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A lithium-rich vapor transport equilibration technique was used to increase the Li/Nb ratio in single crystal LiNbO3 thin film at low temperature (below 600 °C). The extraordinary refractive index ne of the Li-compensated thin film was measured and found to be 2.1983 at 632.8 nm using the prism coupling method, while the ne of the congruent LiNbO3 was 2.2024. The lattice parameter (cr) of Li-compensated LiNbO3 thin film was determined to be 13.8604 Å using high-resolution x-ray (HRXRD) ω - 2θ scan, which was between the values of congruent LiNbO3 (13.8650 Å) and stoichiometric LiNbO3 (13.8562 Å). The Raman spectra showed significant differences in the relative intensity and the FWHM of Raman lines between the Li-compensated thin film and congruent thin film.
© 2015 Optical Society of America
1. Introduction
Lithium niobate (LiNbO3, LN) is one of the most promising optical crystal materials known to humans and it has many applications because of its excellent electro-optic, piezoelectric, pyroelectric, photo-elastic and non-linear optical properties [1]. In recent years, high-refractive-index contrast, which is single-crystal LN thin films on a low refractive index SiO2 cladding layer or other substrates (lithium niobate on insulator, LNOI), have been fabricated using crystal ion slicing and wafer bonding technologies [2–4]. Various photonic devices have been reported in LNOI, including photonic crystals [5], micro-rings [6] and microdisk, resonators [7–10]. However, commercially available LN crystals usually have a congruent composition ([Li]/[Nb] = 48.4/51.6), as indicated by conventional Czochralski method [11], and LNOI, which are split from the bulk LN, are no exception. There are intrinsic defects in congruent LN (CLN) due to the lack of Li [12]. These intrinsic defects affect the optical properties of CLN. There are many reports concerning near stoichiometric LN (SLN) single-crystal [13, 14]. Thus, it is obvious that fabrication of good-quality, Li-compensated LNOI is essential and can be identified by the changes of the physical properties such as the refractive index, lattice parameter and dipole moment. The near stoichiometric LN thin film fabricated by different deposition techniques has been attracted much attention due to improvements in many of its physical properties, which have rendered it superior to congruent LN thin film [15]. As we know, few studies of optical and structural properties of Li-compensated single-crystal LN thin film have been reported in the literature so far. Therefore, research to increase the Li/Nb ratio to make near stoichiometric LNOI (NSLNOI) is necessary.
The lithium-rich vapor transport equilibration (LRVTE) technique has been reported to increase the Li/Nb ratio of LN single-crystal [16]. This technique consists of annealing LN in close proximity to a much larger mass of Li2O powder. Given sufficient time (100 h) and sufficiently high temperatures (1100 °C), the Li/Nb ratio in the bulk LN increased via a mechanism involving vapor transport and solid-state diffusion. However, for LNOI, the annealing temperature should be kept below 550 °C to prevent any detachment between the SiO2 layer and LN substrate caused by thermal mismatching of these two materials [17, 18]. Because the thickness of the LN thin film is only around 0.5 µm, the Li diffusion can be much shallower than that of bulk LN material. The LRVTE at temperature below 550 °C in LNOI is possible.
In this work, the NSLNOI was fabricated by LRVTE in an oxygen atmosphere at 520 °C on LNOI. The extraordinary refractive index ne of NSLNOI was found to be 2.1983 by the prism coupling method at the wavelength 632.8 nm. The lattice parameter (cr) was found to be 13.8604 Å by HRXRD ω - 2θ scan. Raman spectroscopy, which is highly sensitive to small modifications in the structure of LN, showed significant differences in the relative intensity and the FWHM of Raman lines between NSLNOI and LNOI.
2. Experimental
Figure 1 shows schematic cross-section of a LNOI-wafer. LNOI was fabricated as described in a previous work [17]. The fabrication was performed at the research center of Nanoln. The annealing temperature of LNOI could be around 500 °C.
The LRVTE treatment was carried out in Al2O3 crucible partially filled with the Li2O powder (purity 99.99%). The LNOI to be treated was placed on the other side of the Al2O3 crucible to prevent the direct contact with the Li2O powder. The crucible was sealed by an Al2O3 lid. The parameters of LRVTE for LNOI were given in Table 1. As shown in Table 1, the LRVTE parameter was set to 520 °C/30 h. In the LRVTE process, the Li2O was diffused into the LN thin film. The lithium diffusivity in the crystallographic z direction of LN at 500 °C was 6.9 × 10−16 m2s−1 [19, 20]. After 30 h annealing, the average diffusion distance [21], calculated by the standard random walk equation, was about 20 μm. As a reference, a similar annealing procedure (520 °C/30 h) was performed to another LNOI except the absent of the Li2O powder. For the two samples that were compared here, one was treated at 520 °C/30 h with Li2O, and the other at 520 °C/30 h without Li2O, had dimensions of 8 mm × 10 mm each, and they came from a single piece of LN thin film (16 mm × 10 mm).
