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Investigation of proton exchanged optical waveguides in LiNbO3 using elastic recoil detection
Nuclear Instruments and Methods in Physics Research B61 (1991) 91-93 North-Holland
91
Investigation of proton exchanged optical waveguides in LiNbO, using elastic recoil detection M. Rot&chalk Friedrich-Schiller-
‘, T. Bachmann
and A. Witzmann
Universittit Jena, Max- Wien-Platz
2
I, O-6900 Jena, Germany
Received 11 December 1990
Waveguiding layers were fabricated in LiNbO, by proton exchange and partly subsequent annealing. The measurement of the optical in-depth profiles is described. The near surface hydrogen contents obtained by elastic recoil detection were compared with the change of the extraordinan,_ optical refractive index An,. The dependence was found to have a nonlinear shape because of a crystalline phase transition during the annealing procedure.
1. Introduction In spite of the established knowledge that even slight deviations from the stoichiometric composition of LiNbO, strongly effect the optical, electro-optical and acousto-optical properties of lithium niobate single crystals [l], the difficulty of measuring in-depth concentration profiles of light elements over thin (some pm) surface layers makes it hard to correlate the optical with the structural properties of surface waveguiding layers in LiNbO, single crystals. On the other hand, the proton exchange (PE) [2], now the favourite fabrication method for integrated optical devices in LiNbO, [3,4], is connected with a drastic structural transformation and high 1 : 1 Li t) H exchange rates [5]. After exchange the extraordinary refractive index is increased by An, = 0.12 at the wavelength X = 0.633 pm, while the ordinary index is decreased, therefore waveguiding is not possible for it. The extraordinary index change An, is characterized by a step profile. Generally, the proton exchange involves a subsequent annealing procedure leading to lower index changes and Gaussian index profiles. The annealed waveguides exhibit low attenuation values, long term index stability, high coupling efficiency to optical fibres and no more degradation in the electro-optical effect. These annealed PE-guides are the original source of highly-efficient integrated optical devices. The comparison of the measured composition and index profiles for exchanged layers showed that An, is not linearly dependent on the hydrogen amount x in Li,_,H,NbO,, except for small values of x [6]. The
’ Institut flir Angewandte Physik. 2 Institut fur Festkiirperphysik. 0168-583X/91/$03.50
concentrations were measured by chemical means in this case. Using the elastic recoil detection (ERD) technique the hydrogen and lithium in-depth profiles of PE-guides were examined by others [7]. These investigations yielded a H-concentration c after exchange of more than 20 at.% corresponding to x > 1 (more than the Li-concentration in the crystal). However, this does not agree with the results in refs. [5,6], publishing x I 0.75 in Li, _,H,Nb03. We investigated various Li,_,H,NbO, layers in Xcut LiNbO, concerning their optical profiles by m-line spectroscopy and concerning their near surface hydrogen concentration by ERD-analysis. 2. Experimental The proton exchange was carried out in X-cut LiNbO, either in pure benzoic acid melt or in melts containing 1 mol% of lithium benzoate in order to obtain layers having different initial hydrogen contents. The samples were exchanged at 250°C for 1 min (pure) and at 180°C for 6 h (1 mol%) resulting in single-mode optical waveguides. Five samples were fabricated of the second kind, four of them were subsequently annealed under different conditions, i.e. at temperatures from 300°C up to 4OO’C for 1 h in a dry oxygen atmosphere. Generally, the annealed PE-guides exhibit two guided modes in a Gaussian profile. However, for exact calculations of the An,-profile more information is needed. Therefore, we used m-line spectroscopy measurements [8] at four wavelengths from the blue up to the red. The well-known WKB-method for the Gaussian profiles [8] and the b/V-method [8] for the step profiles then yielded the calculated index changes An, and profile depths.
