Plasma-enhanced atomic layer deposition of AlPO ₄ /AlP _<i>x</i> O _<i>y</i> : comparing dual source and supercycle approaches for composition control

In pursuit of developing a plasma-enhanced atomic layer deposition (PEALD) process for AlPO₄, we explored two different approaches, both employing an O₂ plasma as the co-reactant. First-principles density functional theory (DFT) calculations indicate that TMA-phosphine adducts are stable, with ethyl or isopropyl groups on the phosphine. The adducts were thermally characterized, with the newly synthesized [Me₃AlPⁱPr₃] (TMAPIP) featuring a promising one-step evaporation. Therefore, it was tested as a dual-source precursor at 120 °C, providing both Al and P atoms for the resulting AlP_<i>x</i>O_<i>y</i> layers, thereby simplifying the process design. Although the P content of the PEALD-deposited films was limited to a few percent, this might be advantageous for P doping of Al₂O₃. The second approach, therefore, involved a supercycle (SC) process design, in which the number of phosphorus reagent sub-cycles using P(NMe₂)₃ as the precursor was varied alongside a single Al₂O₃ cycle with TMA; in both cases, O₂ plasma was used as the co-reactant. Simple gas-phase DFT calculations show that P(NMe₂)₃ reacts favorably with the chemisorbed Al species present in the second sub-cycle. The SC method enabled the incorporation of significantly higher amounts of P over a broad temperature range from 60 °C to 240 °C. The deposition of stoichiometric AlPO₄ was ultimately achieved by varying the number of phosphorus cycles, allowing the composition to be precisely adjusted <i>via</i> the deposition temperature.