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Abstract
The formation mechanism of twinning and defects in He+/H+ ion implanted z-cut and x-cut KTP samples are studied. A series of parallel linear defects observed on the surface of ion-implanted z-cut KTP samples is related to twinning boundaries formed in the KTP structure. The implanted He+/H+ ions tend to aggregate along these twinning boundaries that are normally parallel to a (100) plane and perpendicular to an x-axis direction, where K atoms are loosely bonded. The lattice between twinning boundaries is tilted at some angle to the rest of the crystal. Implanted He+/H+ ions and target K atoms are sensitive to temperature change, and diffuse easily along z-axis in stagger paths when temperature is elevated. Various implantation parameters are employed, including different ion fluences and beam current densities, with an aim to find the optimum condition for KTP thin film exfoliation. It is found that the temperature window for layer splitting of KTP samples is small and low, and implanted He+ ions are more likely to aggregate into bubbles or cracks that layer splitting requires when implantation is performed along x-axis.
© 2017 Optical Society of America
1. Introduction
Potassium titanyl phosphate (KTiOPO4 or KTP) with excellent nonlinear optical and electro-optical properties has attracted many attentions [1, 2], and its thin film or hetero-structure is in high demand because of their applications in nonlinear optics and optoelectronic integrated devices [3, 4]. Due to the complex crystalline structure of KTP, it is difficult to use conventional film growth method, such as chemical vapor deposition, sputtering, or pulsed laser deposition, to grow KTP thin film with good single crystalline property on substrates [5–7]. Therefore, ion implantation in conjunction with wafer bonding method has been applied to fabricate high quality single crystalline thin film and hetero-structure.
Ion implantation has developed to be a promising technology for many useful applications: waveguide formation [8, 9] and materials processing or modification [10–12]. High-ion fluences implantation into a crystal is known to form a large concentration of dislocations and defects. These defects can be usefully employed to alter the lattice or chemical composition, and in some cases to form an implanted buried layer to cause exfoliation of single-crystal thin films [13–16]. The technique of implantation-assisted lift-off of thin complex oxide films, such as smart-cut [13, 14], and crystal-ion-slicing (CIS) [16], uses an implantation of high ion fluences (1016-1017 ions/cm2) with keV energy to create a thin sacrificial layer with a small straggle depth. Light ions such as H and He are chosen for implantation because they can diffuse easily in the target samples and aggregate into bubbles or cracks that layer splitting requires [17–20].
Despite we have already achieved layer-splitting in H+/He+-implanted KTP in both x-cut and z-cut [21,22], the exfoliated KTP thin films are inferior in physical properties, either too small or with destructive crystalline structure. In order to fabricate KTP thin film or hetero-structure with promising properties, a better understanding of the physics and chemistry of the material alteration resulting from ion implantation is needed. Some research has confirmed that strain and stress caused by interstitial atoms result in lattice damage [23, 24], but how the strain is formed is still unclear. In this letter, we study how the implanted He+/H+ ions distribute in KTP based on its structural character, explain twinning formation in KTP, and then provide the optimum condition for film exfoliation.
The goal of this paper is to understand the detailed response of the lattice structure to He+/H+ -implantation for both z-cut and x-cut KTP. Thus implantation with various ion fluences and beam current densities are employed in order to find the optimum condition for layer splitting. Atomic force microscopy (AFM), Rutherford backscattering spectroscopy/channeling (RBS/C) and transmission electron microscopy (TEM) were applied to examine both surface topography and inner structural modification. In this paper, a detailed study of the crystal after ion implantation in KTP is carried out, with particular emphasis on understanding the nature of He+/H+ ions distribution and defect network formation. Our results show that He+/H+ ions tend to aggregate along (100) plane, perpendicular to x-axis, and He+/H+ ions and K atoms are more likely to diffuse along z-axis in stagger paths. The temperature window for layer splitting of KTP samples is low, and ion implantation performed along x-axis into KTP samples is more likely to exfoliate KTP thin film with better property.
