from zirconia to yttria sampling the ysz phase diagram using sputter deposited thin films

The films were analyzed usingtransmission electron microscopy, revealing that, while the films with 8 mol% and 20 mol% yttria retain their crystal structure from the bulk compound tetrag

thin films

Thomas Götsch, Wolfgang Wallisch, Michael Stöger-Pollach, Bernhard Klötzer, and Simon Penner,

Citation: AIP Advances 6, 025119 (2016); doi: 10.1063/1.4942818

View online: http://dx.doi.org/10.1063/1.4942818

View Table of Contents: http://aip.scitation.org/toc/adv/6/2

Published by the American Institute of Physics

From zirconia to yttria: Sampling the YSZ phase diagram using sputter-deposited thin films

Thomas Götsch,1Wolfgang Wallisch,2Michael Stöger-Pollach,2

Bernhard Klötzer,1and Simon Penner1, a

1Institute of Physical Chemistry, University of Innsbruck, Innrain 80/82,

A-6020 Innsbruck, Austria

2University Service Center for Transmission Electron Microscopy (USTEM), Vienna

University of Technology, Wiedner Hauptstraße 8–10, A-1040 Vienna, Austria

(Received 30 November 2015; accepted 12 February 2016; published online 22 February 2016)

Yttria-stabilized zirconia (YSZ) thin films with varying composition between 3 mol%and 40 mol% have been prepared by direct-current ion beam sputtering at a substratetemperature of 300◦C, with ideal transfer of the stoichiometry from the target tothe thin film and a high degree of homogeneity, as determined by X-ray photo-electron and energy-dispersive X-ray spectroscopy The films were analyzed usingtransmission electron microscopy, revealing that, while the films with 8 mol% and

20 mol% yttria retain their crystal structure from the bulk compound (tetragonaland cubic, respectively), those with 3 mol% and 40 mol% Y2O3undergo a phasetransition upon sputtering (from a tetragonal/monoclinic mixture to purely tetragonalYSZ, and from a rhombohedral structure to a cubic one, respectively) Selectedarea electron diffraction shows a strong texturing for the three samples with loweryttria-content, while the one with 40 mol% Y2O3 is fully disordered, owing tothe phase transition Additionally, AFM topology images show somewhat similarstructures up to 20 mol% yttria, while the specimen with the highest amount ofdopant features a lower roughness In order to facilitate the discussion of thephases present for each sample, a thorough review of previously published phasediagrams is presented C 2016 Author(s) All article content, except where other-wise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4942818]

However, the influence the amount of yttria in the solid solution on the relevant properties,such as crystal structure and conductivity is often neglected By doping zirconium oxide, ZrO2, withyttrium oxide, Y2O3, a tetravalent ion (Zr4 +) is substituted by a trivalent one (Y3 +) Due to charge

neutralization, oxygen vacancies are formed, increasing the ionic conductivity These vacancies alsoplay a role in stabilizing the often desired tetragonal or cubic structures.7

YSZ has been proposed to exist in various crystal structures, some of which are shown inFigure1 In Figure1(a), monoclinic ZrO2/YSZ is shown (for YSZ, the only difference is that some

of the Zr are substituted by Y and some O are missing, and that the lattice parameters will thusdiffer).8In this structure, edge-sharing polyhedra are formed by seven-fold coordinated Zr atoms.For tetragonal zirconia,9 displayed in Figure 1(b), the coordination number of Zr increases by

a Electronic mail: Simon.Penner@uibk.ac.at

2158-3226/2016/6(2)/025119/20 6, 025119-1 © Author(s) 2016.

