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Experimental characterization of ZnO thin films and identification of intermediate frequency peaks of ZnO / SiO2 / Si SAW devices

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  • Save American Journal of M aterials Science 2013, 3(4): 100-103 DOI: 10.5923/j.materials.20130304.06 Experimental Characterization of ZnO Thins Films and Identification of Frequency Peaks in ZnO/SiO2/Si SAW Devices S. Bensmaine*, B. Benyoucef Unité de Recherche M atériaux et Energies Renouvelables (URM ER). Université de Tlemcen, BP 119, Tlemcen, 13000, Algeria Abstract In this work, we investigate experimentally the different properties of ZnO thins films deposited by radio-frequency magnetron with Zinc target (sputtering method) and we calculate the frequency peaks in ZnO/SiO2/Si structure. We study the effect of the power and the pressure deposition on the thin films properties. We characterized by X-ray diffraction the film crystalline quality and by scanning electron microscopy (SEM) the film morphology. The X-ray diffraction analyses has revealed that the ZnO film is polycrystalline in nature having a hexagonal wurtzite type crystal structure and (002) orientation. The SEM showed that the structure of the films is colu mnar. The thickness of the films is measured by profilo meter apparatus (Dektak3 ST). The principal application of ZnO thin films is the SAW (surface acoustic wave) devices. We realized e xpe rimentally the ZnO/SiO2/Si structure with the optima l para meters. The frequency response of the device is measured by the network analyzer. We obtained the frequency of 476 MHz for a phase velocity of 5712 ms-1. Keywords Th in Films Zinc Oxide, Characterization, SAW Devices, Frequency Peak 1. Introduction Zinc Oxide is one of the most used thin film piezoelectric material. It belongs to the hexagonal wurt zite crystal type, having 6mm symmetry. Th is structure can be considered as two inter-inserted hexagonal structures (zinc and oxygen) spaced by (3/8) c fro m each other, where c is the main symmetry axis of the crystal. It is generally treated as a semiconductor as a result of excessive zinc. ZnO is very versatile and due to its high coupling coefficient, many applications exist, like SAW devices[1], bulk acoustic wave devices, gas sensors, infrared detectors, tactile sensor array s [2]. ZnO can be deposited using various physical and chemical vapour deposition techniques (pu lsed-enhanced chemical vapour deposition, pulse laser deposition, ion-beam assisted d ep os it io n , mo lecu lar b eam ep it a xy , s p u t t erin g …). Radio-frequency magnetron sputtering technique is one of the most widely used to its reproducibility and efficiency[3, 4, 5]. The growth methods of ZnO films are co mpatib le with a wid e ran g e o f s u bs t rates (py rex, q u art z, s il ico n , sapphire…). Zn O is a wide-bandgap oxide semiconductor with a d irect energy gap of about 3.37 eV. The zinc o xide has emerged as one of the most promising materials, due to its * Corresponding author: s_bensmaine@yahoo. fr (S. Bensmaine) Published online at Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved optical and electrical properties associated with the high chemical and mechanica l stability. The physical properties of the films depend on the sputtering parameters, such as the substrate temperature, the oxygen partial pressure and the sputtering power[6]. Therefore, it is mean ingful to report in this paper, the effects of the pressure and power on ZnO thins films structural and morphological p roperties using r.f. magnetron sputtering. We investigate experimentally with the optimal parameters the ZnO/Si and ZnO/SiO2/Si structures. The ZnO c-axis with respect to the surface normal was showed to be a necessary requirement towards shear wave mode excitation in SAW. The sensing mechanis m consists of a perturbation of the surface along which the waves propagate. The frequency characterization of the SAW devices is obtained with a network analy zer and using equation (1) f= v (1) λ where f is the measured centre frequency, λ the considered used wavelength (μm) and v the phase velocity calculated. 