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Effect of pre-annealing of lithium niobate on the structure and optical characteristics of proton-exchanged waveguides
Optical Materials 88 (2019) 176–180
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Effect of pre-annealing of lithium niobate on the structure and optical characteristics of proton-exchanged waveguides
T
Aleksei Sosunov∗, Roman Ponomarev, Oksana Semenova, Igor Petukhov, Anatoly Volyntsev Perm State University, 614990, 15 Bukirev St, Perm, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium niobate Proton exchange Structure Pre-annealing Optical modulators
The influence of pre-annealing of lithium niobate in the range of 400–800 °C was studied at each step of formation of proton-exchange waveguides. Pre-annealing affects the structure and optical properties of lithium niobate. There is a relationship between the initial structure of lithium niobate and the optical characteristics of proton-exchange waveguides. The linear dependence between the deformations of the annealing proton exchange waveguides and the refractive index is established. Pre-annealing leads to an improvement in the structure of the subsurface layer of lithium niobate, as well as a slight decrease in the refractive index due to an increase in the homogeneity and a lower concentration of protons inside the crystal. The crystal structure becomes most uniform at a temperature of 500 °C. The results can be useful for different optical crystals used for applications.
1. Introduction Lithium niobate (LN, LiNbO3) is a key material in the photonic industry because of its advantageous combination of functional properties (high Curie temperature – 1145 °C, high electro-optical coefficients – r13 = 10 p.m./V and r33 = 32 p.m./V, wide transparency window – 0.35–5.5 μm, etc.) and its commercial availability. Proton exchange often used to form optical waveguides because it has several merits (low optical loss – 0.2 dB/cm, this is a low-temperature process (about 300 °C) compared to the metal diffusion; low photorefraction; maintains of the light polarization. However, there are number of demerits, for example high sensitivity of waveguide parameters to surface quality [1], significant anisotropy of supported radiation modes and multistep process. These characteristics provide the opportunity to use LN as a substrate for the fabrication of integrated optical devices, such as electrooptical modulators, switches, diffraction gratings, nonlinear optical frequency converters and electric field sensors [2–4]. It is still an open question about the phenomena of drift in the optical modulator based on proton exchanged waveguides in LN [5–7]. J. P. Salvestrini analyzed various sources of drift in commercially available optical modulators. The different origins have been compared in terms of phase shift and the different corresponding orders of magnitude have been given, pointing out the predominant role of the dc drift. Large role played electrical inhomogeneities at the surface of the LN
∗
substrate by highlighting the link between the time dependence of the dc drift in an LN optical modulator and the electrical conductivity measured at the surface and in the volume of the LN substrate [8]. The drift of optical modulators has related to the instability of the refractive index of proton-exchanged waveguides [9]. The refractive index depends on the proton exchange process, and it depends on the structure of the crystal. It is necessary to investigate the relationship between the structure of the surface layer and the results of proton exchange process. Authors [10–14] showed significant differences in the structure and properties of the surface and LN substrate. Modification of the structure and properties of the LN surface occurs during the processing (polishing) of the wafers. Lithium niobate wafers “optical quality” still have a disorder in the layer under the surface. In practice, annealing/wet annealing is widely used to treatment the structure and various characteristics of LN, including for LN thin films [15] and hybrid optical waveguides [16]. In particular, the use of LN pre-annealing with a Crmask allows for improving the surface roughness of the ridge waveguides, because the mask inherits all defects in the surface during its subsequent etching [17]. In this paper we make pre-annealing lithium niobate to eliminate surface stresses and to study the effect on the structure and optical characteristics of proton-exchanged waveguides.
