Ferroelectricity in Simple Binary ZrO2 and HfO2

Johannes Muller,*,† Tim S. Boscke,‡,¶ Uwe Schroder,‡,§ Stefan Mueller,§ Dennis Brauhaus,∥,▽

Ulrich Bottger,∥ Lothar Frey,⊥ and Thomas Mikolajick§,#

†Fraunhofer Center for Nanoelectronic Technology, Dresden, Germany‡Qimonda GmbH, Dresden, Germany§Namlab gGmbH, Dresden, Germany∥RWTH Aachen, Aachen, Germany⊥Fraunhofer Institute of Integrated Systems and Device Technology, Erlangen, Germany#Chair of Nanoelectronic Materials, University of Technology Dresden, Dresden, Germany

*S Supporting Information

ABSTRACT: The transition metal oxides ZrO2 and HfO2 as well as theirsolid solution are widely researched and, like most binary oxides, areexpected to exhibit centrosymmetric crystal structure and therewith lineardielectric characteristics. For this reason, those oxides, even thoughsuccessfully introduced into microelectronics, were never considered to bemore than simple dielectrics possessing limited functionality. Here we reportthe discovery of a field-driven ferroelectric phase transition in pure, sub 10nm ZrO2 thin films and a composition- and temperature-dependenttransition to a stable ferroelectric phase in the HfO2−ZrO2 mixed oxide.These unusual findings are attributed to a size-driven tetragonal toorthorhombic phase transition that in thin films, similar to the anticipated tetragonal to monoclinic transition, is lowered toroom temperature. A structural investigation revealed the orthorhombic phase to be of space group Pbc21, whosenoncentrosymmetric nature is deemed responsible for the spontaneous polarization in this novel, nanoscale ferroelectrics.

KEYWORDS: Ferroelectric, hafnium oxide, zirconium oxide, phase transition, thin film

The binary oxides HfO2 and ZrO2 have been extensivelystudied for more than a century. Especially their

martensitic phase transition from tetragonal to monoclinicand its implications on the mechanical properties of thematerial system have always been and still are of great scientificand commercial interest. The mechanical strain released duringthe volume expanding transformation to the monoclinic phase(P21/c) can be directly utilized in the transformationtoughening of “ceramic steel”.1 To avoid this defect generatingmechanism when using the pure oxides, stabilization of the hightemperature polymorphs of tetragonal (P42/nmc) or cubic(Fm3m) symmetry is usually pursued.2−4

Only in recent years, driven by microelectronic scaling andthe industry’s strive to find a suitable “high-k” replacement forintegrated gate and capacitor dielectrics, extensive research hasbeen conducted on HfO2 and ZrO2 based thin films.5,6 In thecourse of this material development it was found that for thinlayers in the range of several nanometers the tetragonal tomonoclinic transition temperatures, as estimated from ceramicbulk samples, are significantly lowered.7−9 This size-inducedphase transition is frequently observed in the context of free aswell as confined ZrO2 nanoparticles and was attributed to thelower surface energy developed by the high temperaturepolymorphs of HfO2 and ZrO2.

10−12 However, as in the bulkphase diagram,13 the tetragonal to monoclinic transition

temperature for HfO2 is still higher compared to the one ofZrO2. In sub 20 nm films this usually leads to tetragonal,undoped ZrO2 and partially monoclinic HfO2 thin films14 thatstill require, similar to stabilized bulk ceramics, small amountsof group III,15 IV,16 or rare earth element17−20 dopants to reacha full stabilization of the high temperature polymorphs.Recently, however, we reported that, until a complete

stabilization of those theoretically predicted21,22 and exper-imentally confirmed16,23 “higher-k” polymorphs in HfO2 thinfilms is reached, those phase transitions are accompanied by theoccurrence of ferroelectricity.24 Several dopants such as Si,24,25

Y,26 and Al27 as well as the admixture of 50 mol % ZrO228 were

identified to provoke ferroelectricity in HfO2 that due to itscentrosymmetric phase relation (P21/c − P42/nmc − Fm3 m)was widely believed to be paraelectric. The occurrence offerroelectricity in binary oxides is of high scientific interest andhas so far theoretically been predicted only for alkaline oxides.29

The discovery of ferroelectricity in thin films of the broadlycommercialized binary oxides of hafnium and zirconium isunexpected and affects multiple fields of application. Especiallyin the context of ferroelectric memories, which since their

