Influence of noble-metal surfactants on metal-oxide thin film growth

Abstract

Metal-oxide thin films are widely used for a variety of emerging applications and technologies such as electronic-device building and photocatalysts thanks to their diverse physical and chemical properties. However, impurities, interstitials, and other defects can alter the electronic structure in semiconductor or insulator crystals used in photocatalysis, leading to in-gap states and thus limiting their potential usage in applications. Defect-related trapping of photoelectrons in oxide semiconductors remains a challenge in photoelectrochemistry, as the presence of trap sites and unoccupied in-gap states result in a loss of charge carriers due to recombination. This ultimately leads to a vastly shortened photocarrier lifetime,which in turn severely reduces the achievable energy efficiency of photocatalyticoxide materials. To avoid the problem of thermally generated defects, methods for growing thin films at low temperatures were developed. However, low temperatures greatly affect the kinetics of growth: due to a decreased mobility of surfaceatoms, the amount of time adatoms need to find thermodynamically stable latticepositions increases, leading to more bulk defects if not enough time was providedduring deposition. Therefore, an atomically well-ordered and defect-free surface with a high mobility of surface atoms at low temperatures is essential for growingdefect-free films.In this thesis, the influence of Ir as a potential surfactant on SrTiO3 homoepitaxialgrowth via pulsed laser deposition (PLD) was investigated using reflectionhigh-energy electron diffraction (RHEED) to monitor the growth behavior andatomic-force microscopy (AFM) to examine the resulting surface morphology. In addition, the surface structure was analyzed using co-axial impact collision ionscattering spectroscopy (CAICISS). A difference in the growth mode when usingIr-doped SrTiO3 was found for multiple deposition temperatures and a shorter periodicity of the RHEED oscillations when using Ir-doped SrTiO3 compared to non-doped SrTiO3 is reported. Step-flow growth is achieved earlier for Ir-dopedSrTiO3 at 800 °C. The segregation mechanisms of Ir during thin film growth of Ir-doped SrTiO3 was investigated and a comparison with the literature was performed. The observation of clustering on the surface is only reported for are petition rate of 1 Hz at 600 °C or for 0.5 Hz with multiple higher temperature depositions. In addition, the importance of well-defined substrate surfaces and the tight control of growth parameters such as the laser ablation repetitionrate, the surface morphology and the Ir concentration in the deposited films was highlighted.