Thermal stress-assisted annealing to improve the crystalline quality of an epitaxial YSZ buffer layer on Si

Abstract

Yttria-stabilised zirconia (YSZ) is an excellent buffer layer that can be epitaxially grown on Si through low-cost deposition techniques, such as pulsed laser deposition (PLD) and sputtering, for the realisation of novel electronics, where a variety of epitaxial functional oxides are integrated on Si. However, the crystalline quality of the as-grown YSZ on Si is poorer than that of the SrTiO3 buffer layer grown by molecular beam epitaxy (MBE). Here, we report a simple annealing technique for achieving high-quality single-crystal oxides on a Si substrate. We significantly improved the crystalline quality of YSZ/Si through the mechanical strain energy arising naturally from the large difference between the thermal expansion coefficients of YSZ (similar to 9 x 10(-6) K-1) and Si (similar to 3 x 10(-6) K-1), which acts as an extra energy source for defect annihilation. A rapid heating rate (similar to 110 degrees C s(-1)) allows this strain energy to maximally build up in the YSZ layer at the annealing temperature, and the defects are effectively annihilated during annealing. We further demonstrate the integration of high-quality epitaxial CeO2 and orthorhombic or tetragonal Y:HfO2 thin films on YSZ/Si substrates through the thermal stress-assisted annealing method. Our results not only provide insights into the manipulation of thermal stresses to engineer epitaxial heterostructures but also provide opportunities to integrate high-quality complex oxides on Si for the commercialisation of novel electronics.