Selective formation of competitive c -axis and a -axis oriented Li Nb O 3 epitaxial filmson Al 2 O 3 ( 11 2 ¯ 0 )Housei Akazawa Citation: Journal of Vacuum Science & Technology A 27, 51 (2009); doi: 10.1116/1.3021365 View online: http://dx.doi.org/10.1116/1.3021365 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/27/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Epitaxial integration of photoresponsive Bi0.4Ca0.6MnO3 with Si(001) J. Appl. Phys. 109, 063913 (2011); 10.1063/1.3561371 Specific mechanism for strain relaxation dependent on crystallization route of LiNbO 3 films on Al 2 O 3 ( 0001 ) J. Vac. Sci. Technol. A 26, 281 (2008); 10.1116/1.2841486 Microstructure and nonbasal-plane growth of epitaxial Ti 2 Al N thin films J. Appl. Phys. 99, 034902 (2006); 10.1063/1.2161943 In-plane electro-optic anisotropy of (1−x) Pb(Mg 1/3 Nb 2/3 )O 3 –x PbTiO 3 thin films grown on (100)- cut LaAlO3 Appl. Phys. Lett. 74, 3764 (1999); 10.1063/1.124172 Fabrication and optical characterization of Pb ( Mg 1/3 Nb 2/3 ) O 3 - PbTiO 3 planar thin film optical waveguides Appl. Phys. Lett. 72, 2927 (1998); 10.1063/1.121496

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Selective formation of competitive c-axis and a-axis oriented LiNbO3

epitaxial films on Al2O3„112̄0…Housei Akazawaa�

NTT Microsystem Integration Laboratories, 3-1 Morinosato Wakamiya, Atsugi-shi,Kanagawa 243-0198, Japan

�Received 27 August 2008; accepted 20 October 2008; published 8 December 2008�

The crystallographic orientation of epitaxial LiNbO3 films on the Al2O3�112̄0� substrate can becontrolled either toward the c-axis or a-axis depending on the situation at crystallization.Crystallization during high-rate sputter deposition at elevated temperatures produced c-axis-orientedfilm with 60°-rotated twin domains. Minimizing the surface energy when the growing surface wasexposed to abundant ion flux selected �0001� terminated mosaic crystal. In contrast, solid-phasecrystallization or as crystallization during a low-rate sputter deposition yielded a-axis-oriented filmcoexisting with 180° polarization twin domains. The enhanced migration of atoms in low-densityamorphous film as well as pseudomorphic lattice matching growth under thermal equilibratedconditions led to the self-organization of relaxed crystalline domains. © 2009 American VacuumSociety. �DOI: 10.1116/1.3021365�

I. INTRODUCTION

LiNbO3 �LN� is a representative electro-optic materialwidely employed in photonic devices. Z-cut LN crystalshave been the primary choice for fabricating periodicallypoled domain structures to achieve quasiphase matching.1,2

In wavelength converting devices, including wavelengthconverters and frequency doubling green lasers, LN crystal issubject to passive use once it has been poled; light merelytransmits the waveguide to convert its wavelength. In othercategories of devices, such as optical frequency modulatorsand fast electro-optic switches, however, device operation iselectrically manipulated. Since the electrodes are on the sur-face, the electric-field direction at the LN waveguide is par-allel to the surface. If the polarization of LN crystal matchesthe electric field, an r33 value that is three times larger thanr13 is feasible.3 For this reason, a periodically poled structurewith polarization parallel to the surface has been fabricatedusing off-cut and X-cut MgO:LN crystals.4,5

By using an analogy to bulk crystal devices, there havebeen a number of studies regarding LN film growth on sap-phire, especially the Al2O3�0001� substrate, which is a stan-dard template crystal matching the hexagonal crystal struc-tures of compound semiconductors and oxides. The inherentproblem with LN epitaxial film on Al2O3�0001� with itsc-axis normal to the surface is that sapphire is a genuineinsulator having a resistivity higher than 1013 � cm. Control-ling polarization by using an external electric field is thusimpossible. If LN film with its c-axis parallel to the surfacecan be obtained, depositing metal electrodes on the surfacewould allow an electric field to be applied both for polingand device operation. To assist visualization of the epitaxialrelation, Fig. 1 illustrates the specifications for a hexagonallypacked lattice, which is common to Al2O3 and LN. The fun-damental lattice unit of the sapphire A-plane, i.e.,

