oxygen transport in epitaxial srtio /srti xfexo...

Manuscript prepared for J. Sens. Sens. Syst.with version 2014/09/16 7.15 Copernicus papers of the LATEX class coperni-cus.cls.Date: 25 May 2016

Oxygen Transport in Epitaxial SrTiO3/SrTi1−xFexO3

Multilayer Stacks

Michal Schulz1, Timna Orland2, Alexander Mehlmann2, Avner Rothschild2, and Holger Fritze1

1Institute of Energy Research and Physical Technologies, Clausthal University of Technology, Goslar, Germany.2Department of Materials Science and Engineering, Technion – Israel Institute of Technology, Haifa, Israel.

Correspondence to: Michal Schulz ([email protected])

Abstract. In this work nano-ionic materials made of strontium titanate (SrTiO3, STO) and solid solution ofstrontium ferrite in STO (SrTi1−xFexO3, STF) are grown on single crystalline STO substrates and character-ized. Since STF exhibits an oxygen deficiency and, simultaneously, enables oxygen interstitial defects, a spacecharge area close to the STO | STF interface is present. Oxygen tracer diffusion experiments and impedancespectroscopy at temperatures from 500 to 700 °C and at oxygen partial pressure ranging from 10−3 to 10−23 barconfirm fast transport of the oxygen and, thereby, enhanced ionic conductivity at the interface. There, a p-typeconductivity of about 400 S/m at 700 °C is found. Such structures open new possibilities in design of nano-ionicmaterials for oxygen sensors and energy conversion at temperatures lower than that of conventional materialssuch as yttrium-doped zirconia.

1 Introduction

Increasing worldwide concerns about environmental degra-dation, global warming, need for new renewable energysources and safety issues regarding e.g. nuclear power leadsto search for new materials for sensor, energy conversion5

and storage applications. Currently used materials such as yt-trium stabilized zirconia (YSZ) yield an outstanding perfor-mance in regard of ionic transport (Ioffe et al., 1978; Badwal,1992; Kilo et al., 2004; Gerstl et al., 2011), but on the otherhand require high operating temperatures of above 700 °C.10

This simulates growing interest in nano-sized ionic materi-als which could, for example, reduce the operating tempera-tures of such devices to economically viable temperatures ofabout 500 °C. Lowering the temperature and enhancing ionicconductivity leads to systems with e.g. shorter startup time,15

shorter response time, reduced heat dissipation, lower inter-nal resistance of the cells as well as prolonged life time. Suchnano-sized ionic systems, however, remain in early stage ofresearch and development largely because of material limita-tions. Improved understanding and control of the interfacial20

ionic transport and innovative means to tailor their ionic con-ductivity are required to facilitate further development.

This work focuses on investigation of nano-ionic struc-tures comprising alternating layers of metal oxides having

oxygen deficiency and excess. Consequently, increase of25

concentrations of mobile defects in form of oxygen vacan-cies and interstitials and thus enhance ionic conductivity isexpected. The Debye screening length should be in the or-der of several ten nanometers resulting in effects on the oxy-gen transport visible by oxygen exchange experiments and30

impedance measurements.The transport properties in these materials are investigated

using oxygen exchange experiments and impedance spec-troscopy as a function of temperature and of oxygen par-tial pressure. The material systems selected for this work35

are strontium titanate (SrTiO3, STO) and solid solutions ofstrontium ferrite (SFO) in STO (SrTi1−xFexO3, STF) with20 mol-% and 50 mol-% SFO. Diffusion barriers are made ofthin lanthanum aluminate (LaAlO3, LAO) and gold layers.

2 Models40

2.1 Space charge at the interface

STO single crystals show a perovskite structure with closely-packed anion sublattice. Because of size constrains strontiumtitanate cannot accommodate interstitial defects O′′i , but, onthe other hand is tolerant to vacancy defects V••O . STF mixed45

crystals have a double perovskite structure with inherent oxy-

2 Schulz et al.: Oxygen Transport in Epitaxial STO/STF Multilayer Stacks

STF20 or STF50

STO

LAOAu

char

ge c

arri

erco

nent

rati

on

18-O concentration

distance fromopening in the structure

(a)

(b)

(a)

(b)

(b)

Figure 1. Schematic illustration of an interface between STF layerand STO. The oxygen depth profiles along the interface are ex-pected to be much broader close to the interface (a) than in bulkSTO or STF (b).

gen deficiency (brownmillerite structure of SFO) which pro-vides sufficient room for oxygen interstitials.

