Electrets and otherrelated phenomena

Structure-Properties relationships in

LiNbO3-based multilayered ferroelectric structuresV. Bornand, Ph.Papet and F. Henn

Laboratoire de Physicochimie de la Matiere Condensee LPMC UMR CNRS 5617UM II, Place E. Bataillon, C.C. 003 - 34095 Montpellier Cedex 5 - France

Abstract. We report the growth of LiNbO3 thin films onto1n203:Sn-coated <111>-Si substrates by standard radio-frequency sputtering. Multi-layer procedures, up to 4 successivedeposits, have been developed that can subsequently improve thestructural and macroscopic ferroelectric properties of such as-grown composite structures. The enhancement of polarization,as high as 40 FlIC.cm-2 in 4-stacked layers, is attributed to c-oriented seed-layer-induced crystallization (self-polarization)and interfacial (migratory) polarization.

I. INTRODUCTION

In the past few years, ferroelectric thin films andsuperlattices have become the subject of increasingimportance in order to develop new active materials, adaptivestructures and self-organized architectures for the newgeneration of ferroelectric random access memories (FRAM)or surface acoustic wave (SAW) thin film devices. No doubtthat what may be called "the nanoscale century" will followthis trend, thus leading to a new technical and technologicalrevolution, particularly in termns of thin film processing andmaterials.LiNbO3 (LN) is expected to be an ideal alternative for

small-scale systems. In an attempt to both develop novelintegrated electro-optical systems and offer new functionalityfor future LN-based devices, the stacked-layer growth of LNthin films onto In203:Sn (ITO)-coated <111>-Si substrateshas been investigated by radio-frequency (r.f.) sputtering.Breakthroughs in material processing and LN capabilities arethe driving idea behind the choice of such an underlyingtemplate. Indeed, with both its low refractive index and goodelectrical conductivity for low SnO2 content (<10%), ITOappears as an efficient material for transparent electrodes incontemporary and emerging thin films [1].A possibility to enhance the c-oriented growth and

modulate the polarization response consists in performingmulti-step procedures and imposing stacked-layerconfiguration by interrupting the growth and recoveringambient temperature and pressure conditions betweendifferent deposits [2]. Up to 4 stacked LN layers weresuccessively and successfully deposited onto ITO/Sisubstrates. In the present work, structural and macroscopicferroelectric properties of 2-step and 4-step grown systemsare compared and discussed on the basis of correlatednucleation and polarization effect. These experiments canpotentially contribute to the better understanding of themechanisms responsible for orientation selection onto ITO/Si

and illustrate the crucial role of space-charge like defects onpolarization in ferroelectric thin-film capacitors.

II. RESULTS AND DISCUSSION

Selective orientationDue to the lack of atomic registry and lattice match

between LN (rhombohedral, R3c) and ITO (cubic, Ia3),polycrystalline structures are observed on LN mono-layersdirectly grown onto ITO [2-3]. One way to promote orientedgrowths lies in multi-step procedures, with the first-sputteredLN layer as thin as possible to prevent both grain growth andheterogeneous nucleation. As previously reported, LN bi-layers corresponding to 1h (first deposit) and 5h (seconddeposit) deposition times, at 600°C, can exhibit degrees oforientation along the c-axis as high as 50% [2]. This thin LNhomo-template appears as a suitable seed-layer for the nextnucleation-growth process, thus resulting in orientedmaterials (Fig. la). The deposition of 4 successive thin layerscan further improve this texturation, with degrees oforientation, estimated through the factor of Lotgering J(006),as high as 94% (Fig. lb).

0~~~~'4=10,0 12,5 15,0 1755 20,0 22,5 25,0 27,5

0 (degrees)

Fig. 1. X-ray diffraction patterns of LiNbO3 thin films sputtered onto ITO-covered <1I1 >-Si substrates and annealed in air at 750°C for lh.

(a) lh/600°C + 5h/600°C (b) four steps, 3*)h30/300°C + l*lh30/600°CThe relative intensity ofLN (006) peak increases for multi-layered structures.

Parasitic phases can be suppressed in as-deposited composite systems.LN; TITO; A-< 1I 1 >-Si; *parasitic phase

To explain this spontaneous orientation, differentmechanisms can be considered, such as (1) space-chargefield, (2) self-polarization, or (3) collective nucleation, re-crystallization and polarization effect. Beyond its ferroelectricnature, LiNbO3 is a well-known ionic conductor [4]. Due tothe easy diffusion of low-sized low-charged Li ions, a

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gradient of Li vacancies can be formed in the LN films nearthe interface(s), which can induce a space charge field (SCF)in the direction perpendicular to the substrate [5]. Even if theboundaries of this interdiffusion region seems to be quitenarrow as revealed by depth profile analysis by secondary ionmass spectroscopy [2], this SCF may promote the formationof c-oriented crystallites. The higher the number of layers, thestronger the built-in-field, the greater the self-orientationeffect (Fig. 1).

