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LiNbO3 and LiTaO3 growth by the capillary liquid epitaxial technique
Journal of Crystal Growth 50 (1980) 291—298 © North-Holland Publishing Company
LiNbO3 AND LITaO3 GROWTH BY THE CAPILLARY LIQUID EPJTAXIAL TECHNIQUE T. FUKUDA and H. HIRANO Toshiba Research and Development (‘enter, Toshiba c’orporation, 1 Konwkai Toshiba-cho, Kawasaki, Japan Received 12 October 1978; manuscript received in final form 14 March 1979
LiNbO3 films have been grown on LiTaO3 substrates and LiTaO3 films have been grown on LiNbO3 substrates by the capillary liquid epitaxial (CLE) technique. This new technique and advanced variations of the method, are presented. A further extension of the CLE technique has been used to grow multiple layer structure films, striped films on substrates, and multiple ribbons. The crystals obtained are evaluated as optical waveguide materials or composite materials for surface acoustic wave use. The unique aspects of the CLE technique and possible future applications are discussed.
1. Introduction
lary liquid epitaxial (CLE) technique was proposed by the authors. The method was further advanced to grow a thin film on ribbon or multiple ribbons in one furnace using CLE and EFG techniques simultaneously. This paper is concerned with the growth and characterization of LiNbO3 and LiTaO3 crystals by the CLE technique. Various extensions of the method are described. Growth characteristics and material properties are described and the key aspects and future prospects of the techniques are discussed.
Recently, the EFG technique has received much attention in connection with potentially low cost in producing shaped crystals. There is current interest in Si ribbon growth for solar cells and n-A1203 ribbon for silicon on sapphire (SOS) devices [l—4]. The technique has more recently been applied by the authors [5] to LiNbO3 ribbons, which have been of considerable interest for optical, piezoelectric and surface acoustic wave device applications. A few characteristic features, which led to further advanced techniques, were revealed. One is that the impurity concentration in the solid approaches that in the liquid, so that the effective distribution coefficient is nearly unity [61, as predicted previously by Swartz et al. [71.This fact suggests that the EFG technique is very advantageous in growing solid-solution crystals without compositional variations. This method is successfully applied to LiTa~Nb1_~O3 solid solution crystal growth [8]. Another feature is that the actual growth rate must be decreased, as opposed to the potentially achievable rapid growth rate, if highquality crystals are required. For example, the growth speed of LiNbO3 ribbon [5] for piezoelectric use or ct-Al203 ribbon [3] for SOS devices is remarkably slow, as compared with that reported by LaBelle et a!. [9,10]. To retain the advantageous features of EFG, ribbons must be grown multiply [3] or must be thinner. As a reply to the requirement, the capil-
2. CLE growth procedure 2.1. Growth setup and geometric consideration The growth setup used for CLE growth is the same as used in the preparation of LiNbO3 ribbon crystals [5], which has been described elsewhere [11]. Briefly, the growth setup comprises a 50 mm diameter X 30 mm long Pt crucible, a slit-shape Pt capillary with a 0.5 mm width and 30—50 mm height, a conical Pt afterheater, ceramic insulators and a substrate pulling mechanism. The geometry of the growth is shown in fig. 1. Growth is initiated when the tip of the substrate touches the liquid in the crucible, with about 0.5 mm separation between the substrate and capillary plate. When the substrate is pulled, the solution is mixed with that in the capillary slit, on the top of 291
/
292
T. Fukuda, H. Hirano LiNbO
3 and LiTaO3 growth by CLE technique
Pulling
I
4
l
film
Substrate
\\
LiNbO3 fiIm~~ ~ ~_~~/LiTOO3
Crystal
Subsfrate)
Pt die
Substrate ~
a
__________
Fig. 1. The geometry of CLE growth.
the die. Therefore, the capifiary die is used as a reservior to feed a layer of liquid between the exterior of the die and an adjacent substrate. Temperature adjustment is accomplished by monitoring the liquid temperature using a PtfPt—13% Rh thermocouple. Fig. 2 shows the geometries for an improved CLE technique and a multiple-layer growth technique. In the improved CLE technique, the capillary die comprises of two parallel vertical plates of different length suitably spaced to provide the capifiary action (see fig. 2a). The liquid rises through the capifiary die top and then growth is initiated. The substrate plate constitutes the die wall complementing the lower end portion of the shorter capillary plate. Fig. 2b, shows that a film or ribbon (LiNbO3) or ribbon (LiTaO3) crystal can be grown using the improved CLE and EFG techniques simultaneously.
