Tin oxide atomic layer deposition from tetrakis(dimethylamino)tin and water

Abstract

Due to the abundance and usefulness of tin oxide for applications such as transparent conductors, sensors, and catalysts, it is desirable to establish high quality atomic layer deposition (ALD) of this material. ALD allows for uniform, conformal coating of complex topographies with ultrathin films and can broaden the applicability of tin oxide to systems such as nanostructured solar cells. The present work examines the ALD of tin oxide by means of the precursor tetrakis(dimethylamino)tin and water as a counter-reactant. Low temperature growth in the range of 30–200 °C on Si(100) and glass substrates is studied. It is found that the growth rate increases with reduced temperature, up to ∼2.0 Å/cycle at 30 °C, as compared to 0.70 Å/cycle at 150 °C. The ALD process is established to be saturated even at the lowest temperature studied, for which the film contamination levels are below the detection limits of x-ray photoelectron spectroscopy. As-deposited films are smooth (rms roughness of 33 Å for a 460 Å thick film deposited on Si at 150 °C) and amorphous according to x-ray diffractometry. However, post-annealing to 600 °C in nitrogen leads to the formation of polycrystalline rutile SnO2. The optical constants obtained from variable angle spectroscopic ellipsometry suggest that as-deposited films have amorphous type bandgaps, which decrease with increasing temperature and film thickness. High quality, amorphous SnO2 films with moderately tunable optical properties are thus obtained by ALD at temperatures as low as 30 °C using a commercially available tin precursor and water.