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Deposition of ZnO multilayer on LiNbO3 single crystals by DC-magnetron sputtering
Applied Surface Science 257 (2011) 10233–10238
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Deposition of ZnO multilayer on LiNbO3 single crystals by DC-magnetron sputtering M. Shirazi a,∗ , M.T. Hosseinnejad b , A. Zendehnam a , Z. Ghorannevis b , M. Ghoranneviss b a b
Thin Film Laboratory, Physics Department, Science Faculty, Arak University, Arak, Iran Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 1 May 2011 Received in revised form 4 July 2011 Accepted 4 July 2011 Available online 12 July 2011 Keywords: LiNbO3 ZnO multilayer Thermal oxidation PL SEM AFM
a b s t r a c t Zinc oxide (ZnO) thin films were deposited on LiNbO3 (LN) single crystals with 200 nm thicknesses by three different ways, where coating of zinc (Zn) film was followed by thermal oxidation for four, two, and one steps with 50, 100, and 200 nm thicknesses repeatedly. Sample, which was produced at 4-step of deposition and oxidation of Zn layer, showed high transmittance and low structural defect due to a lower photoluminescence intensity and Urbach energy. Average grain size in X-ray diffraction (XRD), scanning electron microscopy (SEM) micrograph, and atomic force microscopy (AFM) images for multilayer of ZnO was lower than monolayer of ZnO thin films. Applying multilayer coating technique leads to decrease of surface roughness and scattering on light on surface and fabrication of LiNbO3 waveguides with lower optical loss. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Lithium niobate (LiNbO3 ) or LN crystals are widely used in photonic and integrated optical devices because of their high electro-optic, acousto-optic and second order nonlinear coefficients and piezoelectric effect, besides the fact that large and excellent optical quality single crystals are readily available [1–3]. In fact LN single crystals has been widely used as substrates for such integrated optical devices as holographic data storage, second harmonic generation, acoustic devices, optical switches, modulators, nonlinear wavelength converters and optical waveguides. Optical waveguides in LN have been intensively studied for applications including integrated and nonlinear optics, telecommunication systems and fiber sensors [4–6]. Several methods for fabricating low-loss waveguides in lithium niobate have been experimented. They are: out-diffusion of lithium oxide from the crystal surface, ion exchange, proton exchange and Ti-diffusion. Each one of these techniques has some limits of applicability, proton exchange only increases in the extraordinary refractive index, ion implantation requires the use of ion accelerators. Ti-diffused waveguides guide ordinary and extra ordinary polarization but suffer optical damage in the visible part of the spectrum [7,8]. Zn diffusion is an effective technique for the fabrication
∗ Corresponding author. E-mail address: [email protected] (M. Shirazi). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.027
of LiNbO3 waveguide. Zn:LiNbO3 waveguide supports propagation of both TE and TM modes, and exhibit high resistance to photorefractive damage [9,10]. Nevado and Lifante [11], fabricated Zn:LiNbO3 waveguide with using Zn vapor. Young et al. [12], reported the fabrication of planner waveguide by diffusion of Zinc oxide (ZnO) films into LN substrate in a Li-rich atmosphere. Yoon and Eknoyan [13], indicated surface degradation resulting from diffusion of metallic Zn into LN, and chose ZnO instead of Zn to limit this. However, much remains unknown about the optical and structural properties of ZnO thin films deposited on LiNbO3 . The understanding of structural phase behavior of ZnO layer is very important for the development of highly efficient ZnO:LiNbO3 waveguides. Nonetheless reports regarding the structural and optical properties of ZnO thin layer on the LiNbO3 substrate are rare [14–16].In our previous works optical and structural properties of ZnO deposited on LN substrate were investigated before and after the diffusion of Zn into LN substrate [17,18]. In present work DC-magnetron sputtering was used for deposition of the Zn layer on the LN substrate and thermal oxidation of the Zn films was performed to produce the ZnO thin films. As we know, one of the important parameters in fabrication of lowloss LiNbO3 waveguide is the thickness of the Zn thin films. Since the critical ZnO film thickness for the diffusion of Zn into LN is 70–200 nm [19], the ZnO thin films with 200 nm thickness (measured by RBS) were produced. In present work ZnO thin films have been produced by thermal oxidation of the sputtered zinc films on the LN substrate with thickness (200 nm) were obtained with coat-
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M. Shirazi et al. / Applied Surface Science 257 (2011) 10233–10238
