on Si(100) by using a planar 2 inch magnetron source (Angstrom Science) operated by RFpower supply (Huttinger Gmbh.). The residual pressure provided by the vacuum system was 1 × 10−4 Pa. A pure silicon circular target with thickness of 3 mm was mechanically clamped to the water-cooled electrode. N2 was used as reactive sputtering gas with an RF power of 100 W and the deposition rate was 2nm/min. A Si3 N4 layer 0.5 µm thick was deposited on top of the Si(100) wafers. This thickness was selected because, after calculations, the effect of the silicon substrate becomes negligible for silicon nitride layers thicker than 0.3 µm. After Si3 N4 deposition, ZnO films were grown by reactive sputtering at room temperature. The films were deposited at the same time on Si(100) and on the previous Si3 N4 layer, in order to compare V SAW in ZnO films grown under identical conditions. The planar magnetron source was operated by a DC-power supply and the working chamber was pumped down to 1 × 10−4 Pa. DC reactive sputtering from a pure zinc metallic target (99.99% purity) was carried out in a mixed atmosphere of argon and oxygen (60% Ar–40% O2 ). By using a current intensity of 20 mA (∼6 W), a deposition rate of about 1 nm/min was obtained. The thickness of the ZnO thin film, measured by X-ray reflectivity, was 73 nm. Simulations of SAW velocity behavior in ZnO/Si3 N4 layered structure showed that in this range of ZnO thickness, the effects of the Si3 N4 layer were appreciable. The crystallinity and crystallographic orientation of the ZnO films were determined by X-ray diffraction. A D8Brucker-AXS diffractometer operated at the Kα emission of a Cu anode has been used. The degree of c-axis orientation normal to the substrate was evaluated from analysis of X-ray rocking curves. The experimental Brillouin Light Scattering set-up has already been described elsewhere [18]. It can be summarized as follows: the light source was a 2060 Beamlok Spectra Physics Ar+ ion laser provided with an intracavity temperature stabilized singlemode and single-frequency z-lok etalon (λ0 = 514.5 nm). The scattered light was analyzed using a Sandercock-type 3 + 3 tandem Fabry–Pérot interferometer [19]. The incident polarization direction was chosen to be in-plane (p-polarization) while no polarization analysis of the scattered light was made. The typical values for finesse and contrast were 150 and 109 , respectively. 3. Results and discussion Fig. 1(a) shows the typical diffraction diagram of the sputtered ZnO films. Samples exhibit the characteristic peak of the ZnO hexagonal wurtzite-type structure. Fig. 1(b) gives the X-ray rocking curves for the (002) peak to show a c-axis preferred orientation. In order to study the influence of buried interlayers on the SAW propagation velocity (from now on SAW velocity), angular dependent HRBS was the experimental technique chosen [20]. ZnO films are very transparent materials thus making it extremely difficult to obtain information on SAW velocity by means of HRBS. There are two main scattering mechanisms that couple light with acoustic phonons; the elasto-optic coupling mechanism, and the ripple mechanism [21]. In transparent materials the first one dominates and
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Fig. 1. (a) Room temperature θ –2θ X-ray diffraction spectra of ZnO films deposited on Si(100) and on Si3 N4 /Si(100). (b) Rocking curves of ZnO/Si(100) and ZnO/Si3 N4 /Si(100) samples. The detector angle is the position of the (002) peak of ZnO.
bulk acoustic waves can be studied. In opaque materials (for instance metals) the ripple mechanism is the one to be observed allowing the study of surface acoustic waves. In the case of ZnO films, it is clear that the dominant mechanism is the elasto-optic coupling and, in order to obtain information about the SAWs, it is necessary to find a way to enhance the ripple scattering mechanism. A successful way to do this is to deposit a very thin metallic film on the transparent sample [22–25]. It has been shown that the thin metallic film will reproduce the main features of the SAWs of the transparent material. This technique was already employed for AlN and synthetic diamond films [22,25]. After that reported knowledge, an Al thin film was deposited on each of the different ZnO samples via DC magnetron sputtering. The Al film thickness was determined to be 50 nm by X-ray Reflectivity. The azimuthal angle dependent Brillouin spectroscopy was performed for all the samples at backscattering with a sagittal angle of 55◦. The sample rotates about the normal to the film surface. Brillouin spectra have been recorded under a geometric arrangement that makes k × h = 1 (k is the scattering wave vector and h is the thickness of the Al thin film).
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Fig. 2. (a) Brillouin spectra of Al/ZnO/Si(100) and Al/ZnO/Si3 N4 /Si(100) samples taken at scattering angle of α = 55◦ . (b) Brillouin frequency shift of surface acoustic phonons of Al/ZnO/Si(100) and Al/ZnO/Si3 N4 /Si(100) samples, as a function of the azimuth angle.
