Publi Structure Finale

8/14/2019 Publi Structure Finale

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Structural and physical characterization of hexagonal rare-earth manganites

ReMnO3 films (Re = Tb, Dy, Ho, Er, Y) grown by MOCVD

I. Glard1, G. Huot1, N. Jehanathan2, O. I. Lebedev2, G. Van Tendeloo2, F. Ducroquet3 and C.

Dubourdieu1, a)

1. Laboratoire des Matriaux et du Gnie Physique, CNRS, Grenoble INP, 3 parvis Louis Nel,

38016 Grenoble, France

2. Electron Microscopy for Materials Research (EMAT), University of Antwerp,

Groenenborgerlaan 171, B2020 Antwerpen, Belgium

3. Institut de Microlectronique, Electromagntisme et Photonique, CNRS, Grenoble INP, 3

parvis Louis Nel, 38016 Grenoble, France

a) Author to whom correspondence should be addressed; electronic mail:

[email protected]

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Hexagonal manganites ReMnO3 thin films, with R = Y, Er, Ho, Dy and Tb, were grown by liquid

injection MOCVD on (111) YSZ and (111) Pt-buffered Si substrates. On both substrates the

films had the direction of the ferroelectric polarization (c-axis) perpendicular to the substrate

plane. Hexagonal DyMnO3 and TbMnO3 phases were obtained through epitaxial phase

stabilization and exhibit a high crystalline quality and smooth surfaces over a large deposition

temperature range (800-925C), while the quality of the other ReMnO3 phases was very

temperature-dependent. A study of the microstructure by high-resolution transmission electron

microscopy evidenced the presence of secondary orientations. These inclusions are epitaxially

oriented relatively to the matrix and have a lateral size of ~ 10-20 nm. All investigated films were

antiferromagnetic. Ferromagnetic ordering of the rare-earth magnetic moments was observed at

low temperature (< 10 K). A dielectric constant of 17-20 for YMnO3 films was deduced from

capacity voltage measurements. The leakage current density was found to be dependent on the

type of rare-earth element in ReMnO3.

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I. INTRODUCTION

Multiferroic compounds exhibit simultaneously at least two ferroic orders in the same phase such

as ferroelectricity and ferromagnetism [1,2]. By extension, antiferrroic orders are also included.

These materials are of interest for combining multiple functionalities in a same device. Moreover,

if the electric and magnetic orders are coupled, giving rise to a magnetoelectric effect, it could be

possible to have a mutual control of the properties: controlling a polarization by a magnetic field

or controlling a magnetization by an electric field. This opens the route to completely new

devices, such as electrically-controlled magnetic data storage. The field of magnetoelectrics, and

more specifically of magnetoelectric multiferroics, has experienced a strong revival since few

years [3-6]. Hexagonal rare-earth (Re) manganites ReMnO3 - with Re = Y, Sc and Ho-Lu - are

among the few oxides, which are both ferroelectric and magnetic. The paraelectric to ferroelectric

transition takes place at a temperature of ~ 900-1000 K, and the paramagnetic to

antiferromagnetic transition takes place at ~ 70-100 K. The hexagonal structure can be described

as dense oxygen-ion packing (ABCACB) with Mn3+ ions having a five-fold trigonal bipyramidal

coordination and with Re3+ ions having a sevenfold mono-capped octahedral coordination [7,8].

In the form of thin films, YMnO3 has been the most studied compound of the series, originally

for its ferroelectric properties and as a possible candidate for gate oxide in ferroelectric-field

effect transistors [9,10]. The YMnO3 films are grown by pulsed laser deposition [11-31], rf

magnetron sputtering [9,10,32-40], chemical solution deposition [41-47], molecular beam epitaxy

[48,49] sol-gel method [50-52] or metal organic chemical vapor deposition [53-56]. Fewer

studies have been devoted to other hexagonal ReMnO3 thin films. Among these are thin films of

HoMnO3 [56-66], YbMnO3 [15,26,39,45,46,67,68], ErMnO3 [69-71] and YMnO3/HoMnO3

superlattices [56]. A few years ago, we reported the use of epitaxial phase stabilization for

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growing in the hexagonal form the ReMnO3 compounds, which normally crystallize in an

orthorhombic perovskite phase [72-74], such as DyMnO3, GdMnO3, EuMnO3 or SmMnO3. Other

reports have followed using this method for the growth of hexagonal DyMnO 3 [21,56,61-63,75],

TbMnO3 [56,62,63,76,77], GdMnO3 [21,61-63,78,79] and ScMnO3 [61].

