Trimethylaluminum and Oxygen Atomic Layer Deposition on Hydroxyl-Free Cu(111)

Atomic layer deposition (ALD) of alumina using trimethylaluminum (TMA) has technological importance in microelectronics. This process has demonstrated a high potential in applications of protective coatings on Cu surfaces for control of diffusion of Cu in Cu₂S films in photovoltaic devices and sintering of Cu-based nanoparticles in liquid phase hydrogenation reactions. With this motivation in mind, the reaction between TMA and oxygen was investigated on Cu(111) and Cu₂O/Cu­(111) surfaces. TMA did not adsorb on the Cu(111) surface, a result consistent with density functional theory (DFT) calculations predicting that TMA adsorption and decomposition are thermodynamically unfavorable on pure Cu(111). On the other hand, TMA readily adsorbed on the Cu₂O/Cu­(111) surface at 473 K resulting in the reduction of some surface Cu¹⁺ to metallic copper (Cu⁰) and the formation of a copper aluminate, most likely CuAlO₂. The reaction is limited by the amount of surface oxygen. After the first TMA half-cycle on Cu₂O/Cu­(111), two-dimensional (2D) islands of the aluminate were observed on the surface by scanning tunneling microscopy (STM). According to DFT calculations, TMA decomposed completely on Cu₂O/Cu­(111). High-resolution electron energy loss spectroscopy (HREELS) was used to distinguish between tetrahedrally (Al_tet) and octahedrally (Al_oct) coordinated Al³⁺ in surface adlayers. TMA dosing produced an aluminum oxide film, which contained more octahedrally coordinated Al³⁺ (Al_tet/Al_oct HREELS peak area ratio ≈ 0.3) than did dosing O₂ (Al_tet/Al_oct HREELS peak area ratio ≈ 0.5). After the first ALD cycle, TMA reacted with both Cu₂O and aluminum oxide surfaces in the absence of hydroxyl groups until film closure by the fourth ALD cycle. Then, TMA continued to react with surface Al–O, forming stoichiometric Al₂O₃. O₂ half-cycles at 623 K were more effective for carbon removal than O₂ half-cycles at 473 K or water half-cycles at 623 K. The growth rate was approximately 3–4 Å/cycle for TMA+O₂ ALD (O₂ half-cycles at 623 K). No preferential growth of Al₂O₃ on the steps of Cu(111) was observed. According to STM, Al₂O₃ grows homogeneously on Cu(111) terraces