Effects Of Annealing Treatment On Structures And Optical Properties Of LaAlO₃ Thin Films

For a long time, perovskite oxide materials attracted many interests in recent years for their remarkable dielectric, ferroelectric, electro-optic, ferromagnetic, superconducting. Lanthanum aluminates(LaAlO₃)is one of the perovskite oxide materials and itself is an insulator with a wide band gap. LaAlO₃ crystals have been used for substrates of high-temperature superconductors and electro-optic devices because of good lattice matching, large dielectic constant, wide optical-band gap and good thermal property. Recently years, amorphous LaAlO₃ thin films with pseudo-cubic perovskite has been the subject of study due to its new identity as a promising candidate for the high k gate oxide, which is intended to replace silicon dioxide (SiO2) in the metal-oxide-semiconductor field-effect-transistor (MOSFET) application. Moreover, LaAlO₃ crystal has gained extensive application with electron parts of apparatus, catalyzing, batteries of high temperature fuel.In this paper, LaAlO₃ thin films were fabricated on p-Si (100) substrates by using radio-frequency magnetron sputtering deposition method at room temperature, and argon is taken as sputtering gas. Then the samples were annealed in air atmosphere with different temperature, respectively.The structure and the morphology of the LaAlO₃ thin films which annealed at different temperature were investigated separately by X-ray diffracation (XRD) and atomic force microscopy (AFM), and then studied the effects of annealing ttreatment on the crystal state and suface roughness of LaAlO₃ thin films.The results of XRD and AFM show that the annealing temperature considerably affects the microstructures of thin films. Here,LaAlO₃ films growen on Si substrates are amorphous, however, the films changed gradually from amorphous to crystalline above 900oC and higher temperature can improve the crystalline quality of LaAlO₃ films. As the annealing temperature rises up, the surface roughness of films are also increaseed.The optical properties of LaAlO₃ thin films annealed at different temperature were characterized with the optical absorption and photoluminescence spectra. Two emission peaks located at 368nm and 470nm in photoluminescence (PL) spectra are observed, respectively. The intensities of PL peaks increased with increasing annealing temperature. It may be attributed to the improved crystallinity of obtained LaAlO₃ thin films. According to the absorption spectra and the calculated defect's energy levels in the LaAlO₃ films, it can be proposed that the 368nm UV emission originates from the energy levels of oxygen vacancies to the top of valence band and the 470nm blue emission is derived from electron transition between the level of Al La anti-site and the top of valence band.