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LiNbO3 optical waveguides deposited on sapphire by electric-field-assisted pulsed laser deposition
Vacuum 58 (2000) 396}403
LiNbO optical waveguides deposited on sapphire by electric-"eld-assisted pulsed laser deposition夽 R.I. Tomov , T.K. Kabadjova , P.A. Atanasov *, S. Tonchev, M. Kaneva, A. Zherikhin, R.W. Eason Institute of Electronics, Bulgarian Academy of Sciences, 72 Tzarigradsko shosse, 1784 Soxa, Bulgaria Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko shosse, 1784 Soxa, Bulgaria Scientixc Research Centre for Technological Lasers, RAN, 142092, Pionerskaya 2, Nictl RAN, Troitsk, Moscow region, Russia Department of Physics and Optoelectronics Research Centre, University of Southampton, Southampton S017 1BJ, UK
Abstract Oriented LiNbO "lms have been grown on sapphire (1 1 0 2) substrates by low electric-"eld-assisted pulsed laser deposition. KrF excimer laser (248 nm) has been utilized for the ablation of monocrystalline stoichiometric target. The deposition was performed at all substrate temperatures in the range of 550}8003C in an oxygen atmosphere. The ablation threshold of 2.5 J/cm for the monocrystalline target was determined from the experimental results presenting ablation rate vs. laser #uence dependence. Electro-optical properties of these "lms appeared to be strongly dependent on the process parameters (substrate temperature, oxygen partial pressure, electric "eld), demonstrating the importance of the epitaxial growth control during the deposition. The "lms structure and morphology were characterized by XRD and AFM, respectively. The waveguide properties of the "lms were studied experimentally. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Laser ablation; Thin "lms; Lithium niobate
1. Introduction Due to its broad range of unique piezoelectric, electro-optic, nonlinear optical and photorefractive properties, lithium niobate is a material of great technological interest for optical applications 夽
Paper presented at the 11th International School on Vacuum, Electron and Ion Technologies, 20}25 September 1999, Varna, Bulgaria. * Corresponding author. Tel.: #359-2-7144-382; fax:#359-2-975-3201. E-mail address: [email protected] (P.A. Atanasov). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 1 9 6 - 2
R.I. Tomov et al. / Vacuum 58 (2000) 396}403
397
[1]. A bulk LiNbO has already been utilized for producing a variety of optical devices. With the progress of thin-"lm technology and growth of high-quality thin "lms, lithium niobate appears to have high potential for many di!erent applications as surface acoustic waves (SAW) devices, optical waveguide devices, holographic memory using photorefractive e!ect, etc. [1}4]. Because of its anisotropic EO properties the control of the "lms growth orientation is very important [5]. A di!erent epitaxial growth orientation is required to obtain large EO e!ect depending on the direction of light propagation and applied electric "eld. Since LiNbO and sapphire have similar crystal structure and appropriate combination of optical parameters for waveguide propagation, sapphire appears to be an excellent substrate for growing epitaxial thin "lms. LiNbO thin "lms have been studied by rf magnetron sputtering [6], liquid-phase epitaxy [7], molecular beam epitaxy [8] and sol}gel processes [9]. Among other techniques laser ablation deposition has been successfully applied [10}15]. The laser ablation has been shown to be very appropriate for the complex oxide "lms deposition due to its intrinsic feature of easily transfering the target stoichiometry to the substrate. But even with the pulsed laser ablation deposition it has been found to be di$cult to get epitaxial LiNbO "lms since Li is highly volatile. Using temperatures of 700}8003C, Li-de"cient phases, such as LiNb O , have been often obtained [16]. Li-de"cient phase can be suppressed by depositing "lms at temperature below 5003C but lower deposition temperature usually led to lower quality of crystal structure and bad adhesion. In this work we report on the preparation of quality thin-"lms LiNbO on sapphire substrate at moderate deposition temperatures (500}6003C) from a single-crystal target, which has a congruent melting composition (&48.4 mol% Li O). Low electric "eld was applied along the normal to the substrate surface in order to stimulate preferential growth of optical axis.
