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A systematic study of the chemical etching process on periodically poled lithium niobate structures
Materials Science and Engineering B 118 (2005) 150–154
A systematic study of the chemical etching process on periodically poled lithium niobate structures N. Argiolas a , M. Bazzan a , A. Bernardi a , E. Cattaruzza a , P. Mazzoldi a , P. Schiavuta a , C. Sada a, ∗ , U. Hangen b a
INFM-MATIS and Physics Department, University of Padova, Via Marzolo 8, 35131 Padova, Italy b Surface, Rheinstr. 7, D-41836 H¨ uckelhoven, Germany
Abstract A systematic analysis on the dynamics of the chemical etching of periodically poled lithium niobate (PPLN) structures grown by off-center Czochralski technique was carried out on crystals prepared under different experimental growth conditions. The etched depth reaches values close to 600 nm and it does not further increase even after long etching times. However, the lateral etching cannot be neglected when the etching times are higher than 5 min. The estimation of the domain widths distribution can be affected by artifacts if the etching conditions are not properly chosen. The best structures are obtained for erbium oxide doping level of 0.3 mol% into the starting melt and the period depends on the pulling and rotational rates instead of on the growing rate. This results support the role of the thermoelectric field in the domain formation at the Curie isotherm. © 2005 Elsevier B.V. All rights reserved. Keywords: Periodical poling; Lithium niobate; Crystal growth; Atomic force microscopy; Etching
1. Introduction Periodically poled lithium niobate (PPLN) is a promising material for non-linear optical quasi-phase-matched devices [1,2] and recently has attracted great interest also for efficient volume-phase grating recording [3] and soliton propagation. The most common technique to prepare periodic structures consists of applying an electric field either during the material growth (about 0.4 V/cm) or after that, even at room temperature [2,4]. In alternative, the so-called modified Czochralski technique [5–9] can be exploited to grow PPLN crystals with the desired period. Whatever the fabrication process is, the device performance is strongly influenced by the quality of the periodic structures. The test of the domain distribution along the PPLN material is therefore mandatory before starting the device packaging. According to [10], in fact, a spatial resolution of at least 1% should be used to map the periodic structure. In order to test the domains ∗
Corresponding author. Tel.: +39 049 8277037; fax: +39 049 8277003. E-mail address: [email protected] (C. Sada).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.12.088
distribution, the chemical etching followed by the direct observation of domains using an optical microscope has been widely employed to characterize PPLN structures [11]. In fact, as the etching rate is faster for a negative Z-face than for a positive one [12], domains with opposite polarization states can easily be identified through topographic measurements. Techniques such as scanning force microscopy (SFM) are therefore extremely useful to test the PPLN structure with typical lateral resolution of tens of nanometers. Since the SFM analysis is limited to a small scanned region (less than 100 m × 100 m), it is more convenient to combine it with a scanning made with a standard profilometer after the chemical etching process [13]. In this case, the range is typically of centimeters with a resolution of some hundreds of nanometers (about 1% of the typical width of the domains used in first-order quasi-phase-matching devices). Both SFM images and profilometer line-scan give reliable measurements of the domain dimensions only if the chemical etching does not introduce artifacts. In literature, it is usually assumed that the lateral erosion of the domains walls can be neglected, the domains width being independent
N. Argiolas et al. / Materials Science and Engineering B 118 (2005) 150–154
of the etching time. In this work, we present the results of a systematic study of the chemical etching on PPLN structures grown by the off-center Czochralski technique. We will show that the lateral erosion cannot be neglected and it may lead to overestimate the negative domains width. The best conditions to carry out the chemical etching are finally presented.