FIG 1 Various crystal structures of YSZ as found in several phase diagrams Based on Refs 8 13

one, resulting in distorted cubes Cubic zirconia (Figure 1(c)), exhibiting the CaF2structure, alsofeatures cubically coordinated zirconium atoms (undistorted).10 It is to be noted that the images

in Figures1(a)to Figure1(c)depict ZrO2, but are also valid for YSZ, where a certain fraction ofthe Zr atoms are simply replaced by Y (and the occupancies of the O sites are reduced as well).These three structures comprise the majority of the phase diagram at the zirconia-rich side (seebelow for a discussion of the phase diagrams) There have been several other proposals in the past,most describing ordered structures, such as Zr3Y4O12, in a rhombohedral structure,11 as depicted

in Figure1(d) This structure is made up of a mixture of edge-sharing octahedrally and seven-foldcoordinated zirconia/yttria polyhedra This structure results from an ordering of the vacancies inthe defective fluorite structure.11Additionally, the existence of a cubic pyrochlore (Figure1(e)) hasbeen suggested, with the composition Zr2Y2O7,12 consisting of alternating layers of edge-sharingdistorted [ZrO6] octahedra and [YO8] cubes In Figure1(f), the structure of pure yttria (Y2O3) isshown This substance crystallizes in a body-centered cubic structure and contains edge-sharingdistorted[YO6] octahedra

In order to investigate the influence of the yttria-content on the crystal structure, the morphologyand surface topology, as well as the epitaxial growth properties, we decided to employ our modelthin film systems with the goal of establishing a thin film “phase diagram” of YSZ, forming therequired basis for future investigations regarding other properties of these thin films, and, by aimingfor epitaxially-grown films, preparing model systems for future studies as eventual catalysts Thethin film phase diagram is especially important since there is a vast number of applications of YSZthin films, as outlined above It appears that, while the number of publications dealing with zirconia

is large, many of them only focus on pure ZrO2,14,15 or present results on just one stoichiometry

of yttria-stabilized zirconia.16,17 In many cases, supported films and not free-standing ones (as inour case) are used.16To the best of our knowledge, there have been no systematic studies regardingthe Y-influence over a wider stoichiometry range, but only for limited compositional variationssuch as in Ref 18 For the present study, we hence restricted ourselves to the low-yttria side,featuring sample compositions ranging from 3 mol% yttria up to 40 mol%, so as to just focus on thetechnologically-relevant materials Additionally, to facilitate the discussion of the compositions andcrystal structures obtained in our study, an overview of often-used concentration quantities, as well as

a thorough discussion of existing phase diagrams, going more into detail than existing publicationssuch as the one by Chevalier et al.,19are presented in the next sections.

II YSZ STOICHIOMETRY QUANTITIES

There are various ways to present the composition of solid mixtures such as yttria-stabilizedzirconia, such as the molar percentage of one of the constituents, the atomic percentages or the

Zr/Y ratio The practically most useful quantity probably is the molar percentage of Y2O3(mol%

Y2O3) as it is directly related to the preparation procedure Also often used is the atomic percentage

of yttrium (at% Y), for it is the quantity that is obtained from elemental composition analysessuch as XPS or EDX Another composition often found, especially in phase diagrams, is the molarpercentage of YO1.5(mol% YO1.5) It should be noted that, against common belief, this value is notsimply twice that of mol% Y2O3because n(Y2O3) and n(YO1.5) are found in the numerator, but also

in the denominator of the equations for the molar percentages, respectively

In Table I, the conversions between all mentioned concentration quantities are listed Eachequation describes the calculation of the property in the left column from that in the top row

III DISCUSSION OF EXISTING PHASE DIAGRAMS

A large amount of phase diagrams concerning the zirconia-yttria system are reported in ature, but these do not all agree with each other and, in fact, contain many contradictory exam-ples Hence, we strive to give a brief overview of the published proposals in order to facilitatethe discussion of our findings later The focus of this small review will thus be put on thoseyttria-concentrations that were also used in our study: 3 mol%, 8 mol%, 20 mol% and 40 mol%

liter-Y2O3 These samples will be referred to as 3YSZ, 8YSZ, 20YSZ and 40YSZ, respectively, with thenumber denoting the amount of yttria (in mol% Y2O3) present One of the first phase diagrams ofthis system was published by Duwez et al.20in 1951, featuring no ordered phases such as Zr3Y4O12.Also, according to this diagram, no pure cubic zirconia could exist Rather, the monoclinic poly-morph would transform into its tetragonal counterpart at approximately 1000◦C, which then wouldmelt at about 2700◦C The first occurrence of the cubic form of the yttria-doped variant would be