Corresponding author. E-mail address: [email protected] (A. Sosunov).
https://doi.org/10.1016/j.optmat.2018.11.018 Received 9 September 2018; Received in revised form 12 November 2018; Accepted 13 November 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Optical Materials 88 (2019) 176–180
A. Sosunov et al.
2. Experimental 2.1. Sample preparation The samples used for the investigations were congruent X-cut LiNbO3 1-mm-thick 3-inch wafer (Crystal Technology, Inc.). Wafer was cut into a 15 × 10 mm2 pieces. Treatment of LiNbO3 is performed by pre-annealing in a wide temperature range of 400–800 °C in increments of 100°С. The samples were heated with speed 400 °C/h to the required temperature, then for 3 h they were annealed, and after that they slowly cooled with the furnace. In addition, there was a control sample without pre-annealing. Long-term exposure is compensated by a high heating rate to avoid additional structural defects, dislocations, cracking, etc. Pre-annealing temperature was chosen within a wide range, despite the cation sublattice can be disturbed higher than 600 °C [18,19] and the reduction process is accompanied by the formation of oxygen vacancies and Li loss [20]. We consider that the effect of temperature on the refractive index of lithium niobate well describes Sellmeier equation. There are large numbers of different modifications of Sellmeier equation. However, Sellmeier equation can be modified to describe the dependence of the refractive index on a combination of factors — wavelength, temperature, and composition crystal. Zelmeyer equation in the original version: m
n2 (λ ) = 1 +
∑ i=1
(λ2
Fig. 1. Fracture of sample.
Properties of single-crystal materials are largely determined by their internal structure. The most important information about the structural defects and strain can be obtained by using special X-ray diffraction techniques to investigate the fine structure of the crystals. The research of X-ray diffraction was carried out by the precise double-crystal spectrometer method. X-rays reflected from two series of flat single crystals of which the first was monochromator, had a perfect structure, and the second was sample. Monochromator was dislocation-free silicon crystal, set in the position corresponding to reflection Kβ-line cobalt radiation (λ = 1.62075 Å) from the crystallographic plane (111). The use of a single β-line eliminated the possibility of X-ray reflections overlapping, which is observed for α-line (doublet). During shooting, the crystal was rotated around the goniometer axis about the reflecting position with the angle θ for the registration θ/2θ-curves. Diffraction reflections were obtained for the first and second order with indices (110) and (220), respectively. The resolution of this method is 0.0001 deg. All measurements were carried out at room temperature, voltage – 30 kV, anode current– 10 mA, using a slit width of 0.05 mm.
λ2αi − λi2)
where λ-wavelength of the incident radiation in nm, and αi is a constant depending on the concentration of oscillators of the i-type in a unit volume, called the force oscillator resonant wavelength λi. Samples were cleaned in isopropyl alcohol and deionized water before and after proton exchange. The entire surface of the samples was protonated by submerging the wafers into benzoic acid at 170 °C during 2 h in closed zirconium reactor. The reactor was placed in a furnace on rotating platform to mix benzoic acid and cooled at rate of 10 °C/min after completion of the proton exchange (PE) process. Annealing was carried out at 350 °C during 5.5 h, and then slowly cooling in the furnace.
3. Results 3.1. Scanning electron microscopy
2.2. Methods
Fracture allows to fixing the position of the structure in its natural form without distortion. Structure of the cross-section of a crystal LN after fracture and different pre-annealing temperature is shown in Fig. 2. There is investigations show the presence of a stressed subsurface layer with a depth of about 6 μm for the pristine sample. The pre-annealing temperature is increased, while the depth of the layer decreases to 1 μm or disappears completely at 500 °C and above. It is because of high crystal defects concentration. Pre-annealing is apparently a good tool for treating LN. SEM results indicate that the temperatures 500–800 °C is optimal. The defective surface sublayer becomes blurred and decreases in thickness with increasing temperature (Table 1).
Electron microscopy was carried out using a Hitachi S3400 N SEM operating at 20 kV. Imaging and analysis tools included a conventional diffraction detector. We researched cross-section of LiNbO3 samples after fracture. Scratched on the surface of LN with a diamond pyramid, and then sample was broken along her. Analysis of the subsurface layer SEM was performed on the opposite side of the scratch (−X-scratch, and +X-analysis). Fig. 1 shows a schematic representation of the fracture. Electrical conductivity of the surface of a dielectric is necessary for conducting SEM analysis. SC7620 magnetron sputter with AuePd target was used to deposit thin conducting layer on the sample. The thickness of the conducting layer was set to a minimum, firstly, in order that the investigated structure was not hidden under the metallic layer, and, secondly, ensured the flow of charges from the surface of the samples. In this study, thickness of layer was about 2 nm. Increment of the refractive index of planar waveguide was determined using mode spectroscopy with an accuracy of ± 0.0002. Accuracy depends on the number of excited modes and form of allocation of the refractive index along the depth of the layer. Firstly, sets of effective (mode) refractive indices (Nm) at λ = 632 nm with prism coupling method [21] were determined, and secondly, using the obtained values of Nm, profile of the refractive index along depth of the waveguide layer was reconstructed by means of the IWKB method [22].
