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Thin Solid Films 461 (2004) 17–21

Effect of target compositions on the crystallinity of h-FeSi2 prepared by

ion beam sputter deposition method

K. Yamaguchia,*, A. Heyab, K. Shimuraa, T. Katsumatac, H. Yamamotoa, K. Hojoua

aTokai Research Establishment, Japan Atomic Energy Research Institute, 2-4 Shirakata-Shirane, Tokai-mura, Ibaraki-ken 319-1195, Japanb Industrial Research Institute of Ishikawa, 2-1 Kuratsuki, Kanazawa 920-8203, Japan

cGraduate School of Science and Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi 316-8511, Japan

Available online 27 April 2004

Abstract

Targets with the elemental composition of Fe, Fe2Si and FeSi2 were employed in the present study to grow h-FeSi2 film on Si (100)

substrate by means of ion beam sputter deposition (IBSD) method. The results revealed that when FeSi2 target was employed, a Si-rich phase,

a-FeSi2 (Fe2Si5), was predominant at temperatures above 973 K, while h-FeSi2 phase was observed only in the limited temperature range at

around 873 K. In this case, Si was originated both from the sputtered target and the substrate, thus, the supply of Si was considered to be

excessive to sustain h structure. On the other hand, the films prepared with Fe target became polycrystalline as they grow thicker than 100

nm. In order to optimize the supply of Fe and Si for epitaxial growth, Fe2Si target was employed, where highly (100)-oriented h-FeSi2 layerof 120 nm in thickness was obtained at 973 K.

D 2004 Elsevier B.V. All rights reserved.

PACS: 68.47.Fg; 68.55.Nq; 81.05.Hd; 81.15.Cd

Keywords: Ion beam sputter deposition; h-FeSi2; Crystal structure; Target compositions

1. Introduction

The growth of epitaxial layers of h-FeSi2 has attracted

considerable attention in connection with its optoelectronic

and microelectronic application [1]. The present authors

have employed ‘‘ion beam sputter deposition’’ (IBSD)

method to grow highly oriented h-FeSi2 thin film on a

single crystal Si (100) substrate [2–5]. In earlier studies

[4,5], it was shown that when the substrate is appropriately

treated prior to deposition, and that the substrate tempera-

ture and the deposited thickness of sputtered atoms from Fe

target are appropriately chosen, a high quality film, that is, a

continuous and highly oriented h-FeSi2 (100) film can be

obtained in the temperature range of 873–973 K. However,

as the substrate temperature increased or the deposited

thickness decreased, presence of a phase became predom-

inant. Furthermore, when the deposited thickness of Fe

increased, the film became polycrystalline.

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.02.052

* Corresponding author. Tel.: +81-29-282-6474; fax: +81-29-282-

6716.

E-mail address: yamag@popsvr.tokai.jaeri.go.jp (K. Yamaguchi).

Above observation seemed to indicate the importance of

Fe and Si inter-diffusion in the formation process of iron

silicides. In the present study, an attempt was made to

control the supply of Fe and Si atoms in the silicidation

process by employing various chemical compositions of

Fe–Si targets, namely, FeSi2 and Fe2Si, in addition to pure

Fe. All the targets were bombarded by Ar+ ion beam and the

sputtered atoms were deposited on Si substrate in the

temperature range between RT (room temperature) and

1173 K. The crystal properties of the films were investigated

by means of X-ray diffraction (XRD).

2. Experimental

The single crystal Si (100) substrates (n-type, purity;

99.999%) used to prepare iron silicides were supplied from

Nilaco, whose electrical resistivity is evaluated to be 10–

20 V cm at RT. They were ultrasonically cleansed in

acetone before being installed in ultra-high vacuum vessel

of the IBSD apparatus (Omegatron, OMS-1500IBS). Prior

to deposition, the Si substrate was either thermally etched

at as high as 1273 K, sputter etched at room temperature

Table 1

Summary of the crystal structure of FeSi2 films synthesized in the

temperature range of 773–973 K and the deposited Fe thickness of 8–30 nm

Fe thickness (nm) Substrate temperature (K)

773 873 973

8 (B) B B�A15 B B B�A

(313)+(331)

30 B B B

(202)+(220) (204)+(240)

(313)+(331)

Note that h alone indicates that the film has strong (h00) [h=4, 6, 8] peaks,

whereas other observed peaks of h phase are specifically listed in the table.

(h) indicates that only very small peaks of h (h00) were observed.

K. Yamaguchi et al. / Thin Solid Films 461 (2004) 17–2118

using 3 keV Ne+ ions, followed by thermal annealing at

1073 K for 30 min, or chemically etched based on RCA

method [4]. A mass-separated 35 keV Ar+ ion beam was

then irradiated to the target of either Fe, an amorphous

FeSi2 (both were supplied by Furuuchi Chemical, purity;

>99.9%), or Fe2Si (Toshima Manufacturing, purity;

>99.9%) and the sputtered atoms were deposited on the

substrate in the temperature range of RT-1173 K. The base

pressure of the deposition chamber was less than 5.0�10�8

Pa, whereas the residual gas pressure during deposition was

below 2.7�10�5 Pa.

The film thickness was monitored in-situ with a quartz

crystal microbalance (ULVAC, CRTM-1000) located near

the substrate. The deposited thickness of Fe at RT was

confirmed at several Ar+ ion fluences by cross-sectional

transmission electron microscopy (TEM, JEOL, JEM-

3010F) observation, as well as by use of multiple-beam

interferometer (Mizojiri Optical, BM-3N). If the deposited

Fe is completely reacted with Si to form h-FeSi2, a

deposited thickness of 30 nm Fe layer, for an example,

Fig. 1. XRD patterns of the films formed at the substrate tem

corresponds to h-FeSi2 thickness of about 100 nm, based

on the consideration of the densities of Fe (7.87 g cm�3)

and FeSi2 (4.93 g cm�3 for h phase). For the compound

targets, namely Fe2Si and FeSi2 targets, it was necessary to

check whether Fe and Si atoms from the target are

deposited according to its elemental composition. For this

purpose, a germanium (Ge) substrate was used to deposit

atoms from Fe2Si target at RT in the IBSD apparatus. The

deposited film was analyzed using electron probe micro

analyzer (EPMA, JEOL, JXA-8100) and was confirmed

that the Fe/Si ratio in the film was nearly 2. An XPS

analysis also revealed that the surface composition ratio of

Fe/Si was close to 2. These observations confirmed that

the Fe2Si target was likely to be sputtered in accordance

with its elemental composition. The deposited thickness of

FeSi2 was evaluated by multiplying the measured thickness

of Fe2Si by a factor of 2, taking into consideration the

difference of their densities.

The crystal structure of the deposited film was analyzed

by means of h–2h X-ray diffraction (XRD, MAC Science,

MXP3T) using Cu Ka radiation, while the morphology of

the films was observed by means of TEM and scanning

electron microscope (SEM, JEOL, JAMP-7830F).

3. Results

3.1. Fe target

More syntheses of h-FeSi2 films by IBSD method

have been performed using Fe target than any other

target. In the following, a brief summary of the results

with Fe target is described, where thin films of FeSi2were formed on Si (100) substrate in the temperature

peratures in the range of RT-1173 K using FeSi2 target.

Fig. 2. SEM image of a-FeSi2 islands formed on Si (100) substrate at

1173 K.

Fig. 3. XRD patterns of the films formed at the substrate temperatures in the

range of 873–973 K with the deposited FeSi2 thicknesses of (a) 15 nm, (b)

30 nm, and (c) 60 nm. Note that in these figures, closed circles denote the

peaks of h-FeSi2 (h00) (h=4, 6, 8), open circles denote those of other h-FeSi2 phases, whereas asterisks denote those of a phase.

K. Yamaguchi et al. / Thin Solid Films 461 (2004) 17–21 19

range of 773–973 K with the deposited Fe thickness of

8–30 nm [5]. The XRD analyses revealed that under the

conditions of the present study, the dominant phase was

h-FeSi2. The film with best crystal orientation was

obtained either at 873 K with the deposited Fe thickness

of 15 nm, or at 973 K with the deposited Fe thickness of

30 nm. On the other hand, increasing the substrate tem-

perature and decreasing the thickness from the above

conditions resulted in the film with combined phases of

h- and metallic a-FeSi2. In addition, decrease of the

temperature and increase of the thickness appeared to

induce the film to grow in various crystal orientations, so

that the film became polycrystalline. The dependence of

the crystal structure of FeSi2 films on the substrate tem-

perature and the deposited Fe thickness is tabulated in

Table 1. The obtained results are considered to indicate

importance of the mass balance of Si and Fe atoms in the

formation of h-FeSi2 film.

3.2. FeSi2 target (Fe/Si=1:2)

Fig. 1 compares the XRD patterns of the films formed

at the substrate temperatures in the range of RT-1173 K

with the deposited Fe thickness of 24 nm. The substrates

were thermally etched at 1273 K for 3 min before depo-

sition. The figure clearly indicates that no formation of

h phase was observed below 773 K. It was anticipated that

an employment of FeSi2 target directly leads to the forma-

tion of h-FeSi2 phase at low temperature where Si diffusion

from the substrate is unlikely to occur. However, the fact

was that silicidation rate was so small in this temperature

range that no discernible peaks of h phase were observed. In

order to enhance the silicidation rate, temperature must be

increased. At 873 K, small peaks of h (400) and (800) are

visible, together with those of a (001) and a (111). Those

peaks of h phase became smaller when the substrate

temperature was raised to 973 K and disappeared above

1073 K. Appearance of a-FeSi2 (Fe2Si5) is considered to be

a consequence of incorporating additional Si atoms from the

substrate to form Si-rich a phase.

SEM observations revealed that the surfaces of the films

synthesized below 873 K were fairly smooth, whereas it

became rougher as the temperature was increased above 973

K, and finally became discontinuous above 1073 K as fine

grains of a crystallites aggregated to form islands. Such a

film is shown in the SEM image in Fig. 2, where small

islands (up to several micrometers) of a-FeSi2 are seen to

align in the directions of either Si h010i or Si h001i.

Fig. 4. SEM images of the h-FeSi2 films synthesized at 973 K with the

deposited Fe2Si thicknesses of (a) 15 nm, (b) 30 nm, and (c) 60 nm.

K. Yamaguchi et al. / Thin Solid Films 461 (2004) 17–2120

Furthermore, high-resolution images of silicide–substrate

interface are obtained for some of the films. Although not

shown here, a-FeSi2 grain was observed to penetrate deep

into the substrate, forming a distinctive interface.

3.3. Fe2Si target (Fe/Si=2:1)

Using the Fe2Si target, FeSi2 film was synthesized in the

temperature range of 873–973 K, with the deposited Fe2Si

thickness of 15–60 nm. All the experiments using this target

were performed on the substrates treated by chemical

etching. The XRD patterns of the film formed with the

Fig. 5. Relation of crystal structure and the deposited thickness of Fe2Si. T

deposited Fe2Si thicknesses of 15, 30 and 60 nm are shown

in Fig. 3(a–c), respectively. When the deposited thickness

of Fe2Si was 15 nm (Fig. 3(a)), only very small peaks of h(400), h (331)+(313), h (204)+(240) and h (800) were

observed at 923 K, indicating that the film is polycrystalline.

At 973 K, on the other hand, those of h (h00) (h=4, 6 and

8), which are in epitaxial relationship with Si (100) plane,

became apparent, although the appearance of a (001) and a

(111) indicates that the film was composed of a and hphases.

When the deposited thickness of Fe2Si was increased to

30 nm (Fig. 3(b)), the trend was more evident that the film

grown at lower temperatures (below 923 K) was essentially

polycrystalline. Only at 973 K the presence of h (h00)

became apparent. Further increasing the Fe2Si thickness,

one still finds that with peaks of h (202)+(220), h(313)+(331) and h (204)+(240), the films were polycrystal-

line, but the intensity of h (h00) increased markedly when

the film was synthesized at 973 K, see Fig. 3(c).

The SEM images were taken for the films synthesized at

973 K with various film thicknesses and are shown in Fig. 4.

It appears that all the films were composed of islands. At the

deposited thickness of 15 nm (Fig. 4(a)), the islands were

about 100 nm in size, whereas they became larger as the

thickness increased: i.e. one can see that some islands grew

to be as large as several micrometers in size at the thickness

of 60 nm (Fig. 4(c)). Since the deposition rate was kept

constant in these syntheses, the size of the grain is most

likely to be connected with the annealing which takes place

concurrently with the deposition process. Together with the

results of XRD analyses, it can be deduced that such large

grains were composed of h-FeSi2, and that while the grains

increased in their size, the crystal orientation of the films

was aligned to h (100) plane, so that the polycrystalline

nature of the film gradually became highly oriented.

The above observations are depicted in Fig. 5 to show

how the crystal properties of the films changed with the

he values in the parenthesis denote the thickness in terms of FeSi2.

K. Yamaguchi et al. / Thin Solid Films 461 (2004) 17–21 21

increase of deposited thickness at 973 K when Fe2Si target

was used. It is evaluated in terms of the intensity ratio of the

characteristic peaks of h (h00) planes and those of other

planes. In other words, the ratio of a (100) and h (800)

represents a measure of preferential growth of a phases,

whereas that of h (220) (+(202)) and h (800) represents the

degree of polycrystallinity of h-FeSi2 film. It is shown in the

figure that the orientation of the film improved with the

deposited thickness. In addition, a data point was taken from

the case of Fe target and plotted in the figure so that a

comparison can be made on the dependence of the crystal

property on the target compositions. The degree of the

orientation at the h-FeSi2 thickness of 120 nm turned out

to be as good as (or even excels) that of 100 nm with using

Fe target.

4. Discussion

Unfortunately, the substrates were pre-treated differently

before the deposition in the present study. According to

present authors’ experience with Fe target, difference of

the film properties due to sputter etching and chemical

etching of the substrates was small, while they were very

poor when the substrates were etched thermally [4]. So,

care must be taken to interpret the data obtained with

FeSi2 target. Nevertheless, taking into consideration of the

facts that a phases have also been observed in the films

whose substrates were treated by sputter etching or chem-

ical etching, and that even with use of Fe target, a

continuous and highly oriented layer of h-FeSi2 has never

been obtained when thermal etching was employed. This

indicates that the appearance of a phase is not a conse-

quence of surface treatments. Nevertheless, in the case of

FeSi2 target, a small peak of a phase can be seen at as low

as 873 K, while such was not the case with Fe target.

Therefore, it may well be concluded that inability to obtain

single phase of h-FeSi2 film in the case of FeSi2 target is

primarily due to an inappropriate mass balance of Fe and

Si at the interface of silicide and the substrate where the

reaction takes place.

A general trend observed in the cases of Fe and Fe2Si

targets was similar: i.e. the result obtained at 973 K with the

Fe thickness of 30 nm is comparable to that obtained with

Fe2Si thickness of 60 nm at the same temperature. It should

be noted that in these cases, film thicknesses in terms of

h-FeSi2 were 100 and 120 nm, respectively. Since in these

cases, the obtained films were highly oriented, it is implied

that this high-quality condition can be extended thicker if

one employs an appropriate target composition. Under the

conditions of the present study, moderately increasing the

Fe/Si ratio from 0.5 appeared to favor the formation of

highly oriented h-FeSi2 film. However, obtaining thicker

film with high crystal orientation by just changing the target

compositions has its own limit, since in the case of Fe2Si,

target Si atoms from the substrate must be incorporated to

form disilicide, when it is considered that h-FeSi2 itself actsas a barrier for inter-diffusion of Fe and Si atoms [6]. Such

being the case, h-FeSi2 is often used as a template, upon

which a stoichiometric ratio of Fe and Si is supplied from

their sources to form thicker layer [7,8].

5. Conclusion

Ion beam sputter deposition (IBSD) method was em-

ployed to form a film of h-FeSi2 on Si (100) substrate in the

wide temperature range of RT-973 K, employing various

elemental compositions of Fe–Si target. The XRD analyses

revealed that when FeSi2 target was used, the dominant

phase was a-FeSi2. It was believed that high deposition

temperature induced Si atoms to migrate from the substrate,

in addition to those from the sputtered target, thus, the

supply of Si was excessive to sustain h structure. On the

other hand, the films prepared with Fe target became

polycrystalline as they grow thicker than 100 nm. Consid-

ering that the mass balance of Fe and Si atoms is not fully

optimized for epitaxial growth, Fe2Si target was employed.

The obtained result was encouraging where highly (100)-

oriented h-FeSi2 layer of 120 nm in thickness was obtained

at 973 K, while the grains were observed to grow in size

rather than to aggregate into small islands. Observed de-

pendence on the target compositions should shed more light

on clarifying the role of Fe and Si inter-diffusion in the

formation of h-FeSi2.

Acknowledgements

The authors are grateful to Mr. T. Nakanoya of Japan

Atomic Energy Research Institute and Mr. M. Haraguchi of

Furuya Metal for their contribution to the present work.

This work was in part supported by the Grant-in-Aid for

Scientific Research (#15560731), Japan Society for the

Promotion of Science (JSPS).

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