Distinguishing the Rhombohedral Phase from Orthorhombic Phases in Epitaxial Doped HfO2 Ferroelectric Films

Dis inguishing he Rhombohed al Phase om O ho hombic Phases in Epi axial Doped H O2Fe oelec ic Films Ad ian Pe a u,*Ole G onenbe g, Ul ich Schu mann, Lo enz Kienle, Ra i D oopad, and He mann Kohls ed Ci e This: ACS Appl. Ma e . In e aces 2024, 16, 42534−42545 Read Online ACCESS Me ics & Mo e A icle Recommenda ions * sı Suppo ing In o ma ion ABSTRACT: Epi axial s ain plays an impo an ole in he s abiliza ion o e oelec ici y in doped ha nia hin ilms, which a e eme ging candida es o Si-compa ible nanoscale de ices. He e, we epo on epi axial e oelec ic hin ilms o doped H O2 deposi ed on La0.7S 0.3MnO3-bu e ed S TiO3subs a es, La0.7S 0.3MnO3S TiO3-bu e ed Si (100) wa e s, and igonal Al2O3subs a es. The in es iga ed ilms appea o consis o ou domains in a hombohed al phase o ilms deposi ed on La0.7S 0.3MnO3-bu e ed S TiO3subs a es and wo domains o hose deposi ed on sapphi e. These indings a e suppo ed by ex ensi e ansmission elec on mic oscopy cha ac e iza ion o he in es iga ed ilms. The doped ha nia ilms show e oelec ic beha io wi h a emanen pola iza ion up o 25 μC/cm2and hey do no equi e wake-up cycling o each he pola iza ion, unlike he epo ed polyc ys alline o ho hombic e oelec ic ha nia ilms. KEYWORDS: e oelec ic H O2, ul a hin ilm epi axy, ha nia, epi axial H 0.5Z 0.5O2 hin ilms, Y-doped e oelec ic H O2. 1. INTRODUCTION Fe oelec ic ma e ials a e o inc easing echnological impo ance ha ing con inuously ex ending applica ions includ- ing e oelec ic andom-access memo ies 1,2 o s o age, senso s and ac ua o s, 3,4 op ics, 5,6 and elec onic de ices o neu o-inspi ed elec onics such as mem is o s, necessa y o building up elec onic synapses o neu omo phic compu - ing. 7−9 Recen ly disco e ed e oelec ici y in doped ha nia ul a hin ilms opens new pe spec i es o he ealiza ion o special de ices like e oelec ic ield-e ec ansis o s and e oelec ic unnel junc ions 10,11 due o hei compa ibili y wi h he Si echnology. Single-c ys al Y-doped H O2 e oelec ic ilms deposi ed on y ium-s abilized zi conia ha e al eady been epo ed, exhibi ing he o ho hombic pola phase wi h he space g oup Pca21(o-phase) and a pola iza ion o 16 μC/cm2, gene ally conside ed esponsible o e oelec ici y in hin doped ha nia ilms. 12 On he o he hand, epi axial H 0.5Z 0.5O2 ilms deposi ed on La0.7S 0.3MnO3/S TiO3(subs a e) by pulsed lase deposi ion (PLD) ha e ecen ly been e- po ed. 13,14 These ilms do no c ys allize in he commonly epo ed o-phase, bu in a pola hombohed al phase ( -phase), s abilized ia epi axial s ain, and was iden i ied wi h a la ge P o 34 μC/cm2. Mo eo e , hese hombohed al ilms do no equi e “wake-up” cycling o es ablishing e oelec ic swi ch- ing unlike mos o he epo ed polyc ys alline ilms wi h an o ho hombic pola phase. Recen ly, polyc ys alline H - (Z )1+xO2c ys allizing in a hombohed al e oelec ic phase wi h a low coe ci e ield ha e been epo ed. 15 Meanwhile, epi axial doped ha nia ilms we e epo ed by some au ho s, being deposi ed on se e al single c ys al subs a es like y ia- s abilized zi conia (111)YSZ, (100)YSZ, 16 LSMO-bu e ed LaAlO3, 14,17,18 S TiO3, 13,14,18,19 NdScO3, NdGaO3, MgO, 18 YAlO3, (LaAlO3)0.3−(S 2AlTaO6)0.7 (LSAT), DyScO3, 14,18 S TiO3-bu e ed Si, 20 and GaN-bu e ed Si. 21 An in e es ing s udy on epi axial doped ha nia ilms deposi ed on a ious La0.7S 0.3MnO3(LSMO)-bu e ed subs a es e eals ha TbScO3and GdScO3a e e y good candida es o epi axial s ess s abiliza ion o he e oelec ic phase. 18 In mos o hese s udies, he s ess-s abilized me as able pola o ho hombic phase (Pca21) was epo ed o be esponsible o he e oelec ici y in doped ha nia ilms, 12,17,19,20,22−26 whe eas he hombohed al (space g oups R3mo R3) phases a e less epo ed. The eason migh be he ac ha he o ho hombic phase is ha d o dis inguish om he hombohed al phase. In his a icle, we ocus on he challenge o dis inguishing be ween hese phases. Recei ed: June 24, 2024 Re ised: July 23, 2024 Accep ed: July 28, 2024 Published: Augus 5, 2024 Resea ch A iclewww.acsami.o g © 2024 The Au ho s. Published by Ame ican Chemical Socie y 42534 h ps://doi.o g/10.1021/acsami.4c10423 ACS Appl. Ma e . In e aces 2024, 16, 42534−42545 This a icle is licensed unde CC-BY 4.0 Downloaded ia CHRISTIAN-ALBRECHTS-UNIV KIEL on Sep embe 9, 2024 a 12:30:29 (UTC). See h ps://pubs.acs.o g/sha ingguidelines o op ions on how o legi ima ely sha e published a icles. Apa om he epi axial ha nia ilms deposi ed on sapphi e, ITO-bu e ed YSZ, and GaN, only epi axial ilms deposi ed on LSMO-bu e ed subs a es ha e been epo ed