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A waveguiding layer formed by H+ implantation in LiNbO3
Vacuum/volume43/number 11/pages 1069 to 1070/1992
0042-207X/92$5.00+.00 @ 1992 PergamonPress Ltd
Printed in Great Britain
A w a v e g u i d i n g layer f o r m e d by H + i m p l a n t a t i o n in LiNbO3 T i a n h a o S h a o a n d X i n y u a n J i n a g , Shanghai Institute of Metallurgy, Academia Sinica, Shanghai 200050.
P R China and L Z h a n g , School of Mathematical and Physical Sciences, University of Sussex, Brighton BN1 9QH, UK and X i q i F e n g , Shanghai Institute of Ceramics, Academia Sinica, Shanghai 200050, P R China
A planar optical layer has been formed in LiNb03 crystallites by H + implanatation. The refractive index profiles have been determined using a dark-mode technique at 0.6328 Itm. H ÷ implantation has fabricated a monomode waveguide in the LiNb03 substrate, in which a real waveguide mode is guiding with TE polarized light. In the nuclear stopping region, radiation damage and an index reduction exist. But this is not true in the electronic stopping region, the lattice there is not destroyed and an index enhancement occurs. This means that an optical "well" is formed in the LiNb03 substrate to confine optical modes between the buried layer and the surface. After annealing in flowing oxygen, the optical absorption of the waveguide was decreased greatly. This waveguide is thermally stable up to 400°C and its loss is probably low ( ~ 1 dB cm-l).
1. Introduction
3. Results and discussion
Ion implantation of energetic light ions (e.g. He +) has been used to fabricate optical waveguides in many insulating materials ~ 3. Ion implantation into these materials causes refractive index changes in the implanted region as a result of radiation damage and additional physical and chemical effects on the index from the implanted ion itself 4. For the extraordinary index (ne) of H +implanted LiNbO3 crystallite, there is a damage reduction at the projected ion range from the nuclear stopping but an index enhancement in the electronic stopping region. The form of the ne profile further suggests that a combined effect of several processes forms an optical 'well' which confines the light beam between the guiding layer and the surface of the LiNbO3 substrate.
3.1. Optical waveguides. Figure 1 shows the extraordinary refractive index (ne) profiles determined for H+-implanted LiNbO3 crystallites. The results indicate that the index ne decreases in the nuclear stopping region. The depth of the index change corresponds to the projected ion range (Rp), which shows that H + implantation has formed a buried damaged layer in the LiNbO3 substrate at the end of ion's track. It is why the destructive effect in the nuclear stopping region causes the radiation damage there. However, this is not true in the electronic stopping region, where not only is the lattice hardly destroyed but the index (ne) of the surface layer is also slightly enhanced. The electronic stopping effect comes from interactions between electrons and atoms in
2. Experimental method The LiNbO3 samples of Y-cut, X-propagating crystallites, with a typical size of 30 x 5 x 1 mm were optically polished. Planar waveguide dark modes (TE) of the sample were observed and measured using a standard futile single-prism coupling technique 5, in which a laser beam at a wavelength of 0.6328 /~m is coupled into the surface layer of the crystallite. The extraordinary refractive index (no) of the unimplanted LiNbO 3 sample is 2.2031 at 0.6328/~m. Samples were then implanted uniformly with H + ions at room temperature, and at energies 180, 500 and 600 keV to dosages of 1, 3 and 5 x 10 ~6 cm 2, After implantation with a variety of ion energies and doses, samples were annealed in flowing oxygen to remove the damage from the guiding layer and compensate oxygen loss during the implantation and annealing processes. The stress of the lattice of H+-implanted LiNbO3 samples was measured using an X-ray double crystalline spectrometer.
2.0
180 keY
500 keY 600 keY
1.5
x ._>
1.0
0.5
A AA
o_--,1
2/3
j.
5
-0.5 Damage depth (pm)
Figure I. The extraordinary refractive index (no) profiles determined for H ~-implantedLiNbO3 (Y-cut), 3 x 1016cm 2. 1069
Tianhao Shao et al. W a v e g u i d i n g
layer
the lattice and the electronic excitement. Instead. this shows that the electronic excitement can e n h a n c e the e x t , a o r d i n a r y refractive index of the electronic stopping region. Hc + i m p l a n t a t i o n in LiNbO~ causes the n~ of the electronic stopping region to increase (;. As a similar result, our data show that the % profile of H + - i m p l a n t c d LiNbO.> in the case of the electronic energy, slightly increases the index but is a c c o m p a n i e d by a s h a r p nuclear d a m a g e peak, which was considered to be a confining barrier o f reduced index. D a r k - m o d e spectrum observation indicates that one s h a r p rhode with very low scattering is a real waveguide mode. However, these modes are not sufficient to be confined only by the buried barrier. It seems that a combined effect which is c o m p o s e d of the electronic excitement and the nuclear d a m a g e in the H +-implanted LiNbO~ substrate exists. This effect increases the difference in refractive index (An) between the guiding layer and the substrate, and contributes to confining optical modes. It means that an optical "well" has been formed in the near-surface region of the H + - i m p l a n t e d L i N b O : crystallite. Because the sample ends have n o t been polished for tong enough, the observation of this bright mode by the end coupling m e t h o d implies that the loss of this mode is very low, which is p r o b a b l y lower than 1 dB cm ~.
