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Complementary X-ray topography and near-surface diffraction for investigations into the structure of ion-implanted optical waveguides
Nuclear Instruments and Methods
a3__ __
in
Physics Research B 97 (1995) 337-341 NOM
B
Beam Interactions with Materials 8 Atoms
@ ELSEVIER
Complementary X-ray topography and near-surface diffraction for investigations into the structure of ion-implanted optical waveguides
*,
R.S. Lowther-Harris a,b, S.D. Brown a, P.W. Haycock a, P.J. Chandler ‘, L. Zhang ‘, C.C. Tang b, R.P. Findlay a, L. Babsail ‘, M. Rodman ‘, J. Knight ‘, P.D. Townsend ’ ” DrparlmentofPhysics,School of Scienceand Engineering, Keele University, Keele, Stuflordshire, ST5 SBG UK h DRAL, Daresbury Laboratory, Daresbury. Warrington, Cheshire. WA4 3AD, UK ’ School of Mathematical and Physical Sciences, Unirtersity of Sussex. Falmer, East Sussex, BNI 9QH. UK
Abstract Ion implantation is now a well established technique for the production of applications. However, in some materials, such as lithium niobate, it leads waveguiding region. We have investigated the structure of the waveguide and the niobate by carrying out complementary X-ray topography and near surface X-ray indicate radiation enhanced annealing in the waveguide itself and the upper region of amorphisation in the damaged layer.
1. Introduction In lithium niobate (LiNbO,), waveguide fabrication by implantation with helium ions produces refractive index anomalies in the waveguiding layer which have yet to be fully explained. It has been suggested these may be due to lithium diffusion towards the nuclear damage region [l]. In order to examine directly the structure and defects deeper in the waveguide region and into the damaged layer itself, the surface of an implanted crystal was progressively etched with hydrofluoric acid and the samples were examined using the complementary techniques of X-ray topography and near-surface X-ray diffraction, both at the DRAL SRS.
2. Experimental
details
All experiments were performed on a single crystal of y-cut LiNbO, 45 X 40 X 0.8 mm, with one large face polished and all other faces ground. The sample was implanted with He+ ions at room temperature to a dose of 1.5 X lCIlh ions cm-‘, at an energy of 1.5 MeV. The ordinary refractive index profile closely follows the structural damage profile caused by implantation and peaks under these conditions at a depth of 3.35 p,rn [2,3].
* Corresponding author. Tel. +44 782 621111, ext. 7923, fax + 44 782 711093. e-mail: phd53Qpotter.cc.keeIe.ac.uk. 0168-583X/95/$09.50
optical waveguides for integrated optics to anomalous optical properties in the damaged region in ion-implanted lithium diffraction on etched crystals. The results of the substrate, as well as a high degree
The topography experiments were carried out on station 7.6 of the DRAL SRS using the arrangement described by Fisher and Barnes [4]. Grazing incidence diffraction experiments were performed on station 2.3 at the DRAL SRS, which was modified from its usual powder diffraction geometry to allow in-plane diffraction measurements with the [OO.l] direction aligned in the scattering plane. All diffraction results were obtained as multiple w scans, stepping 20 between each scan, before being converted to reciprocal space grids in hexagonal units, LiNbO, having a rhombohedral lattice. Initial diffraction results were obtained from unimplanted LiNbO, as a reference. After implantation, the sample was cut into 4 equal size pieces and each piece was etched to a different depth. Kawabe et al., [s], showed that a 48% HF solution at 50°C etches unimplanted LiNbO, crystal at the rate of 8 nm min-‘. This was verified by our trials which gave a figure of approximately 1 pm removal after 2 h. The 4 pieces were etched for l/4, 2, 4 and 5 h respectively in order to reveal successively more of the waveguiding region and, finally, the damaged layer itself.