2. Experimental A schematic diagram of our set-up is shown in Fig. 1. A KrF excimer laser `Lambda Physika LPX-200 operating at 248 nm wavelength with pulse duration of 25 ns was utilized. Laser beam was introduced into the vacuum chamber through a plane-parallel quartz window. In our experiments, the laser-energy density on the target surface was adjusted to 2.5}3.5 J/cm. The
Fig. 1. Schematic diagram of oxygen-plasma-assisted laser deposition set-up with cw CO laser irradiation of substrate surface.
398
R.I. Tomov et al. / Vacuum 58 (2000) 396}403
distance between the target and the substrate was optimized to 4}5 cm. The substrate was "xed on a resistive heater oriented plane-parallel to the target. The substrate temperature was controlled by a thermocouple in contact with the holder. The experiment performed with thermocouple glued by silver dag directly onto the substrate surface showed 50}703C lower temperature compared to that measured on the holder. The "lms were deposited at substrate temperatures in the range of 550}8003C in an oxygen atmosphere. The deposition chamber was "rst evacuated down to a residual pressure of 10\ mbar. After deposition the "lms were annealed in oxygen (0.5 mbar) for 30 min in order to reduce the oxygen de"ciency. It is well known that oxygen vacancies are responsible for the absorption in the visible spectrum. The texturing of the monocrystalline LiNbO pellet surface during the deposition process was avoided by rotation. A low electric "eld (&few V/cm) directed normally to the substrate surface was applied in-situ during the growth by introducing a positively biased grid electrode placed between the grounded substrate holder and the target. The substrate-grid distance was "xed to 8 mm. XRD was used to determine the crystallinity of the deposited "lms. AFM was used to examine the surface morphology of deposited "lms. The e!ective indices of the waveguide were determined by the prism coupling method [17]. Using a SrTiO prism and HeNe laser beam (632.8 nm) TE and TM modes propagation have been observed.
3. Results and discussions The ablation threshold of monocrystalline LiNbO target was determined in an oxygen atmosphere. Fig. 2 shows the dependence of the ablation rate vs. laser #uence (E). The curve is built by the experimental data obtained at 0.02 mbar oxygen pressure and with target rotation. The ablation threshold is evaluated by a linear "tting to be approximately 2.5 J/cm. There are two important technological parameters responsible for di!erent phases formation, that is, the substrate temperature and the oxygen partial pressure during the deposition. Fig. 3 shows the H}2H X-ray di!raction patterns of the lithium niobate "lms deposited on
Fig. 2. Experimentaly dependence of the ablation rates vs. laser #uence.
R.I. Tomov et al. / Vacuum 58 (2000) 396}403
399
Fig. 3. XRD spectra of LiNbO thin-"lms deposited by electric-"eld-assisted laser ablation deposition at (a) 6003C, 0.1 mbar, O and (b) 8003C, 0.1 mbar O .
a (1 1 0 2) sapphire substrates at oxygen pressure 0.1 mbar and various temperatures (600 and 8003C) and annealed in oxygen pressure for 30 min. As shown in Fig. 3a the "lm deposited at lower temperature shows (0 1 2) and (0 2 4) peaks of LiNbO in good alignment with substrate vectors and small (2 0 2) peak indicating the disorientation of some grains. Even at this moderate temperature, peaks of Li-de"cient phase, LiNb O are observed. The "lm deposited at 8003C shows dominating peaks of Li-de"cient phase as well as strong peaks of oxygen de"cient phase LiNbO (see Fig. 3b). The presence of both (0 1 2) and (3 0 0) LiNbO peaks has been observed with an intensity ratio I(300)/I(012)"2. When the deposition was performed without electric "eld assistance (3 0 0) peak appeared as dominant thus con"rming the results of Shibata et al. [10] that the cation arrangements seems to be a dominant factor for the growth on the sapphire (1 1 0 2) at the proposed conditions. The oxygen partial pressure appeared to be an important parameter for controlling phase formation. Increasing the oxygen pressure to 0.2 mbar at 8003C a strongly oriented Li-de"cient almost single phase was obtained (see Fig. 4). This result suggests that during the plasma expansion additional Li losses were produced. In addition to the Li losses due to re-evaporation, some of the low-weight Li atoms have been lost due to the increased collisions at higher oxygen pressure. Apparently, the cumulative e!ects on the target composition during
400
R.I. Tomov et al. / Vacuum 58 (2000) 396}403
Fig. 4. XRD spectra of LiNbO thin-"lm deposited by electric-"eld-assisted laser ablation deposition at 8003C, 0.2 mbar O .