Received: May 30, 2012Revised: July 12, 2012Published: July 19, 2012

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introduction strongly rely on the challenging integration ofperovskite-based materials, these fully CMOS compatible andhighly scalable HfO2-based ferroelectrics have the potential tooffer a novel and much simpler approach.30,31

Here we report the discovery of a composition- andtemperature-dependent ferroelectric phase transition coveringthe full mixing range of HfO2−ZrO2 thin films. A stableferroelectric phase is observed for the solid solution at roomtemperature, whereas a field driven transition is observed inpure ZrO2. A structural investigation by grazing incidence X-raydiffraction (GI-XRD) suggests a composition-dependenttetragonal to orthorhombic to monoclinic phase change inHfO2−ZrO2 thin films when starting from Zr-rich composi-tions. As further revealed by in situ high temperature GI-XRD,confinement of the thin films by a TiN electrode assists in thesuppression of the tetragonal to monoclinic transition duringcooling, resulting in an alternative tetragonal to orthorhombicpathway. This noncentrosymmetric, orthorhombic, and therebypotentially ferroelectric Pbc21 phase of the mixed oxide wasfound to further stabilize with decreasing temperature. This ledto a stable ferroelectric phase at 80 K in Zr-rich samples thatexhibited only field-driven ferroelectric transition at roomtemperature. Those nonlinear and temperature-dependentdielectric characteristics found in pure ZrO2 and in its solidsolution with HfO2 confirm the existence of a tetragonal toorthorhombic phase transition and further underline theassumption that the occurrence of ferroelectricity in HfO2and ZrO2 based thin films is an intrinsic property of thosematerial systems and cannot be explained by doping relateddefect dipoles or ionic mobility.We utilized atomic layer deposition (ALD) to prepare a set

of 9 nm HfO2−ZrO2 thin films on TiN, spanning a widecomposition range of the solid solution starting and endingwith the pure oxides (Figure 1A). All films crystallized duringdeposition of a TiN top electrode at 500 °C. The polycrystal-line nature was confirmed by high-resolution transmissionelectron microscopy (HR-TEM; Figure 1B). Polarization−voltage (PV) as well as small signal capacitance−voltage (CV)characteristics of TiN-based metal−insulator−metal (MIM)capacitors of those films are depicted in Figure 2A.Just as expected from the centrosymmetry of its monoclinic

structure, the pure HfO2 film shows a linear relation betweenthe displacement current and the applied electric field as well asa fairly constant capacitance in this field range. However, asZrO2 content increases, characteristic ferroelectric PV and CVhysteresis evolve until for a nearly equal mixture of ZrO2 andHfO2 a remanent polarization of 17 μC/cm2 is reached. Thecoercive field of this sample was approximately 1 MV/cm.Further increasing the ZrO2-content in the solid solution leadsto a thinning of the hysteresis loop at zero bias,phenomenologically best described as superimposed antiferro-electric-like characteristics. This thinning continues until forpure ZrO2 the remanent polarization has completely vanishedand only a distinct double-loop hysteresis remains. When takinga direct look at the current flowing on and off the capacitorduring a triangular voltage excitation, as depicted in Figure 2B,the polarization switching can clearly be separated from leakagecurrent contributions that often lead to a confusion of simpleparaelectric materials with ferroelectrics.32

A summary of the composition-dependent phase transitionwitnessed in Figure 2A and B is given in Figure 2C. Theremanent polarization, the dielectric constant, and themonoclinic phase fraction estimated from the peak area relation

of the coinciding tetragonal 011t and orthorhombic 111o againstthe monoclinic 111m reflection are plotted versus the ZrO2content. The underlying grazing-incident X-ray diffraction (GI-XRD) measurements are given in the Supporting Information.It is clearly observed that with the surface energy drivendestabilization of the monoclinic phase toward pure ZrO2 thedielectric constant steeply increases and the remanent polar-ization is maximized at the phase boundary of this transition.However, when directly comparing the monoclinic phasefraction to the evolution of permittivity, one notices that themonoclinic phase has almost completely vanished above 50 mol% ZrO2 admixture while permittivity still increases towardZrO2. This gives a first indication that the tetragonal phase suchas present in the pure ZrO2 is not immediately reached. As willbe elaborated further, the phase transition is possibly bridgedby an additional phase of intermediate permittivity.A field-driven ferroelectric transition in pure ZrO2 and an

adjacent ferroelectric transition with HfO2 admixture is trulysurprising and has so far neither for bulk ceramics nor for thinfilms of ZrO2 or HfO2−ZrO2 been reported. As alreadymentioned, the temperature-driven and well-understood P21/c− P42/nmc − Fm3m transition does not allow for a stablespontaneous polarization. However, detailed GI-XRD measure-ments depicted in Figure 3A reveal that the assumption of asimple tetragonal to monoclinic phase transition does not provesatisfactory for the observations made in this work. As expectedthe pure HfO2 shows a predominantly monoclinic P21/c crystalstructure, with the 111m and −111m reflections being the mostprominent feature in the diffractogram. Likewise the structure