3. Results and discussion 3.1. Topography-edge An edge of each etched piece was examined using white beam X-ray topography, a technique which highlights defects in crystals. Generally the darker the reflected
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0168-583X(94)00394-7
VII. ELECTRONICS MATERIALS
R.S. Lowrher-Harris et al. /Nucl. Instr. and Meth. in Phys. Res. B 97 (1995) 337-341
338
Fig. 1. Edge topograph. (iO.S)-implanted lithium niobate
Fig. 2. Surface topograph. (jO.O)-implanted lithium niobate, etched 4 h. pattern, the greater the number of extended defects, although amorphous material also produces light images since it does not lead to Bragg diffraction. Fig. 1 shows a (70.8) reflection topograph from the implanted sample edge. It can be seen that the sample is split into distinct regions: (1) a mottled region; (2) a clearer region which appears to be missing; and (3) a thin black line separating the two. The mottled region represents diffraction from the bulk material with its inherent distribution of defects, thought to be extended defects. The top right hand corner (4) is unimplanted where the sample was masked during implantation and the much clearer region at the top left of the picture (2) is the region below the implanted surface, which appears clearer due to radiation enhanced annealing from the implantation process. This annealing extends to a depth of many tens of microns below the sample surface, which is significantly further than the range of the helium ions. This is in agreement with previous results obtained by topography and with Raman spectroscopy which showed annealing to occur to a depth of 90 pm [6], and with similar work on implanted lithium tantalate [7]. In between the mottled, bulk material and the much clearer annealed region, there is a thin line a few microns wide (3) considerably darker than any other part of the topograph. This corresponds to the high concentration of defects which have been swept out from the region above. It appears that implantation leads to crystal lattice improvement in the waveguiding region, with some defects swept out to the surface where they are lost and some to a depth of several tens of microns below the surface.
unimplanted corner is still present in the bottom left of the picture (1) but the implanted surface now appears very non-uniform. It is known that implantation dramatically enhances the etch rate of lithium niobate [5,8], thus once the etchant reaches the damaged region from the crystalline guiding region, it will etch through significantly more quickly. What we are seeing in Fig. 2 is evidence that we have stopped the etching at the end of the guiding layer, just as it is beginning to rapidly attack some areas of the damaged layer. The darker shades (2) are thus areas of low defect density as would be found in the waveguiding region, while the clear areas (3) correspond to amorphous material in the damaged layer. After 5 hours, all the damaged layer has been removed and the etchant is attacking the substrate material at the depth where the helium ions have come to rest, just beyond the layer of nuclear damage. This is evident in Fig. 3 which shows the (30.0) reflection. Here uniformity has returned to the majority of the picture (1) but is of a darker shade than the unimplanted corner (21, indicating a reduc-
3.2. Topography-surface Topographs were also taken of the etched faces. The two shallowest etched pieces led to topographs with a uniform appearance and a clear distinction between the lighter implanted area and the more dense unimplanted corner of the sample, confirming the improvement of crystal structure in the guiding region. Fig. 2 shows the (30.0) reflection from the sample etched for 4 h. The
0’
Fig. 3. Surface topograph. 5 h.
(30.0)-implanted
mm
‘5
lithium niobate, etched
R.S. Lowther-Harris et al. /Nucl. Instr. and Meth. in Phys. Res. B 97 (1995) 337-341
339
Fig. 4. (00.6) peak-unimplanted lithium niobate, unetched. (Inset) Detail of Bragg peak to show side feature
tion in primary extinction [9]. The X-ray penetration depth here is around 100 A. The far lighter region around the unimplanted corner was above the etchant but was at-
Fig. 5. (00.6) peak-implanted
tacked by the HF vapour. This led to a reduced etch depth, which appears to correspond to the damaged layer in the light area (3).
lithium niobate, shallow etched.
VII. ELECTRONICS
MATERIALS
340
R.S. Lowther-Harris et
Instr. and Meth. in Phys. Res. i3 97 (1995) 337-341
Fig. 6. @0.6) peak-implanted lithium niobate, etched 5 h. 3.3. Difiaction
4. Conclusion
Fig. 4 shows the (00.6) Bragg peak for the lithium niobate sample before implantation. This exhibits splitting with a distinct side peak parallel to the main peak (inset). After implantation, the sample was studied again and the splitting of the Bragg peak was found to have disappeared. This is consistent with an improvement of the crystal lattice in the form of a reduced mosaic structure and lower defect density as a result of the annealing caused by the implantation process, as seen from the topography results. In the sample etched to a depth of _ 125 A (15 min), the Bragg peak is sharply defined, Fig. 5, which is to be expected as we have etched just below the surface of the guiding region. The X-ray penetration depth is 360 A. After etching for 5 h we would expect to be at the far side of the nuclear damage, on the helium ion sites. Diffractometry of this region should indicate increased defect density in the form of a distorted Bragg peak. Fig. 6 shows the (00.6) Bragg peak from this sample which is indeed broader than Fig. 5 and exhibits splitting of 0.006 A-’ in both the [OO.l] and [hh.O] directions. The half width of the Bragg peak in Fig. 6 is four times that of Fig. 5, in the [hh.O] direction and the ratio of intensities between Figs. 4, 5 and 6 is 3:2:1 respectively. All 4 etched pieces were studied by topography and it was found that the annealed region was present in each. This observation and the Bragg peak being much sharper for the shallowest etched, implanted sample, leads us to infer that the etching process itself has not done any damage to the remaining crystal.
We have looked for the first time directly at the structure of the waveguiding region and implanted layer of ion-implanted optical waveguides in lithium niobate. We have found an improvement of the crystal lattice in the waveguiding region and a reduction in long range crystallinity in the nuclear damaged layer, implying structural damage caused by the implantation. These observations correlate with optical refractive index changes for the same regions which give rise to the waveguiding properties. We have also seen for the first time, evidence of annealing effects far beyond the implanted layer, extending tens of microns into the bulk material, as a result of defects having been swept away by the implantation and resulting in a narrow concentration of defects a few microns thick at the edge of this annealed region. This radiation enhanced annealing may contribute to the refractive index anomalies in the waveguides.
Acknowledgements The authors wish to thank Dr. D. Laundy and Dr. S.P. Collins for assistance with data collection. This work is supported by the EPSRC.
References [ll L. Zhang, P.J. Chandler and P.D. Townsend, J. Appl. Phys. 70 (1991) 1185.
R.S. Lowther-Harris et al. /Nucl. Instr. and Meth. in Phys. Res. B 97 (1995) 337-341 [2] G.T. Reed and A.L. Weiss, Nucl. Instr. and Meth. B 19/20 (1987) 907. [3] L. Zhang, P.J. Chandler and P.D. Townsend, Nucl. Instr. and Meth. B 59/60 (1991) 1147. [4] G.R. Fisher and P. Barnes, Phil. Mag. B 61 (1990) 217. [5] M. Kawabe, M. Kubota, K. Masuda and S. Namba, J. Vat. Sci. Technol. 15 (1978) 1096. [6] SD. Brown, R.S. Lowther-Harris, P.W. Haycock, J.F. Kelly, P.J. Chandler. P. Barnes and M.E. Unwin, to be published.
341
[7] R.S. Lowther-Harris, SD. Brown, P.W. Haycock and R.P. Findlay, to be published. [8] G. Gotz and H. Karge, Nucl. Instr. and Meth. 209/210 (1983) 1079. [9] B.D. Cullity, Elements of X-ray diffraction. 2nd ed. (Addison-Wesley, 1978) p. 270.
VII. ELECTRONICS
MATERIALS
a3__ __
in
Physics Research B 97 (1995) 337-341 NOM
B
Beam Interactions with Materials 8 Atoms
@ ELSEVIER
Complementary X-ray topography and near-surface diffraction for investigations into the structure of ion-implanted optical waveguides
*,
R.S. Lowther-Harris a,b, S.D. Brown a, P.W. Haycock a, P.J. Chandler ‘, L. Zhang ‘, C.C. Tang b, R.P. Findlay a, L. Babsail ‘, M. Rodman ‘, J. Knight ‘, P.D. Townsend ’ ” DrparlmentofPhysics,School of Scienceand Engineering, Keele University, Keele, Stuflordshire, ST5 SBG UK h DRAL, Daresbury Laboratory, Daresbury. Warrington, Cheshire. WA4 3AD, UK ’ School of Mathematical and Physical Sciences, Unirtersity of Sussex. Falmer, East Sussex, BNI 9QH. UK
Abstract Ion implantation is now a well established technique for the production of applications. However, in some materials, such as lithium niobate, it leads waveguiding region. We have investigated the structure of the waveguide and the niobate by carrying out complementary X-ray topography and near surface X-ray indicate radiation enhanced annealing in the waveguide itself and the upper region of amorphisation in the damaged layer.
1. Introduction In lithium niobate (LiNbO,), waveguide fabrication by implantation with helium ions produces refractive index anomalies in the waveguiding layer which have yet to be fully explained. It has been suggested these may be due to lithium diffusion towards the nuclear damage region [l]. In order to examine directly the structure and defects deeper in the waveguide region and into the damaged layer itself, the surface of an implanted crystal was progressively etched with hydrofluoric acid and the samples were examined using the complementary techniques of X-ray topography and near-surface X-ray diffraction, both at the DRAL SRS.
2. Experimental
details
All experiments were performed on a single crystal of y-cut LiNbO, 45 X 40 X 0.8 mm, with one large face polished and all other faces ground. The sample was implanted with He+ ions at room temperature to a dose of 1.5 X lCIlh ions cm-‘, at an energy of 1.5 MeV. The ordinary refractive index profile closely follows the structural damage profile caused by implantation and peaks under these conditions at a depth of 3.35 p,rn [2,3].
