Formation and stability of lanthanum oxide thin films deposited from β-diketonate precursor

Formation and stability of lanthanum oxide thin ®lmsdeposited from b-diketonate precursor

Minna Nieminen*, Matti Putkonen, Lauri NiinistoÈLaboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, P.O. Box 6100, FIN-02015 Espoo, Finland

Received 18 November 2000; accepted 15 January 2001

Abstract

Lanthanum oxide thin ®lm deposition by atomic layer epitaxy (ALE) was studied at 180±4258C on soda-lime glass and

Si(1 0 0) substrates using a b-diketonate type precursor La(thd)3 and ozone. The chemical constituents of the ®lms were

analyzed by TOF-ERDA, RBS and FTIR while XRD and AFM were used to determine the crystallinity and surface

morphology. Films grown below 2758C were amorphous La2O2CO3, while at deposition temperatures above 3008C XRD

patterns indicated that cubic La2O3 phase was formed. All the ®lms were transparent and uniform with only small thickness

variations. Carbonate type impurity was found in all ®lms, but the carbon content of the ®lms decreased with growth

temperature being 3 at.% in ®lms grown above 4008C. Hexagonal La2O3 was obtained when the ®lms grown on silicon

substrates were annealed at 8008C or above in a nitrogen ¯ow. The as-deposited cubic and annealed hexagonal La2O3 ®lms

were found to be chemically unstable in ambient air since a transformation to monoclinic LaO(OH) and hexagonal La(OH)3

was detected, respectively. # 2001 Elsevier Science B.V. All rights reserved.

Keywords: Lanthanum oxide; Thin ®lm; ALE deposition; b-Diketonate

1. Introduction

In spite of the high application potential of lantha-

num oxide thin ®lms, only a few studies have been

made to prepare them. La2O3 ®lms have been depos-

ited by different physical thin ®lm growth methods

such as electron-beam evaporation [1], pulsed-laser

evaporation [2] and vacuum evaporation [3,4]. Oxida-

tion of lanthanum overlayers has also been used to

prepare lanthanum oxide ®lms [5,6]. The spray meth-

ods [7±9] have been attempted as well, but to the best

of our knowledge only three articles concerning the

CVD growth of La2O3 have been published [10±12].

Lanthanum oxide thin ®lms are reported to be

mechanically stable [3], possess high electrical break-

down ®eld strengths [1,4,8] and dielectric constants

[1]. La2O3 thin ®lms are also optically transparent

over a wide wavelength range from ultraviolet to

infrared [3,8,12]. The potential applications of

La2O3 thin ®lms include dielectric layers in device

applications [1] and protective [13] or optical [8]

coatings. Recently, it has been reported that La2O3

thin ®lms can also be used as coating layers in order to

improve the CO2-sensing characteristics of SnO2 thick

®lm gas sensors [14].

It appears that the growth of good quality La2O3

®lms has not been demonstrated by the CVD method

[10±12]. The main problem seems to be the relatively

large carbon residue in the ®lms. The deposition of

lanthanum oxide ®lms from La(thd)3 and water at

Applied Surface Science 174 (2001) 155±165

* Corresponding author. Tel.: �358-9-451-2605;

fax: �358-9-462-373.

E-mail address: [email protected] (M. Nieminen).

0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 1 4 9 - 0

5708C resulted in poorly crystalline, pale brown ®lms

[10]. Lanthanum oxide carbonate ®lms were obtained

when the growth of the ®lms was carried out at 9008Cby a modi®ed CVD process from La(acac)3 dissolved

in butanol [11]. At lower deposition temperatures,

amorphous or poorly crystallized phases were found

[11]. Amorphous, transparent ®lms were obtained

from La(thd)3 by PECVD method in an oxygen

plasma at 4008C, but a carbon content of 1.6 wt.%

was still observed [12]. It is dif®cult to compare the

quality of ®lms between different studies since the

actual chemical composition of the ®lms was analyzed

only in the PECVD study. According to XRD results,

crystalline La2O3 ®lms (monoclinic [9] or hexagonal

[7]) were obtained only when the deposition tempera-

ture was above 5008C, whereas lower deposition

temperatures led to poorly crystallized or amorphous

®lms. A postannealing in oxygen at 8008C for 6 h was

needed to obtain crystalline, hexagonal La2O3 ®lm

[8]. Only in two articles the chemical stability of

lanthanum oxide ®lms has been discussed. Suzuki

et al. [7] reported that when hexagonal La2O3 ®lms

were left in air, they reacted with CO2 and with time an

increasing amount of La2(CO3)3 was observed by

XRD. In the work done by De Asha et al. [6] lantha-

num oxide ®lms were found to adsorb water in dis-

sociative fashion, leading to extensive surface

hydroxylation.