In this paper we present a detailed study of the growth and properties of YMnO3, ErMnO3,

HoMnO3, DyMnO3 and TbMnO3 thin films on two types of substrates. The effect of the

deposition temperature on the crystallinity and on the surface morphology is investigated. The

local microstructure is studied. Finally, the magnetic and the dielectric properties of the films are

discussed.

II. EXPERIMENTAL

ReMnO3 (Re = Y, Er, Ho, Dy, Tb) films were grown on two different substrates: ZrO2(Y2O3) cut

along the (111) plane and (111) Pt (150 nm)/ TiO 2 (20 nm)/ SiO2 (300 nm)/ p-type (001) Si -

noted respectively (111) YSZ and (111) Pt/Si. The synthesis was achieved by liquid injection

metal organic chemical vapor deposition [80]. Mn(tmhd)3 and Re(tmhd)3 precursors were mixed

in an organic solvent (monoglyme) and injected with a micro-valve into an evaporator held at

250C. Flash evaporation occurs and the vapors of the reactive species are transported with argon

towards the heated surface of the substrate, in a total pressure of 0.66 kPa and with an oxygen

partial pressure of 0.33 kPa. Films of variable thickness, from 5 up to 500 nm were prepared.

Different growth temperatures were investigated ranging from 800 to 925C. For TbMnO3 and

DyMnO3 films a higher substrate temperature favors the phase stabilization and increases the

critical thickness above which the stabilization is no longer possible [74]. After deposition

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samples were in-situ annealed at the same temperature as the growth under one atmosphere of di-

oxygen.

The crystalline structure of the films was studied by x-ray diffraction. /2 -scans and -scans

were performed using CuK radiation. Rocking curves ( -scans) were performed using FeK

radiation. The films morphology was studied by atomic force microscopy in tapping mode.

Selected samples were observed by high-resolution transmission electron microscopy with a

JEOL4000EX microscope operated at 400 kV. Cross-section samples were prepared by

mechanical grinding, down to a thickness of about 20 m, followed by ion-milling. The samples

were cut parallel to a cubic plane of the substrate, perpendicular to the contact plane. The

magnetic measurements were done in a commercial SQUID with the magnetic field

perpendicular or parallel to the substrate plane. Electrical measurements were performed on

capacitive structures using top gold electrodes, which were evaporated through a shadow mask.

C(V) curves were recorded using a HP4284A precision LCR meter. I(V) curves were measured

using a HP 4156B parameter analyzer.

III. RESULTS AND DISCUSSION

The crystalline structure of the films depends strongly on several parameters such as the growing

temperature, the average composition of the films and their thickness. The change in lattice

parameters as a function of thickness and composition of the films will be described elsewhere.

We focus here on the effect of the processing temperature on the quality of the epitaxy and on the

differences found with the nature of the rare-earth. We investigated a temperature range of 800-

925C. Note that the upper limit is rather high and that most physical vapor deposition (PVD)

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techniques, such as pulsed laser deposition (PLD), usually do not reach such a high temperature.

A. Films grown on (111) YSZ substrates

Single crystals of YSZ (with a = 0.514 nm) cut along the (111) plane (from CrysTec [81]) were

chosen because the corresponding in-plane lattice exhibits a low lattice mismatch and an

excellent coincidence of the oxygen atoms with the lattice in (a, b) plane of hexagonal

manganites. Figure 1 is a sketch of the (111) plane of the substrate and of the three possible

arrangements of a ReMnO3 manganite on this plane. With these arrangements the hexagonal

manganite cell is in good coincidence with (111) YSZ lattice. The lattice mismatch is in the range

4 to +4 % for all rare-earths, respectively from La to Lu [72,73]. At room temperature, the

lowest lattice parameter mismatch is for Sm (+0.1%) and then it increases for Tb (+1.7%), Dy

(+1.9%), Ho (+2.6%), Y (+2.7%) and Er (+2.9%). The DyMnO3 and TbMnO3 hexagonal phases

are obtained through epitaxial phase stabilization. The stabilization is possible thanks to the free

energy gained from the formation of a coherent interface [82,83]. Since the contribution of the

surface energy is inversely dependent on the film thickness, the stabilization occurs only for a

limited thickness. Above this critical thickness, the growth reverts to the bulk thermodynamically

stable phase (cubic perovskite).

