Chemisorption of Tetrakis(dimethylamido)hafnium on Hydroxylated HfO2: A Comparative Theoretical Study with Zr and Ti Precursors

The chemisorption mechanism of tetrakis(dimethylamido)hafnium (Hf(NMe2)4) on hafnium oxide (HfO2) was investigated using density functional theory (DFT) calculations and was compared with the Zr and Ti precursors with the identical ligands. Simulations were performed on a 2x2 slab model of monoclinic HfO2 (-111) constructed with a realistic, optimized surface hydroxyl (OH) coverage of 4.45 OH·nm-2. Three sequential ligand exchange reactions were simulated, revealing that the first and second steps are exothermic with low activation energies of 0.20 and 0.32 eV, respectively. In contrast, the third step was found to be endothermic and nonspontaneous at below 503{degree sign}C, leading to the termination of the reaction at ALD process temperatures and the formation of -O2Hf(NMe2)2 as the final chemisorbed species. This result is consistent with in-situ experimental observations. A comparative analysis with other group IV metal precursors showed that the number of HNMe2 released from the precursor during chemisorption depends on the metal center. While Hf(NMe2)4 has more favorable reaction energies in the first two steps, this trend reverses in the third step, where it exhibits lower reactivity than both Zr(NMe2)4 and Ti(NMe2)4. This reduced reactivity is attributed to the lower density of hydroxyl groups and the dissociation of the Hf-O bond of the substrate. These results highlight the importance of realistic surface models with optimized hydroxyl densities for quantitatively reliable simulations of ALD surface chemistry.