at 5 mol% Y2O3 At low temperatures, a miscibility gap between cubic and monoclinic YSZ wouldexist, which would react towards the tetragonal polymorph upon reaching the respective eutectoidictemperature between 400◦C and 500◦C Thus, 3YSZ would be either monoclinic or tetragonal,depending on whether the latter was frozen in a metastable state, and all other samples of interestwould be pure cubic YSZ (since that region extends from 7.5 mol% to more than 50 mol% Y2O3,where a two-phase area between cubic YSZ and bcc yttria starts)

In 1963, Fan et al.21published an incomplete version of the low-temperature region of the phasediagram, differing strongly from that by Duwez et al by containing a compound of the composition

Zr2Y2O7at 33.3 mol% Y2O3, exhibiting a cubic pyrochlore structure Also, the zirconia-rich region

of the diagram features dissimilarities in the progression of the phase boundaries, featuring no cubic

TABLE I Conversion between commonly-used concentration values for yttria-stabilized zirconia y always refers to the quantity in the left column, x to that in the top row.

vert.: y, horiz.: x mol% Y 2 O 3 mol% YO 1.5 at% Y (incl O) at% Y (excl O) Zr /Y Ratio mol% Y 2 O 3 — y = x

YSZ at lower temperatures, but rather suggesting the existence of solid solutions of zirconia and tria, respectively, in the pyrochlore, as well as the stability of monoclinic solid solutions until yttriaconcentrations corresponding to the pyrochlore Also, the pyrochlore and Y2O3show a miscibilitygap between approximately 47 mol% and 33 mol% Y2O3 On the basis of this phase diagram, each

yt-of our samples would contain the pyrochlore structure, as all the specimens up to 20YSZ would be

a mixture between either monoclinic or tetragonal YSZ and Zr2Y2O7, and 40 YSZ would consist ofonly the pyrochlore structure, which would exhibit a certain degree of flexibility regarding the exactcomposition However, no further study was able to confirm the existence of the Zr2Y2O7phase

At the end of the 1960s, another systematic study was performed by mixing ZrO2and Y2O3

in 5 mol% steps and heating the mixture with a CO2laser.22 – 24The pyrochlore suggested by Fan

et al was not found, with cubic YSZ seemingly taking its place (also including a region of cibility between cubic YSZ and yttria, but at higher concentrations), and the zirconia-rich regionshows similarities to Duwez’ version In contrast to the proposal by Fan et al.,21monoclinic YSZ

immis-is only found up to a very small amount of yttria, with the cubic polymorph becoming the stablephase at room temperature already below 10 mol% Y2O3 It has to be noted, though, that the databelow 500◦C is sparse in this study This may be the reason for the omission of the eutectoidicdecomposition of the tetragonal phase, as seen in Duwez’ version (and many later diagrams), forinstance At 8 mol%, 20 mol% and 40 mol% Y2O3, a cubic structure should be found and, for thesample with the lowest yttria content, namely 3 mol%, tetragonal YSZ would be the stable form,with small amounts of monoclinic YSZ at low temperatures

A slightly different low-yttria region was found by Srivastava et al.,25 who, like Duwez andco-workers, observed a eutectoidic transformation at around 4 mol% Y2O3at 565◦C, below whichtetragonal YSZ would decompose into the monoclinic and cubic variants, which is where 3YSZwould be located Otherwise, no large differences from the previous diagram are to be found, withthe cubic YSZ/Y2O3two-phase region starting only at 80 mol% instead of already at a bit morethan 40 mol% in the case of the previously mentioned study, but also comprising all other threeconcentrations investigated in this work

Also in the early 1970s, Rouanet26could not validate the existence of a Zr2Y2O7compoundeither, but instead located a new ordered phase: Zr3Y4O12, with a hexagonal crystal structure.This new phase, also called the δ phase (derived from the zirconia-scandia system),11 is located

at 40 mol% Y2O3, which would correspond directly to our 40YSZ sample, and is stable up to

