Preferred Orientation (006) of LiNbO 3 Thin Film Deposited on Sapphire using. R.F. Sputtering

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1 Preferred Orientation (006) of LiNbO 3 Thin Film Deposited on Sapphire using R.F. Sputtering Yeou-Yih Tsai a, Yee-Shin Chang b, *, Guo-Ju Chen c, Yi-Shiuan Shieh d, Chia-Liang Kuo e a Department of Electronic Engineering, Kao Yuan University,Lujhu, Kaohsiung, 821 Taiwan b Department of Electronic Engineering, National Formosa University,Huwei, Yunlin 632 Taiwan c Department of Materials Science and Engineering, I-Shou University,Kaohsiung, 840 Taiwan d Taiwan Semiconductor Manufacturing Company Limited, Tainan 74147, Taiwan e Department of Materials Science and Engineering, National Cheng Kung University,Tainan, 701 Taiwan ABSTRACT The sputter deposition of LiNbO 3 on sapphire substrate (0001) was performed using a radio frequency sputter deposition technique with various deposition temperatures and ratios of Ar/O 2. After deposition, the specimens were annealed at 500 and 600 for 1 and 2 hr in an O 2 atmosphere. The XRD analysis results indicate that when the growth temperature was above 450, preferred orientation (006) easily grew at Ar/O 2 ratios of 8/2 and 6/4; the optimum condition was 500 for an Ar/O 2 ratio of 8/2. The surface morphology of LiNbO 3 film seems to become rougher with increasing annealing temperatures and time. Post annealing at 500 for 2 hr in an O 2 atmosphere increased the LiNbO 3 preferred orientation intensity, and the transmittance increases from 56 % to about 70 %. Increasing annealing temperature and time led to the formation of the Li deficiency phase of LiNb 3 O 8, which caused the intensity of preferred orientation (006) and the transmittance to decrease. Keywords:Lithium niobate,x-ray diffraction, Thin film, Preferred orientation, Optical properties 1. Introduction Lithium niobate (LiNbO 3 ) has been found to be an excellent material for pyroelectric, piezoelectric, and nonlinear optical devices. It is a very attractive material for the fabrication of optical wave-guide devices. Single-crystal lithium niobate is a very important ferroelectric material with a Curie temperature of 1483 K and a rhombohedral structure with planar sheets of oxygen atoms in a distorted hexagonal close-packed arrangement 1. In bulk form, LiNbO 3 has been used in many applications, most of which require high quality materials with low optical loss, high homogeneity, and high purity. Recently, thin films of LiNbO 3 have been studied for use in an integrated form with unique pyroelectric, piezoelectric, and nonlinear optical properties, which would make it an ideal material for the fabrication of surface acoustic wave (SAW) and optical devices. Particularly, the electromechanical coupling coefficient (k 2 ) of single-crystal LiNbO 3 is far larger (5.5%) than those of quartz, AlN, and LiTaO 3 (0.14, 0.7 and 0.64%, respectively). 2 Thin films are used in a wide range of applications,

2 including electronic, magnetic, and optical devices and systems. In these applications, the distributions of grain sizes and orientations strongly affect the properties, performance, and reliability of the films. LiNbO 3 thin films have been prepared by radio-frequency (R.F.) magnetron sputtering 3, pulsed laser deposition 4-6, chemical vapor deposition 7, liquid phase epitaxial growth 8-9, and the sol gel method Sputter deposition techniques, using either DC or RF, are most commonly used for the preparation of polycrystalline LiNbO 3 thin films due to their advantages of lower deposition temperature, less film damage, higher deposition rates, and compatibility with IC processing. Some properties of a thin film are characterized by its preferred orientation. Many researches have reported that (006) oriented LiNbO 3 films could be grown on sapphire (0001) and MgO (111) because sapphire has the same crystal structure as that of LiNbO 3 (006) and MgO (111) has a similar oxygen packing plane to that of LiNbO 3 (006) They were grown using the sol-gel method, but the processes are generally complicated and the reagents used are very expensive. In the present study, LiNbO 3 preferred orientation (006) was deposited on sapphire using R. F. Sputtering. The structure, surface morphology, and optical properties of the resulting LiNbO 3 thin film were examined. 2. Experimental Procedure 2.1. Preparation of LiNbO 3 Target It is difficult to grow epitaxial LiNbO 3 films because Li is highly volatile. In order to prevent the Li atom from evaporating during the target preparation at a high sintering temperature, the LiNbO 3 target with Li/Nb=1.1 was prepared using the conventional solid-state reaction method with Li 2 CO 3, and Nb 2 O 5 powders used as the starting materials with a purity of %, supplied by Aldrich Chemical Company, Inc. and Acros Organics. The starting materials were mixed in ethanol by ball milling with zirconia balls in polyethylene jars for 24 hr, and then dried at 120. The mixed powders were calcined at 800 for 10 hr to form the LiNbO 3 phase and further milled for 24 hours, also with zirconia balls in polyethylene jars, after furnace cooling. The obtained ground powders were then added to 3 wt % of polyvinyl alcohol (PVA) solution as a binder. After drying at 120, they were pressed into discs of 2.5 inches in diameter and 5 mm thick (with a pressure of about 5 MPa), followed by cold isostatic pressing of 200 MPa for 20 minutes in rubber bags. Finally, the discs were sintered at 1000 for 24 hr Deposition of LiNbO 3 Thin Films LiNbO 3 thin films were deposited using a radio frequency (R.F.) magnetron sputter deposition technique on sapphire substrates using a LiNbO 3 target with a purity of 99.99%, a Li/Nb ratio of 1.1, and a diameter of 2 inches. Initially, the sputter deposition chamber was evacuated to a pressure lower than torr. Deposition of LiNbO 3 thin films was then carried out at a pressure of 5 mtorr using an R.F. power of 150 W and a working distance of 45 mm. Various ratios of Ar/O 2 (8/2, 6/4, and 5/5) were used. During the deposition of LiNbO 3 thin film, the substrate temperatures were 450 and 500, and the deposition time was 30 min. After deposition, an annealing treatment was carried out under an oxygen atmosphere at different temperatures and times to avoid possible oxygen deficiency and to improve the optical properties of the as-grown LiNbO 3 films Characterizations The crystal structure of specimens was analyzed using X-ray diffractometry (XRD, Rigaku Dmax-33 X-ray diffractometer) with Cu-Kα radiation with a source power of 30 kv and a current of 20 ma to identify the possible phases formed after heat-treatment. A Tenco α-step 500 instrument was used to measure the film