Seed Crystal Puuinq
LiMbO3 crystal lLiNbO3 film)
LiTaO3 ribbon crystal
Crucible
Heater
°
LNb03
°
tuvo~
0
Crucible
Heater o
2.2. Substrate materials and growth conditions LiNbO3 thin fihns were grown from a LiNbO3— LiVO3 molten solution. A mixture of 50 mole% U2C03, 10 mole% Nb2O~and 40 rnol% V205 was used as a starting material. The mixture was heated in a Pt crucible by rf heating and the solution ternperature was adjusted to a value suitable for growth (850—900°C).Film thickness was controlled by solution temperature and pulling speed. After growth was terminated, the furnace was cooled to room temperature at about 200°C/h. For LiTaO3 substrates, mirror.polished plates (typical dimensions 15 X 30 X 2 mm) were fabricated from Czochralski grown boules. The following orientations were used:
~
LiTaO3
~
/rnelt
/
//
0 0
b
0
Fig. 2. The geometries for improved variations of the CLE technique (a) and multiple-layer growth technique (b).
(00l)(100), (100)(210), (130°rotated Y plate)(210) and (170° rotated Y plate)(210), where ( ) and ( show the plate plane and puffing direction, respectively. LiTaO3 thin films were grown from a LiTaO3— LiVO3 molten solution, as were LiNbO3 films from UNbO3—LiVO3 molten solution. For substrates, LiNbO3 plate crystals (dimensions 15 X 30 X 2 mm) were fabricated from Czochralski-grown boules.
1’. Fukuda, H. Hirano / LiNbO
3 and LiTaO3 growth by CLE technique
293
LITaO3 Substrate
24.3~ ION BEAM ETCHING
20gm wide and 0.75 ~m deep groves etched with an ion beam, along the (100) threetion. Fig. 3. A (O01)(100) LiTaO3 substrate with
Orientations were (001)(100), (131°rotated Y plate) (210), and (210)(l 12.2°rotated Y). For several advanced experiments, based on the CLE technique, multiple layer structure films or striped films on substrates and multiple ribbons were grown. The following substrates were used: LiNbO 3 films on (001)(100) LiTaO3 plates, LiTaO3 films on (00l)(lOO) LiNbO3 plates, and (00l)(l00) LiTaO3 substrates with 200 or 25 pm width and 0.75 pm depth along (100) direction as shown in fig. 3 (made by ion beam etching). LiNbO3 thick films were grown on (001 )( 100) L1TaO3 plates or as-grown ribbons, from a LiNhO3 melt instead of a LiNbO3—LiVO3 solution.
3. CLE growth and crystal quality 3.1. LiNbO3 film on LiTaO3 substrate LiNbO3 epitaxial thin films have been successfully grown onto LiTaO3 substrates. Fig. 4 shows a typical
as-grown film on a (001)(100) plate substrate and the cross-sectional profile of an area near the substrate— film boundary measured by a flatness tester. The film thickness, when grown at 970°C and 3 mm/ mm pulling speed, was about 2 pm and almost constant, except near the fIlm edge (see fig. 4b). The film surface was smooth, clear, and a mirror-polished plane. The side view of the film—substrate boundary observed by optical microscopy was very sharp. An X-ray rocking curve from the (006) reflection showed clearly separated four peaks of CuKa1 and CuKcv2 radiation from the film and substrate [11]. These results suggest that films obtained were high quality. The thickness of the film is a function of the solution temperature and pulling speed, as has been reported in detail elsewhere [111. The lower the solution temperature, the thicker the film obtained. But as the temperature became lower, many small hilocks appeared on the film surface and slip boundaries near edge were observed. Rapid pulling produced a gradual decreases in thickness within the crystal, while slower pulling produced a gradual increase in thickness.
T. Fukuda, H. Hfrano / LiNbO
294
3 and LiTaO3 growth by CLE technique
~
a
that the film did not suffer bad effects of large supercooling by lowering the solution temperature. The improvement observed as the result of changing the die length (1) (where 1 means the part of slit-shape capillary with a 0.5 mm width) for growth on the (170° rotated Y plate)(2l0) plate is shown in fig. 5. Fig. 6 shows typical etch patterns for LiNbO3 films grown on (—Z)LiNbO3 (‘°(00l)LiNbO3) plates (-fZ)LiNbO3 plates and (+Z)LiTaO3 plate crystals, respectively. Etching was carried out for 15 minutes at the bouling point of the etchant (HF : HNO3 = 2 : 1). For crystals that were examined, it was observed that the film surface side was always (—Z) plane over the whole the plate, irrespective of substrate orientation. It is suggested that the film grown by CLE technique is single domain. This may be attributed to the fact that the growth is initiated in the ferroelectric phase. The lattice constant, C0, of a LiTa~Nb1~O3 film on a (00l)(l00) LiTaO3 plate, which was grown using the mixture of LiNbO3 (10 mole%), LiTaO3 (10 mole%) and V205 (80 mole%), was measured by X-ray diffraction. The C0 was 13.80 A, which was nearly the same as that of the bulk LiTa05Nb05O3 crystal [7]. This suggests that keff of CLE grown film from these systems approaches unity, as is indicated in the EFG growth [5]. 3.2. LiTaO3 film on L1NbO3 substrate
LiNbO3 film 2~I
LiToO3 substrate
~
~ b
Fig. 4. (a) A typical as-grown LiNbO3 film on a LiTaO3 substrate and (b) the cross-sectional profile of an area near the substrate—film boundary.
