Liquid phase epitaxial growth of LiNbO3 thin film using Li2O-B2O3 flux system

Journal of Crystal North-Holland

Growth

,ounNa,or CRVSTAL GROWTH

132 (1993) 48-60

Liquid phase epitaxial growth of LiNbO, thin film using L&O-B,O, flux system Atsuo Yamada a, Hitoshi Tamada a and Masaki Saitoh b a Sony Corporation Research Center, 174, Fujitsuka-cho, Hodogaya-ku, Yokohama

240, Japan b Sony Corporation Corporate Research Laboratories, 6-7-15, Kitashinagawa, Shinagawa-ku, Tokyo 141, Japan Received

27 November

1992; manuscript

received

in final form 28 April

1993

LiNbO, thin film optical waveguides were fabricated by liquid phase epitaxy. In order to improve the crystallinity and optical transparency of the film, a 5 mol% MgO-doped LiNbO, substrate and Li,O-B,O, flux system were used instead of a conventional combination of LiTaO, substrate and Li,O-V20, flux system. Dependence of film properties on the melt composition was investigated in detail. As a result, high crystallinity with a small linewidth of 11.4 arc set in the X-ray rocking curve, and good surface morphology with perfect mirror surface were realized by choosing a suitable melt composition. The chemical composition of the grown film was estimated to be nearly stoichiometric from results of several characterizations, and the Li/Nb ratio of the film could not be widely changed by changing the Li,O/NbzO, ratio in the melt. The as-grown film shows a perfect transparency for the wavelength down to the fundamental absorption edge resulting in a small guided optical wave propagation loss of less than 1 dB/cm at a wavelength of 458 nm. Ferroelectric domain inversion phenomena at the film-substrate interface, and preliminary results of trial on controlling film properties by Mg, Zn, Na and Ti doping are also shown and discussed.

1. Introduction LiNbO, shows excellent properties as an optical device which utilizes electro-optic effects or nonlinear optic effects such as optical switches, modulators, second harmonic generation (SHG) devices, and so on. Although these functions can be achieved by introducing a bulk LiNbO, single crystal into the optical system, functional devices using optical-waveguide system which integrates many functions on one substrate (integrated optics) are more attractive because of its advantage on size, efficiency, and stability. Waveguide fabrication is a key technique to construct these devices. Up to now, techniques based on diffusion process such as Ti indiffusion [I] and H+ exchange [2] have been extensively studied. However, the thin film optical waveguide fabrication technique has not been established 0022-0248/93/$06.00

0 1993 - Elsevier

Science

Publishers

yet in spite of many attractive advantages as follows. First, a step index profile and an independent controllability of the waveguide depth and the width are favorable for a waveguide design. Second, it is possible to introduce useful dopants [3,41 and to fabricate multi-layered structures [5], which extend the area of the device application. Furthermore, Ti indiffused and H+ exchanged waveguides have their intrinsic drawback of large photorefractive damage and reduced optical nonlinearity, respectively. Therefore, it seems very important to develop a LiNbO, thin film optical waveguide which has a quality to be used as an optical device. Many attempts have been made for this purpose by various methods like RF sputtering [6-81, chemical vapor deposition [9], or liquid phase epitaxy (LPE) [lo-131. However, previously reported LiNbO, thin film optical waveguides had

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A. Yamada et al. / LPE growth of LiNbO, thin film using Li,O-B,O,