2. Experimental Periodically poled Er:LiNbO3 single crystals were grown along the X-axis with a pulling speed (vp ) ranging from 2 to 5 mm/h and with a rotational rate (ω) between 5 and 15 rpm so that the growth rate is maintained almost constant. The growth conditions were chosen to have the designed period L in the range 4–10 m of the PPLN structure. The starting melt was made of 99.999% stoichiometric lithium niobate powders with different content of Er2 O3 salts in the range between 0 and 0.35 mol%. The growth temperature was controlled by a computer algorithm keeping constant the mass variation of the melt in order to obtain a crystal with fixed radius. Each crystal was cut in slices with the optical axis perpendicular to their major surface (Z-cut) and polished with standard techniques. Finally, these slices were etched in acid solution (HF:HNO3 = 1:2 (v/v)) at 80 ◦ C for different times (0–50 min.). The main details of the domain structures were investigated mapping the etched surfaces using the scanning force microscopy technique. An SFM Autoprobe CP (Park Scientific Instruments) was used in contact mode to scan an area of 32 m × 32 m. Finally, we measured the reduced elastic modulus (Er ) and hardness (H) of the domains by means of a Hysitron Triboscope nanoindenter used in the Berkovich geometry. The indenter tip is pressed in a controlled way on the surface and load versus depth curve is measured; from the analysis of that curve is possible to determine Er and H [14,15]. The nanoindenter head can also provide in situ atomic force microscopy (AFM) images,
151
enabling us to view the surface topography before and after the indentation. It is therefore possible to select accurately the domain to be measured. Moreover, we investigated domains distribution of the PPLN structures by mapping the etched surfaces along the crystal using a Tencor profilometer (with a lateral resolution equal to few nanometer and a depth resolution of about 1 nm). The scan was performed along the direction normal to the border domain with a scanning speed equal to 1 m/s and a sampling rate 500 Hz. The measured scan is however the result of a “convolution” of the Profilometer tip shape with the real pattern profile of the etched PPLN crystal. To obtain the real PPLN pattern, a deconvolution procedure was applied, as reported in Ref. [16]. In particular, the position of the border domain is given by analyzing the numerical derivative of the deconvoluted scanned profile. At each border domain corresponds an edge: the procedure allows identifying the edge position and returns the domain wall location and domain width or period of the structure as a function of the position. Further details on the procedure used in the present work are reported in Ref. [16].
3. Results and discussions Several crystals were grown under different experimental conditions by varying the Er2 O3 content into the starting melt between 0 and 0.35 mol% and by changing the offcenter in the range of 0–8 mm. The periodicity of the PPLN structure was checked by means of the chemical etching procedure. In order to determine the best etching conditions, a systematic study was performed varying the etching temperature and time, respectively. The topographic investigation of the PPLN structure was performed on the sample grown from the melt containing an Er2 O3 concentration equal to 0.35 mol%. This sample was etched for different times at temperatures in the range 80–110 ◦ C in a mixture of HNO3 :HF (2:1 (v/v)). In particular, the etching times were varied in the
Fig. 1. AFM image of as prepared PPLN sample, after the polishing procedure and without etching. PPLN growth conditions: off-center 4 mm, Er2 O3 in the melt 0.6 mol%. The evidence of domains pattern is due to the different hardness to mechanical polishing shown by the positive and negative domains, respectively.
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range 0–3000 s at steps of 30 s, respectively. We performed SFM analysis of the etched surface to reconstruct the domains width and check the effect of the lateral etching. In Fig. 1, we report the SFM image relative to an as-polished sample (i.e. without etching). As it can be clearly seen, a surface topography is evident, showing a difference in the height of the two domains of some nm (while this effect was not observed in the Y-cut samples). Such a difference was ascribed to the different hardness to the mechanical polishing procedure [17]. To check this hypothesis, we performed an indentation test on an as-polished sample to measure Er and H of both the positive and negative domains. We took a sample AFM image before the indentation procedure to distinguish accurately the domains and a second AFM image after the indentation to check the deformation produced by the indenter tip. The measured values are the following: H = (13.2 ± 0.6) GPa and Er = (182 ± 3) GPa for the higher domains and H = (12.9 ± 0.3) GPa and Er = (186 ± 2) GPa for the lower ones. The values are compatible within errors, showing no difference in the hardness of the two phases of PPLN. The indentation tests suggest that some other mechanisms should be considered to explain the surface topography shown in Fig. 1. A reasonable explanation could be a different abrasive efficiency of the alumina particles during the polishing procedure. Indeed, it could exist an electrostatic charging effect of such particles depending on the domain they are facing. Work is in progress to clarify this aspect. In Figs. 2 and 3, the AFM images of the PPLN structure etched for 2.5 and 50 min, respectively, are shown. The topographic investigation pointed out that the modifications due to the lateral erosion let the negative domains width increase in spite of the positive ones. In the first 3 min, the depth etched