1250◦C, where it decomposes peritectoidically At the low-yttria side of the diagram, no eutectoid

is visible in the diagram, which is due to the omission of the low-temperature part (only data above

1000◦C are shown) Judging from the limited information available, 3YSZ would be tetragonal,8YSZ a mixture between tetragonal and cubic polymorphs, and 20YSZ would be found in its cubicstructure The eutectoid, however, was published again by Scott,27with the two-phase region belowthe eutectoid extending to 10 mol% Y2O3 The ordered phase, Zr3Y4O12, on the other hand, is notfeatured in this diagram Instead, two cubic bcc-phases are found between 80 mol% and 95 mol%

Y2O3, which, according to later work of the same author,28 are due to contaminations causingnon-equilibrium effects Scott also examined the concentration regions where the various poly-morphs could be kept in a metastable state at room temperature, which results in the informationthat only tetragonal YSZ is possible between 3 mol% and 6 mol% Y2O3— at lower concentrations,the monoclinic analogue is obtained, and, at higher concentrations, cubic YSZ becomes more stable(with small regions of overlap between the polymorphs) This would mean that 3YSZ would betetragonal, 8YSZ, 20YSZ and 40YSZ already cubic

Gorelov investigated the low-yttria content region more closely,29 and found four ZrO2morphs (and, hence, also four YSZ solid solutions) instead of the usual three In addition to the mono-clinic one, and the already known tetragonal analogue, which is stable between 1200◦C and 2300◦C,another tetragonal phase was observed between 2300◦C and 2500◦C, where it transforms to cubiczirconia This new tetragonal phase is primarily distinguished by differences in lattice parameters:the c axis is compressed and a is increased Both tetragonal phases decompose eutectoidically uponcooling, with the high-temperature one forming cubic and the low-temperature tetragonal compound,and the latter yielding monoclinic and cubic YSZ Due to the limitation on concentrations below

poly-10 mol%, no conclusion can be drawn for 20YSZ or 40YSZ, but 8YSZ should be cubic according to

this diagram, and 3YSZ could be either a mixture between monoclinic and cubic YSZ, or one of thetetragonal polymorphs, depending on the ability to keep them metastable at low temperatures.Stubican and coworkers found a eutectoidic decomposition of cubic YSZ into monoclinic zir-conia and Zr3Y4O12at temperatures as low as 400◦C,30 – 32which could have an influence on 3YSZ,8YSZ and 20YSZ However, looking at the higher-temperature reading, the two former could also

be within the tetragonal/cubic two-phase region, and 20YSZ in the cubic area Their version of thephase diagram also contains the peritectoidically transforming Zr3Y4O12phase, again at 40 mol%

In the diagram published by Pascual and Duran,33 on the other hand, this ordered phase wouldtransform dystectoidically into cubic YSZ at 1375◦C However, they also proposed the existence of

a second phase, for which they coined the term “1:6 phase” as it complies with the stoichiometry

of ZrY6O11, which supposedly crystallizes hexagonally, transforms dystectoidically at 1700◦C and

is found at 75 mol% Y2O3 What they could not verify was the miscibility gap between monoclinicZrO2and Zr3Y4O12— that is, there is no eutectoid for cubic YSZ The sample with 3 mol% Y2O3

would be tetragonal at elevated temperatures, 8YSZ would exhibit the cubic polymorph, and 20YSZeither the cubic one too, or a mixture between the cubic and the δ phase

