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Abstract
Developing strong electro-optic (EO) effect materials and devices is vital for high-speed optical communications and integrated photonics. In this work, we explored a chemical solution deposition technique to grow pure perovskite lead zirconate titanate (PZT) films on sapphire substrates. The grown PZT films demonstrated a preferential orientation and a broadband optical transmission window ranging from 600 to 2500 nm. Based on the high-quality film, we subsequently designed and fabricated a PZT Mach-Zehnder interference waveguide EO modulator. The measured half-wave voltage Vπ is 3.6 V at the wavelength of 1550 nm, corresponding to an in-device EO coefficient as high as ∼133 pm/V. The response of the PZT modulator from 6 to 12 GHz has been measured. We foresee that our work may pave the way towards power-efficient, ultra-compact integrated devices, including modulators, switches and sensors.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The exponential increase in the demands of data traffic requires high-speed, energy-efficient and long-term stable optical communication technology. Electro-optic (EO) modulating technologies are emergent as the engines to ameliorate the performance of optical information transmission. Carrier plasma dispersion [1,2], electro-absorption [3,4], and Pockels effect [5,6] have been employed for EO modulators. Among them, Pockels effect can offer ultrafast EO response, pure phase modulation and low absorption loss over an extremely broad optical spectrum [7,8]. Though Pockels effect owns these attractive features, it naturally occurs only in several kinds of crystals that lack inversion symmetry [9,10]. The current state of the art modulators for optical communications and measurement systems mainly bases on lithium niobate (LiNbO3) crystals [11–13]. Nevertheless, LiNbO3 has an EO coefficient limited to ∼30 pm/V. This EO coefficient may eventually cause a high driving voltage and narrow the range of potential applications. Since the driving voltage of a modulator is inversely proportional to EO coefficient [14,15], pursuing low power consumption drives people to explore new materials with large EO coefficients.
Lead zirconate titanate PbZrxTi1−xO3 (PZT) crystals are one promising candidate for EO modulators because of their excellent optical transparency and potential of obtaining high EO coefficient [16,17]. Compared with bulk crystals, thin film waveguides can miniaturize and integrate photonic devices on a single chip [18]. Though PZT materials have been extensively studied in the past [6,19], limited research on exploring PZT waveguide modulators is available. The difficulty lies in that the strong EO effect only occurs in pure perovksite PZT films that are usually grown on metallic intermediate layers [20,21]. Such metallic layers cannot be used for optical waveguides, because the absorption loss from the metal is too large. If PZT waveguide modulators can be developed with the properties of high EO coefficient and low optical loss, it will be valuable for today’s information network and the fast miniaturized components for on-chip optical interconnects.
In this work, we demonstrate that a large EO coefficient can be obtained in PZT crystalline films grown on sapphire substrates via a flexible chemical solution deposition technique. The X-ray diffraction and scanning electron microscopy measurements reveal that the PZT films are dense, uniform, crack-free and preferentially (100) oriented. The high-quality crystalline films enable us to develop a SiO2-PZT Mach-Zehnder interference (MZI) waveguide phase modulator by using the co-planar electrode. At the wavelength of 1550 nm, the measured half-wave voltage is 3.6 V, corresponding to an in-device EO coefficient of ∼133 pm/V. We further proved that modulation over the signal frequency of 6–12 GHz was feasible in the PZT modulator.
2. Experimental section
2.1 PZT precursor preparation
PZT with preferred crystallographic orientations is critical for achieving strong EO effect [22]. In order to obtain high-quality PZT films, we explored a straightforward chemical solution deposition (CSD) technique to grow pure perovskite PZT films on sapphire substrates. As the seed layer for the high-quality PZT film, LaNit solvent was firstly obtained by dissolving lanthanum nitrate hexahydrate [La(NO3)3•6H2O] in 2-methoxy ethanol. The concentration of the solution was adjusted to 0.036 mol/L. PZT (Pb(Zr0.52Ti0.48)O3) precursor solution was prepared by a modified 2-methoxyethanol based chemical solution route [16]. It’s worth noting that 20% mol excess of Pb can compensate the Pb loss during the annealing process. Firstly, the lead acetate hydrate was dissolved in 2-methoxy ethanol. Subsequently, Ti-isopropoxide and Zr-isoproxide solutions were mixed and chelated in stoichiometric proportion under N2 protection. Small amounts of acetyl acetone and formamide were added as chelating agent and addition to improve the film flatness. Finally, it was mixed with the distilled lead acetate to get the PZT precursor solution with a concentration of 0.4 mol/L.
2.2 PZT films deposition
The sapphire substrate was cleaned by piranha solution, deionized water and isopropanol. After cleaning, the LaNit seed film was spin-coated onto the sapphire substrate and annealed at 400–500 °C in a tube furnace for 10 min. In the process of heating, the films experience a sequence of endothermic weight loss to initiate the intermediate phase transformation. With the increase of molar concentration of the solution, the thickness and the inhomogeneity of the seed film will also increase. Herein, we prepared 0.036 mol/L solution and followed a multilayers spin-coating and annealing procedure to avoid this problem. After that, the PZT precursor solution was spin-coated onto the seed film, and heat treated on a hot plate at 200 °C to evaporate organic solvent and 400 °C to burn out the organic components of the gel layer. Each spin-coating and pyrolysis step results in a layer of 40–50 nm, so the process cycle is repeated several times to achieve the ideal thickness.