Figure 1. (A) ZrO2 and HfO2 content in the HfO2−ZrO2 solidsolution measured by XPS plotted against the ALD pulsing ratio of theutilized alkylamide precursors, TEMAH and TEMAZ. The nearlysimilar growth per cycle of both precursors enables a linear and almostdirect stoichiometry control by the ALD cycle ratio. (B) HR-TEMmicrographs of the metal−insulator−metal capacitor used for electricaltesting in this work. The polycrystalline nature of the HfO2−ZrO2 thinfilms as well as of the TiN metal electrodes is clearly visible.

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of the pure ZrO2 can be identified as the tetragonal P42/nmcphase frequently reported in literature.33−36 However, it has to

be noted that especially in thin films separating the tetragonalphase with a c/a ratio close to 1 from the cubic phase proves

Figure 2. (A) PV hysteresis at 1 kHz and small signal CV hysteresis at 10 kHz (50 mV level) of 9 nm thin HfO2−ZrO2 based metal−insulator−metalcapacitors at room temperature. An evolution from paraelectric HfO2 to ferroelectric HfO2−ZrO2 to an antiferroelectric-like behavior in ZrO2 can beobserved in PV as well as in CV characteristics. (B) Current response to a triangular voltage excitation reveals polarization switching to be clearlyseparable from leakage current contributions at high fields. (C) Remanent polarization, dielectric constant, and monoclinic phase fraction in theHfO2−ZrO2 solid solution with respect to the mixing ratio of the oxides. With increasing ZrO2 content the dielectric constant increases due to areduction in monoclinic phase fraction, whereas the remanent polarization is maximized in the transition region.

Figure 3. (A) GI-XRD diffractograms of 9 nm ZrO2, Hf0.5Zr0.5O2, and HfO2 thin films at an incident angle of 0.55°. Starting from ZrO2 acomposition-dependent tetragonal to orthorhombic to monoclinic transition is observed. Strong reflections are labeled in the graph. Referencepowder patterns for HfO2 and ZrO2 were calculated from the literature (P42/nmc, Malek et al.;33 Pbc21, Kisi et al.;

37 P21/c, Ruh et al.38). (B) In situGI-XRD measurements allow the direct observation of a suppressed tetragonal to monoclinic transformation during cooling in samples confined by aTiN electrode.

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rather challenging, usually resulting in an ambiguous interpret-ability of the published results.23 In this work the absence ofdiffractions from the TiN electrode (see sample preparation inSupporting Information) and the high signal-to-noise ratioallow for the identification of the tetragonal phase by the 012treflection at 42° originating from the slightly distorted oxygenlattice in the tetragonal modification.Unexpected diffractions on the other hand appear for the

equal mixture of ZrO2 and HfO2 that was found to exhibit thehighest remanent polarization at room temperature. Severalreflections are indicated that cannot be explained by a simplephase mixture of P21/c and P42/nmc, suggesting the presence ofa third phase stable during the transition from m-HfO2 to t-ZrO2. Similar to in the doped HfO2 systems that were found toexhibit ferroelectricity, this phase best matches the diffractionpattern of an orthorhombic phase with Pbc21 symmetry.24,28,27

The existence of this noncentrosymmetric phase in ZrO2,unfortunately published without data on its dielectric proper-ties, was first and solely reported by Kisi et al.37 in 1989. In aneutron diffraction study they observed the Pbc21 phase insmall particles of Mg stabilized ZrO2 confined in a cubic matrix.A martensitic tetragonal to orthorhombic phase transitioninduced by a displacement of the Zr and O1 ions against theO2 ion was suggested. This tetragonal to orthorhombictransformation eventually circumvents the volume expandingtetragonal to monoclinic transformation not favorable in ahighly constrained environment, such as in a cubic matrix or inour case of a thin film additionally confined by a metalelectrode.The supporting nature of the mechanical confinement