an elementary problem such as poor surface morphology or poor crystallinity, resulting in a large optical propagation loss in the visible spectrum region. Recently, the authors reported LiNbO, thin film optical waveguides with high crystallinity and good surface morphology prepared by liquid phase epitaxy using Li,O-V,O, flux and 5 mol% MgO-doped Z-plate LiNbO, substrate [14]. Unfortunately, there existed a large optical absorption loss at green and shorter wavelengths due to the 3F,-3F, crystal field transition of V3+ ions [15] which are incorporated into the film from the Li,O-V,O, flux. The main purpose of the present research is to completely remove this optical absorption without degrading the crystallinity and the surface morphology. Although the absorption at green wavelengths was significantly reduced by 0, annealing, the effect for the blue and shorter wavelength region was not sufficient [14]. Therefore, it is necessary to remove the origin of the optical absorption by introducing, as a means, another flux system which is free from transition metals. Among many flux systems which have been previously reported, Li20-B,O, was chosen as a likely candidate because it contains no transition metal and the eutectic reaction temperature of about 800°C on the LiNbO,-LiBO, pseudo-binary system [ill is sufficiently lower than the Curie temperature of LiNbO,. From these favorable facts, it was speculated that no optical absorption center would be introduced even if boron was incorporated into the film, and single-poled films are easily obtained without a following poling process. Even though LPE growth of LiNbO, on LiTaO, substrate using Li,O-B,O, flux has been reported [ll], examination on film properties was not fully carried out. Moreover, 5 mol% MgO-doped LiNbO, seems to be more suitable as a substrate because of the smaller difference of the lattice constants and their temperature dependence between film and substrate, In the present study, LPE growth of LiNbO, thin films on 5 mol% MgO-doped LiNbO, substrate using Li,O-B,O, flux was performed, and the dependence of their various properties on the melt composition was investigated in detail. Excellent characteristics as an optical waveguide, especially

49

flwc system

at the short wavelength region, and preliminary results of trial on controlling waveguiding properties by some dopants are also presented.

2. LPE growth of thin films

1

The LPE growth apparatus used in this study is a conventional one equipped with a vertical furnace which has a single zone heater. A schematic diagram of the apparatus is shown in fig. 1. The temperature of the melt was controlled within an accuracy of +0.2”C and was directly measured by a thermocouple protected by a platinum pipe. Li,CO,, Nb,O,, and B,O, powders of 99.99% purity were used as the raw materials. These powders were roughly mixed at the approA1203 rod 7

Therm0 -couple

Heater’

Fig. 1. Diagram

of the LPE furnace.

50

A. Yamada et al. / LPE growth of LiNbO,

priate proportions and transferred to the platinum crucible which was 50 mm in diameter, 120 mm in height, and 1 mm in thickness. The mixture was heated at llOO-1200°C for at least 3 days, and cooled to the temperature which was about 20°C higher than the saturation point. Then the 5 mol% MgO doped Z-plate LiNbO, substrate, which was suspended vertically by a platinum wire, was lowered and held just above the melt surface. The size of the substrate was typically 18 mm X 18 mm. The melt temperature was then set to the growth temperature which was slightly (about 5°C) lower than the saturation point. After an equilibrium state was achieved, the substrate was dipped into the melt during a required time for the growth of a film with suitable thickness. The sample was then withdrawn and slowly cooled to room temperature. The film thickness was controlled by changing the degree of supersaturation and the dipping time. The growth rate was typically 1-2 pm/min, and 3-4 min dipping resulted in the suitable thickness for an optical waveguide. The residue of the flux adhering to the film was washed away with dilute hydrochloric acid. Fig. 2 shows the melt composition of our experiments. First, LPE growth was tried from melts corresponding to 10,15,20, and 50 mol% LiNbO, on the LiNbO,-LiBO, pseudo-binary system.

Liz0

thin film using Li20-B,O,

41

43

flux system

45 Liz0

Fig. 3. Saturation

47

49

51

53

in melt (mol %)

temperature versus Li,O B,O, is fiied at 40 mol%.

content

in melt.

The saturation temperatures, which were determined by extrapolating the line of the growth rate versus growth temperature, were 850, 885, 950, and llOo”C, respectively. Films with mirror-like surface were obtained from the melts containing 15 and 20 mol% LiNbO,. The composition of 20 mol% LiNbO, in the pseudo-binary system corresponds to the point of 50 mol% Li,O, 10 mol% Nb,O,, and 40 mol% B,O, in the ternary system. Next, the Li,O/Nb,O, ratio was also varied along the 40 mol% B,O, fixed line. Film growth was possible in the composition range from 42 to 52 mol% Liz0 (18 to 8 mol% Nb,O,) and the saturation temperature changed from 1045 to 880°C in this range, as shown in fig. 3.