Fig. 5. AFM pictures of LiNbO thin-"lms deposited by electric-"eld-assisted laser ablation deposition at (a) 6003C, 0.1 mbar, O and (b) 8003C, 0.1 mbar O .
multipulse laser ablation took place, as shown for YBa Cu O targets earlier [18], and also \V contribute to the problem of Li-de"cient "lm growth. Films were examined by atomic force microscopy. Fig. 5a and b show an AFM image of two LiNbO thin-"lms deposited at equal oxygen pressure (0.1 mbar) and 600 and 8003C, respectively. The droplets impurities about 1 lm size which are the characteristic of laser ablation method are observed on both samples. The density of droplets decreases with increasing substrate temperature.
R.I. Tomov et al. / Vacuum 58 (2000) 396}403
401
One possible mechanism of such reduction could be spreading and migration of the droplets, resulting in coalescence in the "lm at higher temperature, proposed by Yonezawa et al. [19]. The information about the number of modes propagation and relative thin-"lms process parameters are summarized in Table 1. Fig. 6 shows the waveguide propagation of one TE and one TM modes (HeNe laser * 632.8 nm) in &200 nm thick LiNbO "lm deposited at 6503C. The propagation losses were evaluated to 15 dB/cm. The thicker "lms possessed rough morphology and higher density of droplet contamination and consequently the optical losses rose above Table 1 Deposition parameters and number of waveguide propagation modes Sample
p(O ) (mbar)
¹ (3C)
Number of TE modes
Number of TM modes
1 2 3 4
0.2 0.05 0.2 0.1
600 650 800 800
2 1 1 2
2 1 1 3
Fig. 6. Waveguide optical propagation of TE and TM modes in 200 nm thick LiNbO thin "lm deposited by electric-"eld-assisted laser ablation deposition at 6003C, 0.1 mbar O .
402
R.I. Tomov et al. / Vacuum 58 (2000) 396}403
40 dB/cm due to the light rescattering on these type of defects. The ordinary refractive indices for the "lm deposited at 6503C and 0.05 mbar were determined as n "2.2468 and n "2.28303. On 2# 2+ the "lm deposited at higher temperature and higher pressure (8003C, 0.2 mbar) and consequently containing a larger amount of Li-de"cient phase, n "2.2838 and n "2.1514 have been 2# 2+ measured. These values are comparable with the value reported earlier by Weis et al. [1] n "2.2910. 4. Conclusion Laser ablation of a stoichiometric target assisted by an electric "eld was used for the deposition of LiNbO thin "lms on sapphire substrates. The electric "eld has been shown to in#uence crystallographic orientation of the growing "lms. From XRD measurements, it follows that a substrate temperature and oxygen pressure were the key process parameters for di!erent phase formation and crystal quality of the "lms as well as for the resulting droplets density. Our study shows the waveguide propagation of 1 TE and 1 TM modes in &200 nm thick LiNbO "lm deposited at 6503C. The "lm refraction indices were calculated to be close to that of the bulk LiNbO target. Further experimental studies are necessary in order to decrease the Li-de"cient phase content on the basis of the mechanisms of Li losses.
Acknowledgements This work was partially supported by the contracts: Contract INCO-Copernicus IC15CT98-0807, and Contracts F-719 and VPR-01 with the Ministry of Education and Science, Bulgaria.