3 thickness. The surface morphology was examined using high-resolution scanning electron microscopy (HR-SEM, XL-40 FEG, Philips). Optical transmittance spectra were measured at room temperature using a Hitachi U-3010 UV visible spectrophotometer with a measurement wavelength from 200 ~ 900 nm, and a scan speed of 200 nm/min. 3. Results and discussion 3.1. Phases in samples Fig. 1 shows the XRD results of LiNbO 3 thin film deposited on sapphire at various substrate temperatures and Ar/O 2 ratios. It is easy to deposit preferred orientation (006) of LiNbO 3 thin film on sapphire using R.F. sputtering. According to the study by Thompson and Carel 16, the driving force used to grow preferred orientation thin film decreases the interface energy between the thin film and the substrate. Both LiNbO 3 and sapphire have the same crystal structure (hexagonal closed packed structure), which minimizes surface and interface energies and leads to growth in a preferred orientation. A higher substrate temperature [Fig.1(a) and Fig.1(c)] improves the crystallinity and preferred (006) orientation of LiNbO 3 thin film. Because it enhances atom mobility and causes the re-evaporation of poorly combined structures 17, leading to better crystallinity. Comparing Fig.1(a) with Fig.1(b) or Fig.1(c) with Fig.1(d), the intensity of preferred orientation (006) decreases as the Ar/O 2 ratio increases. At a higher Ar/O 2 ratio, oxygen deficiency occurs in LiNbO 3, which is sub-stoichiometric and loses long-range order domains As the Ar/O 2 ratio decreases, the volume of the crystalline phase and the degree of preferred (006) orientation increases. Fig. 2 shows the XRD pattern of LiNbO 3 thin films annealed at various temperatures and times under an oxygen atmosphere. The as-grown films were deposited at 500 with Ar/O 2 = 8/2. From the XRD results, the LiNbO 3 thin films still possess a structure with preferred orientation (006), and the intensity of (006) seems to increase with increasing annealing temperature and time. Higher annealing temperatures and longer annealing times lead the annealing out of defects which produces a better crystallinity and a higher degree of (006) preferred orientation. There is a little split of the (006) peak at 600, implying that the higher temperature caused the precipitation of the Li deficient phase (LiNb 3 O 8 ). In addition, at 600, longer annealing times lead to a deficiency of Li, causing the intensity of (006) to decrease Surface morphology The surface morphology of LiNbO 3 film is a very important parameter for integrated-optic device applications. Fig. 3 shows a SEM micrograph of LiNbO 3 thin film deposited on sapphire at various substrate temperatures and Ar/O 2 ratios. It appears that a higher substrate temperature favors a film with a larger crystalline size. However, varying the Ar/O 2 ratio seems to produce little effect on the crystalline size. LiNbO 3 film seems to be denser at lower Ar/O 2 ratios, as shown in Fig. 3(a) and 3(b). As discussed later, reduced deposition rates with increasing temperature create the difference in thin film morphology. SEM micrographs of LiNbO 3 thin film deposited on sapphire after annealing at various temperatures and times under an oxygen atmosphere are shown in Fig.4. It appears that a higher annealing temperature favors a LiNbO 3 thin film with larger crystalline size, but some aggregations appear. On the other hand, the film is denser and smoother at a lower annealing temperature. As discussed earlier, this suggests that a higher annealing temperature leads to the precipitation of the Li deficiency phase of LiNb 3 O 8, which makes the morphology of thin film rougher.