o be e o- elec ic. A s udy on o he bu e laye s like LaNiO3, La0.5Ca0.5MnO3, S RuO3, and Ba0.95La0.05SnO3showed a low o no s abilized e oelec ic phase, excep o hose wi h mangani e elec o- des. 27 In addi ion o he epi axial s ain imposed by he subs a e, ac o s such as deposi ion empe a u e, pa ial gas p essu e, and ilm composi ion also play an impo an ole in de e mining he o ma ion o c ys al phases. Fo example, Kaise e al. epo ed ha ei he a monoclinic o a hombohed al phase is s able in H O2 hin ilms g own by molecula beam epi axy on c-cu sapphi e depending on he oxygen pa ial p essu e. DFT simula ions ha e shown ha he hombohed al phase is s abilized by oxygen acancies, no due o he epi axial s ain. 28 Fu he mo e, i was discussed ha a zi conium subs i u ion may ha e he same e ec . Supe la ice and laye s uc u es p o ide a u he imp o e- men o he e oelec ic pe o mance o ha nia-based ilms. Thus, “wake-up” ee e oelec ic capaci o s based on H O2/ Z O2supe la ices 29 o capaci o s wi h imp o ed e oelec ic and dielec ic pe o mances ha e been epo ed. 30,31 In his wo k, he ocus will be on e oelec ic epi axial doped ha nia ilms deposi ed on h ee di e en subs a es o compa e hei s uc u al p ope ies and o de e mine hei c ys al s uc u e. Thus, Z /Y:H O2/La0.7S 0.3MnO3/S TiO3 (100), Z /Y:H O2/La0.7S 0.3MnO3/S TiO3/Si (100), and Z / Y:H O2/Al2O3(0001) sys ems a e in es iga ed he e. The s uc u al cha ac e iza ion o each o hese h ee sys ems (comp ising XRD and TEM esul s) is p esen ed in a sepa a e subsec ion, and he elec ical cha ac e iza ion is shown in he las sec ion o he esul s. 2. EXPERIMENTAL METHODS 2.1. Ma e ials. H 0.5Z 0.5O2and H 0.93Y0.07O2 hin ilms o hicknesses a ying om 2.5 o 12 nm we e deposi ed by pulsed lase deposi ion (PLD) on La0.7S 0.3MnO3-bu e ed S TiO3(001) (STO) subs a es, La0.7S 0.3MnO3-bu e ed Nb (0.5%)-doped S TiO3 (001), S TiO3-bu e ed Si subs a es, and sapphi e (0001) subs a es. The doped H O2and he LSMO ilms we e deposi ed in a single p ocess wi hou b eaking he acuum. The H 0.5Z 0.5O2-and H 0.93Y0.07O2-sin e ed ce amic a ge s we e pu chased om EVO- CHEM whe eas he LSMO a ge was pu chased om P axAi . A K F excime lase o 248 nm in wa eleng h was used o abla ion in a comme cial PLD sys em om Su ace GmbH. The La0.7S 0.3MnO3 ilms we e deposi ed a a subs a e empe a u e o 780 °C, unde 0.15 mba O2, and a a lase luence and equency o 1.4 J/cm2, and 2 Hz, espec i ely. The doped H O2 ilms we e deposi ed a a subs a e empe a u e o 800 °C, unde 1.5 ×10−2mba O2, and a a lase luence o 1.4 J/cm2and a equency o 5 Hz. A e deposi ion, he ilms we e cooled a 5 C/min o oom empe a u e unde 3 mba o oxygen p essu e. Epi axial 20 nm S TiO3bu e laye s we e deposi ed on Si(100) subs a es by molecula beam epi axy (MBE) a Texas S a e Uni e si y. The epi axial oxide g ow h on silicon was achie ed using a codeposi ion p ocess in which bo h he alkaline ea h me al and he Ti shu e s we e opened in a con olled oxygen en i onmen . Since, unde hese g ow h condi ions, he s icking coe icien o he indi idual elemen s is uni y, ca e ul calib a ion o he luxes was pe o med o s oichiome ic oxide ilms. The g ow h a e used was app oxima ely 2A/min and was calib a ed using RHEED in ensi y oscilla ions. 2.2. XRD Cha ac e iza ion. The XRD measu emen s including wide- ange ecip ocal space maps (RSMs) and pole igu es we e acqui ed wi h a Sma Lab di ac ome e (Rigaku) equipped wi h a 9 kW Cu anode X- ay ube and a 2D HyPix-3000 X- ay de ec o . A wo- bounce monoch oma o was used o high- esolu ion XRD scans. 2.3. TEM Sample P epa a ion and Analysis. C oss-sec ional TEM samples we e p epa ed om he S TiO3and Al2O3subs a es using ocused ion beam (FIB) milling in an FEI Helios Nanolab sys em. Be o e Ga-ion e ching, a p o ec i e P coa ing was deposi ed wi h a gas injec ion sou ce inside he FIB sys em. A plan- iew sample on a Si subs a e wi h epi axial S TiO3and LSMO laye s was p epa ed wi h a P ecision Ion Polishing Sys em (PIPS, Model 691 om Ga an Inc.). The FIB samples we e analyzed using a JEOL JEM-2100 wi h elec ons accele a ed o 200 kV ex ac ed om a LaB6 ca hode, while he PIPS sample was analyzed wi h an FEI Tecnai F30 G2a 300 kV and a ield emission gun. 2.4. De ice Fab ica ion. Fe oelec ic capaci o s we e ab ica ed by deposi ing Cu op elec odes by he mal e apo a ion h ough a s encil mask. The de ice size a ies om 25 o 225 μm2. O he e oelec ic capaci o s on Si subs a es we e pa e ned by using UV op ical li hog aphy and A +ion beam e ching. 