Domain Engineering: Periodic Domain Patterning in Lithium Niobate

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1 399 Domain Engineering: Periodic Domain Patterning in Lithium Niobate V.Ya. Shur, R.G.Batchko2, E.L. Rumyantsevl, G.D.Miller2, M.M.Fejer2 and R.L.Byer2 I Institute of Physics and Applied Mathematics, Ural State University, Ekaterinburg , Russia E.L. Ginzton Laboratory, Stanford University, Stanford, CA Abstract - We present our experimental and theoretical investigations of field induced domain structure evolution in lithium niobate. The processes governing the fabrication of short-pitch domain gratings required for nonlinear optics applications are analyzed in details. We propose an enhanced electric-field poling technique based on spontaneous backswitching which allows to produce domain gratings with periods down to 4 pm in 0.5-mm-thick LiNb03 substrate. INTRODUCTION Recently a new branch of ferroelectric technology and science named domain engineering has been developing rapidly. It is aimed at production of desired reproducible domain structures in commercially available ferroelectric materials. Ferroelectric domain engineering is important for quasi-phasematched (QPM) nonlinear optics which requires the short-period domain gratings [ 11. Electric field poling at room temperature, first applied to bulk LiNb03 by Yamada et al. [2], has become conventional due to its repeatability and applicability over a wide range of materials. To date the creation of the domain period of 3-4 pm in pm thick substrates has been reported for electric field poled LiNbO, [3], LiTaO, [4,5] and KTP [6] which shows promise for blue and even UV light applications. Nevertheless, in thicker substrates, difficulties remain to prepare QPM materials for visible and UV wavelengths. With the proper voltage waveform 6.5-pm-period uniform domain structures can be formed throughout 76-mm-diameter 0.5-mm-thick LiNb03 wafers [7]. As it was shown that all domain patterns obtained during switching in external field are of kinetic nature [8] so the comprehensive study of domain evolution at different stages of poling process is required to improve the conventional technique. Moreover the field induced development of the domain structure in LiNbO, is practically unstudied due to extremely high value of the threshold field. We investigate formation and stabilization of the bulk domain patterns produced in LiNbO, by application of high field to photolithographically-defined electrodes. Visualization of the domain patterns by SEM and optical microscope, mathematical treatment of the poling current data and computer simulation of domain kinetics are used for analysis of the stages of domain patterning. The domain evolution during spontaneous backswitching after removing/decreasing of the poling field is explored. EXPERIMENT The periodic bulk domain structures are obtained in standard optical-grade single-domain 0.5-mm-thick LiNbO, wafers of congruent composition cut normal to polar axis. The substrates are photolithographically patterned with periodic strip metal-electrode (NiCr) structure deposited on z+ surface only. The patterned surface is overcoated with a thin insulating layer (photoresist or spin-on-glass) to inhibit domain growth between the strip electrodes (Fig. I). A high voltage pulse producing an electric fields greater than the threshold ( coercive ) field (21.5 kvimm) is applied to the structure through the fixture containing a liquid electrolyte (LiCI) [9,10] =-3 -rings Fig. I: Scheme of experimental setup for poling (1 - liquid electrolyte, 2 - dielectric layer, 3 - lithographically prepared periodic metal electrodes). Both the voltage and current are monitored during the poling. The parameters of the domain structure are controlled by the pulse shape and current value. After complete or partial poling the polar surfaces and cross /98/$ IEEE.