References [1] Weis R, Gaylord T. Appl Phys A 1985;A37:191}203. [2] Feng D, Ming N-B, Hong J-F, Zhu J-S, Yang Y, Wang YN. Appl Phys Lett 1980;37:607. [3] Smith D, Chacravarthy R, Bao Z, Baran J, Jackel J, d'Alessandro A, Fritz D, Huang S, Zou X, Hwang S-M, Willner A, Li K. J Lightwave Technol 1996;14:1005. [4] Granestrand P, Lagerstrom B, Svensson P, Olofsson L, Falk J-E, Stolz B. Photon Technol Lett 1994;6:71. [5] Rauber A. In: Kaldis E, editor. Current topics in materials science, vol. 1. Amsterdam: North-Holland, 1978. p. 481. [6] Shimizu M, Furushima Y, Nishida T, Shiosaki T. Jap J Appl Phys 1993;32 (Part 1): 4111}4. [7] Yamada A, Tamada H, Saitoh M. Appl Phys Lett 1992;61:2848}51. [8] Betts RA, Pitt CW. Electron Lett 1985;21:960}3. [9] Wernberg AA, Gysling HJ, Filo AJ, Blanton TN. Appl Phys Lett 1993;62:946}50. [10] Shibata Y, Kaya K, Akashi K, Kanai M, Kawai T, Kawai S. Appl Phys Lett 1992;61:1000}2. [11] Ogale SB, Dikshit RN, Dikshit SJ, Kanetkar SM. J Appl Phys 1992;71:5718}20. [12] March AM, Harkness SD, Qian F, Singh RK. Appl Phys Lett 1993;62:952}4. [13] Lee SH, Song TK, Noh TW, Lee JH. Appl Phys Lett 1995;67:43}5. [14] Lee SH, Noh TW, Lee JH. Appl Phys Lett 1996;68:472}4.
R.I. Tomov et al. / Vacuum 58 (2000) 396}403 [15] [16] [17] [18] [19]
403
Ghica D, Ghica C, Gartner M, Nelea V, Nartin C, Cavaleru A, Mihailescu IN. Appl Surf Sci 1999;138}139:617}21. Shibata Y, Kaya K, Akashi K, Kanai M, Kawai T, Kawai S. Appl Phys Lett 1993;62:946. Ulrich R. J Opt Soc Amer 1970;60:337}41. Tomov RI, Manolov VP, Atanasov PA, Tsaneva VN, Ouzounov DG, Tsanev VI. Physica C 1997;274(3/4):187}96. Yonezawa Y, Minimakawa T, Matsuda K, Takezawa K, Morimoto A, Shimitzu T. Appl Surf Science 1998;27}129:639}44.
LiNbO optical waveguides deposited on sapphire by electric-"eld-assisted pulsed laser deposition夽 R.I. Tomov , T.K. Kabadjova , P.A. Atanasov *, S. Tonchev, M. Kaneva, A. Zherikhin, R.W. Eason Institute of Electronics, Bulgarian Academy of Sciences, 72 Tzarigradsko shosse, 1784 Soxa, Bulgaria Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko shosse, 1784 Soxa, Bulgaria Scientixc Research Centre for Technological Lasers, RAN, 142092, Pionerskaya 2, Nictl RAN, Troitsk, Moscow region, Russia Department of Physics and Optoelectronics Research Centre, University of Southampton, Southampton S017 1BJ, UK
Abstract Oriented LiNbO "lms have been grown on sapphire (1 1 0 2) substrates by low electric-"eld-assisted pulsed laser deposition. KrF excimer laser (248 nm) has been utilized for the ablation of monocrystalline stoichiometric target. The deposition was performed at all substrate temperatures in the range of 550}8003C in an oxygen atmosphere. The ablation threshold of 2.5 J/cm for the monocrystalline target was determined from the experimental results presenting ablation rate vs. laser #uence dependence. Electro-optical properties of these "lms appeared to be strongly dependent on the process parameters (substrate temperature, oxygen partial pressure, electric "eld), demonstrating the importance of the epitaxial growth control during the deposition. The "lms structure and morphology were characterized by XRD and AFM, respectively. The waveguide properties of the "lms were studied experimentally. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Laser ablation; Thin "lms; Lithium niobate
1. Introduction Due to its broad range of unique piezoelectric, electro-optic, nonlinear optical and photorefractive properties, lithium niobate is a material of great technological interest for optical applications 夽
Paper presented at the 11th International School on Vacuum, Electron and Ion Technologies, 20}25 September 1999, Varna, Bulgaria. * Corresponding author. Tel.: #359-2-7144-382; fax:#359-2-975-3201. E-mail address: [email protected] (P.A. Atanasov). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 1 9 6 - 2
R.I. Tomov et al. / Vacuum 58 (2000) 396}403
397