Finally, the amorphous PZT thin films were annealed in a tube furnace at 400 °C for 10 min and 600 °C for 10 min, under flowing O2 gas to allow crystallization into the perovskite PZT phase. A steady oxygen flow of 200 sccm was maintained throughout the annealing procedure. The ramping rate for heating and cooling of the specimen in the annealing system were 10 °C/min and 5 °C/min.
3. Results and discussion
3.1 PZT film characterization
The obtained 220 ± 10 nm-thick PZT films were evaluated by X-ray diffraction (XRD) with CuK$\mathrm{\alpha }$ radiation (Bruker D8 Advance). The diffractograms are recoded for 2θ angles from 20° to 60°. The thickness of the LaNit seed layer is approximately 5 nm for all the samples. Figure 1(a) represents the phase evolution of the PZT thin films deposited under differe temperatures in the O2 atmosphere. No crystalline peak is observed from the PZT films when the annealing temperature is < 500 °C. In contrast, the PZT film annealed at 550 and 600 °C is crystallized into a pure perovskite phase, with no evdience of any intermediate secondary phase formation. The diffractograms show two strong diffraction peaks along the (100) and (200) crystallographic orientations at 2θ = 21.7° and 44.3°, which are the primary and secondary diffraction peaks of the (100) oriented PZT films [16,20,21]. The inter-planar spacing of PZT layer perpendicular to the film surface is calculated to be ∼4.09 Å from the high resolution XRD [23]. The strongest diffraction peaks are obtained when the annealing temperature is 600 °C. The XRD also revealed that PZT with the morphotropic phase boundary (MPB) structure located between rhombohedral and tetragonal phase [17]. The preferential orientation along the (100) rystallographic direction with sharp diffraction peaks corresponds to a strong EO effect [24]. In addition, we also tested the samples annealed in the the N2 or air gas flow (at 600 °C) and observed weak perovskite peaks as shown in Fig. 1(b). Since EO effect of PZT films strongly depends on the quality and crystallographic orientation, anealing at 600 °C in the O2 atmosphere has been selected as the optimized condition in this work.
Fig. 1. XRD results of the PZT films. (a) Annealed at different temperatures: 500 °C (green), 550 °C (black) and 600 °C (blue). (b) Annealed in different gas: N2 (black), air (red) and O2 (blue) at 600 °C. For comparison, pure sapphire (green) is also given.
The ordinary and extraordinary refractive index (ne and no) and extinction coefficient (κ) versus the wavelength as shown in Fig. 2 was measured by a spectroscopic ellipsometry analyzer (J.A. Woollam RC2 XI+). Here, we will use the refractive index no = 2.40 at 1550 nm for the modulator. Meanwhile, the measured extinction coefficient (k) sharply decreases in the range of 400–600 nm. It is almost zero at the wavelength over 600 nm (k is too low to be measured), revealing that the material has a broadband transmission window. The high refractive index and low material absorption loss enable the PZT to be featuring a sub-wavelength scale light confinement and a dense integration of optical components.
Fig. 2. The ordinary and extraordinary refractive index (red lines) and extinction coefficient (green lines) of PZT thin films at different wavelengths.
Scanning electron microscopy (SEM, Zeiss Gemini500) measurement was used to observe the surface and the cross-section of the PZT film. The surface morphology of the film as shown in Fig. 3(a) confirms dense and uniform polygonal grains without any cracks. The surface roughness of the PZT film was also measured to be < 2.5 nm by the atomic force microscope. According to the Payne-Lacey model [25], the surface scattering loss of a waveguide is caused by surface roughness at the interfaces between waveguide and cladding. In our case, the smooth surface of the PZT film will contribute to a negligible surface scattering loss. Figure 3(b) shows the captured cross-section of the PZT film with a thickness of 220 ± 10 nm, verifying that the PZT film is well crystallized with dense and columnar grains.
Fig. 3. SEM images of the 220 ± 10 nm-thick PZT film: (a) surface morphology and (b) cross-section. The images demonstrate that the PZT film is smooth and well crystallized.
3.2 Design and fabrication of SiO2-PZT waveguide modulators
In order to validate the strong EO effect, we designed and fabricated a PZT MZI waveguide modulator. Figure 4(a) shows the cross-section of the designed MZI waveguide. The thickness of PZT and sol-gel SiO2 are 220 ± 10 and 200 ± 10 nm, respectively. The SiO2 has a width of 2 µm. The modulator employs a co-planar electrode consisting of 200 nm-thick Al with a distance of 10 µm on the top of the PZT. The fundamental TE mode in the waveguide as shown in Fig. 4(a) was simulated by using the optical mode calculation of Rsoft. In the simulations, the refractive indexes are 1.48 and 1.75 for SiO2 and Al2O3, respectively [26]. We can see that the mode tightly concentrates in the PZT layer. According to the simulation, 48.6% of the light is confined in PZT thin layer. The TE mode also indicates that, in order to access the largest EO coefficient, a horizontal electric field is required. The DC electric field distribution obtained by using COMSOL software is illustrated in Fig. 4(b). In the simulations, the permittivity of SiO2 and Al2O3 is set as 3.9 and 10, respectively. The permittivity of PZT is based on the measured value in Ref. [27]. From the DC field distribution, we find that a horizontal electric field crosses the region where the optical field locates. As a result, an EO modulator can be realized by changing the refractive index of the PZT through the co-planar electrode.
