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The modification of LiTaO3 crystal by low-energy He-ion implantation
Nuclear Instruments and Methods in Physics Research B 290 (2012) 54–58
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
The modification of LiTaO3 crystal by low-energy He-ion implantation L.L. Pang a, Z.G. Wang a,⇑, Y.F. Jin a, C.F. Yao a, M.H. Cui a,b, J.R. Sun a, T.L. Shen a,b, K.F. Wei a, Y.B. Zhu a,b, Y.B. Sheng a, Y.F. Li a,b a b
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 22 November 2011 Received in revised form 9 July 2012 Available online 8 September 2012 Keywords: Lithium tantalate He implantation Transmittance Surface Defect
a b s t r a c t The effects of He-ion implantation on the surface morphology and transmittance of LiTaO3 single crystals are investigated. The samples were implanted with 250 keV He-ion at different fluences at room temperature. The results show that the surface morphology and transmittance of implanted samples strongly depend on the ion fluence and the time when the samples expose to the air up to 60 days. When the fluence is above 1.0 1016 He+/cm2, the transmission spectra indicate that a high concentration of defects is created. 3D-profile images show that at the higher fluence a great many triangular stripes appear on the surface of the samples. After 60 days, the recovery of the transmittance occurs and varies with the fluence. For the sample at the fluence of 5.0 1016 He+/cm2, the raised stripes on the surface evolve into narrow cracks. Regional exfoliation, however, occurs on the surface of the sample with the fluence of 1.0 1017 He+/cm2. According to the experimental results and simulation of SRIM 2008 code, the evolvement of defects and the behavior of He are discussed. Published by Elsevier B.V.
1. Introduction In recent years, the behavior of He interstitials in solids has been attracting more attention because He is chemically inert and almost insoluble in most solids. It is well known that after He-ion implantation, a high concentration of defects should be created near the end of the ion range in the materials. As a result, the local, buried, compressive-strain field is formed. The visible macroscopic effects such as swelling, blister, flake and exfoliation will occur on the surface of the materials implanted by He-ion. In nuclear reactor, some structural materials will be destroyed due to such effects under the bombardment of energetic He-ion. However, many techniques related to these effects have been developed, such as smart-cut [1,2], crystal-ion slicing (CIS) [3,4], and ion-implantation optical waveguides [5,6]. Considerable effort has also been made to understand the behavior of He interstitials in many kinds of materials [7–12]. Kling et al. [13] studied the defect evolution and characterization in LiNbO3 implanted with 20 keV He-ion. Ofan et al. [14,15] studied the nature of a striking pattern of extended defects formed in heavy implantation with He-ion and spherical solid He nanometer bubbles in z-cut LiNbO3. It is found that He interstitials accumulate at the twin boundaries and the diameter of spherical He bubble is below a critical radius for balancing the surface and elastic energies as predicted by elastic theory.
⇑ Corresponding author. Tel./fax: +86 931 4969330. E-mail address: [email protected] (Z.G. Wang). 0168-583X/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.nimb.2012.09.003
Ferroelectric and piezoelectric LiTaO3 single crystal is a promising material for fabricating integrated optic devices [16] because of its excellent photorefractive, non-linear and electro-optic effects. Planar waveguides have been fabricated by He-ion implantation in LiTaO3 crystals with the optical axis perpendicular to the guiding surface [17]. It has been reported that high quality of LiTaO3 film could be fabricated successfully by H-ion implantation [18]. The purpose of this work is to study the modification of LiTaO3 crystal induced by He-ion implantation in detail by focusing the transmittance and surface morphology of the crystal after ion implantation, particularly, on understanding the evolvement of defects and the behavior of He interstitials. 2. Experiments The optical grade z-cut congruent LiTaO3 single crystal wafers, with the size of 10 10 1 mm3, were cleaned before ion-implantation. The implantation experiment was performed at room temperature with 250 keV He-ion, delivered by the 320 kV Multidiscipline Research Platform for Highly Charged Ions in Institute of Modern Physics, Chinese Academy of Sciences, at fluences ranging from 1 1015 to 1 1017 He+/cm2. During the implantation, the ion beam was scanned over the area of the samples to insure uniform implantation. The elastic collision between incident ions and atoms of target occurs during ion-implantation, forming the displacement damage (atoms) in the target. As a result, a considerable concentration of defects is created in implanted layer, such as vacancy-interstitial pairs, also known as Frenkel pairs, and im-
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3.2. The transmission spectra
Fig. 1. The damage cross section and He-ion distribution profile simulated by SRIM 2008 code vary with depth in LiTaO3.
planted ions. We have used the SRIM 2008 code [19] to simulate the damage (displacement per atom) induced by implantation and He-ion distribution in the LiTaO3 crystal implanted by 250 keV He-ion. Fig. 1 shows the damage cross section and Heion distribution profile vary with depth in LiTaO3. The properties of the implanted samples, including surface morphology and transmittance, were measured with Micro XAM-3D surface profiler and PerkinElmer Lambda 900 UV/VIS/NIR Spectrometer. The samples had been kept in the test-tubes at room temperature for 60 days in the laboratory with the humidity less than 15%. Then properties of the implanted samples were measured again in order to observe the change of the properties with the exposing time.
