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Journal of Crystal Growth 292 (2006) 355–357 www.elsevier.com/locate/jcrysgro
Growth and characterization of Mn-doped stoichiometric lithium niobate single crystals G. Ravia,, K. Kitamurab, R. Mohankumarb, S. Takekawab, M. Nakamurab, Y. Liub a
Crystal Research Centre, Alagappa University, Karaikudi-3, India Advanced Materials Laboratory, National Institute for Materials Science (AML/NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan
b
Available online 14 June 2006
Abstract Optical quality near stoichiometric lithium niobate (SLN) and Mn-doped SLN crystals have been grown by top-seeded solution growth from 55 mol% Li-rich solution. A uniform segregation of Mn throughout the crystal was obtained. Doping of Mn decreases the Curie temperature while the absorption edge shifts towards the longer wavelength region. The effect of Mn doping on the switching properties has been analyzed. r 2006 Elsevier B.V. All rights reserved. PACS: 77.80.Dj; 77.84.Dy; 78.20.Jq; 81.10.Dn; 81.70.Jb Keywords: A1. Absorption; A2. Top-seeded solution growth; B1. Lithium compounds; B2. Ferroelectric materials; B3. Nonlinear optical
1. Introduction Lithium niobate (LiNbO3) is one of the most interesting materials due to its wide variety of applications, e.g., optical waveguides, surface acoustic wave devices, optical modulators, etc [1]. It is a material of interest for its excellent photorefractive properties and its good mass productivity and is used for recording and erasing the images [2–6]. In two photon-recording process, it is necessary to add transition metal ions or rare earth dopants in LiNbO3 crystal for improving the performance of volume holographic recording [7]. In the present study, pure and Mn-doped near stoichiometric lithium niobate (SLN) crystals have been grown using an induction heating system. The compositional uniformity along axial and radial directions has been checked using ICP and the Curie temperature of the samples was measured using differential scanning calorimetry (DSC). A transition metal element Mn has been chosen as a dopant for increasing photorefractive effect. Recently nonvolatile two-color holographic recording was demonstrated with high sensitivity Corresponding author. Department of Physics, Alagappa University, Karaikudi-630 003, India. Tel.: +91 4565 225205; fax: +91 4565 225202. E-mail address:
[email protected] (G. Ravi).
0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.04.075
in near SLN crystal doped with 8 ppm of Mn [8]. The useful wavelength range for holographic recording has been found from UV–VIS spectra. The coercive field, internal bias field and saturation driving voltages have been determined for pure and doped near SLN crystals.
2. Experiment Near SLN and Mn-doped SLN single crystals have been grown in a conventional RF-heated Czochralski growth system from a platinum crucible at air atmosphere. The growth run was performed for the initial solution composition of Li=Nb ¼ 55 45. For the growth of Mn-doped SLN crystals, two different doping concentrations (10 and 50 ppm) were chosen. The doping concentrations of Mn in the crystals were determined by the ICP spectroscopy method. To confirm stoichiometry of the grown crystals, the Curie temperature was measured using DSC. The samples for optical characterization are taken from the single domain area of the crystals. The absorption edge wavelength of the crystals was measured using a Hitachi U3200 spectrophotometer. The coercive field measurements were performed using a ferroelectric characterization evaluation system (Toyo Corporation Model FCE-1). The
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G. Ravi et al. / Journal of Crystal Growth 292 (2006) 355–357
hysteresis loops were traced for a single-cycle applied field at an applied frequency of 0.01 Hz. The coercive field and internal field values were determined for Mn-doped crystals and compared with the values of nondoped near SLN crystal. Heat flow (mW)
Near SLN crystals of pure and Mn (50 ppm)-doped are shown in Fig. 1a and b. The dimensions of the crystals are 40 mm diameter and 60 mm length. All the grown crystals are transparent, mechanical twin free and have same morphology with three growth ridges. It is well known that the Curie temperature reflects the Nb concentration in LiNbO3 crystals [9]. The Curie temperature of pure, 10 and 50 ppm-doped crystals are 1191, 1188 and 1180 1C (Fig. 2), which correspond to a lithium concentrations 49.6, 49.5 and 49.3% respectively. As Mn ions substitute Nb5+, the concentration of anti-site defect Nb4+ Li increases for the doped crystals, which may cause a reduction in the Curie temperature [10,11]. It can be observed from the transmission spectra (Fig. 3) that the lower cut-off wavelength value is slightly shifted to longer wavelength side for the Mndoped crystals. The absorption edge wavelengths of pure and Mn-doped (10, 50 ppm) near SLN crystals are 305, 308 and 311 nm, respectively. The shift of the absorption edge towards the longer wavelength region may be due to the increased defect density of the doped crystals. When doping MnO in LN, Mn2+ gets incorporated into the
Pure SLN SLN:Mn10 SLN:Mn50
-40
1191.2 °C
1180.3 °C -50
1188.2 °C -60 1160
1180
1200
Temperature (°C) Fig. 2. DSC curves showing Curie temperature of pure and doped SLN samples.