Al2O3�112̄0�, is a rectangle surrounded by orthogonal edges

along the �11̄00� and �0001� directions. The lattice matchingwith LN crystal can be facilitated in both directions. Sincethe a parameters of Al2O3 and LN crystals correspond toaAl2O3

=0.476 and aLN=0.515 nm, the lattice mismatch in thedirection orthogonal to the c direction is 7.9%. Since the cparameters of Al2O3 and LN crystals correspond to cAl2O3

=1.299 and cLN=1.386 nm, the lattice mismatch along the c

direction is 6.5%. Hence, Al2O3�112̄0� is a good candidatefor the substrate to orient the c-axis of LN crystalline filmwithin the surface plane. Only a few papers have reported theorientation of LN films on the sapphire A-plane6–8 andR-plane.9–12 For instance, Lee et al.8 reported the dependenceof the preferred orientation of LN films grown on

Al2O3�112̄0� on the ambient O2 pressure during pulsed-laserdeposition. Fujimura et al.11 demonstrated that LN film de-

posited on the sapphire R-plane, Al2O3�011̄2�, by rf magne-

tron sputtering was variable either with �011̄2�, �112̄0�, and

�101̄0� orientations depending on the target composition,substrate temperature, total sputtering gas pressure, or rfpower. Although the preference for the c-axis orientation un-der sputter deposition at elevated temperatures is a universalphenomenon with LN films,13–16 it is not at all clear whichcrystallographic orientation of LN actually emerges on theAl2O3 substrate other than the C-plane because thermody-namically and kinetically preferred orientations are not thesame in many cases. In the work reported in this article, wefound that the preferred orientation of LN film on

Al2O3�112̄0� was not unique; both c-axis and a-axis orienta-tions emerge depending on the existing conditions when thefilm was crystallized. This variance in orientation suggeststhe possibility that the preferred orientation can be artificiallycontrolled under appropriate conditions.a�Electronic mail: akazawa@aecl.ntt.co.jp

51 51J. Vac. Sci. Technol. A 27„1…, Jan/Feb 2009 0734-2101/2009/27„1…/51/6/$25.00 ©2009 American Vacuum Society

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II. EXPERIMENT

LN films were deposited in an electron-cyclotron-resonance �ECR� plasma sputtering apparatus that featured abranch-and-connection-type ECR plasma source. Details onthe growth conditions are described elsewhere.13 The targetwas sintered with a Li:Nb atomic concentration ratio of 1:1.Sputtering under argon and O2 partial pressures of 4�10−2

and 1.3�10−3 Pa yielded near-stoichiometric LN films. TheLi:Nb composition ratio was 0.485:0.515, as determined byinductively coupled plasma atomic emission spectroscopy.Since the Li:Nb ratio as well as the oxygen concentration infilms markedly changes with the partial pressure of O2 gas,the dependence of the crystallinity on the O2 flow rate was

not assessed. 2 in. Al2O3�112̄0� substrates were degreased inacetone and dried in clean air. After they were loaded intothe deposition chamber, they were annealed at 800 °C for30 min under an O2 ambient gas flow �1�10−2 Pa�, whichwas followed by an additional 30 min of annealing in avacuum �1�10−5 Pa�. Then, the substrate was cooled downto the deposition temperature. Unless otherwise stated, thepowers of the microwaves to produce the plasma and the rfapplied to the target were both 500 W under the standardsputtering conditions, but the rf power was varied when theeffect of sputtered ion flux was examined. A film thicknesssuitable for all evaluation methods was chosen ranging be-tween 0.2 and 1 �m.