Fig. 4 shows the transmission spectra of the samples at different fluences. It is clear that there is an excellent transmittance of 77% in the spectrum of the un-implanted sample. In Fig. 4(a), the change of the transmittance strongly depends upon the implantation fluence. A dramatic decrease of the transmittance, however, is observed when the fluence is at 1.0 1016 He+/cm2, which suggests that many defects produced in crystal. Note that the decrease of the transmittance of implanted samples mainly occurs in the visible light region. With the increase of fluence, the transmittance continuously decreases firstly and then rises almost to the level of un-implanted sample when the fluence reaches 1.0 1017 He+/ cm2. This peculiar behavior, to our knowledge, is observed for the first time. It should be remarked that the samples are kept always at room temperature, and there is no exfoliation observed on the surface of the sample, as shown in Fig. 2(d). It should exclude the possibility that the rise of the transmittance of this sample is attributed to the change of temperature or the exfoliation of the top implanted layer. In Fig. 4(b), it is clear that the transmittance of implanted samples has recovered after 60 days. The recovery of the transmittance varies with the fluence. The transmittance curves of samples at the fluence of 1.0 1015 and 1.0 1017 He+/cm2, nearly overlapping together with that of un-implanted sample, seem that their transmittance recover completely. Typically, a dramatic recovery of the transmittance occurs in the sample at the fluence of 5.0 1016 He+/cm2. Such recovery progress was also observed in the LiTaO3 implanted with Ar-ion, after being annealed in oxygen atmosphere [23].
4. Discussion 3. Results 4.1. The defects produced by He implantation 3.1. 3D-surface morphology Fig. 2 shows 3D-surface morphology images of LiTaO3 samples at fluences of (a) un-implanted, (b) 1.0 1016, (c) 5.0 1016, (d) 1.0 1017 He+/cm2. From the Fig. 2(a), surface roughness of the un-implanted sample is only several nanometers. At the fluence of 1.0 1016 He+/cm2, no obvious change is found on surface and the sample remains the same surface roughness just as un-implanted sample. But it looks like black. Note that in Fig. 2(b) there is a sharp column of which the height is in the micrometer range. That makes this sample look like different from others. However, many stripes with three particular orientations appear on the surface of the sample implanted at the fluence of 5.0 1016 He+/cm2. These stripes, rising about a dozen of nanometers out of the surface of the sample, cross each other and present triangular patterns. When the fluence reaches 1.0 1017 He+/cm2 the stripes become closer and crack to the depth of dozens of nanometers. Such stripes which should be related to domains in the crystal [20–22] are similar to the previous results in LiNbO3 [15]. After 60 days only the surface morphology of the samples with the fluence above 5.0 1016 He+/cm2 has changed, as shown in Fig. 3. In Fig. 3(a), at the fluence of 5.0 1016 He+/cm2, the stripes have already split and become long and narrow cracks with the depth from dozens to hundreds of nanometers. In addition, there are a large number of shallow holes with depth of dozens of nanometers on the surface. While for the sample at the fluence of 1.0 1017 He+/cm2 (see Fig. 3(b)), the triangular or zonal exfoliation along previous cracks on the surface is observed and the edge of exfoliated regions is very regular. The height of steps created by exfoliating is about 0.9 lm, which is close to the He-ion range simulated by SRIM 2008 code [19].
A great number of defects could be created in the He-implanted LiTaO3 crystal, including oxygen, lithium, tantalum vacancy-interstitial pairs and impure He interstitials. The decrease of the transmittance of implanted LiTaO3 in a certain range of fluence can be interpreted in terms of oxygen vacancies (VO) produced during implantation. The VO combines with other defects, for example Li-vacancies, Ta4+, reduced metal, etc. to form the complexes of the VO [24]. These complexes and VO containing different charge contribute to the absorption of light [23,25]. Arnold et al. [28] believed that the charge state of the defects produced by displacement processes has important effects on optical absorption. But some reports [26,27] suggested that the radiation-induced absorption is attributed to VO related polarons in LiNbO3. The concentration of defects created during implantation increases with the increasing fluence, causing a continuing decrease of the transmittance. At the fluence of 1.0 1017 He+/cm2, however, the transmittance of the sample abruptly rises and as of yet, there are scarcely any similar reports found about such abnormal change. From the simulation of SRIM 2008 code, as shown in Fig. 1, near the end of ion range He concentration reaches 3 1021 cm 3, or 3.2 at.% of the LiTaO3 density, and generates a displacement of 1.8 dpa (displacement per atom). A large number of He bubbles, however, are formed in LiNbO3 when He concentration is 2 at.% [14]. And it needs only about 1 at.% in some metals [9,10]. Therefore, it is reasonable to assume that near the end of ion range a lot of defect clusters such as He bubbles, and He-platelets (see the section 4.2) are formed instead of simple vacancy defects. We speculate that the charge state of these defect clusters may be changed greatly, which leads to the weak optical absorption [28]. Note that, as many cracks in the depths under the surface
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Fig. 2. 3D-surface morphology images of LiTaO3 samples (left) at the fluences of (a) un-implanted, (b) 1.0 1016, (c) 5.0 1016, (d) 1.0 1017 He+/cm2 and the scan lines (right, obtained for the solid lines on the surface morphology images).