3.2. X-ray diffraction The sample was placed so that its surface was oriented parallel to the surface of the monochromator. In this position the half-width of the diffraction curve is independent of the natural spectral line width (no disperse), as determined only by the geometrical and physical factors. Also such an arrangement is most convenient for investigating the degree of imperfection of single crystals. The geometric factor can be neglected because of its smallness in comparison to the physical factor in the broadening of the spectral lines for the Gaussian distribution. The physical factor determines deformation of the crystal lattice of the 177
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Fig. 3. Deformations of LiNbO3 after pre-annealing and APE (a); displacement of diffraction lines after PE (b–c).
Fig. 2. Cross-section of LN after fracture and pre-annealing (a – pristine, b–400°С, с– 500–800°С).
Deformations (ε) were calculated by broadening the spectral lines by equation:
Table 1 Depth of subsurface layer from pre-annealing temperature.
ε2 =
Temperature, °C
25 (pristine)
400
500
600
700
800
Depth, um
6
5
0–1
0–1
0–1
0–1
cβ12 β2 − β22 β1 16tg 2θ1 (cβ2 − tβ1)
,
whereβ – half-width of experimental line, t = tg2θ2/tg2θ1, c = cosθ1/ cosθ2, indices 1 and 2 are the first (110) and second (220) order of reflection, respectively. The displacement of the diffraction lines (phases) was calculated from the Wolf-Bragg equation:
material. The structure of LiNbO3 was studied at each step in the development of proton-exchange waveguides (pre-annealing, PE and APE). The deformations were calculated from the broadening (after pre-annealing and APE) and the displacement (after PE) of the diffraction lines.
2dsinθ = nλ, where d – interplanar spacing, θ – reflection angle, n – reflection order and λ – wavelength. 178
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(contrast). Our results show this dependence at a pre-annealing temperature of 500 °C. At this temperature, the minimum refractive index of proton-exchange waveguides (Δne) is inversely proportional to its depth (δ) or homogeneity of the structure. As the pre-annealing temperature rises, the index of refraction and deformation of the crystal lattice increases. This may be due to the violation of the oxygen scaffold and loss of Li atoms. It is necessary to note the relationship between the structure of the LN crystal and its optical characteristics. A linear relationship between the deformations of the crystal lattice of LN and the refractive index of proton-exchange waveguides was established during pre-annealing temperatures. Electro-optical coefficient r33 is proportional to Δne. If Δne higher, then stronger the electro-optical response, that is good for modulators. Our results showed small decrease of Δne from the pre-annealing temperature. However, the decrease is Δne insignificant for the technology of optical modulators manufacturing, and more homogeneous structure lead to significantly affect the stability of different characteristics (refractive index, Vbias and electro-optical coefficients). 4. Conclusion In this paper we demonstrate the trend of the effect of pre-annealing LN at each step of the formation of APE waveguides. Pristine LN has a 6 μm deep disorder layer, which is blurring with pre-annealing. The optimum pre-annealing temperature is about 500 °C. Optical characteristics are reduced insignificantly for technological process. However, a most homogeneous structure is a promising medium for increasing the stability of technology, refractive index and drift phenomena. The results demonstrate a linear relationship between structural changes and the optical properties of APE waveguides, depending on the pre-annealing of LN. We have established a linear relationship between the deformations ε of the subsurface layer of LN and the results of the proton exchange process Δne, δ. These results are relevant for all optical crystals used for buried waveguides formation. The change in temperature or electric field can lead to instability of the refractive index due to the motion of charged defects in area of proton-exchanged waveguides. The ability to influence features of the structure subsurface layer of LN has the prospect of improving the drift characteristics of optical modulators based on proton-exchange waveguides. Our results give more control of the structural characteristics will allow us to minimize the instability of the refractive index of proton-exchanged waveguides. The investigation of the drift characteristics of optical modulators on the pre-annealing temperature of LN will be considered in the next paper.