Atomic layer epitaxy (ALE) [15,16] also known as

atomic layer deposition (ALD) or atomic layer CVD

(ALCVD), is a surface controlled growth technique

where the substrate surface is alternately exposed to

the vaporized reactant pulses. The precursor pulses are

separated by inert purge gas pulses to eliminate gas-

phase reactions and remove reaction products. In an

ideal case, one monolayer (or a distinct fraction of it)

of the ®rst reactant is chemisorbed on the substrate and

this layer reacts with the second precursor pulsed onto

the substrate, resulting in the formation of a solid ®lm.

The ®lm thickness can be controlled by repeating this

reaction cycle.

Our interest to grow La2O3 thin ®lms is two-fold.

Firstly, lanthanum oxide has many attractive proper-

ties for applications and since a successful deposition

of La2O3 by CVD has not been reported, this moti-

vated us to exploit the possibilities of ALE. The

second reason for the present study stems from desire

to obtain more knowledge of the La2O3 growth

process in order to better understand the more com-

plex growth of lanthanum-containing ternary oxides

by ALE, such as LaNiO3, LaCoO3 and LaMnO3, where

La(thd)3 has also been used as a precursor [17±19].

2. Experimental

Film depositions were carried out with a commer-

cial ¯ow-type F-120 atomic layer epitaxy (ALE)

reactor [16,20] manufactured by ASM Microchemis-

try Ltd. The reactant source materials were alternately

introduced into the reactor while nitrogen with a

purity of >99.999% (Schmidlin UHPN N2 generator)

was used as a carrier and purging gas. The source

material for lanthanum was La(thd)3 (thd � 2,2,6,6-

tetramethyl-3,5-heptanedione) which was synthesized

from 99.99% La2O3 by the method described by

Eisentraut and Sievers [21] and puri®ed by sublima-

tion. Thermal behavior of the La(thd)3 precursor has

been studied earlier in connection with LaNiO3

deposition [17]. In this study La(thd)3 was evaporated

from an open glass crucible held at 1708C. Ozone

generated from oxygen gas (99.999%) in an ozone

generator (Fischer model 502) was used as oxidizing

reactant. The ®lm deposition took place at a reduced

pressure of 2±3 mbar in the temperature range of 180±

4258C. The effects of the precursor pulsing times and

the purging time between the reactant pulses on the

®lm growth were studied as well. Soda lime glass

(5 cm� 5 cm) and (1 0 0) silicon were used as sub-

strates.

The thicknesses of the ®lms were evaluated by

®tting the transmittance and re¯ectance spectra [22]

measured with a Hitachi U-2000 double beam spectro-

photometer in a region of 190±1100 nm for silicon

substrates and 370±1100 nm for soda lime glass.

Thickness measurements were veri®ed by pro®lome-

try (Sloan Dektak 3030ST from Veeco Instruments).

The steps were etched by 1 M hydrochloric acid.

A few selected thin ®lm samples were heat-treated

in a rapid thermal annealing furnace PEO 601 (ATV

Technologie GmbH, Germany). The annealing proce-

dure was carried out in nitrogen atmosphere during

0.5±3 h at 600, 800, 900 or 9508C. Also few annealing

experiments were done in air or oxygen atmosphere at

900 or 9508C in a tube furnace. The crystal structure

and crystallite orientation of the as-deposited and

156 M. Nieminen et al. / Applied Surface Science 174 (2001) 155±165

annealed ®lms was determined by X-ray diffraction

measurements with a Philips powder diffractometer

MPD 1880 using Cu Ka radiation. The surface mor-

phology of the ®lms was examined by a Nanoscope III

atomic force microscope (Digital Instruments) using a

scanning area of 2 mm� 2 mm. Samples were mea-

sured in tapping mode (TM) and a scanning frequency

of 1±2 Hz was used. Roughness values were calcu-

lated as root mean square values (RMS).