* Corresponding author. Tel. +44 782 621111, ext. 7923, fax + 44 782 711093. e-mail: phd53Qpotter.cc.keeIe.ac.uk. 0168-583X/95/$09.50
optical waveguides for integrated optics to anomalous optical properties in the damaged region in ion-implanted lithium diffraction on etched crystals. The results of the substrate, as well as a high degree
The topography experiments were carried out on station 7.6 of the DRAL SRS using the arrangement described by Fisher and Barnes [4]. Grazing incidence diffraction experiments were performed on station 2.3 at the DRAL SRS, which was modified from its usual powder diffraction geometry to allow in-plane diffraction measurements with the [OO.l] direction aligned in the scattering plane. All diffraction results were obtained as multiple w scans, stepping 20 between each scan, before being converted to reciprocal space grids in hexagonal units, LiNbO, having a rhombohedral lattice. Initial diffraction results were obtained from unimplanted LiNbO, as a reference. After implantation, the sample was cut into 4 equal size pieces and each piece was etched to a different depth. Kawabe et al., [s], showed that a 48% HF solution at 50°C etches unimplanted LiNbO, crystal at the rate of 8 nm min-‘. This was verified by our trials which gave a figure of approximately 1 pm removal after 2 h. The 4 pieces were etched for l/4, 2, 4 and 5 h respectively in order to reveal successively more of the waveguiding region and, finally, the damaged layer itself.
3. Results and discussion 3.1. Topography-edge An edge of each etched piece was examined using white beam X-ray topography, a technique which highlights defects in crystals. Generally the darker the reflected
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0168-583X(94)00394-7
VII. ELECTRONICS MATERIALS
R.S. Lowrher-Harris et al. /Nucl. Instr. and Meth. in Phys. Res. B 97 (1995) 337-341
338
Fig. 1. Edge topograph. (iO.S)-implanted lithium niobate
Fig. 2. Surface topograph. (jO.O)-implanted lithium niobate, etched 4 h. pattern, the greater the number of extended defects, although amorphous material also produces light images since it does not lead to Bragg diffraction. Fig. 1 shows a (70.8) reflection topograph from the implanted sample edge. It can be seen that the sample is split into distinct regions: (1) a mottled region; (2) a clearer region which appears to be missing; and (3) a thin black line separating the two. The mottled region represents diffraction from the bulk material with its inherent distribution of defects, thought to be extended defects. The top right hand corner (4) is unimplanted where the sample was masked during implantation and the much clearer region at the top left of the picture (2) is the region below the implanted surface, which appears clearer due to radiation enhanced annealing from the implantation process. This annealing extends to a depth of many tens of microns below the sample surface, which is significantly further than the range of the helium ions. This is in agreement with previous results obtained by topography and with Raman spectroscopy which showed annealing to occur to a depth of 90 pm [6], and with similar work on implanted lithium tantalate [7]. In between the mottled, bulk material and the much clearer annealed region, there is a thin line a few microns wide (3) considerably darker than any other part of the topograph. This corresponds to the high concentration of defects which have been swept out from the region above. It appears that implantation leads to crystal lattice improvement in the waveguiding region, with some defects swept out to the surface where they are lost and some to a depth of several tens of microns below the surface.
unimplanted corner is still present in the bottom left of the picture (1) but the implanted surface now appears very non-uniform. It is known that implantation dramatically enhances the etch rate of lithium niobate [5,8], thus once the etchant reaches the damaged region from the crystalline guiding region, it will etch through significantly more quickly. What we are seeing in Fig. 2 is evidence that we have stopped the etching at the end of the guiding layer, just as it is beginning to rapidly attack some areas of the damaged layer. The darker shades (2) are thus areas of low defect density as would be found in the waveguiding region, while the clear areas (3) correspond to amorphous material in the damaged layer. After 5 hours, all the damaged layer has been removed and the etchant is attacking the substrate material at the depth where the helium ions have come to rest, just beyond the layer of nuclear damage. This is evident in Fig. 3 which shows the (30.0) reflection. Here uniformity has returned to the majority of the picture (1) but is of a darker shade than the unimplanted corner (21, indicating a reduc-
3.2. Topography-surface Topographs were also taken of the etched faces. The two shallowest etched pieces led to topographs with a uniform appearance and a clear distinction between the lighter implanted area and the more dense unimplanted corner of the sample, confirming the improvement of crystal structure in the guiding region. Fig. 2 shows the (30.0) reflection from the sample etched for 4 h. The
0’
Fig. 3. Surface topograph. 5 h.