Fig. 4. Waveguide profiles (a); refractive index and depth of the waveguides as functions the pre-annealing temperature (b).
The structural features and their changes are shown in Fig. 3 after each step of manufacture. The deformation (Fig. 3a) is slightly larger after APE because the crystal lattice is not completely restored, but the trend remains the same. The trend of the results obtained after each stage of formation of proton-exchange waveguides is preserved. The set of β-phases (Fig. 3b) formed after the proton exchange process also reflects the original structure. At a temperature of 500 °C, there is a characteristic minimum of deformations of the crystal lattice and also a minimal displacement of the diffraction lines (Fig. 3a,c). From the viewpoint of the morphology of 500 °C is the optimum temperature treatment of LiNbO3 to obtain the most homogeneous subsurface structure and manufacture of proton-exchange waveguides. High temperature leads to more disordered structure, the appearance of new defects and oxygen vacancies. Diffraction line broadening occurs at small angles θ and corresponds to large deformations in the surface layers. The results obtained with SEM in good agreement with the XRD results. Thus, pre-annealing improve the structure of the LN crystal and proton-exchange waveguides.
Funding This work was supported by RFBR [grant number 17-43-590309]. Declaration of interest None.
3.3. Mode spectroscopy References Optical characteristics (increment of refractive index, depth, profile) of proton-exchanged waveguides were characterized by the use of mode spectroscopy and are presented in Fig. 4. The waveguide profiles have a gradient distribution at any pre-annealing temperatures of LN. The difference in refractive index of the waveguide and the cladding is about 0.012–0.015 at depth 4 μm. The concentration of protons that penetrate into the LN in a real experiment always is finite. Thus, refractive index of waveguides is inversely proportional to the defect density of the crystal structure. The lower density of defects, the lower refractive index of the waveguide
[1] M. Gruber, A. Leitner, D. Kiener, P. Supancic, R. Bermejo, Incipient plasticity and surface damage in LiTaO3 and LiNbO3 single crystals, Mater. Des. 153 (2018) 221–231. [2] L. Arizmendi, Photonic applications of lithium niobate crystals, Phys. Status Solidi 201 (2004) 253–283. [3] M. Bazzan, C. Sada, Optical waveguides in lithium niobate: recent developments and applications, Appl. Phys. Rev. 2 (2015) 040603–040624. [4] O. Yavuzcetin, et al., Fabrication and characterization of single mode annealed proton exchanged waveguides in -x-cut lithium niobate, Opt. Mater. 36 (2013) 372–375. [5] X. Yuan, Y. Zhang, J. Zhang, M. Zhang, Any point bias control technique for MZ
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[15] H. Han, L. Cai, H. Hu, Optical and structural properties of single-crystal lithium niobate thin film, Opt. Mater. 42 (2015) 47–51. [16] P.O. Weigel, S. Mookherjea, Reducing the thermal stress in a heterogeneous material stack for large-area hybrid optical silicon-lithium niobate waveguide microchips, Opt. Mater. 66 (2017) 605–610. [17] U.O. Salgaeva, S.S. Mushinsky, A.B. Volyntsev, I.S. Azanova, D.I. Shevtsov, Effect of pre-annealing process on the surface roughness of ridge waveguides formed with wet etching of Z-cut LiNbO3, Ferroelectrics 496 (2016) 143–148. [18] H. Boysen, F. Altorfer, A neutron powder investigation of the high-temperature structure and phase transition in LiNbO3, Acta Crystallogr. Sect. B Struct. Sci. 50 (1994) 405–414. [19] H. Lehnert, H. Boysen, F. Frey, A. Hewat, P. Radaelli, A neutron powder investigation of the high-temperature structure and phase transition in stoichiometric LiNbO3, Z. Kristallogr. 212 (1997) 712–719. [20] S. Bredikhin, S. Scharner, M. Klingler, V. Kveder, B. Red’kin, W. Weppner, Nonstoichiometry and electrocoloration due to injection of Li+ and O2- ions into lithium niobate crystals, J. Appl. Phys. 88 (2000) 5687–5694. [21] H. Onodera, I. Awai, J. Ikenoue, Refractive-index measurement of bulk materials: prism coupling method, Appl. Opt. 22 (1983) 1194–1197. [22] J.M. White, P.F. Heidrich, Optical waveguide refractive index profiles determined from measurement of mode indices: a simple analysis, Appl. Opt. 15 (1976) 151–155.