of this sample (see Fig. 2(d)), and in case the sample exposes to the air after being taken out of the vacuum target chamber, the oxygen in the air could easily diffuse into the sample and recombine with the Vo, leading to the annihilation of Vo [23]. Although a large amount of displacement damage was created during implantation near the surface of the sample, only a small quantity of Vo survives finally, which has very little to contribute to the optical absorption.
On the whole, a weak decrease of transmittance of the sample with 1.0 1017 He+/cm2 is observed. But it should be stressed that in the absence of sufficient evidence and analysis a further discussion is unable to be developed. After 60 days, the raised stripes on the surface of the sample with 5.0 1016 He+/cm2 become deep cracks in company with formation of many holes. As a result, the superficial area contacting
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Fig. 3. 3D-surface morphology images of LiTaO3 samples (left) after exposing the samples to air for 60 days at the fluences of (a) 5.0 1016 He+/cm2, and (b) 1.0 1017 He+/ cm2 and the scan lines (right, obtained for the solid lines on the surface morphology images).
(a)
with oxygen in the air has increased. And it is helpful for the oxygen to diffuse into the inside of the sample. This result in the recombining of oxygen with Vo in this process just like the case of the sample with 1.0 1017 He+/cm2 discussed above. Therefore the dramatic recovery of the transmittance of the sample takes place. But this kind of recombination may scarcely occur in the deeper regions near the end of ion range, since it is difficult for oxygen to diffuse deeply in the sample. Consequently, the slight recovery occurred merely for the sample at the fluence of 1.0 1016 He+/ cm2. Note that for the sample with 1.0 1017 He+/cm2, the complete recovery of the transmittance should be attributed to the exfoliation of the top implanted layer. 4.2. The behavior of He
(b)
Fig. 4. Transmission spectra of LiTaO3 samples implanted at the different fluences, (a) the spectra of as-implanted samples, (b) the spectra after exposing samples to air for 60 days. The ion fluences are given in the figures.
Domains and domain walls are a fundamental property of interest in ferroelectrics materials. In ideal ferroelectric domain walls are well accepted to be only one to two lattice units wide. However, domain walls in real ferroelectrics, showing unexpected property variations, can extend over micrometer length scales [21]. In this work, during the implantation, point defects and He interstitials are sufficiently mobile to migrate locally [29]. Therefore, during their migration, He interstitials are easy to be trapped by the walls of triangular prisms of domains which act as the structure of twin [15]. The accumulation of He interstitials at the twins leads to the increase of local stress. When the stress reaches a certain value, it will result in the large strain along the domain walls, i.e., the raised stripes, as shown in Fig. 2(c). With increasing fluence, the He interstitials continue to accumulate a higher concentration near the domain walls, meanwhile, the strain also increases. Consequently, when the concentration reaches a certain value, the stripes evolve into cracks (see Fig. 2(d)), accompanying the release of some of He. Several interesting phenomena occurring after exposure to air for 60 days can also be explained based on the understanding in
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the behavior of He. The cracking of raised stripes for the sample with 5.0 1016 He+/cm2 should be attributed to the large stress as a result of more He interstitials aggregating near the domain walls. Thus, it indicates that even at room temperature, He interstitials migrate sufficiently and are trapped by the twin boundaries. However, the large-area exfoliation occurring on the sample at the fluence of 1.0 1017 He+/cm2, is similar to the smart cut or lay exfoliation, which have been widely used to fabricate the high quality flim. Firstly, a critical fluence is needed for this exfoliation led by light ions (H/He) implantation. Then a post implantation anneal process is necessary to achieve splitting. So far, this technology, has been applied in many crystals, such as silicon, LiNbO3, InP, GaAs and KTaO3 [1–4,8]. In order to understand the influence of laboratory atmosphere on the exfoliation, we measured its components and no exotic matter was found. The air in laboratory is normal. Therefore, it is impossible to attribute the exfoliation to the reaction of the damaged surface to laboratory atmosphere. According to our results and the analysis above, three potential reasons should be responsible for this phenomenon. (1) As the implantation up to a large fluence of 1.0 1017 He+/ cm2, a high concentration of planar defects (termed ‘‘platelets’’) should be formed at the end of ion range. He-platelets, acting as nucleation sites for micro-crack formation, are the origin of micro-cracks. (2) The high concentration of He deposit (about 3.2 at.