We investigated two process routes for crystallization.The first was sputter deposition at elevated temperatures toinduce instantaneous crystallization. The second was solid-phase epitaxy; amorphous LN film was deposited at 250 °Cand in situ annealed in a vacuum to induce postcrystalliza-tion. These crystallization modes are denoted by “as crystal-lization” and “postcrystallization” in this article. The crystalstructure of LN films was analyzed by x-ray diffraction�XRD� and the epitaxial relation was determined by measur-ing the pole figures and analyzing the transmission electrondiffraction patterns. The real-space image of crystallites wasobserved by cross-sectional transmission electron micros-copy �XTEM�. The specimens were prepared by mechanicalpolishing and argon ion milling. The electron beam was in-

cident along the m-axis, i.e., the �11̄00� direction. The sur-face morphology of the LN film was studied by atomic forcemicroscopy �AFM� operated in the tapping mode.

III. RESULTS AND DISCUSSION

Figure 2�a� depicts the � /2�-scan XRD patterns from aLN film when crystallization occurred during deposition at510 °C. The diffraction signals from the film are only at�006� and �0012� appearing at 2�=38.9° and 80.8°, whichindicates predominant orientation toward the c-axis. Figure2�b� is a similar � /2�-scan XRD pattern from a film post-crystallized at 470 °C. In this case, the �110� and �220� peaksare at 2�=34.9° and 73.9°, indicating a-axis orientation. Fig-ures 3�a� and 3�b� show 2�-scan XRD patterns at �=1.5°corresponding to the same samples as in Figs. 2�a� and 2�b�.Taking the XRD patterns under glancing incidence condi-

FIG. 1. Crystallographic specifications of hexagonally packed lattice.

FIG. 2. � /2�-scan XRD patterns from LN films. �a� As-crystallized at510 °C and �b� postcrystallized at 470 °C.

FIG. 3. 2�-scan XRD patterns from �a� as-crystallized and �b� postcrystal-lized LiNbO3 films at x-ray incidence angle of 1.5°.

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tions eliminates the substrate peaks and highlights misori-ented crystallites. The two patterns are obviously different.Figure 3�a� contains absolutely no peaks while many peaksare apparent in Fig. 3�b�. This striking contrast indicates thatnucleation at the point of solid-phase crystallization does notresult in ideal selection of the �110� orientation, although�110� is the major domain in the following growth stage, butas crystallization has almost ideal selectivity to c-axis orien-tation. The tiny peaks superimposed over the backgroundintensity in Fig. 1�b�, in fact, confirm the coexistence of mis-oriented crystallites.

One important finding is that substrate temperature duringsputtering was not the primary factor controlling the majororientation although the relative volume fraction between thec-axis and a-axis oriented domains depended on temperature.This can be seen in Fig. 4, where the �006� and �110� peakheights are plotted as a function of the temperature duringsputtering for as-crystallized films �Fig. 4�a�� and the anneal-ing temperature for postcrystallized films �Fig. 4�b��. The�006� peak is always higher than the �110� peak according tothe as-crystallization route. The translational energy impartedto argon ions and neutrals in the plasma is around 10–30 eV,which is equivalent to superheating atoms on the growingsurface to more than 105 K. Thus, the difference in the sub-strate temperature will actually make a negligible contribu-tion. On the other hand, the �110� peak is always higher thanthe �006� peak according to the postcrystallization route. Themajor orientation appears to be selected by conditions as towhether or not the substrate is exposed to plasma �electronsand argon ions� or sputtered species during crystallization.

We examined deposition below 300 °C to verify whethermere plasma heating of the substrate was or was not respon-

sible for the preferential c-axis orientation. Decreasing thecrystallization temperature during sputtering merely deterio-rated the crystallinity until the amorphous phase nucleatedwithout any sign of forming an a-axis oriented crystal. Thisexperimental result indicates that the plasma effect was be-yond increasing the substrate temperature. As a control ex-periment, we crystallized amorphous film under exposure toECR plasma that was delivered through the center of thecylindrical target without applying rf power. However, thepresence of plasma during postcrystallization did not affectthe orientation, i.e., orientation was always toward thea-axis. Hence, simultaneous deposition and crystallizationwere the necessary conditions to obtain c-axis orientation.

