Amorphous thin films of ferroelectric oxides

Ren Xu, Yuhuan Xu and John D. MackenzieDepartment ofMaterials Science and EngineeringUniversity ofCalifomia, Los Angeles, CA 90024ABSTRACTAmorphous thin films of ferroelectric LiNbO3 and Pb(ZrO52Tigj4)O3 were prepared on avariety ofsubstrates. Their properties were characterized in comparison with typical ferroelectriccrystal behavior. It was found that these amorphous thin films show ferroelectric-like behaviors wellbefore being crystallized. These findings suggest possible practical applications in areas whereferroelectric crystals are routinely in use. A structural model is proposed to account for theferroelectric-like behaviors in the amorphous phases.1. INTRODUCTIONFerroelectric thin films have received increasing interests in recent years because of their widerange ofpotential applications. Unique properties of ferroelectric materials such as the well-knownpiezoelectric, electro-optic, electro-acoustic, and non-linear optic effects1 made possible a variety ofdevice applications. For instance, Pb(Zr1..T)O3 ferroelectric thin films are being intensely studiedfor non-volatile memory applications. Ferroelectric LiTaO3 single crystals with high pyroelectriccoeffIcient were demonstrated to be a good fast infrared radiation detector material'. Having highlinear electro-optic coefficients, LiNbO3 single crystal films are also used as light modulators in opticalsignal processing applications3'4. Other important applications ofdemonstmtional thin film devices asoptical wave guide, electroacoustic tmnsducer, high frequency surface acoustic wave (SAW)device7, pyroelectric infrared detectors 10, ferroelectric memory cells' 1 ,12 ferroelectricphoto-conductive displays136, optical modulators5J7, and field effect transistors (FET) with aferroelectric gate, and metaVinsulatozfsemiconductor transistor (MIST) device18 have also been widelystudied.One ofthe main obstacles between demonstrational waveguide devices and commercialapplications is the optical loss due to scattering offthe grain boundaries. Consequently, efforts havebeen directed to the fabrication ofsingle crystal LiNbO3 waveguides by techniques such as REsputtering, liquid phase epitaxy and ion-exchange. It is also in principle possible to avoid theproblems ofgrain boundary scattering by completely eliminating crystallinity in these waveguides.From the materials science point ofview, such endeavour involves the synthesis ofan amorphousmaterial that presents at least some ofthe typical ferroelectric or ferroelectric-like behaviors.Ferroelectricity has been commonly associated with crystalline materials, although it has beenpostulated that this phenomenon can occur in an amorphous solid'9. However, experimentaldemonstration ofamorphous ferroelectricity has not been an easy task up to the present. For instance,ciystalline LiNbO3 has been rapidly quenched from the molten state to give a glassy LiNbO320.Dielectric anomaly was observed for this glassy LiNbO3, although increasing ionic conductivity athigh temperatures can complicate the interpretation ofthe results ofthe dielectric measurements.Unfortunately, such glassy samples are minute in size, e.g. 0. 1 cm2 platelets about 10-20 .im thick.Such very small samples and the brittleness of the glassy phase make other measurements verydifficult. A reversible pyroelectricity was also mentioned while no quantitative results were published.Such glassy LiNbO3, when doped with iron, showed from Mossbauer spectroscopy anomaloustemperature dependence of the area under resonance, quardrupole splitting and center