Journal of Crystal Growth 237–239 (2002) 725–729

MoO3–Li2O flux LPE growth of YIG films and itscharacterization

T. Takagi*, M. Fujino, T. Fujii

Murata Manufacturing Co., Ltd., 2-26-10 Tenjin, Nagaokakyo, Kyoto 617-8555, Japan

Abstract

Pure-Yttrium iron garnet (Y3Fe5O12; YIG) films were grown from a MoO3–Li2O flux onto (1 1 1) GGG substrate

using liquid phase epitaxy. By investigating the [Fe2O3]/[Y2O3] molar ratio of the flux, we were able to successfully grow

stoichiometric and high-purity YIG films. Evaluations and calculations of lattice mismatch along the /1 1 1S, /0 0 1S,and /1 1 0S directions using X-ray rocking-curve measurements revealed that these values differed significantly from

one another. This clearly indicated that structural deformation had occurred in the YIG films. The deformation was

larger in the films grown from the MoO3–Li2O flux than in those grown from a PbO–B2O3 flux. However, the impurity

concentration in the former was one hundred times less than that in the latter. Thus, because of this purity, the YIG

films grown from the MoO3–Li2O fluxes are preferable for use in optical and microwave devices. r 2002 Elsevier

Science B.V. All rights reserved.

PACS: 81.15.Lm; 81.10.�h; 68.55�a

Keywords: A3. Liquid phase epitaxy; B1. Oxides; B2. Magnetic materials

1. Introduction

Yttrium iron garnet (Y3Fe5O12; YIG) singlecrystal films grown using liquid phase epitaxy(LPE) have long been used as optical isolators andin microwave devices [1–4]. Researchers have usedLPE to grow YIG from PbO–B2O3(–Bi2O3) [5],BaO–BaF2–B2O3 [6], and MoO3–Li2O fluxes[7,8]. The most commonly used flux is PbO–B2O3(–Bi2O3) because of its high growth rate and

low growth temperature. However, when this fluxis used, Pb ions contaminate the YIG films [9], andwe have found that this Pb contamination has anegative effect on the performance of microwavedevices. To avoid this problem, we used MoO3–Li2O flux to grow the films, as the componentsof this flux hardly disseminate into the YIGfilms during LPE. In this work, we investigatedthe relationship between the [Fe2O3]/[Y2O3]molar ratio of the flux and that of the films. Bydoing so, we were able to successfully growstoichiometric and high-purity (no Li) YIG films,despite the fact that Li was found in films fromprevious experiments [7,8]. We were also able toascertain the effect of impurities on the latticemismatch.

*Corresponding author. Murata Manufacturing Co. Ltd.,

2288 Oshinohara, Yasu, Shiga 520-2393, Japan. Tel.: +81-77-

586-8209; fax: +81-77-587-1923.

E-mail address: takagisc@murata.co.jp (T. Takagi).

0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 9 8 8 - 1

2. Experiments

2.1. Film growth

A cross-section of the furnace used for LPEgrowth is shown in Fig. 1. The cylindrical furnacewas partitioned into three zones so that atemperature bias could be established, both forthe upper and lower zones, to homogenize thethermal conditions in the furnace. A platinumcrucible, 100mm in both diameter and height, wasfilled with a 1700 g melt. A set of donut-shaped Ptreflectors was positioned above the crucible. Table1 shows the typical melt composition used. Themelt was heated for 12 h at 12001C and thencooled to the growth temperature (1060–11601C);

growth rates of 0.04–0.20 mm/min were obtained asa result. Film growth began when the substratewas dipped into the melt after the growthtemperature was maintained for 2 h. The YIGfilms were grown onto a 52mm in diameter (1 1 1)-oriented gadolinium gallium garnet (Gd3Ga5O12;GGG) substrate (a ¼ 1:2383 nm) rotating at50 rpm, with a rotation-switching interval of 15 s.The films were then raised above the melt androtated at 500 rpm to drain off adhering fluxdroplets.