In the same way, the different dynamical properties of theelectrons and ions in the plasma can induce a superimposedelectric field near the surface, all the more important since theselected ambient is rich in oxygen (Ar:02 = 40:60).Consequently, the electrostatic coupling of the growingcolumns of material with this built-in electric field couldfavor a preferential orientation parallel to this field, i.e.perpendicular to the substrate Since the polar direction isalong the <001> crystallographic axis in LiNbO3, (OOh)crystalline planes are preferentially set parallel to thesubstrate. The strong variation of the film orientationaccording to the thermal history during a same deposit is a

qualitative proof of this coupling with the plasma self-polarization [6]. An example is given in Fig. 2 for bi-layeredsystems. By progressively increasing the temperature duringthe second deposit, one can strongly affect the characteristicsof the plasma near the heated surface and the resultingcrystalline texture of the film. This results in the shift of thepreferential orientation from (00h) to (Ohh). Note thatworking at lower temperatures during both the first depositand the first hours of the second deposit can also limitinterdiffusion phenomena at the film/substrate interface (Fig.2b), thus reducing the associated ionic defects.

Finally, structural considerations can provide an additionalexplanation for the observed selective orientation. Re-crystallization process during the successive deposits can

favor a c-oriented, (006) planes possessing one of the fastestgrowth rate of the LN structure [7]. The complexity of thephenomena results form the close interplay between thesedifferent scenarios.

a~~~~~~

C)

12.5 15.0 17,5 20,0 22,5 2570 27,5

0 (degrees)

Fig. 2. X-ray diffraction patterns of two-step (lh+5h) deposited LiNbO3 thinfilms sputtered onto ITO-covered <111>-Si substrates and annealed in air at

750°C for lha) lh!600°C + 5h/600°C (b) lh/300°C + 5h/300°C(3h)+600°C(2h).

Variations of plasma properties during sputtering, induced by changes ofsubstrate temperature during deposition, can shift the preferential orientation

from (hOO) to (hhO).O-LN; Y-ITO; A-<1 11>-Si; *-parasitic phase

Polarization effectThe degree of polarization is known to be determined by

both crystal orientation and degree of texturation along asame direction. Un-polarized films with non-polar <011>-preferential orientation exhibit poor ferroelectric capabilities.On the contrary, the higher the degree of orientation along thec-polar direction, the higher the remanent polarization Pr (Fig.3). Moreover, increasing the number of interfaces in 4-layersystems leads to a further intensity enhancement of Pr,evidencing once again the important role played by space-charge induced built-in-field effect. The recordedmacroscopic polarization in multi-layered materials iscertainly the result of a collective effect combining naturalpolarization and what may be called "migratory" polarization[8]. One can assume the inclusion of a contribution topolarization arising from the transfer of charges across thesuccessive interfaces in addition to its internal self-polarization resulting from the c-preferred orientation [9-10].The charged defects accumulated at the interfaces - realcapacitive phases in parallel to the ferroelectric capacitor -

may create a local electric field under the externally appliedpotential, thus generating a cooperative movement. The resultis an enhancement of the overall macroscopic Pr4Echaracteristics and an elongation of the hysteresis loops toshow higher Pr. The fact that the value of the coercitive fieldis almost identical in 2-step and 4-step c-oriented layers tendsto validate such a scheme. Localized charges create localelectric fields which stimulate the occurrence of self-orientedmicro-clusters at the layer boundaries with local polarization.

c-

4

*:_

N

a

60

40

20

0

-20

-40

-60

-300 -200 -100 0 100 200 300 400Electric Field (kV.cmr&)

Fig. 3. Hysteresis loops versus film orientation and deposition procedure ofLiNbO3 thin films sputtered onto ITO-covered <11 1>-Si.Films of Fig. I and 2. Applied field (Ea): 120 kV.mmn'

LI. CONCLUSION

The multi-layer process is one of the effective approachesto enhance both the selective orientation and macroscopicpolarization of ferroelectric heterostructures. Highly c-

oriented 4-layered LN films could be sputtered onto ITO/Si,that exhibit Pr as high as 40 .tC.cm-2. However, thecomplexity of phenomena in such systems is provided by theinterplay between bulk ferroelectric polarization, domainmotion and space-charge effects at the interfaces. Careftulinvestigation of the mechanisms of formation of the built-in

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- lh+5h - Y'110) - 53%Ih+5h - Y¶006) - 50%

* 4*1h30 - -4006) - 94%

electric field(s) in such stacked layers appears of primaryimportance in understanding their macroscopic ferroelectricproperties and, ultimately, commenting on both the technicalchallenges of such multi-layer approaches and thetechnological reliability of as-deposited systems.

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