At the interface between these two materials oxygen istransferred from STO into STF building mobile Frenkel de-50

fects (Rothschild, 2006)

OO �O′′i +V••O . (R1)

The driving force for this process is a difference in oxygenactivity between STO and STF. The increased vacancy con-centrations at the interface impacts oxygen transport signifi-55

cantly since the oxygen tracer diffusion coefficient is corre-lated with the vacancy diffusion coefficient DV through

DO = f∗DV

cV••O(x)

cO×O(x)

(1)

where cV••O(x), cO×

O(x) and f∗ are concentrations of oxy-

gen vacancies and oxygen and the tracer correlation correc-60

tion factor, respectively. In case of oxygen tracer, the correla-tion factor is very close to unity and therefore set to unity infurther discussion. Since oxygen concentration in bulk, cO×

O,

can be assumed to be nearly constant, the diffusion coeffi-cient of oxygen is proportional to the concentration of the65

vacancies. Therefore, as shown in Fig. 1, the enhanced spacecharge close to the interface is expected to facilitate fast oxy-gen transport in that region. This effect can be controlled bythe concentration of ferrite phase in STF (20 or 50 %) andfurther increased by creating stacks of interleaved STF and70

STO layers and adjusting the thickness of the structures sothat the space charge regions overlap.

2.2 Ionic transport

STO is a perovskite structure with well studied electrochemi-cal properties (e.g. Balachandran and Eror (1981); Chan et al.75

(1981); Wagner et al. (2004); De Souza (2009); Ohly et al.(2006); Gömann et al. (2005)). The oxygen partial pressuredependence of its mixed ionic-electronic conductivity makes

Space charge zone in STO

STO substrateD2

j = 0

j ≠ 0 D2

D2

D1

j = 0

j = 0

j = 0

Figure 2. Diffusion model (not to scale) used to simulate the 2-dimensional diffusion process through space charge zone of STOand crystal bulk. The dotted line covering most of the outer bound-aries represents an area with zero flow, the arrows outside the modelrepresent places, where 1D depth profiles are acquired (see furtherin text).

it a good candidate for high temperature gas sensor appli-cations (Fergus, 2007; Hu et al., 2004). Depending on the80

oxygen partial pressure three regions with different pO2 de-pendence may be regarded (De Souza, 2009).

2.2.1 Low oxygen partial pressures

At low oxygen partial pressures the oxygen atoms leave thecrystal lattice generating oxygen vacancies and free electrons85

compensating the charge:

O×O �V••O +1

2O2 +2e′ (R2)

with the equilibrium

KRed(T ) = p1/2O2n2[V••O ]. (2)

Since the concentration of oxygen vacancies formed by un-90

intended acceptor doping exceeds the amount of vacanciesgenerated due to reduction of STO significantly, it may beassumed that the term [V••O ] is constant. Therefore, the n-type electronic conductivity depends on the oxygen partialpressure as follows95

σn ∝ p−1/4O2. (3)

2.2.2 Intermediate oxygen partial pressures

At intermediate oxygen partial pressures the ionic conduc-tivity dominates with oxygen vacancies being the prevalentmobile charge carriers.100

σion =4q2

kT[O×O]DO (4)

2.2.3 High oxygen partial pressures

At high oxygen partial pressures the oxygen atoms fill thevacancies. As a result of this reaction holes are generated to

Schulz et al.: Oxygen Transport in Epitaxial STO/STF Multilayer Stacks 3

- 2 2 - 2 0 - 1 8 - 1 6 - 1 4 - 1 2 - 1 0 - 8 - 6 - 4 - 2 0 2

- 3

- 2

- 1

0

1

��

log(σ

/ S/m

)