% of the LiTaO3 density according to the Fig. 1.) and the strong ability of He interstitials migration cause the He-platelets develop into micro-cracks [12]. Large inner stress drives gas-containing micro-crack the lateral propagation slowly in terms of a typical Ostwald ripening process [11,12,30] within the next 60 days at room temperature. These micro-cracks are easy to join together in the same plane because of high concentration of He-platelets. Large micro-cracks could be formed in this way. (3) The domains in the LiTaO3 limited the formation of larger micro-cracks, but made the regional splitting easily. As a result, regional exfoliation along the walls of domains occurs. This is why exfoliation presents triangular or zonal regions with regular edges. 5. Conclusions Z-cut LiTaO3 single crystals were implanted with 250 keV Heion at different fluences at room temperature. The surface morphology and transmittance of implanted LiTaO3 crystals were investigated by 3D-profiler and UV/VIS/NIR Spectrometer. The experimental results indicate that the decrease of the transmittance of LiTaO3 crystal results from the defects related to oxygen vacancies created during implantation. In a certain range of fluence, the decrease of the transmittance strongly depends on the
fluence. After the samples have exposed to the air for 60 days, the annihilation of some defects leads to different degree recovery of the transmittance of implanted samples. A relative high concentration of He interstitials is trapped at domain walls, leading to raised stripes with particular orientations along domain walls. When the concentration of He accumulates to a certain value, the raised stripes will evolve into cracks. In the case of heavy implantation with 1.0 1017 He+/cm2, regional exfoliation along cracks occurs after exposure to the air for 60 days. Acknowledgements This work is supported by National Basic Research Program of China (973 Program, 2010CB832902), National Natural Science Foundation of China (10835010), and the Chinese Academy of Sciences. The authors thank Jinyu Li for the help during the ionimplantation experiments. References [1] M. Bruel, Nucl. Instrum. Methods Phys. Res., Sect. B 108 (1996) 313. [2] M. Bruel, Electron. Lett. 31 (1995) 1201. [3] M. Levy, R.M. Osgood, R. Liu, L.E. Cross, G.S. Cargill, A. Kumar, H. Bakhru, Appl. Phys. Lett. 73 (1998) 2293. [4] T. Izuhara, R.M. Osgood, M. Levy, M.E. Reeves, Y.G. Wang, A.N. Roy, H. Bakhru, Appl. Phys. Lett. 80 (2002) 1046. [5] F. Chen, X.L. Wang, K.M. Wang, Opt. Mater. 29 (2007) 1523. [6] F. Chen, J. Appl. Phys. 106 (2009) 081101. [7] R.E. Galindo, A. van Veen, J.H. Evans, H. Schut, J.T.M. de Hosson, Nucl. Instrum. Methods Phys. Res., Sect. B 217 (2004) 262. [8] T.W. Simpson, I.V. Mitchell, G.O. Este, F.R. Shepherd, Nucl. Instrum. Methods Phys. Res., Sect. B 148 (1999) 381. [9] H. Ullmaier, E. Camus, J. Nucl. Mater. 251 (1997) 262. [10] J.D. Hunn, E.H. Lee, T.S. Byun, L.K. Mansur, J. Nucl. Mater. 282 (2000) 131. [11] B. Aspar et al., J. Electron. Mater. 30 (2001) 834. [12] M. Hartmann, H. Trinkaus, Phys. Rev. Lett. 88 (2002) 055505. [13] A. Kling, M.F. da Silva, J.C. Soares, P.F.P. Fichtner, L. Amaral, F. Zawislak, Nucl. Instrum. Methods Phys. Res., Sect. B 175 (2001) 394. [14] A. Ofan, O. Gaathon, L. Zhang, S. Bakhru, H. Bakhru, Y. Zhu, D. Welch, R.M. Osgood Jr., Phys. Rev. B 82 (2010) 104113. [15] A. Ofan, O. Gaathon, L. Zhang, K. Evans-Lutterodt, S. Bakhru, H. Bakhru, Y. Zhu, D. Welch, R.M. Osgood Jr., Phys. Rev. B 83 (2011) 064104. [16] M.H. Li, C.H. Yang, Introduction to Photorefractive Materials Science, Science Press, 2003. p. 163. [17] C. Mignotte, Nucl. Instrum. Methods Phys. Res., Sect. B 229 (2005) 55. [18] W. Liu, D. Zhan, X. Ma, J. Vac. Sci. Technol., B 26 (2008) 206. [19] J.F. Ziegler, SRIM 2008. Available from:
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
The modification of LiTaO3 crystal by low-energy He-ion implantation L.L. Pang a, Z.G. Wang a,⇑, Y.F. Jin a, C.F. Yao a, M.H. Cui a,b, J.R. Sun a, T.L. Shen a,b, K.F. Wei a, Y.B. Zhu a,b, Y.B. Sheng a, Y.F. Li a,b a b
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 22 November 2011 Received in revised form 9 July 2012 Available online 8 September 2012 Keywords: Lithium tantalate He implantation Transmittance Surface Defect