In 1984, Ruh et al.34again focused on the high-ZrO2region of the YSZ phase diagram Theirfindings do not deviate drastically from other proposals What is, however, interesting, is that the cubicphase starts already for yttria-contents below 8 mol% Also, according to this diagram, the tetragonalphase would only be stable up to about 2 mol%, which would mean that 3YSZ would already be amixture between the tetragonal and cubic structures, even at relatively low temperatures Again, asalready found in other references,25,29the tetragonal variant undergoes a eutectoidic decomposition.Other papers by Yoshikawa, Suto et al.35,36also discussed the low-yttria region, and their findingscorrespond well with those of Ruh et al., with the cubic YSZ region starting below 8 mol% Y2O3too,with the same conclusions regarding our investigated samples being drawn as for Ruh’s work Some

of these results are confirmed by another publication from Stubican in 1988,37although the cubicregion of the phase diagram starts only well above 8 mol% Y2O3at lower temperatures (also due tothe eutectoid already published earlier30–32), meaning that both 3YSZ and 8YSZ would be mixturesbetween tetragonal and cubic structures, except for high temperatures in the case of 8 mol% Y2O3,where it would be purely cubic They mention that this deviation from the work of Ruh et al could,however, also be due their use of a hydrothermal method since it was difficult ot obtain equilibrium

at lower temperatures This time, in contrast to their previous proposal, they found that the hedral, ordered phase, Zr3Y4O12, transforms dystectoidically at 1382◦C to the cubic analogue Whatthey couldn’t confirm was the ZrY6O11compound proposed by Pascual and Duran

rhombo-The 1980s also brought about the advent of calculated phase diagrams For instance, Degtyarevand Voronin constructed such diagrams based on the calculated thermodynamic properties of all thephases.38 – 40This included the rhombohedral Zr3Y4O12phase, which transforms dystectoidically tocubic YSZ Also, this diagram features an eutectoidic decomposition of tetragonal, as well as thecubic structure (into monoclinic YSZ and the ordered phase) They also computed the diagramsfor an increased pressure, where the region of stability for the cubic polymorph is enlarged andthe monoclinic form is generally less stable.40At ambient pressures, 3YSZ is located within thetetragonal/cubic mixture area, and 8YSZ at intermediate temperatures as well 20YSZ and 40YSZshould be inside the cubic region, even though none of their diagrams actually shows 40 mol%, as

no hints of the δ phase are visible in the partial diagrams they show

The two eutectoids were also found by the calculations of Du et al,41 who did additionalexperiments that confirmed their computations A year later, the same group published anotherversion,42with the main difference being in the high Y2O3region They, however, employed two

different sets of model parameters, which showed drastic differences in the eutectoidic region tween the monoclinic zirconia and Zr3Y4O12 In another revision of this diagram by Jin and Du,43another miscibility gap was introduced at the low-temperature end below 479◦C, where the orderedphase decomposes into the monoclinic solid solution and Y2O3 This would suggest that, at lowtemperatures, a demixing of the solid solutions would be favoured If high-temperature phases arequenched and brought to room temperature, 3YSZ and 8YSZ would be tetragonal or cubic (if initialtemperatures exceeded 2000◦C), and 20YSZ cubic 40YSZ would crystallize in the rhombohedral

be-Zr YO structure — at least for a limited temperature range between 479◦C and 1376◦C

TABLE II Summary of the YSZ phase diagrams found in literature Abbreviations: cub = cubic, tetr = tetragonal, dystect = dystectoidic, peritect = peritectoidic, eutect = eutectoidic.