Typical examples of /2 -scans were presented in Ref. 52 for 25 to 100 nm thick manganite

films grown at 850 or 900C. All the films are oriented with the c-axis perpendicular to the

substrate plane, which is the direction of the ferroelectric polarization in these hexagonal phases.

Only 00 peaks of the hexagonal LuMnO3-type structure appear. Secondary 110 orientations can

appear for some films, which we relate to the fact that hh0 orientations also appear for the YSZ

substrates, which are not perfect single crystals. The -scans reveal the sixfold symmetry of the

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hexagonal structure. An example of -scans is presented in figures 2(a) and 2(b) for the 112

reflection of a 150 nm thick TbMnO3 film and for the 200 reflection of the substrate respectively.

The films are textured with the following epitaxial relationships: (001) hex. ReMnO3 // (111) YSZ

and hex. ReMnO3 // YSZ.

Rocking curves ( -scans) were performed on the 004 peak of the manganites. A typical rocking

curve, obtained for a 25 nm thick DyMnO3 film, is given in figure 3(b). Two contributions are

found when fitting the curves: a broad peak of small intensity and a sharper peak of high

intensity. The contribution of the sharp peak comes from the (partially) relaxed epitaxial film,

whereas the broad peak may arise from diffuse scattering on defects such as misfit dislocations

[84,85]. In the following, we focus on the values of the sharper contribution, which reveal the

quality of the epitaxy.

For YMnO3 films grown at 800C, the full width at half maximum (FWHM) of the 004 rocking

curve is of 0.17 - 0.22 for films thinner than 50 nm, which denotes a reasonable spread of the c-

axis orientation, and it increases up to 0.46 for 150 nm thick films. For comparison Dho et al.

report FWHM values of 0.06 for films grown also on YSZ [18]. The FWHM values for

HoMnO3 grown at 850C are quite similar to those of YMnO3, ranging from 0.13 for 25 nm up

to 0.31 for 150 nm thick films. Kim et al. report similar values in the range 0.1 - 0.4 [58,59] for

films grown by PLD. A 100 nm ErMnO3 film grown at 850C exhibits a FWHM of 0.12. As

shown in Fig. 3(a), the FWHM value of YMnO3 films (with a thickness of 25 nm) depends on the

deposition temperature. A minimum value is found for films grown at 850C. Epitaxially-

stabilized DyMnO3 and TbMnO3 films exhibit remarkably small c-axis orientation spread, with

FWHM values of respectively 0.06 and 0.09 for 25 nm films. These values increase

respectively to 0.35 and 0.15 for 150 nm thick films. Remarkably too, these values are

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temperature-independent on a large processing window of 825-925C, as illustrated in Fig. 3(a)

for DyMnO3 films (for a similar thickness of 25 nm).

A clear correlation was found between the FWHM trends for the different rare-earth manganites

and the surface morphology, which was investigated by AFM in tapping mode. Figure 4 presents

typical AFM images of the film surface on 2x2 m2 scans and the dependence of the root mean

square (rms) value of the roughness as a function of film thickness. The surface roughness

increases for increasing film thickness. For YMnO3 and HoMnO3, rms values are of the order of

0.1 - 0.5 nm (less than half a monolayer) for 5 nm films and increase up to ~ 3.5 - 4.0 nm for 150

nm thick films. DyMnO3 and TbMnO3 are remarkably smooth with rms values in the range 0.1 (5

nm films) to 0.9 nm (150 nm films), thus lower than one unit cell over the whole thickness range.

The thickness dependence of the roughness is also much lower than for YMnO3 or HoMnO3

films. For the thinner films, steps of ~ 1 nm height (~ 1 unit cell) are observed, evidencing a 2D

growth. Steps were not observed on as-received (111) YSZ substrates but have been seen after

heating a substrate at 800C and then annealing it for 15 minutes under one atmosphere of

oxygen.