by the acid solution is close to 600 nm, corresponding to an etching rate normal to the surface of the order of 3 nm/s. If longer etching times are used, the total etched depth remains almost constant while a progressive lateral chemical erosion occurs. In Fig. 4, we report the etched depth in function of the etching time, while Fig. 5 shows the dependence of the mean negative domains width as a function of the etching time. The scans were performed starting from different points of the border domain, 3 m apart, labelled in the inset with the progressive numbers 1–3, respectively. The results of these three scans are in complete agreement, supporting the repeatability of the measurements. It is important to underline that after 3 min, the acid solution no longer etches the material in the direction normal to the surface but begins to act laterally. For increasing etching times, the depth of holes corresponding to the negative domains still remains close to 600 nm. This unexpected result indicates that there is a limit on the thickness of the etched pattern. Work is in progress to clarify this effect. The lateral erosion let the negative domain widths increase up to 50% when the etching time reaches 50 min. As a consequence, we estimate the lateral etching rate to be close to 0.6 nm/s. It is worth noting that the measurement of the PPLN period is not modified by the etching process, being the sum of the positive domain width (lp ) and the negative one (ln ). This systematic investigation allows us to identify the following best etching conditions: etching temperature and time equal to 80 ◦ C and 5 min, respectively. Under this condition, we etched all the crystals prepared under the different growth conditions. In order to estimate the period of the grown PPLN structure, they were scanned using the profilometer. As previously pointed out the data were processed so that the position of the border domain is given
Fig. 2. AFM image of a PPLN sample etched for 2.5 min at 80 ◦ C. PPLN growth conditions: off-center 4 mm, Er2 O3 in the melt 1.2 mol%.
N. Argiolas et al. / Materials Science and Engineering B 118 (2005) 150–154
153
Fig. 3. AFM image of a PPLN sample etched for 50 min at 80 ◦ C. PPLN growth conditions: off-center 4 mm, Er2 O3 in the melt 1.2 mol%.
by analyzing the numerical derivative of the deconvoluted scanned profile. As reported in Ref. [16], one of the crucial point is to determine the true edge corresponding to the real border domain wall. The systematic study on the etching dynamics underlined that the true edge is located in correspondence of the shoulder at the border domain wall (see sketch in Fig. 6). We observed, in fact, that the chemical etching makes sharp edges in correspondence to negative-to-positive domain interfaces and that edge radius of curvature is smaller than the typical size of a profilometer tip. The radius of curvature in the proximity of the tip apex is in fact about 0.6 m, while the SFM images on PPLN samples show edges with curvature radius of about 0.12 m. This means that the map measured by the profilometer is the result of a “convolution” of the tip shape with the real pattern profile exhibited by the etched PPLN crystal and that the deformation given by the
“convolution” effect cannot be neglected. The data processing is therefore mandatory. We report in Fig. 7, the predicted values of the period Λ(vp ) = vp /ω and the average measured value Λmeas of the PPLN period, respectively. At fixed rotational rate, Λmeas was found to be compatible within experimental error with that expected from the ratio of the pulling rate and the rotational rate (Λ(vp ) = vp /). Instead, if one considers the growth rate vg (equal to the sum of the pulling rate vp and the depletion rate vt of the melt contained in the crucible), the period Λ(vg ) = vg /ω is not compatible (within the experimental errors) with Λmeas . This result suggests that the melt depletion (i.e. lowering of the level inside of the crucible) plays a minor role as the crystal grows. Moreover, it suggests that the modulation in the domain period do not form at the solid–liquid interface but at the Curie isotherm, supporting the role of the thermoelectric field as reported in Ref. [18].
Fig. 4. Dependence of the etched depth on the etching time. The scans were performed starting from three different points, 3 m apart, labelled with the progressive numbers 1–3 as shown in the inset.
Fig. 5. Dependence of the negative domains width on the etching time.
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N. Argiolas et al. / Materials Science and Engineering B 118 (2005) 150–154
the positive and negative domains width values claimed in literature [17] can be partially explained in terms of artifacts induced by the chemical etching. Although, the determination of period is no longer affected by the chemical etching times, the measure of domains width can be strongly influenced by the lateral erosion. The determination of the border domain errors, needed to estimate the second harmonic generation expected from the PPLN structure, depends significantly on the domains width, and therefore on chemical etching procedure. Moreover, the dependence of the period on the pulling and rotational rates instead of the growing rate supports the role of the thermoelectric field in the domain formation at the Curie isotherm.
Acknowledgements Fig. 6. Sketch representing the true edge to be considered for a proper determination of the domain border.
