Study On The Phase Transition And Optical Properties Of Strontium Titanate Thin Films

This thesis concentrates on the phase transition and optical properties of ferroelectric strontium titanate (SrTiO₃) thin films. Since SrTiO₃ is a very promising material in the applications of dynamic random access memories, superconductors and micro/nanoelectronics, it is crucial to study the phase transition and optical properties of SrTiO₃.First, we summarize the fundamentals of macroscopical and microcosmic theories of phase transition for ferroelectrics, with an emphasis on"soft mode"theory. Basic principles on experiments used in our studies such as micro-Raman scattering, photoluminescence (PL), spectroscopic ellipsometry (SE), and atomic force microscopy (AFM) are introduced. We also briefly describe their instruments and apparatus.Next, we review the studies of the optical properties in SrTiO₃, including the relationship between para-ferroelectric phase transition and lattice vibration, the adjustment of para-ferroelectric phase transition temperature (T_C), annealing processes and images, and the origin of PL, etc. According to the earlier research status, the majority of our work is assigned to the following aspects: the impact of annealing temperature (T_A) on T_C, the interfacial layers and surface states of the samples annealed under different T_A in constant oxygen atmosphere, and the origin of the near-band-edge emission (NBE) in PL spectra.Then, we present a detailed investigation on T_C of SrTiO₃ ultrathin films grown by molecular beam epitaxy on Si substrate under various T_A, on the basis of recent understanding of the interfacial layers formation in constant oxygen atmosphere. We show that T_C determined by the ultraviolet Raman spectroscopy is found to enhance linearly with the increasing compressive thermal strain. The present work not only demonstrates that the"strain engineering"room-temperature ferroelectricity in SrTiO₃ films can also be realized through the rapid controlled annealing, in addition to the substitution of substrates in the literature, but also provides an experimental basis for easily tuning T_C of SrTiO₃ films, which is essential for the device design for SrTiO₃.Finally, we discuss the origin of PL in such films by concentrating on the NBE blue emission near 3.2eV at different environmental temperature and under various T_A. It is proved that surface roughness could enhance the PL intensity of NBE. Surface roughness can be obtained either from SE indirectly or from AFM directly. It is found that the PL intensity of NBE increases with surface roughness and oxygen vacancy on the ground of defect-induced electron-hole recombination luminescence (the model of self-trapped excitons). Besides, this kind of intensity decreases as the thermal strain (tensile or compressive) becomes small, which means that strain might be related with the origin of PL. The present work paves the way in experiment for further exploring the origin of PL of ferroelectric thin films.