Surface diffusion of strontium titanate thin films homoepitaxially grown by pulsed laser deposition

Abstract

The complex oxide thin films have been studied extensively to understand and utilize the physical properties such as ferroelectricity, magnetism, and multiferroism. Advance in the film synthesis techniques such as pulsed laser deposition (PLD) facilitates the growth of a wide range of complex oxide layers at the mono-layer accuracy. The fabrications of ultra-thin layers and articial multilayer structures enable functional interfacial phenomena such as high mobility electron systems. Moreover, mist strain control of epitaxial layers not only improves the existing physical properties such as the Tc enhancement in superconductors but also creates new emerging phases such as the vortex array and metastable phases. It is required to understand the growth mechanism to synthesize high quality complex oxide thin films. The thin film growth mechanisms have been studied by experimental observations of surface adatom states and theoretical simulations for surface diffusion that influences the surface formation such as nucleation of fractal islands or compact patterns. Especially, the thermodynamic variables such as the diffusivity and activation energy of surface molecules of complex oxides synthesized by a pulsed laser deposition (PLD) have been studied through a reflection high electron energy diffraction (RHEED). Despite a lot of studies, we still do not completely understand the diffusion process. In this thesis, the surface diffusion process of a homoepitaxial strontium titanate thin film is investigated using the relaxation behavior of the RHEED specular spot intensity after a half-layer growth. The time-evolution of a RHEED recovery can be t into a double exponential curve, indicating the involvement of at least two diffusion mechanisms. Each of characteristic time is investigated as a function of growth temperature and thus the diffusion activation energies are determined according to the Arrhenius plot. In order to understand the experimental results, we performed the surface diusion simulation by using the Monte Carlo algorithm. The simulation includes the possibility of the hopping of an entire cluster as well as the hopping of a broken cluster separated from the mother island. Since the RHEED specular intensity is proportional to the surface step density, the number of clusters for a given cluster size is responsible for a relaxation mechanism. Multiple decays observed in the RHEED recovery are most likely due to the multiple sizes of clusters, each of them has its own activation energy.