We kindly acknowledge Dr. Giacomo Torzo for the fine discussions and the revision of the text. This work has been partially supported by FIRB RBNE01KZ94F.
References
Fig. 7. Domains period in function of the pulling rate vp : Λ(vp ) = vp /ω is the period predicted by the pulling rate, Λ(vg ) = vg /ω is the period predicted by assuming the growth rate and Λmeas is the measured of the PPLN period, respectively.
4. Conclusions We performed a systematic analysis on the dynamics of the chemical etching of PPLN structures grown by the offcenter Czochralski technique. We found that the etched depth saturates to a value close to 600 nm after only 3 min of etching corresponding to an erosion rate close to 3.4 nm/s. Moreover, the lateral etching cannot be neglected when the etching times are higher than 5 min, and it presents a rate close to 0.6 nm/s. The estimation of the domain widths distribution can be therefore affected by artifacts if the etching conditions are not properly chosen. The high mismatch between
[1] J.A. Armstrong, N. Bloemberger, J. Ducuing, P.S. Pershan, Phys. Rev. 127 (1962) 1918. [2] M. Hou´e, P.D. Townsend, J. Phys. D: Appl. Phys. 28 (1995) 1747. [3] M. Taya, M.C. Bashaw, M.M. Fejer, Opt. Lett. 21 (1996) 857. [4] A.V. Golenishev-Kutuzov, V.A. Golenishev-Kutuzov, R.I. Kalimullin, Physics-Uspekhi 43 (7) (2000) 647. [5] D. Feng, N.B. Ming, J.F. Hong, Y.S. Yang, J.S. Zhu, Z. Yang, Y.N. Wang, Appl. Phys. Lett. 37 (7) (1980) 607. [6] N. Ming, F.I. Hong, D. Feng, J. Mater. Sci. 17 (1982) 1663. [7] I.I. Naumova, N.F. Evlanova, S.A. Blokhin, S.V. Lavrishchev, J. Cryst. Growth 187 (1998) 102. [8] V. Bermudez, F. Caccavale, C. Sada, F. Segato, E. Dieguez, J. Cryst. Growth 191 (1998) 589. [9] V. Bermudez, M.D. Serrano, E. Dieguez, J. Cryst. Growth 200 (1999) 185. [10] M.M. Fejer, G.A. Magel, D.H. Jundt, R.L. Byer, Quasi-PhaseMatched 28 (11) (1992) 2631. [11] M. Hou´e, P.D. Townsend, Appl. Phys. Lett. 66 (1995) 2667. [12] K. Nassau, H.J. Levinstein, G.M. Loiacono, J. Phys. Chem. Solids 27 (1966) 983. [13] N. Argiolas, M. Bazzan, C. Sada, J. Cryst. Growth 249 (1–2) (2003) 275. [14] S.A. Syed Asyf, K.J. Wahl, R.J. Colton, Rev. Sci. Instr. 70 (1999) 2408. [15] S.A. Joyce, J.E. Houston, Rev. Sci. Instr. 62 (1991) 710. [16] M. Bazzan, N. Argiolas, A. Bernardi, C. Sada, Mater. Charact. 51 (2003) 177. [17] A.M. Prokhorov, Yu.S. Kuz’minov, Physics and Chemistry of Crystalline Lithium Niobate, Adam Hilger, New York, 1990, p. 127. [18] C. Sada, N. Argiolas, M. Bazzan, Appl. Phys. Lett. 79 (14) (2001) 2163.