All the recorded HRBS spectra are of similar quality independently of the substrate (either Si or Si3 N4 ). Fig. 2(a) shows typical spectra, where the Rayleigh mode (SRM) and a higher order Sezawa mode (SSM) can be observed. Fig. 2(b) gives the azimuthal dependence of the Rayleigh mode for both studied samples. The Surface Acoustic Wave propagation velocity can be obtained straightforwardly from the Brillouin frequency shift ( f ): V SAW =
2π f f λ0 = 2 sin(α) q SAW
(1)
λ0 is the laser wavelength in vacuum, q SAW the scattering wave vector and α is the scattering angle (in this case 55◦ ). In order to evaluate the influence of the different substrates, we have proceeded to simulate the HRBS spectrum of the system Al thin film on ZnO layer on Si and on Si3 N4 substrates. The simulation procedure is based on the Green function algorithm developed by Every et al. [26]. Fig. 3 shows the results of the simulation for Al(50 nm)/ZnO(73 nm) on top of both substrates (Si and Si3 N4 ). The difference in the simulated spectra introduced only by the change of substrate becomes obvious, the one with the Si3 N4
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Fig. 3. (a) Numerical simulation of a Brillouin spectrum of Al/ZnO/Si(100). (b) Numerical simulation of a Brillouin spectrum of Al/ZnO/Si3 N4 . Peaks in the 10–12 GHz range correspond to the surface Rayleigh mode (SRW). These results have been obtained by using the following density and elastic constants of the different materials. Si: (ρ = 2329 kg/m3 ; c11 = 165.6 GPa; c12 = 63.9 GPa; c44 = 79.5 GPa); Si3 N4 : (ρ = 3300 kg/m3 ; c11 = 387.39 GPa; c12 = 143.30 GPa; c44 = 122.05 GPa); ZnO: (ρ = 5676 kg/m3 ; c11 = 209.7 GPa; c33 = 210.9 GPa; c13 = 105.1 GPa; c44 = 42.5 GPa; c66 = 44.3 GPa); Al: (ρ = 2702 kg/m3 ; c11 = 110.57 GPa; c12 = 56.47 GPa; c44 = 27.05 GPa).
substrate being stiffer. The difference is about 13% between the corresponding Rayleigh modes (SRM). The obtained Brillouin shifts for Al(50 nm)/ZnO(73 nm)/Si(100) and Al(50 nm)/ZnO(73 nm)/Si3 N4 (500 nm) systems are10.43 and 11.83 GHz. These results agree with the experimentally determined SRM modes of Al/ZnO/Si and Al/ZnO/Si3 N4 /Si samples shown in Fig. 2(a). There are at least two facts that are interesting in these data; the lack of clear angular symmetry (Fig. 2(b)) and the confirmed difference in elastic properties of the SAWs of ZnO depending on the type of substrate. At least in the case of the Al/ZnO/Si sample, one could expect some influence of the cubic Si substrate (that gives in the simulation an azimuthal oscillation about 2%) but our experimental data give no evidence of this influence. The difference observed in SAW velocity between our two samples (10.16 and 10.43 GHz; that by using Eq. (1)) correspond to SAW velocity values of 3193 m/s and 3276 m/s, respectively) is about 2.5%, far from the difference obtained by the simulations. The reason for these discrepancies may have
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a structural origin. Observing the X-ray data of Fig. 1 there are clear differences between both samples. The information obtained from the diffraction peaks and the rocking curves for the ZnO/Si3 N4 /Si and ZnO/Si samples clearly indicates a much worse orientation of the ZnO crystalline c-axis in the direction perpendicular to the film plane in the case of ZnO/Si3 N4 /Si. Even though the rocking curve of ZnO/Si is clearly better, the misalignment of the ZnO crystalline c-axis with the perpendicular to the film plane is enough to mask the influence of the Si substrate symmetry as the angle dependent HRBS data show. The Si3 N4 layer clearly increases this effect. The reduction of the difference in SAW velocity between both samples is also due to the same reason. Comparing the simulation with the experimental values of the Al/ZnO/Si sample, the small spatial distribution of the ZnO crystalline c-axis induces a clear reduction of the SAW velocity and an approach in velocity of the different SAW modes is observed. The much bigger delocalisation of the ZnO crystalline c-axis in the Al/ZnO/Si3 N4 /Si sample implies a bigger reduction in the value of the SAW velocity as it is observed in the experimental data. However, the effect of the Si3 N4 layer is clear and increases the SAW velocity of the ZnO sample. 4. Conclusions The SAW phase velocity of ZnO thin films has been determined by High Resolution Brillouin Light Scattering spectroscopy in order to prove the effect of an intermediate layer of Si3 N4 . The system formed by ZnO/Si3 N4 /Si(100) presents a SAW phase velocity higher than that corresponding to ZnO/Si(100). That increase is due to the effect of bigger elastic constants of Si3 N4 layer than that of ZnO. The increase is in agreement with the Elastic Theory calculations. There is a small difference between the experimentally determined values and the calculated ones; it should be explained because calculations are made for (001) ZnO films and, on the other hand, the films studied here present a high but not full texture degree. Acknowledgements This work has been supported by the Spanish DGICYT under contract No. MAT200301880 and MAT2003-6147-C04-02 and by CAM-DGI 07N/0077/2002. References [1] H. Nakahata, S. Fujii, K. Higaki, A. Hachigo, H. Kitabayashi, S. Shikata, N. Fujimori, Semicond. Sci. Technol. 18 (2003) 96. [2] I. Sayago, M. Aleixandre, A. Martínez, M.J. Fernández, J.P. Santos, J. Gutiérrez, I. Gràcia, M.C. Hornillo, Synth. Met. 148 (2005) 37. [3] T. Xu, G. Wu, G. Zhang, Y. Hao, Sensors Actuators A 104 (2003) 61. [4] K.H. Yoon, J.W. Choi, D.H. Lee, Thin Solid Films 302 (1997) 116. [5] E. Vasco, J. Rubio-Zuazo, L. Vázquez, C. Prieto, C. Zaldo, J. Vac. Sci. Technol. B 19 (2001) 224. [6] W. Water, S.-Y. Chu, Mater. Lett. 55 (2002) 67. [7] N.W. Emanetoglu, C. Gorla, Y. Liu, S. Liang, Y. Lu, Mater. Sci. Semicond. Process. 2 (1999) 247. [8] M.S. Wu, W.C. Shih, W.H. Tsai, J. Phys. D: Appl. Phys. 31 (1998) 943. [9] M.H. Lee, S.M. Chang, J.B. Lee, J.S. Park, IEEE Freq. Contr. Symp. Proc. (2002) 70.
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