3. Characterization

of film properties

3.1. Crystallinity and lattice mismatch

Nb205 Fig.

2. Melt

40 composition Nb,O,-B,O,

8203

used in experiment ternary system.

for

Li,O-

The crystallinity and the lattice mismatch were characterized by X-ray double crystal method, using (0,0,12) reflection for both first and second crystal. Fig. 4 shows the change of the rocking curve when the Li,O content is varied from 42 to 52 mol% along the 40 mol% B,O, fixed line in the Li*O-Nb,O,-B,03 ternary system. The film crystallinity was evaluated using the full width at half maximum (FWHM) of the film peak, AB, and

A. Yamada

et al. / LPE growth of LiNbO,

the lattice mismatch along the n-axis defined as substrate lattice constant minus film lattice constant, Aa, was calculated as follows. From the Bragg diffraction condition, h = 2d sin 8,, the equation which connects Ad (= dsub - d,,i) and 60 (= esub - t&) is given by the following, assuming that 68 is sufficiently small, i.e., eepi + &, =

8,: Ad/%,,

= -iS@/tan

eB ,

peak anwhere esub and e,,, are the diffraction gles and dsub and depi are the distances of the (0,0,12) planes for the substrate and the epi-film,

Substrate

Substrate

Substrate

thin film using Li,O-B,O,

respectively. given by Au = asub -

Then,

the Ad

‘epi

=

51

flux system

asubdsub

lattice

mismatch

Aa is

l-v G

)

where asub and aepi are the x-axis lattice constants of the substrate and the film, respectively, and v is the Poisson ratio of LiNbO,. Here, we use values of A = 0.15405 nm, asub = 0.51494 nm, and 1, = 0.254. From fig. 4, it is recognized that the high crystallinity with very small FWHM of 11.4 arc set, which is comparable to that of the substrate peak (10.2 arc set), is realized for the film from 52 mol% Li,O melt, and a small Au in the order of lop4 nm indicates a close match between the film and the substrate. For comparison, the lattice mismatch, Au, and the FWHM, de, in the X-ray rocking curve are drawn in fig. 5 in the same scale for both Li,OV,O, and Li*O-B,O, flux systems as a function of the Li,O content along the 40 mol% V,O, or B,O, fixed line. Although it is common for both flux systems that the crystallinity becomes much better and that the lattice mismatch becomes larger as Li,O in the melt increases, the change of Au for films from the Li,O-B,O, flux is restricted to a narrow region with the relatively large lattice mismatch as compared with films from the Li,O-V,O, flux. In this case, a large lattice mismatch means a small lattice constant and hence a large Li/Nb ratio in the film [16]. Therefore, the Li/Nb ratio of a film from the L&O-B,O, flux system is relatively large and rather difficult to control by changing Li20/Nb,0, in the melt. 3.2. Surface morphology

Rocking-curve

angle 0

Fig. 4. X-ray compositions double crystal mol% B,O,; B,O,; Cc) 42 Film thickness

rocking curves for films from various melt on the 40 mol% B,O, fixed line by the X-ray method : (a) 52 mol% Li,O, 8 mol% Nb,O,, 40 (b) 48 mol% Li,O, 12 mol% Nb,O,, 40 mol% mol% Li,O, 18 mol% Nb,O,, 40 mol% B,O,. was about 9, 12, and 9 grn for samples (a), (b), and Cc), respectively.

The surface morphology of a film grown from 50 mol% Li,O melt was investigated by an atomic force microscope (AFM), and the result is shown in fig. 6. A perfectly flat surface is realized in the as-grown state, and the surface of the epi-film is smoother than that of the polished surface of a 5 mol% MgO-doped LiNbO, substrate, indicating the existence of a surface flattening effect during the LPE process.