2.5. Elec ical Cha ac e iza ion. The e oelec ic hys e esis loops (P−Vloops) o he e oelec ic capaci o s we e measu ed by using a Radian Technologies P emie II e oelec ic es e . The es signal used he e consis s o a linea amp wa e o m wi h a pe iod in he ange o 0.1 o 150 ms. Addi ional e oelec ic hys e esis loops we e measu ed wi h an aixACCT TF Analyze 3000. 3. RESULTS AND DISCUSSION 3.1. S uc u al Cha ac e iza ion. 3.1.1. H 0.5Z 0.5O2 (HZO) Films Deposi ed on LSMO-Bu e ed (001)-O ien ed Nb:STO Subs a es. 3.1.1.1. X- ay Di ac ion Analysis. An X- ay di ac ion (XRD) scan o a H 0.5Z 0.5O2(HZO) ilm on LSMO-bu e ed (001)-o ien ed Nb:STO subs a es is depic ed in Figu e 1a. The specula e lec ions 001, 002, and 003 o he Nb:STO subs a e a e he mos in ense ones, ollowed by he specula e lec ions o he epi axial LSMO ilm. The 003 and 006 e lec ions o he HZO ilm a e also p esen . Figu e 1b shows he magni ied 003 HZO egion, whe e he hickness oscilla ions a e clea ly isible, demons a ing good c ys alline Figu e 1. (a) The XRD pa e n o he H 0.5Z 0.5O2/La0.7S 0.3MnO3/ S TiO3(subs a e) he e os uc u e; (b) he de ailed H 0.5Z 0.5O2003 egion wi h hickness oscilla ions; he blue line ep esen s he expe imen al da a, and he ed line is he simula ion o he di ac og am showing a ilm hickness o 8 nm. (c) Simula ion o he La0.7S 0.3MnO3001 egion ( ed line), he ilm hickness is ound o be abou 71 uni cells, i.e., 27 nm. (d) Rocking cu e o he H 0.5Z 0.5O2003 e lec ion; FWHM = 0.032°. ACS Applied Ma e ials & In e aces www.acsami.o g Resea ch A icle h ps://doi.o g/10.1021/acsami.4c10423 ACS Appl. Ma e . In e aces 2024, 16, 42534−42545 42535 quali y and well-de ined in e aces. An a e age HZO ilm hickness o 8 nm is es ima ed om he i o he 003 e lec ion egion, using he o mula o he heo e ical di ac og am, 32 adap ed o in e plana spacing: I n( ) sin( d sin / )/sin( d sin )/ 2 [ ] , whe e dis he in e plana spacing o he XRD peak in ol ed ( he planes a e Figu e 2. (a) Wide ange RSM o he HZO/LSMO/STO (subs a e) he e os uc u e. The spo s belonging o LSMO/STO a e deno ed in da k blue, whe eas he spo s om he HZO ilm a e deno ed in ed. The angles ma ked nex o he HZO spo s ep esen he in-plane o a ion o he co esponding HZO domains; he hkl Mille indices a e assigned he e o he R3mphase. (b) Pole igu e o he HZO {201} and HZO {022} spo s; he black, ed, blue, and g een a ows indica e he ou domains wi h he amilies o spo s associa ed wi h hem. Pole igu e simula ion o he HZO ilms: (c) phase R3m, conside ing (001) ou -o -plane o ien a ion and p esence o ou domains, wi h an in-plane o a ion o 90°wi h espec o one ano he ; (d) phase Pca21conside ing (111) ou -o -plane o ien a ion and p esence o ou domains, wi h an in-plane o a ion o 90°wi h espec o one ano he , and he p esence o {200}, {020}, and {002} spo s. Figu e 3. HZO/LSMO/STO (subs a e) he e os uc u e. (a) 2θscans o he 12 poles a chi = 71.1°and o he specula spo (chi = 0°in black). 2θ scans o he 12 poles a (b) chi = 55.7°and (c) a chi = 35°. ACS Applied Ma e ials & In e aces www.acsami.o g Resea ch A icle h ps://doi.o g/10.1021/acsami.4c10423 ACS Appl. Ma e . In e aces 2024, 16, 42534−42545 42536 pa allel o he sample su ace), θis i s co esponding di ac ion angle, λis he X- ay wa eleng h, and nis he ilm hickness in he numbe o in e plana spacing. The heo e ical di ac og am is plo ed wi h a ed line as shown in Figu e 1b. A simila analysis o he LSMO ilm e ealed an LSMO ilm hickness o 27.4 nm, as illus a ed in Figu e 1c. The ocking cu e o he H 0.5Z 0.5O2003 e lec ion illus a ed in Figu e 1d shows a FWHM o 0.032°, indica ing a good c ys alline quali y o hese ilms. Nex , wide- ange ecip ocal space maps (RSMs) and pole igu es we e measu ed o he same s ack o de e mine he c ys allog aphic phase and he o ien a ion o he HZO ilms. Thus, in he ecip ocal space map o he H 0.5Z 0.5O2/ La0.7S 0.3MnO3/S TiO3(subs a e) he e os uc u es shown in Figu e 2a, he subs a e e lec ions 001, 002, and 003 a e iden i ied and labeled in da k blue colo on he RSM. The ecip ocal space spo s co esponding o he epi axial LSMO laye could no be dis inguished om hose o he STO subs a e, due o lack o esolu ion in he wide ange RSM measu emen s. He e, we like o speci y ha he sample was on pu pose in-plane o a ed by an angle φ= 15°(wi h espec o he in-plane a-di ec ion o he STO subs a e) o a oid he con ibu ion o nonspecula spo s o he STO/LSMO). Fo mo e cla i y, a wide- ange RSM o he same sample measu ed a φ= 45°is shown in Figu e S1, whe e he 111, 112, 113, and 221 spo s o he STO/LSMO a e also p esen . The HZO ilms g ow epi axially ou -o -plane on he LSMO/STO empla e, and he ilms appea o consis o ou kinds o domains, belonging o he same c ys allog aphic R3mphase, i s epo ed by Wei e al., 13 ha ing ou kinds o