Domain Reversal in Stoichiometric Litao3 Prepared by Vapor Transport Equilibration L

APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 19 8 NOVEMBER 2004

Domain reversal in stoichiometric LiTaO3 prepared by vapor transport equilibration L. Tian and Venkatraman Gopalana) Materials Research Institute and Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802 Ludwig Galambos Department of Electrical Engineering, SSPL, Stanford University, Stanford, California 94035-4075 (Received 26 April 2004; accepted 20 September 2004) ͑ ͒ Domain reversal in stoichiometric lithium tantalate LiTaO3 single crystals prepared by vapor transport equilibrium (VTE) method was studied. Starting from a virgin VTE crystal and using water electrodes, the coercive fields were found to be 1.39±0.01 kV/cm and 1.23±0.01 kV/cm for the first poling and the second poling, respectively, indicating a built-in internal field of ␮ 2 0.08 kV/cm. The spontaneous polarization, Ps was 55.2±0.5 C/cm and the Curie temperature was Tc =701±2.5°C. The switching time, ts, exhibits an exponential dependence on the external field, E with activation energy of 10.84±0.22 kV/cm. No domain backswitching was observed. These properties are dramatically different from those of congruent and even near-stoichiometric compositions of lithium tantalate grown by Czhochralski method. © 2004 American Institute of Physics. [DOI: 10.1063/1.1814436]

͑ ͒ Ferroelectric lithium tantalate single crystals LiTaO3 “virgin crystal” will be referred as forward poling (subscript are used in many photonic applications.1–5 Although com- f) and the second reversal as reverse poling (subscript r). monly referred to as LiTaO3, lithium tantalate has a wide The coercive field in this letter is defined in the conventional solid solution region varying from stoichiometric point at way as the field at which 50% domain area is reversed. which Li/͑Li+Ta͒=0.5 to the Ta-rich side in which Li/͑Li The hysteresis loop was obtained by the integration of ͑ ͒ +Ta͒ϳ0.47. In the last decade, the role of defects in both the transient poling current, i= d 2PsA ր dt (Ps is the polar- ͑ ͒ lithium niobate LiNbO3 and lithium tantalate has been ization and A is the area of domain reversal), with a capaci- widely studied.6–10 These defects present in the Nb-rich or tor, which is in series with the sample and power supply.20,22 Ta-rich compositions strongly affect properties such as pho- A HP34401A multimeter with 10 G⍀ input impedance was torefractive optical damage resistance,11–14 Curie used to measure the voltage across the capacitor to prevent temperature,15–17 absorption edge,16–18 lattice parameters,17 leakage of the capacitor. The applied dc field across the crys- refractive indices,16 and coercive fields.19,20 Table I provides tal thickness was linearly ramped up at 2 V/s. The measured ␮ 2 a comparison of the various compositions of LiTaO3 and spontaneous polarization is Ps =55.2±0.5 C/cm . The co- ϳ their properties, including data from this study. ercive fields are Ecf 1.39±0.01 kV/cm for forward poling ϳ Here, we report on the electric-field-induced domain re- and Ec,r 1.23±0.01 kV/cm for reverse poling, respectively. versal in vapor transport equilibrium (VTE)-grown stoichio- These values are a factor of ϳ12 smaller than the NSLT-CZ metric lithium tantalate (SLT) crystals, which according to samples, and a factor of ϳ130 smaller than congruent Table I has very low coercive fields for LiTaO3 single crys- LiTaO3 (CLT) samples [Fig. 2(b)]. Figure 1 shows a linear tals. This will, for example, enable quasi-phase-matched fre- dependence of the coercive field on the crystal composition quency conversion devices through thick bulk samples. The and the Curie temperature. noncongruently melting compositions of lithium tantalate The crystal was successively switched 44 times. The can be grown either by double-crucible-Czochralski (CZ) time interval between two successive domain reversals was method17 or by performing VTE treatment21 on as-grown about 2 min. Figure 2(a) shows the hysteresis loops of po- congruently melting lithium tantalate single crystals.18 In the larization versus electric field for the 1st hysteresis loop latter, Li is indiffused into the crystal by thermal processing. cycle and the 22nd hysteresis loop cycle. Figure 2(b) shows In this work, the samples of stoichiometric lithium tantalate the coercive fields as well as the spontaneous polarization for (SLT) were synthesized by a VTE treatment method by Sili- both forward poling and reverse poling obtained in these 22 con Light Machines.™ The samples are Z-cut crystals with a experimental cycles. These results show that the coercive ͑ ϳ thickness of 0.83 mm. Note that the previously labeled sto- fields at the 2 V/s ramp rate Ecf 1.39±0.01 kV/cm,Ec,r ichiometric LiTaO3 grown by the CZ method in Refs. 16 and ϳ1.23±0.01 kV/cm͒ do not change significantly between 17 are labeled here as near-stoichiometric LiTaO3 (NSLT- cycles. This is in strong contrast to CLT and NSLT-CZ CZ). The VTE-grown LiTaO3 crystals studied here are la- lithium tantalate crystals reported before in Ref. 10, where beled as stoichiometric LiTaO3, or SLT-VTE. the coercive field changes with repeated cycling. Similarly, In the following experiments, liquid electrodes (tape wa- ͑ ͒ the internal field, Eint= Ecf-Ec,r /2–0.08 kV/cm was ob- ter) were used as contacts. The first domain reversal from the served for SLT-VTE, which is only about 6.1% of its coercive field. In contrast, the internal field is about 25% of its 19 10 a)Electronic mail: [email protected] coercive field in CLT and 13% in NSLT-CZ.