2. Experimental Details Various deposition techniques have been for ZnO thin films. Two types are predominant: chemical vapour deposition CVD and physical vapour deposition PVD. Many different kinds of thin films are deposited by evaporation and sputtering, both of wh ich are examples of American Journal of M aterials Science 2013, 3(4): 100-103 101 PVD. Th is processes for deposition of ZnO thin films use the generation of gaseous phase ZnO molecu les fro m a solid ZnO target and the condensation of these molecules on a substrate. During sputtering, the target (Zn or ZnO) at a high negative potential, is bombarded with positive inert gas ions (mostly Ar) created in plas ma. The target material is sputtered away by mo mentum transfer and the ejected surface atoms are deposited (condensed) onto the substrate placed an anode. Sputtering is preferred over evaporation in many applications due to a wider choice of materials to work with and better adhesion to the substrate[7]. Sputtering is employed in laboratories and production settings. Most sputtered ZnO films are polycrystalline and grow preferentially with their crystallographic c-axis perpendicular to the substrate on any material[8]. 2.1. Effect of the Deposition Power and Pressure The ZnO thin films were deposited by r.f. magnetron system on silicon Si (100) substrates . The Zinc target (purity 99.99%) d iameter was 100 mm (4 inch) and 6.35 mm th ick. The distance between the cathode and the substrate holder was 70 mm. The deposition chamber was pu mped down to a base pressure of 5.10-7 mbar by a turbomo lecular pump prior to the introduction of the argon-oxygen gas mixture for ZnO thin films production. The RF power delivered by the RF generator was fixed at 20 watts and 100 watts. The pressure was fixed at 2.10-2mbar, 2.10-3 mbar and 6.10-3 mbar. All these conditions were summarized in table 1 and table 2. Figure 1. X-ray diffractograms of ZnOthin films as function of r.f. power with 2.10-2mbar of pressure The XRD analyses of the ZnO thin films on silicon are shows in fig.1 As can be seen the samples Si1and Si3 shows a low quality of the crystalline structure of ZnO thin films. It could be attributed to the low RF power delivered by the RF generator. Further increasing substrate temperature and time deposition don’t give a better result. The samples Si2 and Si4 shows on the XRD analyses an important peak (002) oriented ZnO crystallites. These samp les have a good crystalline quality. As can be seen the crystalline quality is very sensitive to the RF power[5, 9]. Table 1. Deposition conditions of ZnO/silicon structure by magnetron sputtering. Variation of the RF power (002) Samples Si1 Si2 Si3 Si4 Temp. (°C) 31 200 Power (W) 20 100 20 100 Pressure (mbar) 2.10-2 Ar/O2 (%) 50-50 Time (s) 6000 3000 6000 3000 Table 2. Deposition conditions of ZnO/silicon structure by magnetron sputtering. Variation of the pressure Samples Si5 Si6 Si7 Temp. (°C) 31 Power (W) 100 Pressure (mbar) 2.10-2 2.10-3 6.10-3 Ar/O2 (%) Time (s) 50-50 3000 The crystallographic structure of films were analysed by X-ray Diffract ion using the Cu Kα (with λ = 1.5405 A°) radiation. The mo rphology of the films is obtained by using scanning electron microscopy. The profilo meter apparatus allo ws the measurement of the thickness and the growth rate of the ZnO thin films. 3. Results and Discussion 3.1. Structural Properties Intensity (a.u.) (100) (101) (002) Fi gure 2. X-ray diffractograms of ZnO thin films as funct ion of pressure with 100 watt of power Fig.2 shows the XRD patterns for samples deposited at several pressures (2.10-2mbar, 2.10-3 mbar and 6.10-3 mbar). We observe at low pressure of 2.10-3 mbar only one strong peak at 2θ~34.4°, wh ich can be attributed to the (002) line of the hexagonal ZnO wurtzite phase. This samp le Si6 is highly textured, with c-axis perpendicular to the substrate surface[10]. It can be exp lains that the zinc ato ms and oxygen ato ms undergo fewer collisions, and thus have mo re kinetic energy, allowing them to arrange to form the plan 102 S. Bensmaine et al.: Experimental Characterization of ZnO Thins Films and Identification of Frequency Peaks in ZnO/SiO2/Si SAW Devices (002). The intensity of the (002) peak decreases at high pressure of 2.10-2 mbar. The three peaks at the higher pressure of 2.10-2 mbar correspond to (100), (002) and (101) orientations and their relat ively lo w intensities indicate a poor crystalline quality because the atoms suffer mo re collisions and therefore have less kinetic energy. Moreover the peak intensity decreases while its full width at half maximu m (FW HM) increases. (Dektak3 ST) apparatus. Table 3. Thickness and deposition rate for different samples Si1, Si2, Si3 and Si4 Samples Si1 Si2 Si3 Si4 Power (w) 20 100 20 100 Thickness (nm) 60 693 65 687 Growth rate (nm/s) 1.00 10-5 2.31 10-4 1.08 10-5 2.28 10-4 As can be seen when the RF power decreases, the thickness of the thin films decrease, which exp lains the decreasing of the growth rate. We observe that at 100 watts RF power, we obtained a bigger growth rate so an important flow of Zinc, consequently, the decrease of the time for reaction of the o xygen on the surface o f the thin films. We conclude that increasing the RF power can density the ZnO thin films. All these characterizat