in-plane o ien a ions, o a ed by 90°wi h espec o one ano he . A simila domain s uc u e was epo ed by Nukala e al. o such he e os uc u e. 14 The eason o his domain con igu a ion could be explained by he 4- old symme y imposed by he subs a e, as also desc ibed by Es andi a e al. 18 and obse ed in he e oepi axy o oxides 33 o semiconduc o s. 34 The esul s a e illus a ed in Figu e 2a. The angles ma ked nex o he HZO spo s in he wide- ange RSM ep esen he in-plane o a ion o he co esponding HZO domains. F om he 022 HZO spo , we ex ac ed he la ice pa ame e alues o a= 7.17 and c= Figu e 4. (a) HRTEM mic og aph showing an LSMO back elec ode in ZA [ 54 2] g owing along (012) planes. An FFT analysis shown below in (b) and (c) was pe o med on he wo HZO egions ma ked wi h ci cles. The le g ain is in [100] ZA, and he igh FFT shows a supe posi ion o a [100] and a [ 1 00] ZA which could be explained by a win wi h a 2- old o a ion a he [003] di ec ion. These domains a e indica ed by yellow and ed ec angles. The o ange ellipse in he le FFT in (b) ma ks he addi ional [ 2 22] mino i y g owing di ec ions o he main [003] g owing di ec ion. The SAED pa e n in (d) is simila o ha o FFT in (c). Figu e 5. (a) Wide- ange RSM o a 5.6 nm hick HYO ilm deposi ed on La0.7S 0.3MnO3/S TiO3/Si (subs a e); he e, he obse ed spo s om he HYO ilm a e assigned o (001)-o ien ed ilms o he e oelec ic R3mphase, consis ing o ou ypes o domains wi h 0°, 90°, 180°, and 270°in- plane o ien a ion. The measu emen was acqui ed a φ= 0°. (b) Pole igu e o he HYO {022}, HYO {201}, STO/LSMO {011}, and Si {111} e lec ions; he yellow, ed, whi e, and g een a ows indica e he ou domains wi h he amilies o spo s associa ed wi h hem. The measu ed 2θ ange is 24.9−37.4°. The adial di ec ion ep esen s he χaxis anging om 0° o 90°, whe eas he azimu hal di ec ion ep esen s he φaxis wi h a ange om 0° o 360°. ACS Applied Ma e ials & In e aces www.acsami.o g Resea ch A icle h ps://doi.o g/10.1021/acsami.4c10423 ACS Appl. Ma e . In e aces 2024, 16, 42534−42545 42537 8.82 Å. Mo e p ecise alues will be ex ac ed om he high- esolu ion in-plane and ou -o -plane scans. A pole igu e o he HZO/LSMO/STO (subs a e) he e os uc u e is shown in Figu e 2b. The adial di ec ion ep esen s χ, which anges be ween 51°and 85°, while he azimu hal di ec ion ep esen s φ, which anges be ween 0°and 360°. Please no e ha he pole igu e p esen ed he e is no measu ed o a single e lec ion bu o a wo- he a ange be ween 23°and 37°. Thus, he HZO {201} and HZO {022} amilies o e lec ions a e seen he e. The 12 poles o he HZO {201}, measu ed a a 2θangle o 30.5°and he 12 poles o he HZO {022} measu ed a a 2θangle o 35.6°a e clea ly isible in he pole igu e. F om he c ys al symme y o he R3m phase, one expec s h ee poles o each, spaced by 120°, bu he p esence o ou kinds o c ys allog aphic domains o a ed 90°wi h espec o one ano he would gi e us he obse ed 12 poles. To ep oduce he measu ed da a, we conside ed ou domains o he R3mphase ha ing he (001) o ien a ion and an in-plane o a ion o 90°wi h espec o one ano he and simula ed he pole igu e using he MTEX 35 simula ion ool. The simula ed pole igu e is p esen ed in Figu e 2c. As can be seen, bo h {201} (a chi = 71.15°) and {022} (a chi = 55.7°) spo amilies could be ep oduced in he simula ion o he R3m phase. I one conside s he o ho hombic Pca21phase wi h a single (111)-o ien ed domain, h ee poles o a ed by 120°a e expec ed o he HZO{ 1 1 1}, om he c ys al symme y, as simula ed wi h MTEX and also expe imen ally epo ed, 18 and one pole o each o he {020}, {200}, and {002} e lec ions. The HZO{ 1 1 1} o he Pca21phase would be he co esponding HZO{201} spo s o he R3mphase, and he HZO{020} o he Pca21phase would be he co esponding HZO{022} spo s o he R3mphase. Fu he mo e, i one conside s (111) g own ilms consis ing o ou domains ha ing ou in-plane o ien a ions o a ed 90°wi h espec o one ano he and simula es he pole igu e, one can ep oduce he 12 spo s co esponding o HZO { 1 1 1}, as shown in he simula ed pole igu e om Figu e 2d. To enhance cla i y, he poles om each domain in he simula ion om Figu es 2c,d a e indica ed wi h an a ow o a speci ic colo . Thus, we ha e black, ed, blue, and g een a ows o indica e he ou domains. Addi ionally, 12 poles a e expec ed a a wo- he a angle o abou 35.6°, each o he {020}, {200}, and {002} e lec ions con ibu ing wi h ou spo s in he simula ion, sepa a ed by a 90°angle in phi. The co esponding wo- he a angles o he nonsymme y equi alen 020, 002, and 200 e lec ions o he Pca21phase a e close o each o he . Thus, i is di icul o dis inguish be ween he R3mand Pca21phases based on