Micro-scale domain structure formation by e-beam point writing onthe Y cut surfaces of LiTaO3 crystals.

L.S.Kokhanchik*, D.V. PunegovInstitute of Microelectronics Technology and High Purity Materials RAS, Chernogolovka, Moscow

District, 142432,Russia

ABSTRACT

Periodic micro-scale domain structures were revealed in congruent LiTaO3 crystals by point electron beam writing in ascanning electron microscope. Peculiarities of the polarization reversal at point e-beam irradiation of the Y-cuts inLiTaO3 crystals were investigated. Different distances between the points along Z direction on the polar Y-cuts allowrevealing the long planar domain structures. Various periods of the structures were formed by set of different pointdistances in the X direction.

Keywords: lithium tantalate; polarization reversal; Y-cut; periodic domain structures; electron beam point writing.

1. INTRODUCTIONIn recent years, the engineering of periodic domain structures in lithium niobate and lithium tantalate crystals (LN, LT)for optics, telecommunications and other practical and scientific applications has rapidly increased. The LN and LT withperiodical 1D and 2D domain structure open up a range of possibilities for bulk and waveguide nonlinear opticalinteractions [1]. During ten years after the first electrical poling of bulk LN samples [2] research on periodically poledLN and LT is under intense interest around the world resulting in production of photonic devices. The new generation ofLN and LT based devices demands the formation of periodic domain structures of micro and nano-scale sizes in them.Among different ways of such domain structure formation are: a special laser irradiation [3], scanning probe microscopywith high electric field applied [4], application of a high voltage pulses to lithographically defined metallic electrodes ona surface [5], and irradiation of LN and LT by focused ion and electron beams [6,7].The most fast and available way of reversal polarization of a large number of crystal areas is the -Z cut irradiation by anelectron beam in a scanning electron microscope (SEM). The possibilities of periodic domain structures engineering bythe electron beam in the SEM using -Z cut surfaces have already been studied for a long time [2,8,9]. Several worksreported the formation of 2D domain structures in LT and LN crystals by local surface inversion of the –Z cuts, using ascanning electron microscope and samples covered with a dielectric layer (e-beam resist), which facilitates trappingelectron beam charges. These localized electrons generated the inversion process in the crystals [10,11]. Periodicdomain inversion in Y-cut LN,LT crystals could be achieved by applying electric field to a special electrode on a surface[12-14]. The depth of the domain inversion was 1-2,5 µm with a period of 4.75 µm after 2 kV voltage applying to theelectrodes. The possibility of planar domain formation on the Y-cut LN by needle-shaped electrode was reported in[15,16]. Under scanning over the Y surface, a needle-shaped electrode created planar needle-like domains orientedparallel to the surface. A high voltage was applied between the electrode and the sample surface.

We report domain formations in the Y cuts of congruent LiTaO3 crystals due to electron beam irradiation in the SEM.We have investigated the possibility of planar micro-scale and nano-scale strip-like domain structure engineering at pointe-beam drawing with different irradiation dosages. The smallest period in domain structures formed by this way was ~1micron, which corresponds to the strip like domain width ~ 500 nm.

2. EXPERIMENTALRectangular samples of a congruent lithium tantalate crystal were cut at the right angle to [010] direction and were

*mlk@iptm.ac.ru ; phone +7 (496 52) 44 006; fax +7 (495) 962 8047

Micro- and Nanoelectronics 2007, edited by Kamil A. Valiev, Alexander A. Orlikovsky,Proc. of SPIE Vol. 7025, 70250J, (2008)

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optically polished to thickness 280-300 microns. The (010) surface, Y-cut, is parallel to the spontaneous electricpolarization vector Ps. The Z axis direction coinciding with Ps direction was parallel to the short side of the rectangularsamples. The long sides of the rectangular samples coincided with the X direction. Optical polished plates of Y–cutswere covered by a metal layer (Al) on one side, and on the other hand (-Y side) by a layer 0,4-0,5 micron thick PMMAelectron resist. We have used the electron resist covering for facilitation of Ps switching process at an electron beamirradiation [11]. The metal covered sides were grounded. The -Y sample sides were irradiated in a JSM-840A scanningelectron microscope at a 25 kV accelerating voltage and electron beam currents ~0,1 10-9 A.