provided by the TiN electrode in this process can bedemonstrated by in situ GI-XRD measurements recordedduring cooling. As shown in Figure 3B, a thin film crystallizedinto the orthorhombic phase is stable toward any further heattreatment above 800 °C as long as the TiN electrode is still inplace. However, if this mechanical encapsulation is removed, atransformation to the monoclinic phase can be observed,strongly increasing when temperature drops below 200 °C.These findings are in accordance to the assumption of areversible, temperature-dependent tetragonal to orthorhombictransition. In the constrained system the orthorhombic phasetransforms to the tetragonal phase during heat up and back intothe orthorhombic phase during cooling. If the encapsulation isremoved, the orthorhombic phase is still stable at roomtemperature, as depicted in Figure 2A, but after renewedheating into the tetragonal phase part of the system undergoesthe well-established tetragonal to monoclinic transformation.As already mentioned, compared to this well-established

transition very little research has been conducted on the lesscommon polymorphs of HfO2 and ZrO2. Nevertheless, for pureHfO2 and ZrO2 the stability of the Pbc21 phase was calculatedfrom ab initio by Lowther et al.,39 indicating that the relativestability of this phase with respect to the monoclinic phase lieswithin a narrow range of only ∼10 meV/atom. The calculatedand experimental lattice parameters as well as our resultsestimated from the HfO2−ZrO2 diffractograms in Figure 3A aresummarized in Table 1. Considering the different preparationmethods the estimated lattice parameters of this work are ingood agreement with these previous results.In this context it is interesting to add that already in 1985

Suyama et al.40 published the synthesis of an HfO2−ZrO2 solidsolution, which exhibited pure orthorhombic symmetry.Additionally orthorhombic phases have been observed in

pure41 as well as Y-doped42 HfO2 thin films grown by ALD.A complete summary of earlier sightings of orthorhombicphases in ZrO2 is further given by Heuer et al.

43 However, in allcases the nonlinear dielectric properties and in most cases theexact space groups of those orthorhombic phases were notfurther elaborated.The temperature dependence of the ferroelectric phase

stability was further investigated down to 80 K. Due to areduced leakage current at 80 K, a fully saturated antiferro-electric-like PV hysteresis loop can be recorded for pure ZrO2and is shown in the Supporting Information, Figure 1. To ruleout a distortion of the hysteresis loop due to leakagecontributions, the comparison of this low temperaturehysteresis loop to hysteresis loops at elevated temperatureswas done at a smaller excitation signal (Figure 4). For ZrO2 it

becomes apparent that with a decreasing temperature similar towith increasing HfO2 content the stability of the ferroelectricphase increases. In accordance with observations made on thetemperature dependence of the antiferroelectric hysteresisloops in PLZTS, the critical field for the back switching ofthe spontaneous polarization decreases when with decreasingtemperature a ferroelectric phase is approached.44 For a Zr-richthin film, containing only 22 mol % HfO2, at room temperaturealready being comprised of a field-driven and a stableferroelectric phase, an almost complete transition to a purelyferroelectric phase with decreasing temperature can beobserved. As assumed for the ferroelectric transition in Si-doped HfO2, the higher symmetry tetragonal structure ascompared to the lower symmetry orthorhombic structure is

Table 1. Estimated Lattice Parameters of the InvestigatedHfO2 and ZrO2 Polymorphs Compared to Literature Data

GI-XRD, this workab initio

simulationaneutron

diffractionb

HfO2 ZrO2 Hf0.5Zr0.5O2 HfO2 ZrO2 ZrO2

spacegroup P21/c P42/nmc Pbc21 Pbc21 Pbc21 Pbc21

a 5.14 3.59 5.24 5.3 5.26 5.26b 5.07 3.59 5.01 5.11 5.07 5.07c 5.29 5.17 5.05 5.1 5.08 5.08

aAb initio simulation results obtained by Lowther et al.37 bA neutrondiffraction study conducted by Kisi et al.39

Figure 4. PV hysteresis of ZrO2 revealing a decreasing critical field forpolarization switching when temperature is lowered from 230 to 80 K.For the same temperature range a complete transition from anantiferroelectric-like hysteresis loop to a ferroelectric hysteresis can beobserved in Zr-rich samples of the solid solution.