he (low esolu ion) wide- ange RSM and pole igu es. One way o dis inguish he R3mphase om he Pca21phase is o compa e he in e plana spacing o he 12 spo s measu ed a a chi angle o 71.15°wi h he in e plana dis ance o he specula 111 spo o he Pca21phase. In he case o he Pca21phase, all spo s should ha e he same d-spacing (due o he symme y), whe eas o he R3mphase, he specula spo ( he 003 e lec ion) should ha e a la ge d-spacing alue. 13,14 Symme ic wo- he a scans o he 12 poles measu ed a chi = 71.15°a e plo ed oge he wi h he ou -o -plane e lec ion (chi = 0°) as shown in Figu e 3a. The 12 spo s sha e nea ly he same 2θ alues, whe eas he ou -o -plane e lec ion has a smalle 2θ (la ge d-spacing), which is an indica ion o he R3mphase. Ano he possibili y o dis inguish be ween he wo polymo phs is o measu e symme ic 2θscans a a ound 35° o he 12 spo s a a chi angle o 55.7°. In he case o he R3m phase, he {022} spo s should ha e he same 2θ alues, whe eas o he Pca21phase, he {020}, {200}, and {002}, each con ibu ing wi h ou spo s ( om he ou domains) sepa a ed by 90°in φwi h espec o one ano he (see Figu e 2d), should ha e sligh ly di e en 2θ alues. Simila ly, one can analyze he 12 spo s measu ed a chi = 35°a a 2θo abou 50°. Thus, o he R3mphase, he {204} spo s should show he same 2θ alues whe eas o he Pca21phase, he {022}, {202}, and {220} each con ibu ing wi h ou spo s sepa a ed by 90° in φwi h espec o one ano he should ha e sligh ly di e en 2θ alues. The symme ic wo- he a scans o he 12 poles measu ed a chi = 55.7°and a 2θ alue a ound 35°a e p esen ed in Figu e 3b, and he 12 poles measu ed a chi = 35° and a 2θ alue o a ound 50°a e shown in Figu e 3c. To imp o e he ease o unde s anding o he eade , he 2θscans o each {020}, {200}, and {002} as in Figu e 3b and {022}, {202}, and {220} e lec ions as in Figu e 3c co esponding o he Pca21phase a e depic ed using dis inc colo s: ed, g een, and blue. Thus, in he case o he Pca21phase, he e lec ions o he same colo (sepa a ed in phi by 90°) should sha e he same 2- he a alue and should be sligh ly di e en o each colo . When hese 2θ alues a e all equal, assuming he e oelec ic Pca21phase, would imply a=b=c, which con adic s he epo ed me ics o his c ys allog aphic phase, making i unlikely ha he in es iga ed ilms a e in he Pca21 phase. These esul s a e ins ead in good ag eemen wi h he me ics o he R3mphase. Howe e , he exis ence o a sepa a e Figu e 6. (a) Plan- iew SAED pa e n o Si/STO/LSMO/HYO. The subs a e Si is o ien ed in he [100] zone axis as well as he STO which is o a ed by 45°wi h espec o Si. HYO shows mainly wo ypes o e lec ions. The ed ci cles depic he HYO [001] zone axis wi h {220} e lec ions aligned wi h he STO (101) planes ( ed a ow) while he e lec ions ma ked by he g een ci cles a e aligned wi h he STO (110) planes (g een a ow). (b) shows a high- esolu ion TEM mic og aph in he same iewing di ec ion which is con i med by he FFT in (c) showing he same di ac ion pa e n as in a). In (c), he g een, ed, and yellow ci cles we e used o il e ing he FFT and building a colo ed in e se FFT shown in (d). The majo i y o he domains ha e an [003] ou -o -plane o ien a ion wi h {220} e lec ions ma ked by ed and g een ci cles. The mino i y (yellow) is o ien ed wi h STO (200). ACS Applied Ma e ials & In e aces www.acsami.o g Resea ch A icle h ps://doi.o g/10.1021/acsami.4c10423 ACS Appl. Ma e . In e aces 2024, 16, 42534−42545 42538 e oelec ic hombohed al phase which is no jus a s uc u al dis o ion caused by epi axial s ess dis o ion o he Pca21 o ho hombic phase is s ill unde deba e, as sugges ed by Fina and Sanchez. 36 To gain a clea e unde s anding o how he scans om Figu e 3c ela e o pa icula e lec ions and domains, simula ed pole igu es o bo h R3mand Pca21phases o he 12 spo s a chi = 35°and a 2θo abou 50°a e shown in Figu e S5. 3.1.1.2. T ansmission Elec on Mic oscopy Cha ac e - iza ion. The same HZO ilm on he LSMO-bu e ed (001)- o ien ed Nb:STO subs a e analyzed by XRD was used o TEM cha ac e iza ion. The HRTEM mic og aph in Figu e 4a shows he LSMO back elec ode g owing along (012) planes while he HZO hin ilm g ows mainly in he [003] di ec ion o he R3mphase. Howe e , a mino i y g owing di ec ion along [ 2 22] was also obse ed (ma ked wi h an o ange ellipse in he FFT in Figu e 4b, as also epo ed by Wei e al). 