Electro-optical properties of BaTiO3–SrTiO3 multilayer thin films for waveguide modulators

Jussi Hiltunen∗,a, Mikko Karppinena, Pentti Kariojaa, Jyrki Lappalainenb, Jarkko Puustinenb, Vilho

Lanttob, Harry L. Tullerc

a)VTT Technical Research Centre of Finland, Kaitovayla 1, Fin-90571 Oulu, Finland

b)Microelectronics and Materials Physics Laboratories and EMPART Research Group of Infotech Oulu , University of Oulu, Oulu, P.O.B. 4500 FIN-90014, Finland

c)Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77

Massachusetts Avenue, Cambridge, MA 02139, USA

ABSTRACT

Optical properties of ferroelectric BaTiO3 (BTO) and paraelectric SrTiO3 (STO) multilayer structures were investigated as a possible material choice for thin-film electro-optic devices. It has been demonstrated that dielectric properties can be enhanced by optimizing the stacking periodicity of BTO-STO superlattices, and in this work, it was studied how the shifts in permittivity are transferred to the optical properties. BTO-STO superlattices with stacking periodicity varying between 27 Å and 1670 Å were grown on MgO substrates by pulsed laser deposition. In x-ray diffraction patterns, periodic satellite peaks were observed indicating the formation artificial superlattices. The evolution of electro-optic response with varying stacking periodicity was analyzed by ellipsometric transmission method. The electro-optic response reached a maximum at a stacking periodicity of 105 Å corresponding the individual layer thickness of 13 unit cells. The suitability of superlattices, and also single layer BTO thin films, in planar optical devices were evaluated by fabricating and characterizing Mach – Zehnder waveguide modulators.

Keywords: electro-optics, ferroelectrics, BaTiO3, SrTiO3, superlattice, waveguide

1. INTRODUCTION Oxide ferroelectric materials have attracted both scientific and technological interest due to functional properties, such as, nonlinear permittivity, piezoelectricity and electro-optic activity[1],[2]. BaTiO3 (BTO) commonly serves as a model perovskite ferroelectric and has been intensively studied in both bulk and thin film forms. Research efforts have been focused on attempts to improve the properties of BTO thin films by the formation of solid solutions and multilayer structures with SrTiO3 (STO)[3],[4]. Interestingly, their physical properties can be quite different in compositionally equivalent structures, and in oxide superlattices, stacking structure has been shown to be an important parameter to engineer permittivity[5],[6]. These multilayers have been fabricated with a very accurate control of stacking periodicity, down to the length of about a unit cell, by different methods e.g. molecular-beam-epitaxy (MBE)[7] and pulsed-laser-deposition (PLD)[5].

The formation BTO-STO superlattice structure is illustrated in Figure 1 (a). Layers consisting of BTO or STO unit cells are stacked periodically to form the superlattice. Figure 1 (b) shows an example of the planar optical device, where the functionality is based on the artificial superlattice thin film. The waveguiding structure is manufactured between electrodes and the propagation properties of the guided wave can be tuned by applying voltage over the electrodes. Thin film technology is considered attractive in electronics and photonics applications, given its ability to a) utilize a wide variety of materials not processable in the bulk, and b) achieve high levels of component integration in photonic devices [8]. In this work, the influence of stacking periodicity on the electro-optic response of BTO-STO superlattices was

∗ jussi.hiltunen@vtt.fi www.vtt.fi

Silicon Photonics and Photonic Integrated Circuits, edited by Giancarlo C. Righini, Seppo K. Honkanen, Lorenzo Pavesi,Laurent Vivien, Proc. of SPIE Vol. 6996, 69960H, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.781163

Proc. of SPIE Vol. 6996 69960H-12008 SPIE Digital Library -- Subscriber Archive Copy

• Ba2o Sr2O Ti4 BTO/o 02-

STO

STO/BTO

BTO

(b)

strip-loaded/ridge wavegaide

studied. Furthermore, waveguide modulators based on the single layer BTO films and BTO-STO superlattices were fabricated and characterized.

Fig 1. (a) The crystal structure of periodic BTO-STO superlattices, and (b) example of an active waveguide device based

on the superlattice stack.

2. EXPERIMENTAL SECTION 2.1 Thin film deposition

Pulsed laser deposition was utilized to grow BTO-STO multilayer structures on single crystal MgO substrates with (001) orientation. Two different deposition systems with XeCl (308 nm) or KrF (246 nm) lasers were utilized in this work. Slightly varying processing conditions were used, but generally, the thin film stacks were deposited by alternately focusing the laser beam on nominally stoichiometric BTO and STO targets. Oxygen background pressure was applied and substrate temperature of about 700 to 800 °C was maintained during reactive deposition. Further information on the film growth is provided e.g. in Refs. [9] and [10]. In slab film measurements (Sections 2.2 and 2.3), the films with the total thickness of about 360 nm were used to evaluate the influence of stacking periodicity on the electro-optic response. In waveguide devices (Section 2.4), the thickness of the BTO-STO superlattice films was 180 nm, while single layer BTO films had the thickness of about 400 nm.

