Oxidant-driven growth and crystallization of MoOx in H2-gas-free thermal atomic layer deposition using a high-volatility liquid Mo precursor

A newly designed, highly volatile liquid molybdenum precursor, Mo(CO)5P[OCH(CH3)2]3, was synthesized and implemented for the first time in thermal atomic layer deposition (ALD). This precursor is engineered in the liquid phase to simultaneously achieve high volatility and stable vapor delivery, and there have been no prior reports on the ALD growth mechanism or device-level applicability enabled by this chemistry. Using this precursor, we systematically elucidate oxidant-dependent growth behavior, ligand decomposition pathway, and crystallization characteristics of thermal ALD molybdenum oxide (MoOx, 2 < x < 3) thin films. MoOx was deposited at 280 °C and 1 Torr for 200 cycles under O2 and H2O-based oxidizing environments, and the growth per cycle, structure evolution (XRD, Raman), and chemical states (hard x-ray photoelectron spectroscopy) were systematically quantified. Although both oxidants yielded amorphous MoOx in the as-deposited state, postannealing at 550 °C induced preferentially oriented α-MoO3 crystallization only in the H2O-processed films. Structural and chemical analyses indicate that H2O promotes ligand hydrolysis and surface –OH-mediated Mo–O–Mo condensation, increasing the Mo6+ fraction and enabling the formation of near-stoichiometric α-MoO3, whereas O2 fails to induce equivalent structural reordering pathway. To validate device-level implications, the H2O-derived α-MoO3 was incorporated as a hole-selective contact in PM6:L8-BO near-infrared organic photodetectors, achieving a responsivity of 0.7722 A W−1, detectivity of 4.38 × 1013 Jones, and microsecond-scale temporal response. These results establish a custom-synthesized liquid precursor as a viable platform for H2-free ALD and identify oxidant selection as a primary process knob governing structure ordering and optoelectronic functionality in ALD-grown transition-metal oxides.