Growth of Superconducting Sr₂RuO₄ Thin Films via Thermal Laser Epitaxy

Thermal laser epitaxy (TLE) is a novel technique for thin film deposition which employs continuous wave lasers to simultaneously heat both the substrate and elemental sources. This laser heating approach allows for evaporation or sublimation of nearly all elements from the periodic table, ultrahigh substrate temperatures exceeding 2000 C, and broad compatibility with process gases at a wide range of pressures from UHV up to 1 Torr, among other benefits. As a result, TLE dramatically expands the parameter space available for thin film synthesis compared to existing epitaxy techniques. However, to date it has proven experimentally challenging to achieve simultaneous control of multiple laser based elemental sources with the flux stability and systematic fidelity necessary for the growth of ternary or multernary systems of interest such as complex oxides.<br/><br/>In order to establish the capabilities of TLE for the growth of such complex materials, we demonstrate here the successful epitaxial synthesis of several Ruddlesden-Popper phases of the Sr-Ru-O ternary oxide system via TLE. Near instant thermalization of both source elements and substrates from laser heating allows the process of thermodynamic phase control to be achieved rapidly during film deposition without the need for physical shuttering of sources. Additionally, we find that the “n=1” phase Sr₂RuO₄ can be reliably synthesized at substrate temperatures in excess of 1200 C and in a background environment of pure molecular oxygen, within an adsorption-controlled growth window that is inaccessible to conventional MBE approaches. We show that Sr₂RuO₄ films grown under these conditions demonstrate extremely high structural, electronic, and chemical quality, as evidenced by the appearance of superconductivity at relatively high critical temperatures. In particular, the higher growth temperatures and elemental source fluxes afforded by laser heating allow us to achieve phase pure 214 without higher-N intergrowths typically observed in MBE-grown films, and growth rates more than 10 times faster than MBE. A detailed accounting of the experimental approach, growth thermodynamics and film characterization will be discussed. This work not only demonstrates the feasibility of TLE for the synthesis of high-quality complex oxide thin films, but also suggests new routes to achieving thin film growth in other materials systems that remain as-yet inaccessible to conventional epitaxy techniques.