Year Citation Exp./Theo Ordered Phase(s) Main Features

1951 Duwez et al.20 exp — no cub ZrO 2 , eutect tetr YSZ

1963 Fan et al.21 exp Zr 2 Y 2 O 7 no cub YSZ

1968 Ruh, Rouanet, Skaggs22–24 exp —

1971 Rouanet26 exp Zr 3 Y 4 O 12 peritect Zr 3 Y 4 O 12

1974 Srivastava et al.25 exp — eutect tetr YSZ

1975 Scott27 exp — two cub phases

1978 Gorelov29 exp — two tetr phases, two eutect.

1982 Stubican et al.30–32 exp Zr 3 Y 4 O 12 peritect Zr 3 Y 4 O 12 , eutect cub YSZ

1983 Pascual, Duran33 exp Zr 3 Y 4 O 12 , ZrY 6 O 11 dystect ordered phases

1984 Ruh et al.34 exp — cub YSZ for < 8 mol%Y 2 O 3

1987 Suto, Yoshikawa et al.35,36 exp — cub YSZ for < 8 mol%Y 2 O 3

1988 Stubican37 exp Zr 3 Y 4 O 12 dystect Zr 3 Y 4 O 12

1987 Degtyarev, Voronin38–40 theo Zr 3 Y 4 O 12 dystect Zr 3 Y 4 O 12

1990 Du et al.41,42 theo Zr 3 Y 4 O 12 dystect Zr 3 Y 4 O 12

1992 Jin, Du43 theo Zr 3 Y 4 O 12 dystect Zr 3 Y 4 O 12

1995 Suzuki44 exp Zr 3 Y 4 O 12 peritect Zr 3 Y 4 O 12

1996 Yashima45 exp — no equilibrium < 1200◦C

1997 Suzuki46 exp Zr 3 Y 4 O 12 peritect Zr 3 Y 4 O 12

2005 Fabrichnaya et al.47 theo Zr 3 Y 4 O 12 dystect Zr 3 Y 4 O 12

In an experimental phase diagram, created by Suzuki,44 only minor differences to other posals can be discerned For instance, Zr3Y4O12transforms peritectoidically at 1360◦C This dia-gram also features the eutectoids for the cubic and tetragonal YSZ species and the miscibility gapbetween monoclinic zirconia and Zr3Y4O12, and the crystal structures of our samples would be thesame as for the previously discussed versions Yashima et al.,45 on the other hand, took a closerlook at the low-yttria content region (up to around 20 mol% Y2O3) They found it impossible toreach thermodynamic equilibrium below 1200◦C, and, hence, their diagram does not show equi-librium lines for those temperatures However, like Scott,27they managed to determine the regions

pro-of metastability for each pro-of the solid solutions, according to which cubic YSZ is obtainable forconcentrations above 11 mol% Y2O3(e.g 20YSZ), and the tetragonal structure is obtained above

1 mol% Y2O3(hence, it being the stable phase for 3YSZ and 8YSZ)

Suzuki later published another (partial) phase diagram for YSZ,46investigated using tivity measurements This version also is in good agreement with other publications Fabrichnaya

conduc-et al.47 reported a theoretical diagram in 2005, which differs significantly from other recent posals in our region of interest, in that, for a certain range in Y content, the cubic polymorph (forexample for 20 mol% Y2O3) would not decompose at all upon lowering the temperature (i.e there

pro-is no eutectoid) The remaining regions resemble previous diagrams, such as the dystectoidicallytransforming δ phase, with other implications on our samples being that 3YSZ would crystallizetetragonally and 8YSZ both, cubically and tetragonally

TableIIsummarizes these diagrams, allowing for a quick overview of the major points of each

of them, where the evolutionary nature of the phase diagram proposals can be witnessed, especiallywith regard to the ordered compounds — at first, none had been reported, then there were differentsuggestions, and, soon it became accepted and almost every diagram contained Zr3Y4O12

IV EXPERIMENTAL DETAILS

A Thin film deposition

The thin films have been deposited on NaCl(001) single crystals at elevated temperatures of

623 K to facilitate crystallization and possibly epitaxy using a custom-made sputter-gun (see belowfor details) in a modular high-vacuum apparatus with a base pressure in the low 10−7mbar regime

FIG 2 Overview of the redesigned sputter device, featuring a modular setup with an easily removable shielding (at the top), onto which the filament is mounted, allowing for a significant speed-up in filament changes In addition to a higher mechanical stability, this new design enables the sputtering of insulating targets due to the higher temperatures reached because of the closer proximity to the glowing filament.

