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Abstract
The ion-implantation method can fabricate TiO2 thin films of anatase and rutile structures in an alternate sequence together so that a wide range of applications can be realized. By changing the implantation parameters, the thickness of anatase and rutile thin films can be modulated, and films with a thickness of nano-scale can be achieved. An amorphous TiO2 thin film with a thickness of 13nm was formed in a rutile TiO2 single crystal by low energy He+ ions with high fluences, and this amorphous phase transformed to an anatase and even a rutile phase with thermal treatments. SEM and AFM were used to observe the surface morphology of TiO2 thin film. TEM and Raman scattering techniques confirmed the phase transition process in TiO2 thin films with temperature increase. The design of a combination of TiO2 thin films with both anatase and rutile phases together using the ion-implantation method is also explained.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
TiO2 thin films have been extensively studied because of their interesting optical, electrical and chemical properties [1, 2]. Since a TiO2 thin film with a high refractive index is transparent in visible-light range, it can be used as an antireflection coating on SiO2 thin films [3]. TiO2 films in anatase phase could accomplish the photocatalytic degradation of organic compounds under the radiation of UV, so it has a variety of application prospects in the field of environmental protection [4, 5]. In addition, a TiO2 thin film in rutile phase is known as a good blood compatibility material and can be used as an artificial heart valve [6].
To realize these applications, it is necessary to control the phase transition in TiO2 thin films. It is well known that TiO2 exists in three different phases: anatase, rutile and brookite. Only anatase, rutile and amorphous have been observed in TiO2 thin films up to now [7]. It has been confirmed that TiO2 films consisting of anatase and rutile phases with an appropriate ratio have the best photcatalytic activity [4, 8], so it is important to optimize the preparation process to obtain TiO2 films with optimum phase composition.
Many techniques have been used to fabricate TiO2 thin films, such as sputtering, sol-gel dip coating method, evaporation, chemical vapor deposition, as well as metalorganic chemical vapor deposition (MOCVD) which is a prevalent epitaxial-growth technique with an advantage of large-area deposition [9–17]. However, these methods have limitations in fabricating thin films with high quality crystalline structure. These conventional epitaxial growth methods require good lattice and physical parameters matching between epitaxial film and substrate and usually accompanied by a seriously degraded performance of crystal, resulting in not qualified for practical applications. In this paper, ion-implantation method is introduced firstly to fabricate nano-scale TiO2 thin films consisting of different phases in good crystalline property.
Ion implantation as a promising technology has achieved many useful applications [18–23], and especially, thin film fabrication using implantation assisted with wafer bonding has shown potential values in optoelectronic integrated devices, well-known as “smart-cut” [24, 25], and “crystal-ion-slicing (CIS)” [26]. The principle of this method is: light ions such as H+ and He+ with fluences of 1016-1017 ions/cm2 implanted into a crystal can generate a large concentration of dislocations and defects at the end of ion range, which can trap H+/He+ into bubbles or cracks [27–29]. Subsequent wafer bonding and annealing treatments can promote implanted H+/He+ to aggregate into large cavities, and as a result, the implantation area above the damage layer can be exfoliated from bulk, as a thin film [30–33]. This layer splitting method can be applied to TiO2 to fabricate thin films, but in this paper, versatile ion-implantation method is used flexibly to realize nano-scale TiO2 thin films with different phases combining together to achieve a wide range of functions. It is known that TiO2 films in amorphous and anatase phase is thermodynamically unstable, but can transform to rutile phase via post-annealing treatment. Ion-implantation technique with characteristic of controllability and reproducibility shows its unique advantage in controlling the ratio of different phases in TiO2 films.
In this paper, low energy He+ ions with high ion fluences were implanted into rutile TiO2 single crystal and formed a thin amorphous layer at surface, and this amorphous thin film can transform to anatase thin film after annealing between 400°C~700°C, and then transform into rutile phase after further annealing [8, 34]. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to observe surface morphology and thin film properties; transmission electron microscopy (TEM) and Raman scattering were applied to examine the inner structural modification and phase transition in thin films. How to achieve TiO2 thin films with multi-phase using ion-implantation method is also illustrated in detail.
2. Materials and methods
30keV He+ ions with fluences of 8 × 1016 to 10 × 1016 ions/cm2 were implanted into rutile TiO2 single crystals along [001] axis at room temperature, and the beam current density was kept around 1 μA/cm2 during implantation. Before implantation, all the samples with size of 5mm × 5mm × 0.5mm 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.
Post-implant annealing was performed on samples at temperature from 200°C up to 1000°C in air ambient, in order to achieve phase transition. The phase transition process in TiO2 thin films with temperature increasing is shown in Table 1, which is studied by AFM, Raman scattering and TEM methods, shown in the following Results and discussion part.
Table 1. Phase transformation of TiO2 thin films with temperature increasing
The TiO2 thin film in amorphous phase can be exfoliated easily from rutile TiO2 bulk and bonded on Si substrate. SEM and AFM were used to observe the morphology of TiO2 thin film, as well as the surface topography in different phase state. 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. 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.
Measurements of phase state in TiO2 thin film were performed with Raman spectra on a Labram HR Evolution Raman Spectrometer under a backscattering geometry. A blue line (488 nm) of an Ar+ laser was taken as the excitation source.
3. Results and discussion
3.1. TiO2 amorphous thin film formed by ion implantation
He+-ions with low energy and high ion fluences (8 × 1016 to 10 × 1016 ions/cm2) implanted into TiO2 can directly form a thin film in amorphous phase at surface. This is because the implantation range induced by He+ ions with low energy is very shallow, the damage caused by electronic and nuclear stopping power overlap at the shallow ion implantation range. Additionally, cascading nuclear collision between target atoms and implanted He+ ions with high concentration also contribute to the totally damaged lattice in the implantation range. The relatively high concentration of vacancy or defects at the end of ion range can trap and aggregate implanted He+ ions into bubbles or cracks, so the amorphous thin film can be detached from TiO2 bulk easily through bonding on silicon substrate and thermal treatments. Referring to “smart-cut” method, ion-implanted surface of TiO2 sample was bonded on Si wafer with special epoxy resin glue, and was exposed to thermal treatment. The amorphous thin film was exfoliated after annealing at 200°C for 1hour. Actually, crevices already exist at the end of ion range after ion-implantation, and bonding can peel off the thin film through exerting mechanical stress. Schematic diagram of this process is presented in Fig. 1, which contains three steps: implanting He+-ions into rutile TiO2 single-crystal and forming bubbles or cracks at the end of ion range; spin coating epoxy resin glue on implantation surface and bonding Si substrate on the surface; imposing a series of annealing on the sample stack until TiO2 thin film is exfoliated from bulk.
SEM and AFM methods were used to observe the exfoliated TiO2 thin film and remaining TiO2 sample, shown in Fig. 2. Figure 2(a) presents the morphology of TiO2 thin film bonded on Si substrate and Fig. 2(b) shows the leftover of TiO2 sample with clear boundary, tested by SEM. The boundary of the thin film (top and right sides) is consistent with that in Fig. 2(b) (bottom and right sides), indicating that this thin film was exfoliated from the TiO2 sample. Energy dispersive X-ray spectroscopy (EDX) was also used to detect atomic composition of this thin film, to confirm this TiO2 thin film. After layer exfoliation, a step shape along the boundary of thin film left on the surface of TiO2 matrix, and the height of the step measured by AFM is around 13 nm. The height is corresponding to the thickness of TiO2 thin film, shown in Fig. 2(c) and (d), but lower than the ion implantation range simulated by SRIM. This is because the dynamic annealing effects induced by high ion fluences can effectively promote out-diffusion of implanted He+-ions to shallower implantation depth.
Fig. 2 SEM images of TiO2 thin film (a) and remaining bulk (b), and morphology of TiO2 substrate measured by AFM in three-dimension (c), as well as height graph of step (d).
TEM was also employed to study inner-structure of He+-implanted TiO2 sample, which are presented in Fig. 3. In Fig. 3(a), amorphous TiO2 thin film can be clearly seen at sample surface with thickness of 13 nm. High-resolution TEM image is presented in Fig. 3(b), which presents disordered structure of TiO2 thin film and good single-crystalline lattice of TiO2 substrate.
Fig. 3 TEM images of as-implanted TiO2 sample in (a) and (b), and TEM image (c) and diffraction pattern (d) of TiO2 sample after annealing at 700°C.
3.2. Phase transition in TiO2 thin film with temperature increase
According to previous research, amorphous phase in TiO2 thin film starts to transform to anatase when annealing at 500°C, and completely becomes anatase thin film at 700°C. After further annealing at 900-1100°C, anatase structure changes to rutile structure [8, 34]. In our present experiment, phase transition can be achieved with lower temperature in the thin amorphous TiO2 film, and amorphous phase transforms to anatase after annealing at 400°C. Raman scattering was used to detect the structural modification in 400°C thermal-treated TiO2 samples, shown in Fig. 4.
XRD is the most common used method to test structural properties. It usually reveals the long-range order of materials and gives average structural information within several unit cells. However, our TiO2 film is too thin to be available in XRD measurement. In comparison, Raman scattering as a local probe is very sensitive to crystallinity and microstructures of materials. Comparing with Raman spectrum of as-implanted TiO2 sample in Fig. 4, a clear peak at 144 cm−1 assigned as Eg arisen from the external vibration of the anatase structure was found in Raman spectrum of TiO2 sample after annealing at 400°C, which indicates that an anatase phase exists in the TiO2 thin film after thermal treatment. Therefore, amorphous structure in the thin film at surface starts to transform to anatase structure after annealing at 400°C. A strong peak at 610 cm−1 and a small peak at 450 cm−1 corresponding to Eg and A1g modes of the rutile phase come from the rutile structure of TiO2 substrate [35, 36].
The influence of annealing temperature on surface morphology of TiO2 thin film was also studied by AFM, shown in Fig. 5. Figure 5(a) presents the surface morphology of as-implanted TiO2 sample and column-like structure is found. The diameter of these column-like grains is about 30-50nm, which is consistent with the morphology of amorphous thin film [8]. After annealing at 500°C, these columns aggregate into larger grains with diameter of 50-100nm, which is in agreement with the surface property of anatase structure [34], as shown in Fig. 5(b).
Fig. 5 Surface morphology of TiO2 thin film in as-implanted (a) and after annealing at 500°C observed by AFM.
Imposing further annealing on the TiO2 thin film, anatase phase can be transformed to rutile. TEM was employed to investigate the crystalline modification in the thin film, shown in Fig. 3(c) and (d). Figure 3 (c) shows the inner structure of thin film after annealing at 700°C, and amorphous structure has become ordered lattice. The area circled shows staggered crystalline structure, which is magnified in the inset. The diffraction pattern in Fig. 3(d) is in agreement with the lattice property of TiO2 (100) or (010) facet. Particularly, the diffraction points marked in red circles and magnified in the inset present two points, a major highlight and a side point, which indicate both rutile and anatase structure may co-exist in the thin film. As we know, rutile and anatase crystal both belong to tetragonal lattice system, so the image of bicrystals in Fig. 3(c) and bi-diffraction points in Fig. 3(d) indicate that both anatase and rutile phases appear in the TiO2 thin film after 700°C annealing.
3.3. Design of TiO2 thin film containing both anatase and rutile phases together using the ion-implantation method
According to the above research, we can use ion-implantation method to combine TiO2 thin films in rutile and anatase phase together, shown in Fig. 6. Multi-implantation can be applied on rutile single-crystal TiO2 sample. He+ ions with different energies are implanted into TiO2 sample, and damage layers can be formed at different depths below sample surface (as the position of anatase films in Fig. 6). If ion fluence is large enough, the lattice in damage layers would become totally disordered because of nuclear collisions. The lattice between damage layers may have some defects induced by ion-implantation, but these defects can be recovered by thermal treatments (as the rutile layer between anatase layers in Fig. 6). After a series of thermal treatments, the damage layers with amorphous structure will be transformed to anatase thin films, and there is a rutile thin film sandwiched in anatase films. Therefore, TiO2 thin films with anatase and rutile phase alternately arranged together can be obtained, as illustrated in Fig. 6.
Fig. 6 Diagram of combination of TiO2 thin films in rutile and anatase phase together using ion-implantation method.
Ion-implantation technique is versatile, not only because it can generate amorphous thin films which can be transformed to anatase and even rutile films, but also, it can adjust the thickness of amorphous (anatase) thin films and the rutile thin film between the damage layers through modulating implanting energies. If the energy of implanted ions is increased, the damage layer would locate at deeper position below sample surface. Consequently, the thickness of the rutile film in the middle would increase. Meanwhile, we can also increase the energy and fluence of implanted ions to enlarge the thickness of amorphous layers that can transform to anatase films with annealing. Additionally, with ion-implantation technique, it is possible to obtain TiO2 thin films in amorphous, anatase or rutile phase with only nano-scale thickness, like the 13 nm-thick film illustrated in this paper. According to our research results illustrated above, multi-layer structure with anatase phase and rutile phase together is relatively stable, and the amorphous thin film at surface region can be exfoliated only after bonding and annealing treatments. Consequently, this proposed multi-layer system is reliable. In our future work, we will design TiO2 thin films in amorphous, anatase and rutile phase with the optimum ratio together using ion-implantation method, in order to achieve a wide range of applications.
4. Conclusion
This paper proposes firstly to use ion-implantation method to fabricate TiO2 thin films with anatase and rutile phase together in optimum ratio to achieve a variety of applications. Low energy He ions with high ion fluence were implanted into rutile TiO2 single crystal and formed an amorphous thin film with thickness of 13nm at surface. This amorphous thin film transformed to anatase film after annealing at 400°C, and rutile structure appeared in the film after annealing at 700°C. SEM and AFM were used to observe the morphology of TiO2 thin film, as well as surface topography at different phase state. TEM and raman scattering techniques were used to detect crystalline properties with temperature increasing.
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, JCYJ20170302142547533 and JCYJ20160422103744090); Guangdong Natural Science Foundation (Grant No. 2016A030310059 and 2017A030310316); State Key Laboratory of Nuclear Physics and Technology, Peking University.
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