13 This mino i y g ow h di ec ion is no obse ed in XRD measu e- men s. Di e en domains o he HZO we e p esen wi h (02 2 ) planes o ien ed o he igh o he le , hese planes a e indica ed by a black and a ed a ow, espec i ely. F equen ly bo h o ien a ions supe impose, as can be seen in he FFT in Figu e 4c. This pa e n can be explained by a 2- old win o a ion a ound he [003] g owing di ec ion. Acco dingly, we can see he e a leas wo o he ou domains obse ed in XRD. The missing wo domains canno be dis inguished in his o ien a ion because hey ha e he same di ac ion pa e n. Fo ins ance, he [110] zone axis (ZA) is equi alen o he [100] ZA in a hombohed al symme y. A clea iew o he bounda y o he domains, howe e , was no ound in many mic og aphs examined. The selec ed a ea elec on di ac ion (SAED) pa e n in Figu e 4d was aligned pa allel o he [100] ZA o he STO. In Figu e 7. (a) High- esolu ion 2θ/ωscan o a 4.7 nm hick HZO ilm deposi ed on sapphi e. (b) Wide ange RSM o he same ilm on sapphi e. The spo s belonging o sapphi e a e deno ed in black, whe eas he spo s om he HZO ilm a e deno ed in ed. The angles deno ed nex o he HZO spo s ep esen he in-plane o a ion o he co esponding HZO domains assigned o he R3mphase. The sapphi e subs a e was o a ed by phi = 30°du ing he RSM measu emen . (c) Pole igu e o he HZO {201}, HZO {022}, sapphi e {10 2 } and sapphi e {104} spo s. The adial di ec ion ep esen s he χaxis anging om 0° o 90°, whe eas he azimu hal di ec ion ep esen s he φaxis wi h a ange om 0° o 360°. ACS Applied Ma e ials & In e aces www.acsami.o g Resea ch A icle h ps://doi.o g/10.1021/acsami.4c10423 ACS Appl. Ma e . In e aces 2024, 16, 42534−42545 42539 his o ien a ion, he LSMO is in he [ 2 2 1 ] ZA and he epi axial g ow h o all ilms is clea ly isible. The di ac ion pa e ns o STO and LSMO a e indica ed by black ho izon al lines. Fo HZO, he (003) e lec ions in he g owing di ec ion a e mos p ominen , while in-plane no e lec ions can be e idenced, and also he diagonal planes a e e y ain (ma ked wi h o ange ci cles). Fu he mo e, om his SAED pa e n, a di e ence in he d- alues o (006) and (042) can be obse ed, as illus a ed by he dashed ing a ound he (006) e lec ions. These e lec ions would co espond o he {222} planes o he e oelec ic o ho hombic phase, which canno ha e a d- alue di e ence (in a elaxed s uc u e) due o he o ho hombic symme y. An e ec o an ellip ical dis o ion o he di ac ion pa e n by unp ecise adjus men s o he p ojec o lens sys em can be Table 1. aand cLa ice Pa ame e s o he S ained Doped H O2Films Deposi ed on Di e en sys ems in his Wo k We e Ex ac ed om High-Resolu ion 2θ/ωScans and In- Plane Scans a he e os uc u e a(Å) c(Å) 8 nm HZO/LSMO/STO 7.194 8.899 7.7 nm HYO/LSMO/STO 7.170 8.953 7.6 nm HZO/LSMO/STO/Si 7.236 8.822 5.6 nm HYO/LSMO/STO/Si 7.212 8.883 6.2 nm HYO/Al2O37.207 8.879 2.6 nm HYO/Al2O37.239 8.773 4.7 nm HZO/Al2O37.219 8.805 H O2,R3m(no. 160); a=b;α= 90°;β= 90°;γ= 120° a om x y z occupancy H 0.83335 0.16665 0.25089 1 H 0.00000 0.00000 0.58415 1 O 0.14966 0.85034 0.15904 1 O 0.48699 0.51301 0.32788 1 O 0.00000 0.00000 0.85998 1 O 0.00000 0.00000 0.35364 1 a The c ys allog aphic s uc u e is he one epo ed by Wei e al. in e 13 adap ed o he la ice pa ame e s measu ed in his wo k. Figu e 8. Pole igu e simula ion o he HZO ilms using MTEX. (a) Phase R3m, conside ing (001) ou -o -plane o ien a ion and p esence o wo domains (indica ed by he ed and he blue a ows), wi h an in-plane angula o a ion o 180°wi h espec o one ano he . (b) Phase Pca21 conside ing (111) ou -o -plane o ien a ion, and he p esence o wo domains, wi h an in-plane angula o a ion o 180°wi h espec o one ano he . Figu e 9. In-plane XRD o he HYO(6.2 nm)/Al2O3: (a) in-plane scan showing he HYO(220) and Al2O3(300) e lec ions; he la ice pa ame e s and he FWHM a e gi en in he inse . (b) In-plane phi- scan o he HYO(220) egion showing a 6- old in-plane symme y; he FWHM is 4.1°. ACS Applied Ma e ials & In e aces www.acsami.o g Resea ch A icle h ps://doi.o g/10.1021/acsami.4c10423 ACS Appl. Ma e . In e aces 2024, 16, 42534−42545 42540 uled ou , because he e lec ions o he polyc ys alline P om he FIB p epa a ion a e no ellip ically dis o ed. Supe imposed a e he [100] ZA o STO and he [ 2 2 1 ] ZA o LSMO which a e illus a ed by black e ical lines. The e lec ions o HZO in he diagonal a e e y ain . The e o e, he ec angles om he FFT a e used he e double sized o illus a e he di ac ion pa e n o HZO. The ings in he SAED pa e n in (d) o igina e om he p o ec i e P coa ing used in he FIB. 3.1.2. H 0.93Y0.07O2(HYO) Films Deposi ed on S TiO3- Bu e ed Si (100)-O ien ed Subs a es. 3.1.2.1. X- ay Di ac ion Analysis. Y ium-doped Ha nia ilms H 0.93Y0.07O2(HYO) we e deposi ed on Si (100)-o ien ed subs a es on which a 10 nm S TiO3(STO) was deposi ed by molecula beam epi axy (MBE) a Texas S a e Uni e si y. 37,38 The H 0.93Y0.07O2/La0.7S 0.3MnO3epi axial s ack was deposi ed by PLD, using he