��

Figure 3. Comparison of total conductivity in STO as a functionof oxygen partial pressure and temperature as reported by severalauthors.

compensate the charge:105

V••O +1

2O2 �O×O +2h• (R3)

with the equilibrium

KOx(T ) = p−1/2O2

p2[V••O ]−1. (5)

The concentration change of oxygen atoms at oxygen crys-tals sites is negligible and may be regarded as constant.110

Therefore, the p-type electronic conductivity depends on thepO2 as follows

σp ∝ p1/4O2. (6)

The total conductivity σtot = σn +σion +σp as reportedby several authors is summarized in Fig. 3.115

2.2.4 Transport along the interface

As mentioned previously, the interface is formed by Frenkeldefects. This type of defect is expected to increase the ionicconductivity contribution by providing additional charge car-riers - oxygen interstitials in STF and oxygen vacancies in120

STO. At low oxygen partial pressures the space charge zoneexhibits lower conductivity than the bulk of STO and STF,since excess of V••O shifts the equilibrium of Reaction (R2)towards reactants (i.e. O×O). Finally, due to shift of the equi-librium of the Reaction (R3) towards products, the p-type125

conductivity at high oxygen partial pressures is expected toincrease.

3 Experimental

A scheme of sample is given in Fig. 4. Here, a multilayerstructure deposited on STO substrate is shown. Since the130

STO substrate

STF

STO

STF Au LAO

182OInitial SIMS crater

Figure 4. A scheme of a sample with two STF layers and STO layerdeposited on the substrate.

sample is initially covered with LAO+Au diffusion barrier,there is no path for oxygen transport into the structure. There-fore, a small opening is etched in the barrier and structureunderneath using an Ar ion source. Focused ion beam al-lows to precisely control the shape of the crater. As an ad-135

ditional benefit, the concentration of secondary ions createdduring this process is measured (SIMS measurement) so thata depth-dependent distribution of elements such as Sr, Ti, Oand Fe in as-prepared samples is determined. Whilst subse-quent annealing the 18O isotope may diffuse into the inter-140

face area directly. Finally, during 3D SIMS analysis a largearea around initial crater is etched using SIMS and a 3D dis-tribution of 18-oxygen in the sample is calculated.

3.1 Sample preparation

All layers are synthesized on a (100) face of single crystalline145

10×10×0.5 mm3 SrTiO3 substrates. The STO samples aremasked in order to limit the deposited area to approximately7×7 mm2 and installed in a vacuum chamber. Subsequentlythe samples are heated up to approximately 700 °C. Later,thin layers of target materials are epitaxially grown on the150

STO surface by means of pulsed laser deposition (PLD) us-ing a KrF excimer laser. Depending on the requested arrange-ment either STF50 or STF20 or alternating layers of STFand STO with a thickness of 50 nm are deposited. Finally, a20 nm LAO layer is provided to prevent oxygen diffusion.155

In order to verify the diffusion barrier, a sample withAu deposited on top of LAO was annealed in the 18O en-riched atmosphere at 600 °C for 3600 seconds. Subsequentlya depth profile of 18-oxygen concentration was acquired.The profile was finally compared with results of subsequent160

3D measurements. As shown in Fig. 5 the diffusion of oxy-gen through an undamaged diffusion barrier is largely sup-pressed.

3.2 Oxygen-18 tracer diffusion

The samples used for oxygen diffusion experiments are cut165

into four pieces after film deposition. Subsequently the sam-ples are cleaned and a thin Au layer is deposited using aCressington Sputter Coater 108. As mentioned above the


0 2 0 4 0 6 0 8 0 1 0 0 1 2 00 . 0 0

0 . 0 1

0 . 0 2

0 . 0 3

18 O / (16 O +

18 O)

d e p t h / a . u .