a b s t r a c t The effects of He-ion implantation on the surface morphology and transmittance of LiTaO3 single crystals are investigated. The samples were implanted with 250 keV He-ion at different fluences at room temperature. The results show that the surface morphology and transmittance of implanted samples strongly depend on the ion fluence and the time when the samples expose to the air up to 60 days. When the fluence is above 1.0 1016 He+/cm2, the transmission spectra indicate that a high concentration of defects is created. 3D-profile images show that at the higher fluence a great many triangular stripes appear on the surface of the samples. After 60 days, the recovery of the transmittance occurs and varies with the fluence. For the sample at the fluence of 5.0 1016 He+/cm2, the raised stripes on the surface evolve into narrow cracks. Regional exfoliation, however, occurs on the surface of the sample with the fluence of 1.0 1017 He+/cm2. According to the experimental results and simulation of SRIM 2008 code, the evolvement of defects and the behavior of He are discussed. Published by Elsevier B.V.
1. Introduction In recent years, the behavior of He interstitials in solids has been attracting more attention because He is chemically inert and almost insoluble in most solids. It is well known that after He-ion implantation, a high concentration of defects should be created near the end of the ion range in the materials. As a result, the local, buried, compressive-strain field is formed. The visible macroscopic effects such as swelling, blister, flake and exfoliation will occur on the surface of the materials implanted by He-ion. In nuclear reactor, some structural materials will be destroyed due to such effects under the bombardment of energetic He-ion. However, many techniques related to these effects have been developed, such as smart-cut [1,2], crystal-ion slicing (CIS) [3,4], and ion-implantation optical waveguides [5,6]. Considerable effort has also been made to understand the behavior of He interstitials in many kinds of materials [7–12]. Kling et al. [13] studied the defect evolution and characterization in LiNbO3 implanted with 20 keV He-ion. Ofan et al. [14,15] studied the nature of a striking pattern of extended defects formed in heavy implantation with He-ion and spherical solid He nanometer bubbles in z-cut LiNbO3. It is found that He interstitials accumulate at the twin boundaries and the diameter of spherical He bubble is below a critical radius for balancing the surface and elastic energies as predicted by elastic theory.
⇑ Corresponding author. Tel./fax: +86 931 4969330. E-mail address: [email protected] (Z.G. Wang). 0168-583X/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.nimb.2012.09.003
Ferroelectric and piezoelectric LiTaO3 single crystal is a promising material for fabricating integrated optic devices [16] because of its excellent photorefractive, non-linear and electro-optic effects. Planar waveguides have been fabricated by He-ion implantation in LiTaO3 crystals with the optical axis perpendicular to the guiding surface [17]. It has been reported that high quality of LiTaO3 film could be fabricated successfully by H-ion implantation [18]. The purpose of this work is to study the modification of LiTaO3 crystal induced by He-ion implantation in detail by focusing the transmittance and surface morphology of the crystal after ion implantation, particularly, on understanding the evolvement of defects and the behavior of He interstitials. 2. Experiments The optical grade z-cut congruent LiTaO3 single crystal wafers, with the size of 10 10 1 mm3, were cleaned before ion-implantation. The implantation experiment was performed at room temperature with 250 keV He-ion, delivered by the 320 kV Multidiscipline Research Platform for Highly Charged Ions in Institute of Modern Physics, Chinese Academy of Sciences, at fluences ranging from 1 1015 to 1 1017 He+/cm2. During the implantation, the ion beam was scanned over the area of the samples to insure uniform implantation. The elastic collision between incident ions and atoms of target occurs during ion-implantation, forming the displacement damage (atoms) in the target. As a result, a considerable concentration of defects is created in implanted layer, such as vacancy-interstitial pairs, also known as Frenkel pairs, and im-
L.L. Pang et al. / Nuclear Instruments and Methods in Physics Research B 290 (2012) 54–58
55
3.2. The transmission spectra
Fig. 1. The damage cross section and He-ion distribution profile simulated by SRIM 2008 code vary with depth in LiTaO3.
planted ions. We have used the SRIM 2008 code [19] to simulate the damage (displacement per atom) induced by implantation and He-ion distribution in the LiTaO3 crystal implanted by 250 keV He-ion. Fig. 1 shows the damage cross section and Heion distribution profile vary with depth in LiTaO3. The properties of the implanted samples, including surface morphology and transmittance, were measured with Micro XAM-3D surface profiler and PerkinElmer Lambda 900 UV/VIS/NIR Spectrometer. The samples had been kept in the test-tubes at room temperature for 60 days in the laboratory with the humidity less than 15%. Then properties of the implanted samples were measured again in order to observe the change of the properties with the exposing time.