same deposi ion pa ame e s as hose deposi ed on Nb:S TiO3subs a es. The wide- ange RSM o he doped ha nia ilms deposi ed on La0.7S 0.3MnO3/S TiO3/ Si (subs a e) shows ea u es simila o hose deposi ed on La0.7S 0.3MnO3/S TiO3(subs a e), an ou -o -plane o ien a- ion, and he p esence o ou kinds o domains wi h di e en in-plane o ien a ions, as desc ibed be o e. The wide- ange RSM was acqui ed a φ= 0°, and besides he specula Si(004), he Si(022) spo was isible in his di ac ion geome y. The nonspecula STO/LSMO di ac ion spo s 1 11, 1 12, and 2 21 a e also p esen . The esul s a e shown in Figu e 5a. A pole igu e o HYO{022}, HYO{201}, STO/LSMO{011}, and Si{111} measu ed o a 2θ anging om 34.8° o 47.8°is shown in Figu e 5b. The 12 spo s o he HYO{022} and HYO{201} a ising om he ou domains a e p esen on he pole igu e, simila o he esul s om doped ha nia ilms deposi ed on LSMO-bu e ed STO subs a es. 3.1.2.2. T ansmission Elec on Mic oscopy Cha ac e - iza ion. The TEM p epa a ion o plan- iew samples g own on he STO subs a es was no success ul because in 3 ials he samples b oke apa du ing mechanical polishing be o e inishing he p epa a ion. As a esul , a Z /Y:H O2sample on S TiO3-bu e ed Si was p epa ed. This sample should be Figu e 10. (a) HRTEM mic og aph o Al2O3in [210] ZA and HZO in [100] ZA. Two di e en domains we e ound ia FFT analysis in (b) and (c); again, he [100] ZA is mi o ed a he (003) plane. The SAED pa e n in (d) shows bo h domains, and he Al2O3[210] ZA is indica ed by e ical black lines. In his sys em, he (003) e lec ions a e a he weak compa ed o he STO/LSMO sys em, and he (060) e lec ion is isible in-plane. In FFTs e en he (030) planes. The di ac ion ings o igina e om he P coa ing which was used o p o ec du ing FIB p epa a ion. Figu e 11. Fe oelec ic cha ac e iza ion. Dynamic e oelec ic pola iza ion hys e esis loops o he (a) Cu/H 0.5Z 0.5O2/La0.7S 0.3MnO3/S TiO3, (b) Cu/H 0.93Y0.07O2/La0.7S 0.3MnO3/S TiO3/Si, and (c) Cu/H 0.93Y0.07O2/La0.7S 0.3MnO3/Nb:S TiO3capaci o s. (d) “Wake-up” e ec in es iga ion on a p is ine e oelec ic capaci o . ACS Applied Ma e ials & In e aces www.acsami.o g Resea ch A icle h ps://doi.o g/10.1021/acsami.4c10423 ACS Appl. Ma e . In e aces 2024, 16, 42534−42545 42541 ep esen a i e, as indica ed by he XRD analysis. A SAED pa e n o he plan- iew sample wi h Si o ien ed in he [100] ZA is shown in Figu e 6a. He e, he HYO o ms a simila di ac ion pa e n as epo ed by Wei e al. 13 wi h 12 {120} e lec ions which can be explained by HYO ha ing a leas wo domains o a ed in plane by 90°. The g een and ed ci cles in Figu e 6a indica e wo domains. The domains ma ked wi h g een ci cles a e o ien ed wi h he STO (110) planes while he domains ma ked wi h ed ci cles a e o ien ed wi h he STO (101) planes. Adding he 2- old win o a ion obse ed in he c oss-sec ional TEM analysis esul s in he ou domains obse ed in XRD pole igu es. These ou ypes o domains may o m du ing he c ys al nuclea ion du ing PLD, whe e each nucleus can be o ien ed wi h STO (110), (−110), (1− 10), o (−1−10). The same colo code is used in he FFT shown in Figu e 6c o he HRTEM mic og aph in shown Figu e 6b. These colo ed ci cles we e used o he il e ed in e se FFT in Figu e 6d which illus a es he g anula mic os uc u e o he HYO hin ilm. Howe e , all indi idual g ains a e epi axial. A TEM image in a di e en o ien a ion, il ed away om he [100] zone axis o Si, is shown in Figu e S4. He e, s ong Moi e inges appea which we e again used o a il e ed in e se FFT. F om hese alse colo images, he a e age wid h o 20 domains can be oughly es ima ed o be 10 nm ±3 nm. The high c ys allini y o he HZO, as indica ed by he ocking cu e, a gues o cohe en in e aces be ween hese 10 nm- sized domains as would be he case o wins. In addi ion o he obse ed majo i y e lec ions in he SAED pa e n shown in Figu e 6a om HYO wi h he [003] ou -o - plane o ien a ion, a mino i y is obse ed in he FFTs (see yellow ci cles in Figu e 6c and blue ci cles in Figu e S4b) ha is o ien ed wi h he STO (200) planes. 