A u L A O S T F S T O S T F S T O b u l k

w i t h i n i t i a l S I M S c r a t e r w i t h o u t i n i t i a l S I M S c r a t e r

Figure 5. Result of Au | LAO diffusion barrier test showing two18O depth profiles, one without initial SIMS crater (see text for fur-ther explanation) and one with initial crater.

gold layer in combination with LAO acts as a diffusion bar-rier for oxygen.170

The diffusion path for oxygen is opened by initial SIMSmeasurement. Thereby a rectangle crater of approximately500×750 µm2 is created in the middle of the sample. Themeasurement opens the oxygen diffusion barrier and givesinformation about oxygen isotope abundance, arrangement175

of the deposited layers and interfaces in the as-prepared ma-terial.

The 18O2 diffusion runs are performed in a furnace witha transfer rod which enables quick insertion or removal ofthe sample from the hot area. Prior to the diffusion runs each180

sample is pre-annealed at the same temperature as the tem-perature of subsequent annealing run in order to enable oxy-gen equilibration throughout the area of interest. The processis performed at the same oxygen partial pressure but the an-nealing time is increased by factor of four at least.185

After pre-annealing, the samples are quickly removedfrom the hot zone of the furnace and the atmosphere is re-placed with a 18O2-enriched gas (concentration >85%). Sub-sequently, the samples are transferred again into the hot zoneand annealed. Finally, the samples are removed from the fur-190

nace and cooled down quickly.The diffusion runs are performed at temperatures from 500

up to 700 °C. Higher temperatures are out of scope of thiswork.

3.3 Oxygen transport195

The SIMS measurements are performed with the Hiden An-alytical SIMS Workstation system. Here, a primary Ar+ ionbeam with a spot diameter of approximately 80 µm is used.The Ar ions are accelerated in a 5 kV field, the intensity of theprimary ion beam is set to 100 nA. The analyzed surface area200

for depth profiles is set to 500×750 µm2. Disturbances of the

measurement from crater walls are reduced by gating the ac-quisition area electronically to about 100×100 µm2. In orderto reduce the impact of surface charging during the sputterprocess, an electron flood gun with an energy set to 10 eV is205

used.After diffusion runs the samples are analyzed for the sec-

ond time using a different mode of operation of the SIMSsystem. Here, a larger area of 2.1×1.6 mm2 is scanned at thesame primary ion intensity. The signals from the quadrupole210

mass spectrometer for 16O− and 18O− are recorded for ev-ery beam position on the sample. Using this technique a 3Dprofile of relative 18O concentration through the depositedlayers down to STO bulk is created.

3.4 Conductivity215

For impedance measurements, rods with dimensions of10×2×0.5 mm2 are cut. Subsequently, two platinum wiresare attached using a Pt-paste and the sample is sintered at1000 °C. A SIMS analysis of the rod at the end of experi-ments confirms that the layered structure is not damaged due220

to the diffusion of iron during the sintering process. Finally,the sample is installed in a gas tight tube furnace which isequipped with an oxygen ion pump and in house designedcontrol electronics to adjust the oxygen partial pressure in arange from 10−3 to 10−23 bar (Schulz et al., 2012).225

The samples are annealed at temperatures between 500and 700 °C in CO/CO2 and H2/H2O atmospheres with anoxygen partial pressure adjusted stepwise between 10−23

and 10−3 bar. In order to test the reproducibility, two pO2

ramps are performed for each gas composition and each tem-230

perature. The complex impedance is measured with a So-lartron SI1260 gain phase analyzer in a frequency range from1 Hz to 1 MHz.

The measured sample consists of a STO substrate as wellas thin STF and LAO films on top of it. They form resis-235

tors connected in parallel to each other. Therefore, the totalmeasured conductance Gtot can be regarded as a sum of allcontributing conductances,

Gtot =GSTO +GSTF +GLAO +Giface. (7)

Here, Giface corresponds to the conductance of the interface240

between STO and STF. According to the thicknesses of cor-responding layers (500 µm STO versus 50 nm STF or 20 nmLAO), the contribution in conductivity from STF, LAO or theinterface has to be at least several orders of magnitude higherthan the conductivity of STO in order to expect a measurable245

impact on the total measured conductance.


Figure 6. High resolution TEM image of a STO/STF interface.

4 Results and discussion

4.1 Pulsed laser deposition

A HR-TEM image of the interface between substrate anddeposited STF (see Fig. 6) confirms epitaxial growth of the250

layer.