modulator, Optik 178 (2019) 918–922. [6] L.R. Hofer, D.B. Schaeffer, C.G. Constantin, C. Niemann, Bias voltage control in pulsed applications for Mach-Zehnder electrooptic intensity modulators, IEEE Trans. Contr. Syst. Technol. 25 (2017) 1890–1895. [7] A. Chen, E. Murphy, Broadband Optical Modulators: Science, Technology, and Applications, CRC press, 2011. [8] J.P. Salvestrini, et al., Analysis and control of the DC drift in LiNbO3-based Mach–Zehnder modulators, J. Lightwave Technol. 29 (2011) 1522–1534. [9] H.G. Muller, et al., Reduction of lattice defects in proton-exchanged lithium niobate waveguides, J. Appl. Phys. 110 (2011) 1–7. [10] J. Piecha, A. Molak, U. Breuer, M. Balski, K. Szot, Features of surface layer of LiNbO3 as-received single crystals: studied in situ on treatment samples modified by elevated temperature, Solid State Ionics 290 (2016) 31–39. [11] P. Galinetto, et al., Micro-Raman analysis on LiNbO3 substrates and surfaces: compositional homogeneity and effects of etching and polishing processes on structural properties, Optic Laser. Eng. 45 (2007) 380–384. [12] F. Caccavale, et al., MicroRaman investigation of optical quality lithium niobate wafers, Proc. SPIE 5451 (2004) 67–73. [13] A.V. Sosunov, A.B. Volyntsev, K.B. Tsiberkin, V.A. Yuriev, R.S. Ponomarev, Features of structure and mechanical properties LiNbO3, Ferroelectrics 506 (2017) 24–31. [14] A. Nakamura, E. Tochigi, J. Nakamura, I. Kishida, Y. Yokogawa, Dislocation structure at a {ī2ī0}/ < 10ī0 > low-angle tilt grain boundary in LiNbO3, J. Mater. Sci. 47 (2012) 5086–5096.
180
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Effect of pre-annealing of lithium niobate on the structure and optical characteristics of proton-exchanged waveguides
T
Aleksei Sosunov∗, Roman Ponomarev, Oksana Semenova, Igor Petukhov, Anatoly Volyntsev Perm State University, 614990, 15 Bukirev St, Perm, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium niobate Proton exchange Structure Pre-annealing Optical modulators
The influence of pre-annealing of lithium niobate in the range of 400–800 °C was studied at each step of formation of proton-exchange waveguides. Pre-annealing affects the structure and optical properties of lithium niobate. There is a relationship between the initial structure of lithium niobate and the optical characteristics of proton-exchange waveguides. The linear dependence between the deformations of the annealing proton exchange waveguides and the refractive index is established. Pre-annealing leads to an improvement in the structure of the subsurface layer of lithium niobate, as well as a slight decrease in the refractive index due to an increase in the homogeneity and a lower concentration of protons inside the crystal. The crystal structure becomes most uniform at a temperature of 500 °C. The results can be useful for different optical crystals used for applications.