A systematic study of the chemical etching process on periodically poled lithium niobate structures N. Argiolas a , M. Bazzan a , A. Bernardi a , E. Cattaruzza a , P. Mazzoldi a , P. Schiavuta a , C. Sada a, ∗ , U. Hangen b a
INFM-MATIS and Physics Department, University of Padova, Via Marzolo 8, 35131 Padova, Italy b Surface, Rheinstr. 7, D-41836 H¨ uckelhoven, Germany
Abstract A systematic analysis on the dynamics of the chemical etching of periodically poled lithium niobate (PPLN) structures grown by off-center Czochralski technique was carried out on crystals prepared under different experimental growth conditions. The etched depth reaches values close to 600 nm and it does not further increase even after long etching times. However, the lateral etching cannot be neglected when the etching times are higher than 5 min. The estimation of the domain widths distribution can be affected by artifacts if the etching conditions are not properly chosen. The best structures are obtained for erbium oxide doping level of 0.3 mol% into the starting melt and the period depends on the pulling and rotational rates instead of on the growing rate. This results support the role of the thermoelectric field in the domain formation at the Curie isotherm. © 2005 Elsevier B.V. All rights reserved. Keywords: Periodical poling; Lithium niobate; Crystal growth; Atomic force microscopy; Etching
1. Introduction Periodically poled lithium niobate (PPLN) is a promising material for non-linear optical quasi-phase-matched devices [1,2] and recently has attracted great interest also for efficient volume-phase grating recording [3] and soliton propagation. The most common technique to prepare periodic structures consists of applying an electric field either during the material growth (about 0.4 V/cm) or after that, even at room temperature [2,4]. In alternative, the so-called modified Czochralski technique [5–9] can be exploited to grow PPLN crystals with the desired period. Whatever the fabrication process is, the device performance is strongly influenced by the quality of the periodic structures. The test of the domain distribution along the PPLN material is therefore mandatory before starting the device packaging. According to [10], in fact, a spatial resolution of at least 1% should be used to map the periodic structure. In order to test the domains ∗
Corresponding author. Tel.: +39 049 8277037; fax: +39 049 8277003. E-mail address: [email protected] (C. Sada).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.12.088
distribution, the chemical etching followed by the direct observation of domains using an optical microscope has been widely employed to characterize PPLN structures [11]. In fact, as the etching rate is faster for a negative Z-face than for a positive one [12], domains with opposite polarization states can easily be identified through topographic measurements. Techniques such as scanning force microscopy (SFM) are therefore extremely useful to test the PPLN structure with typical lateral resolution of tens of nanometers. Since the SFM analysis is limited to a small scanned region (less than 100 m × 100 m), it is more convenient to combine it with a scanning made with a standard profilometer after the chemical etching process [13]. In this case, the range is typically of centimeters with a resolution of some hundreds of nanometers (about 1% of the typical width of the domains used in first-order quasi-phase-matching devices). Both SFM images and profilometer line-scan give reliable measurements of the domain dimensions only if the chemical etching does not introduce artifacts. In literature, it is usually assumed that the lateral erosion of the domains walls can be neglected, the domains width being independent
N. Argiolas et al. / Materials Science and Engineering B 118 (2005) 150–154
of the etching time. In this work, we present the results of a systematic study of the chemical etching on PPLN structures grown by the off-center Czochralski technique. We will show that the lateral erosion cannot be neglected and it may lead to overestimate the negative domains width. The best conditions to carry out the chemical etching are finally presented.
2. Experimental Periodically poled Er:LiNbO3 single crystals were grown along the X-axis with a pulling speed (vp ) ranging from 2 to 5 mm/h and with a rotational rate (ω) between 5 and 15 rpm so that the growth rate is maintained almost constant. The growth conditions were chosen to have the designed period L in the range 4–10 m of the PPLN structure. The starting melt was made of 99.999% stoichiometric lithium niobate powders with different content of Er2 O3 salts in the range between 0 and 0.35 mol%. The growth temperature was controlled by a computer algorithm keeping constant the mass variation of the melt in order to obtain a crystal with fixed radius. Each crystal was cut in slices with the optical axis perpendicular to their major surface (Z-cut) and polished with standard techniques. Finally, these slices were etched in acid solution (HF:HNO3 = 1:2 (v/v)) at 80 ◦ C for different times (0–50 min.). The main details of the domain structures were investigated mapping the etched surfaces using the scanning force microscopy technique. An SFM Autoprobe CP (Park Scientific Instruments) was used in contact mode to scan an area of 32 m × 32 m. Finally, we measured the reduced elastic modulus (Er ) and hardness (H) of the domains by means of a Hysitron Triboscope nanoindenter used in the Berkovich geometry. The indenter tip is pressed in a controlled way on the surface and load versus depth curve is measured; from the analysis of that curve is possible to determine Er and H [14,15]. The nanoindenter head can also provide in situ atomic force microscopy (AFM) images,
151
enabling us to view the surface topography before and after the indentation. It is therefore possible to select accurately the domain to be measured. Moreover, we investigated domains distribution of the PPLN structures by mapping the etched surfaces along the crystal using a Tencor profilometer (with a lateral resolution equal to few nanometer and a depth resolution of about 1 nm). The scan was performed along the direction normal to the border domain with a scanning speed equal to 1 m/s and a sampling rate 500 Hz. The measured scan is however the result of a “convolution” of the Profilometer tip shape with the real pattern profile of the etched PPLN crystal. To obtain the real PPLN pattern, a deconvolution procedure was applied, as reported in Ref. [16]. In particular, the position of the border domain is given by analyzing the numerical derivative of the deconvoluted scanned profile. At each border domain corresponds an edge: the procedure allows identifying the edge position and returns the domain wall location and domain width or period of the structure as a function of the position. Further details on the procedure used in the present work are reported in Ref. [16].