The sputtering was performed in 5 × 10−5 mbar Ar, and as targets, pellets of the different YSZsamples (commercial powders with 3 mol%, 8 mol%, 20 mol% and 40 mol% Y2O3, by SigmaAldrich) were used These targets were prepared by pressing the powders onto a spirally-shaped

Ta wire at approximately 20 kN in a KBr pellet press used for infrared spectroscopy studies Bysubmerging the coated sodium chloride crystals in water, the thin films can be floated off, andafterwards collected using TEM gold grids to yield unsupported thin films with nominal thicknesses

of 25 nm

While there were successes in sputtering YSZ with 8 mol% yttria using our previously lished direct-current ion beam sputter device,48 , 49 a redesign was required in order to suit moreinsulating targets, such as some of the oxides used in this study, without having to resort toradio-frequency power supplies, as is often the case for commercial sputtering systems The mainchanges, displayed in Figure2, encompass a new, modular head for the device, where the target can

pub-be brought in closer proximity to the hot filament, causing the target to reach higher temperatures,decreasing the resistivity Also, not visible in the illustration, to limit the heat conductance awayfrom the oxide, the target mounting was altered to split up the structural connection (now usingceramics) from the electric connection (via a very thin Ta wire) The new design also brings about

a higher mechanical stability compared to previous versions, resulting in a better stability of thedeposition process due to less vibrations being transmitted to target and the gun head, allowingfor the growth of more homogeneous films Additionally, the revised shielding (needed to blockthe line of sight between the target and the substrate in order to avoid evaporated or sputteredtungsten (oxide) contaminations) now features a conical center part instead of a cylindrical one,and is lower in height, maximizing the substrate area that is coated Another benefit relating to theshielding stems from the new filament mountings, which are now directly attached to the removableshielding, meaning that the whole filament can now be swapped much quicker than before

B Characterization of the films

The unsupported films were investigated with respect to their crystallographic properties, ture and homogeneity using a FEI Tecnai F20 S-TWIN (high-resolution) analytical (scanning)transmission electron microscope (200 kV), equipped with an EDAX Apollo XLT2 silicon-driftdetector for energy-dispersive X-ray spectrometry (EDX) and a GIF Tridiem electron energy-lossspectrometer

struc-For the X-ray photoelectron spectroscopy (XPS) compositional analyses, a Thermo ScientificMultiLab 2000 spectrometer (with a base pressure in the high 10−11mbar to low 10−10mbar range),fitted with a monochromated Al-Kα X-ray source, an Alpha 110 hemispehrical sector analyzerand an ion gun for sputter-depth profiling (operated at 3 kV using argon), was utilized For theseinvestigations, additional samples had been prepared by depositing the oxides on silicon wafers

The atomic force microscopy (AFM) surface topology images of the unsupported specimens,from which the surface roughness values were calculated, were recorded using a Veeco DigitalInstruments Dimension 3100 in tapping mode For this, Veeco RTESPW silicon cantilevers withforce constants between 20 N m-1and 80 N m-1as well as resonance frequencies from 256 kHz to

317 kHz were employed

C Characterization of the target materials

In order to gain an understanding about the change of crystallographic structure from thetarget to the thin film, X-ray diffraction studies have been performed on the target powders using

a Siemens D5000 diffractometer under ambient conditions, while recording a 2θ range from 20◦to

70◦with a step size of 0.02◦

V RESULTS AND DISCUSSION

A Composition

To check the purity and composition of the thin films, various methods were employed: usingX-ray photoelectron spectroscopy, sputter depth profiles were recorded, energy-dispersive X-ray(EDX) spectra were taken both in TEM, as well as in STEM mode (spectrum imaging), and electronenergy-loss spectrometry was utilized as well Figure 3displays the surface-sensitive XP spectra(Figure3(a)) and EDX spectra (Figure3(b)), which, due to the larger mean free path of X-rays incontrast to electrons of the same energy, corresponds to an integrated composition over the wholethickness range

These two sets of plots show that the samples are contamination-free In Figure3(a), the mainpeaks visible are the O 1s (at 532 eV), the corresponding O KLL auger peak at around 1000 eVbinding energy, the Zr 3d and 3p (180 eV and 331 eV, respectively), as well as the Y 3d (158 eV)and 3p peaks (301 eV) Going from 3YSZ to 40YSZ, the relative change of Zr/Y peak intensitiescan be observed nicely For the thinner films, namely 3YSZ and 40YSZ (and, to a smaller extent,20YSZ), the Si 2p peak at 99 eV and the Si 2s one (149 eV) from the underlying substrate (XPSsamples were deposited directly on silicon wafers) begin to be visible at the surface spectrumalready At 245 eV, the Ar 2p peak can be seen, which is due to incorporated argon in the thin

FIG 3 Surface-sensitive X-ray photoelectron spectra (a) and energy-dispersive X-ray spectra recorded in the TEM (b) both show that the films are impurity-free (note that the NaCl does not originate from the deposition process, but rather from the substrate, and that it can be removed by rinsing with water) and that the stoichiometry is the same as in the target.

films, most likely due to implanted Ar+ions into the target during the sputter-depositing process.