1. Introduction Lithium niobate (LN, LiNbO3) is a key material in the photonic industry because of its advantageous combination of functional properties (high Curie temperature – 1145 °C, high electro-optical coefficients – r13 = 10 p.m./V and r33 = 32 p.m./V, wide transparency window – 0.35–5.5 μm, etc.) and its commercial availability. Proton exchange often used to form optical waveguides because it has several merits (low optical loss – 0.2 dB/cm, this is a low-temperature process (about 300 °C) compared to the metal diffusion; low photorefraction; maintains of the light polarization. However, there are number of demerits, for example high sensitivity of waveguide parameters to surface quality [1], significant anisotropy of supported radiation modes and multistep process. These characteristics provide the opportunity to use LN as a substrate for the fabrication of integrated optical devices, such as electrooptical modulators, switches, diffraction gratings, nonlinear optical frequency converters and electric field sensors [2–4]. It is still an open question about the phenomena of drift in the optical modulator based on proton exchanged waveguides in LN [5–7]. J. P. Salvestrini analyzed various sources of drift in commercially available optical modulators. The different origins have been compared in terms of phase shift and the different corresponding orders of magnitude have been given, pointing out the predominant role of the dc drift. Large role played electrical inhomogeneities at the surface of the LN
∗
substrate by highlighting the link between the time dependence of the dc drift in an LN optical modulator and the electrical conductivity measured at the surface and in the volume of the LN substrate [8]. The drift of optical modulators has related to the instability of the refractive index of proton-exchanged waveguides [9]. The refractive index depends on the proton exchange process, and it depends on the structure of the crystal. It is necessary to investigate the relationship between the structure of the surface layer and the results of proton exchange process. Authors [10–14] showed significant differences in the structure and properties of the surface and LN substrate. Modification of the structure and properties of the LN surface occurs during the processing (polishing) of the wafers. Lithium niobate wafers “optical quality” still have a disorder in the layer under the surface. In practice, annealing/wet annealing is widely used to treatment the structure and various characteristics of LN, including for LN thin films [15] and hybrid optical waveguides [16]. In particular, the use of LN pre-annealing with a Crmask allows for improving the surface roughness of the ridge waveguides, because the mask inherits all defects in the surface during its subsequent etching [17]. In this paper we make pre-annealing lithium niobate to eliminate surface stresses and to study the effect on the structure and optical characteristics of proton-exchanged waveguides.
Corresponding author. E-mail address: [email protected] (A. Sosunov).
https://doi.org/10.1016/j.optmat.2018.11.018 Received 9 September 2018; Received in revised form 12 November 2018; Accepted 13 November 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Optical Materials 88 (2019) 176–180
A. Sosunov et al.
2. Experimental 2.1. Sample preparation The samples used for the investigations were congruent X-cut LiNbO3 1-mm-thick 3-inch wafer (Crystal Technology, Inc.). Wafer was cut into a 15 × 10 mm2 pieces. Treatment of LiNbO3 is performed by pre-annealing in a wide temperature range of 400–800 °C in increments of 100°С. The samples were heated with speed 400 °C/h to the required temperature, then for 3 h they were annealed, and after that they slowly cooled with the furnace. In addition, there was a control sample without pre-annealing. Long-term exposure is compensated by a high heating rate to avoid additional structural defects, dislocations, cracking, etc. Pre-annealing temperature was chosen within a wide range, despite the cation sublattice can be disturbed higher than 600 °C [18,19] and the reduction process is accompanied by the formation of oxygen vacancies and Li loss [20]. We consider that the effect of temperature on the refractive index of lithium niobate well describes Sellmeier equation. There are large numbers of different modifications of Sellmeier equation. However, Sellmeier equation can be modified to describe the dependence of the refractive index on a combination of factors — wavelength, temperature, and composition crystal. Zelmeyer equation in the original version: m
n2 (λ ) = 1 +
∑ i=1
(λ2
Fig. 1. Fracture of sample.
Properties of single-crystal materials are largely determined by their internal structure. The most important information about the structural defects and strain can be obtained by using special X-ray diffraction techniques to investigate the fine structure of the crystals. The research of X-ray diffraction was carried out by the precise double-crystal spectrometer method. X-rays reflected from two series of flat single crystals of which the first was monochromator, had a perfect structure, and the second was sample. Monochromator was dislocation-free silicon crystal, set in the position corresponding to reflection Kβ-line cobalt radiation (λ = 1.62075 Å) from the crystallographic plane (111). The use of a single β-line eliminated the possibility of X-ray reflections overlapping, which is observed for α-line (doublet). During shooting, the crystal was rotated around the goniometer axis about the reflecting position with the angle θ for the registration θ/2θ-curves. Diffraction reflections were obtained for the first and second order with indices (110) and (220), respectively. The resolution of this method is 0.0001 deg. All measurements were carried out at room temperature, voltage – 30 kV, anode current– 10 mA, using a slit width of 0.05 mm.