2.2. Characterization

The film characteristics were measured usinghigh-resolution X-ray diffraction equipment.Monochromatized Cu-Ka radiation emitted froma set of four Ge (2 2 0) single crystals was used tomeasure the X-ray rocking-curves of YIG (8 8 8)and (0 0 12) reflections. We then evaluated thelattice mismatch (Dd=d) between the film and thesubstrate along the /1 1 1S (vertical to the filmsurface) and /0 0 1S directions from Bragg’s angleof film and substrate reflections. In this paper, wedefined Dd=d as

Dd=d ¼dYIG � dGGG

dGGG; ð1Þ

where dYIG and dGGG are the spacing of YIG andGGG, respectively. We calculated the spacing ofthe YIG films, d1 1 1 and d0 0 1; in each direction byusing the above equation. If we regard the YIGfilms as being cubic, the spacing of d1 1 0 (parallelto the film surface) is represented as

d1 1 0 ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi55

d20 0 1 � d21 1 1

q: ð2Þ

The YIG-film thickness was measured using alight-interference thickness measuring system, andinductively coupled plasma atomic-emission spec-trometry was used to determine the chemicalcomposition of the films.

3. Results and discussion

Fig. 2 depicts typical YIG films grown from theMoO3–Li2O flux system, with [Fe2O3]/[Y2O3]equaling (a) 0.07 and (b) 0.18. The film thicknesses

Hea

ter

Un

its

Pt

Cru

sibl

eG

GG

Subs

trat

e

Ref

lect

ors

Fig. 1. Cross-section of furnace used for LPE growth.

Table 1

Typical melt composition

Components Molar ratio

MoO3 63.4–66.0

Y2O3 16.2–16.9

Li2CO3 15.8–16.5

Fe2O3 0.6–4.6

T. Takagi et al. / Journal of Crystal Growth 237–239 (2002) 725–729726

were (a) 2 mm and (b) 9 mm, and the surfaces wereextremely smooth. When the films grown moreover, cracks appeared due to strain from latticemismatch between YIG and GGG.Fig. 3 shows the dependence of the lattice

mismatch along the /1 1 1S direction (Dd=d1 1 1)on the [Fe2O3]/[Y2O3] ratio of the flux. Theabsolute value of Dd=d1 1 1 decreased with anincreasing [Fe2O3]/[Y2O3] ratio, when [Fe2O3]/[Y2O3] was o0.18. It saturated at approximately�1500 ppm, when [Fe2O3]/[Y2O3] was >0.18. Thisvalue (�1500 ppm) is much larger than thatcalculated from the JCPDS data (�240 ppm).1

The dependence of the Fe concentration in thefilms (x) on the [Fe2O3]/[Y2O3] ratio of the flux is

shown in Fig. 4. When [Fe2O3]/[Y2O3] was o0.18,x increased with the [Fe2O3]/[Y2O3] ratio of theflux. Stoichiometric YIG films were successfullygrown when [Fe2O3]/[Y2O3] was >0.18. A com-parison of Figs. 3 and 4 shows that x and Dd=d1 1 1have similar tendencies, indicating that Fe defects,when [Fe2O3]/[Y2O3] was o0.18, caused thecrystal lattice to contract. We made furtherinvestigations to determine why the Dd=d1 1 1 valuewas so high in this case.Table 2 shows the d values of YIG, GGG, and

YIG films grown from the MoO3–Li2O andPbO–B2O3 fluxes, where 1.2383 nm is the latticeconstant of GGG. The d1 1 1 values of the filmswere smaller than that of the YIG JCPDS data inboth fluxes, and the films grown from the MoO3–Li2O flux had smaller d1 1 1 values than thosegrown from the PbO–B2O3 flux. For the MoO3–Li2O flux growth, the d1 1 0 value of the filmscalculated from Eq. (2) was larger than that of theGGG substrate, while for the PbO–B2O3 fluxgrowth, the values were comparable.The results obtained from these measurements

and calculations, clearly indicate that the YIGfilms grown from the MoO3–Li2O flux onto the(1 1 1) GGG substrate expanded along the /1 1 0Sdirection and contracted along the /1 1 1S direc-tion. This appears to be because the thermalexpansion coefficient of YIG (1.04� 10�51C�1)2 is

-5000

-4000

-3000

-2000

-1000

0

0 0.1 0.2 0.3[Fe2O3] /[Y2O3] of flux

∆d/d

of <

111>

/ppm

Fig. 3. Dependence of lattice mismatch along the /1 1 1Sdirection.

3

4

5

6

0 0.1 0.2 0.3[Fe2O3] /[ Y2O3] of flux

X

Y3FexO12

Fig. 4. Dependence of Fe concentration in films (x).