LiNbO3 films, which were grown onto the (100) (210), (131°rotatedY plate)(210), and (170°rotated Y plate)(210) plates using the same growth conditions as employed on the (00l)(l00> plates, were of poor quality having rough surfaces and many defects, The film quality was remarkably improved by adjusting the initiated temperature using dies with different lengths. It is assumed that the appropriate growth temperature was achieved after carrying quickly the solution onto the substrate using capillary action so
LiTaO3 thin films could be also grown with good epitaxy onto LiNbO3 substrates, whose melting point was about 400°Clower than that of the film material. Fig. 7 shows an as-grown film surface on the (001) (100) plate, and the result measured with a flatness tester. The film thickness grown at 1026°C at a pulling rate of 2 mm/mm, was about 2 pm. The film surface and quality observed and measured by optical microscopy and X-ray diffraction were nearly the same as that of LiNbO3 films on LiTaO3 substrates. 3.3. Multiple layer structure films, striped films on substrates, and multiple ribbons
Fig. 8 shows the as-grown film surface flatness and side view of LiTaO3 and LiNbO3 multiple layer structure films on (001)(100) LiTaO3 substrates. The film surface flatness is almost the same as that of the
T. Fukuda.
• ~
i-i’.
Hirano / LiiVbO3
and
LiTaO3 growth
by CLE technique
295
4
.~
4.
~
Eq
~~IaTe~
Fig. 5. Film surfaces of(I 70 rotated Y plate)(210) plate improved by changing the die length (I): (a) (value omitted) 1: (h) 1.31;(c) 1.71.
Hg. 6. Typical etch patterns for a LiNbO3 tOrn on (a) ( -Z) LiNbO3 plate, (hi ~+Z~LiNbO3plate and (c) (+Z)LiTaO3
single layer films used as a substrate (see dotted line in fig. 8). Film thicknesses are about 5 pm. In particular, it is characteristic that the film to film boundary observed by a polariied microscope is very sharp.
LiNhO3 filmswere grown onto (001 )(lOO) LiTaO3 substrates in which striped ditches 200 or 25 pm in width, 0.75 pm in depth along the pulling direction, had been prepared (see fig. 3). The results of growth
plate,respeetively.
a a a
T. Fukuda, H. Hirano / LiNbO
296
3 and LiTaO3 growth by CLE technique
film a
substrate
Fig. 9. A LiNbO3 film grown on a (001)(l00) LiTaO3 substrate with ditches 25 gm wide and 0.75 pm deep.
V%J~J
into a 25 pm wide stripe is shown in fig. 9. In fig. 10, LiTaO3blm 3ate 1mm b LIMbO substr Fig. 7. (a) A typical as-grown film surface on a (001)(100> plate and (b) the result measured by flatness tester.
the flatness of the film surface across the stripe (200 it should be noted that striped ditches were compm width) is shown compared with that of substrate. pletely buried under films and that the film surface was almost flat. From the consideration of the CLE characteristics mentioned above it is suggested that a buried film or layer structure film, as is depicted in fig. 11, can be
grown by combining the CLE technique with etching LiTa
3Im
V
lip
and polishing. Shaped films can also be grown using a shaped die.
LiMb~fitm 1mm
LITaO3 substrate
substrate
Fsubstrate—I-—-_film
Li TQO ____
____
3 Li Nb03
~
/ _____________
LiTaO3 film Fig. 8. The as-grown film surface flatness and side view of a LiTaO3 and LiNbO3 multiple layer structure film on a (001)(100) LiTaO3 substrate.
Fig. 10. The flatness of the film surface across the stripe (200 pm width) is compared with that of substrate.
T. Fukuda, H. Hirano I LiNbO
3 and LiTaO3 growth by CLE technique
ION
BEAM ETCHING
CLE
film
—
4. Materials characteristics
POLISH —
______
\CLE
CUT
CLE
______
______
/~1SH
______
Fig. 11. A buried film or layered structure film are depicted, which can be grown by combining CLE technique, etching
and polishing,
Using the die as shown in fig, 2, LiNbO3 thin films and LiNbO3 thick films were grown on (001)(100) LiTaO3 substrates. Thin films grown from the LiNbO3—LiVO3 system were essentially the same as those grown using the die shown in fig. 1. The film grown from a LiNbO3 melt was 200 pm thick. (001)(100) LiNbO3 on LiTaO3 multiple ribbons were also grown. When grown from the LiNbO3 melt the surface was not smooth, having striation and ripples, as was seen in the EFG grown ribbon [5]. The cornposition profiles perpendicular to the film—substrate boundary have been determined using an X-ray probe microanalyzer. As is shown in fig. 12, there is a sharp transition from the Nb to the Ta containing layer.