λ2αi − λi2)
where λ-wavelength of the incident radiation in nm, and αi is a constant depending on the concentration of oscillators of the i-type in a unit volume, called the force oscillator resonant wavelength λi. Samples were cleaned in isopropyl alcohol and deionized water before and after proton exchange. The entire surface of the samples was protonated by submerging the wafers into benzoic acid at 170 °C during 2 h in closed zirconium reactor. The reactor was placed in a furnace on rotating platform to mix benzoic acid and cooled at rate of 10 °C/min after completion of the proton exchange (PE) process. Annealing was carried out at 350 °C during 5.5 h, and then slowly cooling in the furnace.
3. Results 3.1. Scanning electron microscopy
2.2. Methods
Fracture allows to fixing the position of the structure in its natural form without distortion. Structure of the cross-section of a crystal LN after fracture and different pre-annealing temperature is shown in Fig. 2. There is investigations show the presence of a stressed subsurface layer with a depth of about 6 μm for the pristine sample. The pre-annealing temperature is increased, while the depth of the layer decreases to 1 μm or disappears completely at 500 °C and above. It is because of high crystal defects concentration. Pre-annealing is apparently a good tool for treating LN. SEM results indicate that the temperatures 500–800 °C is optimal. The defective surface sublayer becomes blurred and decreases in thickness with increasing temperature (Table 1).
Electron microscopy was carried out using a Hitachi S3400 N SEM operating at 20 kV. Imaging and analysis tools included a conventional diffraction detector. We researched cross-section of LiNbO3 samples after fracture. Scratched on the surface of LN with a diamond pyramid, and then sample was broken along her. Analysis of the subsurface layer SEM was performed on the opposite side of the scratch (−X-scratch, and +X-analysis). Fig. 1 shows a schematic representation of the fracture. Electrical conductivity of the surface of a dielectric is necessary for conducting SEM analysis. SC7620 magnetron sputter with AuePd target was used to deposit thin conducting layer on the sample. The thickness of the conducting layer was set to a minimum, firstly, in order that the investigated structure was not hidden under the metallic layer, and, secondly, ensured the flow of charges from the surface of the samples. In this study, thickness of layer was about 2 nm. Increment of the refractive index of planar waveguide was determined using mode spectroscopy with an accuracy of ± 0.0002. Accuracy depends on the number of excited modes and form of allocation of the refractive index along the depth of the layer. Firstly, sets of effective (mode) refractive indices (Nm) at λ = 632 nm with prism coupling method [21] were determined, and secondly, using the obtained values of Nm, profile of the refractive index along depth of the waveguide layer was reconstructed by means of the IWKB method [22].
3.2. X-ray diffraction The sample was placed so that its surface was oriented parallel to the surface of the monochromator. In this position the half-width of the diffraction curve is independent of the natural spectral line width (no disperse), as determined only by the geometrical and physical factors. Also such an arrangement is most convenient for investigating the degree of imperfection of single crystals. The geometric factor can be neglected because of its smallness in comparison to the physical factor in the broadening of the spectral lines for the Gaussian distribution. The physical factor determines deformation of the crystal lattice of the 177
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Fig. 3. Deformations of LiNbO3 after pre-annealing and APE (a); displacement of diffraction lines after PE (b–c).
Fig. 2. Cross-section of LN after fracture and pre-annealing (a – pristine, b–400°С, с– 500–800°С).
Deformations (ε) were calculated by broadening the spectral lines by equation:
Table 1 Depth of subsurface layer from pre-annealing temperature.
ε2 =
Temperature, °C
25 (pristine)
400
500
600
700
800
Depth, um
6
5
0–1
0–1
0–1
0–1
cβ12 β2 − β22 β1 16tg 2θ1 (cβ2 − tβ1)
,
whereβ – half-width of experimental line, t = tg2θ2/tg2θ1, c = cosθ1/ cosθ2, indices 1 and 2 are the first (110) and second (220) order of reflection, respectively. The displacement of the diffraction lines (phases) was calculated from the Wolf-Bragg equation:
material. The structure of LiNbO3 was studied at each step in the development of proton-exchange waveguides (pre-annealing, PE and APE). The deformations were calculated from the broadening (after pre-annealing and APE) and the displacement (after PE) of the diffraction lines.
2dsinθ = nλ, where d – interplanar spacing, θ – reflection angle, n – reflection order and λ – wavelength. 178
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(contrast). Our results show this dependence at a pre-annealing temperature of 500 °C. At this temperature, the minimum refractive index of proton-exchange waveguides (Δne) is inversely proportional to its depth (δ) or homogeneity of the structure. As the pre-annealing temperature rises, the index of refraction and deformation of the crystal lattice increases. This may be due to the violation of the oxygen scaffold and loss of Li atoms. It is necessary to note the relationship between the structure of the LN crystal and its optical characteristics. A linear relationship between the deformations of the crystal lattice of LN and the refractive index of proton-exchange waveguides was established during pre-annealing temperatures. Electro-optical coefficient r33 is proportional to Δne. If Δne higher, then stronger the electro-optical response, that is good for modulators. Our results showed small decrease of Δne from the pre-annealing temperature. However, the decrease is Δne insignificant for the technology of optical modulators manufacturing, and more homogeneous structure lead to significantly affect the stability of different characteristics (refractive index, Vbias and electro-optical coefficients). 4. Conclusion In this paper we demonstrate the trend of the effect of pre-annealing LN at each step of the formation of APE waveguides. Pristine LN has a 6 μm deep disorder layer, which is blurring with pre-annealing. The optimum pre-annealing temperature is about 500 °C. Optical characteristics are reduced insignificantly for technological process. However, a most homogeneous structure is a promising medium for increasing the stability of technology, refractive index and drift phenomena. The results demonstrate a linear relationship between structural changes and the optical properties of APE waveguides, depending on the pre-annealing of LN. We have established a linear relationship between the deformations ε of the subsurface layer of LN and the results of the proton exchange process Δne, δ. These results are relevant for all optical crystals used for buried waveguides formation. The change in temperature or electric field can lead to instability of the refractive index due to the motion of charged defects in area of proton-exchanged waveguides. The ability to influence features of the structure subsurface layer of LN has the prospect of improving the drift characteristics of optical modulators based on proton-exchange waveguides. Our results give more control of the structural characteristics will allow us to minimize the instability of the refractive index of proton-exchanged waveguides. The investigation of the drift characteristics of optical modulators on the pre-annealing temperature of LN will be considered in the next paper.
Fig. 4. Waveguide profiles (a); refractive index and depth of the waveguides as functions the pre-annealing temperature (b).
The structural features and their changes are shown in Fig. 3 after each step of manufacture. The deformation (Fig. 3a) is slightly larger after APE because the crystal lattice is not completely restored, but the trend remains the same. The trend of the results obtained after each stage of formation of proton-exchange waveguides is preserved. The set of β-phases (Fig. 3b) formed after the proton exchange process also reflects the original structure. At a temperature of 500 °C, there is a characteristic minimum of deformations of the crystal lattice and also a minimal displacement of the diffraction lines (Fig. 3a,c). From the viewpoint of the morphology of 500 °C is the optimum temperature treatment of LiNbO3 to obtain the most homogeneous subsurface structure and manufacture of proton-exchange waveguides. High temperature leads to more disordered structure, the appearance of new defects and oxygen vacancies. Diffraction line broadening occurs at small angles θ and corresponds to large deformations in the surface layers. The results obtained with SEM in good agreement with the XRD results. Thus, pre-annealing improve the structure of the LN crystal and proton-exchange waveguides.
Funding This work was supported by RFBR [grant number 17-43-590309]. Declaration of interest None.
3.3. Mode spectroscopy References Optical characteristics (increment of refractive index, depth, profile) of proton-exchanged waveguides were characterized by the use of mode spectroscopy and are presented in Fig. 4. The waveguide profiles have a gradient distribution at any pre-annealing temperatures of LN. The difference in refractive index of the waveguide and the cladding is about 0.012–0.015 at depth 4 μm. The concentration of protons that penetrate into the LN in a real experiment always is finite. Thus, refractive index of waveguides is inversely proportional to the defect density of the crystal structure. The lower density of defects, the lower refractive index of the waveguide
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