2 400 sections are etched for minutes by hydrofluoric acid (without heating). The surface relief associated with domain patterns is visualized by optical microscope and SEM. Studying the domain structure in the bulk by optical method, the spatial resolution in x direction is enhanced by choosing appropriate tilted cross-sections. The comparison of domain patterns obtained for different duration of the poling pulse yields the detail information about domain evolution. STAGES OF THE DOMAIN EVOLUTION The analysis of the domain patterns after partial poling makes it possible to distinguish several stages of domain evolution [I 11. The poling process starts with the nucleation (arising of new domains) at z+ polar surface along the electrode edges when polar component of the The stabilization of the domain patterns in conventional poling method requires the special poling voltage waveform for suppressing the backswitching process after removing of the poling field (Fig. 3a) [ 121. MAIN APPROACH Our approach to domain patterning in the bulk ferroelectrics is based on the assumption of key role of screening effects [13]. It is clear, that switching trom the single domain state is achieved through nucleation and growth of reversal domains [8]. Both nucleation and growth are governed by the polar component of local electric field E, [&I31 at the nucleation sites and domain walls respectively. The spatial distribution of the local field E, (r,t) is determined by the sum of z components of: 1) external field E,, (r) produced by the voltage applied to the electrodes, 2) the depolarization field Edrp(r.t) produced by bound charges of instantaneous domain pattern. and 3) screening fields of two types: the external one Eescr(r,t) due to the charge redistribution at the electrodes and the bulk one EbsL,(r.t) which compensate the residual depolarization field by various bulk mechanisms [8,14,15]. big 2. The main stages of domain evolution during switching in single domain plate with stripe electrodes. The depolarization field slows the domain growth while the screening processes reduce its influence. It is important to keep in mind that after complete external screening the bulk residual depolarization field Ed,(r) remains due to the surface dielectric gap existing in any ferroelectric [8, IS]. local field E, exceeds the threshold value [SI (Fig. 2a). The second stage represents forward and sideways growth and merging of the domains undei the electrodes (Fig. 2b). At the third stage the plane walls of formed laminar domains move out of electrodes (Fig. 2c). After the termination of the switching process by rapid decreasing of the poling field two possibilities can be realized: either the stabilization of generated domain structure or the partial backswitching ( flip-back ) of domains to initial state. - E E. s 1F!._ (U U Time [ms] Time [ms] Fig. 3: Poling voltage waveforms for conventional poling (a) and backswitched poling (h). For an infinite single-domain ferroelectric capacitor where L - thickness of dielectric gap, d - thickness of the sample, Ps - spontaneous polarization, E~ - dielectric permittivity of the gap. This residual field can be screened by the charge redistribution in the bulk and aligning of polar defects [ 14,151. The processes are relatively slow (T IO s) [SI so usually obtained retardation of the bulk screening leads to various memory and retardation effects [ 131. For example, after fast enough decreasing of the external field, the spontaneous backswitching process can reconstruct (even completely) the initial domain state under the action of this partially screened residual backswitched field It will be shown later that the manipulation by

3 40 1 screening effects can be used to enhance the fidelity of fine-pitch domain structures. DETAILS OF THE DOMAIN EVOLUTION Nucleation of new domains at the surface It is clear that nucleation density is the main parameter which is important for short-pitch domain gratings. Many attempts to increase it [lo] showed that the density of spike-like domain nuclei under the completely electroded area (far from the electrode edges) depends on the electrode material. At the same time our experiments demonstrate the spatially nonuniform nucleation for electrode gratings with dramatic increase (by the orders of magnitude) of the nucleation density along the electrode edges (Fig. 4a). This behavior can be explained in framework of our approach by known singular spatial distribution of the polar component of switching field E, at the surface near the edges of finite electrodes (Fig. 4b). The experimentally observed linear nuclei density is about l.l~lo mm~i for 22.5 kv/mm. C ,o 0.5 1,o 1,5 2,o Distance bd Fig. 5: The histogram of the distances between the neighboring nuclei fitted by the Gaussian. Enlargement ar?d coalescence under the electrodes The enlargement of the nucleated spike-like domains is achieved through the forward and sideways domain wall motion. The forward growth velocity v, for LiNbO, is always much higher (about 100 times) then the sideways one v,. The ratio of velocities determines the observed spike angle about one degree. In the case of simultaneous growth of the spikes the value of E,(r,t) at the given tip depends on the distances from the neighboring domains. The suppression of the local field in the vicinity of the charged walls of spike domains during the forward growth leads to the observed local deviations of tip propagation after pinning of neighbor tips by defects. This effect can be partly responsible for usually observed difference between zc and i domain patterns. U o* 0, I Btance &rrj Fig. 4: Nucleation near the edges of the electrodes (during backswitching) (a) and calculated spatial distribution of the field polar component