3. Results and discussions Several crystals were grown under different experimental conditions by varying the Er2 O3 content into the starting melt between 0 and 0.35 mol% and by changing the offcenter in the range of 0–8 mm. The periodicity of the PPLN structure was checked by means of the chemical etching procedure. In order to determine the best etching conditions, a systematic study was performed varying the etching temperature and time, respectively. The topographic investigation of the PPLN structure was performed on the sample grown from the melt containing an Er2 O3 concentration equal to 0.35 mol%. This sample was etched for different times at temperatures in the range 80–110 ◦ C in a mixture of HNO3 :HF (2:1 (v/v)). In particular, the etching times were varied in the
Fig. 1. AFM image of as prepared PPLN sample, after the polishing procedure and without etching. PPLN growth conditions: off-center 4 mm, Er2 O3 in the melt 0.6 mol%. The evidence of domains pattern is due to the different hardness to mechanical polishing shown by the positive and negative domains, respectively.
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range 0–3000 s at steps of 30 s, respectively. We performed SFM analysis of the etched surface to reconstruct the domains width and check the effect of the lateral etching. In Fig. 1, we report the SFM image relative to an as-polished sample (i.e. without etching). As it can be clearly seen, a surface topography is evident, showing a difference in the height of the two domains of some nm (while this effect was not observed in the Y-cut samples). Such a difference was ascribed to the different hardness to the mechanical polishing procedure [17]. To check this hypothesis, we performed an indentation test on an as-polished sample to measure Er and H of both the positive and negative domains. We took a sample AFM image before the indentation procedure to distinguish accurately the domains and a second AFM image after the indentation to check the deformation produced by the indenter tip. The measured values are the following: H = (13.2 ± 0.6) GPa and Er = (182 ± 3) GPa for the higher domains and H = (12.9 ± 0.3) GPa and Er = (186 ± 2) GPa for the lower ones. The values are compatible within errors, showing no difference in the hardness of the two phases of PPLN. The indentation tests suggest that some other mechanisms should be considered to explain the surface topography shown in Fig. 1. A reasonable explanation could be a different abrasive efficiency of the alumina particles during the polishing procedure. Indeed, it could exist an electrostatic charging effect of such particles depending on the domain they are facing. Work is in progress to clarify this aspect. In Figs. 2 and 3, the AFM images of the PPLN structure etched for 2.5 and 50 min, respectively, are shown. The topographic investigation pointed out that the modifications due to the lateral erosion let the negative domains width increase in spite of the positive ones. In the first 3 min, the depth etched
by the acid solution is close to 600 nm, corresponding to an etching rate normal to the surface of the order of 3 nm/s. If longer etching times are used, the total etched depth remains almost constant while a progressive lateral chemical erosion occurs. In Fig. 4, we report the etched depth in function of the etching time, while Fig. 5 shows the dependence of the mean negative domains width as a function of the etching time. The scans were performed starting from different points of the border domain, 3 m apart, labelled in the inset with the progressive numbers 1–3, respectively. The results of these three scans are in complete agreement, supporting the repeatability of the measurements. It is important to underline that after 3 min, the acid solution no longer etches the material in the direction normal to the surface but begins to act laterally. For increasing etching times, the depth of holes corresponding to the negative domains still remains close to 600 nm. This unexpected result indicates that there is a limit on the thickness of the etched pattern. Work is in progress to clarify this effect. The lateral erosion let the negative domain widths increase up to 50% when the etching time reaches 50 min. As a consequence, we estimate the lateral etching rate to be close to 0.6 nm/s. It is worth noting that the measurement of the PPLN period is not modified by the etching process, being the sum of the positive domain width (lp ) and the negative one (ln ). This systematic investigation allows us to identify the following best etching conditions: etching temperature and time equal to 80 ◦ C and 5 min, respectively. Under this condition, we etched all the crystals prepared under the different growth conditions. In order to estimate the period of the grown PPLN structure, they were scanned using the profilometer. As previously pointed out the data were processed so that the position of the border domain is given
Fig. 2. AFM image of a PPLN sample etched for 2.5 min at 80 ◦ C. PPLN growth conditions: off-center 4 mm, Er2 O3 in the melt 1.2 mol%.