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stabilized with increasing temperature.25 Especially for the ZrO2system this observation is in good agreement with the reportson the appearance of an orthorhombic phase in ZrO2 atcryogenic temperatures and confirms the temperature depend-ence of the related tetragonal to orthorhombic transitionsuggested by those authors.37,45

Further measurements, especially to higher temperatures,were conducted to determine the Curie temperature of thesystem. The results are given in the Supporting Information.Due to high leakage contributions at elevated temperatures aswell as the technical setup, measurements were limited to 400K. Within this temperature range no paraelectric behavior in PVmeasurements and therewith no characteristic permittivitysignatures indicating the Curie temperature were observed.Nevertheless, the characterization of the Curie temperature aswell as its frequency dispersion would be of great value for adeeper understanding on the nature of the underlying phasetransitions.Ferroelectricity originating from a rarely witnessed, non-

centrosymmetric modification of HfO2 and ZrO2, namely, theorthorhombic Pbc21 phase, seems reasonable and has proven tobe highly reproducible independent of the system investigated.Especially the ferroelectric transition in the ZrO2−HfO2 solidsolution described in this work underlies this assumption, sincedue to the equally sized and completely mixable cations noeffect on the oxygen vacancy density is expected and therewithon ionic conductivity. Even though we believe that true,structure-related ferroelectricity is also responsible for thenonlinear dielectric characteristics in the trivalent andtetravalent doped HfO2 systems we reported earlier, thecontribution of ionic conduction in these undersized andoversized substituted systems might not be negligible. Recentexperiments using electrochemical strain microscopy revealedthat the local O-vacancy distribution, density, and mobility canbe precisely controlled by an electrical field yielding a surprisingelectromechanical responds in otherwise nonpiezoelectricsystems like, for example, yttria-stabilized ZrO2.

46 The authorsdo not want to rule out the significant involvement of O-vacancies to the observed phenomena, especially since thestability of those defects are believed to play an important rolein the stabilization of the individual phases in HfO2 and ZrO2.

47

Nevertheless, given the structural data and the compositionaleffects observed, we believe that the ever-present O-vacanciesmight be actively involved but are unlikely the root cause forthe formation of the polarization in those systems.On the other hand, a true antiferroelectric nature of pure

ZrO2 and its origin in the tetragonal phase remainsquestionable. Nevertheless, our earlier work indicates that theappearance of this field-driven transition is closely linked to theappearance of the tetragonal phase. This antiferroelectric-likebehavior was observed in Si-24 and Al-doped-27 HfO2 thin filmsas well. Both systems are stabilized into the tetragonal P42nmcstructure with sufficient dopant content. The same is true forthe pure ZrO2 layers investigated in this work. Onlyferroelectric, Y-doped HfO2

26 did not exhibit such a field-driven transition and as proven by experiments48 as well asfrom a theoretical point of view47 favors the cubic structureover the tetragonal at high doping levels. In the context of thetetragonal to orthorhombic phase transition described earlier,this leads to the assumption that this transition is not onlytemperature-dependent but can further be induced by anelectric field alone. However, without further experiments asalready suggested earlier, the possibility of a relaxor ferro-

electric, resulting in equally behaved antiferroelectric-likehysteresis loops, cannot be ruled out.49,50

In conclusion, a composition- and temperature-dependentferroelectric phase transition was observed in thin films of theHfO2−ZrO2 solid solution. Hf-rich samples exhibited apredominantly monoclinic structure, whereas Zr-rich sampleswere crystallized into a tetragonal phase. Ferroelectricityappeared in the mixed oxide and was attributed to a tetragonalto orthorhombic transformation during cooling resulting in anoncentrosymmetric, orthorhombic phase of the space groupPbc21. A field-driven and temperature dependent ferroelectrictransition was observed in pure ZrO2 and in the Zr-rich part ofthe phase diagram suggesting a high reversibility of thistetragonal to orthorhombic transformation. These findings shednew light on ZrO2, a dielectric material extensively used inDRAM storage nodes and embedded capacitors, expected toexhibit linear dielectric functionality.

■ ASSOCIATED CONTENT*S Supporting InformationDetailed description of characterization methods and samplepreparation, GI-XRD measurements of the full HfO2−ZrO2composition range, and supporting low and high temperaturePV-hysteresis and CV measurements of the HfO2−ZrO2system. This material is available free of charge via the Internetat http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: johannes.mueller@ieee.org.Present Addresses¶Bosch Solar Energy AG, Erfurt, Germany▽Aixtron, Herzogenrath, GermanyNotesThe authors declare no competing financial interest.T.B. and U.S. were with Qimonda Dresden during the initialstage of the work.

■ ACKNOWLEDGMENTSWe would like to thank Marcus Mildner for preparing the HR-TEM micrographs. The work for this Letter was supportedwithin the scope of technology development by the EFRE fundof the European Community and by funding of the Free Stateof Saxony (Project MERLIN). The authors are responsible forthe content of the paper.

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Nano Letters Letter

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