Table 1. The parameters of LRVTE for LNOI in an oxygen atmosphere
The refractive indices of the NSLNOI and LNOI were measured using a prism coupling method (Model 2010 Prism Coupler). A He-Ne (632.8 nm) laser beam was coupled in the thin film using a rutile prism. During the measurement, the intensity of the incident light reflected from the prism base-sample interface dropped abruptly at a particular angle called the critical angle. These critical angles (αm) were closely related to the effective refractive indices of the planar waveguide modes (2πnpsinαm/λ0 = βm, where λ0 was the wavelength of the laser in air and βm was the propagation constant of a specific mode “m”). If the refractive index of the prism (np) was known and the effective refractive indices were precisely measured, the refractive index and the thickness of the thin film could be calculated [22].
The lattice parameter (cr) of NSLNOI was determined using high-resolution x-ray (HRXRD) ω - 2θ scan. The contribution to the spectrum was expected from the LN thin film and the LN substrate that possessed a periodic lattice structure, while the SiO2 amorphous layer, which was embedded between the LN thin film and LN substrate, had no contribution [17, 23]. Measurements of HRXRD were performed using an AXS HRXRD D5005 system from Bruker Inc. with Cu-Kα1 radiation source (λ = 1.54056 Å). A detailed measurement procedure of HRXRD was performed in the following way. First, a rocking curve was established to assess the crystal plane of the LN substrate, and then the ω - 2θ scan was conducted on (006) peak of LN substrate around 38.94° (2θ) with a step of 0.0005° to find the position of the (006) peak of LN thin film. After using another rocking curve to assess the crystal plane of the LN thin film, ω - 2θ scan was conducted around the position of the (006) peak of LN thin film with a step of 0.0005°. The (0012) peaks of LN thin films were determined throuth a similar process.
The micro-Raman spectra were collected at room temperature using a Jobin - Yvon/Horiba LabRam HR800 spectrometer in a backscattering geometry. The 488 nm beam from the light source was focused by a 100 × microscope objective (NA = 0.9) in a confocal geometry so as to achieve a depth resolution ≤ 0.2 μm. Raman probing was performed using 1800 lines/mm grating. Polarized Raman spectra of NSLNOI and LNOI were recorded in Z(XY)Z backscattering geometries.
3. Results and discussion
3.1 Prism coupling
Figure 2 shows measured relative intensity of the light (TM polarized) reflected from a prism formed by the NSLNOI (520 °C/30 h with Li2O) and LNOI (520 °C/30 h without Li2O) planar waveguides at the wavelength of 632.8 nm. Sharp dips were observed in both NSLNOI and LNOI, corresponding to the guided modes. The effective refractive index ne of LN was very sensitive to Li concentration. The index ne of LNOI (2.2020) was almost equal to that of the bulk LN (2.2024), but NSLNOI (2.1983) was lower than the bulk LN. Because the thicknesses of LNOI and NSLNOI were very similar, the effective index difference in Fig. 2 was from the refractive index change of LN thin film. The TM0 modes showed a δneff = 0.01514 shift between NSLNOI and LNOI. The ordinary and extraordinary refractive indices of bulk LN, LNOI, NSLNOI, and SLN are summarized in Table 2. The decrease in ne indicated an increase of [Li]/[Nb] in the LN crystal [24]. The Li-diffusion in the LRVTE process caused a change in the composition of LN thin film, which in turn increases birefringence (i.e. no remaining a constant while ne decreasing). The error in the calculated refractive indices was less than 0.0003 at room temperature. However, since there was some internal stress in LN thin film caused by the thermal mismatch between SiO2 layer and the LN thin film, and such internal stress could cause refractive index change via elasto-optical effect, the precise determination of Li/Nb ratio was difficult.
Fig. 2 Relative intensity of the light (TM polarized) reflected from a prism formed by the NSLNOI and the LNOI planar waveguides at the wavelength of 632.8 nm.