4.2 Oxygen tracer diffusivity

The SIMS measurements on the samples annealed at hightemperatures show broadening of the interface between STFand STO (see Fig. 7). Already after one hour of annealing255

at 700 °C the width of the interface measured with SIMS in-creases by factor of two. Using this data a (chemical) dif-fusion coefficient of Fe is calculated DFe = 2.9 · 10−20 m2/s(see dashed line in Fig. 7) using numeric solution of theFick’s second law.260

The broadening of the interface determines the applicationlimit of the structures at about 700 °C. It should be avoidedto anneal the samples at temperatures exceeding this limitsignificantly.

As shown in Fig. 8 the 10 hour long annealing runs at265

a temperature lower by 100 K only shows no significantchange in the Fe-profile compared to the as-prepared sam-ple.

4.3 Oxygen diffusion at the interface

The pre-annealing SIMS measurements on the samples con-270

firm existence of all layers of interest both in case of singleas well as multilayer structures. As shown in an exemplarydepth profile (see Fig. 10) there are two clearly distinct areas

0 1 0 2 0 3 0 4 0 5 0 6 00 . 00 . 10 . 20 . 30 . 40 . 50 . 60 . 7

A f t e r a n n e a l i n g B e f o r e a n n e a l i n g F i t c u r v e

Fe / (

Ti + F

e)

D e p t h / n m

Figure 7. Broadening of the STF/STO interface after 60 min an-nealing at 700 °C. The dashed line represents the fit result, blackpoints the initial concentration of Fe at the interface (including mea-surement uncertainty of SIMS), the red triangles represent the con-centration of Fe after annealing.

0 1 0 2 0 3 0 4 0 5 0 6 0 7 00 . 0

0 . 2

0 . 4

0 . 6

0 . 8

1 . 0�

� ��

Fe / S

r

��

Figure 8. Fe-profile in 50 nm STF layer before and after 10 hoursof annealing at 600 °C in air or H2. No measurable change in theprofile indicates good thermal stability of nanostructured STO|STF.The average depth resolution of the measurement is in the range ofabout 10 nm.


0 5 0 1 0 0 1 5 0 2 0 01 0 - 1

1 0 0

1 0 1�

16O

/ 88Sr

� ��

��

0 . 0

0 . 5

1 . 0

1 . 5

2 . 0

56Fe

/ 88Sr

Figure 9. Mean oxygen concentration x in STO and STF50 mate-rials according to the chemical formula SrTiOx or SrTi0.5Fe0.5Ox.The iron profile is shown to indicate boundaries between STF50and STO layers. Large noise due to a weak and noisy Sr referencesignal.

0 5 1 0 1 5 2 01 0 1

1 0 2

1 0 3

1 0 4

1 0 5

1 0 6

1 0 7

1 0 8

A u 1 6 O 1 8 O S r T i F e L a A l

Coun

ts pe

r sec

ond

M e a s u r e m e n t t i m e / m i n

A u L A O S T F S T O S T F S T O

Figure 10. Initial SIMS profile of as-prepared sample with follow-ing structure: Au | LAO | STF50 | STO | STF50 on STO.

with enhanced Fe signal which are attributed to STF layers.Between them there is one STO. The area close to the surface275

with enhanced La and Al signals is the LAO blocking layer.It must be noted that the SIMS count rate of both oxygen iso-topes in the STF layers are significantly lower than in STO.It may be related to slightly different sputter rate in both ma-terials. The concentration of 18O in the initial profile follows280

the natural abundance of this isotope.The 3D analysis of the samples annealed at 600 °C in 18O

shows results dependent on the structure of the samples. Incase of a single STF layer on the STO substrate, a 50 nm thinarea with enhanced oxygen concentration in STO substrate285

close to the interface is observed. In case of structures con-taining a 50 nm STO film between two 50 nm STF layers, asignificantly increased oxygen concentration in the interme-

0 5 0 1 0 0 1 5 00 . 0 0

0 . 0 1

0 . 0 2

��

�

18O

/ (16O

+ 18O)

��

��!��

Figure 11. 18O depth profile obtained after 11700 s annealing at500 °C. Dotted and dashed lines are respectively a fit to analyti-cal solution of Fick’s second law and 2D diffusion simulation withComsol Multiphysics software.