To determine the epitaxial relationship between LN andAl2O3, XRD pole figures were drawn using the �024� diffrac-tion spots. The position of the �024� poles from LN films�Figs. 5�b� and 5�c�� was considered with reference to the�024� poles of Al2O3 �Fig. 5�a��. LN �0001� single crystal hasan intrinsic threefold symmetry, whereas the �024� poles ofour c-axis-oriented LN film exhibit sixfold symmetry. Themost obvious interpretation of this result is that 60°-rotatedtwin domains are present with a 1:1 ratio. Since two �024�poles are just on the �0001� axis, either of the fundamentalvectors of a1, a2, or a3, is parallel to the c direction. On the

other hand, the �024� poles of the LN �112̄0� plane shouldhave a twofold symmetry, but the pole figure in Fig. 5�c� hasa fourfold symmetry. This also reflects that the film consistsof twin domains. To achieve consistency with Fig. 5�c�, the

FIG. 4. Dependence of �006� and �110� XRD peak intensities on �a� depo-sition temperature for as-crystallized films and �b� annealing temperature forpostcrystallized films. The intensities were normalized for a thickness of300 nm.

FIG. 5. Pole figures drawn by using �024� XRD diffraction spot from �a�Al2O3 substrate, �b� c-axis, and �c� a-axis oriented LN films.

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atomic arrangement must be symmetrical when the LN lat-

tice is folded with respect to the �0001� and �11̄00� axes. Thesmaller LN �024� poles in Fig. 5�c� than those in Fig. 5�b�indicate that the domain size of postcrystallized film is largerthan that of as-crystallized film.

The crystallographic model describing the epitaxial rela-tionship for c-axis oriented crystal is illustrated in Fig. 6. The

lattice parameters along the �11̄00� direction of Al2O3 andthe LN A-plane correspond to 0.824 and 0.892 nm. This 8%lattice mismatch imposes compressive strain on the film. Thethree nearest neighbor oxygen atoms are contained in theunit cell in this direction. Along the c direction, five a latticeparameters of LN �5aLN=2.575 nm� coincide with two c lat-tice parameters of Al2O3 �2cAl2O3

=2.582 nm�. The latticemismatch in this direction is only 0.9%. The epitaxial rela-

tion is thus expressed as LN �0001� �Al2O3�112̄0� forthe direction normal to the surface, and LN

�112̄0� �Al2O3�0001� or LN �12̄10� �Al2O3�0001� for in-plane alignment. The unit cells are indicated in the figure.The coexistence of 60°-rotated twin domains A and B indi-cates that both domains are energetically equivalent. Therelative vertical positions of Li and Nb atoms in the unit celldetermine the polarization, either up or down with respect tothe surface plane. Domains A and B can be exchanged bysubstituting Li with Nb and vice versa. They also overlapthrough rotating one lattice by 60°.

A crystallographic model describing the epitaxial relation-ship for the a-axis oriented crystal is illustrated in Fig. 7.Since the a- and c-lattice parameters of Al2O3 and LN are

comparable, the unit cell is commonly rectangles with the

shorter side along the �11̄00� direction and the longer sidealong the �0001� direction, thus achieving commensuratestructure. Domain A corresponds to a state where polariza-tion occurs from the right to the left, while domain B is astate where it is inverted by 180°. Domains A and B can beexchanged by substituting Li with Nb atoms and vice versa,and also by folding the lattice with respect to the �0001� and

�11̄00� axes. The epitaxial relation is expressed as LN

�112̄0� �Al2O3�112̄0� for the direction normal to thesurface, and LN �0001� �Al2O3�0001� or LN

�0001̄� �Al2O3�0001� for the in-plane direction. The exactposition of individual Nb and Li atoms in relation to theAl2O3 lattice is not determined Figs. 6 and 7. We only con-sidered the periodicity of lattice units in terms of latticematching at the interface.

Figures 8�a� and 8�b� are XTEM images ofc-axis-oriented film. In the bright-field image �Fig. 8�a��, thenear-interface region appears dark since the crystal was di-vided into small mosaic domains that indicate a texturedstructure, in a similar manner to LN crystals epitaxiallygrown on Al2O3�0001�.14 The magnified image shows thateach domain width near the interface is typically 10 nm. Thedomain width becomes larger farther from the interface. Thedark-field image lit by the �006� spot �Fig. 8�b�� has frag-mentary bright regions, which do not form a continuoussingle domain.