Table 2. Refractive indices and thicknesses (λ = 632.8 nm)
3.2 HRXRD
At room temperature, the CLN belonged to the ferroelectric phase and had a group of R3c (3m point group) with lattice parameters of ar = 5.1505 Å, cr = 13.8650 Å, and the SLN lattice parameters (ar = 5.1474 Å, cr = 13.8562 Å) were smaller [25]. For a set of Miller-Bravais indices (hkl), the spacing dhkl of the corresponding planes is given by the following equation [26]:
dhkl was the lattice spacing of (hkl) plane. h, k, and l were miller indices. a and c stood for the hexagonal lattice vector lengths.Multiple-order reflections of HRXRD were found to allow accurate measurement of the lattice parameter. In single-crystal diffraction experiments, the absolute diffraction angle was generally not accurate due to the uncertainty of zero point, and this zero error could be eliminated by multiple-order reflections [27]. There were (006), (0012) symmetric reflections from the crystal planes of the LN thin film. The following equation was deduced using Bragg’s law:
Here, λ was the wavelength of X-ray, d001 was the lattice spacing of (001) plane, θ006 was the measured Bragg angle from (006) plane, θ0012 was the measured Bragg angle from (0012) plane, and δθ was the zero error (Bragg angle correction because of the zero setting of instrumental alignment). The zero error δθ was almost consistent for l = 6 and 12 after calibration, and could be easily obtained from Eq. (2) [28]. Therefore, using d006 and d0012, the perpendicular parameter (cr) could be deduced using Eq. (1). The lattice parameter of substrate was not included in this deduction procedure. In this way, the influence of substrate during the determination of lattice parameter of the thin film was excluded.Figure 3 shows the results of measurement of the ω - 2θ scan from the (006) and (0012) crystal planes of NSLNOI (520 °C/30 h with Li2O), and LNOI (520 °C/30 h without Li2O), respectively. The peak positions of (006) and (0012) from the LN thin films were used to calculate the angle correction δθ in Eq. (2). The precision of lattice parameter was affected by the angle measurement resolution of the equipment and the Bragg angle position error caused by fitting errors [29]. A step resolution of 0.0005° resulted in a lattice parameter resolution of 0.0004 Å. As shown in Table 3, the lattice parameter (cr) of LNOI was 13.8661 Å, which was larger than that of the CLN (13.8650 Å). This was attributed to the internal stress in the LN thin film. The lattice parameter (cr) of NSLNOI was 13.8604 Å, which was between the values of CLN (13.8650 Å) and SLN (13.8562 Å). During LRVTE process, the Li diffused into the LN thin film, and improved the composition of Li, which led to the lattice parameter close to that of SLN.
Fig. 3 Measured diffraction peaks by ω - 2θ scan from the (006) and (0012) crystal planes of NSLNOI, and LNOI, respectively. The 2θ positions of bulk material of the (006) and (0012) crystal planes are marked by the vertical lines .
Table 3. Lithium niobate lattice parameters at room temperature
3.3 Raman spectroscopy
Raman scattering measurements were performed on NSLNOI (520 °C/30 h with Li2O) and LNOI (520 °C/30 h without Li2O). Figure 4 shows the polarized Raman spectra recorded in Z(XY)Z scattering configuration. The light propagation in Z direction could prevent photo-induced changes in the refractive index in LN crystals, which could affect the Raman scattering [30]. The five lines at 152, 237, 430, 578, and 739 cm−1 were assigned to E mode and the three lines at 274, 332, and 873 cm−1 were assigned to A mode [31]. According to the previous results [32], the O-Nb-O bending modes below 432 cm−1 were strongly coupled with the Li-O stretching and O-Li-O bending modes; the frequencies in the range of above 550 cm−1 might be assigned to the Nb-O stretching modes involving essentially oxygen atom shift, so they were not affected by LRVTE. Only frequencies in the 100 - 550 cm−1 range were influenced by LRVTE. The intensity of the lines in NSLNOI was almost half of those in LNOI. As Li/Nb ratio increased, the intensity of the lines decreased, which was consistent with the results reported by Bhatt et al. [33]. Intensity of the Raman line was related to polarizability of the unit cell [34]. The vibration spectrum line width of LN was a sensitive indicator for the Li/Nb ratio [35]. The FWHM of the lines in the 100 - 550 cm−1 range in NSLNOI were narrower than that of LNOI. For example, the linewidths at the 332 cm−1 peak were shown in the inset of Fig. 4. The linewidths were 18.95 cm−1 (LNOI) and 16.5 cm−1 (NSLNOI), respectively. The linewidth decreased with increasing Li content. This was due to the reduction of the disordering of the cation sublattice in NSLNOI. The changes in linewidth lines width and intensity showed that SLN had fewer defects than CLN [36].
Fig. 4 Comparison of the polarized Raman spectra recorded for NSLNOI and LNOI in Z(XY)Z scattering configuration. (Inset) The enlarged peaks at 332 cm−1. The FWHM in NSLNOI and LNOI is 18.95 cm−1 and 16.5 cm−1, respectively.
4. Conclusions
Because SLN has fewer intrinsic defects than CLN because of the compensation of Li, some physical properties in SLN such as electro-optic coefficient and nonlinear optical coefficient will increase. In this paper, LRVTE was used to compensate for the Li content in LNOI at low temperature (520 °C). After LRVTE, the ne of LN thin film was 2.1983, which deviated from the LNOI (2.2020) and moved towards that of the SLN (2.1898). The lattice parameter (cr) of the LRVTE LNOI was 13.8604 Å, which deviated from the LNOI (13.8661 Å) and moved towards that of the SLN (13.8562 Å). The Raman spectra showed that in the 100 - 550 cm−1 range, the linewidth and intensity decreased, which confirmed that the Li composition in LN thin film was increased by LRVTE process.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) (Grants No. 11275116, No. 11375105 and No. 51272135).
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