52

A. Yamada et al. / LPE growth of LiNbO,

thin film using Li,O-B,O,

3.3. Refractive index

n, is almost independent of the Li/Nb ratio [18]. Therefore, guiding selectivity for an ordinary wave is explained by a large Li/Nb composition ratio in a film, which is supported by a corresponding large lattice mismatch described in section 3.1.

Measurement of refractive index was tried by m-line spectroscopy [171 at a wavelength of 633 nm using a rutile prism as a light-film coupler. TE modes, which correspond to an ordinary wave, were successfully excited for the as-grown film, and the corresponding m-line spots were clearly observed. By measuring the synchronous angles to the prism face, the refractive index for the ordinary wave, no, was calculated to be 2.286 for a film grown from the melt consisting of 50 mol% Li,O, 10 mol% Nb,O,, and 40 mol% B,O,. On the other hand, the refractive index for the extraordinary wave, n =, could not be quantified, because the TM mode, which corresponds to an extraordinary wave, could not be excited at all for all samples. This indicates that n, of films grown from Li,O-B,O, flux is smaller than that of the 5 mol% MgO-doped LiNbO, substrate (2.192) at 633 nm, whereas no is sufficiently larger than that of the substrate (2.276). It is well known that the Li/Nb composition variation of LiNbO, has a strong influence on n,, i.e., the higher the Li/Nb ratio is, the smaller n, becomes, whereas 2.210 . . . I

. . ,

(a) Li2OWO5

. . , . I .

5

flux system

3.4. Ferroelectric domain structure The etching technique can identify a ferroelectric domain structure because the -2 and -Y end surfaces of LiNbO, show a deeply etched pattern, while the +Z and + Y end surfaces show a scarcely etched pattern [193. The cross section, which corresponds to the -Y surface of the substrate, was optically polished and then etched in a 1HF + 4HN0, solution at 90°C for 1 min. The polished surfaces were examined using a differential interference microscope. In case the film region is scarcely etched, it is judged that the spontaneous polarization of the film is opposite to that of the substrate. In this way, the ferroelectric domain structure was examined for films grown from melts of various compositions along the 40 mol% B,O, fixed line. Films grown from the melts containing 50 5,...,...,...,...,...,200

system

P 160

I 48

. ..‘...‘...‘...I0 50

2

oL...,...,...,...~...lo

52

54

56

42

Li20 in melt (mol%) Fig. 5. The lattice mismatch An, the linewidth AB, and the refractive stoichiometric LiNbO,, 2.189) versus Li,O content on the 40 mol% V,O, Li20-Nb,O,-B,03 ternary system. Domain inversion phenomena are domain inversion occurs only at the + 2 surface of the substrate (see fig. - Z surface of the substrate (see fig. 7a); ( f Z) domain

44

46 Liz0

48

50

52

in melt (mol%)

index n, (A: ne of the substrate, 2.192; B: n, or B,03 fixed line in the (a) Li,O-Nb,O,-V,Os also shown using notations defined as follows: 5 of ref. [14]): (- Z) domain inversion occurs only instability, multi-domain (see fig. 7b).

of or (+ at

the (b) Z) the

A. Yarnada et at. / LPE growth of LiNbO 3 thin film using Li20-B203 flux system

and 52 mol% Li20 show a domain inversion with a distinct boundary at the - Z surface of the substrate, while single poled films along the same

53

direction as the substrate were grown on the + Z surface of the substrate, as shown in fig. 7a. The domain became unstable when LizO in the melt

20 ,00 (nm) 10 (rim) 0

(a,

(nm)

~VU~

20 )00 (nm) I0

I

(nm) 0 (L,

(nm)

,- ~,,~v

Fig. 6, AFM observation of surface morphology for a 5 ~zm thick film (a) grown from the melt consisting of 50 tool% Li20 , 10 tool% Nb2Os, 40 tool% B203, and of the polished surface of the substrate (b).