The smoother and better-oriented films in the case of DyMnO3 and TbMnO3 (compared to

YMnO3 and HoMnO3) may originate in the lower lattice mismatch with the substrate. While no

significant effect could be observed on the /2 -scans for the different rare-earth and growing

temperatures, the film quality is well benchmarked with the cross correlation found between the

FWHM of the 004 rocking curves and the rms value of the roughness as shown in Fig. 3(a). From

this analysis, the substrate temperature appears to be quite critical for the growth of YMnO 3 and

HoMnO3 films.

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TEM images and electron diffraction (ED) patterns were performed on selected films. For all

films a relatively flat interface is observed and the ED patterns reveal an epitaxial growth. Figure

5(a) shows a low magnification TEM image of a ~ 100 nm thick YMnO 3 film on (111) YSZ. This

film appears to have a columnar growth that develops as the film thickness increases. Such a

growth was not observed for a TbMnO3 film of similar thickness. These observations are in good

agreement with the much higher roughness observed for YMnO3 films compared to the

epitaxially-stabilized ones. Figure 5(b) is the corresponding ED pattern taken at the film-substrate

interface. The bright reflections correspond to the YSZ substrate; the other spots are originating

from the YMnO3 film and indexed based on a hexagonal structure with P63cm space group. The

following epitaxial relationship was deduced from the ED patterns for all investigated ReMnO3

films: [001]ReMnO3 // [111]YSZ and (-110)ReMnO3 // (01-1)YSZ. This relation is consistent with the

epitaxial relationship determined from x-ray diffraction.

The extra weak spots observed for the YMnO3 film are due to a secondary orientation of the

growth. The HRTEM image of Fig. 5(c) shows the interface between the YMnO3 film and the

YSZ substrate. Nano-inclusions of secondary orientations are observed, which are epitaxially

grown relatively to the film matrix, with the following relationship: [-1-11]incl. // [001]matrix and (-

110)incl. // (-110)matrix. We already observed such nano-inclusions in DyMnO3 hexagonal-stabilized

films [74]. We recently showed that these inclusions have a significant impact on the overall

ferroelectric response of the films measured by second harmonic generation (SHG) and that they

appear to be polarized with a preferred sense of the ferroelectric polarization along the c direction

[86]. They were also detected by SHG in PLD-grown ReMnO 3 films on (111) YSZ and no

systematic trend for their occurrence could be determined as a function of growth temperature,

nature of rare-earth or deposition technique [86].

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Finally, Fig. 5(d) and 5(e) show the HRTEM images of HoMnO3 (~15 nm) and TbMnO3 (~110

nm) films. The films appear to be free of defects within the bulk of the film. The interface

between the (111) YSZ substrate and the film is flat and well defined. The structure at the

interface will be discussed in details elsewhere [87].

B. Films grown on (111) Pt/Si substrates

YMnO3, ErMnO3, HoMnO3 and DyMnO3 thin films were synthesized on (111) Pt-buffered Si

substrates purchased at Inostek [88]. Such stack is used as a conductive substrate, which allows

electrical measurements. Deposition of Pt on Si was preferred to other substrates (Pt-buffered

SrTiO3 or alumina) for future integration purposes. However, Pt is not epitaxially grown on

silicon; it is only 111 oriented (perpendicular to the Si surface) without in-plane epitaxy. All the

manganite films were grown oriented with the c-axis - the direction of the ferroelectric

polarization - perpendicular to the substrate plane [56]. Only 00 peaks of the hexagonal

structure appear for films thinner than 100 nm, but secondary 110 and 112 orientations can

appear for the thicker films in a larger amount than on (111) YSZ substrates. Films are mainly

00 oriented but -scans show that there is no in-plane texture since the substrate itself is not

single crystalline. The hexagonal structure and the quality of the growth along the c-direction

were also confirmed by high-resolution TEM observations. Despite the absence of an in-plane

texture for the substrate, the epitaxial stabilization was achieved for DyMnO 3 thanks to the local

epitaxy of DyMnO3 on the 111-oriented Pt grains.