0003-6951/2004/85(19)/4445/3/$22.004445 © 2004 American Institute of Physics Downloaded 21 Mar 2006 to 146.186.113.217. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp 4446 Appl. Phys. Lett., Vol. 85, No. 19, 8 November 2004 Tian, Gopalan, and Galambos

TABLE I. A comparison of congruent (CLT), near-stoichiometric (NSLT-CZ), and stoichiometric (SLT-VTE) crystals. Measurements not referenced below are from this study.

Near-stoichiometric Stoichiometric Congruent (CLT) (NSLT-CZ) (SLT-VTE)

Growth Technique Czochralski (CZ) method Double-crucible Czochralski (CZ) method VTE treatment on CZ-grown CLT Composition, [Li]/([Li]ϩ[Ta]) 0.48510 0.49820 ϳ0.5 Curie Temperature ͑°C͒ 601±220 685±120 701±2.5 Coercive fields ͑kV/cm͒, 211.55±2.81 (forward)19 17 (forward)20 1.61±0.06 (forward) 296 K (ramp rate 15 V/s) 125.99±1.65 (reverse)19 15 (reverse)20 1.48±0.06 (reverse) Coercive fields ͑kV/cm͒, 1.39±0.01 (forward) 296 K (ramp rate 2 V/s) ¯¯1.23±0.01 (reverse) Internal fields ͑kV/cm͒ 44.28±2.0819 1.020 0.08 UV Absorption edge ͑nm͒ 27518 26018 256 Activation field 51.4±5.9 (forward)10 26.48±2.1 (forward)10 10.84±0.22 (forward) ͑kV/cm͒ 36.93±5.0 (reverse)10 33.4±2.1 (reverse)10 10.84±0.22 (reverse) ϳ1.7 s (forward)10 ϳ700 ms (forward)10 Ͻ1ms(both forward and Stabilization time ϳ0.1–0.3 s (reverse)10 ϳ100 ms (reverse)10 reverse), if present at all ͑␮ 2͒ 19 20 Ps C/cm 60±3 55±3 55.2±0.5

The domain switching time was measured by applying CLT crystals, where the observed stabilization time was as step voltages across the sample. The HP 33210A function high as 1.7 s for forward poling and ϳ0.1–0.3 s for reverse generator and Trek 20/20C amplifier was used as power sup- poling. ply. The transient current pulse resulting from the domain The domain structure of LiTaO3 changes with its com- reversal process was measured by Tektronics TDS 340A os- position. The domain in CLT can be seen under an optical cilloscope. The rise time of the applied field across the microscope without any applied voltage.23 The domain shape ϳ ␮ ϳ sample was 50 s. The current sensitivity was 20 nA. is triangular with walls perpendicular to the y axis. The vis- Figure 3 shows the electric field, E, versus switching time for both forward poling and reverse poling. Switching time is defined as the total time required for switching 95% of the total electrode area here. The switching time for ferroelectric domain reversal can be described as: ␦ ͑t ͒ = ͑t ͒ expͩ r,f ͪ s r,f 0 r,f ϯ E Eint where E−Eint is for forward poling, and E+Eint is for reverse poling. Figure 3 shows that the switching times for both forward poling and reverse poling have the same activation ͑␦ ␦ ͒ energy r = f =10.84±0.22 kV/cm . The domain backswitching discussed in Ref. 10 was also studied in the SLT-VTE crystals. The shortest pulse width of the applied field was about 1 ms. The result showed that there is only a switching current and no backswitching current for both forward and reverse domain reversals. This means that the stabilization time, if any, for domains for Ͻ SLT-VTE samples is tstab 1 ms. This is in strong contrast to

FIG. 2. (a) Hysteresis loops for 1st experimental cycle and 22nd experimental cycle of stoichiometric LiTaO3 single crystals (SLT-VTE). Cycling fre- quency was ϳ4 minutes/cycle. (b) Coercive fields and spontaneous polar- ͑ ͒ ͑ ͒ FIG. 1. Composition dependence of forward Ec,f and reverse Ec,r coer- ization measured in 22 successive experimental cycles. Coercive fields ͑ ͒ ͑ϳ ͒ ͑ϳ ͒ cive fields, and the Curie temperature TC in lithium tantalate single Ec,f 1.39±0.01 kV/cm and coercive field Ec,r 1.23±0.01 kV/cm do crystals. not change with cycling. Downloaded 21 Mar 2006 to 146.186.113.217. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp Appl. Phys. Lett., Vol. 85, No. 19, 8 November 2004 Tian, Gopalan, and Galambos 4447

defect field and the coercive field are proportional to the density of these defect dipoles. In SLT-VTE crystals, the TaLi and VLi are in very small amounts, and thus the coercive fields, internal field, and stabilization times are proportion- ately smaller. We also note that the recently reported25 thickness dependence of the switching times in CLT suggests that the relaxation time for these defect complexes is substan- tially lower than 1 ms when the defect is within ϳ250–500 nm of the crystal surface. This results in smaller stabilization times, as well as lower asymmetry between for- 10 Ͻ ward and reverse defect fields, ED for crystals of 500 nm thickness. The authors would like to acknowledge Silicon Light Machines™ for providing the SLT-VTE samples, and Oxide ͑ ͒ FIG. 3. The switching time ts f,r as a function of field E±Eint, where the Corporation for providing the NSLT-CZ. They also thank R. negative sign is for forward poling and the positive sign for reverse poling. The measured internal field E =0.08 kV/cm. Miles, G. Miller, M. Fejer, and G. Foulon for bringing this int material to our attention. This research was supported by the National Science Foundation though Grant Nos. DMR- ibility of a domain wall in CLT under EOIM without any 9984691, DMR-0349632, ECS-9988685, and DMR- applied voltage suggests the presence of optical birefrin- 0103354. gence at the domain wall, which indicates the presence of 24 local strains and electric fields. In NSLT-CZ crystals, the 1D. A. Scrymgeour, Y. Barad, V. Gopalan, K. T. Gahagam, Q. Jia, T. E. extremely low birefringence only gives a very faint contrast Mitchell, and J. M. Robinson, Appl. Opt. 40, 6236 (2001). at the domain walls.10 The domain shapes are hexagonal, and 2D. A. Scrymgeour, A. Sharan, V. Gopalan, K. T. Gahagan, J. L. Casson, R. can only be seen with an external applied field Sander, J. M. Robinson, F. Muhammad, P. Chandramani, and F. Kiamilev, Appl. Phys. Lett. 81, 3140 (2002). ͑ϳ10 kV/cm͒. However, the domain walls cannot be seen in 3 K. T. Gahagan, D. A. Scrymgeour, J. L. Casson, V. Gopalan, and J. M. SLT-VTE without or even with an external applied field. The Robinson, Appl. Opt. 40, 5638 (2001). field required for domain-wall motion in SLT-VTE is very 4K. T. Gahagan, V. Gopalan, J. M. Robinson, Q. X. Jia, and T. E. Mitchell, low ͑ϳ0.8 kV/cm͒. Even with applied fields, the birefrin- Appl. Opt. 38, 1186 (1999). 5 gence due to electro-optic effect is extremely small (⌬n M. Kawas, V. Gopalan, T. E. Schlesinger, and D. D. Stancil, J. Lightwave ϳ −6 −7 Technol. 15, 1716 (1997). 10 to 10 ) to induce sufficient optical contrast. The do- 6A. M. Prokhorov and Y. S. Kuziminov, Physics and Chemistry of Crys- main shape in SLT-VTE is also hexagonal with wall parallel talline Lithium Niobate (Hilger, Bristol, 1990). to the crystallographic y axis as revealed by etching tech- 7O. F. Schirmer, O. Thiemann, and M. Wohlecke, J. Phys. Chem. Solids 52, 185 (1991). nique. The optical imaging of domain walls and the domain 8 shapes in CLT, NSLT-CZ, and SLT-VTE crystals suggested N. Iyi, K. Kitamura, F. Izumi, J. K. Yamamoto, T. Hayashi, H. Asano, and S. Kimura, J. Solid State Chem. 101, 340 (1992). that there is a clear correlation between the optical birefrin- 9V. Gopalan and T. E. Mitchell, J. Appl. Phys. 83, 941 (1998). 24 gence, and lithium stoichiometry in the crystals. 10S. Kim, V. Gopalan, K. Kitamura, and Y. Furukawa, J. Appl. Phys. 90, These dramatic differences in domain reversal properties 2949 (2001). 11 among CLT, NSLT-CZ, and LT-VTE can be understood in M. Jazbinsek, M. Zgonik, S. Takekawa, M. Nakamura, K. Kitamura, and H. Hatano, Appl. Phys. B: Lasers Opt. 75,891(2002). terms of the nonstoichiometric defect model proposed in Ref. 12P. Bernasconi, G. Montemezzani, P. Gunter, Y. Furukawa, and K. Kita- 10, where the authors proposed the structure of a defect com- mura, Ferroelectrics 223, 373 (1999). plex comprised of a niobium/tantalum antisite surrounded by 13Y. Furukawa, K. Kitamura, K. Niwa, H. Hatano, P. Bernasconi, G. Mon- three Li+ vacancies in the nearest neighborhood, plus one temezzani, and P. Guner, Jpn. J. Appl. Phys., Part 1 38, 1816 (1999). 14 independent Li+ vacancy along the polar z direction. This P. Dittich, G. Montemezzani, M. Habu, M. Matsukura, S. Takekawa, K. Kitamura, and P. Gunter, Opt. Commun. 234, 131 (2004). defect complex is assumed to be comprised of a dipole mo- 15P. F. Bordui, R. G. Norwood, C. D. Bird, and J. T. Carella, J. Appl. Phys. ment, which has two contributions: (a) The contribution to 78, 4647 (1995). 16 the electrical dipole arising only from the TaLi antisite defect, I. G. Kim, S. Takekawa, Y. Furukawa, M. Lee, and K. Kitamura, J. Cryst. 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