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Abstract
Lanthanum modified lead zirconate titanate (PLZT) thin films were prepared by sol-gel and the plasma annealing process. The PLZT thin films are pure perovskite phase and highly crystalline. The polarization-electric loops confirmed quadratic PLZT thin films. The optical properties were determined by a spectroscopic ellipsometry analyzer, showing an absorption coefficient of near zero and an energy gap of around 3.6 eV. The insertion losses of the optical devices based on PLZT films are less than 5 dB. These films have the potential to be applied in the field of integrated nanophotonic devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Lanthanum modified lead zirconate titanate (PLZT) thin films with the unique ferroelectric, dielectric and electro-optic properties have been studied extensively over the past few decades [1–4]. The chemical formula of PLZT is Pb1-xLax (ZryTi1-y)1-x/4O3, which is generally abbreviated as x/y/(1-y). La3+ replaces the ions at the A-site and induces vacancies at the B-site to balance the charges and enhance the domain reorientation [5,6]. While the ferroelectric thin films have been mainly exploited for electronic applications, the increasing constraints on power consumption have resulted in an increasing interest in the exploration of the PLZT thin films for optical applications [7,8]. In order to be used in the electro-optic devices such as optical memories, sensors, spectral filters, and optical switches, the ferroelectric materials must possess excellent optical properties which are closely related to ferroelectric properties [9–11]. Earlier studies [12,13] have shown that the PLZT x/y/(1-y) with x = 0.08∼0.12 and y = 0.65 exhibits a large electro-optic coefficient, high transmittance, good temperature stability and high-speed response (<1 ns), which have been widely used in the fabrication of optoelectronic devices.
The PLZT ferroelectric films can be prepared by a variety of deposition methods, such as sol–gel [14], RF sputtering [15], chemical vapor deposition (CVD) [16], and pulsed laser deposition (PLD) [17]. However, the preparation of high quality PLZT thin films still remains a challenge. Among these techniques, the sol-gel method is attractive because of its low temperature and low cost. However, the PLZT thin films by sol-gel process often suffer some problems such as the formation of pyrochlore phase, crack and porosity. Du et al. [18] reported that the addition of polyvinylpyrrolidone (PVP) helps the formation of the perovskite phase. Jeyaseelan [19] and Lu [20] have reported that the cracking issue can be resolved to some extent by using chemical additives in the solution. It is essential to obtain pure perovskite films in order to achieve excellent ferroelectric and optical properties [21,22]. A lot of endeavors have been made to eliminate pyrochlore phase, improve crystallinity and obtain crack free PLZT thin films by sol-gel methods. In this paper, in order to obtain high quality thin films, monoethanolamine and PVP have been added as chemical additives in the sol-gel solution. Combined the plasma annealing process, thin films with excellent optical properties were obtained.
The plasma is generated by applying the voltage and mainly composed of high energy ions, electrons and neutral particles [23–25]. In general, the produced glow plasma activates the reactant particles which are subsequently deposited on the substrate surface. Compared with the conventional processes, there are several advantages of the surface plasma annealing technology, including (1) the treatment occurs only on the surface without changing the intrinsic properties of the matrix, (2) shorter reaction times and higher reaction rates, (3) simpler process and better controllability, and (4) environment-friendly. The surface plasma treatment improves the properties of the thin films, such as ultra-compact and high-speed ferroelectric domain conversion.
Most of related researches on PLZT films paid attention to electrical properties, whereas optical properties of these films are much less investigated so far [26–28]. In this paper, we mainly focus on the investigation of the ferroelectricity, optical properties of PLZT films and the applications of the prepared PLZT films on the optical devices with very low insert losses. This work will pave the way for the use of PLZT thin films in the field of integrated nanophotonic devices which are miniaturized and highly efficient.
2. Experimental
The high quality PLZT (x/65/35) thin films were deposited on the indium tin oxide/silicon dioxide (ITO/SiO2) substrates by modified sol-gel and the plasma annealing. Compared with the traditional sol-gel method, this modified sol-gel method includes four steps. Firstly, the solution A was prepared by dissolving Pb(Ac)2 and La(NO3)3 in the ethylene glycol monomethylether according to their stoichiometric ratio. Secondly, the solution B was prepared by dissolving zirconium n-butoxide and titanium tetraisopropanolate in the ethylene glycol monomethylether. Then, the solution A and B were mixed by the magnetic stirring. At last, a small amount of acetylacetone was added as the chelating ligand, and the monoethanolamine was added as the stabilizing agent. The pH values were adjusted by HAc. In order to get crack free films, PVP was applied to retard the condensation reaction and promote the structural relaxation, which led to the reduced film stress during the plasma thermal treatment. The Pt/PLZT/ITO/SiO2 optical waveguide devices were fabricated based on the PLZT thin films deposited on the ITO/SiO2 substrate, and the insert losses of the devices were investigated.