80
Transmittance (%)
3. Results and discussion
-30
60
40
Pure SLN SLN:Mn(10 ppm) SLN:Mn(50 ppm)
20
0 300
450 Wave length (nm)
Fig. 3. Optical absorption edge for the pure and Mn-doped SLN crystals.
Fig. 1. The grown (a) pure and (b) Mn-doped (50 ppm) SLN crystals.
Nb5+ sites and would act as a lower valent species on a higher valent site [10,11]. Mn doping also induces an absorption band extending from 450 nm to the absorption edge. The crystals possess nearly 70% transmittance above 450 nm. The single domain nature was identified from the etched thick wafer of the crystal (Fig. 4). A wafer cut from the bottom of the crystal shows the same patterns. This indicates that the grown samples are completely single phases, except at the 2 mm periphery. This is one of the major advantages in near SLN compared to congruent LN (CLN) crystals because the poling procedure at high temperature in CLN sometimes introduces cracks or scattering centers into the crystal. The reflection optical photograph does not show any ferroelectric domain reversal at the facets. The hysteresis loops for pure and Mn-doped samples have a high rectangularity and has strong internal field. Table 1 shows the results obtained from hysteresis
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may be due to the increased defect density of the doped crystals. The lattice defects diffuse to the domain wall regions, thus pinning the domain near the walls may be the possible reason for the above effects in doped crystals [13–15]. It is interesting to note that the doped crystals grown from the solution containing Li-58 mol% did not show any appreciable change in coercive and internal bias fields compared to pure SLN crystal (4.0 kV/mm). This is due to less number of intrinsic anti-site defects in pure and doped crystals grown from Li-58 mol% solution. The detailed studies of coercive and internal bias fields for the crystals grown from different Li concentrations are under progress. 4. Conclusion
Fig. 4. A polished thick boule (12 mm) of single domain crystal.
Table 1 Switching characteristics of pure and doped samples
While stoichiometry remains a key problem for Czochralski grown LiNbO3 single crystals, bulk crystals with near stoichiometry could be grown from Li-rich solution by top-seeded solution growth technique. Highly homogeneous, single domain optical-quality Mn-doped near SLN crystals was grown without any inclusion, cracking and mechanical twin problems. A significant variation of Curie temperature and the absorption edge was found for the Mn-doped crystals. An increase in coercive field and internal bias field was obtained for the Mn-doped crystals. References
Crystal (thickness ¼ 0.3 mm)
Coercive field at 0.01 Hz Ec (kV/mm)
Internal bias field Eint (kV/mm)
Saturation drive voltage (kV)
Pure SLN (Li:58 mol%) Pure SLN (Li:55 mol%) SLN:Mn-10 (Li:55 mol%) SLN:Mn-50 (Li:55 mol%)
3.95 7.95 9.55 9.90
0.05 1.03 1.05 1.10
1.5 3.0 3.8 4.0
measurement. The coercive field for the doped crystals is increased (9.90 kV/mm) compared to nondoped near SLN crystal grown from Li-55 mol% (7.95 kV/mm). When these coercive field values are compared with the available data for LN crystals grown from congruent melt and grown from Li-58 mol% are 21 and 4 kV/mm, respectively [12]. It is also found that increasing the amplitude of applied voltage increases the coercive field and spontaneous poloarization values. Moreover, the driving voltage required for switching is found to increase for the doped crystals along with the increase of internal bias field. This increase of coercive field, driving voltage and internal field
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