Effects of Plasma Reactants on Atomic Layer Deposition of Lithium Phosphate and Lithium Phosphorus Oxynitride Electrolyte Films

The effects of plasma reactants on the plasma-assisted atomic layer deposition (ALD) of lithium phosphate are investigated in relation to the fabrication of high-quality lithium phosphorus oxynitride (LiPON) thin films for potential use as a solid-state electrolyte (SSE) in both microbatteries and neuromorphic devices. Our ALD processes enable the incorporation of nitrogen into a lithium phosphate matrix, using lithium tert-butoxide and tris(dimethylamino)phosphine as the lithium and phosphorus precursors, respectively, in a deposition temperature window of 220–300 °C. With O₂ plasma, polycrystalline lithium phosphate films, with a relatively well-arranged pyrophosphate, are deposited. Amorphous LiPON films, with a mixture of pyrophosphates and orthophosphates, are obtained when Ar or NH₃ plasma is used. When the NH₃ flow rate increases, the nitrogen composition increases up to ∼13%, while residual carbon is kept below a few percent. For a Li_2.5PO_1.9N_0.8 film deposited at 300 °C with NH₃ plasma, the ionic conductivity is measured as 1.65 ± 0.42 × 10^–6 S/cm at 25 °C, with an activation energy of 0.66 eV. This conductivity is the highest value of any ALD LiPON film reported to date. Our ALD processes exhibit a high level of controllability of the molecular structures of the phosphorus oxynitride matrix with high ionic conductivity, which makes them suitable for realizing high-performance Li SSE thin films.