Fig. 2. YIG films grown from (a) [Fe2O3]/[Y2O3]=0.07 and (b)

[Fe2O3]/[Y2O3]=0.18.

1 JCPDS No.43-0507.

2Deltronic Crystal Industries, Inc., Product brochure,

‘‘Yttrium iron garnet.’’

T. Takagi et al. / Journal of Crystal Growth 237–239 (2002) 725–729 727

larger than that of GGG (0.9� 10�51C�1).3 Wetherefore surmised that YIG has a larger latticeconstant than GGG at 11001C. Thus, when theYIG films contracted during the cooling periodafter LPE growth, the GGG substrate preventedcontraction along the /1 1 0S direction, but notalong the /1 1 1S direction. This would haveresulted in the structural deformation of the YIGfilms as described above. This appears to suggestthat fitting the YIG lattice to GGG at the growthtemperature would result in the structural defor-mation of the films at room temperature. Thiswould mean that the d1 1 0 value of YIG filmswould have to be equal to or less than that ofGGG because the thermal expansion coefficient ofYIG is larger than that of GGG. However, ourresults showed the opposite; the d1 1 0 value of YIGwas larger than that of GGG.Table 3 shows the results obtained by analyzing

the chemical composition of the YIG films. Theconcentrations of Pt and Zr observed in the filmswere less for the MoO3–Li2O flux than for thePbO–B2O3 flux. This appears to be because the Ptcrucible was virtually unaffected by the MoO3–Li2O flux. In previous works [7,8] Li was foundin such films. However, the Mo and Li contentsof the flux components in our films was less thanthe detectable lower limits when [Fe2O3]/[Y2O3] equaled 0.18. With the PbO–B2O3 fluxgrowth, the impurity concentration (Pb, Pt, Zr) inthe films was over one hundred times greaterthan with the MoO3–Li2O flux. This concentrationforces the YIG lattice to expand because the

ionic radii of these impurities are larger thanthose of Fe and Y. This difference in the im-purity concentration affects the degree of latticedeformation and thus, it is the presence of Pb, Pt,and Zr impurities that causes the YIG lattice toswell.

4. Conclusion

The pure-YIG films we grew from the MoO3–Li2O flux had a thickness ranging from 2 to 9 mmand extremely smooth surfaces. The chemicalcomposition of the films became stoichiometricwhen [Fe2O3]/[Y2O3] was >0.18. Measurements ofthe X-ray rocking-curves of the YIG films revealedthat the films grown from the MoO3–Li2O fluxonto (1 1 1) GGG substrate expanded along the/1 1 0S direction and contracted along the/1 1 1S direction. This deformation was attribu-ted to a difference in the thermal expansioncoefficients of YIG and GGG. That is to say,when YIG films contracted during the coolingperiod after LPE growth, the GGG substrateprevented contraction along the /1 1 0S directionbut not along the /1 1 1S direction. The impurityconcentrations of these films were very low; i.e.,one hundred times less than that of films grownfrom the PbO–B2O3 flux. Thus, the YIG growthmethod using MoO3–Li2O flux is a good methodfor obtaining high-purity YIG crystal films. Webelieve that these high-purity YIG films arepreferable for use in optical and microwavedevices.

Table 2

d values of YIG, GGG, and YIG films

d/nm

/1 1 1S /0 0 1S /1 1 0S

MoO3–Li2O flux 0.7137 1.2376 0.8758

PbO–B2O3 flux 0.7146 1.2381 0.8756

GGG sub. 0.7149 1.2383 0.8756

YIG–JCPDS data 0.7148 1.2380 0.8754

Table 3

Chemical compositions of YIG films

[Fe2O3]/[Y2O3] MoO3–Li2O PbO–B2O3

0.07 0.18

Impurity concentration/wt ppm Mo o5 o2 FLi 150 o5 FPt 19 16 3000–5000

Zr 57 34 100–200

Pb F F 3000–8000

3Shinetsu Chemical Co. LTD., Product brochure, ‘‘Gadoli-

nium gallium garnet.’’

T. Takagi et al. / Journal of Crystal Growth 237–239 (2002) 725–729728

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T. Takagi et al. / Journal of Crystal Growth 237–239 (2002) 725–729 729