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.
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 620083, Russia E.L. Ginzton Laboratory, Stanford University, Stanford, CA 94305
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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.
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.
INTRODUCTION
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 (2 1.5 kVimm) is applied to the structure through the fixture containing a liquid electrolyte (LiCI) [9,10].
EXPERIMENT
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 200-300 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
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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-
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sections are etched for 5-1 0 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
big 2 . The main stages of domain evolution during switching in single domain plate with stripe electrodes. 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.
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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].
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].
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 - 10’- 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
Fig. 3 : Poling voltage waveforms for conventional poling (a) and backswitched poling (h).
It will be shown later that the manipulation by
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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
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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 ~ l O ’ m m ~for i 22.5 kV/mm.
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Fig. 5 : The histogram of the distances between the neighboring nuclei fitted by the Gaussian.
Enlargement ar?dcoalescence 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
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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).
Fig. 6: Hexagonal domains in LiNbO;: experimental pattrrn (a). the scheme of layer-by-layer domain growth ( b ) . T h i n arrows - directions of step propagation: thick arrons directions of wall motion.
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.
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
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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.
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.
Domain wall moiion out of electrodes
The spread of the formed domain walls out of electrode for thick sample (the electrode period A