- 2 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 00 . 0 1

0 . 0 2

0 . 0 3 ��

�

18O

/ (16O

+ 18O)

��

��

Figure 12. 18O depth profie after 3600 s annealing at 700 °C.

diate STO layer is observed (see Fig. 13). Even larger 18Oconcentration is measured in the vicinity of the initial pre-290

annealing SIMS crater.Based on the SIMS data an oxygen diffusion coefficient

along the interface at 600 °C of DO = 3.4 · 10−10 m2/s isfound. For comparison, the oxygen diffusion coefficient DO

in YSZ measured at the same temperature by Kilo et al.295

(2004) is approximately two orders of magnitude lower (ca.2 . . .3 · 10−12m2/s).

4.4 Oxygen partial pressure dependent conductivity

The conductivity measurements are performed in two differ-ent gas buffer compositions, CO/CO2 and H2/H2O. Both gas300

mixtures are applied to figure out if the conductivity is af-fected by e.g. diffusion of hydrogen or hydroxyl groups inthe crystals.


00.2

0.40.6

0.81.0

1.21.4

1.61.8

2.0

x / mm

Au

LAO

STF

STO

STO

STF

Figure 13. A cross section of 3D 18O profile showing enhancedoxygen concentration in the intermediate STO layer. In the middleof the profile the pre-annealing SIMS crater is visible.

Figure 14. 18Two cross sections of oxygen distribution showinginfluence of the oxygne diffusion coefficient across the interface onthe shape of concentration depth profile.

0 . 0 0 . 2 0 . 4 0 . 60 . 0 0 0

0 . 0 0 5

0 . 0 1 0

0 . 0 1 5

0 . 0 2 0

0 . 0 2 5 ��

�

18O

/ (16O

+ 18O)

��0 . 0 0 . 2 0 . 4 0 . 6

�

��

��

��

Figure 15. Impact of oxygen diffusion coefficient along the inter-face on the oxygen depth profile acquired 100 µm from the initialSIMS crater. Higher diffusion coefficient (D1 and 10D1 in this ex-ample) do not cause any significant change in the profile, whereaslower coefficients (0.1D1 and 0.01D1) cause unequal oxygen dis-tribution in the interface area.

- 5

- 4

- 3

log(p O2

/ bar)

0 . 60 . 81 . 01 . 21 . 41 . 61 . 82 . 0

R / M

Ohm

0 1 2 3 4 5 6 7

t i m e / h

Figure 16. Exemplary plot showing the change in samples resis-tance during changes in the oxygen partial pressure. Large pO2

noise is caused by operating the oxygen sensor at unusually lowtemperature of 500 °C.

After the requested temperature stabilizes, the pO2is

firstly decreased stepwise from 10−3 to 10−23 bar and then305

increased stepwise back to 10−3 bar. At each step, the oxy-gen partial pressure is held constant until the measuredimpedance stabilizes, that is until the changes in the mea-sured resistance drops below 1 %. After two complete pO2

ramps new temperature is adjusted. This process is shown310

exemplarily in Fig. 16 where the changes in oxygen partialpressure and samples resistance are shown as a function oftime.

The results show that there is only a slight difference inthe conductivity measured in two different gas mixtures. In315


- 2 4 - 2 2 - 2 0 - 1 8 - 1 6 - 1 4 - 1 2 - 1 0 - 8 - 6 - 4 - 2- 4 . 5- 4 . 0- 3 . 5- 3 . 0- 2 . 5- 2 . 0- 1 . 5- 1 . 0- 0 . 50 . 0

- 1 / 4

��

�log

(σ / S

/m)

��

��

1 / 4

Figure 17. Conductivity of the sample as a function of oxygenpartial pressure and temperature. The measurements are performedin CO/CO2 (closed symbols) and H2/H2O (open symbols) atmo-spheres.