Figures 8�c� and 8�d� are similar bright- and dark-fieldXTEM images of a-axis-oriented film after a sufficientlylong crystallization time. The film consists of large crystal-

FIG. 6. Atomic-arrangement model representing lattice matching betweenc-axis-oriented LN epitaxial film and Al2O3 substrate. Unit cells of LNcrystal are indicated by rhomboids. Twin domains A and B can be inter-changed by rotating domains by 60° or interchanging Li and Nb.

FIG. 7. Atomic-arrangement model representing lattice matching betweena-axis-oriented LN epitaxial film and Al2O3 substrate. Unit cells of LNcrystal are indicated by rectangles. Polarization is denoted by arrows. Twindomains A and B are interchanged by folding arrangement with respect tothe �1−100� axis.

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line domains with a typical horizontal width of 200 nm,which are about 20 times wider than those in Figs. 8�a� and8�b�. Although all domains are nearly �110� oriented, somedomains are not bright in the dark-field image because of theslightly inclined a-axis from the surface normal. Tilting ofdomains has also been observed in LN /Al2O3�0001� �Ref.14� and ZnO /Al2O3�0001�.17 Some area near the surface re-mained noncrystalline, which are the regions that failed tocrystallize. The solid-phase crystallization being terminatedbefore the crystalline front reaches the surface suggests lowatomic density in the film deposited at 250 °C. The atomicdensity of our postcrystallized LN films being lower thanthat of bulk crystal has previously been confirmed throughthe precise evaluation of refractive indices by using prismcoupling. Postcrystallized films have refractive indices rang-ing between 2.02 and 2.03, lower than that of bulk crystal�no=2.286 and ne=2.203�.15 Similarly, Nashimoto et al.7 ob-

tained a-axis oriented, porous LN film on Al2O3�112̄0� bypostannealing sol-gel derived films at 400 °C. With the de-velopment of crystallization from the interface, the crystal-lized part becomes dense as atoms in the still noncrystallizedregion will be supplied into the crystallites, leaving a sparsesurrounding region that fails to become crystallized.

Figures 9�a� and 9�b� are high-resolution XTEM imagesobtained by focusing on the interfaces of c-axis and a-axisoriented films. In Fig. 9�a�, the few atomic layers of LNcrystal terminated at the interface appear very sharp andmatch the substrate lattice over a long range. Instantaneouscrystallization when sputtered atoms are deposited resultedin long-range lattice matching between LN and Al2O3 crys-tals, which exerted strong compressive strain. This was themain cause for the crystal to subdivide into numerous mosaicdomains. In fact, the diffraction angles of �006� peak re-

vealed that as-crystallized films had an elongated c-axislength compared to that of bulk crystals, which means thatthe film suffered from compressive strain within the a-bplane. On the other hand, postcrystallized LN films had alattice parameters close that of bulk crystals, within �0.03 Ådeviating from the value of bulk crystal. Almost completestrain relaxation even if the lattice mismatch is as large as

7.9% along the �11̄00� direction and 6.5% along the �0001�direction cannot reasonably be explained unless the interac-tion between LN film and the substrate is weakened. Thecrystalline lattice in Fig. 9�b� is unclear and lattice disorder isperiodically introduced at the interface, which efficiently ac-commodates the lattice mismatch strain. During the postan-nealing, self-organized crystalline domains will be formed soas to minimize the strain energy.

The AFM images in Fig. 10 illustrate how the surfacemorphology of LN film was affected by various treatments.The plan view AFM images are consistent with the cross-sectional images in Fig. 8. Figure 10�a� represents the sur-face structure of amorphous film deposited at 250 °C. Somecraters are distributed over the surface while the surrounding

FIG. 8. XTEM images of ��a� and �b�� as-crystallized and ��c� and �d��postcrystallized films. �b� and �d� correspond to dark-field images lit by the

�006� and �110� spots. Electron beam incidence is from the �11̄0� direction.