ing of zinc (Zn) film repeatedly, where coating of the Zn film was followed by thermal oxidation for four, two, and one steps with 50, 100, and 200 nm thicknesses repeatedly. This deposition technique presents interesting advantages such as a high deposition rate, a low substrate temperature, a good adhesion of the thin films on the LN substrate, and therefore a good packing density of the films. In addition the other advantage of using multilayer technique is a shorter period of time and lower temperature for oxidation of Zn thin films. In this work the effects of this way of deposition on the optical and the structural properties of ZnO thin films are investigated. 2. Experimental details In order to produce ZnO thin films, Zn was sputtered on LN substrates, followed by thermal oxidation of the Zn using a conventional oven in an open air with average humidity of 60%. For deposition of the Zn, a vacuum system with base pressure of 10−6 mbar was employed (Hind High Vacuum, H.H.V.12 MSPT), and a circular flat disc (diameter 125 mm and thickness 3 mm) of pure Zn (99/9%) was used as the sputtering target. For plasma formation, research grade argon (purity 99/99%) was employed at pressures of 10−2 to 10−1 mbar. To change the coating rate, the discharge current was varied from 200 to 1200 mA, and the distance between Zn target and substrate (LN) was changed from 5 to 15 cm. Conditions were optimized to obtain densely packed, continuous and uniform films with a coating rate of 1.95 nm/s. All the samples in this work were deposited under these optimized conditions. Z-cut commercial congruent LiNbO3 supplied by Focktek with dimensions of 20 mm × 5 mm × 0.5 mm and very good optical quality polished surfaces were employed as the substrate for the deposition. Before using the LN substrate, a 2-step ultrasonic cleaning process (acetone and methanol) was carried out. Before Zn deposition, the plasma discharge was run for a few minutes, and to ensure a high purity coating, the produced plasma was checked by a spectrometer with high resolving power. Only when Zn and Ar (atoms, and ions) were the only lines present, deposition of Zn on LN was started. ZnO thin films with 200 nm thicknesses (measured by RBS) were produced, and deposition of Zn on LN was carried out at the 300 K substrate temperature. ZnO thin films were deposited on the LiNbO3 single crystals with the thickness (200 nm), where coating of zinc (Zn) film was followed by thermal oxidation for four, two, and one steps with 50, 100, and 200 nm thicknesses repeatedly. For the optical measurements a double beam spectrophotometer (Camspec, model M350) with wavelength range of 200–1100 nm was employed. Photoluminescence (PL) spectra, (Stellar Net EPP-200) were taken at room temperature under 340 nm xenon lamps as the excitation source. The ZnO film structure was studied by X-ray diffraction (XRD), (X’pert PW 3373) ˚ radiation. To investigate surface morpholusing Cu K␣ ( = 1.54 A) ogy of the samples, scanning electron microscopy (SEM) (model Cambridge S360) and atomic force microscopy (Park Scientific Instrument Auto Probe model CP) was also performed in air using the force constant mode.
Table 1 Characterization of deposition and oxidation of LN1, LN2 and LN3 samples. Sample characterization
Zn thickness (nm)
Temperature of annealing (◦ C)
LN1 LN2 LN3
200 100 50
400 400 400
1 layer of Zn (1-steps) 2 layer of Zn (2-steps) 4 layer of Zn (4-steps)
carried out at every step of coating with the oxidation temperature of 400 ◦ C. Fig. 1 shows variation of optical transmittance spectra of these ZnO samples against wavelength (200–1100 nm). The optical transmittance of sample LN3 is higher than other two, which can be due to the thermal oxidation of sample LN3 carried out in 4-step. When the ZnO thin film is post annealed, transmittance of samples reduces (see Fig. 2). The reduction can be due to evaporation of oxygen or desorption of oxygen molecules from the grain boundaries at temperatures over 500 ◦ C [20,21]. As observed in Fig. 2, the transmittance of LN3 samples varies only slightly. To explain this, we note that during the 4-steps, Zn layers are coated on LN3 samples (50 nm in each step) and exposed to thermal oxidation. Consequently, the sub-layers receive sufficient oxygen during the oxidation, and evaporation of oxygen during the post annealing does not cause any significant change in transmittance of the LN3 samples. On the other hand, in LN1 samples, coating and thermal oxidation occur in a single step (200 nm in 1-step), therefore, absorption of oxygen by Zn atoms during oxidation mainly takes place on the surface. Evaporation of oxygen during post annealing would naturally cause a significant drop in transmittance. The absorption coefficient (˛) is calculated using the transmission spectra and Eq. (1) [22], ˛=
1 ln d
T 0
T
(1)
where T, and To are the transmittance of the sample with ZnO thin film and the substrate (LN) without coating, respectively, and d is the thin film thickness. The fundamental absorption edge of the films corresponds to the electron transitions from valence band to conduction band and this edge can be used to calculate the optical band gap of the thin films. In the direct transition, the optical band gap (Eg ) was calculated by Tauc’s relationship (see Eq. (1)), [22], where A is a constant, and h is the photon energy, and Eg is the energy of band gap. 2
(˛h) = A(h − Eg )
(2)
To calculate Eg a graph of (˛h)2 versus h was plotted. Extrapolation of linear portion to the energy axis gives the Eg . Fig. 3 shows
3. Results and discussion To study of the effects of multilayer coating of ZnO films on the optical and structural properties, three samples LN1, LN2 and LN3 with the following conditions, this is presented in Table 1, were produced. These samples were produced under similar conditions (deposition rates, Ar pressure, discharge current, and voltage). In fact, LN1, LN2 and LN3 samples produced by deposition of Zn layer at one, two and three steps respectively, and thermal oxidation was
Fig. 1. Optical transmittance spectra of LN1, LN2 and LN3 samples at 400 ◦ C.