2. Materials and methods
200keV He+ ions at a range of fluences from 3 × 1016 to 6 × 1016 ions/cm2 were implanted into both z-cut and x-cut KTP samples at room temperature, and the beam current density was kept around 3~4 μA/cm2 during implantation. In order to investigate the impact of ion beam current on lattice damage evolution, He+ ions with ion fluence of 6 × 1016 ions/cm2 were irradiated into KTP samples at different beam current densities, which are 1, 3, 9 μA/cm2. Table 1 summarizes He+-implantation parameters. The H+-implantation parameters are shown in Ref [21]. Before implantation, all the samples with size of 5mm × 5mm × 1mm were optically polished and cleaned. During the implantation the ion beam was electrically scanned to ensure a uniform implantation over the samples, and samples were tilted by 7° off the beam direction in order to minimize the channeling effect. The implantation process was performed at LC-4 Ion Implanter in Institute of Semiconductors of Chinese Academy of Sciences. Post-implant annealing was performed on samples at temperature from 200°C up to 500°C in air ambient.
Table 1. Summary of the irradiation parameters
Damage analysis was carried out by using RBS/channeling measurement, which was performed by 2MeV He2+ ions at a scattering angle of 165° in 1.7 MV tandem accelerator of Peking University. In channeling measurements, samples were carefully angle oriented to minimize backscatter for channeling measurements, and the backscattered count (yield) was recorded as a function of channel (energy). The lattice structural modifications induced by ion implantation were detected by transmission electron microscopy (TEM) method in cross-section, which was performed using Tecnai G2 F20 S-Twin at 200kV with a field emission gun. TEM samples were prepared with conventional polishing and ion-beam milling techniques. Stopping and Range of Ion Matter (SRIM 2008) was used to simulate the vacancy distribution in ion-implanted KTP. The surface morphology of the irradiated KTP samples was observed using optical microscopy (OM) and atomic force microscopy (AFM) methods. AFM was performed under ambient conditions on a Bruker Multi-mode VIII microscope (Bruker Corporation, Billerica, MA), operating in tapping mode at a cantilever frequency of 250 ± 10 kHz.
3. Results and discussion
3.1. Surface characterization by optical microscopy and AFM
It is known that a striking series of linear defects was detected by OM and AFM on the surface of He+-implanted LiNbO3 sample, which are due to the resulting interstitials and associated defect clusters caused by high concentrations of irradiated species [25]. Under our irradiation condition, similar phenomenon was found in z-cut KTP implanted by 200keV He+ with ion fluence of 6 × 1016 ions/cm2 at 9μA/cm2, which is shown in Fig. 1.
Fig. 1 Surface morphology of He-implanted z-cut KTP with fluence of 6 × 1016 ions/cm2 observed by OM (a) and AFM (d, e), as well as OM results of z-cut KTP implanted with fluence of 5 × 1016 ions/cm2 (b), and 6 × 1016 ions/cm2 after thermal treatment (c).
Using OM, a series of parallel lines with spacing of 2~6μm was observed, shown in Fig. 1(a). Considering the low energy (200keV) of implanted He+ ions, irradiated He+ ions have relatively low electronic stopping power (31eV per ion and Å), smaller than the electronic-energy-deposition threshold value for ionization-simulated damage (between 65 and 110 eV per ion and Å) [26]. Therefore, irradiation-induced damage observed in targets, like these linear defects, is mostly from nuclear collision. AFM was also applied to study these surface morphology modification in detail, which are shown in Fig. 1(d) and (e). Figure 1(d) displays a typical tapping-mode-AFM topography map and a scan line that is marked with a yellow circle. Figure 1(e) presents the cross-section of the linear defects indicated by the scan line, showing that the implantation causes triangular “tunnels” raised on the sample surface with base width of 0.7μm and height of 10nm. These parallel lines act as boundaries of twin domains, and the asymmetric slope of the raised topography indicates the tilting of crystal plane between twin boundaries relative to the rest of the crystal [25, 27].