3.1.3. H 0.5Z 0.5O2Films Deposi ed on Al2O3(0001) Subs a es. 3.1.3.1. X- ay Di ac ion Analysis. Nex , Y- and Z -doped H O2 ilms we e deposi ed on igonal Al2O3(0001) single c ys al subs a es. A 5 nm hick H 0.5Z 0.5O2 ilm deposi ed on Al2O3(0001) is cha ac e ized by XRD expe imen s, as p esen ed in Figu e 7. The high- esolu ion ω-2θscan shown in Figu e 7a allowed o he p ecise measu emen o he cla ice pa ame e , lis ed in Table 1, and de e mina ion o he ilm hickness based on he hickness- oscilla ion pe iod, which in his case was 4.7 nm. A wide- ange RSM shown in Figu e 7b was measu ed a an in-plane o a ion angle o φ= 30°. Thus, in addi ion o he specula 006 spo , he 113, 116, and 119 spo s o he Al2O3subs a e a e obse ed a his in-plane phi angle. The HZO ilm appea s o consis o wo kinds o domains, belonging o he same -phase R3mno. 160, o a ed in-plane by 180°wi h espec o one ano he and by 30°wi h espec o he Al2O3c ys allog aphic cell. The co esponding angles o hese wo domains, which we e ob ained om he simula ion o he RSM a e deno ed nex o indexed HZO spo s in he igu e. The pole igu e o he same sample is p esen ed in Figu e 7c. The HZO {201}, HZO {022}, sapphi e {10 2 } and sapphi e {104} spo s om he la ice planes symme ically equi alen a e ma ked on he pole igu e. The 2θ anges om 25° o 37.8°. The sapphi e {10 2 } and {104} spo s e lec he igonal symme y o he phase, esul ing in h ee spo s on he pole igu e o each amily. The HZO {201} and HZO {022} spo s should also show h ee spo s each, acco ding o he c ys allog aphic symme y o he R3mphase, bu six spo s a e obse ed expe imen ally. This can be explained by he p esence o wo kinds o domains, belonging o he same igonal phase R3m, o a ed in-plane by 180°wi h espec o one ano he , as deduced o he wide- ange RSM simula ion discussed abo e. This domain con igu a ion has been epo ed in p io s udies on HZO ilms deposi ed on GaN and sapphi e, 14 also being imposed by he symme y o he subs a e. To e i y ha , a pole igu e o he HZO ilms was simula ed using MTEX, acco ding o he p oposed assump ion, and he esul s a e shown in Figu e 8a. The simula ion co esponds o he indings ob ained om he expe imen s. The simula ion o he Pca21phase o a (111) o ien ed monodomain ilm gi es h ee poles om he {1 1 1}, and one poles om each {020}, {002} and {200} e lec ion amilies. I one conside s wo (111) domains o a ed in-plane by 180° wi h espec o one ano he , he simula ions ep oduce he obse ed six {111} poles, as well as he six poles o he {020}, {002}, and {200} ones, as shown in Figu e 8b. Also, in his case, i is di icul o dis inguish be ween he R3mand Pca21 c ys allog aphic phases om he pole igu e XRD measu e- men s. Conside ing he ela i ely uni o m in ensi y dis ibu ion o he e lec ions co esponding o a speci ic amily o spo s, one can s a e ha he olume ac ion o he wo ypes o HZO domains appea s o be equally dis ibu ed. The measu ed wide- ange RSM o he HZO deposi ed on sapphi e could also be assigned o a e oelec ic o ho hombic phase like, o example, he one epo ed by Xu e al. 39 Wi h his assump ion, he ilms a e epi axial and ha e an (111) ou - o -plane o ien a ion. Such an example is illus a ed in Figu e S2, whe e he obse ed spo s in he RSM om he H O2 ilms deposi ed on sapphi e a e assigned o he abo e-men ioned c ys allog aphic phases. To p ecisely de e mine he in-plane la ice pa ame e s o he deposi ed ilms, we pe o med in-plane X- ay di ac ion measu emen s. Thus, in he case o doped ha nia ilms deposi ed on sapphi e, he HYO (220) and Al2O3(300) e lec ions we e obse ed, as can be seen in Figu e 9a. This means ha he (220) c ys allog aphic planes o he epi axially doped ha nia ilms a e pa allel wi h he (300) planes o he unde lying sapphi e subs a es and pe pendicula o he HYO (003) planes which a e pa allel wi h he sample su ace. The epi axial ela ionship in his case conside ing he R3m c ys allog aphic phase is [220] HZO(001) //[300] Al2O3(001). I one would conside he e oelec ic o ho- hombic phase Pca21ins ead o he igonal phase (R3m), he (111) c ys allog aphic plane is pa allel wi h he sample su ace, whe eas he (20 2 ) plane o ms an angle o 89.84°wi h he (111) plane, e y close o 90°. Fo his eason, based on he in- plane XRD measu emen s, one canno dis inguish be ween he e oelec ic o ho hombic phase and he hombohed al one. F om hese high- esolu ion in-plane scans, one can p ecisely de e mine he in-plane la ice pa ame e o he hombohed al HYO ilms deposi ed on sapphi e, which is 7.207 Å. Nex , he 2-The aChi/Phi axis was ixed a he HYO(220) peak posi ion, and a phi-scan was pe o med, wi h a comple e 360°phi-axis scan ange. The 3- old in-plane symme y expec ed o he hombohed al ilms combined wi h he p esence o he wo 180°in-plane o a ed domains is consis en wi h he obse ed six e lec ions spaced by 60° om his scan and wi h he esul s om he pole igu es o he HZO ilms deposi ed on sapphi e, discussed abo e. The in-plane phi scan shown in Figu e 9b is equi alen o an in-plane ocking cu e, and he wid h o he e lec ions gi es us a measu e o he in-plane mosaici y. In he ACS Applied Ma e ials & In e aces www.acsami.o g Resea ch A icle h ps://doi.o g/10.1021/acsami.4c10423 ACS Appl. Ma e . In e aces 2024, 16, 42534−42545 42542