Rocking curves were performed on the 004 peak of the manganite. The curves can be fitted with

a single peak with quite large FWHM, of typically 1.5 up to 2.8. The substrate exhibits a

FWHM for the 111 reflection of 2.1, which explains the large FWHM of the manganite. Figure

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6 shows typical AFM images obtained for HoMnO3 and DyMnO3 films. The roughness of the

thin films (5 to 10 nm) and the grain size are larger than for epitaxial films on (111) YSZ. For

example, rms roughnesses of 0.9, 1.4 and 1.5 nm are measured for 5 nm films of YMnO3,

HoMnO3 and DyMnO3 respectively. For thicker films (150 nm) the values are in the range 2.4 up

to 3.6 nm. The substrate itself exhibits a quite large rms roughness of 1.0 nm.

Figure 7 shows the low and high magnification TEM images of the cross-sectional sample of a

HoMnO3 film on (111) Pt-buffered Si. The thickness of the film is about 22 nm. The contrast

variations at the grain boundaries of Pt are due to the different orientations of the grains. Figure

7(b) shows the ED pattern taken at the interface between the HoMnO3 film and Pt. It shows that

there is locally an epitaxial growth of the film on the (111) Pt substrate, although no epitaxy was

observed at a macroscopic scale by scans. The HoMnO3 phase grows epitaxial on Pt single

crystalline grains, with a well defined in-plane texture. The local relationship deduced from the

ED is the following: [001]HoMnO3 // [-111]Pt and (-110)HoMnO3 // (01-1)Pt (the bright spots on the

diffraction pattern are indexed to the Pt substrate and the weaker spots to that of the HoMnO 3

film in a hexagonal structure with P63cm space group). A similar result was found for the

stabilized hexagonal phases and this local epitaxy explains why the stabilization is possible on

these non single-crystalline Pt substrates.

Figure 7(c) shows the HRTEM image of this film. The bulk of the film appears to be free of

defects. The distinct contrast variation observed at the interface is an artifact due to Ar milling

damage during the TEM sample preparation.

C. Magnetic measurements

Magnetic measurements were performed on the epitaxial films grown on (111) YSZ substrates.

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The magnetization was measured in a SQUID magnetometer under 0.2 Tesla as a function of

temperature in zero-field cooled (ZFC) and field-cooled (FC) conditions. The magnetic field was

usually applied parallel to the c-axis but measurements in perpendicular configurations were also

performed.

Figures 8(a) and (b) present the magnetization and the inverse of the magnetization versus

temperature obtained in ZFC conditions for 200 nm TbMnO3 and 100 nm ErMnO3 films

respectively (with H//c). There are no clear anomalies or cusps in the magnetization-versus-

temperature curves that could indicate a magnetic transition; only a strong increase of the

magnetization is observed below ~ 50 K. A similar behavior is observed in bulk polycrystalline

(powder) YMnO3 samples [89, 90], while a cusp in the susceptibilities both in the (a,b) plane or

along c is observed in single crystals of YMnO3 [91, 92]. The change in susceptibility is actually

quite small in single crystals [91, 92] and thus hardly detectable in thin films. Since the cusp is

sharper for c while ab exhibits a less definite cusp and a broad peak for T< TN [91], we show

here the magnetization with the field applied perpendicular to the substrate plane (H//c).

However, measurements with H parallel to (a,b) plane display no cusp either.

A paramagnetic behavior is observed above ~ 100 K, with a linear behavior (Curie-Weiss law) of

the inverse of the magnetization with temperature. A deviation of this linearity is observed below

~ 100 K, with a negative paramagnetic Curie-Weiss temperature p, which indicates

antiferromagnetic interactions. Our p values determined from measurements with H//c range

typically between -200 and -20 K for the different investigated compounds. For example, p is

of - 22 K for the 200 nm TbMnO3 film shown in Fig. 8(a). This temperature is similar to the one

measured for PLD-grown film by Lee et al. [76]. For a same compound such as YMnO3, the

Curie-Weiss temperatures could be quite different depending on film thickness: p of -46 and

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-200 K were measured (H// c) for thicknesses of 450 and 200 nm respectively, which might come

from different defects or stoichiometry in the films. Values reported in the literature for bulk

YMnO3 are actually also widespread: -705 K (from ab) [91], - 567 K [92], - 510 K [90] and

-312 K [89]. The large difference found even in single crystalline samples [91,92] could originate

from different types and amounts of impurities in the samples. Our epitaxial films display smaller

| p | values than the corresponding single crystals.