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
92
M. Rottschalk
et al. / ERD investigations
The ERD-measurements were carried out at the Van de Graaff accelerator of the University Jena using a 1.4 MeV He+-ion beam. Forward scattered protons were registered by a Si-surface barrier detector covered with a 4 pm Al-stopper foil at a scattering angle of 30° (angle of incidence and emergence were 75O with respect to the surface normal of the sample). For the calculation of the H-concentration c a computer code for the simulation of ERD-spectra was applied to fit the experimental spectra. The calculations base on the 4He-H recoil cross section data by Benenson [9] and a polynomal fit of the stopping powers of H and He published by Ziegler [lo]. Mainly because of the experimental inaccuracy of the recoil cross section which is non-Rutherford (and because of the sensitivity of the measurement to the geometrical arrangement) the systematical experimental error of the H-concentration c is about 20%.
I
t
’
40
0.4
0
LiNbO,
(11 25CWlmin ,Omol% (21 18O’C/6h , 1 mol% annealing of sample (2) (3) SlO’C/lh (4) 3ZO’C/l h 151 3W’C/lh
08
1.2
lb
2.0
Fig. 1. Calculated refractive index profiles An,(z) of proton exchanged LiNbO, layers. Mel% denotes the amount of lithium benzoate in benzoic acid and .z, the l/e-value of the profile.
resulting in waveguides with lower An,-values that better match a single-mode fibre in the strip waveguide case. Fig. 2 shows the ERD-spectra obtained from virgin and proton exchanged LiNbO,. The measured yield depends strongly on the exchange and annealing conditions. Higher temperatures during the annealing procedure lead to lower hydrogen contents in the near surface region of the samples due to in-depth diffusion of the hydrogen as can seen in fig. 1. Additionally, peaks were
Fig. 1 shows the An,-profiles for the only exchanged samples (1) and (2) resulting in single mode optical waveguides and for the annealed samples [(3)-(6)] exhibiting two guided modes in a Gaussian profile. It is clearly seen that these annealing conditions have lead to the whole possible range of An, at the surface. Diffusing the exchange region deeper into the crystal also reduces the hydrogen concentration at the surface, just
12
waveguides
proton exchanged
3. Results and discussion
100
of LiNb03
depth z [nml 50 0 I
I
proton exchanged
LiNbO, untreated virgincrystal 250"C/lmin,0 mol% 18O'C/6h , 1 mol% annealingof sample n(2) 300"C/lh 320"C/lh 340"C/lh
80
60
100
channe\ Fig. 2. ERD-spectra obtained from proton exchanged LiNbO, samples. Additionally, the spectrum of the untreated, virgin sample is depicted.
M. Rot&chalk et al. / ERD investigations
of LiNbO,
waveguides
Table 1 Optical and ERD-results for Li,_,H,NbO,, the data were taken at a depth of z = 100 nm; An,-profile, cu is calculated assuming an atomic density of 9.5 X lo** at./cm3 for Li, _,H,NbO, Sample
Optical
z, denotes
the l/e-value
of the
ERD-results
results
Profile
2,
An, 0.1230 0.1100 0.0810 0.0390 0.0220 0.0035
Step Step Gaussian Gaussian Gaussian Gaussian
(1) (2) (3) (4) (5) (6)
93
CH
b-4
fat.%]
[lo** cmm3]
0.265 0.270 0.460 0.720 1.190 4.700
13.6 10.8 6.3 5.5 2.4 0.4
1.29 1.03 0.60 0.52 0.23 0.04
detected at the position of the sample surface in the energy spectra, especially for the samples containing no or a low amount of hydrogen. This observation may be explained by water adsorption from air and by the interaction of the incident He+-beam with the residual gases in the vacuum chamber leading to adsorption processes of hydrogen containing compounds on the sample surfaces. Therefore, the H-concentrations c were determined behind the surface peaks by averaging the contents from channel 35 to 45, corresponding to a depth of = 100 nm. In fig. 3 An, near the surface is outlined as a function of the hydrogen concentration c in at.%. We notice the nonlinear shape of the curve caused by the rhombohedral /?- to a-phase transition as it was described for Li,_,H,NbO,-powder samples [5]. Outside the phase transition region the dependence is found to be linear in agreement with [6] for small values of An, and H-concentration c, respectively. Additionally, we found a linear behavior also for high values of An, and
x
0.680 0.540 0.315 0.275 0.120 0.020
In table 1 the optical and ERD-results are summarized. The optical values correspond to fig. 1, while the ERD-results for the relative H-concentration c in at.% and the absolute hydrogen concentration cu are related to the composition and atomic density of Li l_XH,NbO, (9.5 x 1O22 at./cm3), respectively. The x-values for Li, _,H,NbO, were calculated from c using the known Li-density in the crystal (20 at.% corresponding to 1.9 X 1O22 at./cm3). For values below x = 0.12 we have pure a-Li,_,H,Nb03, above x = 0.55 occurs only /3-Li,_,H,NbO, [5]. The region below x = 0.12, that corresponds to An, < 0.02, is only relevant for integral optical devices because the substitution of hydrogen for lithium produces almost no change in the lattice cell constants. Now it is clear that the reported results in [5] are of higher reliability compared with ref.
