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.

The effect of substrate temperature on atomic layer deposited zinc tin oxide

“…For the pure SnO x films the thickness, and thereby the growth rate, decreases with increasing deposition temperature, from~1.3 Å/cycle at 90°C to~0.6 Å/cycle at 180°C. This decrease in growth rate with increasing deposition temperature correlates well with the findings of [18], which also uses TDMASn and water as precursors, but Si(100) as substrates, and shows that our pure SnO x behaves as expected.…”

“…Similar changes of the band gap as in Fig. 1(d) are also attributed to quantum confinement effects for X-ray amorphous SnO x [18] and for Zn 2 SnO 4 nanoparticles [11,26]. Furthermore, it is shown that films consisting of wurtzite isotropic shaped ZnO nanoparticles in size range up to 9 nm are small enough to display quantum confinement effects as well, and that the band gap, E g (in eV), decreases with increasing particle diameter, d (in nm), according to the expression E g = 3.30 + 0.293 / d + 3.94 / d 2 [25,27].…”

“…One can find support for this hypothesis in the literature, where the crystal size and quality are previously found to increase as a function of deposition temperature for ALD grown ZnO [20]. Furthermore, the formation of small crystallites after annealing is also suggested in the case of X-ray amorphous SnO x [18], and the formation of crystalline Zn 2 SnO 4 is observed after annealing of ALD grown ZTO layers, where a phase separation occurs at lower annealing temperatures for more zinc rich films [14].…”

“…Pure SnO x films as-deposited by TDMASn and water are found to be X-ray amorphous when deposited at 120°C [2] and at 150°C [13,18], while binary ZnO films as-deposited by DEZn and water crystallizes in the hexagonal wurtzite structure [19]. Furthermore, previous work on ALD ZTO films, as-deposited by TDMASn, DEZn and water demonstrates a crystalline-to-amorphous transition when mixing ZnO cycles with SnO x cycles [2,13], with films being X-ray amorphous above a [Sn]/ [13].…”

Incomplete elimination of precursor ligands during atomic layer deposition of zinc-oxide, tin-oxide, and zinc-tin-oxide

“…36 This may seem in contradiction to the observation of incomplete removal of the -N(CH 3 ) 2 (dimethylamino, DMA) ligands. However, we propose the following explanation: Density functional theory (DFT)…”