From the analyses of the different films and thicknesses investigated, we concluded that a precise

determination of TN is not possible due to uncertainties related to the subtraction of the strong

paramagnetic contribution of the substrate. Recently, we clearly evidenced by neutron diffraction

the antiferromagnetic transition in 450-500 nm thick YMnO3 ErMnO3 and HoMnO3 films [92],

corresponding to the ordering of the Mn3+ magnetic moments in the (a,b) basal plane in a

triangular frustrated configuration. The ordering temperature TN was determined precisely from

the change in intensity of the maximum of a magnetic Bragg peak. For a thickness of 450-500

nm, TN in YMnO3 and ErMnO3 films is 66 K and 68 K respectively (about 10 K lower than the

bulk value), while for HoMnO3 film, an antiferromagnetic transition was evidenced at 50 K [93].

The magnetization curves reported in figure 8 show a rather high magnetization of tens of

emu/cm3, which is typically one order of magnitude larger than values reported for other

hexagonal manganite films [60,71,76]. We did not observe such large values on all investigated

films. A possible origin could be the presence of Mn3O4 inclusions. The hausmannite Mn3O4

compound is ferrimagnetic at about 42 K [94]. We deposited a 50 nm thick Mn3O4 film on (111)

YSZ in similar conditions as the ones used for the manganite growth. Its magnetization versus

temperature curve is shown in Fig. 8(c). A magnetic transition is observed at 43.5 K, which is

consistent with the bulk value [94]. This result also indicates that one should be careful in

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assigning any magnetic transition around 40-44 K in manganite films as Mn3O4 may possibly

contribute (amounts below 1% might not be detected by conventional x-ray diffraction).

In Fig. 8(a), a maximum of the magnetization is observed at ~ 8 K, which is ascribed to the

ferromagnetic ordering of the rare-earth magnetic moments. The ordering temperature is in good

agreement with the value of 7 K previously reported for TbMnO 3 films [76]. Such a peak was

also measured by Posadas et al. for a YbMnO3 film grown on (001) ZnO [39] or by Lee et al. for

a GdMnO3 film grown on (111) YSZ [78]. We confirmed a ferromagnetic behavior at low

temperature on TbMnO3 films by measuring the magnetization versus applied magnetic field at 4

K. A hysteresis loop was obtained and was no more observed at 30 K, as shown in Fig. 8(d) for a

500 nm TbMnO3 film.

Figures 8(b) show the difference between ZFC and FC magnetizations (measured with H//c) as a

function of temperature for a 100 nm ErMnO3 film. No difference is expected in the case of an

antiferromagnetic compound. However, a splitting is sometimes observed below 50 K as shown

in Fig. 8(b). A similar behavior was observed by other groups: by Noh and coworkers for

GdMnO3, TbMnO3, DyMnO3, ErMnO3 and HoMnO3 films [60,71,75,77,78], by Fujimura and

coworkers for YbMnO3 films [26,67], by Han and Lin [64] and by Drr and coworkers [66] for

HoMnO3 films. It was also reported for bulk samples, for instance by Munoz et al. for a ScMnO3

powder [89] or Fontcuberta et al. for YbMnO3 single crystal [96]. This difference between FC

and ZFC curves has been reported to be probably due to a spin-glass behavior [60,71,75,77]. It

could, however, also be caused by a weak ferromagnetic contribution in the films that could arise

from spin canting as also suggested for bulk samples [95, 96]. The presence of a ferromagnetic

component will be further investigated by neutron diffraction.

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D. Electrical measurements

Electrical measurements were performed at room temperature on MOS structures prepared by

evaporating gold on top of the manganite films. Part of the Pt/Si substrates had been masked

during deposition so that the contact could be easily made on the bottom electrode. Ferroelectric

properties are not addressed as they have been discussed elsewhere from second harmonic

generation measurements [86]. We focus here on C(V) and J(V) characteristics. Measurements

were performed for YMnO3 films of varying thickness. The equivalent oxide thickness (EOT)

was calculated from the capacitance measured at 0 V bias. It represents the thickness of a SiO 2

film that would give the same capacity as the one measured. Considering the contributions of the

manganite film and of an interface between the film and the substrate (two capacitors in series),

the EOT is given by: EOT = ET IL + (3.9/ ) x t

where t is the film thickness, is the relative dielectric permittivity of the film, and ET IL is the

equivalent electrical interfacial layer thickness.