54

A. Yamada et al. / LPE growth of LiNbO, thin film using Li,O-B,O,

flux system

(a 1 Film ,_._______________ Substrate

1 Opm I

I (W

Film _____._______.______. Substrate

Fig. 7. Differential interference micrographs of etched Y surface of LiNbO, films grown from the melts consisting of (a) 50 mol% Li,O, 10 mol% Nb,O,, and 40 mol% B,O,, and of (b) 42 mol% Li,O, 18 mol% Nb,O,, and 40 mol% B,O,. (- Z) for a film grown on - Z surface of the substrate; (+ Z) for a film grown on + Z surface of the substrate

was smaller than 50 mol%, that is, the partial domain inversion at the +Z surface of a substrate or the multi-domain structure was observed, as shown in fig. 7b. These phenomena are also shown in fig. 5, using the notation of +Z, -Z, and fZ along with those of films from the Li,O-V,O, flux [14]. As a whole, it is recognized that the value of the lattice mismatch, i.e. the Li/Nb ratio of the film, is a good parameter for describing the domain inversion phenomena. The mechanism of self-poling and domain inversion will be discussed in section 5.2.

3.5. Film-substrate

interjace profile

In order to investigate the sharpness of the film-substrate interface, depth profiles of Mg and Nb were measured by secondary ion mass spectrometry (SIMS). Fig. 8 is the result for a film grown at 940°C from the melt consisting of Li,O 50 mol%, Nb,O, 10 mol%, and B,O, 40 mol%. There exists an approximately 400 nm thick transient layer formed by Mg diffusion from the substrate to the film. However, if the thickness of a practical optical waveguide (typically

A. Yamada et al. / LPE growth of LiNbO,

1 .o

1.5

2.0

thin film using Li,O-B,O,

250

Depth (rm)

Fig. 8. Substrate-film interface profile of Mg and Nb measured by secondary ion mass spectrometry (SIMS) for a film which is grown from the melt consisting of 50 mol% Li,O, 10 mol% Nb,O,, and 40 mol% B,O, at 940°C and cooled to room temperature at about SO”C/min. Boron content was below detection limit.

3-5 pm) is taken into account, the profile of this LPE film Although boron could not be this measurement because of (10 ppm), a very small amount

it can be said that is nearly a step. detected at all in its detection limit might be included.

3.6, Optical transmittance Fig. 9 shows the optical transmission spectra measured by a spectrophotometer. A beam was incident on the sample with the direction normal to the film surface, and the wavelength was swept from 550 to 185 nm. In this way, the total transmittance for the as-grown film and the substrate was measured. The thickness of the substrate and the film was 1 mm and about 10 pm, respectively. A similar measurement was also made for a 1 mm thick substrate without a film. As expected, there exists no significant optical absorption for a film grown from the Li,O-B,O, flux, at least to the fundamental absorption edge of the 5 mol% MgO-doped LiNbO, substrate, while considerable absorption of V3+ was clearly observed for the film grown from the Li,O-V,O, flux. propagation

loss

Fig. 10 shows the guided-optical-wave propagation loss for the TE mode at a wavelength of

350 400 Wavelength

450 (nm)

Fig. 9. Optical transmittance spectra near the absorption edge: (a) 1 mm thick 5 mol% MgO doped LiNbO, substrate; (b) 10 pm thick LiNbO, thin film grown from Li,O-B,O, flux with a substrate; (c) 10 pm thick LiNbO, thin film grown from Li,O-V,O, flux with a substrate. Transmittance is not calibrated for Fresnel losses.

458 nm. Planar waveguides of 20, 10, and 5 mm length with 5 mm width were cut from one sample with a film thickness of 7 pm, and then both ends of the waveguides were optically polished. A 180 mW laser beam was coupled into each waveguide using two cylindrical lenses arranged perpendicular to each other (f = 250 mm followed by f = 12.7 mm), where the size of the beam waist was 5 pm x 50 pm. After optimizing the optics alignment, the stray light was eliminated by a slit, and the throughput power from the waveguide was selectively measured. The propagation loss

_‘OOO -

1

3.7. Guided-wave

flux system

0

I”““I”‘I’I”111’1111 5 IO 15 Propagation distance

20

25

(mm)

Fig. 10. Measurement of guided-optical-wave propagation loss at a wavelength of 458 nm.

