Atomic Layer Deposition Of Cobalt Phosphate From Cobaltocene, Trimethylphosphate, And O-2 Plasma

Electrodeposited cobalt phosphate has been reported in the literature as a robust alternative to noble metal-based electrocatalysts for the O-2 evolution reaction. In parallel, atomic layer deposition (ALD) has been acknowledged as a key technology for the preparation of thin films for energy applications. With the present work, the authors have addressed the preparation of cobalt phosphate thin films by a plasma-assisted ALD process. The process developed consists of cobaltocene (step A) and trimethyl phosphate (step C) exposures alternated by O-2 plasma (steps B and D) in an ABCD fashion. The process shows a linear growth with a growth per cycle of 1.12 +/- 0.05 angstrom at 300 degrees C and no nucleation delay. The ALD saturation behavior has been demonstrated for each dosing step, and the process shows minimal inhomogeneity on 100mm diameter wafers in terms of film thickness (<1.0%) and refractive index (<0.5%). The chemical characterization of the layers deposited shows that the composition is close to the stoichiometric (Co3.1P2O8.3 for a deposition temperature of 200 degrees C) and the oxidation states of cobalt and phosphorus and agrees with those reported in the literature. The cobalt-to-phosphorous ratio has been found to correlate with the values of absorption coefficient (k) and refractive index (n) of the layers. Furthermore, the authors have gained insights into the surface reactions occurring during each ALD step by quadrupole mass spectrometry investigation. The results suggest that cobaltocene undergoes associative adsorption and the cyclopentadienyl ligands are removed during subsequent O-2 plasma exposure. Moreover, the authors have indirectly identified cyclopentadienone as an intermediate of the oxidation and removal of cyclopentadienyl ligand. On the other hand, the trimethyl phosphate dosing is characterized by chemisorption via the elimination of methanol.