[1]. A bulk LiNbO has already been utilized for producing a variety of optical devices. With the progress of thin-"lm technology and growth of high-quality thin "lms, lithium niobate appears to have high potential for many di!erent applications as surface acoustic waves (SAW) devices, optical waveguide devices, holographic memory using photorefractive e!ect, etc. [1}4]. Because of its anisotropic EO properties the control of the "lms growth orientation is very important [5]. A di!erent epitaxial growth orientation is required to obtain large EO e!ect depending on the direction of light propagation and applied electric "eld. Since LiNbO and sapphire have similar crystal structure and appropriate combination of optical parameters for waveguide propagation, sapphire appears to be an excellent substrate for growing epitaxial thin "lms. LiNbO thin "lms have been studied by rf magnetron sputtering [6], liquid-phase epitaxy [7], molecular beam epitaxy [8] and sol}gel processes [9]. Among other techniques laser ablation deposition has been successfully applied [10}15]. The laser ablation has been shown to be very appropriate for the complex oxide "lms deposition due to its intrinsic feature of easily transfering the target stoichiometry to the substrate. But even with the pulsed laser ablation deposition it has been found to be di$cult to get epitaxial LiNbO "lms since Li is highly volatile. Using temperatures of 700}8003C, Li-de"cient phases, such as LiNb O , have been often obtained [16]. Li-de"cient phase can be suppressed by depositing "lms at temperature below 5003C but lower deposition temperature usually led to lower quality of crystal structure and bad adhesion. In this work we report on the preparation of quality thin-"lms LiNbO on sapphire substrate at moderate deposition temperatures (500}6003C) from a single-crystal target, which has a congruent melting composition (&48.4 mol% Li O). Low electric "eld was applied along the normal to the substrate surface in order to stimulate preferential growth of optical axis.
2. Experimental A schematic diagram of our set-up is shown in Fig. 1. A KrF excimer laser `Lambda Physika LPX-200 operating at 248 nm wavelength with pulse duration of 25 ns was utilized. Laser beam was introduced into the vacuum chamber through a plane-parallel quartz window. In our experiments, the laser-energy density on the target surface was adjusted to 2.5}3.5 J/cm. The
Fig. 1. Schematic diagram of oxygen-plasma-assisted laser deposition set-up with cw CO laser irradiation of substrate surface.
398
R.I. Tomov et al. / Vacuum 58 (2000) 396}403
distance between the target and the substrate was optimized to 4}5 cm. The substrate was "xed on a resistive heater oriented plane-parallel to the target. The substrate temperature was controlled by a thermocouple in contact with the holder. The experiment performed with thermocouple glued by silver dag directly onto the substrate surface showed 50}703C lower temperature compared to that measured on the holder. The "lms were deposited at substrate temperatures in the range of 550}8003C in an oxygen atmosphere. The deposition chamber was "rst evacuated down to a residual pressure of 10\ mbar. After deposition the "lms were annealed in oxygen (0.5 mbar) for 30 min in order to reduce the oxygen de"ciency. It is well known that oxygen vacancies are responsible for the absorption in the visible spectrum. The texturing of the monocrystalline LiNbO pellet surface during the deposition process was avoided by rotation. A low electric "eld (&few V/cm) directed normally to the substrate surface was applied in-situ during the growth by introducing a positively biased grid electrode placed between the grounded substrate holder and the target. The substrate-grid distance was "xed to 8 mm. XRD was used to determine the crystallinity of the deposited "lms. AFM was used to examine the surface morphology of deposited "lms. The e!ective indices of the waveguide were determined by the prism coupling method [17]. Using a SrTiO prism and HeNe laser beam (632.8 nm) TE and TM modes propagation have been observed.
3. Results and discussions The ablation threshold of monocrystalline LiNbO target was determined in an oxygen atmosphere. Fig. 2 shows the dependence of the ablation rate vs. laser #uence (E). The curve is built by the experimental data obtained at 0.02 mbar oxygen pressure and with target rotation. The ablation threshold is evaluated by a linear "tting to be approximately 2.5 J/cm. There are two important technological parameters responsible for di!erent phases formation, that is, the substrate temperature and the oxygen partial pressure during the deposition. Fig. 3 shows the H}2H X-ray di!raction patterns of the lithium niobate "lms deposited on
Fig. 2. Experimentaly dependence of the ablation rates vs. laser #uence.