Electron beam drawing on the Y cut surface was performed using the NanoMaker program. The size and rectangularforms of irradiation areas, the shape and motive of drawing (arrangement of irradiated areas), and also the irradiationdosage D [µC/cm2] were set by the program. Square-like points of (0,5 0,5) µm2, (1 1) µm2 and (2 2) µm2 wereirradiated. The distances between the irradiated points in different motives varied from 5 to 25 microns in the Zdirection, those in the X direction from 2,5 to 20 microns. To choose an optimum mode of irradiation, various dosages ofirradiation in the range D = 200-1750 µC/cm2 were used. The direction of electron probe jumps from point to point to oropposite the Z axis was taken into account during drawing. On the fig1 is shown the schema of the experimental setupused.

Fig. 1. The schematic experimental setup during e-beam drawing on Y cut surface of LT.

Domain structures drawn by an electron beam were investigated in scanning and optical microscopes after hot etching ofthe samples with HF and HNO3 (1:2) [17] . Before the procedure, the resist and Al layers were removed by acetone andby corresponding acids. Potential domain images could be observed before and after etching in the SEM low voltagemode (1 and 2 keV of e-beam energy were used).

3. RESULTS3.1 POLARIZATION REVERSAL OF POLAR Y-CUTS AT POINT E-BEAMIRRADIATION

Needle-shaped or drop-shaped domains were formed in a surface layer of the LT crystal samples after a point electronbeam irradiation. The domains were elongated in the direction parallel to the Z axis and were ~15 to 25 microns long. Atbig irradiation dosages, the domains could be as long as 50-100 microns.

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The domains had elongated potential images in SEM before chemical etching of the samples. However, these potentialimages were quickly screened by an electron beam at viewing. Some areas, which irradiated using of one of the motives,are presented in Fig.2 in the SE mode at small SEM magnification.

Fig.2. SEM potential images of six motives observed before sample etching. Large isolated drop-shaped domains are placesof e-beam standing before and after the motive drawing.

Critical charge density D for polarization reversal in the crystal area Sirrd = 1µm2 irradiated by the electron beam energyUirrd=25 keV was about 400 - 500 µC/cm2. Narrow (~1µm) domains 15-25 microns long were formed at the pointirradiation. The width of elongated strip-domains grows with an increase in charge dose D per one point of irradiation.At irradiation of a 4 µm2 area the minimum width of the resulting domains was ~2 µm. The dependence of strip-likedomain widths on D magnitudes at point irradiation with these different square areas is shown on Fig.3.

Fig.3. Domain width vs point irradiation dosages (D) at electron beam irradiation: Uirrd=25 keV, I=0,01x109A.Sirrd=(1x1)µm2 – squares; Sirrd=(2x2)µm2 – circles.

Chemical etching of the surface showed that switched needle-like domains could consist either of one micro-domainwhen irradiated at small dosages or several merged micro-domains received at bigger dosages. See Fig.4.

The results obtained point out the possibility of polarization reversal of the Y-cuts of congruent lithium tantalate crystalsat a point electron beam irradiation. In the -Z cuts of LN and LT, switching of spontaneous polarization is achieved dueto the effect of normal component of an electric field between an introduced point negative charge and a grounded metal

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electrode at the sample bottom [2]. In the Y cuts the switching of spontaneous polarization apparently occurs due to thetangential part of the electric field which is antiparallel to the +Z crystal axis (E Z). See schematic presentation of thepolarization reversal in Y-cut in fig.5.

Fig 4. Optical images of two long domain structures after etching received by point writing with motive (15x15) µm in Xand – Z directions on the sample surface : Sirrd =1µm2 ; Uirrd = 25 keV; the dosage grows with each jumping of thebeam in –Z direction (from right to left of the presented sample surface).

We believe the switching occurs in the surface layer ~ 1,5-2 µm thick because of the depth of electron penetration R forlithium tantalate ( =7,4 g/cm3) at 25 keV is about 1,5-2 microns [18]. The direction of spontaneous polarization in theY-cut is parallel to the Z direction: Ps Z. The electric field in the surface layer switches the vector of spontaneouspolarization only from one site of the point charges, where the direction Ps and under the condition that themagnitude E exceeds the strength of coercive field (about Ec =2,1 107 V/m for congruent LT [19]) In our investigationdose minimum for switching conditions at the irradiation point area Sirrd=1 µm2 corresponds to an introduced pointcharge of 5 10-12 C. At small distances (~2-3 µm) from the charge, the electric field will be about 5x107 V/m, which isquite enough for switching conditions.

Fig.5. Schematic presentation of polarization reversal in Y-cut surface layer by introduced e-beam negative charge at pointsurface irradiation in SEM.

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Left part of the fig.5 presents the introduced point charge, vector Ps position and electric field ones before polarizationreversal; right part is the same crystal area after polarization reversal with switching field Eloc direction.