Actually, twin boundaries already exist in as-growth KTP crystals [27, 28], and become apparent shown as these linear defects after He+ ions implantation. Following presents the detailed study on these twin boundaries in order to better understand the mechanism of implanted He+ ions movement in KTP structure. KTP is known to show a highly anisotropic ionic conductivity which is greatly enhanced along z-axis direction [27]. This has been attributed to the motion of loosely bounded K+ ions through the structure along paths that are parallel to z-axis direction, which can be promoted by the dynamic annealing effect during ion implantation process. However, due to the special structure of KTP, it has showed that there are not simple one-dimensional paths along z-axis direction but rather that the K sites and hole sites lie on complex interconnecting diffusion paths which weave through the frame work allowing a net movement of K ions in this direction [27], so the implanted He+ ions tend to aggregate along these stagger K diffusion paths, which is shown in Fig. 2 (a). A series of linear defects observed on ion-implanted sample surface are parallel to y-axis direction, which means that these twinning boundaries are like twin-mirror plane parallel to (100) plane, and perpendicular to x-axis direction [28]. According to the AFM results illustrated above, the tilting of raised twinning boundaries indicates that the lattice structure between twin boundaries may be dislocated relative to the rest of lattice, like the twinning formed in He+-implanted LiNbO3 [25]. A schematic illustration of a twinning band formed in He+-implanted KTP is presented in Fig. 2(b).
Actually, it has been showed that these twinning boundaries can also be formed parallel to (010) plane [27], which means that the linear defects on sample surface caused by ion implantation may also be parallel to x-axis direction. This is confirmed in z-cut KTP sample implanted by He ions with ion fluence of 5 × 1016 ions/cm2 at 3~4μA/cm2, where linear defects on surface parallel to both x-axis and y-axis direction were observed by OM, shown in Fig. 1(b). However, it is found that the He+ ions are more likely to aggregate along (100) plane because the thermal expansion coefficient of KTP along x-axis is larger than that along y-axis or z-axis. The space between crystalline planes perpendicular to x-axis may expand because of the dynamic annealing during ion implantation, so the implanted He+ ions tend to aggregate in these spaces parallel to (100) plane. This also explains why the interplanar space of He+-implanted KTP shrinks along the z-axis but expands along x-axis [23].
The evolution of these linear defects versus annealing temperature was also investigated. It was found that the number of these parallel lines observed by OM was decreased after annealing at temperature of 200°C for half an hour (shown in Fig. 1(c)), and they almost disappeared after further thermal treatments, like annealing at temperature above 350°C. This damage evolution phenomenon was also detected in 117keV H+-implanted z-cut KTP with ion fluence of 8 × 1016 ions/cm2 [21]. This indicates that a recovery process that involves thermal-activated dislocation motion in combination with He+ ions migration starts. However, in terms of LiNbO3 crystal, the density of these extended defects appearing on surface reaches a maximum value for an annealing temperature of 250°C and is fully eliminated by a temperature of 380°C [25], which indicates that the effect of thermally activated mechanical twinning and glide dislocation nucleation may be augmented by thermal stresses when the sample is annealed to temperature higher than a critical value, and these defects related to stress relief can be recovered with higher annealing temperature. In comparison, it seems that the critical temperature value for the maximum defects density formed in He+/H+-implanted KTP is lower than 200°C, so the temperature window for damage accumulation to achieve layer-splitting is lower and narrower than that of LiNbO3. As a result, layer exfoliation occurred during ion implantation process for 117keV H+-implanted z-cut KTP sample [21]. This active dynamic annealing effect may be attributed to KTP’s special structure, in which K+ ions are loosely bounded and easily diffuse along z-axis with thermal impetus [27]. The movement of K ions can create many hole sites that help accumulate implanted He+/H+ ions, so layer splitting can be achieved when the density of accumulated He+/H+ ions is large enough.
