Growth-temperature-dependent coalescence determines structural phase of mist-chemical-vapor-deposition-grown SnO2 thin films

Abstract

We grew SnO2 thin films on (001) YSZ substrates by a mist chemical vapor deposition (mist CVD) method and investigated their structural properties by measuring synchrotron x-ray diffraction. We found that the structural phases of the films depend on the growth temperature. SnO2 deposited at 400 °C was found to be epitaxially grown on the substrate and to have the high-pressure-stabilized structure [i.e., columbite structure with the (100)-orientation]. On the other hand, films deposited at 700 °C have a mixture of the epitaxially grown columbite structure and a polycrystalline rutile structure. We also found that while films deposited at both temperatures are grown in island-growth manners, the size of the islands in the initial stage of growth depends on the growth temperature. The islands of the films grown at 400 °C have diameters of 59 nm ± 10 nm, while the islands of the films grown at 700 °C have diameters 2 times larger (110 nm ± 16 nm). These observations indicate that the coalescence of the deposited mist particles resulting from the higher temperature growth, which makes the island larger, leads to partial relaxation of the substrate-induced strain. This explains why the films deposited at 700 °C consist of a mixture of the columbite and rutile structures. Our results highlight the significance of the growth-temperature-dependent coalescence and its induced strain relaxation as the factor determining the structural phase of mist-CVD-grown films.