ALD of Tantalum and Titanium Phosphates Using Tris(dimethylamino)phosphine

Atomic layer deposition (ALD) of metal phosphates is increasingly explored for energy applications, yet the mechanisms governing phosphorus incorporation remain poorly understood. In this work, we introduce two novel ALD processes for amorphous tantalum phosphate and titanium phosphate thin films using tris(dimethylamino)phosphine (TDMAP) as the phosphorus precursor and metal alkylamides as precursors. Both processes employ a four-step ALD cycle, consisting of TDMAP/O2-plasma and metal alkylamide/O2-plasma subcycles, and exhibit a pronounced temperature dependence, with the growth per cycle decreasing as the temperature increases from 150 to 300 °C. In situ characterization indicates ligand exchange reactions during the TDMAP pulse and enhanced P uptake at lower temperatures due to higher reactive site density. The resulting films deposited at 150 °C are a tantalum oxyphosphate and a phosphate-containing titanium oxide, both exhibiting substantially higher phosphorus-to-oxygen and phosphorus-to-metal ratios than their trimethylphosphate-based counterparts. A key finding of this work is that phosphorus incorporation can be further increased by replacing the O2-plasma pulse following the TDMAP exposure with a H2O pulse, shifting the composition toward phosphate-rich stoichiometries intermediate between ortho- and metaphosphates. In situ measurements suggest that less extensive ligand removal during hydrolysis reduces the availability of active sites for subsequent metal precursor adsorption, thereby lowering metal incorporation. These findings demonstrate that tuning the surface chemistry established between the TDMAP and metal precursor exposures provides a straightforward and effective approach for tailoring the composition of metal phosphate films.