Two complementary ion beam techniques, Ruther-

ford backscattering spectrometry (RBS) and time-of-

¯ight elastic recoil spectrometry (TOF-ERDA) were

used at the Accelerator Laboratory of the University of

Helsinki to determine the ®lm composition and stoi-

chiometry. Both techniques yielded information about

concentrations and depth pro®les of the main compo-

nents in the ®lms. By combining the results of these

two methods, multiple scattering effects of heavy ions

in the sample and telescopes could be minimized for

TOF-ERDA. The results obtained were consistent

within experimental accuracy.

The RBS experiments were carried out with 4He�

ions from the 2.5 MV Van de Graaff accelerator

working at 2.0 MeV. A scattering angle of 1708 was

used. The ®lm thicknesses together with the lantha-

num and heavier impurity concentrations are readily

revealed by the RBS spectra, while the amount of

oxygen and lighter impurities such as carbon and

hydrogen are obtained more accurately from the

TOF-ERDA analysis. A beam of 197Au9� ions at

48 MeV for TOF-ERDA was obtained from a 5 MV

tandem accelerator EGP-10-II. The sample surface

was tilted 208 and recoils were detected at 408 with

respect to the incoming beam. In TOF-ERDA ele-

ments with different masses were separated by mea-

suring the velocity and energy for each detected recoil.

After identi®cation of the recoil mass, the elemental

velocity spectrum was converted to energy spectrum.

Depth pro®les were deduced for each element by

using known geometry, scattering cross-sections and

parameterized stopping powers [23].

Structural information of the ®lms was obtained

from Fourier transform infrared (FTIR) spectra. The

transmission spectra of the ®lms were obtained using a

Nicolet Magna-750 Fourier transform infrared spec-

trometer equipped with a deuterated-triglycine-sulfate

(DTGS) detector. The substrate contribution was sub-

tracted from the measured sample spectra.

3. Results and discussion

3.1. In¯uence of deposition parameters

The dependence of the growth rate on the growth

temperature is presented in Fig. 1. The pulse durations

of La(thd)3 and O3 were 0.8 and 2 s, respectively. Up

to 2258C the growth rate increases with temperature.

In the temperature region from 225 to 2758C a con-

stant growth rate of 0.36 AÊ per cycle at both substrates

was obtained. When the temperature was raised above

2758C the growth rate increased with temperature.

Since the ®lms were uniform without a notable thick-

ness pro®le within the substrate length of 10 cm also

in these higher temperatures a CVD-type growth could

be excluded. When the deposition temperature was

raised above 4258C a CVD-type growth was observed

and the deposition was no longer feasible. All the ®lms

grown at different deposition temperatures on soda

lime glass substrates were highly transparent in the

wavelength region of 370±1100 nm.

In order to verify the self-controlled nature of the

®lm growth, the effect of source and purge pulse

durations on the ®lm growth rate at 2508C were

studied. The dependence of the growth rate on O3

pulse (La(thd)3 0.8 s) and La(thd)3 pulse (O3 2.0 s) is

presented in Fig. 2. The saturation of the growth rate at

a constant level is achieved when the ozone pulse

duration is longer than 1 s. The ®lm growth rate was

found to be independent of the La(thd)3 pulse duration

between 0.5 and 1.5 s, con®rming that the growth is

self-controlled. The purge gas pulse durations (0.8±

3 s) had no effect on the growth rate. The narrow

plateau between 225 and 2758C where the ®lm growth

is independent of temperature, precursor pulse and

purge times is an indication of an ALE-window [16].

The dependence of the ®lm thickness on the number

of reaction cycles at 2508C is shown in Fig. 3. A linear

relation with growth rate of 0.36 AÊ per cycle is

obtained. In order to prove the controlled growth of

the ®lms also above ALE window, the dependence of

the ®lm thickness on the reaction cycles was studied at

350 and 3758C (Fig. 3). It was found out that the

thicknesses of the ®lms grown above the ALE window

can be controlled by the number of deposition cycles.

When La(thd)3 pulse time at higher deposition tem-

peratures was varied between 0.8±1.5 s no marked

difference on the ®lm growth was observed.

M. Nieminen et al. / Applied Surface Science 174 (2001) 155±165 157

Fig. 1. Dependence of the growth rate on the deposition temperature. Pulse durations were 0.8 s for La(thd)3 and 2 s for O3. Growth rate of

0.36 AÊ per cycle was obtained between 225±2758C. Dotted line represent the carbon content of the ®lms measured by TOF-ERDA.