Fig. 4. (a) Simulated optical model profile and (b) DC electric field distribution (arrows denote the direction of the DC electric field) in the waveguide modulator.
Based on the design, the modulator was fabricated on a sapphire substrate. As schematically shown in Fig. 4(a), a 5 nm-thick LaNit seed layer was firstly coated on the sapphire substrate. Then, a PZT film was deposited on the seed layer. The preparations of the LaNit and PZT layers followed the process aforementioned in the section above. Subsequently, sol-gel SiO2 was spin-coated on the PZT film, and then baked in air at 120 °C for 2 h and 150 °C for another 2 h to form a 200 ± 10 nm-thick film [28]. The MZI waveguide was fabricated by using photolithography and dry-etching (Samco RIE-101iPH) by CHF3 gas. Finally, the co-planar Al electrodes on one of the MZI arms were obtained via the thermal evaporation and lift-off.
3.3 EO measurement of the waveguide modulators
To evaluate the EO properties, the device was measured by an end-face coupling system. Input laser (1550 nm) was coupled into the waveguide through a polarization maintaining fiber. The polarization of the light was controlled by a polarizer between the laser and the polarization maintaining fiber. Output light was collected by a CCD camera to characterize the quality of the optical mode. Figure 5(a) shows the captured output light from the CCD when the input light is TE polarization, in which a clear single mode pattern can be observed.
Fig. 5. (a) TE mode output light collected by a CCD camera and (b) measured spectrum of half-wave voltage Vπ. The Vπ is 3.6 V with an electrode length L of 1 cm.
The Vπ of the modulator was obtained by measuring the relationship between the applied AC voltage and transmission light intensity. From a clear modulation output function as shown in Fig. 5(b), a Vπ of 3.6 V at 3 kHz was measured. With an electrode length L of 1 cm, VπL of the modulator is 3.6 V·cm. In an EO modulator, the EO coefficient r can be calculated by the equation:[29,14]
where λ is the operation wavelength, g the inter-electrode distance, and Γ the overlap integral between the applied electric field and the optical mode. For the in-device EO coefficient rin-device [14], n3 = neff3 = 8.12 and Γ = 0.4. Based on the measured VπL and Eq. (1), the rin-device is estimated to be 133 ± 5 pm/V. We also noted that some works used n = nPZT=2.40 to deduce the material EO coefficient rmaterial. In this case, rmaterial will be around 80 pm/V.Then, we measured the high frequency response of the PZT modulator by using the sideband measurement technique [30]. RF signals were provided by a signal generator (Anritsu MG3697C) and applied onto the travelling-wave electrode via a picoprobe (GGB Industries Inc. 40A-GSG-200-DS,). The input light at 1.55 um was adjusted to TE polarization and the output light was fed into an optical spectrum analyzer (Yokogawa AQ6370D). When the RF signal with a frequency of ω was applied to the modulator, the transmission light shows the sideband spectrum with the peak spacing of the same ω [14]. Figure 6 shows the measured transmission spectra by applying ω in the range of 6–12 GHz to the modulator. It can be observed that clear sidebands appear in the spectra, equally spaced around the main peak. The high frequency response indicates the potential application of our modulator at high speed if a traveling-wave electrode is applied in future [31].
In Table 1, we compared the performance of our PZT films with prior works, all of them have recently demonstrated EO properties. According to the wavelength dependences of r in the well-known EO crystals, r at the wavelength of 632.8 nm is around 1.2–1.5 times of the r at 1550 nm [32,33]. From the table, it can be seen that our PZT film has the potential of being applied in high-speed integrated photonics.
Table 1. Electro-optical properties of PZT in recent works
4. Conclusions
In conclusion, the PZT thin films have been fabricated on sapphire wafer by utilizing the chemical solution deposition technique. This straightforward film fabrication offers well commercial promise. The XRD and SEM characterizations demonstrate strong preferential thin film growth, with dense, crack free and polygonal crystal grains. The PZT films have a negligible optical absorption from 600 to 2500 nm. Based on the PZT films, we have designed and fabricated a sol-gel SiO2 / PZT MZI waveguide modulator, one of the arms with co-planar electrodes. The measured ${V_\pi }$ is 3.6 V with an electrode length L of 1 cm at the wavelength of 1550 nm, corresponding to an in-device EO coefficient of ∼133 pm/V. The sideband test indicates the possibility of applying the device at high-speed modulation. The above results reveal that the modulator should have a prospect towards future information network and on-chip optical interconnects.
Acknowledgment
F. Qiu would like to memorialize Emeritus Professor Tadashi Narusawa (Kochi University of Technology) via this work.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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