171.
References
PI D.M. Smith, SPIE Proc. 460 (1984) 22. PI J.L. Jackel, C.E. Rice and J.J. Veselka, Appl. Phys. Lett. 41 (1982) 607.
131 P.G. Suchoski,
0.06 egionof nonlinearchange
0
0
2
4
6
8
10
12
14
16
H-concentrntionc [at%1
Fig. 3. Dependence of An, (taken at a depth of 100 nm) on the hydrogen concentration measured in at.!&.
T.K. FindakIy and F.J. Leonberger, Optics Lett. 13 (1988) 1050. [41 M. RottschaIk, A. Rasch and W. Karthe, J. Opt. Commun. 10 (1989) 138. 151 C.E. Rice and J.L. Jackel, Mater. Res. Bull. 19 (1984) 591. Lett. 19 161 J.L. Jackel, C.E. Rice and J.J. Veselka,.Electron. (1983) 387. [71 C. Canali, A. Camera, G. Mazzi, P. Mazzoldi and G. Arnold, IOOC-ECOC ‘85, Venezia, Italy, p. 43. [S] Integrated Optics, ed. T. Tamir (Springer Verlag, Berlin, Heidelberg, New York 1979) p. 223, p. 61 and p. 19. [9] R.E. Benenson, L.S. Wielunski and W.A. Lanford, Nucl. Instr. and Meth. B15 (1986) 453. [lo] J.F. Ziegler, The Stopping and Ranges of Ions in Matter, Vols. 3 and 4 (Pergamon, New York, 1977).
91
Investigation of proton exchanged optical waveguides in LiNbO, using elastic recoil detection M. Rot&chalk Friedrich-Schiller-
‘, T. Bachmann
and A. Witzmann
Universittit Jena, Max- Wien-Platz
2
I, O-6900 Jena, Germany
Received 11 December 1990
Waveguiding layers were fabricated in LiNbO, by proton exchange and partly subsequent annealing. The measurement of the optical in-depth profiles is described. The near surface hydrogen contents obtained by elastic recoil detection were compared with the change of the extraordinan,_ optical refractive index An,. The dependence was found to have a nonlinear shape because of a crystalline phase transition during the annealing procedure.
1. Introduction In spite of the established knowledge that even slight deviations from the stoichiometric composition of LiNbO, strongly effect the optical, electro-optical and acousto-optical properties of lithium niobate single crystals [l], the difficulty of measuring in-depth concentration profiles of light elements over thin (some pm) surface layers makes it hard to correlate the optical with the structural properties of surface waveguiding layers in LiNbO, single crystals. On the other hand, the proton exchange (PE) [2], now the favourite fabrication method for integrated optical devices in LiNbO, [3,4], is connected with a drastic structural transformation and high 1 : 1 Li t) H exchange rates [5]. After exchange the extraordinary refractive index is increased by An, = 0.12 at the wavelength X = 0.633 pm, while the ordinary index is decreased, therefore waveguiding is not possible for it. The extraordinary index change An, is characterized by a step profile. Generally, the proton exchange involves a subsequent annealing procedure leading to lower index changes and Gaussian index profiles. The annealed waveguides exhibit low attenuation values, long term index stability, high coupling efficiency to optical fibres and no more degradation in the electro-optical effect. These annealed PE-guides are the original source of highly-efficient integrated optical devices. The comparison of the measured composition and index profiles for exchanged layers showed that An, is not linearly dependent on the hydrogen amount x in Li,_,H,NbO,, except for small values of x [6]. The
’ Institut flir Angewandte Physik. 2 Institut fur Festkiirperphysik. 0168-583X/91/$03.50