N. Argiolas et al. / Materials Science and Engineering B 118 (2005) 150–154
153
Fig. 3. AFM image of a PPLN sample etched for 50 min at 80 ◦ C. PPLN growth conditions: off-center 4 mm, Er2 O3 in the melt 1.2 mol%.
by analyzing the numerical derivative of the deconvoluted scanned profile. As reported in Ref. [16], one of the crucial point is to determine the true edge corresponding to the real border domain wall. The systematic study on the etching dynamics underlined that the true edge is located in correspondence of the shoulder at the border domain wall (see sketch in Fig. 6). We observed, in fact, that the chemical etching makes sharp edges in correspondence to negative-to-positive domain interfaces and that edge radius of curvature is smaller than the typical size of a profilometer tip. The radius of curvature in the proximity of the tip apex is in fact about 0.6 m, while the SFM images on PPLN samples show edges with curvature radius of about 0.12 m. This means that the map measured by the profilometer is the result of a “convolution” of the tip shape with the real pattern profile exhibited by the etched PPLN crystal and that the deformation given by the
“convolution” effect cannot be neglected. The data processing is therefore mandatory. We report in Fig. 7, the predicted values of the period Λ(vp ) = vp /ω and the average measured value Λmeas of the PPLN period, respectively. At fixed rotational rate, Λmeas was found to be compatible within experimental error with that expected from the ratio of the pulling rate and the rotational rate (Λ(vp ) = vp /). Instead, if one considers the growth rate vg (equal to the sum of the pulling rate vp and the depletion rate vt of the melt contained in the crucible), the period Λ(vg ) = vg /ω is not compatible (within the experimental errors) with Λmeas . This result suggests that the melt depletion (i.e. lowering of the level inside of the crucible) plays a minor role as the crystal grows. Moreover, it suggests that the modulation in the domain period do not form at the solid–liquid interface but at the Curie isotherm, supporting the role of the thermoelectric field as reported in Ref. [18].
Fig. 4. Dependence of the etched depth on the etching time. The scans were performed starting from three different points, 3 m apart, labelled with the progressive numbers 1–3 as shown in the inset.
Fig. 5. Dependence of the negative domains width on the etching time.
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N. Argiolas et al. / Materials Science and Engineering B 118 (2005) 150–154
the positive and negative domains width values claimed in literature [17] can be partially explained in terms of artifacts induced by the chemical etching. Although, the determination of period is no longer affected by the chemical etching times, the measure of domains width can be strongly influenced by the lateral erosion. The determination of the border domain errors, needed to estimate the second harmonic generation expected from the PPLN structure, depends significantly on the domains width, and therefore on chemical etching procedure. Moreover, the dependence of the period on the pulling and rotational rates instead of the growing rate supports the role of the thermoelectric field in the domain formation at the Curie isotherm.
Acknowledgements Fig. 6. Sketch representing the true edge to be considered for a proper determination of the domain border.
We kindly acknowledge Dr. Giacomo Torzo for the fine discussions and the revision of the text. This work has been partially supported by FIRB RBNE01KZ94F.
References
Fig. 7. Domains period in function of the pulling rate vp : Λ(vp ) = vp /ω is the period predicted by the pulling rate, Λ(vg ) = vg /ω is the period predicted by assuming the growth rate and Λmeas is the measured of the PPLN period, respectively.
4. Conclusions We performed a systematic analysis on the dynamics of the chemical etching of PPLN structures grown by the offcenter Czochralski technique. We found that the etched depth saturates to a value close to 600 nm after only 3 min of etching corresponding to an erosion rate close to 3.4 nm/s. Moreover, the lateral etching cannot be neglected when the etching times are higher than 5 min, and it presents a rate close to 0.6 nm/s. The estimation of the domain widths distribution can be therefore affected by artifacts if the etching conditions are not properly chosen. The high mismatch between
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