Fig. 2. Dependence of the growth rate at 2508C on the O3 and La(thd)3 pulse durations.


ALE method has also been used for controlled

depositions onto porous, high surface area substrates

in order to get information of the growth mechanisms

of different processes. These studies give also some

insight into the ALE thin ®lm depositions on ¯at

substrates, but it should be noted that different deposi-

tion conditions such as pressure and surface composi-

tion may affect results. In ALE depositions onto

porous SiO2, decomposition of La(thd)3 has been

reported to start at temperatures higher than 3008C[24,25], which is in accordance with our results. In the

recent study of Kukli et al. [26] a controlled ALE

growth of lanthanum sul®de ®lms from La(thd)3 and

H2S was found at temperature region of 360±4108C.

They reported that only above 4508C decomposition

of the precursor and uncontrolled CVD-type growth

was detected. We believe that the increased thin ®lm

growth rate at higher deposition temperatures is

caused by a partial decomposition of the lanthanum

precursor and not by different surface reaction path-

ways, for instance. By analogy with Y(thd)3 as

revealed by a mass-spectrometric study [27,28], the

La(thd)3 may lose in the gas phase one or two of its

three ligands which makes it possible that the density

of surface coverage by the precursor and consequently

the growth rate increases. Therefore, we believe that

La(thd)3 is only partially decomposing and a surface

controlled growth mechanism is still valid also at

higher deposition temperatures, in La2O3 case up to

4258C. Recently, a similar phenomenon was detected

in the ALE growth of Y2O3 thin ®lms [29].

3.2. Characterization of as-grown ®lms

X-ray diffraction (XRD) measurements made

immediately after deposition on ®lms grown on both

substrates revealed that ®lms deposited below 3008Cwere amorphous. At 3008C, the XRD pattern showed

only one weak peak (d � 3:27 AÊ ) which is the (2 2 2)

re¯ection of the cubic La2O3 [30,31], see Fig. 4. The

®lms grown at 3258C were cubic, polycrystalline

La2O3 with the (2 2 2) re¯ection as the strongest peak.

When the deposition temperature was raised to 3508Cand above, the XRD patterns indicate that the ®lms

Fig. 3. Dependence of the ®lm thickness on the number of cycles at 250, 350 and 3758C. The pulse durations for La(thd)3 and O3 were 0.8 and

2 s, respectively. The reproducibility was tested with two samples at 3758C using 1750 cycles.


have a preferred (1 0 0) orientation since the (4 0 0)

re¯ection is very intense compared with the few

additional peaks, namely (2 2 2), (4 4 0), (6 1 1) and

(6 2 2), in the XRD pattern, see also Fig. 5. Changing

the substrate type at 300±4258C appeared to impose

no difference in the texture of the ®lms. These results

are in agreement with our previous experiments [18].

In the literature cubic (low temperature modi®cation)

La2O3 phase is reported to exist below �4008C while

hexagonal (low temperature modi®cation) La2O3 exist

between �400 and 20408C [32].

Stoichiometry and impurity residues of the ®lms

deposited on silicon substrates at various temperatures

were quantitatively determined by TOF-ERDA and

RBS. No impurities other than carbon and hydrogen

were detected. The carbon content of the ®lms was

dependent on the deposition temperature (Fig. 1). In

every case, carbon levels were distributed evenly

throughout the ®lm. All the ®lms grown at different

temperatures contained less than 1 at.% of hydrogen

in the bulk of the ®lms. The atomic composition of

®lms grown at temperatures between 225±2758C was

Fig. 4. XRD patterns of cubic La2O3 ®lms deposited at 300, 325 and 3508C on amorphous soda lime glass. Miller indices of cubic La2O3 are

given.

Fig. 5. (a) XRD pattern of ®lm grown at 3508C on silicon substrate; (b) same ®lm after storage of few weeks. Miller indices of cubic La2O3

and monoclinic LaO(OH) are given. The re¯ections of LaO(OH) are marked with asterisk.


about 22 at.% La, 66 at.% O and 11.5 at.% C, which is

nearly the same as in La2O2CO3 phase. In the thermo-

analytical literature La2O2CO3 phase is reported to

be stable during dynamic heating under vacuum con-

ditions from �300 to �5008C, above which its

decomposition to oxide begins [33]. The carbon con-

tent (8±10 at.%) of ®lms grown between 300±3258Cis not enough for the ®lms to be La2O2CO3 phase.