2.2 Crystal structure analysis

Figure 2 shows the low angle regions of the XRD θ-2θ diffraction patterns. The lowest plot corresponds to the film with the shortest stacking periodicity. In the three lowest patterns, the formation of periodic satellite peaks due to superlattice structure was observed. In these cases, the periodicity of the BTO-STO pairs could be calculated based on the XRD measurements, with values of 27, 49 and 105 Å obtained. When the layer thickness was further increased, the satellite peaks were no longer observed and the periodicity was estimated by comparing the number of deposited layers with the measured total film thickness resulting in periodicities of 314 and 1670 Å.

The out-of-plane lattice parameters were evaluated by simulating the diffraction scans. It was estimated that when the periodicity was below 105 Å, the lattice parameters for BTO and STO were 4.20-4.22Å and 3.91, respectively. In terms of unit cells, this would mean the stacking periodicity of (BTO)3-4/(STO)3-4, (BTO)6/(STO)6 and (BTO)13/(STO)13 for the films of 27, 49 and 105 Å periodicities, respectively. The c-lattice parameter of BTO bulk is 4.038 Å, which implies a highly strained lattice parameter in these superlattices[11]. Contrary, the STO lattice parameter was relatively close to the bulk value of 3.905 Å [12]. Besides the lattice parameter mismatch between BTO and STO layers, also difference in

Proc. of SPIE Vol. 6996 69960H-2

d0)02'(0C0C

15 20 25 30

Diffraction angIe [20]35

temperature expansion coefficient between the substrate and the film [13] together with the thin film deposition conditions [14] are known to have an influence on the lattice parameters. Values obtained from the simulation of diffraction patterns were also supported by the measurement results of the films with 314 and 1670 Å periodicities. In these patterns, only the characteristic BTO and STO reflections were observed. BTO peaks were broad indicating a strong relaxation of the out-of-plane lattice parameter. In the film with 314 Å periodicity, the BTO (001) reflection showed a double peak form. This is attributed to the relaxation of the strained crystal from the lattice parameter of 4.21 Å to 4.08 Å. Similar strain relaxation was also observed in BTO-STO stacks of 1670 Å periodicity. In this structure, the maximum XRD peak intensity corresponds to a lattice parameter of about 4.08 Å implying that most of the BTO lattice volume is partly relaxed. The evolution of epitaxy breakdown with decreasing layer periodicity is also worth noting. In the films of long periodicity (314 Å and 1670 Å), only (001)-plane based reflections were observed, while in films of short periodicity, the intensity of the (110)/(101) peak increased. This is assumed to be related to the number of BTO-STO interfaces, which increases with decreasing stacking periodicity.

Fig 2. X-ray diffraction θ-2θ pattern of the BTO-STO thin film stack with different stacking periodicity. The numbers

on the peaks represents the multiplies of the superlattice satellite reflections. 2.3 Characterization of the electro-optical properties

Electro-optic response of the slab films were measured with a similar setup used, e.g. by Adachi et al [15] and illustrated in Figure 3. In this arrangement He-Ne laser (633 nm) sourced beam was directed into the gap between electrodes. The polarization state angle of the incoming beam was at 45° relative to the applied electric field direction. In-plane induced shift in birefringence ∆n due to electro-optic effect causes the phase retardation φ between perpendicular field components and can be expressed as

nd∆=λπϕ 2

, (1)

where λ is the wavelength and d the film thickness. By using a quarter wave plate the polarization state is returned back to linear, but with slightly different angle α from 45 deg and this shift was measured. The phase retardation shift φ due to electro-optic effect can be obtained by doubling the polarization angle shift in this measurement arrangement [15]. The setup was equipped with a phase lock amplifier that was frequency matched with a chopper modulating the laser light.

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-sample A/4 1/

1Polarization state

oeffi

cien

t (p

m/V

) a

oi

C)

—4

) 0

0 0

0

Bire

frin

genc

e shi

ft [1

O]

/I I I 111111 I I 111111

10 100 1000

Period icity (A)

Fig 3. Measurement setup used to characterize the electro-optic response in slab films.

Insert in Figure 4 shows an example of the electro-optic response induced in-plane birefringence shift as a function of applied electric field. The measurements were carried out with electric field sweep between -3 to 3 V/µm, but only the positive side is shown insert, since the curves were centrosymmetric. The shifts in refractive index were nonlinear in all measurements, but the commonly used first order effective term r in expression

rEnn 3

21

=∆ (2)

was used to compare the strength of the electro-optic response[16]. This was obtained by using the linearized part between 2.5 and 3 V/µm (region between the dashed lines). Calculated r values as a function of stacking periodicity are plotted in Figure 4. As the stacking periodicity decreased from 1064 Å to 106 Å the effective electro-optic response increased from 12 up to 51 pm/V. After reaching a maximum point, the r value decreased again with the decreasing periodicity.

Fig 4. The linearized electro-optic coefficient as function of stacking periodicity in BTO-STO superlattices. The insert

shows an example of the shift in birefringence as a function of electric field.