Fig. 4 shows the transmission spectra of the samples at different fluences. It is clear that there is an excellent transmittance of 77% in the spectrum of the un-implanted sample. In Fig. 4(a), the change of the transmittance strongly depends upon the implantation fluence. A dramatic decrease of the transmittance, however, is observed when the fluence is at 1.0 1016 He+/cm2, which suggests that many defects produced in crystal. Note that the decrease of the transmittance of implanted samples mainly occurs in the visible light region. With the increase of fluence, the transmittance continuously decreases firstly and then rises almost to the level of un-implanted sample when the fluence reaches 1.0 1017 He+/ cm2. This peculiar behavior, to our knowledge, is observed for the first time. It should be remarked that the samples are kept always at room temperature, and there is no exfoliation observed on the surface of the sample, as shown in Fig. 2(d). It should exclude the possibility that the rise of the transmittance of this sample is attributed to the change of temperature or the exfoliation of the top implanted layer. In Fig. 4(b), it is clear that the transmittance of implanted samples has recovered after 60 days. The recovery of the transmittance varies with the fluence. The transmittance curves of samples at the fluence of 1.0 1015 and 1.0 1017 He+/cm2, nearly overlapping together with that of un-implanted sample, seem that their transmittance recover completely. Typically, a dramatic recovery of the transmittance occurs in the sample at the fluence of 5.0 1016 He+/cm2. Such recovery progress was also observed in the LiTaO3 implanted with Ar-ion, after being annealed in oxygen atmosphere [23].
4. Discussion 3. Results 4.1. The defects produced by He implantation 3.1. 3D-surface morphology Fig. 2 shows 3D-surface morphology images of LiTaO3 samples at fluences of (a) un-implanted, (b) 1.0 1016, (c) 5.0 1016, (d) 1.0 1017 He+/cm2. From the Fig. 2(a), surface roughness of the un-implanted sample is only several nanometers. At the fluence of 1.0 1016 He+/cm2, no obvious change is found on surface and the sample remains the same surface roughness just as un-implanted sample. But it looks like black. Note that in Fig. 2(b) there is a sharp column of which the height is in the micrometer range. That makes this sample look like different from others. However, many stripes with three particular orientations appear on the surface of the sample implanted at the fluence of 5.0 1016 He+/cm2. These stripes, rising about a dozen of nanometers out of the surface of the sample, cross each other and present triangular patterns. When the fluence reaches 1.0 1017 He+/cm2 the stripes become closer and crack to the depth of dozens of nanometers. Such stripes which should be related to domains in the crystal [20–22] are similar to the previous results in LiNbO3 [15]. After 60 days only the surface morphology of the samples with the fluence above 5.0 1016 He+/cm2 has changed, as shown in Fig. 3. In Fig. 3(a), at the fluence of 5.0 1016 He+/cm2, the stripes have already split and become long and narrow cracks with the depth from dozens to hundreds of nanometers. In addition, there are a large number of shallow holes with depth of dozens of nanometers on the surface. While for the sample at the fluence of 1.0 1017 He+/cm2 (see Fig. 3(b)), the triangular or zonal exfoliation along previous cracks on the surface is observed and the edge of exfoliated regions is very regular. The height of steps created by exfoliating is about 0.9 lm, which is close to the He-ion range simulated by SRIM 2008 code [19].
A great number of defects could be created in the He-implanted LiTaO3 crystal, including oxygen, lithium, tantalum vacancy-interstitial pairs and impure He interstitials. The decrease of the transmittance of implanted LiTaO3 in a certain range of fluence can be interpreted in terms of oxygen vacancies (VO) produced during implantation. The VO combines with other defects, for example Li-vacancies, Ta4+, reduced metal, etc. to form the complexes of the VO [24]. These complexes and VO containing different charge contribute to the absorption of light [23,25]. Arnold et al. [28] believed that the charge state of the defects produced by displacement processes has important effects on optical absorption. But some reports [26,27] suggested that the radiation-induced absorption is attributed to VO related polarons in LiNbO3. The concentration of defects created during implantation increases with the increasing fluence, causing a continuing decrease of the transmittance. At the fluence of 1.0 1017 He+/cm2, however, the transmittance of the sample abruptly rises and as of yet, there are scarcely any similar reports found about such abnormal change. From the simulation of SRIM 2008 code, as shown in Fig. 1, near the end of ion range He concentration reaches 3 1021 cm 3, or 3.2 at.% of the LiTaO3 density, and generates a displacement of 1.8 dpa (displacement per atom). A large number of He bubbles, however, are formed in LiNbO3 when He concentration is 2 at.% [14]. And it needs only about 1 at.% in some metals [9,10]. Therefore, it is reasonable to assume that near the end of ion range a lot of defect clusters such as He bubbles, and He-platelets (see the section 4.2) are formed instead of simple vacancy defects. We speculate that the charge state of these defect clusters may be changed greatly, which leads to the weak optical absorption [28]. Note that, as many cracks in the depths under the surface
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Fig. 2. 3D-surface morphology images of LiTaO3 samples (left) at the fluences of (a) un-implanted, (b) 1.0 1016, (c) 5.0 1016, (d) 1.0 1017 He+/cm2 and the scan lines (right, obtained for the solid lines on the surface morphology images).