3.2. Lattice damage in He+-implanted z-cut KTP
We used RBS/Channeling and TEM techniques to investigate lattice structural modification in z-cut KTP samples after ion implantation. Figure 3(a) and (b) show the damage profile versus depth under different ion fluences and different current densities. The damage profiles that are the relative number of displaced lattice atoms are extracted from RBS spectra by using a multiple-scattering dechanneling model based on Feldman’s procedure and applied for all target elements in the crystal [29, 30], and total target vacancies per ion is 124 and total target displacements per ion is 125. Clearly, the impact from different ion fluences and beam current densities on the lattice damage formation in He+-implanted z-cut KTP is almost the same, resulting in similar damage profiles. The peak damage ratio of lower than 40% from a depth of 0.7~1.0μm below sample surface indicates that these implantation parameters only induce medium lattice damage in z-cut KTP samples. There are some factors contributing to this inconspicuous lattice damage, including shallow implantation depth due to relatively low implantation energy and dynamic annealing effect due to high current density, which can promote out-diffusion of He+ ions. The diffusion of He+ ions can be augmented with higher fluence, and some He+ ions may aggregate at near-surface region during out-diffusion process, and cause lattice damage there. Therefore, a small damage peak at near-surface region was found for z-cut KTP samples with higher ion fluence. Crystallographically driven fissure and cracks are observed on surface because He+ ions can easily aggregate along (100) plane where K atoms are loosely distributed. Simulation profile by SRIM is also given in Fig. 3(a), showing that the experimental data consists with theoretical results. For He+-implanted z-cut KTP, similar lattice damage profiles independent on ion fluences imply that the dynamic annealing effect can promote He+ ions diffusion and recover lattice damage. As we know, the substrate temperature increases as the exposure to irradiated species increases. Linear defects on surface were observed only on KTP samples with high ion fluences (6?1016 and 5?1016 ions/cm2), because strong thermal effect facilitates the diffusion of He ions and promotes He ions to aggregate along twin boundaries. As implanted ions transfer the energy to the host samples during implantation, lattice damage accumulates due to energy deposition. This effect is emphasized when current density is low [23], because in a relative low temperature situation energy is easy to deposit but not to dissipate. In our study, the lattice disorder under different beam current densities is similar, which indicates that diffusion of He+ ions and K atoms occurs at a rather low temperature and becomes saturated when temperature is elevated . The present results show that the temperature window for layer splitting of z-cut KTP is smaller and lower than that of LiNbO3 samples (around 250°C) that are already succeeded in layer splitting.
Fig. 3 The damage profile of z-cut KTP implanted by He+ ions with fluences of 3 × 1016, 5 × 1016 and 6 × 1016 ions/cm2 in (a), and implanted by He+ ions with fluence of 6 × 1016 ions/cm2 at current densities of 1, 3 and 9μA/cm2 in (b).The SRIM simulation result is also shown in (a) in solid line.
TEM was also employed to study inner-structure of z-cut KTP implanted by He+ ions with fluence of 6 × 1016 ions/cm2 at 9μA/cm2, which are presented in Fig. 4. All these TEM images are in cross-section geometry, and ion implantation direction is from left to right indicated in the figure. Figure 4(a) shows the lattice structure near surface with good single-crystalline structure. It indicates that the near-surface region that corresponds to exfoliated thin film is almost unperturbed, so the KTP thin film that we intend to achieve can preserve good optical and physical properties. Figure 4(b) displays the lattice structure in damage layer at the end of ion range, corresponding to damage peak in Fig. 3 from RBS. Defects generated by inelastic collisions between implanted He+ ions and target atoms, such as He nanobubbles, cracks and dislocations, are mainly distributed in damage layer. However, it shows that the lattice in this region was not fully amorphous, and both crystalline and disordered structure can be seen, which is consistent with RBS results. The implanted He ions were also straggled to substrate region that below damage layer, as shown in Fig. 4(c). Since the sample was inclined to crack along the defects when it was thinned to the thickness required for high-resolution imaging (100<nm), it was difficult to obtain high-resolution TEM imaging of twin boundaries. However, high-contrast imaging of twin boundaries is achieved and shown in Fig. 4(d). He+ ions tend to trap at these twin boundaries, forming dislocation pileups shown in Fig. 4(d). The weaving shape of the twinning boundary confirms the complex interconnecting diffusion paths of K+ ions, as shown in Fig. 2(a).