5: > in ci 5 I—FLii
297
Th
Optical experiments using waveguides have been carried out using the LiNbO3 film on (00l)(l00) LiTaO3 substrate. A 6328 He—Ne laser beam was successfully fed into even without polishing the as-grown film, the by film, a rutile prism coupler. As is 2 pm thick film on the left and travels to the right. shown in fig. 13, the laser light is coupled into the LiTaO3 films on LiNbO3 substrates and LiNbO3 films on LiTaO3 substrates are interesting as cornposite materials for surface acoustic wave use. Fig. 14 shows the transmitted field intensity pattern for a SAW filter made of a LiTaO3 film on a (131°rotated Y plate)(2l0) LiNbO3 substrate. The substrate has a large coupling coefficient but large temperature stability. Regular interdigital electrodes of aluminum are patterned on the as-grown film surface [13]. The electrode could be made nearly the same as on the polished surface of the bulk crystal in an acoustic frequency of about 60 MHz. The film thickness was about 2 pm, and SAW were propagated about 20 mm along the X ( (210)) direction. It was found that the composite material worked sufficiently as a SAW filter, with low transmission loss and without any troublesome spurious signal caused by layer structure. Temperature stability was improved as a compromise between those of film and substrate.
Nb
—LiTaO3 LiNbO3— Fig. 12. The composition profiles determined by X-ray probe microanalyser.
Fig. 13. Optical waveguiding. A 6328 He--Ne laser beam was fed into the LiNbO3 film by a rutile prism coupler.
a a a a a a aa a a a 7’. Fukuda, H. Hirano
298
Frequency
I
very sharp. The film quality is revealed to be high, as
MHz) 100
50 I
/ LiNbO3 and LiTaO3 growth by CLE technique I
I
observed by X-ray and microscopic measurements. effective distribution coefficient keff is nearly unity. (5) The films can be grown rapidly. Shaped films can also be grown by using the appropriate shaped die.
-
-
(3) Multiple The films layer are structure likely to films, be single striped domain. films (4) on subThe strates and multiple ribbons are gronw by advanced
—10 ~spaceFIe’d’ntens~ty -
-
—
— - — - —
-
—-
—-
—
-—
Fig. 14. The transmitted field intensity pattern for a SAW filter made of LiTaO3 film on (131° rotated Y ptate)(210) LiNbO3 substrate.
The measured velocities in the LiNbO3 film on a (001)(100) LiTaO3 substrate and a LiTaO3 film on a (00l)(l00) LiNbO3 substrate, increased and decreased with thickness of film, respectively [12]. These results suggest that fabrication of a slow ridge waveguide or a fast slot waveguide will be possible if the guide shown in fig. 4 in ref. [14] is made up, using a combination of the variation of the CLE technique described in section 3.3 and etching.
5. Summary and conclusion LiNbO3 films on LiTaO3 substrates and LiTaO3 films on LiNbO3 substrates were grown by the CLE technique. In summary: (1) High melting point films (LiTaO3) can be grown onto low melting point substrates (LiNbO3) with good epitaxy. (2) The film surface is smooth and the film-to-substrate boundary is
variations of
the CLE technique. This has proven to
be posite a promising materialstechnique for optical for the waveguide preparation or of surface cornacoustic wave applications.
References 11] H.B. Serreze, J.C. Swartz, G. Entine and K.V. Ravi, Mater. Res. Bull. 9 (1974) 1421. 12] K.V. Ravi, J. Crystal Growth 39 (1977) 1. [3] K. Hoshikawa and K. Wada, Oyo Butsuri 46 (1977) 938
(in Japanese).
[4] J.C. Swartz, B. Siegel, A.D. Morison and H. Lingertat, J. Electron. Mater. 3 (1974) 309. [5] T. Fukuda and H. Hirano, Mater. Res. Bull. 10 (1975) [61 801. S. Matsumura and T. Fukuda, J. Crystal Growth 34 (1976) 350. [71 J.C. Swartz, T. Surek and B. Chalmers, J. Electron. Mater. 4 (1975) 255. [8] T. Fukuda and H. Hirano, J. Crystal Growth, 35 (1976) 127. 19] HE. LaBelle, Jr. and A.l. Mlavsky, Mater. Res. Bull. 6 (1971) 571. 110] H.E. LaBeile, Jr., Mater. Res. Bull. 6 (1971) 581. [11] T. Fukuda and H. Hirano, Appl. Phys. Letters 28 (1976) 575. [121T. Fukuda and H. Hirano, NCCG-8 (1977) p. 6. [13] H. Hirano, T. Fukuda, S. Matsumura and S. Takahashi, Proc. Ferro. Ferro Material. App!. 1(1978) 81. [14] E.A. Ash, R.M. Delarue and R.F. Humphryes, IEEE Trans. MTT-17 (1969) 882.
1
LiNbO3 AND LITaO3 GROWTH BY THE CAPILLARY LIQUID EPJTAXIAL TECHNIQUE T. FUKUDA and H. HIRANO Toshiba Research and Development (‘enter, Toshiba c’orporation, 1 Konwkai Toshiba-cho, Kawasaki, Japan Received 12 October 1978; manuscript received in final form 14 March 1979
LiNbO3 films have been grown on LiTaO3 substrates and LiTaO3 films have been grown on LiNbO3 substrates by the capillary liquid epitaxial (CLE) technique. This new technique and advanced variations of the method, are presented. A further extension of the CLE technique has been used to grow multiple layer structure films, striped films on substrates, and multiple ribbons. The crystals obtained are evaluated as optical waveguide materials or composite materials for surface acoustic wave use. The unique aspects of the CLE technique and possible future applications are discussed.
1. Introduction
lary liquid epitaxial (CLE) technique was proposed by the authors. The method was further advanced to grow a thin film on ribbon or multiple ribbons in one furnace using CLE and EFG techniques simultaneously. This paper is concerned with the growth and characterization of LiNbO3 and LiTaO3 crystals by the CLE technique. Various extensions of the method are described. Growth characteristics and material properties are described and the key aspects and future prospects of the techniques are discussed.
Recently, the EFG technique has received much attention in connection with potentially low cost in producing shaped crystals. There is current interest in Si ribbon growth for solar cells and n-A1203 ribbon for silicon on sapphire (SOS) devices [l—4]. The technique has more recently been applied by the authors [5] to LiNbO3 ribbons, which have been of considerable interest for optical, piezoelectric and surface acoustic wave device applications. A few characteristic features, which led to further advanced techniques, were revealed. One is that the impurity concentration in the solid approaches that in the liquid, so that the effective distribution coefficient is nearly unity [61, as predicted previously by Swartz et al. [71.This fact suggests that the EFG technique is very advantageous in growing solid-solution crystals without compositional variations. This method is successfully applied to LiTa~Nb1_~O3 solid solution crystal growth [8]. Another feature is that the actual growth rate must be decreased, as opposed to the potentially achievable rapid growth rate, if highquality crystals are required. For example, the growth speed of LiNbO3 ribbon [5] for piezoelectric use or ct-Al203 ribbon [3] for SOS devices is remarkably slow, as compared with that reported by LaBelle et a!. [9,10]. To retain the advantageous features of EFG, ribbons must be grown multiply [3] or must be thinner. As a reply to the requirement, the capil-
2. CLE growth procedure 2.1. Growth setup and geometric consideration The growth setup used for CLE growth is the same as used in the preparation of LiNbO3 ribbon crystals [5], which has been described elsewhere [11]. Briefly, the growth setup comprises a 50 mm diameter X 30 mm long Pt crucible, a slit-shape Pt capillary with a 0.5 mm width and 30—50 mm height, a conical Pt afterheater, ceramic insulators and a substrate pulling mechanism. The geometry of the growth is shown in fig. 1. Growth is initiated when the tip of the substrate touches the liquid in the crucible, with about 0.5 mm separation between the substrate and capillary plate. When the substrate is pulled, the solution is mixed with that in the capillary slit, on the top of 291
/
292
T. Fukuda, H. Hirano LiNbO
3 and LiTaO3 growth by CLE technique
Pulling
I
4
l
film
Substrate
\\
LiNbO3 fiIm~~ ~ ~_~~/LiTOO3
Crystal
Subsfrate)
Pt die
Substrate ~
a
__________
Fig. 1. The geometry of CLE growth.
the die. Therefore, the capifiary die is used as a reservior to feed a layer of liquid between the exterior of the die and an adjacent substrate. Temperature adjustment is accomplished by monitoring the liquid temperature using a PtfPt—13% Rh thermocouple. Fig. 2 shows the geometries for an improved CLE technique and a multiple-layer growth technique. In the improved CLE technique, the capillary die comprises of two parallel vertical plates of different length suitably spaced to provide the capifiary action (see fig. 2a). The liquid rises through the capifiary die top and then growth is initiated. The substrate plate constitutes the die wall complementing the lower end portion of the shorter capillary plate. Fig. 2b, shows that a film or ribbon (LiNbO3) or ribbon (LiTaO3) crystal can be grown using the improved CLE and EFG techniques simultaneously.