The X-ray diffraction was carried out using Bruker D8 Advance with monochromatic Cu Kα radiation to analyze the phase structure. The transmittance of PLZT thin films were measured by the ultraviolet-visible diffuse reflection absorption spectrophotometer (UV-2500). The refractive index and the extinction coefficient were examined by using the spectroscopic ellipsometry analyzer (SE-850). The polarization versus electric field (P–E) hysteresis loops were measured by Radiant Precision LC equipment with the microprobe station. Finally, The Pt/PLZT/ITO/SiO2 optical devices were fabricated, and the insert losses were tested at Accelink Technologies Company.
3. Results and discussion
The room temperature X-ray diffraction (XRD) patterns for PLZT thin films are presented in Fig. 1. Figure 1(a) and (b) show the XRD of the films prepared with different annealing procedures and with different La contents by the plasma annealing at 650°C, respectively. From Fig. 1(a) and (b), the cubic perovskite structures (PDF NO.46-0336) for all the PLZT thin films were confirmed. For the films prepared from the plasma annealing at 650 °C, there exist pure perovskite phases with different La contents. The intensity of the peak (110) is far stronger than that of the peak at 2θ=35°, which belongs to ITO substrate. By comparison, for the films treated by the conventional annealing, there exist the pyrochlore phase and stronger diffraction peaks of ITO substrates. The pyrochlore phase is detrimental to the microstructure, crystallinity, electrical and optical properties, and deteriorates the performance of the optical devices. The main peak (2θ=31°) of the PLZT films by plasma annealing at 650 °C is more intensive and sharper than those by conventional annealing even at 700 °C. It is clear that the plasma annealing is better than the conventional annealing for PLZT thin films. The reason is that for the conventional annealing the samples were heat treated in the air atmosphere, but for the plasma annealing the samples were heat treated under high vacuum and weak oxygen atmosphere in order to reduce oxygen vacancies. Especially, there are high energy particles who promote the reaction on the surface of the PLZT thin films while plasma annealed. From Fig. 1(b), the XRD patterns of PLZT (x/65/35) films show the (111) (200) (211) peaks of perovskite structure, and an obvious (110) oriented structure.
Fig. 1. XRD patterns of PLZT films by (a) conventional annealing at 600, 650, 700 °C, plasma annealing at 650 °C and (b) plasma annealing at 650 °C with different La contents.
Figure 2 shows the polarizing microscope images of PLZT (9/65/35) films prepared from different annealing processes. The surfaces of the PLZT films by both conventional and plasma annealing are smooth without holes. It can be clearly seen that the films through plasma surface treatment are crack free, they contain less defects and fewer porosity, especially in the edge part of the samples. That is because the produced glow plasma activates the particles which are deposited on the substrate surface. And meanwhile the ions on the surface of the films can acquire high kinetic energy due to the hits of the plasma and be easy to move, and then the surface can be further densification. As the result, the cubic perovskite structure PLZT thin films were less defects and almost crack-free.
Fig. 2. The polarizing microscope images of PLZT (9/65/35) thin films by (a) conventional annealing at 650 °C and (b) plasma annealing at 650 °C.
Figure 3 shows the SEM images of the PLZT (9/65/35) films prepared by different annealing methods. Compared with conventional annealing, the surface of PLZT thin films by plasma annealing was obviously denser and more uniform. This is because when the plasma hits the PLZT films, the ions on the surface of the films acquire high kinetic energy, which is not only conducive to accurate atomic arrangement in accordance with the cubic lattice to reduce surface defects, but also promotes the uniform dispersion of ions and fine grains.
Fig. 3. The SEM images of PLZT (9/65/35) thin films by (a) conventional annealing at 650 °C and (b) plasma annealing at 650 °C.