R.I. Tomov et al. / Vacuum 58 (2000) 396}403
399
Fig. 3. XRD spectra of LiNbO thin-"lms deposited by electric-"eld-assisted laser ablation deposition at (a) 6003C, 0.1 mbar, O and (b) 8003C, 0.1 mbar O .
a (1 1 0 2) sapphire substrates at oxygen pressure 0.1 mbar and various temperatures (600 and 8003C) and annealed in oxygen pressure for 30 min. As shown in Fig. 3a the "lm deposited at lower temperature shows (0 1 2) and (0 2 4) peaks of LiNbO in good alignment with substrate vectors and small (2 0 2) peak indicating the disorientation of some grains. Even at this moderate temperature, peaks of Li-de"cient phase, LiNb O are observed. The "lm deposited at 8003C shows dominating peaks of Li-de"cient phase as well as strong peaks of oxygen de"cient phase LiNbO (see Fig. 3b). The presence of both (0 1 2) and (3 0 0) LiNbO peaks has been observed with an intensity ratio I(300)/I(012)"2. When the deposition was performed without electric "eld assistance (3 0 0) peak appeared as dominant thus con"rming the results of Shibata et al. [10] that the cation arrangements seems to be a dominant factor for the growth on the sapphire (1 1 0 2) at the proposed conditions. The oxygen partial pressure appeared to be an important parameter for controlling phase formation. Increasing the oxygen pressure to 0.2 mbar at 8003C a strongly oriented Li-de"cient almost single phase was obtained (see Fig. 4). This result suggests that during the plasma expansion additional Li losses were produced. In addition to the Li losses due to re-evaporation, some of the low-weight Li atoms have been lost due to the increased collisions at higher oxygen pressure. Apparently, the cumulative e!ects on the target composition during
400
R.I. Tomov et al. / Vacuum 58 (2000) 396}403
Fig. 4. XRD spectra of LiNbO thin-"lm deposited by electric-"eld-assisted laser ablation deposition at 8003C, 0.2 mbar O .
Fig. 5. AFM pictures of LiNbO thin-"lms deposited by electric-"eld-assisted laser ablation deposition at (a) 6003C, 0.1 mbar, O and (b) 8003C, 0.1 mbar O .
multipulse laser ablation took place, as shown for YBa Cu O targets earlier [18], and also \V contribute to the problem of Li-de"cient "lm growth. Films were examined by atomic force microscopy. Fig. 5a and b show an AFM image of two LiNbO thin-"lms deposited at equal oxygen pressure (0.1 mbar) and 600 and 8003C, respectively. The droplets impurities about 1 lm size which are the characteristic of laser ablation method are observed on both samples. The density of droplets decreases with increasing substrate temperature.
R.I. Tomov et al. / Vacuum 58 (2000) 396}403
401
One possible mechanism of such reduction could be spreading and migration of the droplets, resulting in coalescence in the "lm at higher temperature, proposed by Yonezawa et al. [19]. The information about the number of modes propagation and relative thin-"lms process parameters are summarized in Table 1. Fig. 6 shows the waveguide propagation of one TE and one TM modes (HeNe laser * 632.8 nm) in &200 nm thick LiNbO "lm deposited at 6503C. The propagation losses were evaluated to 15 dB/cm. The thicker "lms possessed rough morphology and higher density of droplet contamination and consequently the optical losses rose above Table 1 Deposition parameters and number of waveguide propagation modes Sample
p(O ) (mbar)
¹ (3C)
Number of TE modes
Number of TM modes
1 2 3 4
0.2 0.05 0.2 0.1
600 650 800 800
2 1 1 2
2 1 1 3
Fig. 6. Waveguide optical propagation of TE and TM modes in 200 nm thick LiNbO thin "lm deposited by electric-"eld-assisted laser ablation deposition at 6003C, 0.1 mbar O .