4 3.3. Deposition rate The thickness of LiNbO 3 thin films deposited at various substrate temperatures and Ar/O 2 ratios were measured using α-step, which in turn gives the deposition rate. The deposition rate (Table 1) of LiNbO 3 thin films appears to decrease slightly with substrate temperature because a higher substrate temperature leads to the re-evaporation of loosely adsorbed atoms 17. The deposition rate seems to decrease slightly with decreasing the Ar/O 2 ratio. During sputter deposition, plasma or glow discharge forms through the avalanche ionization of the gas and the degree of such ionization determines the plasma density. It is well known that the ionization energy of oxygen is higher than that of argon 20. As a result, as the Ar/O 2 ratio decreases, the plasma density or the positive ion density decreases. Furthermore, O -2 ions give a lower sputtering yield than Ar + ions do, which also contributes to the decrease in the deposition rate Optical transmittance Many dielectric materials are intrinsically transparent or even opaque because of interior reflection and refraction. A transmitted light beam is deflected in direction and appears diffuse as a result of multiple scattering events. The internal scattering of light may occur at grain boundaries, two phase boundaries, on pores in the material. Polycrystalline specimens, in which the index of refraction is anisotropic, normally appear translucent. The transmission curves from the ultraviolet to the infrared range of the spectrum for the LiNbO 3 thin film annealed in an oxygen atmosphere at various temperatures and times are shown in Fig. 5. The results indicate that the optical transmittance of as-grown LiNbO 3 thin film is about 56 %. LiNbO 3 is a birefringent, which is a kind of uniaxial negative crystal (n e <n o ). The n e and n o vary with the wavelength and temperature of incident light difference 21. For a wavelength of nm, n o and n e are 2.28 and 2.20, respectively. As mentioned above, LiNbO 3 is an optically anisotropic crystal that appears translucent. Annealing at 500 under an oxygen atmosphere for 2 hr produces an average transmittance of about 70 % in the nm range. Moreover, at a higher annealing temperature, the Li-deficient phase of LiNb 3 O 8 forms and becomes finely dispersed. The beam dispersion occurs across phase boundaries when the two phases have different refractive index, causing the transmittance to decrease. 4. Conclusions Sputter deposition of LiNbO 3 on sapphire substrate (0001) was performed using a radio frequency sputter deposition technique with various deposition temperatures and ratios of Ar/O 2. The XRD analysis results indicate that when the growth temperature was above 450, preferred orientation (006) easily grew at Ar/O 2 ratios of 8/2 and 6/4. The optimum condition for growing preferred orientation of LiNbO 3 thin film is at 500 for an Ar/O 2 ratio of 8/2. The surface morphology of LiNbO 3 film seems to become rougher with increasing annealing temperature and time. Post annealing at 500 for 2 hr in an O 2 atmosphere is the best condition for preferred orientation LiNbO 3 thin film deposited on sapphire. The transmittance increased from 56 % to about 70 % in the nm range. Since Li is highly volatile, the Li deficiency phase of LiNb 3 O 8 formed, causing the intensity of preferred orientation (006) and transmittance to decrease. Acknowledgements The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. (NSC E CC3).

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6 Table 1 Deposition rates of LiNbO 3 thin films. Ar/O 2 ratio Deposition rate (Å/min) (= 4) (= 1.5) (= 1) Intensity (006) (006) (006) LiNbO 3 (d)500 O C-6/4 (c)500 O C-8/2 Intensity (006) (006) (006) (006) (deg) LiNbO 3 (d)600 O C-2hr (c)600 O C-1hr (b)500 O C-2hr (a)500 O C-1hr Fig. 2 XRD pattern of LiNbO 3 thin films annealed for various temperatures and times under an oxygen atmosphere. (b)450 O C-6/4 (006) (a)450 O C-8/ (deg) Fig. 1 XRD pattern of LiNbO 3 thin films deposited on sapphire for various substrate temperatures and Ar/O 2 ratios.

7 (a) (a) (b) (b) (c) (c) Fig. 3 The SEM micrographs of LiNbO3 thin film Fig. 4 The SEM micrographs of LiNbO3 thin film deposited on sapphire at various substrate temperatures deposited on sapphire after annealing at (a) 500 /1hr, (b) and Ar/O2 ratios. (a) 500, Ar/O2= 8/2 (b) 500, 600 /1hr, (c) 600 /2hr under an O2 atmosphere. Ar/O2= 6/4 (c) 450, Ar/O2= 8/2.

8 Fig. 5 The transmittance of LiNbO 3 thin film measured after annealing treatment.