Titanium Oxide Film Growth And Ferroelectric Integration

Typical ferroelectric materials such as BST and the third generation semiconductors such as GaN were the most important functional materials,have stimulated interest in exploiting their attractive physical properties for applications. Recently, electronic information systems are quickly developed to further micromation in order to enhance integration density and reliability, realize higher velocity and lower power consumption. The integrated devices between BST and GaN could be fabricated by depositing solid films alternately, and the integrated ferroelectric BST/GaN heterojunction, which have all-in-one of multifunction properties, could accelerate the miniaturization and monolithic development of electronic systems. However, perovskite BST and wurtzite GaN have absolutely different kinds of crystal lattice with large lattice mismatch, otherwise, the fabrication technics are greatly different from each other, BST films were usually deposited at high temperature under oxygen atmosphere, while GaN materials were fabricated at low temperature with high vacuum. So it's important to find a fabrication method to implement the integration of BST with right orientation on GaN.In this paper, we choose TiO₂ to be applied as buffer layer for the integration of BST thin films on GaN, because it has various advantages such as high thermal and chemical stability, and has three polymorphic forms: rutile, anatase and brookite.We control the crystalline phase and orientation of TiO₂ buffer layer to induce the epitaxial growth of highly oriented BST thin films.First, The TiO₂ thin films were prepared by pulsed laser deposition method, The dependence of the crystalline phase and surface morphology of prepared films upon the effect of growth parameters (deposition temperature, oxygen pressure, pulse laser energy intensity) were systematically studied. Rutile TiO₂ (200) has been grown epitaxially on Al₂O₃ (0006) under optimum deposition conditions.Then, ferroelectric BST thin films were deposited on Al₂O₃ (0006) with rutile TiO₂ buffer layer by laser molecular beam epitaxy (L-MBE). Reflective high energy electron diffraction (RHEED) was equipped to in-situ monitor the growth mode and the interface characters. The influence of deposition temperature and thickness of buffer layer for BST thin films was studied. XRD, in-situ RHEED and AFM were employed to characterize the crystal structure of BST films with TiO₂ buffer layer, it was revealed that BST had high orientation of (111), the epitaxial relationship of the heterostructure was estimated to be (111)[1-10]BST//(100)[001]TiO₂//(0001)[11-20]Al₂O₃. The C-V, I-V analysis and P-E tests indicated that the BST (111) thin films had better ferroelectric characteristic with TiO₂ buffer layer.Finally, based the integration of BST/TiO₂/Al₂O₃ heterostructure, the epitaxial BST/TiO₂/GaN all-in-one multiplelayers have been fabricated. The out-of-plane of BST showed the high orientation of (111), several Bragg-reflection spots superposed on sharp streak diffraction patterns was observed, suggesting that the BST film deposited on TiO₂ buffered GaN was grown in the Stranski-Krastanov growth mode, and the interface characters of the heterostructure was very sharp by HRTEM and SEM. Therefore, it was estimated that TiO₂ buffer layer greatly enhanced the crystallization properties even at relative lower deposition temperature. At the same time, the BST thin films induced by TiO₂ buffer layer had better ferroelectric characteristic, lower leakage current, respectively.