56

A. Yamada et al. / LPE growth of LiNbO,

thin fibn using Li20-B,O,

jkx

was measured to be less than 1 dB/cm, which is sufficiently small for many applications to optical devices. As for the case of Li,O-V20, flux, there existed about 10 dB/cm loss at this wavelength even after an 0, annealing [14]. Therefore, change of the flux system from Li,O-V20, to L&O-B,O, resulted in a drastic improvement of the waveguiding property, and this effect would be more remarkable at shorter wavelengths, as expected from fig. 9.

.,.

0

.,

.,

10

5

Fig. 11. Doping

15

characteristics

Results of quantitative analysis by electron probe micro-analyzer (EPMA) showed that Zn is more difficult to introduce into the film than Mg (fig. 111, which appears to be caused by the difference in ionic radius (Li+: 0.078 nm, Mg*+:

20

of Mg and Zn.

0.078 nm, Zn2+: 0.083 nm>. The segregation coefficients, US, of Mg and Zn for Li, which are defined as

Mg

1

Zn

[ Li + Zn I fi,m

[ Li + Mg film aMg=

[

Mg

1

LLi + Mg I

melt

3

a~~

=

[

Zn

1

’

LLi + Zn I me,t

were about 0.33 and 0.14, respectively. These values are much smaller than those when these dopants are incorporated into a congruent bulk crystal [231. The surface morphology was smooth and clear up to the doping levels of 3 mol% Mg and 0.5 mol% Zn to the film, but adhering of the residue of the flux to the film surface became observed in the case of higher doping. Unfortunately, the limit of solid solubility of Zn was too small to make n, of the film exceed that of the 5 mol% MgO doped LiNbO, substrate. 4.2. Na doping

Fig. 12 shows the results of a measurement of method for the Na doped films. The lattice constant of the film becomes larger as Na,O in the melt increases, suggesting substitution of Na+ (0.098 nm) for Li+ (0.078 nm). Although the film crystallinity was not deteriorated by the large amount of Na doping, good surface morphology is maintained only up to 2 mol% doping to the melt, due to a Aa and AtI by X-ray double-crystal

4.1. Mg and Zn doping

.,

,.

MgO, ZnO in melt (mol%)

4. Doping experiment Doping of Mg*+, Zn*+, Na+, and Ti4+ was tried. Mg *+ has the effect of decreasing both IZ~ and n, of LiNbO, [14], and Zn2+ and Ti4+ are, in contrast, known as the dopants which increase both no and n, [1,20]. Although the effect of Na+ doping was not clear, lowering of T, has already been reported [31, so an increase of IZ, was readily expected. If sufficient Zn*+, Na+, or Ti4+ were introduced to the film, guiding for extraordinary wave would be realized. Moreover, Mg*+ and Zn*+ are also effective dopants for improving the resistance against photorefractive damage [20-221. Therefore, it seems very important to know the fundamental doping characteristics of these cations. In this section, we report the results of basic experiments investigating the solid solubility of these dopants when Li,O-B,O, flux is used. The starting melt composition was chosen to be Li,O 50 mol%, Nb,O, 10 mol%, and B,O, 40 mol%. Dopant oxides were pressed and directly added into the melt. In order to get an equilibrium state, the melt was stirred overnight before the film growth was carried out. This process was repeated for higher dopant concentrations until crystal growth became impossible.

system

A. Yanada

et al. / LPE growth of LiNbO,

thin film using Li,O-B,O,

flux system

(0.069 nm) but

+4 (0.064 nm), radius of Nb5+ is 0.069 nm. coefficient, LY,of Ti4+ for Nb5+, EPMA analysis, was about 0.4, fined as

