Gas-phase aluminium acetylacetonate decomposition: revision of the current mechanism by VUV synchrotron radiation

Abstract

Although aluminium acetylacetonate, Al(C₅H₇O₂)₃, is a common precursor for chemical vapor deposition (CVD) of aluminium oxide, its gas-phase decomposition is not well-known. Here, we studied its thermal decomposition in a microreactor by double imaging photoelectron photoion coincidence spectroscopy (i²PEPICO) between 325 and 1273 K. The reactor flow field was characterized by CFD. Quantum chemical calculations were used for the assignment of certain species. The dissociative ionization of the room temperature precursor molecule starts at a photon energy of 8.5 eV by the rupture of the bond to an acetylacetonate ligand leading to the formation of the Al(C₅H₇O₂)₂⁺ ion. In pyrolysis experiments, up to 49 species were detected and identified in the gas-phase, including reactive intermediates and isomeric/isobaric hydrocarbons, oxygenated species as well as aluminium containing molecules. We detected aluminium bis(diketo)acetylacetonate-H, Al(C₅H₇O₂)C₅H₆O₂, at m/z 224 together with acetylacetone (C₅H₈O₂) as the major initial products formed at temperatures above 600 K. A second decomposition channel affords Al(OH)₂(C₅H₇O₂) along with the formation of a substituted pentalene ring species (C₁₀H₁₂O₂) as assigned by Franck–Condon simulations and quantum chemical calculations. Acetylallene (C₅H₆O), acetone (C₃H₆O) and ketene (C₂H₂O) were major secondary decomposition products, formed upon decomposition of the primary products. Three gas-phase aromatic hydrocarbons were also detected and partially assigned for the first time: m/z 210, m/z 186 (C₁₄H₁₈ or C₁₂H₁₀O₂) and m/z 146 (C₁₁H₁₄ or C₉H₆O₂) and their formation mechanism is discussed. Finally, Arrhenius parameters are presented on the gas-phase decomposition kinetics of Al(C₅H₇O₂)₃, aided by numerical simulation of the flow field.