It is well known that polarization reversal from the single domain state is achieved by nucleation of new domains andtheir growth. The local switching field Eloc is determined by the sum of external field (Eext ), depolarization field (Edep)produced by bound charges, and screening fields: Escr –external screening field and Eb -bulk screening field [20,21].Eloc = Eext+ Edep+ Escr +Eb (1)

The depolarization field Edep slows down the domain growth while the screening process reduces its influence. Insamples areas with better screening, domains will be longer, than in other ones.

In our case, Eext is a tangential part of the electric field induced by a point negative charge which is antiparallel to the +Zcrystal axis (E Z). Needle-like domains grow at switching until the magnitude of the switching field becomes equal tothe depolarization field of which is decreased by the screening effect. The domains grow near the surface after electronbeam irradiation of the surface. In this case, noneqilibrium electron-hole pairs induced by the surface e-beam irradiationwould exert the main influence on the screening process. Near the surface, the screening process proceeds more quickly,than in the bulk. Therefore, needle-like domains would be longer in the part closer to the surface. The domain imagesshow that not only do they become narrower by the end of the growth, but their tips outcrop on the surface.

3.2 LONG PERIODIC DOMAIN STRUCTURE FORMATION

3.2.1 INFLUENCE OF BEAM JUMP DIRECTION

For long and good quality periodic domain structure formations the direction of an electron beam jumping from point topoint should be as close as possible to the Z axis position in an irradiated sample because of the direction of needle-likedomain growth coincides with that. During long domain structure formation moving of an electron beam should beagrees closely,also, with the domain growth direction –Z one. Moving of the electron probe against the domain growthresulted in that the length of subsequent domains decreased by some times and the domain structure is interrupted.(Fig.6)

Fig.6 Optical images of the –Y polar cut surfaces obtained after e-beam drawing and chemical etching: the (2,5x25)µmmotive is repeated 10 times, Sirrd = 0,25 µm2 ; the order of the e-beam point position is numerated and coincides withthe +Z direction.

Fig.6 shows how the length of needle-like domains decreases gradually and shapes of the domain end change when theelectron beam jumps do not coincide with the domain growth direction (the –Z direction). The resulted periodicstructure was interrupted after fourth jump of the e-beam.

The observed dependence of the resulted domain length on the direction of the electron beam movement is very similarto that of domain growth on the direction of a metal electrode movement reported in [16]. We believe that thedistribution of electric fields in the surface layer is determined by the combination of fields presented in (1). But, if theelectron beam moves from point to point, the sum of depolarization electric fields of the previous domains wouldinfluence the switching field Eloc. A schematic presentation of this effect is given in fig.7. If the electron beam ispositioned to the right of a previous domain, the growth of a new domain is not subjected to any influence and the newneedle-like domain grows like the previous one: in the same direction and of the same length. If the probe is positionedto the left of a previous domain, the depolarization fields of previous domains would affect the growth of a new domain.The depolarization fields of the previous domains are directed contrary to the switching field of new domains. As theexperimental results showed, this influence is significant and it is summarized from several domains. Fig.6 shows that

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the length of subsequent domains gradually decreased from 35 to 10-12 microns in the (2,5x25) µm motive, whichrepeated 10 times.

Fig.7 Schematic presentation of the domain length dependence on the direction of e-beam jumps: left- jump direction

coincides with the -Z axis direction; right- the direction coincides with the +Z axis direction.

3.2.2 CONSTRUCTION OF DIFFERENT DOMAIN STRUCTURES

Domain structures with different intervals between opposite strip-domains can form at drawing several rows of pointsequences along the X axis and Z one, provided that different distances between rows in the X direction are set. Thedistance between rows of several points was varied in the investigation from 20 to 2,5 microns. Optical images of etcheddomain structures received after an electron beam drawing with different motives and different irradiation point areas arepresented in figures 8(a-c). The most successful periodically poled planar structures were obtained in our investigation atsmaller interval steps in X direction and Sirrd = 0,25 µm.

Fig 8(a,b,c) Optical images of etched periodic domain structures after different e-beam drawing motives: a) Sirrd=1µm2 ,motive ( 5x15) µm ; b) Sirrd=0,25 µm2 ,motive (5x25) µm ; c) Sirrd=0,25 µm2, motive (2,5x25) µm

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Our results show that the reduction of the distances between irradiation points along the X axis in the motives down to 5µm increased the length of the needle-like domains resulted. Simultaneously, this reduction decreased the critical dosefor domain switching. Our experiments showed that areas of Sirrd = 0,25 µm2 at D= 500-1000 µC/cm2 did not switch atlonger distances between the points arranged along the X axis. But the areas Sirrd = 0,25 µm2 were switched and formedat D=400 µC/cm2 the periodic domain structures in the surface layer at the 2,5 µm distances between the points.