402
R.I. Tomov et al. / Vacuum 58 (2000) 396}403
40 dB/cm due to the light rescattering on these type of defects. The ordinary refractive indices for the "lm deposited at 6503C and 0.05 mbar were determined as n "2.2468 and n "2.28303. On 2# 2+ the "lm deposited at higher temperature and higher pressure (8003C, 0.2 mbar) and consequently containing a larger amount of Li-de"cient phase, n "2.2838 and n "2.1514 have been 2# 2+ measured. These values are comparable with the value reported earlier by Weis et al. [1] n "2.2910. 4. Conclusion Laser ablation of a stoichiometric target assisted by an electric "eld was used for the deposition of LiNbO thin "lms on sapphire substrates. The electric "eld has been shown to in#uence crystallographic orientation of the growing "lms. From XRD measurements, it follows that a substrate temperature and oxygen pressure were the key process parameters for di!erent phase formation and crystal quality of the "lms as well as for the resulting droplets density. Our study shows the waveguide propagation of 1 TE and 1 TM modes in &200 nm thick LiNbO "lm deposited at 6503C. The "lm refraction indices were calculated to be close to that of the bulk LiNbO target. Further experimental studies are necessary in order to decrease the Li-de"cient phase content on the basis of the mechanisms of Li losses.
Acknowledgements This work was partially supported by the contracts: Contract INCO-Copernicus IC15CT98-0807, and Contracts F-719 and VPR-01 with the Ministry of Education and Science, Bulgaria.
References [1] Weis R, Gaylord T. Appl Phys A 1985;A37:191}203. [2] Feng D, Ming N-B, Hong J-F, Zhu J-S, Yang Y, Wang YN. Appl Phys Lett 1980;37:607. [3] Smith D, Chacravarthy R, Bao Z, Baran J, Jackel J, d'Alessandro A, Fritz D, Huang S, Zou X, Hwang S-M, Willner A, Li K. J Lightwave Technol 1996;14:1005. [4] Granestrand P, Lagerstrom B, Svensson P, Olofsson L, Falk J-E, Stolz B. Photon Technol Lett 1994;6:71. [5] Rauber A. In: Kaldis E, editor. Current topics in materials science, vol. 1. Amsterdam: North-Holland, 1978. p. 481. [6] Shimizu M, Furushima Y, Nishida T, Shiosaki T. Jap J Appl Phys 1993;32 (Part 1): 4111}4. [7] Yamada A, Tamada H, Saitoh M. Appl Phys Lett 1992;61:2848}51. [8] Betts RA, Pitt CW. Electron Lett 1985;21:960}3. [9] Wernberg AA, Gysling HJ, Filo AJ, Blanton TN. Appl Phys Lett 1993;62:946}50. [10] Shibata Y, Kaya K, Akashi K, Kanai M, Kawai T, Kawai S. Appl Phys Lett 1992;61:1000}2. [11] Ogale SB, Dikshit RN, Dikshit SJ, Kanetkar SM. J Appl Phys 1992;71:5718}20. [12] March AM, Harkness SD, Qian F, Singh RK. Appl Phys Lett 1993;62:952}4. [13] Lee SH, Song TK, Noh TW, Lee JH. Appl Phys Lett 1995;67:43}5. [14] Lee SH, Noh TW, Lee JH. Appl Phys Lett 1996;68:472}4.
R.I. Tomov et al. / Vacuum 58 (2000) 396}403 [15] [16] [17] [18] [19]
403
Ghica D, Ghica C, Gartner M, Nelea V, Nartin C, Cavaleru A, Mihailescu IN. Appl Surf Sci 1999;138}139:617}21. Shibata Y, Kaya K, Akashi K, Kanai M, Kawai T, Kawai S. Appl Phys Lett 1993;62:946. Ulrich R. J Opt Soc Amer 1970;60:337}41. Tomov RI, Manolov VP, Atanasov PA, Tsaneva VN, Ouzounov DG, Tsanev VI. Physica C 1997;274(3/4):187}96. Yonezawa Y, Minimakawa T, Matsuda K, Takezawa K, Morimoto A, Shimitzu T. Appl Surf Science 1998;27}129:639}44.