ions have permitted us to choose good deposition parameters, which are su mmarized in table 4. In order to increase the roughness of the silicon substrate for Zn O th in films gro wth, we use an amorphous SiO2 film. The SiO2 deposition conditions were: the chamber pressure at 6×10–3 mbar, the substrate temperature at 39 °C, the argon-oxygen gas mixture at (20 sccm/20 sccm), the r.f (a) power at 100 W and the time of deposition was fixed at 720 s. Table 4. Deposition conditions of Zinc Oxide by magnetron sputtering Samples Si8 Temp. (°C) 200 Power (W) 200 Pressure (mbar) 4.10-3 Ar/O2 (%) 50-50 Time (s) 7500 4. Application as SAW Devices 4.1. Frequency Peak Determination (b) Figure 3. Typical image of c-axis ZnO film on silicon. (a) :at 100watts RF power and 200°C temperature substrate Si4. (b) : at a pressure of 2.10-3mbar Si6 The cross sectional micrograph of c-axis ZnO thin films shows a columnar growth structure, fig.3. The image presents polycrystalline ZnO thin films growth with their c-axis perpendicular to the silicon substrate. 3.2. Thickness and Growth Rate The thickness of the films and the growth rate are summarised in the table 3. For each sample, we have measurements the thickness of the ZnO thin films using the Figure 4. SEM image of a completed IDT s American Journal of M aterials Science 2013, 3(4): 100-103 103 deposited at low operating pressure of 2x10-3 mbar shows a single orientation (002) corresponding to the c-axis perpendicular to the substrate. With the optimal deposition conditions, we have fabricated a SAW device with a ZnO/SiO2/Si structure. The best frequency of 476 MHz is obtained for a phase velocity of 5712 ms-1 and 12 μm wavelength. REFERENCES [1] Ki Hyun Yoon, Ji-Won Choi,Dong-Heon Lee, Thin Solid Films 302 (1997)116-121. [2] Y. E. Lee, J. B. Lee, Y. J. Kim, H. K. Yang, J. C. Park, H. J. Figure 5. The frequency response of the SAW device fabricated on a structure ZnO/SiO2/Si Kim, J. Vac. Sci. Technol. A 14, 1943 (1996).J. Breckling, Ed., The Analysis of Directional Time Series: Applications to Wind Speed and Direction, ser. Lecture Notes in Statistics. To check the piezoelectricity of the ZnO thin films, a Berlin, Germany: Springer, 1989, vol. 61. SAW device was performed on a structure ZnO/SiO2/Si. Fig. [3] M . Link, M . Schreiter, J. Weber, R. Gabl, D. Pitzer, R. Primig, 4 shows SEM image of a comp leted IDTs (inter-d igital and W. Wersing ,M . B. Assouar, O. Elmazria, Journal of transducers). We have used the network analyzer for the frequency response of the device[11]. We have obtained the Vacuum Science & Technology A: Vacuum, Surfaces, and Films 24 (2006) 218. result shows in Fig. 5, which presents the insertion loss (dB) [4] A. M osbah, A. M oustaghfir, S. Abed, N. Bouhssira, M .S. versus frequency (MHz). Aida, E. Tomasella and M . Jacquet, Surface and Coatings Fig. 5 shows the frequency response of the SAW device Technology, 200 (2005) 293. taken between 300 and 600 MHz. The two peaks correspond [5] S. Bensmaine, L. LeBrizoual, O. Elmazria, B. Assouar, B. to the mode of propagation. The first peak corresponds to a Benyoucef, Journal of Electron Devices, Vol. 5, (2007), pp. frequency of 384 M Hz for a phase velocity equal to 4608 104-109. m.s-1 and the second peak corresponds to the frequency of 476 M Hz for a phase velocity of 5712 ms-1. [6] Y. Yoshino, T. M akino, Y. Katayama, T. Hata. Vacuum 59, 538, (2000). 5. Conclusions In summary, we have deposited ZnO films by r.f magnetron system on silicon (100) substrates. We have observed that the crystalline quality improves with the increase of the injected RF power because there is more to create reactive species, such as argon ion. We have observed that increasing the power is used to obtain a good crystalline quality of the thin films because the atoms undergo fewer collisions and can have a better kinetic energy. The crystalline quality of the sample is small, in v iew of low power and even if you double the deposition time and increases the deposition temperature. The crystalline orientation of Zn O films is strongly influenced by the deposition pressure. So, the Zn O film [7] M . J. M adou, Fundamentals of M icrofabrication, CRC Press, Bacaraton, FL (2002). [8] J. G. E. Gardeniers, Z. M . Rittersma, G. J. Burger, J. Appl. Phys. 83, 7844 (1998). [9] Y. Yoshino, T. M akino, Y. Katayama, T. Hata, Vacuum 59, 538 (2000). [10] L. LeBrizoual, T. Lamara, F. Sarry, M . Belmahi, O. Elmazria, J. Bougdira, M . Rémy, P. Alnot, Physica Status Solidi (a), vol202/11, p.p 2217-2223 (2005). [11] S. Bensmaine, L. LeBrizoual, O. Elmazria, J.J. Fundenberger, M . Belmahi, B. Benyoucef. Diamond and Related M aterials. Vol 17, p.p 1420–1423 (2008).

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