Furthermore, XRD patterns indicate that cubic La2O3

is formed between 300±3258C. Therefore, we believe

that these ®lms are most likely a mixture of La2O3 and

La2O2CO3. Films deposited between temperatures of

350±4258C were very similar to each other. The ®lms

contained around 3 at.% carbon as an impurity and the

La to O ratio in these ®lms was 0.48, when the

stoichiometric ratio is 0.67, indicating an excess of

oxygen in the ®lms. The in¯uence of deposition

temperature on the ®lm composition is summarized

in Fig. 6.

In order to get structural and chemical information

FTIR spectra were also measured immediately after

deposition from the as-grown ®lms deposited on

silicon substrates. A typical FTIR spectrum is pre-

sented in Fig. 7(a). The spectra of all ®lms grown at

different temperatures consisted of infrared bands at

850 cmÿ1 and a doublet with transmittance maxima at

around 1390 and 1470 cmÿ1. These bands can be

identi®ed as being due to unidentately bound carbo-

nate species [34,35]. Intensity of these bands

decreased with the increasing deposition temperature

which is consistent with the decrease of carbon

content in the ®lms when the deposition temperature

was increased. A peak around 450 cmÿ1 observed in

the spectra can be indexed as being due to La2O3 [36].

Fig. 6. A summary of the in¯uence of deposition and annealing temperature on ®lm composition. Also the effect of ambient air on ®lm

quality is shown.

Fig. 7. (a) FTIR spectrum of a ®lm deposited at 3508C on silicon

substrate; (b) same ®lm after storage of few weeks; (c) FTIR

spectrum of a ®lm heat-treated at 8008C in nitrogen atmosphere for

30 min; (d) annealed ®lm after exposure to air.


FTIR results suggests that carbon is mainly present as

carbonate type impurity in the ®lms, which explains

also the oxygen excess and resulting deviation from an

ideal stoichiometry as observed by TOF-ERDA mea-

surements. In the CVD studies of metal b-diketonates,

element-type carbon has been observed, but in our

case the use of strong oxidizer is believed to convert it

to carbonate. A similar type of oxygen excess in

connection with the carbon content was found in

ALE grown Y2O3 ®lms as well [29].

AFM images were recorded for two types of ®lm

series (Table 1). First the dependence of surface

roughness on ®lm thickness was studied and images

were collected from ®lms whose thickness varied

from 71 to 303 nm when grown at 2508C on silicon

and soda lime glass substrates. A very small increase

of roughness as a function of thickness was detected

on ®lms grown on silicon. The roughness values were

of the same order of magnitude for ®lms grown on

both substrates, viz. 1.2 nm. In the second series of

AFM samples, a comparison was made between ®lms

grown at different temperatures. As a representative

example, an AFM image of ®lm grown at 3508C is

presented in Fig. 8. A small increase of roughness was

detected as the deposition temperature increased.

AFM images were recorded also for ®lms stored in

a desiccator over a several months, but no differences

were detected compared to the as-deposited ones.

Table 1

Roughness as a function of ®lm thickness and deposition temperature on silicon substrates

Temperature

(8C)

Thickness

(nm)

Number of

cycles

RMS value for films

on Si(1 0 0) (nm)

RMS value for films

on soda lime (nm)

200 75 3000 0.6

250 71 2000 0.7 1.2

350 75 1160 1.1

250 142 4000 1.1 1.4

250 303 9000 1.2 1.2

Fig. 8. TM-AFM image of a 75 nm thick ®lm deposited on silicon at 3508C. Image size: 2mm� 2 mm. Depth scale: 20 nm from black to

white.


3.3. The chemical stability of as-grown ®lms

During the storage of ®lms it was noticed that the

crystalline ®lms obtained at deposition temperatures

above 3258C are chemically unstable. The outlook of

the ®lms appeared to be the same since no difference

in the interference colors of the ®lms was observed.