concentrations were measured by chemical means in this case. Using the elastic recoil detection (ERD) technique the hydrogen and lithium in-depth profiles of PE-guides were examined by others [7]. These investigations yielded a H-concentration c after exchange of more than 20 at.% corresponding to x > 1 (more than the Li-concentration in the crystal). However, this does not agree with the results in refs. [5,6], publishing x I 0.75 in Li, _,H,Nb03. We investigated various Li,_,H,NbO, layers in Xcut LiNbO, concerning their optical profiles by m-line spectroscopy and concerning their near surface hydrogen concentration by ERD-analysis. 2. Experimental The proton exchange was carried out in X-cut LiNbO, either in pure benzoic acid melt or in melts containing 1 mol% of lithium benzoate in order to obtain layers having different initial hydrogen contents. The samples were exchanged at 250°C for 1 min (pure) and at 180°C for 6 h (1 mol%) resulting in single-mode optical waveguides. Five samples were fabricated of the second kind, four of them were subsequently annealed under different conditions, i.e. at temperatures from 300°C up to 4OO’C for 1 h in a dry oxygen atmosphere. Generally, the annealed PE-guides exhibit two guided modes in a Gaussian profile. However, for exact calculations of the An,-profile more information is needed. Therefore, we used m-line spectroscopy measurements [8] at four wavelengths from the blue up to the red. The well-known WKB-method for the Gaussian profiles [8] and the b/V-method [8] for the step profiles then yielded the calculated index changes An, and profile depths.
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
92
M. Rottschalk
et al. / ERD investigations
The ERD-measurements were carried out at the Van de Graaff accelerator of the University Jena using a 1.4 MeV He+-ion beam. Forward scattered protons were registered by a Si-surface barrier detector covered with a 4 pm Al-stopper foil at a scattering angle of 30° (angle of incidence and emergence were 75O with respect to the surface normal of the sample). For the calculation of the H-concentration c a computer code for the simulation of ERD-spectra was applied to fit the experimental spectra. The calculations base on the 4He-H recoil cross section data by Benenson [9] and a polynomal fit of the stopping powers of H and He published by Ziegler [lo]. Mainly because of the experimental inaccuracy of the recoil cross section which is non-Rutherford (and because of the sensitivity of the measurement to the geometrical arrangement) the systematical experimental error of the H-concentration c is about 20%.
I
t
’
40
0.4
0
LiNbO,
(11 25CWlmin ,Omol% (21 18O’C/6h , 1 mol% annealing of sample (2) (3) SlO’C/lh (4) 3ZO’C/l h 151 3W’C/lh
08
1.2
lb
2.0
Fig. 1. Calculated refractive index profiles An,(z) of proton exchanged LiNbO, layers. Mel% denotes the amount of lithium benzoate in benzoic acid and .z, the l/e-value of the profile.
resulting in waveguides with lower An,-values that better match a single-mode fibre in the strip waveguide case. Fig. 2 shows the ERD-spectra obtained from virgin and proton exchanged LiNbO,. The measured yield depends strongly on the exchange and annealing conditions. Higher temperatures during the annealing procedure lead to lower hydrogen contents in the near surface region of the samples due to in-depth diffusion of the hydrogen as can seen in fig. 1. Additionally, peaks were
Fig. 1 shows the An,-profiles for the only exchanged samples (1) and (2) resulting in single mode optical waveguides and for the annealed samples [(3)-(6)] exhibiting two guided modes in a Gaussian profile. It is clearly seen that these annealing conditions have lead to the whole possible range of An, at the surface. Diffusing the exchange region deeper into the crystal also reduces the hydrogen concentration at the surface, just
12
waveguides
proton exchanged
3. Results and discussion
100
of LiNb03
depth z [nml 50 0 I
I
proton exchanged
LiNbO, untreated virgincrystal 250"C/lmin,0 mol% 18O'C/6h , 1 mol% annealingof sample n(2) 300"C/lh 320"C/lh 340"C/lh
80
60
100
channe\ Fig. 2. ERD-spectra obtained from proton exchanged LiNbO, samples. Additionally, the spectrum of the untreated, virgin sample is depicted.
M. Rot&chalk et al. / ERD investigations
of LiNbO,
waveguides
Table 1 Optical and ERD-results for Li,_,H,NbO,, the data were taken at a depth of z = 100 nm; An,-profile, cu is calculated assuming an atomic density of 9.5 X lo** at./cm3 for Li, _,H,NbO, Sample
Optical
z, denotes
the l/e-value
of the
ERD-results
results
Profile
2,
An, 0.1230 0.1100 0.0810 0.0390 0.0220 0.0035
Step Step Gaussian Gaussian Gaussian Gaussian
(1) (2) (3) (4) (5) (6)
93
CH
b-4
fat.%]
[lo** cmm3]
0.265 0.270 0.460 0.720 1.190 4.700
13.6 10.8 6.3 5.5 2.4 0.4
1.29 1.03 0.60 0.52 0.23 0.04
detected at the position of the sample surface in the energy spectra, especially for the samples containing no or a low amount of hydrogen. This observation may be explained by water adsorption from air and by the interaction of the incident He+-beam with the residual gases in the vacuum chamber leading to adsorption processes of hydrogen containing compounds on the sample surfaces. Therefore, the H-concentrations c were determined behind the surface peaks by averaging the contents from channel 35 to 45, corresponding to a depth of = 100 nm. In fig. 3 An, near the surface is outlined as a function of the hydrogen concentration c in at.%. We notice the nonlinear shape of the curve caused by the rhombohedral /?- to a-phase transition as it was described for Li,_,H,NbO,-powder samples [5]. Outside the phase transition region the dependence is found to be linear in agreement with [6] for small values of An, and H-concentration c, respectively. Additionally, we found a linear behavior also for high values of An, and
x
0.680 0.540 0.315 0.275 0.120 0.020
In table 1 the optical and ERD-results are summarized. The optical values correspond to fig. 1, while the ERD-results for the relative H-concentration c in at.% and the absolute hydrogen concentration cu are related to the composition and atomic density of Li l_XH,NbO, (9.5 x 1O22 at./cm3), respectively. The x-values for Li, _,H,NbO, were calculated from c using the known Li-density in the crystal (20 at.% corresponding to 1.9 X 1O22 at./cm3). For values below x = 0.12 we have pure a-Li,_,H,Nb03, above x = 0.55 occurs only /3-Li,_,H,NbO, [5]. The region below x = 0.12, that corresponds to An, < 0.02, is only relevant for integral optical devices because the substitution of hydrogen for lithium produces almost no change in the lattice cell constants. Now it is clear that the reported results in [5] are of higher reliability compared with ref.
171.
References
PI D.M. Smith, SPIE Proc. 460 (1984) 22. PI J.L. Jackel, C.E. Rice and J.J. Veselka, Appl. Phys. Lett. 41 (1982) 607.
131 P.G. Suchoski,
0.06 egionof nonlinearchange
0
0
2
4
6
8
10
12
14
16
H-concentrntionc [at%1
Fig. 3. Dependence of An, (taken at a depth of 100 nm) on the hydrogen concentration measured in at.!&.
T.K. FindakIy and F.J. Leonberger, Optics Lett. 13 (1988) 1050. [41 M. RottschaIk, A. Rasch and W. Karthe, J. Opt. Commun. 10 (1989) 138. 151 C.E. Rice and J.L. Jackel, Mater. Res. Bull. 19 (1984) 591. Lett. 19 161 J.L. Jackel, C.E. Rice and J.J. Veselka,.Electron. (1983) 387. [71 C. Canali, A. Camera, G. Mazzi, P. Mazzoldi and G. Arnold, IOOC-ECOC ‘85, Venezia, Italy, p. 43. [S] Integrated Optics, ed. T. Tamir (Springer Verlag, Berlin, Heidelberg, New York 1979) p. 223, p. 61 and p. 19. [9] R.E. Benenson, L.S. Wielunski and W.A. Lanford, Nucl. Instr. and Meth. B15 (1986) 453. [lo] J.F. Ziegler, The Stopping and Ranges of Ions in Matter, Vols. 3 and 4 (Pergamon, New York, 1977).