The EOT determined from measurements performed at 10 kHz is plotted in Fig. 9 as a function

of YMnO3 film thickness. A linear dependence is obtained. The dielectric permittivity of the

films can be extracted from the slope and is found to be r ~ 17. For measurements performed at

1 kHz, the relative dielectric permittivity is calculated to be 21. These values are quite close to

the YMnO3 bulk value of 20 [97,98]. The frequency dependence may indicate a resistive

contribution of the sample, which is not taken into account in the simple model use to extract the

capacitance from the measured signal. The intercept of the linear curve in Fig. 9 with the Y-axis

gives an electrical interfacial layer thickness of 1.1 nm. It may be due to a reaction at the

interface between the manganite and Pt.

Typical J(V) curves are shown in Fig. 10(a) for 25, 50 and 150 nm thick YMnO3 films. The

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leakage current density JL is of 4.10-6 A/cm2 at 1 V for the 150 nm film and increases with

decreasing film thickness up to 5.10-3 A/cm2 for the 10 nm film. Other groups have also reported

quite large leakage currents for hexagonal YMnO3 [15,43,55]. For example, Fujimura et al.

reported a value of 8.10-5 A/cm2 at -5 V for an epitaxial film grown on Pt(111)/Al2O3 (001) by

PLD [67]. More generally, complex ferroelectric oxides exhibit larger leakage current densities

than dielectric binary metal oxides such as HfO2, ZrO2, Ta2O5 or Y2O3. For comparison in MIM

capacitor structures, a 40 nm Y2O3 crystalline film exhibits a leakage current density of 10-7

A/cm2 at 1 V [99] and a 8 nm HfO2 film has a leakage current density of ~ 10-10 A/cm2 at 1V

[100]. The origin of the leakage currents in ReMnO3 films is still unclear but we think that the

cationic stoichiometry plays a very important role in the leakage current mechanisms, through

oxygen content changes and structural defects both related to possible off-stoichiometry. The role

of the electrode nature is also not known. In case of BiFeO3, it has been shown that no dominant

leakage mechanism could be determined for asymmetric capacitive structures using a top Pt

electrode [101].

For the other ReMnO3 manganite films, leakage currents were acquired only for 500 nm thick

films (450 nm in the case of YMnO3) and are shown in Fig. 10(b). Measurements in thinner films

were usually not reliable due to excess leakage currents. YMnO3 films exhibit the lowest leakage

currents. For the other rare-earths, we observe that the leakage current density increases with

increasing ionic radius of the rare-earth element; the leakage currents are the most important in

the stabilized films. It should be noted that few data are available for current densities at room

temperature in the literature for ReMnO3 films other than YMnO3. Drr et al. reported a

resistivity of 106-107 .cm at 300 K and high leakage currents for 300 nm HoMnO3 films

deposited on Pt(111)/YSZ(111) by PLD [57]. Also high leakage currents above 150 K are

mentioned for epitaxially-stabilized TbMnO3 films (50 nm) by Lee et al. [76,77]. At room

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temperature, leakage currents of 10-6 A/cm2 at 1 V for a 260 nm HoMnO3 film directly on (100)

Si [65], 10-3 A/cm2 at 1 V for a 210 nm ErMnO3 film directly on (100) Si [70] or 10-4 A/cm2 at 2

V for a 100 nm YbMnO3 film on (111)Pt/sapphire [67] were reported.

Again the role of the off-stoichiometry should be worked out in these ReMnO 3 films in order to

distinguish contributions of intrinsic (such as rare-earth type) versus extrinsic origins in the

leakage mechanisms.