The transmittance of the PLZT thin films prepared through conventional and plasma annealing has been shown in Fig. 4(a). The thickness of the PLZT thin films is about 300 nm and we test the transmittance on the ITO glass substrate. Meanwhile, the other similar ITO glass substrate was as the reference. The transmittance shows little change with the wavelength range from 500 nm to near-infrared. Compared with the conventional annealing, the transmittance of the PLZT thin films by plasma annealing exhibits great improvement of the transparency from 68.2% to 89.2% at 800 nm. The thin films through plasma surface treatment are crack free and have fewer defects, which reduced the scattering and the absorption of the light.
Fig. 4. The transmittance and polarization-electric field hysteresis loops of PLZT (9/65/35) thin films by conventional and plasma annealing at 650 °C.
The ferroelectricity is related to the spontaneous polarization of the materials. Upon the application of the electric field, two types of current signals could be observed, i.e., leakage current and the current due to domain switching phenomena. The former give rise to the hysteresis loop not fully saturation and the latter confirms the ferroelectric nature [29,30]. Figure 4(b) shows the P-E hysteresis loops of the PLZT (9/65/35) thin films prepared by conventional and plasma annealing. The remnant polarization (Pr) is 14.8 µC/cm2 and coercive field (Ec) is 72.0 kV/cm for the sample from the plasma annealing process. The Pr is 14.3 µC/cm2 and Ec is 51.3 kV/cm for the films yielded from the conventional annealing process. The PLZT thin films by plasma annealing show higher polarization and smaller leakage current, due to less defects and crack free nature, as illustrated in Fig. 2 and Fig. 3.
As the films through plasma surface treatment exhibit excellent optical and ferroelectric properties, we prepared a series of PLZT (x/65/35) thin films with different La contents by sol-gel process and plasma annealing. Figure 5 presents the P-E hysteresis loops of PLZT (x/65/35) thin films by plasma annealing at 650 °C. With increasing La content from x = 8 to 10 mol%, the shape of the P-E loops becomes slimmer, while the saturation polarization and Pr decreases dramatically. With the increase of La content, the phase structure of PLZT transforms from the ferroelectric to paraelectric phases which leads to a reduction in the spontaneous polarization [29,31]. The Pr is 31.1µC/cm2 and the Ec is 89.3 kV/cm of the PLZT (8/65/35) films. The hysteresis loop for PLZT (9/65/35) films shows a typical quadratic type with fine hysteresis loop and lower remnant polarization (Pr is 18.2µC/cm2 and Ec is 42.5 kV/cm). Since the quadratic type hysteresis loop is closely related to the electro-optic properties, we conclude that the PLZT (9/65/35) films could be used as optical switches and applied in optical communications. For the PLZT (10/65/35) thin films, the Pr is 6.9µC/cm2, which indicates that the ferroelectric phase is changing into the paraelectric phase with the poor ferroelectricity when La content is increased to 10mol%.
Fig. 5. The polarization-electric field hysteresis loops of PLZT (x/65/35) thin films by plasma annealing at 650 °C.
Figure 6 illustrates excellent transmittance of all the PLZT thin films with different La contents and the thickness of the PLZT thin films is about 300 nm. The transmittance sharply increases below the wavelength of 500 nm, and exhibits high transmittance within a wide wavelength range from 500 nm to near-infrared. With increasing La content, the transmittance firstly goes up to 90.5% for the PLZT (9/65/35) films which possesses the highest transparency, and then drops to 77.7% at 800 nm for the PLZT (10/65/35). With La3+ dissolved in the gap of PZT to produce single phase solid solution, the transmittance increased. The excellent optical transmittance indicates that the films have a smooth surface and uniform thickness. With excess La3+ dopant, the oxygen vacancies emerge, which leads to the scattering of light.
The refractive index (n) and the extinction coefficient (k) are measured by using the spectroscopic ellipsometry analyzer (SE-850). As shown in Fig. 7, the refractive index and the extinction coefficient of PLZT films sharply decrease in the ultraviolet range, then tend to level off at the wavelength higher than 500 nm. The refractive indexes of the PLZT films with a La content ranging from 8, 8.5, 9 and 10mol% are 1.86, 1.85, 1.84 and 1.83 respectively, at 632.8 nm. The corresponding extinction coefficients of the PLZT films are 0.007, 0.006, 0.005 and 0.004 at 632.8 nm. The n and k show a trend of slight decrease as the La content increases. To further understand the optical properties of the thin films, we calculated the absorption coefficient (α) and the energy gap (Eg) of the films with different La contents, as presented in Figs. 7 and 8. Generally, the absorption coefficient and the energy gap (Eg) are defined as follows [32]:
where λ is the wavelength, h is the Planck constant, ν is the frequency, B is the constant.Fig. 7. The optical properties (a) refractive index, (b) extinction coefficient and (c) absorption coefficcient of PLZT (x/65/35) films.