(30.0)-implanted
mm
‘5
lithium niobate, etched
R.S. Lowther-Harris et al. /Nucl. Instr. and Meth. in Phys. Res. B 97 (1995) 337-341
339
Fig. 4. (00.6) peak-unimplanted lithium niobate, unetched. (Inset) Detail of Bragg peak to show side feature
tion in primary extinction [9]. The X-ray penetration depth here is around 100 A. The far lighter region around the unimplanted corner was above the etchant but was at-
Fig. 5. (00.6) peak-implanted
tacked by the HF vapour. This led to a reduced etch depth, which appears to correspond to the damaged layer in the light area (3).
lithium niobate, shallow etched.
VII. ELECTRONICS
MATERIALS
340
R.S. Lowther-Harris et
Instr. and Meth. in Phys. Res. i3 97 (1995) 337-341
Fig. 6. @0.6) peak-implanted lithium niobate, etched 5 h. 3.3. Difiaction
4. Conclusion
Fig. 4 shows the (00.6) Bragg peak for the lithium niobate sample before implantation. This exhibits splitting with a distinct side peak parallel to the main peak (inset). After implantation, the sample was studied again and the splitting of the Bragg peak was found to have disappeared. This is consistent with an improvement of the crystal lattice in the form of a reduced mosaic structure and lower defect density as a result of the annealing caused by the implantation process, as seen from the topography results. In the sample etched to a depth of _ 125 A (15 min), the Bragg peak is sharply defined, Fig. 5, which is to be expected as we have etched just below the surface of the guiding region. The X-ray penetration depth is 360 A. After etching for 5 h we would expect to be at the far side of the nuclear damage, on the helium ion sites. Diffractometry of this region should indicate increased defect density in the form of a distorted Bragg peak. Fig. 6 shows the (00.6) Bragg peak from this sample which is indeed broader than Fig. 5 and exhibits splitting of 0.006 A-’ in both the [OO.l] and [hh.O] directions. The half width of the Bragg peak in Fig. 6 is four times that of Fig. 5, in the [hh.O] direction and the ratio of intensities between Figs. 4, 5 and 6 is 3:2:1 respectively. All 4 etched pieces were studied by topography and it was found that the annealed region was present in each. This observation and the Bragg peak being much sharper for the shallowest etched, implanted sample, leads us to infer that the etching process itself has not done any damage to the remaining crystal.
We have looked for the first time directly at the structure of the waveguiding region and implanted layer of ion-implanted optical waveguides in lithium niobate. We have found an improvement of the crystal lattice in the waveguiding region and a reduction in long range crystallinity in the nuclear damaged layer, implying structural damage caused by the implantation. These observations correlate with optical refractive index changes for the same regions which give rise to the waveguiding properties. We have also seen for the first time, evidence of annealing effects far beyond the implanted layer, extending tens of microns into the bulk material, as a result of defects having been swept away by the implantation and resulting in a narrow concentration of defects a few microns thick at the edge of this annealed region. This radiation enhanced annealing may contribute to the refractive index anomalies in the waveguides.
Acknowledgements The authors wish to thank Dr. D. Laundy and Dr. S.P. Collins for assistance with data collection. This work is supported by the EPSRC.
References [ll L. Zhang, P.J. Chandler and P.D. Townsend, J. Appl. Phys. 70 (1991) 1185.
R.S. Lowther-Harris et al. /Nucl. Instr. and Meth. in Phys. Res. B 97 (1995) 337-341 [2] G.T. Reed and A.L. Weiss, Nucl. Instr. and Meth. B 19/20 (1987) 907. [3] L. Zhang, P.J. Chandler and P.D. Townsend, Nucl. Instr. and Meth. B 59/60 (1991) 1147. [4] G.R. Fisher and P. Barnes, Phil. Mag. B 61 (1990) 217. [5] M. Kawabe, M. Kubota, K. Masuda and S. Namba, J. Vat. Sci. Technol. 15 (1978) 1096. [6] SD. Brown, R.S. Lowther-Harris, P.W. Haycock, J.F. Kelly, P.J. Chandler. P. Barnes and M.E. Unwin, to be published.
341
[7] R.S. Lowther-Harris, SD. Brown, P.W. Haycock and R.P. Findlay, to be published. [8] G. Gotz and H. Karge, Nucl. Instr. and Meth. 209/210 (1983) 1079. [9] B.D. Cullity, Elements of X-ray diffraction. 2nd ed. (Addison-Wesley, 1978) p. 270.
VII. ELECTRONICS
MATERIALS