Seed Crystal Puuinq
LiMbO3 crystal lLiNbO3 film)
LiTaO3 ribbon crystal
Crucible
Heater
°
LNb03
°
tuvo~
0
Crucible
Heater o
2.2. Substrate materials and growth conditions LiNbO3 thin fihns were grown from a LiNbO3— LiVO3 molten solution. A mixture of 50 mole% U2C03, 10 mole% Nb2O~and 40 rnol% V205 was used as a starting material. The mixture was heated in a Pt crucible by rf heating and the solution ternperature was adjusted to a value suitable for growth (850—900°C).Film thickness was controlled by solution temperature and pulling speed. After growth was terminated, the furnace was cooled to room temperature at about 200°C/h. For LiTaO3 substrates, mirror.polished plates (typical dimensions 15 X 30 X 2 mm) were fabricated from Czochralski grown boules. The following orientations were used:
~
LiTaO3
~
/rnelt
/
//
0 0
b
0
Fig. 2. The geometries for improved variations of the CLE technique (a) and multiple-layer growth technique (b).
(00l)(100), (100)(210), (130°rotated Y plate)(210) and (170° rotated Y plate)(210), where ( ) and ( show the plate plane and puffing direction, respectively. LiTaO3 thin films were grown from a LiTaO3— LiVO3 molten solution, as were LiNbO3 films from UNbO3—LiVO3 molten solution. For substrates, LiNbO3 plate crystals (dimensions 15 X 30 X 2 mm) were fabricated from Czochralski-grown boules.
1’. Fukuda, H. Hirano / LiNbO
3 and LiTaO3 growth by CLE technique
293
LITaO3 Substrate
24.3~ ION BEAM ETCHING
20gm wide and 0.75 ~m deep groves etched with an ion beam, along the (100) threetion. Fig. 3. A (O01)(100) LiTaO3 substrate with
Orientations were (001)(100), (131°rotated Y plate) (210), and (210)(l 12.2°rotated Y). For several advanced experiments, based on the CLE technique, multiple layer structure films or striped films on substrates and multiple ribbons were grown. The following substrates were used: LiNbO 3 films on (001)(100) LiTaO3 plates, LiTaO3 films on (00l)(lOO) LiNbO3 plates, and (00l)(l00) LiTaO3 substrates with 200 or 25 pm width and 0.75 pm depth along (100) direction as shown in fig. 3 (made by ion beam etching). LiNbO3 thick films were grown on (001 )( 100) L1TaO3 plates or as-grown ribbons, from a LiNhO3 melt instead of a LiNbO3—LiVO3 solution.
3. CLE growth and crystal quality 3.1. LiNbO3 film on LiTaO3 substrate LiNbO3 epitaxial thin films have been successfully grown onto LiTaO3 substrates. Fig. 4 shows a typical
as-grown film on a (001)(100) plate substrate and the cross-sectional profile of an area near the substrate— film boundary measured by a flatness tester. The film thickness, when grown at 970°C and 3 mm/ mm pulling speed, was about 2 pm and almost constant, except near the fIlm edge (see fig. 4b). The film surface was smooth, clear, and a mirror-polished plane. The side view of the film—substrate boundary observed by optical microscopy was very sharp. An X-ray rocking curve from the (006) reflection showed clearly separated four peaks of CuKa1 and CuKcv2 radiation from the film and substrate [11]. These results suggest that films obtained were high quality. The thickness of the film is a function of the solution temperature and pulling speed, as has been reported in detail elsewhere [111. The lower the solution temperature, the thicker the film obtained. But as the temperature became lower, many small hilocks appeared on the film surface and slip boundaries near edge were observed. Rapid pulling produced a gradual decreases in thickness within the crystal, while slower pulling produced a gradual increase in thickness.
T. Fukuda, H. Hfrano / LiNbO
294
3 and LiTaO3 growth by CLE technique
~
a
that the film did not suffer bad effects of large supercooling by lowering the solution temperature. The improvement observed as the result of changing the die length (1) (where 1 means the part of slit-shape capillary with a 0.5 mm width) for growth on the (170° rotated Y plate)(2l0) plate is shown in fig. 5. Fig. 6 shows typical etch patterns for LiNbO3 films grown on (—Z)LiNbO3 (‘°(00l)LiNbO3) plates (-fZ)LiNbO3 plates and (+Z)LiTaO3 plate crystals, respectively. Etching was carried out for 15 minutes at the bouling point of the etchant (HF : HNO3 = 2 : 1). For crystals that were examined, it was observed that the film surface side was always (—Z) plane over the whole the plate, irrespective of substrate orientation. It is suggested that the film grown by CLE technique is single domain. This may be attributed to the fact that the growth is initiated in the ferroelectric phase. The lattice constant, C0, of a LiTa~Nb1~O3 film on a (00l)(l00) LiTaO3 plate, which was grown using the mixture of LiNbO3 (10 mole%), LiTaO3 (10 mole%) and V205 (80 mole%), was measured by X-ray diffraction. The C0 was 13.80 A, which was nearly the same as that of the bulk LiTa05Nb05O3 crystal [7]. This suggests that keff of CLE grown film from these systems approaches unity, as is indicated in the EFG growth [5]. 3.2. LiTaO3 film on L1NbO3 substrate
LiNbO3 film 2~I
LiToO3 substrate
~
~ b
Fig. 4. (a) A typical as-grown LiNbO3 film on a LiTaO3 substrate and (b) the cross-sectional profile of an area near the substrate—film boundary.