IV. CONCLUSION

Hexagonal thin films of YMnO3, ErMnO3, HoMnO3, DyMnO3 and TbMnO3 were grown on (111)

YSZ and (111) Pt-buffered Si substrates by liquid injection MOCVD. The epitaxially stabilized

hexagonal phases TbMnO3 and DyMnO3 exhibit a high crystalline quality and smooth surfaces

over a large substrate temperature range while YMnO3, ErMnO3 and HoMnO3 growth requires an

optimized narrow temperature range. Interfaces between epitaxial films and the substrate are

smooth and well defined on (111) YSZ. On (111)Pt-buffered Si substrates, a local epitaxy is

observed on the Pt grains, which allows the growth of textured films and epitaxial stabilization

for DyMnO3 and TbMnO3. Nano-inclusions with a lateral expansion of about 10-20 nm

corresponding to secondary orientations were revealed by TEM. This finding may have a

significant implication in the ferroelectric response of such films. YMnO3 films exhibit a

dielectric permittivity of 20 similar to the bulk value and the lowest leakage current among the

different rare-earth manganites investigated. The nature of the rare-earth as well as the overall

stoichiometry of the films are thought to play a major role in the conduction mechanisms. Such

effects will be further investigated as the level of leakage currents is a major concern for practical

applications of these compounds.

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ACKNOWLEDGMENTS

This work was performed at LMGP in the frame of the European Network of Excellence FAME

Functionalized Advanced Materials and Engineering: Hybrids and Ceramics (No. FP6-500159-

1) and of the European Specific Targeted Research Project MaCoMuFi Manipulating the

Coupling in Multiferroic Films (No. NMP3-CT-2006-033221). CrysTec (Germany) and SAFC

Hitech (UK) are acknowledged for providing the substrates and the CVD precursors respectively.

The authors acknowledge financial support form the European Union under the Framework 6

program under a contract for an Integrated Infrastructure Initiative (Reference 026019 ESTEEM).

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Figure captions:

Figure 1: (a) Sketch of the surface of a (111) YSZ substrate with the O atoms positions (circles) -

(b) sketch of the three possible arrangements of a hexagonal ReMnO3 film on this surface.

Figure 2: -scans performed (a) on the 112 reflection of a 150 nm thick TbMnO3 film and (b) on

the 200 reflection of the YSZ substrate.

Figure 3: (a) FWHM values of the 004 rocking curves (square) and rms roughness values

(triangle) as a function of the growth temperature for YMnO3 (closed symbols) and DyMnO3

(open symbols) 25 nm films grown on (111) YSZ. Dashed lines are a guide for eyes. (b) Rocking

curve of a 25 nm thick DyMnO3 film grown on (111) YSZ at 900C.

Figure 4: Top: 2x2 m2 scan AFM images of TbMnO3 and DyMnO3 films grown on (111) YSZ

substrates - Bottom: rms roughness values as a function of YMnO3, HoMnO3, DyMnO3 and

TbMnO3 film thickness.

Figure 5: TEM images of ReMnO3 films grown on (111) YSZ: (a) Low magnification TEM

image of a 100 nm YMnO3 film grown at 800C and (b) its corresponding ED pattern taken at the

interface region with the substrate - (c) High resolution TEM image of the same film at the

interface with the substrate - HRTEM images of (d) a 15 nm HoMnO3 film deposited at 850oC

and (e) a 110 nm TbMnO3 film grown at 900oC.

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Figure 6: 2x2 m2 scan AFM images of HoMnO3 and DyMnO3 films grown on (111) Pt-

buffered Si substrates.

Figure 7: (a) Low magnification cross-section TEM image of a HoMnO3 film on (111) Pt and (b)

its corresponding ED pattern; (c) High resolution TEM image of the same film at the interface

with the substrate.

Figure 8: (a), (b) and (c): magnetization measured as a function of temperature under 0.2 T (H//c)

in field-cooled (FC) or zero-field cooled (ZFC) conditions for films of (a) TbMnO3 (200 nm), (b)

ErMnO3 (100 nm) and (c) Mn3O4 (50 nm), all grown on (111) YSZ. The plot of the inverse of the

magnetization is also presented except for Mn3O4 film (d) Magnetization hysteresis measured

as a function of applied magnetic field (H applied along (a,b)) at 4 and 30 K on a 500 nm

TbMnO3 film on (111)YSZ.

Figure 9: (a) Equivalent oxide thickness (EOT) plotted as a function of YMnO 3 film thickness.

The EOT was calculated from the C(V=0) measured at 100 kHz (V=0). The films were grown at

800C.

Figure 10: J(V) curves (a) of YMnO3 films grown at 800C with different thicknesses and (b) of

different 500 nm thick ReMnO3 films (450 nm in the case of YMnO3) grown at 850C.

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