of this sample (see Fig. 2(d)), and in case the sample exposes to the air after being taken out of the vacuum target chamber, the oxygen in the air could easily diffuse into the sample and recombine with the Vo, leading to the annihilation of Vo [23]. Although a large amount of displacement damage was created during implantation near the surface of the sample, only a small quantity of Vo survives finally, which has very little to contribute to the optical absorption.
On the whole, a weak decrease of transmittance of the sample with 1.0 1017 He+/cm2 is observed. But it should be stressed that in the absence of sufficient evidence and analysis a further discussion is unable to be developed. After 60 days, the raised stripes on the surface of the sample with 5.0 1016 He+/cm2 become deep cracks in company with formation of many holes. As a result, the superficial area contacting
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Fig. 3. 3D-surface morphology images of LiTaO3 samples (left) after exposing the samples to air for 60 days at the fluences of (a) 5.0 1016 He+/cm2, and (b) 1.0 1017 He+/ cm2 and the scan lines (right, obtained for the solid lines on the surface morphology images).
(a)
with oxygen in the air has increased. And it is helpful for the oxygen to diffuse into the inside of the sample. This result in the recombining of oxygen with Vo in this process just like the case of the sample with 1.0 1017 He+/cm2 discussed above. Therefore the dramatic recovery of the transmittance of the sample takes place. But this kind of recombination may scarcely occur in the deeper regions near the end of ion range, since it is difficult for oxygen to diffuse deeply in the sample. Consequently, the slight recovery occurred merely for the sample at the fluence of 1.0 1016 He+/ cm2. Note that for the sample with 1.0 1017 He+/cm2, the complete recovery of the transmittance should be attributed to the exfoliation of the top implanted layer. 4.2. The behavior of He
(b)
Fig. 4. Transmission spectra of LiTaO3 samples implanted at the different fluences, (a) the spectra of as-implanted samples, (b) the spectra after exposing samples to air for 60 days. The ion fluences are given in the figures.
Domains and domain walls are a fundamental property of interest in ferroelectrics materials. In ideal ferroelectric domain walls are well accepted to be only one to two lattice units wide. However, domain walls in real ferroelectrics, showing unexpected property variations, can extend over micrometer length scales [21]. In this work, during the implantation, point defects and He interstitials are sufficiently mobile to migrate locally [29]. Therefore, during their migration, He interstitials are easy to be trapped by the walls of triangular prisms of domains which act as the structure of twin [15]. The accumulation of He interstitials at the twins leads to the increase of local stress. When the stress reaches a certain value, it will result in the large strain along the domain walls, i.e., the raised stripes, as shown in Fig. 2(c). With increasing fluence, the He interstitials continue to accumulate a higher concentration near the domain walls, meanwhile, the strain also increases. Consequently, when the concentration reaches a certain value, the stripes evolve into cracks (see Fig. 2(d)), accompanying the release of some of He. Several interesting phenomena occurring after exposure to air for 60 days can also be explained based on the understanding in
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the behavior of He. The cracking of raised stripes for the sample with 5.0 1016 He+/cm2 should be attributed to the large stress as a result of more He interstitials aggregating near the domain walls. Thus, it indicates that even at room temperature, He interstitials migrate sufficiently and are trapped by the twin boundaries. However, the large-area exfoliation occurring on the sample at the fluence of 1.0 1017 He+/cm2, is similar to the smart cut or lay exfoliation, which have been widely used to fabricate the high quality flim. Firstly, a critical fluence is needed for this exfoliation led by light ions (H/He) implantation. Then a post implantation anneal process is necessary to achieve splitting. So far, this technology, has been applied in many crystals, such as silicon, LiNbO3, InP, GaAs and KTaO3 [1–4,8]. In order to understand the influence of laboratory atmosphere on the exfoliation, we measured its components and no exotic matter was found. The air in laboratory is normal. Therefore, it is impossible to attribute the exfoliation to the reaction of the damaged surface to laboratory atmosphere. According to our results and the analysis above, three potential reasons should be responsible for this phenomenon. (1) As the implantation up to a large fluence of 1.0 1017 He+/ cm2, a high concentration of planar defects (termed ‘‘platelets’’) should be formed at the end of ion range. He-platelets, acting as nucleation sites for micro-crack formation, are the origin of micro-cracks. (2) The high concentration of He deposit (about 3.2 at.