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Fig. 2. Optical transmittance spectra of LN1, LN2 and LN3 samples at 700 ◦ C.
plot of (˛h)2 versus photon energy (h) for the samples produced. The optical band gaps determined from these curves for all samples are 3.25 ev, which are in good agreement with the reported values [23]. When these samples were post annealed, values of Eg for all the samples remain constant. In the exponential edge region, where ˛ ≤ 104 cm−1 , Urbach rule is expressed as [24]: ˛(h) = ˛0 Exp
h − E 0
Eu
(3)
Fig. 5(a) and (b) shows room temperature PL spectra for all the samples, at 400 ◦ C temperature of oxidation and 700 ◦ C temperature of the post annealing. The strong near-band-edge (NBE) emission in the UV region result from excitons (free or boundexciton states), while the weak deep-level (DL) emission attributed to oxygen vacancies in structure of the ZnO thin films [27]. The intensity of NBE and DL emission at LN1, which produce in 1-step is higher and this reveals that multilayer of ZnO has lower defect and oxygen vacancy in comparison with the ZnO thin films monolayer. To study the structural properties of these ZnO samples, XRD was carried out. Fig. 6 shows XRD spectra of these ZnO films, which all show hexagonal structure. The grain size of crystallites was calculated using a well known Scherrer’s equation [21]:
where ˛0 and E0 are constant, h is the photon energy, and Eu is the Urbach energy. The Urbach energy term, Eu , in the exponential function (Eq. (3)) determines the steepness of Urbach tail. Indeed, a plot of ln(˛) versus photon energy for h < Eg should be linear and Urbach energy can be obtained from the slope. This parameter is a function of temperature and the degree of structural disorder of the thin films [25,26]. Fig. 3 shows variations of the ln(˛) versus h (for T = 400 ◦ C temperature of oxidation) for LN1, LN2 and LN3 samples. Variations of Eu against the temperature of post annealing for all the samples are presented in Fig. 4. Urbach energy of sample LN1 is higher than LN3 and LN2 at temperature 400 ◦ C, and its values increase with the temperature. These rises of Eu are attributed to the structural and the thermal disorders in the ZnO films. These results show that LN3 multilayer, which was produced at 4-steps, has more optical stability with the temperature in comparison with the other two samples (LN2 and LN1).
where D is crystallite size, is the incident X-ray wavelength ˚ K␣(Cu)), is the diffraction angle and ˇ is full half width ( = 1.54 A, at maximum (FWHM). In fact, sample LN1, which was produced at 1-step of coating and oxidation, has higher average grains and average grain size decreases in sample LN2 and LN3, which were produced in the 2-step and 4-step respectively (Table 2). Figs. 4 and 6 show a decrease in the Urbach energy when average crystal size increases. To explain this effect, we note that with a larger average crystal size, the lattice strain is more relaxed. Therefore, the degree of structural disorder increases, and based on the Cody model [26] (which implies linearly decreasing variations of of Eu with the
Fig. 3. Relationship of (˛h)2 with photon energy (h) for LN1, LN2 and LN3 samples at 400 ◦ C. (The inset shows the variation of ln(˛) against h at 400 ◦ C for LN1, LN2 and LN3 samples).
Fig. 4. Urbach energy as a function of post annealing temperature for LN1, LN2 and LN3 samples.
D=
0.9 ˇ cos()
(4)
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Fig. 6. XRD spectra of LN1, LN2 and LN3 samples.
Table 2 Variation of average grain size calculated by Scherrer’s equation, for LN1, LN2 and LN3 samples. Sample identity
Crystallite size (nm)
LN1 LN2 LN3
33 25 20
Table 3 Variation of RMS roughness measured by AFM, for LN1, LN2 and LN3 samples.
Fig. 5. Room temperature PL spectra of ZnO thin films at (a) 400 ◦ C and (b) 700 ◦ C.
degree of structural disorder of a particular material) the Urbach energy decreases. Surface morphology of ZnO films was investigated by SEM and atomic force microscopy (AFM) techniques. Fig. 7 shows SEM
Sample identity
Ave roughness (nm)
RMS roughness (nm)
LN1 LN2 LN3
14.5 11.4 8
18.6 14.4 10.3
micrograph of ZnO films, indicating that the magnitude of grain boundaries in the LN1 sample is larger than sample LN2 and LN3. For the fabrication of LiNbO3 waveguides, optical loss due to the surface scattering resulted from rough surface are of major concern [28]. In this line AFM images of all samples of the ZnO films was obtained, which is shown in Fig. 8. The particle sizes of sample LN1 are clearly larger than sample LN2 and LN3, thus the surface roughness of sample LN1 is higher than sample LN2 and LN 3 (see Table 3). In LN3 samples, step by step coating and thermal oxidation cause a more uniform layer with more packed surface with less voids, compared to other samples. In contrast, in LN1 samples, the 1-step coating and thermal oxidation cause the atoms not
Fig. 7. SEM micrograph of LN1, LN2 and LN3 samples.
M. Shirazi et al. / Applied Surface Science 257 (2011) 10233–10238
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Fig. 8. AFM images of LN1, LN2 and LN3 samples.
to gain enough energy to be placed in appropriate (uniform but packed) spots and therefore, the LN1 samples have more voids and porosity. The results of PL analysis confirm a much larger rate of defects and oxygen vacancies in LN1 samples than LN2 and LN3 samples (see Fig. 5). Hence, for LN1 samples, coarser surface with more roughness is expected. Thus the multilayer coating technique results in decreasing of the surface roughness and scattering on light on the surface optical LN waveguides. 4. Conclusion ZnO films deposition with 200 nm thicknesses were carried out by multilayer method at one, two and four steps with repeated coating of the Zn film followed by thermal oxidation, with 200, 100, and 50 nm thicknesses repeatedly. The optical and structural characteristics of the prepared samples were investigated by XRD, SEM, AFM, PL analysis. From structural point of view 1-step coated sample (LN1) showed more intense and sharper XRD peaks, which leads to the larger average grain size. SEM and AFM micrographs also showed that grain size and surface roughness for LN1 sample was larger than the sample LN3 which was produced at 4-step of coating and the obtained results were in good agreements with previous results. On the other hand, the multilayer coating technique leads to decreasing of the surface roughness, which results in decreasing of the scattering on light on the surface and fabrication of low loss LiNbO3 waveguides. Our results suggest that deposition of ZnO film by multilayer method produces samples with better optical stability even at higher temperatures of the post annealing, and the structural defect especially oxygen vacancies in this sample is lower.
Acknowledgment The authors would like to thank Iran National Science Foundation (INSF) for their interest and support of this work. References [1] T. Volk, M. Whlecke, Lithium Niobate Defects, Photo Refraction and Ferroelectric Switching , ©Springer-Verlag, Berlin, Heidelberg, 2008. [2] Y. Kuzminov, Lithium Niobate Crystals: Physico-Chemical Aspects of Technology , Cambridge International Science Publishing, 1999. [3] L. Arizmendi, Phys. Status Solidi A 201 (2004) 253–283. [4] D. Janner, D. Tulli, M. García-Granda, M. Belmonte, V. Pruneri, Laser Photonics Rev. 3 (2009) 301–313. [5] R.C. Twu, C.C. Huang, W.S. Wang, IEEE Photonics Technol. Lett. 12 (2000) 161–165. [6] E. Cantelar, R.E. Di paolo, J.A. Sanz-garcıa, P.L. Pernas, R. Nevado, G. Lifante, F. Cusso, Appl. Phys. B 73 (2001) 515–517. [7] V. Hinkov, E. Ise, J. Lightwave Technol. 4 (1986) 444–447. [8] I.P. Kaminow, J.R. Carruthers, Appl. Phys. Lett. 22 (1973) 326–328. [9] B. Herreros, G. Lifante, Appl. Phys. Lett. 66 (1995) 1449–1451. [10] R. Nevado, C. Sada, F. Segato, F. Caccavale, A. Kling, J.C. Soares, E. Cantelar, F. Cusso, G. Lifante, Appl. Phys. B 73 (2001) 555–558. [11] R. Nevado, G. Lifante, Appl. Phys. A 72 (2001) 725–728. [12] W.M. Young, M.M. Fejer, M.J.F. Digonnet, A.F. Marshall, R.S. Feigelson, J. Lightwave Technol. 10 (1992) 1238–1246. [13] D.W. Yoon, O. Eknoyan, Lightwave Technol. 6 (1988) 877–880. [14] Y. Wang, H. Wang, S. Li, S. Hang, J. Xu, J. Ye, R. Zhang, J. Cryst. Growth 248 (2005) 319–323. [15] H. Yamamoto, N. Saigaa, K. Nishimori, Appl. Surf. Sci. 169 (2001) 517–520. [16] Y. Wang, H. Wang, S. Liu, H. Liu, S. Zhau, Y. Hang, J. Xu, J. Ye, S. Gu, R. Zhang, J. Cryst. Growth 277 (2005) 378–381. [17] L. Aghli-Moghadam, A. Baghizadeh, G. Nabiyoni, A. Farashiani, A. Zendehnam, Appl. Phys. A 97 (2009) 805–810. [18] A. Zendehnam, M. Shirazi, M. Sadat, L. Aghli-Moghadam, World Appl. Sci. J. 10 (2010) 1473–1479. [19] T. Suhara, T. Fujieda, M. Fujimura, H. Nishihara, Jpn. J. Appl. Phys. 39 (2000) 864–865. [20] S.Y. Chu, W. Water, J.T. Liaw, J. Eur. Ceram. Soc. 23 (2003) 1593–1598. [21] N. Croitoru, A. Seidman, K. Yassin, Thin Solid Films 150 (1987) 291–301.
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[22] N. Ekem, S. Korkmaz, S. Pat, M.Z. Balbag, E.N. Cetin, M. Ozmumcu, Int. J. Hydrogen Energy 3 (2009) 5218–5222. [23] F.K. Shan, B.C. Shin, S.W. Jang, Y.S. Yu, J. Eur. Ceram. Soc. 24 (2004) 1015– 1018. [24] F. Urbach, Phys. Rev. 92 (1953) 1324.
[25] B. Pejova, Mater. Chem. Phys. 119 (2010) 367–376. [26] G.D. Cody, T. Tiedje, B. Brooks, Y. Goldestein, Phys. Rev. Lett. 47 (1981) 1480–1483. [27] J. Lim, K. Shin, H. Woo Kim, C. Lee, J. Lumin. 109 (2004) 181–185. [28] G.H. Lee, Opt. Express 10 (2002) 556–559.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Deposition of ZnO multilayer on LiNbO3 single crystals by DC-magnetron sputtering M. Shirazi a,∗ , M.T. Hosseinnejad b , A. Zendehnam a , Z. Ghorannevis b , M. Ghoranneviss b a b
Thin Film Laboratory, Physics Department, Science Faculty, Arak University, Arak, Iran Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 1 May 2011 Received in revised form 4 July 2011 Accepted 4 July 2011 Available online 12 July 2011 Keywords: LiNbO3 ZnO multilayer Thermal oxidation PL SEM AFM
a b s t r a c t Zinc oxide (ZnO) thin films were deposited on LiNbO3 (LN) single crystals with 200 nm thicknesses by three different ways, where coating of zinc (Zn) film was followed by thermal oxidation for four, two, and one steps with 50, 100, and 200 nm thicknesses repeatedly. Sample, which was produced at 4-step of deposition and oxidation of Zn layer, showed high transmittance and low structural defect due to a lower photoluminescence intensity and Urbach energy. Average grain size in X-ray diffraction (XRD), scanning electron microscopy (SEM) micrograph, and atomic force microscopy (AFM) images for multilayer of ZnO was lower than monolayer of ZnO thin films. Applying multilayer coating technique leads to decrease of surface roughness and scattering on light on surface and fabrication of LiNbO3 waveguides with lower optical loss. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Lithium niobate (LiNbO3 ) or LN crystals are widely used in photonic and integrated optical devices because of their high electro-optic, acousto-optic and second order nonlinear coefficients and piezoelectric effect, besides the fact that large and excellent optical quality single crystals are readily available [1–3]. In fact LN single crystals has been widely used as substrates for such integrated optical devices as holographic data storage, second harmonic generation, acoustic devices, optical switches, modulators, nonlinear wavelength converters and optical waveguides. Optical waveguides in LN have been intensively studied for applications including integrated and nonlinear optics, telecommunication systems and fiber sensors [4–6]. Several methods for fabricating low-loss waveguides in lithium niobate have been experimented. They are: out-diffusion of lithium oxide from the crystal surface, ion exchange, proton exchange and Ti-diffusion. Each one of these techniques has some limits of applicability, proton exchange only increases in the extraordinary refractive index, ion implantation requires the use of ion accelerators. Ti-diffused waveguides guide ordinary and extra ordinary polarization but suffer optical damage in the visible part of the spectrum [7,8]. Zn diffusion is an effective technique for the fabrication
∗ Corresponding author. E-mail address: [email protected] (M. Shirazi). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.027
of LiNbO3 waveguide. Zn:LiNbO3 waveguide supports propagation of both TE and TM modes, and exhibit high resistance to photorefractive damage [9,10]. Nevado and Lifante [11], fabricated Zn:LiNbO3 waveguide with using Zn vapor. Young et al. [12], reported the fabrication of planner waveguide by diffusion of Zinc oxide (ZnO) films into LN substrate in a Li-rich atmosphere. Yoon and Eknoyan [13], indicated surface degradation resulting from diffusion of metallic Zn into LN, and chose ZnO instead of Zn to limit this. However, much remains unknown about the optical and structural properties of ZnO thin films deposited on LiNbO3 . The understanding of structural phase behavior of ZnO layer is very important for the development of highly efficient ZnO:LiNbO3 waveguides. Nonetheless reports regarding the structural and optical properties of ZnO thin layer on the LiNbO3 substrate are rare [14–16].In our previous works optical and structural properties of ZnO deposited on LN substrate were investigated before and after the diffusion of Zn into LN substrate [17,18]. In present work DC-magnetron sputtering was used for deposition of the Zn layer on the LN substrate and thermal oxidation of the Zn films was performed to produce the ZnO thin films. As we know, one of the important parameters in fabrication of lowloss LiNbO3 waveguide is the thickness of the Zn thin films. Since the critical ZnO film thickness for the diffusion of Zn into LN is 70–200 nm [19], the ZnO thin films with 200 nm thickness (measured by RBS) were produced. In present work ZnO thin films have been produced by thermal oxidation of the sputtered zinc films on the LN substrate with thickness (200 nm) were obtained with coat-
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M. Shirazi et al. / Applied Surface Science 257 (2011) 10233–10238
ing of zinc (Zn) film repeatedly, where coating of the Zn film was followed by thermal oxidation for four, two, and one steps with 50, 100, and 200 nm thicknesses repeatedly. This deposition technique presents interesting advantages such as a high deposition rate, a low substrate temperature, a good adhesion of the thin films on the LN substrate, and therefore a good packing density of the films. In addition the other advantage of using multilayer technique is a shorter period of time and lower temperature for oxidation of Zn thin films. In this work the effects of this way of deposition on the optical and the structural properties of ZnO thin films are investigated. 2. Experimental details In order to produce ZnO thin films, Zn was sputtered on LN substrates, followed by thermal oxidation of the Zn using a conventional oven in an open air with average humidity of 60%. For deposition of the Zn, a vacuum system with base pressure of 10−6 mbar was employed (Hind High Vacuum, H.H.V.12 MSPT), and a circular flat disc (diameter 125 mm and thickness 3 mm) of pure Zn (99/9%) was used as the sputtering target. For plasma formation, research grade argon (purity 99/99%) was employed at pressures of 10−2 to 10−1 mbar. To change the coating rate, the discharge current was varied from 200 to 1200 mA, and the distance between Zn target and substrate (LN) was changed from 5 to 15 cm. Conditions were optimized to obtain densely packed, continuous and uniform films with a coating rate of 1.95 nm/s. All the samples in this work were deposited under these optimized conditions. Z-cut commercial congruent LiNbO3 supplied by Focktek with dimensions of 20 mm × 5 mm × 0.5 mm and very good optical quality polished surfaces were employed as the substrate for the deposition. Before using the LN substrate, a 2-step ultrasonic cleaning process (acetone and methanol) was carried out. Before Zn deposition, the plasma discharge was run for a few minutes, and to ensure a high purity coating, the produced plasma was checked by a spectrometer with high resolving power. Only when Zn and Ar (atoms, and ions) were the only lines present, deposition of Zn on LN was started. ZnO thin films with 200 nm thicknesses (measured by RBS) were produced, and deposition of Zn on LN was carried out at the 300 K substrate temperature. ZnO thin films were deposited on the LiNbO3 single crystals with the thickness (200 nm), where coating of zinc (Zn) film was followed by thermal oxidation for four, two, and one steps with 50, 100, and 200 nm thicknesses repeatedly. For the optical measurements a double beam spectrophotometer (Camspec, model M350) with wavelength range of 200–1100 nm was employed. Photoluminescence (PL) spectra, (Stellar Net EPP-200) were taken at room temperature under 340 nm xenon lamps as the excitation source. The ZnO film structure was studied by X-ray diffraction (XRD), (X’pert PW 3373) ˚ radiation. To investigate surface morpholusing Cu K␣ ( = 1.54 A) ogy of the samples, scanning electron microscopy (SEM) (model Cambridge S360) and atomic force microscopy (Park Scientific Instrument Auto Probe model CP) was also performed in air using the force constant mode.
Table 1 Characterization of deposition and oxidation of LN1, LN2 and LN3 samples. Sample characterization
Zn thickness (nm)
Temperature of annealing (◦ C)
LN1 LN2 LN3
200 100 50
400 400 400
1 layer of Zn (1-steps) 2 layer of Zn (2-steps) 4 layer of Zn (4-steps)
carried out at every step of coating with the oxidation temperature of 400 ◦ C. Fig. 1 shows variation of optical transmittance spectra of these ZnO samples against wavelength (200–1100 nm). The optical transmittance of sample LN3 is higher than other two, which can be due to the thermal oxidation of sample LN3 carried out in 4-step. When the ZnO thin film is post annealed, transmittance of samples reduces (see Fig. 2). The reduction can be due to evaporation of oxygen or desorption of oxygen molecules from the grain boundaries at temperatures over 500 ◦ C [20,21]. As observed in Fig. 2, the transmittance of LN3 samples varies only slightly. To explain this, we note that during the 4-steps, Zn layers are coated on LN3 samples (50 nm in each step) and exposed to thermal oxidation. Consequently, the sub-layers receive sufficient oxygen during the oxidation, and evaporation of oxygen during the post annealing does not cause any significant change in transmittance of the LN3 samples. On the other hand, in LN1 samples, coating and thermal oxidation occur in a single step (200 nm in 1-step), therefore, absorption of oxygen by Zn atoms during oxidation mainly takes place on the surface. Evaporation of oxygen during post annealing would naturally cause a significant drop in transmittance. The absorption coefficient (˛) is calculated using the transmission spectra and Eq. (1) [22], ˛=
1 ln d
T 0
T
(1)
where T, and To are the transmittance of the sample with ZnO thin film and the substrate (LN) without coating, respectively, and d is the thin film thickness. The fundamental absorption edge of the films corresponds to the electron transitions from valence band to conduction band and this edge can be used to calculate the optical band gap of the thin films. In the direct transition, the optical band gap (Eg ) was calculated by Tauc’s relationship (see Eq. (1)), [22], where A is a constant, and h is the photon energy, and Eg is the energy of band gap. 2
(˛h) = A(h − Eg )
(2)
To calculate Eg a graph of (˛h)2 versus h was plotted. Extrapolation of linear portion to the energy axis gives the Eg . Fig. 3 shows
3. Results and discussion To study of the effects of multilayer coating of ZnO films on the optical and structural properties, three samples LN1, LN2 and LN3 with the following conditions, this is presented in Table 1, were produced. These samples were produced under similar conditions (deposition rates, Ar pressure, discharge current, and voltage). In fact, LN1, LN2 and LN3 samples produced by deposition of Zn layer at one, two and three steps respectively, and thermal oxidation was
Fig. 1. Optical transmittance spectra of LN1, LN2 and LN3 samples at 400 ◦ C.
M. Shirazi et al. / Applied Surface Science 257 (2011) 10233–10238
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Fig. 2. Optical transmittance spectra of LN1, LN2 and LN3 samples at 700 ◦ C.
plot of (˛h)2 versus photon energy (h) for the samples produced. The optical band gaps determined from these curves for all samples are 3.25 ev, which are in good agreement with the reported values [23]. When these samples were post annealed, values of Eg for all the samples remain constant. In the exponential edge region, where ˛ ≤ 104 cm−1 , Urbach rule is expressed as [24]: ˛(h) = ˛0 Exp
h − E 0
Eu
(3)
Fig. 5(a) and (b) shows room temperature PL spectra for all the samples, at 400 ◦ C temperature of oxidation and 700 ◦ C temperature of the post annealing. The strong near-band-edge (NBE) emission in the UV region result from excitons (free or boundexciton states), while the weak deep-level (DL) emission attributed to oxygen vacancies in structure of the ZnO thin films [27]. The intensity of NBE and DL emission at LN1, which produce in 1-step is higher and this reveals that multilayer of ZnO has lower defect and oxygen vacancy in comparison with the ZnO thin films monolayer. To study the structural properties of these ZnO samples, XRD was carried out. Fig. 6 shows XRD spectra of these ZnO films, which all show hexagonal structure. The grain size of crystallites was calculated using a well known Scherrer’s equation [21]:
where ˛0 and E0 are constant, h is the photon energy, and Eu is the Urbach energy. The Urbach energy term, Eu , in the exponential function (Eq. (3)) determines the steepness of Urbach tail. Indeed, a plot of ln(˛) versus photon energy for h < Eg should be linear and Urbach energy can be obtained from the slope. This parameter is a function of temperature and the degree of structural disorder of the thin films [25,26]. Fig. 3 shows variations of the ln(˛) versus h (for T = 400 ◦ C temperature of oxidation) for LN1, LN2 and LN3 samples. Variations of Eu against the temperature of post annealing for all the samples are presented in Fig. 4. Urbach energy of sample LN1 is higher than LN3 and LN2 at temperature 400 ◦ C, and its values increase with the temperature. These rises of Eu are attributed to the structural and the thermal disorders in the ZnO films. These results show that LN3 multilayer, which was produced at 4-steps, has more optical stability with the temperature in comparison with the other two samples (LN2 and LN1).