α was calculated through equation above, and which shows the similar changes with the n and k. α is near zero when the wavelength is greater than 500 nm, indicating that there is no light absorbed by PLZT thin films. As shown in Fig. 8, the energy gap of the thin films is the intersection of tangent and hν axis according to Eq. 2, after the linear extrapolation is carried out. With increasing the La content from 8 to 10mol%, Eg dropped from 3.676 to 3.613 eV. This could be attributed to the decrease of grain domain, which led to a reduction in the spontaneous polarization. With increasing the La content, the oxygen vacancy increases which would capture electrons and result in the decrease of Eg. Also, the formation of impurity band [33] by the superposition of impurity wave function, and even the impurity band further overlapping into the forbidden energy gap could result in the decrease of Eg. In general, the formation of oxygen vacancy [34] and impurity band with increasing La content affects the optical properties of PLZT thin films.
It is highly desirable to reduce the absorption loss by using ITO electrodes to replace the traditional metal electrodes. Introduction of a PLZT thin buffer layer between the waveguide and the electrode could prevent optical absorption and scattering, and obtain a compact structure with low insertion loss. By sol-gel processing and plasma annealing, we have prepared the PLZT (x/65/35) ferroelectric thin films with excellent optical properties, including the refractive index of greater than 1.8, the absorption coefficient of near zero, and the energy gap of around 3.6 eV. On the basis of the outstanding optical properties of the PLZT thin films, we have designed a type of PLZT waveguide optical device as shown in Fig. 9. ITO glasses were chosen as the substrate and the transparent nano-scale Pt as the bottom electrode. The thickness of the PLZT buffer is around 40 nm and the waveguide layer is about 300 nm. After the PLZT core layer was prepared by sol-gel and surface plasma treatment processing, the Pt transparent electrode was deposited above the waveguide layer by magnetron sputtering. The dimensions of the PLZT thin films are 1.5 cm length and 1.5 cm width, and the thickness is about 340 nm. Based on the previous results, the PLZT optical devices have high transmittance. If the insertion losses of them are also small, the PLZT optical devices prepared in this research will lay a good foundation for the fabrication of optical switch in the next step, and can be used in the field of integrated nanophotonic.
Figure 10 illustrates the optical path about the test system of insertion losses of the PLZT optical devices. The inherent insertion losses of the optical devices must be small enough to meet the demand of the switching engine imposing on the rest of the optical system. They are two types of the insertion losses test, i.e., transmission and reflective type. For the transmission type, the light path system is easy to be built and has less system error, so we use this system to test the insertion losses. Figure 11 displays the insertion losses of the Pt/PLZT/ITO/SiO2 optical device. The fabry-perot cavity (oscillations in insert loss) is due to the interference of the reflection light from different interface within the optical devices. The insertion losses of the devices we fabricated are less than 5 dB. Because of its excellent properties and ease of fabrication, this PLZT thin film optical device has a great application prospect, for example as a spectrometer or a Mach−Zehnder interference optical switch.
4. Conclusions
In this study, we prepared crack-free PLZT ferroelectric thin films by modified sol-gel and plasma annealing process. The highest transmittance of the PLZT (9/65/35) thin films is 90.5%. The surfaces of the films are smooth and uniform. The P-E hysteresis loops of the PLZT (9/65/35) thin films show typical quadratic type with fine hysteresis loop and lower remnant polarization, e.g. the Pr of 18.2µC/cm2 and Ec of 42.5 kV/cm. The absorption coefficient is near zero and the energy gap is around 3.6 eV. The insertion losses of the prepared PLZT optical devices are found to be less than 5 dB. Given its excellent ferroelectric and optical properties, it is thus anticipated that the prepared PLZT films with optimized composition are promising for applications in the integrated nanophotonic devices.
Funding
Wuhan Applied Basic Research Project (2015010101010009); National Natural Science Foundation of China (NSFC) (61271141).
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Grant No. 61271141) and Wuhan Applied Basic Research Project (Grant No. 2015010101010009). We should also thank Accelink Technologies Company.
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