case of carbon monoxide atmosphere, the n-type conductiv-ity at low oxygen partial pressures is slightly lower comparedto the hydrogen containing atmosphere. One can recognizethree regions in the conductivity data

– n-type conductivity below 10−16 bar with σ ∝ p−1/4O2320

and EA = 3.4 eV (CO) / 2.7 eV (H2)

– ionic conductivity in intermediate range with EA =0.74 eV (CO) / 0.66 eV (H2)

– p-type conductivity above 10−9 bar with σ ∝ p1/4O2and

EA = 0.46 eV (CO) / 0.50 eV (H2)325

The activation energies for p- and n-type electronic con-ductivity are found to be lower than those of pure STO crys-tals. Balachandran and Eror (1981) reports activities of re-spectively 1.56 and 4.56 eV, Chan et al. (1981) reports acti-vation energy of p-type conductivity of 0.98 eV. The activa-330

tion energy of ionic conductivity and thus of oxygen vacan-cies diffusion is lower then 0.99 eV measured in pure STOby Ohly et al. (2006).

The band-gap enthalpy Eg determined from intrinsic min-ima of the electronic conductivity (σn +σp) is found to be335

3.22 eV. The value is in good agreement with band-gap en-thalpies reported by other authors (3.36 eV (Balachandranand Eror, 1981), 3.30 eV (Chan et al., 1981), 3.30 eV (Ohlyet al., 2006)).

The measurement in hydrogen and carbon monoxide con-340

taining atmospheres is shown in Fig. 17. Here, the modeledn-type conductivity contribution at 510 °C does not exceedthe ionic conductivity and is, therefore, not shown.

In comparison to pure STO material (see Fig. 18) one canobserve that the n-type conductivity is nearly unchanged. The345

thickness of the deposited structures as well as the thickness

- 2 2 - 2 0 - 1 8 - 1 6 - 1 4 - 1 2 - 1 0 - 8 - 6 - 4 - 2- 4 . 5- 4 . 0- 3 . 5- 3 . 0- 2 . 5- 2 . 0- 1 . 5- 1 . 0- 0 . 50 . 0

M e a s u r e d σ E l e c t r o n i c c o n t r i b u t i o n t o σ i n S T O C u r r e n t w o r k - e l e c t r o n i c c o n t r i b u t i o n t o σ

log(σ

/ S/m

)

l o g ( p O 2 / b a r )

Figure 18. Comparison of electronic STO conductivity (dotted lineafter Ohly et al. (2006)) with measurement on the STF layer on STOsubstrate (solid) and total conductivity (points).

of the space charge zone are in the nanometer range whichis orders of magnitude lower than the thickness of the STOsubstrate. Since the impedance measurement provides a totalconductivity only, decrease of σn at low oxygen partial pres-350

sures cannot be measured properly. On the other hand, strongenhancement of the p-type and ionic conductivity by severalorders of magnitude at medium and high oxygen partial pres-sures results in measurable increase of total conductivity (in-cluding STO substrate) by a half order of magnitude. Using355

the conductance from Eq. (7) and thickness of all layers onecan calculate the p-type conductivity of interface area. At atemperature of 700 °C a conductivity at the interface of about400 S/m for a space charge zone thickness of 30 nm is found.In this calculation the STF conductivity is taken from Roth-360

schild (2006).

4.4.1 Linking the diffusion data with ionic conductivity

Assuming that ionic contribution in STF films on STO is at-tributed to Frenkel defects, one can calculate the oxygen dif-fusion coefficient from the conductivity data. The 3D SIMS365

measurements reveal a 50 nm thick layer of oxygen enhanceddiffusivity in STO in the vicinity of interface. Using thisnumber and the ionic contribution to σ one obtains an oxy-gen diffusion coefficient of DO = 2.0 · 10−10 m2/s. Havingthe uncertainty of the measurements in mind, the value is370

in a good agreement with the fit of oxygen concentrationobtained by SIMS measurements at the same temperature(3.4 · 10−10 m2/s at 600 °C).

