Preparation And Characterization Of LiNbO₃ Thin Films On Silicon Substrate By Pulsed Laser Deposition

As an important ferroelectric material for fabrication of SAW and integrated optical devices, LiNbO₃ exhibits excellent electro-optical, piezoelectric and nonlinear optical properties. Generally speaking, LiNbO₃ wave-guided structures are demanded in the optical devices. However, there are several problems in the traditional wave-guide films so that there are increasing interests on the fabrication hetero-structure LiNbO₃ films. Compared with bulk materials, hetero-structure LiNbO₃ films possess a series of advantages, like available step index profiles in the multilayer structures. Up to now, there have been various techniques which were used to prepare the hetero-structure LiNbO₃ films. Among them, pulsed laser deposition (PLD) method shows some advantages over the others, for example, it is easier to prepare epitaxial stoichiometric films due to the high compositional consistency with the target. As the base in the microelectronic technology, Si is an interesting host for preparing LiNbO₃ films, because it provides an ideal and cheap substrate for large area processing of devices which can be integrated in the current semiconductor technology, and this will make it possible to develop integrated electro-optics technology. Therefore, this work on Si-based LiNbO₃ films is of great interests and potentials.In this paper, besides the introduction of past and current research on LiNbO₃ films, we investigated growth and properties of Si-based LiNbO₃ films by PLD. Comparing with Si-based LiNbO₃films, we also investigated growth of LiNbO₃thin films on sapphire substrate, and obtained the following results:1. Without the application of induced electrical field and any buffer layer, we firstly prepared the high-quality c-axis oriented LiNbO₃ films on SiO₂/Si substrate by PLD. Through systematically analyzing the influence of experimental parameters on the films growth, the optimal growth conditions were determined: substrate temperature (T_S) is ⁶00℃, oxygen pressure (P_o) is ³0 Pa, laser energy density is 3.2-3.7 J/cm², laser frequency is ³ Hz, substrate-to-target distance (L)is 4 cm, and the thickness of amorphous SiO2 layer is 230nm.2. Stoichiometric LiNbO3 film is a key factor during the investigation. From the results of XPS and SIMS, the as-grown films originated from the stoichiometric ceramic target in the optimal growth conditions exhibit ideal stoichiometric composition and uniform in the cross-sectional profile. Moreover, the film growing mechanism was also discussed, and the growth pattern was supposed.3. The as-grown film deposited on SiCVSi substrate exhibits a very low optical loss (1.14 dB/cm), which is lower than those reported earlier. Moreover, because the surface morphology is believed to be an important factor on the optical loss, we also investigate the effect of the Ts and laser frequency on the film surface morphology.4. High-quality LiNbO3 films for SAW devices were firstly deposited on Si (111) substrate by PLD. Through the investigations of the growth parameters, the film surface morphology and the refractive index, we found that the films exhibited a smooth surface with the rms of 4.87 nm and the refractive index (no) were 2.279, when the Ts was higher than 600 °C. Moreover, the effects of post-annealing were also investigated.5. High-quality c-axis oriented LiNbC>3 films were firstly deposited on ZnO/Si substrates. Compared with the films deposited on SiCVSi and Si (111) substrates, the films on ZnO/Si substrates also exhibited good crystalline quality in the same optimal growth condition. However, the films on ZnO/Si substrates exhibits worse surface morphology than those on SiCVSi and Si (111) substrates and the value of rms are 6.67 nm. The refractive index (no) was 2.255.6. The fabrications of LiNbC>3 films on sapphire were investigated and the effects of Ts on the film growth were analyzed. The results of XRD <ï¿¡-scan show that the films deposited at sapphire exhibit 3-fold symmetry and epitaxial growth with the relationships of (001) LiNbC>3 II (001) AI2O3. Through analyzing the film surface morphology and optical properties, we found the film deposited at 600 °C has the stoichiometric composition and the value of rms is 2.71 nm which is smoother than that deposited on Si substrate. The refractive index of the film was measuredby UV-VIS spectrum as 2.283. Finally, we compared with the effects of different substrates such as SiCVSi, Si (111), ZnO/Si, and sapphire on the crystal quality and the surface morphology of the film.