Fig. 4 TEM images of z-cut KTP implanted by He+ ion with fluence of 6 × 1016 ions/cm2 at 9μA/cm2, including areas near surface (a), in damage region (b), in the substrate (c), and twinning boundary (d).
3.3. Lattice damage in He+-implanted x-cut KTP
No linear defects but only some small bulges were observed on the surface of x-cut KTP samples, which also implies that the twinning boundaries are parallel to (100) plane, perpendicular to x-axis. RBS/Channeling technique was also used to study the lattice damage in He+-implanted x-cut KTP samples. The RBS results of different ion fluences and current densities for z-cut KTP samples are almost the same; in contrast, for x-cut KTP samples, the results are different. The lattice damage increases gradually with fluence rising, and also moves towards surface, as shown in Fig. 5(a). The broadening of damage profile also indicates that He+ ions tend to diffuse towards surface. Similarly, with the current density of implanted He+ ions decreasing, the damaged region broadens towards surface and the damage ratio increases, as shown in Fig. 5(b). These RBS results indicate that lower current density can effectively inhibit dynamic annealing effect during ion implantation process, thereby curbing He+ ions out-diffusion and forming evident lattice damage. The difference of lattice damage formation along x-axis and z-axis also confirms that implanted He+ ions tend to diffuse along z-axis. Therefore, He+ ions are more likely to aggregate into bubbles or cracks that layer splitting requires if ion implantation is performed along x-axis of KTP. However, layer exfoliation can also be achieved for z-cut KTP during implantation, but only with exfoliated film in small area [21].
Fig. 5 The damage profile of x-cut KTP implanted by He+ ions with fluences of 3 × 1016, 4 × 1016, 5 × 1016 and 6 × 1016 ions/cm2 in (a), and implanted by He+ ions with fluence of 6 × 1016 ions/cm2 at current densities of 1, 3 and 9μA/cm2 in (b).
The inner structure of He-implanted x-cut KTP with fluence of 6 × 1016 ions/cm2 at 9μA/cm2 was investigated by TEM, shown in Fig. 6. Figure 6(a) shows that the damage layer (bright band) exists at depth of 800~900nm below sample surface, consistent with RBS results in Fig. 5(b). The lattice close to sample surface corresponding to exfoliated thin film preserves good crystalline property, as shown in Fig. 6(b). The lattice structure in damage layer is not completely amorphous, and ordered crystalline structure still can be detected, shown in Fig. 6(c) and Fig. 6(d).
Fig. 6 TEM images of x-cut KTP implanted by He+ ion with fluence of 6 × 1016 ions/cm2 at 9μA/cm2, including the whole implantation region (a), area near surface (b), the damage region (c), and diffraction pattern in the damage region (d).
4. Conclusion
The nature of distribution of implanted He+ ions in both x-cut and z-cut KTP samples is studied, and formation mechanism of twinning boundaries after ion implantation is explained in this paper. Implanted He+ ions tend to aggregate along twin boundaries, parallel to (100) plane, where K atoms can diffuse along weaving paths when temperature is elevated. The lattice between twinning boundaries is tilted to some angle relative to the rest of crystal. Due to the dynamic annealing effect, He+-implantation along z-axis cannot cause obvious lattice damage, and implantation with different ion fluences and current densities result in similar lattice disorder. Experimental results show that temperature window for layer splitting of KTP samples is low and small. Due to the anisotropic diffusion of He+ ions in KTP, ion-implantation into x-cut KTP samples are more suitable for thin film exfoliation.
Funding
National Natural Science Foundation of China (Grant No. 61575129, No. 51272135 and No. 11475105); China Postdoctoral Science Foundation (Grant Nos. 2016M602510, 2015M582408 and 2016M602511); Shenzhen Science and Technology Planning (Grant No. JCYJ20170302142929402, JCYJ20160328144942069 and JCYJ20160422103744090); Guangdong Natural Science Foundation (Grant No. 2016A030310059); State Key Laboratory of Nuclear Physics and Technology, Peking University.
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