60
~T 0
103 ~Ym_CU t)
40 "S .c
#,
20
\
(a)
I
I
1O0
200
°~0%o I
,
I
300
400
500
Annealing temperature (°C) 10
H + - - L i N b O a (X - cut) A
8
o x
50 keY 1 x 1016 c m 2
6
oo ~ 4 "S
3.2. Disorder and stress of the lattice. H* i m p l a n t a t i o n induces radiation d a m a g e and stress of the lattice in LiNbO~ crystal ~. The extent of the d a m a g e and the stress of the lattice are directly p r o p o r t i o n a l to the impurity c o n c e n t r a t i o n and the density of the nuclear deposited energy. W h e n H + ions were implanted into the lattice, they collided with a t o m s there and finally became interstitial atoms. These a t o m s in the lattice were driven out of their regular positions so that some disorder occurred in the implanted region. The a c c u m u l a t i o n of a great n u m b e r of H ions m a d e the lattice relaxed and the volume e x p a n d e d in the d a m a g e d region. It seems that an index-reduced buried layer as a barrier formed in the LiNbO~ substrate, which could be used for confining modes p r o p a g a t i n g between the guiding layer and the surface. However, from the results o f annealing the samples, this is not always true in H ' - i m p l a n t e d LiNbO~ crystals. Figure 2(a) shows that the stress o f lattice decreases with the annealing temperature. W h e n the annealing t e m p e r a t u r e increased, the stress induced by H + i m p l a n t a t i o n was very sensitive to the annealing t e m p e r a t u r e and nearly exponentially decreased because of the elimination of the d a m a g e in the crystallite. However, the TE mode observation shows that the propagation c o n s t a n t (fl/k) seemed to be unchangeable even t h o u g h the annealing t e m p e r a t u r e changed fiom 350 to 4 5 0 C . According to these results, it can be concluded that TE polarized light was well confined only by a high index layer produced by H + i m p l a n t a t i o n and was not confined by an optical buried barrier. Because the buried layer here nearly disappeared due to the annealing effect so that it could hardly form a sufficient barrier as an optical 'well" to confine modes. Also, the stress of the lattice was not very sensitive to the annealing time. Figure 2(b) shows that the stress of lattice hardly changes with the annealing time. It further indicates that the guiding layer still can confine polarized light due to electronic excitement even if the buried index-reduced layer disappears s. This waveguide is not only structured by a low index
1070
.c_
2 O0
(b/ 0
I 1
I 2
I 3
A n n e a l i n g t i m e (h)
Figure 2. The stress of the lattice changes in H '-implanted LiNbO~: (a) with difl'erent annealing temperatures, (Y-cut. 50 keV, 3 x 10 ~' cm :): and (b) with different annealing times (X-cut. 50 keV, I × 10 ~" cm :).
optical barrier beneath the surface of the substratc, but is also due to the change of the proprieties in the near-surface region of the L i N b O , crystallite.
Acknowledgements The authors are grateful to the High Energy Ion h n p l a n t a t i o n G r o u p , Beijing Institute o f Semiconductor, A c a d e m i a Sinica, l\~r performing the H + i m p l a n t a t i o n , and to the Ion Beam Laboratory, Shanghai Institute of Metallurgy, Academia Sinica, for tinancial support.
References ip D Townsend, Rep Prog Phys, 50, 501 (1987). : G Gotz and H Karge, Nm'l ll~slrum ,V[elh. 209/210, 1079 (I 987). ~S M Mahdavi, P J Chandler and P D Townsend..I Phys D: Appl Phys. 22, 1354 (1989). 4A B Eaik, P J Chandler, P D Townsend and R P Webb, Radial Eft: 46, 235 (1986). >P K Tien. R Uh'ich and R J Martin, App/Phys Letl, 14, 291 (1969). "B L Weiss and C N Ahmad, Nucl Instrum Meth, B30, 51 (1988). ' Shao Tianhao, Jiang Xinyuan, Jiang Binyao and Xu Zhcngquan, 1'ro~ (.'-MRS "90, Vol 4, p 499. Elsevier, Amsterdam (1990). s p .I Chandler, L Zhang and P D Townsend, Nuel lnstrum Meth. B46, 69 (1990).
0042-207X/92$5.00+.00 @ 1992 PergamonPress Ltd
Printed in Great Britain
A w a v e g u i d i n g layer f o r m e d by H + i m p l a n t a t i o n in LiNbO3 T i a n h a o S h a o a n d X i n y u a n J i n a g , Shanghai Institute of Metallurgy, Academia Sinica, Shanghai 200050.
P R China and L Z h a n g , School of Mathematical and Physical Sciences, University of Sussex, Brighton BN1 9QH, UK and X i q i F e n g , Shanghai Institute of Ceramics, Academia Sinica, Shanghai 200050, P R China
A planar optical layer has been formed in LiNb03 crystallites by H + implanatation. The refractive index profiles have been determined using a dark-mode technique at 0.6328 Itm. H ÷ implantation has fabricated a monomode waveguide in the LiNb03 substrate, in which a real waveguide mode is guiding with TE polarized light. In the nuclear stopping region, radiation damage and an index reduction exist. But this is not true in the electronic stopping region, the lattice there is not destroyed and an index enhancement occurs. This means that an optical "well" is formed in the LiNb03 substrate to confine optical modes between the buried layer and the surface. After annealing in flowing oxygen, the optical absorption of the waveguide was decreased greatly. This waveguide is thermally stable up to 400°C and its loss is probably low ( ~ 1 dB cm-l).
1. Introduction
3. Results and discussion
Ion implantation of energetic light ions (e.g. He +) has been used to fabricate optical waveguides in many insulating materials ~ 3. Ion implantation into these materials causes refractive index changes in the implanted region as a result of radiation damage and additional physical and chemical effects on the index from the implanted ion itself 4. For the extraordinary index (ne) of H +implanted LiNbO3 crystallite, there is a damage reduction at the projected ion range from the nuclear stopping but an index enhancement in the electronic stopping region. The form of the ne profile further suggests that a combined effect of several processes forms an optical 'well' which confines the light beam between the guiding layer and the surface of the LiNbO3 substrate.
3.1. Optical waveguides. Figure 1 shows the extraordinary refractive index (ne) profiles determined for H+-implanted LiNbO3 crystallites. The results indicate that the index ne decreases in the nuclear stopping region. The depth of the index change corresponds to the projected ion range (Rp), which shows that H + implantation has formed a buried damaged layer in the LiNbO3 substrate at the end of ion's track. It is why the destructive effect in the nuclear stopping region causes the radiation damage there. However, this is not true in the electronic stopping region, where not only is the lattice hardly destroyed but the index (ne) of the surface layer is also slightly enhanced. The electronic stopping effect comes from interactions between electrons and atoms in
2. Experimental method The LiNbO3 samples of Y-cut, X-propagating crystallites, with a typical size of 30 x 5 x 1 mm were optically polished. Planar waveguide dark modes (TE) of the sample were observed and measured using a standard futile single-prism coupling technique 5, in which a laser beam at a wavelength of 0.6328 /~m is coupled into the surface layer of the crystallite. The extraordinary refractive index (no) of the unimplanted LiNbO 3 sample is 2.2031 at 0.6328/~m. Samples were then implanted uniformly with H + ions at room temperature, and at energies 180, 500 and 600 keV to dosages of 1, 3 and 5 x 10 ~6 cm 2, After implantation with a variety of ion energies and doses, samples were annealed in flowing oxygen to remove the damage from the guiding layer and compensate oxygen loss during the implantation and annealing processes. The stress of the lattice of H+-implanted LiNbO3 samples was measured using an X-ray double crystalline spectrometer.
2.0
180 keY
500 keY 600 keY
1.5
x ._>
1.0
0.5
A AA
o_--,1
2/3
j.
5
-0.5 Damage depth (pm)
Figure I. The extraordinary refractive index (no) profiles determined for H ~-implantedLiNbO3 (Y-cut), 3 x 1016cm 2. 1069
Tianhao Shao et al. W a v e g u i d i n g
layer
the lattice and the electronic excitement. Instead. this shows that the electronic excitement can e n h a n c e the e x t , a o r d i n a r y refractive index of the electronic stopping region. Hc + i m p l a n t a t i o n in LiNbO~ causes the n~ of the electronic stopping region to increase (;. As a similar result, our data show that the % profile of H + - i m p l a n t c d LiNbO.> in the case of the electronic energy, slightly increases the index but is a c c o m p a n i e d by a s h a r p nuclear d a m a g e peak, which was considered to be a confining barrier o f reduced index. D a r k - m o d e spectrum observation indicates that one s h a r p rhode with very low scattering is a real waveguide mode. However, these modes are not sufficient to be confined only by the buried barrier. It seems that a combined effect which is c o m p o s e d of the electronic excitement and the nuclear d a m a g e in the H +-implanted LiNbO~ substrate exists. This effect increases the difference in refractive index (An) between the guiding layer and the substrate, and contributes to confining optical modes. It means that an optical "well" has been formed in the near-surface region of the H + - i m p l a n t e d L i N b O : crystallite. Because the sample ends have n o t been polished for tong enough, the observation of this bright mode by the end coupling m e t h o d implies that the loss of this mode is very low, which is p r o b a b l y lower than 1 dB cm ~.