% of the LiTaO3 density according to the Fig. 1.) and the strong ability of He interstitials migration cause the He-platelets develop into micro-cracks [12]. Large inner stress drives gas-containing micro-crack the lateral propagation slowly in terms of a typical Ostwald ripening process [11,12,30] within the next 60 days at room temperature. These micro-cracks are easy to join together in the same plane because of high concentration of He-platelets. Large micro-cracks could be formed in this way. (3) The domains in the LiTaO3 limited the formation of larger micro-cracks, but made the regional splitting easily. As a result, regional exfoliation along the walls of domains occurs. This is why exfoliation presents triangular or zonal regions with regular edges. 5. Conclusions Z-cut LiTaO3 single crystals were implanted with 250 keV Heion at different fluences at room temperature. The surface morphology and transmittance of implanted LiTaO3 crystals were investigated by 3D-profiler and UV/VIS/NIR Spectrometer. The experimental results indicate that the decrease of the transmittance of LiTaO3 crystal results from the defects related to oxygen vacancies created during implantation. In a certain range of fluence, the decrease of the transmittance strongly depends on the
fluence. After the samples have exposed to the air for 60 days, the annihilation of some defects leads to different degree recovery of the transmittance of implanted samples. A relative high concentration of He interstitials is trapped at domain walls, leading to raised stripes with particular orientations along domain walls. When the concentration of He accumulates to a certain value, the raised stripes will evolve into cracks. In the case of heavy implantation with 1.0 1017 He+/cm2, regional exfoliation along cracks occurs after exposure to the air for 60 days. Acknowledgements This work is supported by National Basic Research Program of China (973 Program, 2010CB832902), National Natural Science Foundation of China (10835010), and the Chinese Academy of Sciences. The authors thank Jinyu Li for the help during the ionimplantation experiments. References [1] M. Bruel, Nucl. Instrum. Methods Phys. Res., Sect. B 108 (1996) 313. [2] M. Bruel, Electron. Lett. 31 (1995) 1201. [3] M. Levy, R.M. Osgood, R. Liu, L.E. Cross, G.S. Cargill, A. Kumar, H. Bakhru, Appl. Phys. Lett. 73 (1998) 2293. [4] T. Izuhara, R.M. Osgood, M. Levy, M.E. Reeves, Y.G. Wang, A.N. Roy, H. Bakhru, Appl. Phys. Lett. 80 (2002) 1046. [5] F. Chen, X.L. Wang, K.M. Wang, Opt. Mater. 29 (2007) 1523. [6] F. Chen, J. Appl. Phys. 106 (2009) 081101. [7] R.E. Galindo, A. van Veen, J.H. Evans, H. Schut, J.T.M. de Hosson, Nucl. Instrum. Methods Phys. Res., Sect. B 217 (2004) 262. [8] T.W. Simpson, I.V. Mitchell, G.O. Este, F.R. Shepherd, Nucl. Instrum. Methods Phys. Res., Sect. B 148 (1999) 381. [9] H. Ullmaier, E. Camus, J. Nucl. Mater. 251 (1997) 262. [10] J.D. Hunn, E.H. Lee, T.S. Byun, L.K. Mansur, J. Nucl. Mater. 282 (2000) 131. [11] B. Aspar et al., J. Electron. Mater. 30 (2001) 834. [12] M. Hartmann, H. Trinkaus, Phys. Rev. Lett. 88 (2002) 055505. [13] A. Kling, M.F. da Silva, J.C. Soares, P.F.P. Fichtner, L. Amaral, F. Zawislak, Nucl. Instrum. Methods Phys. Res., Sect. B 175 (2001) 394. [14] A. Ofan, O. Gaathon, L. Zhang, S. Bakhru, H. Bakhru, Y. Zhu, D. Welch, R.M. Osgood Jr., Phys. Rev. B 82 (2010) 104113. [15] A. Ofan, O. Gaathon, L. Zhang, K. Evans-Lutterodt, S. Bakhru, H. Bakhru, Y. Zhu, D. Welch, R.M. Osgood Jr., Phys. Rev. B 83 (2011) 064104. [16] M.H. Li, C.H. Yang, Introduction to Photorefractive Materials Science, Science Press, 2003. p. 163. [17] C. Mignotte, Nucl. Instrum. Methods Phys. Res., Sect. B 229 (2005) 55. [18] W. Liu, D. Zhan, X. Ma, J. Vac. Sci. Technol., B 26 (2008) 206. [19] J.F. Ziegler, SRIM 2008. Available from: