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Si substrates with a ZnO buffer layer by pulsed laser deposition process
ARTICLE IN PRESS Physica B 405 (2010) 1619–1623
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Physica B journal homepage: www.elsevier.com/locate/physb
Preparation of C-axis textured LiNbO3 thin films on SiO2/Si substrates with a ZnO buffer layer by pulsed laser deposition process Wen-Ching Shih , Xiao-Yun Sun Graduate Institute in Electro-Optical Engineering, Tatung University, No. 40, Sec. 3, Chungshan North Road, Taipei 104, Taiwan, R.O.C
a r t i c l e in fo
abstract
Article history: Received 13 October 2009 Received in revised form 17 December 2009 Accepted 18 December 2009
Highly C-axis textured LiNbO3 thin films were grown on SiO2/Si substrates with a ZnO buffer layer by XeCl excimer pulsed laser deposition technique. We fixed the target–substrate spacing at 40 mm, oxygen flow rate of 20 sccm and the repetition rate of 5 Hz, and then changed the substrate temperature, energy density and oxygen pressure to obtain the optimum deposition conditions. The effect of post-annealing process on improving the structural properties of the as-deposited LiNbO3 thin films was also studied. The ZnO buffer layers and LiNbO3 films were characterized by X-ray diffraction (XRD), atomic force microscopy and scanning electron microscopy to analyze their crystalline structure, surface roughness and surface morphology, respectively. For the optimum deposition parameters such as substrate temperature of 700 1C, energy density of 4.6 J/cm2 and oxygen pressure of 150 Pa, the full width at half maximum intensity (FWHM) of LiNbO3(0 0 6) peak was 0.221. The FWHM of LiNbO3(0 0 6) peak was further reduced to 0.191 via. in-situ post-treatment at 750 1C for 30 min. The results could be useful for integrating the surface acoustic wave device, optical waveguide and semiconductor device on the same Si substrate. & 2009 Elsevier B.V. All rights reserved.
Keywords: LiNbO3 Pulsed laser deposition ZnO Sputtering
1. Introduction Lithium niobate (LiNbO3) is a well-known ferroelectric material with unique piezoelectric, pyroelectric, acoustooptic, electrooptic, and nonlinear optical properties [1–3]. For optical waveguide application, thin film offers the advantage of providing strong beam confinement by using the large refraction index difference between the thin film and the substrate, which gives rise to a higher power density, and integration of optoelectronic components in integrated semiconductor circuits. Until now, LiNbO3 thin films have been successfully grown by different deposition techniques: molecular beam epitaxy, RF magnetron sputtering, metal organic chemical vapor deposition, liquid phase epitaxy, sol–gel, and pulsed laser deposition (PLD) [4–20]. Among these techniques, PLD has been known as one of the most promising deposition methods, since it has the advantages of preserving compositional consistency between the target and the film. From the technological point of view, it would be more desirable to grow LiNbO3 films on Si substrates, because this will make it possible to develop integrated devices in which the surface acoustic wave (SAW) devices, optical waveguide and semiconductor devices may be incorporated on the same Si substrate. In general, it is very difficult to deposit highly oriented LiNbO3 thin film on Si substrate
Corresponding author. Tel.: + 886 2 25925252; fax: + 886 2 25956393.
E-mail address: [email protected] (W.-C. Shih). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.12.054
owing to the lattice mismatch between them, thereby decreasing their electric properties and limiting their applications. Matsubara et al. have shown that ZnO thin films could be epitaxially grown on LiNbO3(0 0 0 1) substrates by RF magnetron sputtering [21]. Kang et al. have also demonstrated the growth of high-quality ZnO heteroepitaxial films on LiNbO3(0 0 0 1) substrates by electron cyclotron resonance-assisted molecular beam epitaxy [22]. These results show that LiNbO3 may be a promising substrate for the highquality epitaxial growth of ZnO film. Moreover, the temperature necessary for epitaxy was found to be lower than that required for films on sapphire. At room temperature the lattice parameters are (in the hexagonal cell) a=5.1494 A˚ and c=13.862 A˚ for LiNbO3 while a=3.252 A˚ and c=5.213 A˚ for ZnO. Previous works [21,23,24] have shown that the lattice misfit in the plane perpendicular to the ZnO 8.5%, o1 0 1 04-direction and the LN o1 1 2 04-direction is while the corresponding lattice misfit between ZnO and sapphire is about 15.53%. In this paper, we report the optimization of PLD process so as to grow the highly C-axis textured LiNbO3 thin films on SiO2/Si substrates with a ZnO buffer layer and study the effect of postannealing process on improving the structural properties of the LiNbO3 films.
2. Experimental procedure In the present study, an RF planar magnetron sputtering system (ANELVA SPF-210HS) was used to prepare the ZnO buffer layer. ZnO (doped with Li2CO3 (0.5–1 wt%)) ceramic disk, 4 in in
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diameter, was used as the sputtering target. The ZnO target is doped with Li2CO3 to increase the resistivity of the ZnO film [25]. For SAW application, the resistivity of the piezoelectric film should be high enough. A polished wafer of (1 0 0) Si with a 0.2 mm SiO2 buffer layer was used as the substrate. The thickness of the ZnO buffer layer was approximately 0.2 mm. The deposition parameters for the ZnO buffer layer were kept identical for further optimization of obtaining C-axis textured LiNbO3 thin film. The deposition parameters for growing ZnO buffer layers on SiO2/Si substrates are RF power of 120 W, substrate temperature of 420 1C, gas flow ratio (Ar/O2) of 1 under the fixed setting of total sputtering gas pressure of 600 Pa, and distance between the 4 in target and substrate of 45 mm. We try to use the furnace annealing to improve the quality of ZnO buffer layers. The annealing temperature was varied from 500 to 1000 1C for 30 min, in 1 atm O2 ambient pressure. LiNbO3 thin films were grown by the PLD technique using a 308 nm XeCl excimer laser from Lamda Physik. Laser pulses with a pulse width of 10 ns and repetition rate of 5 Hz were used.
Polycrystalline LiNbO3 ceramic target (25 mm in diameter) was prepared by sintering stoichiometric mixture (Li:Nb= 1:1) of Li2CO3 and Nb2O5 powders in air at 1190 1C for 2 h. We fixed the target-to-substrate spacing at 40 mm and oxygen flow rate of 20 sccm, then changed the substrate temperature (550–750 1C), energy density (4.1–4.9 J/cm2) and oxygen pressure (1–1500 Pa), to study the effect of deposition parameters on the growth of C-axis textured LiNbO3 thin films. Since the results of the X-ray diffraction (XRD) analysis are affected by the film thickness, all the films are controlled to have similar thickness of about 300 nm. After deposition, the LiNbO3 films were in-situ annealed at 750 1C for different periods (18–60 min) in an oxygen pressure of 500 Pa to improve the film quality. The deposited ZnO buffer layer and LiNbO3 film were characterized by XRD and atomic force microscopy (AFM) to analyze their crystalline structure and surface roughness, respectively. The surface morphology of the layered structure was analyzed using field emission scanning electron microscopy (SEM; Hitachi S-5000).
3. Results and discussion 3.1. Characteristics of sputtered ZnO buffer layers
Fig. 1. X-ray diffraction pattern of 0.2 mm-thick ZnO thin films on (1 0 0) Si substrates with a 0.2 mm-thick SiO2 buffer layer (substrate temperature of 420 1C, deposition pressure of 600 Pa, RF power of 120 W, gas ratio (Ar/O2) of 1.0, deposition time of 10 min).
Fig. 1 shows typical XRD pattern of the as-deposited ZnO thin film on the Si(1 0 0) substrate with a 0.2 mm-thick SiO2 buffer layer, revealing that the ZnO buffer layers are highly C-axis textured. The measurement was carried out at 40 kV and 15 mA with a Siemens D5000 diffractometer using a nickel-filtered copper Ka1 radiation and the X-ray radiation wavelength l of ˚ Fig. 2a shows variation of the XRD patterns of ZnO buffer 1.5406 A. layers with annealing temperature where the post-treatment was proceeded in high-temperature furnace with annealing time of 30 min in 1 atm of O2. Fig. 2b shows the variation of the normalized XRD intensity and FWHM of the ZnO(0 0 2) peak with the annealing temperature. The intensity of diffraction peak corresponding to ZnO(0 0 2) phase increased monotonically with the annealing temperature, whereas the FWHM of ZnO(0 0 2) peak decreases monotonically with the annealing temperature. Both phenomenons indicate clearly that not only the crystallinity but also the texture characteristics of the ZnO buffer layers were improved with furnace annealing, achieving a FWHM value of the
Fig. 2. (a) X-ray diffraction patterns of ZnO buffer layer annealed at different temperatures for 30 min by furnace annealing. (b) Dependence of the normalized XRD intensity and FWHM of the ZnO(0 0 2) peak on the annealing temperature.
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ZnO(0 0 2) peak as small as 0.151 for the films furnace annealed at 1000 1C for 30 min. We summarized the results of XRD and AFM for the as-deposited and annealed ZnO buffer layers (furnace annealed at 1000 1C for 30 min) in Table 1. From Table 1 we can see that the X-ray intensity and FWHM value of the ZnO(0 0 2) peak become larger and smaller after post-annealing, respectively. Besides, the diffraction peak of ZnO(0 0 2) was found to be slightly shifted from 34.31 to the large diffraction angle of 34.41, which is closer to the diffraction angle of 34.421 of the ZnO(0 0 2) single crystal after postannealing. The results show that the quality of the as-deposited ZnO films can be improved markedly after the postannealing process. However, the rootmean-square (RMS) surface roughness of the annealed ZnO buffer layers by AFM measurement was increased from 1.9 nm to 10.8 nm. The result may be due to the increase of the grain size with increasing the annealing temperature.
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to the re-evaporation of the adsorbed Li on the film surface with a rising substrate temperature [26]. The XRD patterns of the LiNbO3 films prepared under various energy densities are shown in Fig. 4. The other deposition conditions are fixed at substrate temperature of 700 1C and oxygen pressure of 150 Pa. The maximum relative intensity was obtained from the LiNbO3 film prepared at energy density of 4.6 J/cm2. The intensity of the LiNbO3(0 0 6) peak monotonically increases as energy density increases up to 4.6 J/cm2 and then decreases again as energy density increases further. Kakehi et al. have reported that the Li concentration in a deposited LiNbO3 film was largely influenced by oxygen radicals, which were produced by the interaction between the incident excimer laser and the oxygen [16]. With increasing laser fluence, the re-evaporation of Li atoms in the film was suppressed by the oxidation of Li atoms due to oxygen radicals. Fig. 5 shows the XRD patterns of the LiNbO3 thin films with different oxygen pressures. The other deposition conditions are
3.2. Characteristics of PLD-deposited LiNbO3 thin films To study the influence of substrate temperature on crystallite orientation, LiNbO3 films were grown in the temperature range of 550–750 1C, increasing the temperature in steps of 50 1C. The other deposition conditions are fixed at energy density of 4.6 J/cm2 and oxygen pressure of 150 Pa. Fig. 3 shows the dependence of the XRD patterns on the substrate temperature. At 550 1C, the as-deposited LiNbO3 film is amorphous. As the temperature becomes higher, the orientation and the crystallinity of LiNbO3 thin film are strongly increased. At 650 1C, completely C-axis textured and good crystalline quality LiNbO3 films are achieved. However, increasing the substrate temperature to 750 1C, the intensity of LiNbO3(0 0 6) peak decreases. This is due Table 1 Summary of the results of XRD and AFM for (a) as-deposited and (b) annealed ZnO film (furnace annealed at 1000 1C for 30 min). Sample
Bragg angle (deg)
X-ray intensity (counts)
FWHM (deg)
RMS surface roughness (nm)
As-deposited ZnO film Annealed ZnO film
34.3 34.4
5008 18006
0.26 0.15
1.9 10.8
Fig. 3. XRD patterns of the LiNbO3 thin films deposited on the ZnO (0 0 2) buffer layer with different substrate temperatures (energy density of 4.6 J/cm2 and oxygen pressure of 150 Pa).
Fig. 4. XRD patterns of the LiNbO3 thin films deposited on the ZnO (0 0 2) buffer layer with different energy densities (substrate temperature of 700 1C and oxygen pressure of 150 Pa).
Fig. 5. XRD patterns of the LiNbO3 thin films deposited on the ZnO (0 0 2) buffer layer with different oxygen pressures (substrate temperature of 700 1C and energy density of 4.6 J/cm2).
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fixed at substrate temperature of 750 1C and energy density of 4.6 J/cm2. Films grown in vacuum were found to be amorphous, while very low intensity diffraction peaks were observed for films grown at a pressure of 1 Pa. When the oxygen pressure was increased, a transition from the LiNb3O8 to the LiNbO3 phase was observed. Films grown at 10 Pa present only three low-intensity diffraction peaks, which can be identified as the LiNbO3(1 0 4), LiNb3O8(6 0 2) and LiNbO3(0 0 6) peaks. The film grown at higher pressure (150 Pa) shows a strong (0 0 6) peak in addition to the weaker peak related to the (1 0 4) diffraction peak of the LiNbO3 phase. Dai et al. have reported that the adatom (or absorbed molecular) energy is an important factor that controls the crystal orientation of the LiNbO3 films during the nucleation stage, and the oxygen pressure also affects the growth orientation through changing the adatom energy [27]. By changing the ambient pressure, we identified the optimum condition window to deposit a single phase LiNbO3 film by using PLD. While the mechanism by which the substrate temperature and energy density affect the crystallinity of the LiNbO3 films is evident, how the oxygen pressure influences the texture characteristics of the films is not obvious. In general, the species contained in the laser ejected plume are subjecting pronounced interaction with the ambient oxygen species. The inelastic scattering in the plume-to-ambient interaction can markedly lower the kinetic energy of the laser ejected species, which, in turn, reduces the crystallization kinetics for the deposited films. Too few scattering results in too high kinetic species, which degraded the thin film crystallinity via the bombardment damage, a phenomenon same as RF-sputtering process. In contrast, too abundant scattering leads to too low kinetic species, which cannot crystallize when arriving the substrate, a phenomenon similar to thermal evaporation thin film deposition process. Therefore, Fig. 5 implies that PO2 =10 Pa is too low, which results in high kinetic energy for the laser ejected species that bombardment damaging the LiNbO3 films, whereas PO2 = 1500 Pa is too high, which leads to low kinetic energy for the species that cannot completely crystallized. Only when PO2 = 150 Pa can produce laser ejected species with proper kinetic energy that can be crystallized well. From the XRD measurement (Figs. 3–5), the optimum deposition parameters are substrate temperature of 700 1C, energy density of 4.6 J/cm2 and oxygen pressure of 150 Pa. The FWHM of the LiNbO3(0 0 6) peak of the sample fabricated under the optimum deposition parameters was only 0.221. The PLDed
Fig. 7. AFM image (3000 3000 nm2) of LiNbO3 thin film deposited on ZnO (0 0 2) buffer layer at the optimum deposition conditions (substrate temperature of 700 1C, energy density of 4.6 J/cm2, oxygen pressure of 150 Pa, annealing temperature of 750 1C and annealing time of 30 min).
LiNbO3 films were post-treated by in-situ annealing in the deposition chamber with annealing temperature of 750 1C and oxygen pressure of 500 Pa to improve the film quality. Fig. 6 shows variation of the XRD patterns of LiNbO3 thin films with annealing time. The results show that the in-situ annealed LiNbO3 films with annealing time of 30 min possess the largest normalized intensity and good C-axis texture characteristics (FWHM= 0.193). Fig. 7 shows the AFM image of the LiNbO3 thin film deposited at the optimum deposition conditions. The RMS surface roughness of the LiNbO3 thin film was about 15 nm, which is very close to that of the annealed ZnO buffer layer (10.8 nm). These results imply clearly that the morphology of the LiNbO3 film inherited that of the ZnO buffer layer. To obtain a thicker film, we changed the deposition time from 30 min to 2 h, while the other deposition conditions mentioned earlier were maintained. Fig. 8 shows the X-ray diffraction pattern of the thick LiNbO3 layer deposited on the ZnO(0 0 2) buffer layer. The FWHM of the LiNbO3(0 0 6) peak of the sample fabricated under the optimum deposition conditions was only 0.18o. The quality of the LiNbO3 film was not degraded with the increasing of the deposition time from 0.5 to 2 h. Fig. 9 shows the crosssectional view of the SEM image (10 kV, 10 000) of the LiNbO3 thin film and the ZnO buffer layer. The thicknesses of the LiNbO3 thin film and ZnO buffer layer were about 1.2 and 0.2 mm, respectively. The optimum deposition conditions obtained from this study will be the basis for the future investigation of optoelectronic devices and waveguides based on LiNbO3 material. Besides, a more conscientious study of the crystal quality of the LiNbO3 film obtained from selected area diffraction patterns by transmission electron microscopy observations is also encouraged as promising future work.
4. Conclusions
Fig. 6. XRD patterns of LiNbO3 thin films annealed at different time durations (annealing temperature of 750 1C).
In summary, highly C-axis textured LiNbO3 thin film was successfully grown on SiO2/Si substrates with a ZnO buffer layer by PLD. The optimum deposition parameters are substrate temperature of 700 1C, energy density of 4.6 J/cm2 and oxygen pressure of 150 Pa. The FWHM of the LiNbO3(0 0 6) peak of the sample fabricated under optimum deposition conditions was only 0.221. Post-annealing process further improved the crystalline quality of LiNbO3 film. The FWHM of LiNbO3(0 0 6) peak was reduced to 0.191 after in-situ post-annealing process. The results could be useful for integrating the SAW device, optical waveguide and semiconductor device on the same Si substrate.
ARTICLE IN PRESS W.-C. Shih, X.-Y. Sun / Physica B 405 (2010) 1619–1623
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Fig. 8. X-ray diffraction pattern of the LiNbO3 thin film deposited at the optimum deposition conditions with deposition time of 2 h.
Fig. 9. Cross-sectional view of SEM image (10 kV, 10 000) of LiNbO3 film (1.2 mm) and ZnO buffer layer (0.2 mm) on SiO2/Si substrate.
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Physica B journal homepage: www.elsevier.com/locate/physb
Preparation of C-axis textured LiNbO3 thin films on SiO2/Si substrates with a ZnO buffer layer by pulsed laser deposition process Wen-Ching Shih , Xiao-Yun Sun Graduate Institute in Electro-Optical Engineering, Tatung University, No. 40, Sec. 3, Chungshan North Road, Taipei 104, Taiwan, R.O.C
a r t i c l e in fo
abstract
Article history: Received 13 October 2009 Received in revised form 17 December 2009 Accepted 18 December 2009
Highly C-axis textured LiNbO3 thin films were grown on SiO2/Si substrates with a ZnO buffer layer by XeCl excimer pulsed laser deposition technique. We fixed the target–substrate spacing at 40 mm, oxygen flow rate of 20 sccm and the repetition rate of 5 Hz, and then changed the substrate temperature, energy density and oxygen pressure to obtain the optimum deposition conditions. The effect of post-annealing process on improving the structural properties of the as-deposited LiNbO3 thin films was also studied. The ZnO buffer layers and LiNbO3 films were characterized by X-ray diffraction (XRD), atomic force microscopy and scanning electron microscopy to analyze their crystalline structure, surface roughness and surface morphology, respectively. For the optimum deposition parameters such as substrate temperature of 700 1C, energy density of 4.6 J/cm2 and oxygen pressure of 150 Pa, the full width at half maximum intensity (FWHM) of LiNbO3(0 0 6) peak was 0.221. The FWHM of LiNbO3(0 0 6) peak was further reduced to 0.191 via. in-situ post-treatment at 750 1C for 30 min. The results could be useful for integrating the surface acoustic wave device, optical waveguide and semiconductor device on the same Si substrate. & 2009 Elsevier B.V. All rights reserved.
Keywords: LiNbO3 Pulsed laser deposition ZnO Sputtering
1. Introduction Lithium niobate (LiNbO3) is a well-known ferroelectric material with unique piezoelectric, pyroelectric, acoustooptic, electrooptic, and nonlinear optical properties [1–3]. For optical waveguide application, thin film offers the advantage of providing strong beam confinement by using the large refraction index difference between the thin film and the substrate, which gives rise to a higher power density, and integration of optoelectronic components in integrated semiconductor circuits. Until now, LiNbO3 thin films have been successfully grown by different deposition techniques: molecular beam epitaxy, RF magnetron sputtering, metal organic chemical vapor deposition, liquid phase epitaxy, sol–gel, and pulsed laser deposition (PLD) [4–20]. Among these techniques, PLD has been known as one of the most promising deposition methods, since it has the advantages of preserving compositional consistency between the target and the film. From the technological point of view, it would be more desirable to grow LiNbO3 films on Si substrates, because this will make it possible to develop integrated devices in which the surface acoustic wave (SAW) devices, optical waveguide and semiconductor devices may be incorporated on the same Si substrate. In general, it is very difficult to deposit highly oriented LiNbO3 thin film on Si substrate
Corresponding author. Tel.: + 886 2 25925252; fax: + 886 2 25956393.
E-mail address: [email protected] (W.-C. Shih). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.12.054
owing to the lattice mismatch between them, thereby decreasing their electric properties and limiting their applications. Matsubara et al. have shown that ZnO thin films could be epitaxially grown on LiNbO3(0 0 0 1) substrates by RF magnetron sputtering [21]. Kang et al. have also demonstrated the growth of high-quality ZnO heteroepitaxial films on LiNbO3(0 0 0 1) substrates by electron cyclotron resonance-assisted molecular beam epitaxy [22]. These results show that LiNbO3 may be a promising substrate for the highquality epitaxial growth of ZnO film. Moreover, the temperature necessary for epitaxy was found to be lower than that required for films on sapphire. At room temperature the lattice parameters are (in the hexagonal cell) a=5.1494 A˚ and c=13.862 A˚ for LiNbO3 while a=3.252 A˚ and c=5.213 A˚ for ZnO. Previous works [21,23,24] have shown that the lattice misfit in the plane perpendicular to the ZnO 8.5%, o1 0 1 04-direction and the LN o1 1 2 04-direction is while the corresponding lattice misfit between ZnO and sapphire is about 15.53%. In this paper, we report the optimization of PLD process so as to grow the highly C-axis textured LiNbO3 thin films on SiO2/Si substrates with a ZnO buffer layer and study the effect of postannealing process on improving the structural properties of the LiNbO3 films.
2. Experimental procedure In the present study, an RF planar magnetron sputtering system (ANELVA SPF-210HS) was used to prepare the ZnO buffer layer. ZnO (doped with Li2CO3 (0.5–1 wt%)) ceramic disk, 4 in in
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diameter, was used as the sputtering target. The ZnO target is doped with Li2CO3 to increase the resistivity of the ZnO film [25]. For SAW application, the resistivity of the piezoelectric film should be high enough. A polished wafer of (1 0 0) Si with a 0.2 mm SiO2 buffer layer was used as the substrate. The thickness of the ZnO buffer layer was approximately 0.2 mm. The deposition parameters for the ZnO buffer layer were kept identical for further optimization of obtaining C-axis textured LiNbO3 thin film. The deposition parameters for growing ZnO buffer layers on SiO2/Si substrates are RF power of 120 W, substrate temperature of 420 1C, gas flow ratio (Ar/O2) of 1 under the fixed setting of total sputtering gas pressure of 600 Pa, and distance between the 4 in target and substrate of 45 mm. We try to use the furnace annealing to improve the quality of ZnO buffer layers. The annealing temperature was varied from 500 to 1000 1C for 30 min, in 1 atm O2 ambient pressure. LiNbO3 thin films were grown by the PLD technique using a 308 nm XeCl excimer laser from Lamda Physik. Laser pulses with a pulse width of 10 ns and repetition rate of 5 Hz were used.
Polycrystalline LiNbO3 ceramic target (25 mm in diameter) was prepared by sintering stoichiometric mixture (Li:Nb= 1:1) of Li2CO3 and Nb2O5 powders in air at 1190 1C for 2 h. We fixed the target-to-substrate spacing at 40 mm and oxygen flow rate of 20 sccm, then changed the substrate temperature (550–750 1C), energy density (4.1–4.9 J/cm2) and oxygen pressure (1–1500 Pa), to study the effect of deposition parameters on the growth of C-axis textured LiNbO3 thin films. Since the results of the X-ray diffraction (XRD) analysis are affected by the film thickness, all the films are controlled to have similar thickness of about 300 nm. After deposition, the LiNbO3 films were in-situ annealed at 750 1C for different periods (18–60 min) in an oxygen pressure of 500 Pa to improve the film quality. The deposited ZnO buffer layer and LiNbO3 film were characterized by XRD and atomic force microscopy (AFM) to analyze their crystalline structure and surface roughness, respectively. The surface morphology of the layered structure was analyzed using field emission scanning electron microscopy (SEM; Hitachi S-5000).
3. Results and discussion 3.1. Characteristics of sputtered ZnO buffer layers
Fig. 1. X-ray diffraction pattern of 0.2 mm-thick ZnO thin films on (1 0 0) Si substrates with a 0.2 mm-thick SiO2 buffer layer (substrate temperature of 420 1C, deposition pressure of 600 Pa, RF power of 120 W, gas ratio (Ar/O2) of 1.0, deposition time of 10 min).
Fig. 1 shows typical XRD pattern of the as-deposited ZnO thin film on the Si(1 0 0) substrate with a 0.2 mm-thick SiO2 buffer layer, revealing that the ZnO buffer layers are highly C-axis textured. The measurement was carried out at 40 kV and 15 mA with a Siemens D5000 diffractometer using a nickel-filtered copper Ka1 radiation and the X-ray radiation wavelength l of ˚ Fig. 2a shows variation of the XRD patterns of ZnO buffer 1.5406 A. layers with annealing temperature where the post-treatment was proceeded in high-temperature furnace with annealing time of 30 min in 1 atm of O2. Fig. 2b shows the variation of the normalized XRD intensity and FWHM of the ZnO(0 0 2) peak with the annealing temperature. The intensity of diffraction peak corresponding to ZnO(0 0 2) phase increased monotonically with the annealing temperature, whereas the FWHM of ZnO(0 0 2) peak decreases monotonically with the annealing temperature. Both phenomenons indicate clearly that not only the crystallinity but also the texture characteristics of the ZnO buffer layers were improved with furnace annealing, achieving a FWHM value of the
Fig. 2. (a) X-ray diffraction patterns of ZnO buffer layer annealed at different temperatures for 30 min by furnace annealing. (b) Dependence of the normalized XRD intensity and FWHM of the ZnO(0 0 2) peak on the annealing temperature.
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ZnO(0 0 2) peak as small as 0.151 for the films furnace annealed at 1000 1C for 30 min. We summarized the results of XRD and AFM for the as-deposited and annealed ZnO buffer layers (furnace annealed at 1000 1C for 30 min) in Table 1. From Table 1 we can see that the X-ray intensity and FWHM value of the ZnO(0 0 2) peak become larger and smaller after post-annealing, respectively. Besides, the diffraction peak of ZnO(0 0 2) was found to be slightly shifted from 34.31 to the large diffraction angle of 34.41, which is closer to the diffraction angle of 34.421 of the ZnO(0 0 2) single crystal after postannealing. The results show that the quality of the as-deposited ZnO films can be improved markedly after the postannealing process. However, the rootmean-square (RMS) surface roughness of the annealed ZnO buffer layers by AFM measurement was increased from 1.9 nm to 10.8 nm. The result may be due to the increase of the grain size with increasing the annealing temperature.
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to the re-evaporation of the adsorbed Li on the film surface with a rising substrate temperature [26]. The XRD patterns of the LiNbO3 films prepared under various energy densities are shown in Fig. 4. The other deposition conditions are fixed at substrate temperature of 700 1C and oxygen pressure of 150 Pa. The maximum relative intensity was obtained from the LiNbO3 film prepared at energy density of 4.6 J/cm2. The intensity of the LiNbO3(0 0 6) peak monotonically increases as energy density increases up to 4.6 J/cm2 and then decreases again as energy density increases further. Kakehi et al. have reported that the Li concentration in a deposited LiNbO3 film was largely influenced by oxygen radicals, which were produced by the interaction between the incident excimer laser and the oxygen [16]. With increasing laser fluence, the re-evaporation of Li atoms in the film was suppressed by the oxidation of Li atoms due to oxygen radicals. Fig. 5 shows the XRD patterns of the LiNbO3 thin films with different oxygen pressures. The other deposition conditions are
3.2. Characteristics of PLD-deposited LiNbO3 thin films To study the influence of substrate temperature on crystallite orientation, LiNbO3 films were grown in the temperature range of 550–750 1C, increasing the temperature in steps of 50 1C. The other deposition conditions are fixed at energy density of 4.6 J/cm2 and oxygen pressure of 150 Pa. Fig. 3 shows the dependence of the XRD patterns on the substrate temperature. At 550 1C, the as-deposited LiNbO3 film is amorphous. As the temperature becomes higher, the orientation and the crystallinity of LiNbO3 thin film are strongly increased. At 650 1C, completely C-axis textured and good crystalline quality LiNbO3 films are achieved. However, increasing the substrate temperature to 750 1C, the intensity of LiNbO3(0 0 6) peak decreases. This is due Table 1 Summary of the results of XRD and AFM for (a) as-deposited and (b) annealed ZnO film (furnace annealed at 1000 1C for 30 min). Sample
Bragg angle (deg)
X-ray intensity (counts)
FWHM (deg)
RMS surface roughness (nm)
As-deposited ZnO film Annealed ZnO film
34.3 34.4
5008 18006
0.26 0.15
1.9 10.8
Fig. 3. XRD patterns of the LiNbO3 thin films deposited on the ZnO (0 0 2) buffer layer with different substrate temperatures (energy density of 4.6 J/cm2 and oxygen pressure of 150 Pa).
Fig. 4. XRD patterns of the LiNbO3 thin films deposited on the ZnO (0 0 2) buffer layer with different energy densities (substrate temperature of 700 1C and oxygen pressure of 150 Pa).
Fig. 5. XRD patterns of the LiNbO3 thin films deposited on the ZnO (0 0 2) buffer layer with different oxygen pressures (substrate temperature of 700 1C and energy density of 4.6 J/cm2).
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fixed at substrate temperature of 750 1C and energy density of 4.6 J/cm2. Films grown in vacuum were found to be amorphous, while very low intensity diffraction peaks were observed for films grown at a pressure of 1 Pa. When the oxygen pressure was increased, a transition from the LiNb3O8 to the LiNbO3 phase was observed. Films grown at 10 Pa present only three low-intensity diffraction peaks, which can be identified as the LiNbO3(1 0 4), LiNb3O8(6 0 2) and LiNbO3(0 0 6) peaks. The film grown at higher pressure (150 Pa) shows a strong (0 0 6) peak in addition to the weaker peak related to the (1 0 4) diffraction peak of the LiNbO3 phase. Dai et al. have reported that the adatom (or absorbed molecular) energy is an important factor that controls the crystal orientation of the LiNbO3 films during the nucleation stage, and the oxygen pressure also affects the growth orientation through changing the adatom energy [27]. By changing the ambient pressure, we identified the optimum condition window to deposit a single phase LiNbO3 film by using PLD. While the mechanism by which the substrate temperature and energy density affect the crystallinity of the LiNbO3 films is evident, how the oxygen pressure influences the texture characteristics of the films is not obvious. In general, the species contained in the laser ejected plume are subjecting pronounced interaction with the ambient oxygen species. The inelastic scattering in the plume-to-ambient interaction can markedly lower the kinetic energy of the laser ejected species, which, in turn, reduces the crystallization kinetics for the deposited films. Too few scattering results in too high kinetic species, which degraded the thin film crystallinity via the bombardment damage, a phenomenon same as RF-sputtering process. In contrast, too abundant scattering leads to too low kinetic species, which cannot crystallize when arriving the substrate, a phenomenon similar to thermal evaporation thin film deposition process. Therefore, Fig. 5 implies that PO2 =10 Pa is too low, which results in high kinetic energy for the laser ejected species that bombardment damaging the LiNbO3 films, whereas PO2 = 1500 Pa is too high, which leads to low kinetic energy for the species that cannot completely crystallized. Only when PO2 = 150 Pa can produce laser ejected species with proper kinetic energy that can be crystallized well. From the XRD measurement (Figs. 3–5), the optimum deposition parameters are substrate temperature of 700 1C, energy density of 4.6 J/cm2 and oxygen pressure of 150 Pa. The FWHM of the LiNbO3(0 0 6) peak of the sample fabricated under the optimum deposition parameters was only 0.221. The PLDed
Fig. 7. AFM image (3000 3000 nm2) of LiNbO3 thin film deposited on ZnO (0 0 2) buffer layer at the optimum deposition conditions (substrate temperature of 700 1C, energy density of 4.6 J/cm2, oxygen pressure of 150 Pa, annealing temperature of 750 1C and annealing time of 30 min).
LiNbO3 films were post-treated by in-situ annealing in the deposition chamber with annealing temperature of 750 1C and oxygen pressure of 500 Pa to improve the film quality. Fig. 6 shows variation of the XRD patterns of LiNbO3 thin films with annealing time. The results show that the in-situ annealed LiNbO3 films with annealing time of 30 min possess the largest normalized intensity and good C-axis texture characteristics (FWHM= 0.193). Fig. 7 shows the AFM image of the LiNbO3 thin film deposited at the optimum deposition conditions. The RMS surface roughness of the LiNbO3 thin film was about 15 nm, which is very close to that of the annealed ZnO buffer layer (10.8 nm). These results imply clearly that the morphology of the LiNbO3 film inherited that of the ZnO buffer layer. To obtain a thicker film, we changed the deposition time from 30 min to 2 h, while the other deposition conditions mentioned earlier were maintained. Fig. 8 shows the X-ray diffraction pattern of the thick LiNbO3 layer deposited on the ZnO(0 0 2) buffer layer. The FWHM of the LiNbO3(0 0 6) peak of the sample fabricated under the optimum deposition conditions was only 0.18o. The quality of the LiNbO3 film was not degraded with the increasing of the deposition time from 0.5 to 2 h. Fig. 9 shows the crosssectional view of the SEM image (10 kV, 10 000) of the LiNbO3 thin film and the ZnO buffer layer. The thicknesses of the LiNbO3 thin film and ZnO buffer layer were about 1.2 and 0.2 mm, respectively. The optimum deposition conditions obtained from this study will be the basis for the future investigation of optoelectronic devices and waveguides based on LiNbO3 material. Besides, a more conscientious study of the crystal quality of the LiNbO3 film obtained from selected area diffraction patterns by transmission electron microscopy observations is also encouraged as promising future work.
4. Conclusions
Fig. 6. XRD patterns of LiNbO3 thin films annealed at different time durations (annealing temperature of 750 1C).
In summary, highly C-axis textured LiNbO3 thin film was successfully grown on SiO2/Si substrates with a ZnO buffer layer by PLD. The optimum deposition parameters are substrate temperature of 700 1C, energy density of 4.6 J/cm2 and oxygen pressure of 150 Pa. The FWHM of the LiNbO3(0 0 6) peak of the sample fabricated under optimum deposition conditions was only 0.221. Post-annealing process further improved the crystalline quality of LiNbO3 film. The FWHM of LiNbO3(0 0 6) peak was reduced to 0.191 after in-situ post-annealing process. The results could be useful for integrating the SAW device, optical waveguide and semiconductor device on the same Si substrate.
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Fig. 8. X-ray diffraction pattern of the LiNbO3 thin film deposited at the optimum deposition conditions with deposition time of 2 h.
Fig. 9. Cross-sectional view of SEM image (10 kV, 10 000) of LiNbO3 film (1.2 mm) and ZnO buffer layer (0.2 mm) on SiO2/Si substrate.
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