LiNbO3 films, which were grown onto the (100) (210), (131°rotatedY plate)(210), and (170°rotated Y plate)(210) plates using the same growth conditions as employed on the (00l)(l00> plates, were of poor quality having rough surfaces and many defects, The film quality was remarkably improved by adjusting the initiated temperature using dies with different lengths. It is assumed that the appropriate growth temperature was achieved after carrying quickly the solution onto the substrate using capillary action so
LiTaO3 thin films could be also grown with good epitaxy onto LiNbO3 substrates, whose melting point was about 400°Clower than that of the film material. Fig. 7 shows an as-grown film surface on the (001) (100) plate, and the result measured with a flatness tester. The film thickness grown at 1026°C at a pulling rate of 2 mm/mm, was about 2 pm. The film surface and quality observed and measured by optical microscopy and X-ray diffraction were nearly the same as that of LiNbO3 films on LiTaO3 substrates. 3.3. Multiple layer structure films, striped films on substrates, and multiple ribbons
Fig. 8 shows the as-grown film surface flatness and side view of LiTaO3 and LiNbO3 multiple layer structure films on (001)(100) LiTaO3 substrates. The film surface flatness is almost the same as that of the
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Fig. 5. Film surfaces of(I 70 rotated Y plate)(210) plate improved by changing the die length (I): (a) (value omitted) 1: (h) 1.31;(c) 1.71.
Hg. 6. Typical etch patterns for a LiNbO3 tOrn on (a) ( -Z) LiNbO3 plate, (hi ~+Z~LiNbO3plate and (c) (+Z)LiTaO3
single layer films used as a substrate (see dotted line in fig. 8). Film thicknesses are about 5 pm. In particular, it is characteristic that the film to film boundary observed by a polariied microscope is very sharp.
LiNhO3 filmswere grown onto (001 )(lOO) LiTaO3 substrates in which striped ditches 200 or 25 pm in width, 0.75 pm in depth along the pulling direction, had been prepared (see fig. 3). The results of growth
plate,respeetively.
a a a
T. Fukuda, H. Hirano / LiNbO
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film a
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Fig. 9. A LiNbO3 film grown on a (001)(l00) LiTaO3 substrate with ditches 25 gm wide and 0.75 pm deep.
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into a 25 pm wide stripe is shown in fig. 9. In fig. 10, LiTaO3blm 3ate 1mm b LIMbO substr Fig. 7. (a) A typical as-grown film surface on a (001)(100> plate and (b) the result measured by flatness tester.
the flatness of the film surface across the stripe (200 it should be noted that striped ditches were compm width) is shown compared with that of substrate. pletely buried under films and that the film surface was almost flat. From the consideration of the CLE characteristics mentioned above it is suggested that a buried film or layer structure film, as is depicted in fig. 11, can be
grown by combining the CLE technique with etching LiTa
3Im
V
lip
and polishing. Shaped films can also be grown using a shaped die.
LiMb~fitm 1mm
LITaO3 substrate
substrate
Fsubstrate—I-—-_film
Li TQO ____
____
3 Li Nb03
~
/ _____________
LiTaO3 film Fig. 8. The as-grown film surface flatness and side view of a LiTaO3 and LiNbO3 multiple layer structure film on a (001)(100) LiTaO3 substrate.
Fig. 10. The flatness of the film surface across the stripe (200 pm width) is compared with that of substrate.
T. Fukuda, H. Hirano I LiNbO
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ION
BEAM ETCHING
CLE
film
—
4. Materials characteristics
POLISH —
______
\CLE
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CLE
______
______
/~1SH
______
Fig. 11. A buried film or layered structure film are depicted, which can be grown by combining CLE technique, etching
and polishing,
Using the die as shown in fig, 2, LiNbO3 thin films and LiNbO3 thick films were grown on (001)(100) LiTaO3 substrates. Thin films grown from the LiNbO3—LiVO3 system were essentially the same as those grown using the die shown in fig. 1. The film grown from a LiNbO3 melt was 200 pm thick. (001)(100) LiNbO3 on LiTaO3 multiple ribbons were also grown. When grown from the LiNbO3 melt the surface was not smooth, having striation and ripples, as was seen in the EFG grown ribbon [5]. The cornposition profiles perpendicular to the film—substrate boundary have been determined using an X-ray probe microanalyzer. As is shown in fig. 12, there is a sharp transition from the Nb to the Ta containing layer.