5 Conclusions

In this work epitaxial structures containing interleaved STO375

and STF layers grown epitaxially on STO substrates aresynthesized and described by oxygen tracer diffusion ex-


periments and conductivity measurements at different gases,temperatures and oxygen partial pressures. The 3D sec-ondary ion mass spectrometry shows increased 18O concen-380

tration in the vicinity of the interface (thickness: 50 nm) be-tween STF and STO which in turn confirms that oxygen dif-fusion coefficient in that area increases significantly.

Conductivity measurements in H2/H2O and CO/CO2

show that there is only slight impact of the atmosphere on385

the resistance of the structures. The materials show an in-creased p-type and ionic conductivity in comparison to pureSTO. Oxygen diffusion coefficient calculated from the con-ductivity data shows good agreement with that of SIMS mea-surements.390

Acknowledgements. This joint research project was financiallysupported by the state of Lower-Saxony and the VolkswagenFoun-dation, Hannover, Germany.

References

Badwal, S.: Zirconia-based solid electrolytes: microstructure, sta-395

bility and ionic conductivity, Solid State Ionics, 52, 23–32, 1992.Balachandran, U. and Eror, N.: Electrical Conductivity in Strontium

Titanate, J. Solid State Chem., 39, 351–359, 1981.Chan, N.-H., Sharma, R., and Smyth, D.: Nanostoichiometry in

SrTiO3, J. Electrochem. Soc., 128, 1762–1768, 1981.400

De Souza, R.: The formation of equilibrium space-charge zones atgrain boundaries in the perovskite oxide SrTiO3, Phys. Chem.Chem. Phys., 11, 9939–9969, 2009.

Fergus, J. W.: Perovskite oxides for semiconductor-based gas sen-sors, Sensors & Actuators: B. Chemical, 123, 1169–1179, 2007.405

Gerstl, M., Frömling, T., Schintlmeister, A., Hutter, H., and Fleig,J.: Measurement of 18O tracer diffusion coefficients in thin yttriastabilized zirconia films, Solid State Ionics, 184, 23–26, 2011.

Gömann, K., Borchardt, G., Schulz, M., Gömann, A., Maus-Friedrichs, W., Lesage, B., Kaïtasov, O., Hoffman-Eifert, S., and410

Schneller, T.: Sr diffusion in undoped and La-doped SrTiO3

single crystals under oxidizing conditions, Phys. Chem. Chem.Phys., 7, 2053–2060, 2005.

Hu, Y., Tan, O., Cao, W., and Zhu, W.: A low temperature nano-structured SrTiO3 thick film oxygen gas sensor, 2004.415

Ioffe, A., Rutman, D., and Karpachov, S.: On the nature of the con-ductivity maximum in zirconia-based solid electrolytes, Electro-chemica Acta, 23, 141–142, 1978.

Kilo, M., Argirusis, C., Borchardt, G., and Jackson, R.: Oxygen dif-fusion in yttria stabilised zirconia — experimental results and420

molecular dynamics calculations, Phys. Chem. Chem. Phys., 5,2219–2224, 2004.

Ohly, C., Hoffmann-Eifert, S., Guo, X., Schubert, J., and Waser,R.: Electrical Conductivity of Epitaxial SrTiO3 Thin Films as aFunction of Oxygen Partial Pressure and Temperature, J. Am.425

Ceram. Soc., 89, 2845–2852, 2006.Rothschild, A.: Electronic Structure, Defect Chemistry, and Trans-

port Properties of SrTi1−xFexO3−y Solid Solutions, Chem.Mater., 18, 3651–3659, 2006.

Schulz, M., Brillo, J., Stenzel, C., and Fritze, H.: Oxygen partial430

pressure control for microgravity experiments, Solid State Ionics,

225, 332–336, http://www.sciencedirect.com/science/article/pii/S0167273812002445, 2012.

Wagner, S., Menesklou, W., Schneider, T., and Ivers-Tiffee, E.:Kinetics of oxygen incorporation into SrTiO3 investigated by435

frequency-domain analysis, 2004.

http://www.sciencedirect.com/science/article/pii/S0167273812002445



oxygen transport in epitaxial srtio /srti xfexo...

Documents