We can conclude that the reduction of distances between points down to 5 microns favors more successful Ps switchingin irradiated areas of LT. First, the length of growing strip-domains increased considerably. Second, for rows of smallirradiation area (Sirrd =0,25 µm2), Ps could be switched at D= 500 µC/cm2 , which was impossible at larger intervals inthe X direction between them, using such D.

We explain this by two reasons. Probably, this can be connected with mutual strengthening of field intensities of theneighboring point charges, which allows achieving the critical magnitude of electric field for spontaneous polarizationswitching at smaller doses.

On the other hand, at a closer arrangement of irradiated sites, the density of free nonequilibrium charges in the surfacelayer would considerably increase. This can promote an appreciable reduction of the switching fields due to muchsuccessful screening processes by the charges induced at e-beam irradiation. It is well known the influence of lightillumination on decreasing of the coercive field in photo-ferroelectrics. The magnitude of a switching electric fields canbe significantly reduced due to the screening of depolarization fields during polarization reversal by nonequilibriumcharge carriers produced at light irradiation of the ferroelectric samples [22].

4. CONCLUSIONPossibility of switching of spontaneous electric polarization by e-beam point drawing on the Y-cut surfaces of LTcrystals are shown and discussed in this work. Peculiarities of the switching of spontaneous electric polarization on theY-cuts were investigated at different point doses and the point arrangements during electron beam drawing. Differentperiodic domain structures on the Y-cuts were formed by point irradiation at drawing of several rows of point sequencesalong the X axis and Z one. The dependence of long domain structure continuity on the e-beam jump direction at thedrawing was observed and explained.

REFERENCES

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10 Li Xijum, Terabe Kazuya, Hatano Hideki, Kitamura Kenji, “ Electron-Beam Domain Writing in StochiometricLiTaO3 Single Crystal by Utilizing resist Layer”, Jap.J.of Appl.Phys., 45, L399-L402 (2006).11 Y.Glickman, E.Winebrand, A.Arie, G.Rosenman, ”Electron-beam-induced domain poling in LiNbO3 for two-dimensional linear frequency conversion”, Appl.Phys.Lett. 88, 011103 (2006).12 K. Nakamura, H.Shimizu, “Poling of Ferroelectric Crystals by Using Interdigital Electrodes and Its Application toBulk-Wave Transducers” Ultrasonics Symposium.1983; 527 – 530 (1983).13 Shinichiro Sonoda, Isao Tsuruma, Masami Hatori, “Second harmonic generation in electric poled X-cut MgO-dopedLiNbO3 waveguides”, Appl.Phys.Lett. 70,3078-3080 (1997).14 Shinichiro Sonoda, Isao Tsuruma, Masami Hatori, “ Second harmonic generation in a domain-inverted MgO-dopedLiNbO3 waveguide by using a polarization axis inclined substrate”, Appl.Phys.Lett. , 71, 3048-3050 (1997).15 V.G. Zalessky, S.O. Fregatov, A.B. Sherman, “Local charge formation in LiNbO3 by needle-like probe”, Phys.SolidState.43,1739 (2001).16 V.G. Zalessky, S.O. Fregatov, ”Micrometer-scale ferroelectric domain formation and injection of space charge in Y-cut LiNbO3 crystals”, Physica B, 371, 158-162 (2006).17 K. Nassau, H.J. Levinstein, V. Loiacono, “The domain structure and etching of ferroelectric lithium niobate”,Appl.Phys.Lett., 6, 228-229 (1965).18 H.J. Leamy,” Charge collection scanning electron microscopy”, J.Appl.Phys.,53,R51-R80 (1982).19 V.Gopalan, T.E.Mitchell, K.E. Sicakfu, “Switching kinetics of 1800 domains in congruent LiNbO3 and LiTaO3crystals”, Solid State Communications,109,111-117 (1999).20 V.M. Fridkin, Ferroelectrics Semiconductors, Consult.Bureau, New York and London,1980.21 V.Ya.Shur “Farst polarization reversal process, evolution of ferroelectric domain structure in thin films”. In: C.A.Paz de Araujo, J.F. Scott, G.W.Taylor, Ferroelectric Thin films,Synthesis and Basic Properties. Gordon and Breach,New York, 1996.22 V.M. Fridkin , Foto-segnetoelectriki. Nauka, Moskva,1979.

Acknolegements: This work was supported by the Russian Foundation for Basic Research (Contract 06-02-16104)

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