where D is crystallite size, is the incident X-ray wavelength ˚ K␣(Cu)), is the diffraction angle and ˇ is full half width ( = 1.54 A, at maximum (FWHM). In fact, sample LN1, which was produced at 1-step of coating and oxidation, has higher average grains and average grain size decreases in sample LN2 and LN3, which were produced in the 2-step and 4-step respectively (Table 2). Figs. 4 and 6 show a decrease in the Urbach energy when average crystal size increases. To explain this effect, we note that with a larger average crystal size, the lattice strain is more relaxed. Therefore, the degree of structural disorder increases, and based on the Cody model [26] (which implies linearly decreasing variations of of Eu with the
Fig. 3. Relationship of (˛h)2 with photon energy (h) for LN1, LN2 and LN3 samples at 400 ◦ C. (The inset shows the variation of ln(˛) against h at 400 ◦ C for LN1, LN2 and LN3 samples).
Fig. 4. Urbach energy as a function of post annealing temperature for LN1, LN2 and LN3 samples.
D=
0.9 ˇ cos()
(4)
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Fig. 6. XRD spectra of LN1, LN2 and LN3 samples.
Table 2 Variation of average grain size calculated by Scherrer’s equation, for LN1, LN2 and LN3 samples. Sample identity
Crystallite size (nm)
LN1 LN2 LN3
33 25 20
Table 3 Variation of RMS roughness measured by AFM, for LN1, LN2 and LN3 samples.
Fig. 5. Room temperature PL spectra of ZnO thin films at (a) 400 ◦ C and (b) 700 ◦ C.
degree of structural disorder of a particular material) the Urbach energy decreases. Surface morphology of ZnO films was investigated by SEM and atomic force microscopy (AFM) techniques. Fig. 7 shows SEM
Sample identity
Ave roughness (nm)
RMS roughness (nm)
LN1 LN2 LN3
14.5 11.4 8
18.6 14.4 10.3
micrograph of ZnO films, indicating that the magnitude of grain boundaries in the LN1 sample is larger than sample LN2 and LN3. For the fabrication of LiNbO3 waveguides, optical loss due to the surface scattering resulted from rough surface are of major concern [28]. In this line AFM images of all samples of the ZnO films was obtained, which is shown in Fig. 8. The particle sizes of sample LN1 are clearly larger than sample LN2 and LN3, thus the surface roughness of sample LN1 is higher than sample LN2 and LN 3 (see Table 3). In LN3 samples, step by step coating and thermal oxidation cause a more uniform layer with more packed surface with less voids, compared to other samples. In contrast, in LN1 samples, the 1-step coating and thermal oxidation cause the atoms not
Fig. 7. SEM micrograph of LN1, LN2 and LN3 samples.
M. Shirazi et al. / Applied Surface Science 257 (2011) 10233–10238
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Fig. 8. AFM images of LN1, LN2 and LN3 samples.
to gain enough energy to be placed in appropriate (uniform but packed) spots and therefore, the LN1 samples have more voids and porosity. The results of PL analysis confirm a much larger rate of defects and oxygen vacancies in LN1 samples than LN2 and LN3 samples (see Fig. 5). Hence, for LN1 samples, coarser surface with more roughness is expected. Thus the multilayer coating technique results in decreasing of the surface roughness and scattering on light on the surface optical LN waveguides. 4. Conclusion ZnO films deposition with 200 nm thicknesses were carried out by multilayer method at one, two and four steps with repeated coating of the Zn film followed by thermal oxidation, with 200, 100, and 50 nm thicknesses repeatedly. The optical and structural characteristics of the prepared samples were investigated by XRD, SEM, AFM, PL analysis. From structural point of view 1-step coated sample (LN1) showed more intense and sharper XRD peaks, which leads to the larger average grain size. SEM and AFM micrographs also showed that grain size and surface roughness for LN1 sample was larger than the sample LN3 which was produced at 4-step of coating and the obtained results were in good agreements with previous results. On the other hand, the multilayer coating technique leads to decreasing of the surface roughness, which results in decreasing of the scattering on light on the surface and fabrication of low loss LiNbO3 waveguides. Our results suggest that deposition of ZnO film by multilayer method produces samples with better optical stability even at higher temperatures of the post annealing, and the structural defect especially oxygen vacancies in this sample is lower.
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