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297
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Optical experiments using waveguides have been carried out using the LiNbO3 film on (00l)(l00) LiTaO3 substrate. A 6328 He—Ne laser beam was successfully fed into even without polishing the as-grown film, the by film, a rutile prism coupler. As is 2 pm thick film on the left and travels to the right. shown in fig. 13, the laser light is coupled into the LiTaO3 films on LiNbO3 substrates and LiNbO3 films on LiTaO3 substrates are interesting as cornposite materials for surface acoustic wave use. Fig. 14 shows the transmitted field intensity pattern for a SAW filter made of a LiTaO3 film on a (131°rotated Y plate)(2l0) LiNbO3 substrate. The substrate has a large coupling coefficient but large temperature stability. Regular interdigital electrodes of aluminum are patterned on the as-grown film surface [13]. The electrode could be made nearly the same as on the polished surface of the bulk crystal in an acoustic frequency of about 60 MHz. The film thickness was about 2 pm, and SAW were propagated about 20 mm along the X ( (210)) direction. It was found that the composite material worked sufficiently as a SAW filter, with low transmission loss and without any troublesome spurious signal caused by layer structure. Temperature stability was improved as a compromise between those of film and substrate.
Nb
—LiTaO3 LiNbO3— Fig. 12. The composition profiles determined by X-ray probe microanalyser.
Fig. 13. Optical waveguiding. A 6328 He--Ne laser beam was fed into the LiNbO3 film by a rutile prism coupler.
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298
Frequency
I
very sharp. The film quality is revealed to be high, as
MHz) 100
50 I
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I
observed by X-ray and microscopic measurements. effective distribution coefficient keff is nearly unity. (5) The films can be grown rapidly. Shaped films can also be grown by using the appropriate shaped die.
-
-
(3) Multiple The films layer are structure likely to films, be single striped domain. films (4) on subThe strates and multiple ribbons are gronw by advanced
—10 ~spaceFIe’d’ntens~ty -
-
—
— - — - —
-
—-
—-
—
-—
Fig. 14. The transmitted field intensity pattern for a SAW filter made of LiTaO3 film on (131° rotated Y ptate)(210) LiNbO3 substrate.
The measured velocities in the LiNbO3 film on a (001)(100) LiTaO3 substrate and a LiTaO3 film on a (00l)(l00) LiNbO3 substrate, increased and decreased with thickness of film, respectively [12]. These results suggest that fabrication of a slow ridge waveguide or a fast slot waveguide will be possible if the guide shown in fig. 4 in ref. [14] is made up, using a combination of the variation of the CLE technique described in section 3.3 and etching.
5. Summary and conclusion LiNbO3 films on LiTaO3 substrates and LiTaO3 films on LiNbO3 substrates were grown by the CLE technique. In summary: (1) High melting point films (LiTaO3) can be grown onto low melting point substrates (LiNbO3) with good epitaxy. (2) The film surface is smooth and the film-to-substrate boundary is
variations of
the CLE technique. This has proven to
be posite a promising materialstechnique for optical for the waveguide preparation or of surface cornacoustic wave applications.
References 11] H.B. Serreze, J.C. Swartz, G. Entine and K.V. Ravi, Mater. Res. Bull. 9 (1974) 1421. 12] K.V. Ravi, J. Crystal Growth 39 (1977) 1. [3] K. Hoshikawa and K. Wada, Oyo Butsuri 46 (1977) 938
(in Japanese).
[4] J.C. Swartz, B. Siegel, A.D. Morison and H. Lingertat, J. Electron. Mater. 3 (1974) 309. [5] T. Fukuda and H. Hirano, Mater. Res. Bull. 10 (1975) [61 801. S. Matsumura and T. Fukuda, J. Crystal Growth 34 (1976) 350. [71 J.C. Swartz, T. Surek and B. Chalmers, J. Electron. Mater. 4 (1975) 255. [8] T. Fukuda and H. Hirano, J. Crystal Growth, 35 (1976) 127. 19] HE. LaBelle, Jr. and A.l. Mlavsky, Mater. Res. Bull. 6 (1971) 571. 110] H.E. LaBeile, Jr., Mater. Res. Bull. 6 (1971) 581. [11] T. Fukuda and H. Hirano, Appl. Phys. Letters 28 (1976) 575. [121T. Fukuda and H. Hirano, NCCG-8 (1977) p. 6. [13] H. Hirano, T. Fukuda, S. Matsumura and S. Takahashi, Proc. Ferro. Ferro Material. App!. 1(1978) 81. [14] E.A. Ash, R.M. Delarue and R.F. Humphryes, IEEE Trans. MTT-17 (1969) 882.
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