60
~T 0
103 ~Ym_CU t)
40 "S .c
#,
20
\
(a)
I
I
1O0
200
°~0%o I
,
I
300
400
500
Annealing temperature (°C) 10
H + - - L i N b O a (X - cut) A
8
o x
50 keY 1 x 1016 c m 2
6
oo ~ 4 "S
3.2. Disorder and stress of the lattice. H* i m p l a n t a t i o n induces radiation d a m a g e and stress of the lattice in LiNbO~ crystal ~. The extent of the d a m a g e and the stress of the lattice are directly p r o p o r t i o n a l to the impurity c o n c e n t r a t i o n and the density of the nuclear deposited energy. W h e n H + ions were implanted into the lattice, they collided with a t o m s there and finally became interstitial atoms. These a t o m s in the lattice were driven out of their regular positions so that some disorder occurred in the implanted region. The a c c u m u l a t i o n of a great n u m b e r of H ions m a d e the lattice relaxed and the volume e x p a n d e d in the d a m a g e d region. It seems that an index-reduced buried layer as a barrier formed in the LiNbO~ substrate, which could be used for confining modes p r o p a g a t i n g between the guiding layer and the surface. However, from the results o f annealing the samples, this is not always true in H ' - i m p l a n t e d LiNbO~ crystals. Figure 2(a) shows that the stress o f lattice decreases with the annealing temperature. W h e n the annealing t e m p e r a t u r e increased, the stress induced by H + i m p l a n t a t i o n was very sensitive to the annealing t e m p e r a t u r e and nearly exponentially decreased because of the elimination of the d a m a g e in the crystallite. However, the TE mode observation shows that the propagation c o n s t a n t (fl/k) seemed to be unchangeable even t h o u g h the annealing t e m p e r a t u r e changed fiom 350 to 4 5 0 C . According to these results, it can be concluded that TE polarized light was well confined only by a high index layer produced by H + i m p l a n t a t i o n and was not confined by an optical buried barrier. Because the buried layer here nearly disappeared due to the annealing effect so that it could hardly form a sufficient barrier as an optical 'well" to confine modes. Also, the stress of the lattice was not very sensitive to the annealing time. Figure 2(b) shows that the stress of lattice hardly changes with the annealing time. It further indicates that the guiding layer still can confine polarized light due to electronic excitement even if the buried index-reduced layer disappears s. This waveguide is not only structured by a low index
1070
.c_
2 O0
(b/ 0
I 1
I 2
I 3
A n n e a l i n g t i m e (h)
Figure 2. The stress of the lattice changes in H '-implanted LiNbO~: (a) with difl'erent annealing temperatures, (Y-cut. 50 keV, 3 x 10 ~' cm :): and (b) with different annealing times (X-cut. 50 keV, I × 10 ~" cm :).
optical barrier beneath the surface of the substratc, but is also due to the change of the proprieties in the near-surface region of the L i N b O , crystallite.
Acknowledgements The authors are grateful to the High Energy Ion h n p l a n t a t i o n G r o u p , Beijing Institute o f Semiconductor, A c a d e m i a Sinica, l\~r performing the H + i m p l a n t a t i o n , and to the Ion Beam Laboratory, Shanghai Institute of Metallurgy, Academia Sinica, for tinancial support.
References ip D Townsend, Rep Prog Phys, 50, 501 (1987). : G Gotz and H Karge, Nm'l ll~slrum ,V[elh. 209/210, 1079 (I 987). ~S M Mahdavi, P J Chandler and P D Townsend..I Phys D: Appl Phys. 22, 1354 (1989). 4A B Eaik, P J Chandler, P D Townsend and R P Webb, Radial Eft: 46, 235 (1986). >P K Tien. R Uh'ich and R J Martin, App/Phys Letl, 14, 291 (1969). "B L Weiss and C N Ahmad, Nucl Instrum Meth, B30, 51 (1988). ' Shao Tianhao, Jiang Xinyuan, Jiang Binyao and Xu Zhcngquan, 1'ro~ (.'-MRS "90, Vol 4, p 499. Elsevier, Amsterdam (1990). s p .I Chandler, L Zhang and P D Townsend, Nuel lnstrum Meth. B46, 69 (1990).