FIG. 9. XTEM lattice images at near interface of �a� as-crystallized and �b�postcrystallized films. Electron beam incidence is from the �11̄0� direction.

FIG. 10. AFM images of LN films. �a� As-deposited at 250 °C, �b� afterpostannealing at 460 °C, �c� after HF etching of a-axis oriented film, �d�as-deposited at 350 °C, �e� as-deposited at 460 °C, and �d� as-deposited at510 °C. The black-to-white gray scales are �a� 50 nm, �b� 50 nm, �c� 50 nm,�d� 30 nm, �e� 50 nm, and �f� 30 nm.

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regions are very flat. When the film was crystallized throughpostannealing, the boundaries between the crystalline grainsbecame conspicuous while the craters were retained �Fig.10�b��. a-axis-oriented LN films were wet etched in a 50%HF solution at room temperature to confirm the orientation.A high rate of etching occurred preferentially from thec-domain surface and the etched front extended into the film.This process created concave pores almost parallel to thesurface seen in the AFM image in Fig. 10�c�. The deep hon-eycombed pores extended into the film from the inclineddirection, which suggests a slightly tilting a-axis from thesurface normal. When the film was deposited at 350 °C,which exceeded the threshold of crystallization, we obtainedthe AFM image in Fig. 10�d�. The flowery mounds, whichare separated by about 2 nm, are self-organized around thenucleation centers. When deposition was done at 460 °C, theflowery patterns disappeared and the morphology defined bysmaller disordered grains became prevalent �Fig. 10�e��,which coincides with the formation of mosaic crystal seen inthe XTEM image of Fig. 8�a�. Increasing the deposition tem-perature further to 510 °C produced fine crystalline grains�Fig. 10�f��.

Figure 11 shows the evolution in the XRD pattern whenthe rf power applied to the target was decreased from500 to 120 W at a fixed microwave power of 500 W. Themicrowave power defined the plasma density whereas the rfpower was almost proportional to the flux of the sputteredatoms and ions arriving at the substrate. We can see that the�110� peak intensified concurrently with the attenuation ofthe �006� peak, which means that the preferred orientationtransferred from the c-axis to the a-axis when the flux of thesputtered species was decreased. Therefore, these circum-stances, where the high flux of the sputtered species includ-ing ions and neutrals are impinging on the surface duringcrystallization, are responsible for the c-axis orientation.

When the ion flux is sufficiently low so that the effect of ionscannot overcome the thermal effect, a-axis orientation willpredominate. The chemical potential of the LN /Al2O3 epi-taxial system is given as the sum of surface energy, interfaceenergy, and the strain energy of crystalline lattice. The sur-face energy of the closed-packed c-plane of the hexagonallattice is the lowest of the other planes. In this respect, c-axisorientation is energetically preferred when the surface crys-tallization governs the crystallization process. In the case ofcrystallization from the interface, minimizing the interfaceenergy determines the orientation, thus commensurate struc-ture �a-axis orientation� will be preferred.

IV. CONCLUSION

The conditions for LN film growth on Al2O3 resulting inc-axis and a-axis orientations have been clarified. Bothc-axis and a-axis orientations can be facilitated through

matching the lattice along the �11̄00� and �0001� directions.c-axis orientation was achieved when the film was crystal-lized during deposition under a high flux of the sputteredspecies. The long-range interfacial matching resulted instrong compressive strain, which was relieved by dividingthe crystal into small mosaic texture domains. The existenceof 60°-rotated twin domains was confirmed. The a-axis ori-entation was achieved though the solid-phase crystallizationof predeposited noncrystalline film or as crystallization undera low flux of the sputtered species. The atomic arrangement

was symmetrical with respect to the �11̄00� and �0001� axes,yielding a mixture of 180°-inverted twin domains. The do-mains were 20 times wider than those that were oriented inthe c-axis. The lattice mismatch strain was relieved due tolattice disorder near the interface.

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FIG. 11. Evolution of � /2�-scan XRD patterns of as-crystallized LN filmswith changing rf power applied to target.

56 Housei Akazawa: Selective formation of competitive c-axis and a-axis oriented 56

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