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The trend line of the electro-optic response resembles the observations of the dependence of permittivity on stacking periodicity with reaching a maximum at a specific individual layer thickness [17],[18]. On the other hand, increasing permittivity with decreasing stacking periodicity down to about two unit cells has been reported [5]. First-principle theoretical studies suggest that the internal dipole structure can be modified by manipulating the stacking periodicity[19], and also both the out-of-plane and in-plane permittivity can vary significantly along with the strain shift [20]. In this work, the stress state relaxation with increasing stacking periodicity was observed and can possibly also contribute to the increase of the tuning properties. Potentially, a certain stacking structure can thereby lead to an optimal response for a specific electric field direction and explain the maximum in electro-optic response at a certain stacking periodicity. 2.4 Manufacture and characterization of test devices

To evaluate the suitability of the deposited films in optical guided wave devices, Mach – Zehnder waveguide interferometers were fabricated and characterized [9],[10]. This included devices on single layer BTO films and on BTO-STO superlattices with 11 unit cell periodicity. Directly etched [21] and strip-loaded type [22] optical waveguide structures based on ferroelectric thin films have been demonstrated. A strip-loaded type device was chosen, since this allows for the use of easily patterned materials for the guiding structure rather than having to pattern the ferroelectric layer itself. A cross section image of the fabricated Mach – Zehnder structure is shown in Figure 5. SixNy layers with thickness of 270 nm were grown by plasma enhanced chemical vapor deposition (PECVD) and lithographically patterned and etched by reactive ion etching (RIE). A refractive index of 1.79 was measured at 1550 nm wavelength for a SixNy slab film prepared in the same manner as the above SixNy layer. Al electrodes adjacent to the active arm were sputter deposited and lithographically patterned and etched with standard aluminium wet etchant.

Fig 5. Cross section scanning-electron-microscope image of the active arm of the modulator with SixNy strip-loaded

waveguide structure. Under an applied electric field, the refractive index of the active layer is tuned and the phase shift is induced between different Mach – Zehnder arms and intensity at the output varies as a function of applied voltage. Figure 6 shows the measurement setup used to characterize the waveguide properties of the test devices. A fiber coupled laser operating at 1550 nm wavelength was used as a light source in the waveguide device measurements. The intensity was modulated with a chopper located between two free space optical fiber connectors. The measurement setup was also equipped with a fiber coupled polarization state controller. The correct polarization at the input fiber was confirmed with the external polarizator before the actual waveguide measurement. TE polarized light was end-fire coupled into the Mach-Zehnder waveguide modulator from the lensed input fiber. Proper waveguide operation was verified by imaging the output intensity distribution with a microscope objective coupled infrared camera presented. During the electro-optic measurements, a voltage sweep was applied across the electrodes and the microscope objective collected light was directed to the optical power meter instead of the camera. The intensity was read from the phase lock amplifier that was frequency matched with the chopper.

Proc. of SPIE Vol. 6996 69960H-5

(X

C

,O0/

0

Inte

nsity

[au.

]

Fig 6. Measurement setup used to characterize the Mach-Zehnder waveguide devices.

Localized intensity peak was observed at the device output facet indicating proper waveguide operation. The intensities at the Mach-Zehnder interferometer output as a function of applied electric field are plotted in Figure 7. By using the expression[22]

Γ=

π

λVLng

reff 3 (3)

the effective electro-optic coefficient value reff can be extracted. Refractive index n was 2.24 for BTO and 2.25 for BTO-STO superlattice films at used wavelength. The electrode length L and the separation g of the electrodes were 3 mm and 9-15 µm, respectively. Vπ is the voltage required to cause a 180 degree phase shift. Γ is an overlap factor describing the portion of the light experiencing the electro-optic effect and was obtained by using a commercial simulation software (FIMMWAVE). Estimated Γ values were 0.74 and 0.39 for single layer BTO films (400 nm) and BTO-STO superlattice films (180 nm), respectively. By substituting appropriate terms in Equation (3), reff value of 23 pm/V was obtained for the BTO film and 72 pm/V for BTO-STO superlattice. The latter value is considerably higher than the standard bulk LiNbO3 r33 value of 30.9 pm/V [23].

Fig 7. Measured optical intensity at Mach-Zehnder waveguide modulator output facet as a

function of applied electric field.

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3. CONCLUSIONS In conclusion, BTO-STO multilayer thin films with stacking periodicity between 27 and 1670 Å were grown on single crystal MgO (001) substrates. XRD measurements suggested a highly strained out-of-plane lattice parameter of 4.20-4.22 Å in the BTO layers due to interface induced stress in the case of short stacking periodicity. On the contrary, STO lattice parameter values were observed to be relatively close to that of the bulk value. The electro-optic response was found to increase at first with decreasing stacking periodicity reaching a maximum value in the superlattice stack with ~10 nm periodicity. Further reduction in stacking periodicity resulted in a decrease in the effective electro-optic coefficient. A strip-loaded Mach-Zehnder waveguide modulators were fabricated on the single layer BTO films and BTO-STO stacks with individual layer thickness of 11 unit cells. Proper waveguide operation was confirmed from the localized optical intensity at the device output facet. An effective electro-optic coefficients of 23 and 73 pm/V were measured for the BTO films and BTO-STO superlattice, respectively.

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