51

while the ionic The segregation calculated from where (Y is de-

Ti (YTi=

[ Nb + Ti I fi,m Ti ’ [ Nb +Ti

2.0 0

0 1

2

3

4

5

6

NazO in melt (mol%)

Fig. 12. Lattice mismatch along the x-axis, Aa, and FWHM of the film peak A@ versus the amount of Na,O doped to the melt.

staying behind of deliquescent flux on the film surface. Domain inversion always took place at the -Z surface of the substrate and guiding of an extraordinary wave could not be realized. 4.3. Ti doping Fig. 13 shows Aa and A0 in the X-ray rocking curve as a function of Ti concentration in the film. Increase of Aa indicates that the valence state of Ti is neither +2 (0.076 nm) nor +3

Although the film crystallinity was not so degraded by doping, deterioration of the surface morphology was observed when more than 0.5 mol% Ti was doped to the film. Domain inversion occurs only at the -Z surface of the substrate when Ti is not doped, as described in section 3.4. As the Ti content in the film increases, the domain structure becomes unstable and finally stabilized, where an inversion occurs only at the +Z surface of the substrate. These phenomena are shown in fig. 13. Although ne of the film seems to be larger than that of the substrate when more than 1.5 mol% Ti are introduced to the film, waveguiding experiments for these films were impossible because of their poor surface morphology.

5. Discussion

5.1. Li/Nb

0

0.5

1

1.5

2

TI concentraGon

2.5

3

3.5

(mol%)

Fig. 13. Lattice mismatch along the r-axis, Aa, and FWHM of the film peak A0 versus Ti concentration in the film. The meanings of notations + Z, - Z, and + Z are the same as in fig. 5.

1 me,t

composition ratio

As concluded from Aa measurements (fig. 51, the Li/Nb composition ratio of films grown from Li,O-B,03 flux is rather larger than that of films from Li,O-V,O, flux, in spite of the same LiZO/Nb,O, ratio for both flux melts. This might come from the difference in chemical properties of melts caused by the difference of melt structure in such a way that B,O, forms covalent networks while V,O, is ionic in the molten state [241. Although the exact value of the Li/Nb ratio in the film is not determined, nearly stoichiometric LiNbO, (n, = 2.189 + 0.0004) [251 must have been grown, considering that IZ, is at most less than 2.192, and Aa is correspondingly large and approaches to saturation, as shown in fig. 5. The

58

A. Yamada et al. / LPE growth

of LiNbO, thin film using Li,O-B,O,,

small diffusion coefficient of Mg and the small segregation coefficients of Mg and Zn for Li sites, as described in sections 3.5 and 4.1, respectively, are thought to be caused by the resultant small defect density. 5.2. Ferroelectric domain structure Domain inversion or instability, which was observed for films from both Li,O-B,Os and Li,O-V,O, flux systems, can be systematically explained by an internal self-poling field induced by the difference of the spontaneous polarization between film and substrate [26,27], as follows. The ferroelectric phase transition temperature T, changes linearly from 1020 to 1180°C as the Li/Nb ratio increases from 0.85 to 1.00 [28]. This means that spontaneous polarization of the film at the growth temperature becomes larger as the composition ratio Li/Nb increases. According to simple electrostatics, an internal electric field is induced by the difference of the spontaneous polarization between film and substrate [26], and the direction of this electric field is parallel to the +Z axis of the substrate when P, of the film (P,,,,,) is larger than that of the substrate (Ps,&. In this case, the domain inversion occurs only on the -Z surface of the substrate. In contrast, an electric field antiparallel to the +Z axis is induced when t,,,i,m is smaller than Ps,sub, and the domain inversion takes place only on the +Z surface. Therefore, we consider that the variation of domain inversion phenomena arises from the variation of the Li/Nb composition ratio in the film, and this is why the lattice mismatch Aa is a good parameter for describing domain inversion phenomena as shown in fig. 5. In case Ps,film is to the close to P, sub, which probably corresponds region of ha from 30 X lop5 to 40 X lop5 nm in fig. 5, the induced internal electric field would not be strong enough to grow a perfectly domain-inverted or single-poled film. Consequently, the domain structure becomes unstable and sometimes a multi-domain structure is observed. Since T, of LiNbO, is lowered by Ti incorporation, the domain structures of Ti doped films described in section 4.3 can be also explained by this model.

