Structural, electrical and optical properties of SnOx films deposited by use of atmospheric pressure plasma jet

Structural, electrical and optical properties of undoped tin oxide thin films (SnOx) deposited on silicon and glass slides by use of an atmospheric pressure plasma jet at substrate temperatures varying between 300 °C and 500 °C were determined. One role of the used plasma is the promotion of SnO2 formation and reduction of organic residues from the tetra-n-butyltin (IV) precursor by supplying a highly reactive oxygen-containing atmosphere. Moreover, film properties were compared to SnOx films prepared by a vacuum direct current sputtering procedure. Scanning electron microscopic investigations showed high specific surface area and porous films at 300 °C substrate temperature. With increasing this temperature a decrease in film thickness and roughness of the films could be observed. These experiments were supplemented by transmission electron microscopy and showed the formation of a dense layer close to the substrate with a more porous layer on top of it. Spectral ellipsometry further supported these results and allowed to determine the thickness of the films as well as the thickness of the dense and porous part. X-ray diffraction pattern indicated the presence of a predominantly crystalline film structure (SnO2, cassiterite) at 500 °C substrate temperature with an average grain size of ~4 nm. Comparing to this, film deposited by vacuum sputtering method were more crystalline with average grain sizes of ~12-14 nm. Additionally, the presence of SnO2 on the surface was demonstrated by X-ray photoelectron spectroscopy. Films with an average transmittance of >85% and low resistivities of 7.2 × 10−2 Ωcm even at lower substrate temperatures of 300 °C could be achieved, whereby electrical film properties (conductivity) were superior to those of films prepared by sputter deposition. First attempts to show gas sensing properties were conducted and revealed a response toward ammonia gas atmospheres.