For 40YSZ, two very small peaks are found at 1070 eV and 508 eV, which are Na 1s and NaKLL peaks, originating from the sample preparation process when transferring it into the XPSchamber This can be confirmed by sputtering the film for a few seconds, after which the sodiumcontamination vanishes Hence, it was only on the top of the surface and does not arise from the thinfilm deposition process

In the EDX spectra (Figure3(b)), the peaks from other elements than Zr, Y or O can readily beexplained as well: the very small Na and Cl signals are remnants of the sodium chloride substrates(and, hence, not from the deposition process itself) and can be minimized by introducing additionalcleaning steps simply using water Au stems from the TEM gold grid, upon which the films areplaced, Cu from the specimen holder, and since the pole piece of the lens contains Fe and Co, theseconsequently yield X-ray fluorescence peaks This fluorescence is due to the measurement positionsbeing chosen close to the grid bars in order to minimize charging effects The proximity to thegold grid causes the high-energy Au X-ray lines to be emitted, in turn resulting in the emission ofX-ray fluorescence from the pole pieces for two of the samples (3YSZ and 40YSZ) Thus, it can beconcluded that the films are of high purity, and the composition is the same as in the target as well,

as the data in TableIIIshow

There, the stoichiometries, as obtained from various methods, are shown: the mean of an XPSdepth profile, the quantification results from the EDX spectra shown in Figure3(b), the integratedEDX spectra from spectrum images, and the quantification of the Zr and Y L3edges The EDXspectra have been quantified using the Cliff-Lorimer method with the software Digital Micrograph

by Gatan, using the k-factors contained therein Calculating the mean of all these values, onecan see that the yttria content in the thin film correlates well with that in the target: for 3YSZ,3.3(7) mol% Y2O3is measured, for 8YSZ 8(2) mol%, for 20YSZ 17(2) mol%, and 40YSZ contains38(3) mol% yttria Looking at the separate values, it immediately comes to mind that the EELSquantification is off the most, for example only yielding 13.8 mol% Y2O3for 20YSZ This is due tothe difficulties arising when quantifying EELS edges, which, in contrast to for instance the Gausspeaks in EDX spectroscopy, is not as straightforward Also, the L3 edges are found at 2080 eVenergy-loss (yttrium) and 2222 eV (zirconium), respectively, where the intensity in the spectrum isalready extremely low All other methods are in good agreement with the target values, with theEDX results in TEM mode corresponding better than those in STEM, because a larger area on thespecimen was sampled, allowing us to use a higher beam current without destroying the thin filmsdue to charging and, thus, obtain a better signal-to-noise ratio

Because not only the composition is relevant, but also its depth and spacial homogeneity, XPSdepth profiling and EDX spectrum imaging were employed (Figure 4) The XPS depth profile(Figure4(a)), which was normalized to the film thickness (determined by taking the inflection point

of a sigmoidal fit of the Si 2p peak intensity) in order to allow for easier comparison of the profiles ifthe films do not have the same thicknesses, shows that the composition does not change drastically.The slight increase in yttria-content at higher etch depths comes from the increased silicon content,which results in the other peak intensities diminishing, making the quantification less reliable For40YSZ, this plot shows that the XPS measurements yield too low Y2O3concentrations, as alreadyseen in TableIII For the other specimens, the values correspond well with the compositions fromthe targets, indicated by the dashed lines

TABLE III Composition of the thin films, as determined by various methods The mean of these values correlates with the target compositions.