flux system

5.3. Ability as an optical waveguide for device application As expected, LiNbO, thin film optical waveguides which are free from transition metal incorporation are obtained, and high crystallinity comparable to that of the substrate, good surface morphology with a perfect mirror surface, single domain structure, and a nearly step index profile are realized at the same time by choosing a suitable melt composition. Consequently, optical absorption at blue-green wavelengths, which existed when Li,O-V,O, flux was used, is removed without any post-heat-treatment like 0, annealing, and negligibly small propagation loss of less than 1 dB/cm at a wavelength of 458 nm is realized. Moreover, a nearly stoichiometric chemical composition of the film promises the shift of the fundamental absorption edge to shorter wavelength region as compared with that of the congruent one [29,30]. These features sufficiently meet a condition as an optical waveguide used in a short wavelength region. In addition, the amount of incorporated boron from Li,O-B,O, flux was less than the detection limit of SIMS analysis, so it is expected that we can make use of those properties of LiNbO, that it originally has. However, TE mode selectivity of this film restricts the number of available tensor elements, resulting in a restriction of the device efficiency and a degree of freedom in design. In particular, rj3, a matrix element of the nonlinear optical coefficient of LiNbO,, which is the largest of all inorganic optical materials, cannot be used in this case. In order to support an extraordinary wave as well, it is necessary to increase n, of the film. But neither controlling Li/Nb ratio nor Zn, Na, or Ti doping for this purpose was successful. Another approach for supporting both ordinary and extraordinary polarizations is to decrease n, of the substrate. MgO-doped LiNbO, with an almost stoichiometric composition is considered to be a favorable substrate because of a probably smaller n, than the MgO-doped congruent LiNbO, used in this study. Unfortunately, growth of a stoichiometric bulk LiNbO, single crystal of sufficiently large size and high quality is extremely difficult

A. Yamada

et al. / LPE growth of LiNbO_, thin film using Li,O-B,O,

[30], and a highly MgO-doped one with almost stoichiometric compositions will be still more so. In contrast, it was clarified in this study that nearly stoichiometric LiNbO, thin films with high crystallinity and good surface morphology can be easily obtained by LPE using Li,O-B,O, flux up to about 3 mol% MgO doping. This indicates that waveguiding for both polarizations is realized by growing a non-doped LiNbO, waveguiding layer on top of a MgO-doped film.

6. Summary Single crystal LiNbO, thin film was successfully grown on a 5 mol% MgO-doped LiNbO, substrate by liquid phase epitaxy using Li,OB,O, flux. Dependence of film properties on the melt composition was investigated in detail and films of high crystallinity and good surface morphology were obtained. Results of several characterizations suggested the good potential of this film as an optical waveguide, especially for applications to blue or a shorter wavelength region owing to the excellent transparency of the grown film. In addition, domain inversion phenomena at the film-substrate interface were systematically explained by a self-poling field induced by a difference of spontaneous polarization between the film and the substrate. The only flaw of this optical waveguide is the selectivity for an ordinary wave. Although attempts for directly supporting an extraordinary wave by some kind of dopant were not successful, the approach of fabricating double-layered structure using MgOdoped under-layer could be promising. Further investigations for supporting an extraordinary wave and optical damage effect, which is an important factor for a LiNbO, waveguide especially used in the short wavelength region, are now in progress.

Acknowledgements The authors thank Y. Murakami for useful advice in the measurement using X-ray doublecrystal method, S. Miwa and I. Nomachi for SIMS

flux system

59

analysis, M. Takasu for EPMA measurements, and K. Bessho for AFM observation.

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