However, a peak caused by hydroxyl group stretching

[34] in LaO(OH) around 3450 cmÿ1 appeared in the

IR spectra of ®lms stored for a few days in a desic-

cator, see Fig. 7(b). At the same time XRD patterns

revealed that the intensity of (4 0 0) re¯ection of cubic

La2O3 was slowly decreasing and (0 0 1), (0 0 2) and

(1 2 0) re¯ections of the monoclinic LaO(OH) phase

[37] were appearing (Fig. 5). These results indicate

that the cubic La2O3 ®lms are absorbing water from

the air, leading to at least partial transformation to

lanthanum oxide hydroxide. If the ®lms were kept in

air the phenomenon was enhanced.

Also, TOF-ERD analyses indicated that the ®lms

are absorbing water from the air. When the ®lms

grown at 3508C or above were measured after a

few days of storage in a desiccator, the hydrogen

content was found to be very high on the surface of

the ®lms, even 20 at.%. However, only after several

weeks of storage the hydrogen content in the bulk of

the ®lm was increased to the same level. Films grown

at 3758C and 4008C were measured after weeks of

storage in a desiccator, and an atomic composition of

about 22 at.% La, 21 at.% H, 54 at.% O (and 3 at.% C)

was detected. This is quite close to the atomic com-

position of LaO(OH) which contains 25, 25 and

50 at.% of La, H and O, respectively. According to

IR spectra, XRD patterns and TOF-ERD analyses, the

®lms grown at deposition temperatures below 3508Cseemed to remain unchanged during storage.

FTIR spectra revealed also that there was no change

in the carbonate group bands of ®lms stored for few

days in a desiccator compared to the as-deposited

®lms. Therefore, carbonate type impurity in the ®lms

is believed to be caused by the precursor and not by the

atmosphere during storage.

3.4. Composition and chemical stability of annealed

®lms

The amorphous ®lms deposited at 2508C and crys-

talline ®lms grown at 3508C on silicon substrates were

annealed at different temperatures in nitrogen atmo-

sphere. The extent and character of crystallization was

dependent on the deposition and annealing tempera-

tures. At annealing temperature of 6008C a mixture of

cubic and hexagonal La2O3 was obtained in both

cases. At 8008C the ®lm grown at 3508C crystallized

as hexagonal La2O3 with a preferred (1 0 1) orienta-

tion (Fig. 9(a)) while the amorphous ®lm crystallized

Fig. 9. (a) XRD pattern of ®lm grown at 3508C and annealed at 8008C in nitrogen atmosphere for 30 min. All the diffraction peaks can be

identi®ed as hexagonal La2O3. (b) Same annealed ®lm after exposure to air for a few days. The XRD pattern consists only of the diffraction

peaks of hexagonal La(OH)3.


as a mixture of cubic and hexagonal lanthanum oxide.

If the annealing time was increased from 30 min to 3 h

also small peaks of La2O2CO3 were seen in the XRD

pattern of a ®lm grown at 2508C and annealed at

8008C. Only after increasing the annealing tempera-

ture to over 9008C the hexagonal La2O3 phase was

obtained from amorphous ®lm. However, if annealing

time was increased to over 30 min also lanthanum

oxide carbonate crystallized together with hexagonal

lanthanum oxide. Changing the annealing temperature

from 800 to 9508C did not have an in¯uence on the

crystallization of ®lms grown at 3508C. Annealing in

nitrogen did not change the appearance of the ®lms,

either. A summary of the in¯uence of annealing

temperature on ®lm composition is shown in Fig. 6.

IR spectrum of ®lm deposited at 3508C and

annealed at 8008C is seen in Fig. 7(c). The carbonate

group peaks disappeared completely when the anneal-

ing temperature was 8008C for ®lm grown at 3508Cand 9008C for ®lm grown at 2508C. At the same time,

peaks at around 900 cmÿ1 were appearing in the

spectrum. These are the combination bands of La-O

streching vibrational modes [34]. The peak at around

450 cmÿ1 can be indexed as infrared peak of La2O3

[36]. The new peak appearing around 1070 cmÿ1 is

due to oxidized silicon.