E, near z+ surface (b). The analysis of experimental domain patterns at the first stage of poling reveals the strong correlation in spatial distribution of the nuclei (Fig. 5). This effect can be explained while taking into account suppression of the local field in the vicinity of individual recently switched domain [SI. The estimations show that the spatial distribution constant characterizing the decay of suppressing component of depolarization field AEz(x) with the distance from recently nucleated spike domain is of the order of spike length. Fig. 6: Hexagonal domains in LiNbO;: experimental pattrrn (a). the scheme of layer-by-layer domain growth (b). Thin arrows - directions of step propagation: thick arrons - directions of wall motion. The patterns observed on z+ surface demonstrate that the growing domains in LiNbO: are of hexagonal shape. The similarity of observed domain evolution with growth of regular crystals under small oversaturation has been discussed in [ 16,171. It was shown that domain growth can be explained in terms of layer-by-layer growth through propagation of individual steps along the walls (Fig. 6). In this case the obtained domain shape is determined by the crystal symmetry. In LiNbO, existence of trigonal anisotropy in the plane normal to

4 402 the polar direction leads to the preferable motion of the steps in three y directions [16]. As a result six plane domain walls are formed (Fig. 6). It is important to point out, that orientation of strip electrodes is always chosen along one of these symmetry directions. Due to such orientation the layerby-layer growth prevails after coalescence of the nucleated domain chains under the edges of electrodes leading to formation of the couples of strip domains with plane walls at z surface. In thick substrates the coalescence of individual domains and formation of the strip ones at z+ is terminated when the spike tips are far from 2-. Experiments show that the coalescence occurs when the tips grow in polar direction at the distance about 50- IO0 pm for the typical distance between nuclei 0.9 pm. The pairs of domain walls formed along the edges of electrodes move towards each other until complete switching of the electroded area on z+. In 0.5-mm-thick substrates this coalescence is terminated before the tips reach z- surface for electrode width less then 3 pm. The regular-shaped laminar domains are formed only after completion of the forward growth. Nevertheless the process of domain evolution is not terminated at this moment. The sufficient undesirable enlargement of the laminar domains out of electrodes is always observed. Domain wall moiion out of electrodes The spread of the formed domain walls out of electrode for thick sample (the electrode period A << d) can be considered as a domain wall motion in uniform electric field. It is clear that the spatial inhomogeneity of external field E,, (which is so important for nucleation) exists only in the surface layer about several A thick. The wall velocity is determined by the polar component of electric field E, [IS]. Absence of external screening in the area between electrodes and very slow bulk screening in LiNbO, at room temperature lead to decrease of E, during shifting due to increasing of uncompensated part of depolarization field. The domain wall stops when The Equation (6) allows to determine the voltage dependence of the domain broadening Ax(Ee, - Et,,). It must be stressed that any conductivity of real insulating layer can sufficiently increase the domain broadening effect. As a result the observed wall shift must depend on the technology of deposition and composition of the layers. We have found experimentally that for short period patterning the considered domain spreading by plane wall motion is not always the case. The spreading amplitude often varies from electrode to electrode and abnormally large shifts occur in some regions (Fig. 7a). For studying the early stages of domain evolution in this case we analyze the domain patterns after very short partial backswitching. This possibility is based on the experimentally confirmed fact that domain nucleation during backswitching is identical to that for poling (switching). The optical observations show that domain kinetics in this situation demonstrates the propagation of set of domain fingers oriented in one of y directions and their subsequent merging (Fig. 7a). The higher resolution observation of the etched surface by SEM shows that these optically observed domain fingers are really the chains of spike-like domains with diameter about 100 nm (Fig. 7b). The linear density of nucleated domains in chains is about IO4 mm-. It is clear that for a short-pitch patterning the finger assisted mechanism of the wall motion leads to complete switching due to wall merging between electrodes and to disruption of the periodicity of domain patterns. The finger assisted mechanism of the wall motion is a pronounce manifestation of the correlated nucleation effect [ 191. The calculation of AE,(x) near the wall of the spike-like domain (Fig.8) demonstrates a pronounced field maximum at a distance about thickness of the surface dielectric gap [ 81. where Et,, - the threshold field for sideways motion of the plane domain wall. Assuming that conductivity of insulating layer and bulk screening can be ignored the average bulk value of E, can be estimated for used experimental setup (Fig. 1), loum, E, = E,, - Edep = U/d - 4 (L,/d) (Ax/A) Ps (E, E ~Y (6) where U - applied voltage, Li -thickness of insulating layer, Ax - shift of the wall from electrode, - dielectric permittivity of insulating layer. Fig. 7: Finger assisted mechanism of dornain spreading out of electrodes. Domain patterns observed by: optical microscope (a) and SEM (b).

5 403 Evolution of the domain structure during backswitching Distance (arb.un.1 Fig. 8: Spatial distribution of the polar component of the local field E, (x) at the surface in the vicinity of the spike-like domain. The combined action of all above mentioned mechanisms of domain broadening leads to observed variations of local domain width (domain duty cycle) and wall shape. The inhibition of external screening of depolarization field in between the electrodes by optimization of the parameters of insulating layer and the shape of electrode edge is the way to prevent the undesirable domain broadening. Stabilization of the domain structure after poling Any generated domain structure has to be stabilized using long enough exposure in constant electric field a bit lower than the threshold value. In the conventional poling the stabilization of obtained domain structure is achieved by suppression of the backswitching process (Fig. 3a). It was demonstrated experimentally that the stabilization step duration more than 50 ms is enough to block essentially the backswitching process [20]. Under the assumption that the depolarization field is the driving force of the backswitching the step duration must be of the order of screening time constant [ 13,141. The experimental dependence of the backswitching charge on the stabilization time was analyzed (Fig. 9). The experimental data demonstrate the exponential decay with time constant about ms. Rapid removal or strong decreasing of the external field leads to spontaneous backswitching of the domains to initial state. In contrast to the conventional approach we demonstrate the opportunity to use the spontaneous backswitching for domain patterning [ 1 I]. This radical change of approach to the problem is based on ideas experimentally confirmed in our earlier works [8,13]. The modification of the voltage waveform (Fig. 3b) is required to apply backswitching for domain patterning. As a result of application of the first (switching) part of the waveform the laminar domains are formed. Their width sufficiently exceeds the width of elec:rodes. The backswitching process takes place within this switched area during the low-applied-field stage and starts with growth of residual (nonswitched) domains. In addition the nucleation under the edges of electrodes occurs so as during switching (Fig. 2a). The domain patterns are frozen in during the stabilization part of the waveform (Fig. 3b). This method allows to freeze in the patterns corresponding to the different stages of backswitching process by the variation of the zero-field part duration. The controlled shift of the plane domain walls of residual domains allows to generate short-pitch domain patterns with higher fidelity in thick substrates. Frequency multiplication We obtain in addition the new methods of domain patterning with multiplication of the domain pattern frequency as compare to the electrode one. First, frequency doubling of the domain pattern can be obtained as a result of appearance of additional backswitched domain under the whole electrode on z+ (Fig. loa) =. (a) 0 r=ll ms - (b) r=34ms a Stabilization time [ms] J - Fig. 9: Time dependence of backswitching charge fraction during stabilization: E = 24 kv/rnrn (a) and E = 20.6 kv/mm (b). Experimental points fitted by exponential function. Fig. 10: Results of backswitched poling. Frequency doubling: z view (a) and y cross-section (b). Frequency tripling: z+ view (c) and y cross-section (d). Erasing : cross-section (e) and splittizg : cross-section (0.

6 In this case the backswitched domains usually do not reach i. Their length depends on the pulse parameters and width of electrodes and typically is about pm (Fig. lob). Moreover we demonstrate that a frequency tripling can be realized (Fig. 10d). In this case the additional domain strips arise just under the edges of the electrodes. Their length is about pm (Fig. 1Oc). Detailed analysis of the behavior of individual domains during backswitching reveals two distinct variants of their evolution: erasing and splitting. The erasing process presents formation of backswitched domains in earlier switched area without any variation of the external shape of switched domain (Fig. 10e). In contrast, after the splitting the growing backswitched domain cuts the initial switched one conserving its volume (Fig ANALYSIS OF THE SWITCHING CURRENTS The switching current data yield additional information about domain evolution [2 I]. For switching we use the conventional experimental setup (Fig. 1) but without metal electrodes on the surface. The switched area is controlled by O-rings having diameter 2-4 mm. Choosing of the small area and low field increasing rate (de/dt = kv/mm-ms) allows to obtain switching data without current limits. Our approach to analysis of the switching current data in linear increasing field [22] allows us to obtain in this case the parameters characterizing the domain kinetics in LiNbO,. The observation of domain patterns shows that the nucleation takes place only at the electrode edge and all growing domains are of hexagonal shape. It means that in this case the switching process starts from 2D domain growth and undergoes the geometrical catastrophe with the decrease of growth dimensionality [2 I ] after complete switching along the electrode edge. The current for 2D growth during switching in the linear increasing field and nucleation only at the boundary of the switched area can be fitted by the following formula [21,22]. Field [kv/mrn] j[mal I I I I Time [rns] Fig. 11: The switching current for poling using the circular electrode. Experimental points fitted by Equations (7) and (9). Then the following expression is obtained j[e,,(t)] = A (t - At) + B (t - At)3 (9) where At = E,, (de,, Idt) I, AIB = - W(p de,, /dt), R - radius of electroded area. Fitting of experimental data by Equations (7) and (9) (Fig. 11) allows to extract the mobility p = cm*ns and the start field value E,, = 23.4 kvimm. The observed deviations from the theoretical dependencies (especially at the first stage) can be attributed to the current jumps at the moments of the merging of the hexagonal-domain walls. 4 pm PATTERNING BY BACKSWITCHED POLING We apply the backswitching method for 4 pm periodic poling of the LiNb mm-thick samples. It is seen that the domain patterns on z+ and z- are practically ideal (Fig. 12). The cross-section shows that this structure is preserved through the bulk. It must be pointed out that to our knowledge early attempts to produce the regular structures in 0.5 mm thick LiNbO, with 4 pm period by conventional poling method have not met with success. where to - the time constant of the switching process. After the geometrical catastrophe (complete merging of all isolated domains along the edge) the domain evolution is determined by the motion of the one domain wall (Fig. 1 la). We can use the linear field dependence of the wall sideways motion velocity due to comparatively narrow range of switching fields where p - mobility of the domain wall, E,, - start field. Fig. 12: The 4 pm domain patterns produced by backswitched poling : y cross-section (a) and z- view (b). 404

7 CONCLUSION Carried complex investigations yield new important information about the domain kinetics in LiNbO? during poling. The domain evolution during spontaneous backswitching after removing of the poling field was investigated for the first time. Backswitched poling in LiNb03 enables higher fidelity and shorter period bulk domain patterning than can be achieved with conventional poling. Backswitched domain periods down to 4 pm are demonstrated in 0.5-mm-thick LiNbO, substrates. The proposed technique based on the controlled backswitching process demonstrates the new possibilities in the domain engineering and can be extended to other ferroelectric materials and device application. ACKNOWLEDGMENTS The authors are grateful to L. Eyres for SEM observations of domain patterns and also to S. Makarov, E. Nikolaeva and E. Shishkin for technical assistance. This work was supported in part by Grant Russian Foundation of Basic Research and by DARPA/ONR through the Center of Nonlinear Optical Materials (CNOM) at Stanford University under ONR Grant REFERENCES R.L. Byer, Quasi-phasematched nonlinear interactions and devices. Journal of Nonlinear Optical Physics & Materials. vol. 6, pp , M. Yamada, N. Nada, M. Saitoh, and K. Watanabe, Firstorder quasi-phase matched LiNbO, waveguide periodically poled by applying an external field for efficient blue second-harmonic generation. Applied Physics Letters, vol. 62. pp , February J.U. Kang, W.K. Bums, Y.J. Ding, and J.S. Melinger, First observation of backward second-harmonic generation in periodically poled bulk LiNbO,, presented at the Conference on Lasers and Electro-Optics. Baltimore, Maryland, May 18-23, K. Mizuushi and K. Yamamoto. Generation of 340-nm light by frequency doubling of a laser diode in bulk periodically poled LiTaO,, Optics Letters, vol. 21, pp , January J.-P. Meyn and M.M. Fejer, Tunable ultraviolet radiation by second-harmonic generation in periodically poled lithium tantalate, Optics Letters, vol. 22, pp: , August W.P. Risk and S.D. Lau, Periodic electric field poling of KTiOP, using chemical patterning. Applied Physics Letters, vol. 69, pp , December G.D. Miller, R.G. Batchko, W.M. Tulloch, D.R Weise, M.M. Feier, and R.L. Byer, 42Y efficient single-pass cw second harmonic generation in periodically poled lithium niobate, Optics Letters, vol. 22, pp , December [8] V.Ya. Shur. in Ferroelectric Thin Films. Synthesis and Basic Properties. New York: Gordon&Breach ch. 6. pp [9] L.E. Myers, R.C. Eckardt, M.M. Fejer, R.L. Byer, and W.R. Bosenberg. Multigrating quasi-phase-matched optical parametric oscillator in periodically poled LiNbO,, Optic Letters. vol. 21, pp April [IO] G.D. Miller, R. G. Batchko, M. M. Fe.jer. and R. L. BJer. Visible quasi-phasematched harmonic generation by electric-field-poled lithium niobate, SPIE Proceedings on Solid State Lasers and Nonlinear Crystals. vol pp [I I] V.Ya. Shur, E.L. Rumyantsev. R.G. Batchko. G.D. Miller. M.M. Fejer, and R.L. Byer, Physical basis of the domain engineering in the bulk ferroelectrics. Ferroelectrics (in press). [I21 V.Ya. Shur and E.L. Rumyantsev, Arising and evolution of the domain structure in ferroelectrics. Journal of the Korean Physical Society, vol. 32. pp. S February [I31 V.Ya. Shur and E.L. Rumyantsev, Kinetics of ferroelectric domain structure: retardation effects. Ferroelectrics. vol. 191, pp P.V. Lambeck, G.H. Jonker. Ferroelectric domain stabilization in BaTiO, by bulk ordering of defects, Ferroelectrics, vol. 22, pp I V.M. Fridkin, Ferroelectrics Semiconductors. New York and London: Consultants Bureau. 1980, 264 p. [I61 V.Ya. Shur and E.L. Rumyantsev. Crystal growth and domain structure evolution, Ferroelectrics, vol pp. 1-7, [I71 V.Ya. Shur, A.L. Gruverman, V.V. Letuchev. E.1,. Rumyantsev and A.L. Subbotin, Domain structure of lead germanate, Ferroelectrics. vol. 98. pp [I81 V.Ya. Shur. A.L. Gruverman, V.P. Kuminov and N.A. Tonkachyova Dynamics of plane domain walls in lead germanate and gadolinium molybdate. Ferroelectrics, vol. 11 I, pp , [I91 V.Ya. Shur, A.L. Gruverman, N.Yu. Ponomarev and N.A. Tonkachyova, Change of domain structure of lead germanate in strong electric field. Ferroelectrics. vol. 126, pp [20] L.E. Myers, Quasi-Phasematched Optical Parametric Oscillators In Bulk Periodically Poled Lithium Niobate. Ph. D. thesis, The Stanford University [21] V.Ya. Shur, E.L. Rumyantsev and S.D. Makarov. Investigation of phase transformation kinetics in real finite systems: Application to ferroelectric switching. Journal of Applied Physics, vol. 84. pp July [22] V.Ya. Shur, E.L. Rumyantsev, S.D. Makarov, A.L. Subbotin and V.V. Volegov, Transient current during switching in increasing electric field as a basis for a new testing method, Integrated Ferroelectrics, vol. IO, pp ,