The annealed ®lms containing hexagonal lantha-

num oxide reacted very quickly with air. Only after a

few hour exposure to air two peaks at 3600 and

640 cmÿ1 were detected in IR spectrum, see

Fig. 7(d). The intensity of these peaks increased

steadily with time. The peaks can be assigned to

the OH stretching and La-OH bending modes of the

La(OH)3 phase [38]. Also, after prolonged time, car-

bonate group bands appeared in the spectrum, indicat-

ing that ®lm was also reacting with carbon dioxide in

air. The formation of La(OH)3 phase was obvious

from the XRD patterns as well, see Fig. 9(b). After

a few days the X-ray diffraction peaks corresponding

to hexagonal La2O3 phase completely disappeared and

the hexagonal La(OH)3 pattern [39] was the only

observable one. During the same time the quality of

the annealed ®lms greatly changed. First the ®lms

became opaque in color and then after prolonged time

the ®lms started to peel off from the silicon substrate.

TOF-ERD analysis of a ®lm grown at 2508C and

annealed at 9508C gave after a storage for few days

an atomic composition of 15 at.% La, 51 at.% O,

32 at.% H and 1.6 at.% C indicating also the formation

of La(OH)3 phase. Simultaneously, the carbon content

of the annealed ®lms was reduced, according to IR

analysis, from 11.4 to 1.6 at.% as expected.

Besides the annealing temperature, the annealing

atmosphere had an in¯uence on the ®nal state of the

material (1 0 1) oriented hexagonal La2O3 thin ®lms

were obtained when the ®lms were annealed in oxygen

atmosphere at 900±9508C. When the ®lms were heat-

treated in air the ®lms crystallized as a lanthanum

oxide carbonate phase. Therefore, it can be concluded

that relatively high temperatures and a CO2-free atmo-

sphere are needed to yield a pure hexagonal lantha-

num oxide thin ®lm.

The chemical unstability of the La2O3 ®lms in

ambient air is consistent with the results obtained

for La2O3 powder material [38]. The rare earth ses-

quioxides (RE2O3, RE � element of atomic number

57±71) are known to be basic and tend to absorb water

vapor and carbon dioxide from the atmosphere. The

order of basicity decreases with increasing atomic

number; lanthanum sesquioxide being the most basic

and therefore the most unstable one of them. Bernal

et al. [38] have studied the reactivity of La2O3 powder

material with CO2 and H2O. In their study, it was

found out that hexagonal La2O3 reacted very quickly

with ambient air forming La(OH)3 after less than 24 h

of exposure. Carbonate group bands were also seen in

the IR spectra, but the XRD pattern consisted only of

La(OH)3 phase.

4. Conclusions

In this study, it has been shown that thin ®lms of

La2O3 can be grown on soda lime and silicon sub-

strates at a relatively low temperature of 4258C and

even below this. The deposition temperature had a

marked in¯uence on the composition of the ®lms. The

®lms grown below 2758C were amorphous, and the

structural and chemical analysis revealed that the ®lms

contained mostly chemically stable La2O2CO3 phase.

When the deposition temperature was raised to 3008Cor above, the XRD patterns of the ®lms clearly

indicated that a cubic La2O3 phase was formed. At

the highest deposition temperature of 4258C the ®lms

contained 3 at.% of carbon, which is much less than

the carbon content in ®lms grown by CVD methods,


however. The infrared measurements revealed that the

carbon impurity of the ®lms was in the form of

carbonate. Similar type carbonate group impurity

has recently been found in ALE grown Y2O3 thin

®lms [29], where the amount of carbonate is much

smaller than in La2O3 ®lms, however.

The ®lms grown at 3508C or above were chemically

unstable in ambient air, and according to chemical

analysis as well as IR and XRD measurements a

transformation from cubic La2O3 to monoclinic

LaO(OH) was observed. A hexagonal (1 0 1) oriented

La2O3 phase was obtained when the ®lms were

annealed at 8008C or above in a nitrogen atmosphere.

However, the annealed ®lms were chemically even

more unstable than as-deposited crystalline ®lms, and

a rather quick transformation from hexagonal La2O3

to hexagonal La(OH)3 was observed.

Acknowledgements

The authors wish to thank Dr. Eero Rauhala for

RBS measurements and Mr. Timo Sajavaara for TOF-

ERD analysis. Furthermore, we wish to express our

gratitude to Mr. Jaakko NiinistoÈ for AFM measure-

ments. The authors are grateful to Professor P. Hau-

tojaÈrvi, Laboratory of Physics, for providing facilities

for AFM measurements.

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Documents

Formation and stability of lanthanum oxide thin films deposited from β-diketonate precursor