Surface reactions of TiCl4 and Al(CH3)3 on GaAs(100) during the first half-cycle of atomic layer deposition

GaAs(100) was exposed to pulses of trimethylaluminum (TMA, Al(CH₃)₃) and titanium tetrachloride (TiCl₄) to mimic the first half-cycle of atomic layer deposition (ALD). Both precursors removed the 9.0 ± 1.6 Å-thick mixed oxide consisting primarily of As₂O₃ with a small Ga₂O component that was left on the surface after aqueous HF treatment and vacuum annealing. In its place, TMA deposited an Al₂O₃ layer, but TiCl₄ exposure left Cl atoms adsorbed to an elemental As layer. This suggests that oxygen was removed by the formation of a volatile oxychloride species. A small TiO₂ coverage of approximately 0.04 monolayer remained on the surface for deposition temperatures of 89 °C to 135 °C, but no TiO₂ was present from 170 °C to 230 °C. The adsorbed Cl layer chemically passivated the surface at these temperatures and blocked TiO₂ deposition even after 50 full ALD cycles of TiCl₄ and water vapor. The Cl and As layers desorbed simultaneously at higher temperature producing peaks in the temperature programmed desorption spectrum in the range 237–297 °C. This allowed TiO₂ deposition at 300 °C in single TiCl₄ pulse experiments. On the native oxide-covered surface where there was a higher proportional Ga oxide composition, TiCl₄ exposure deposited TiO₂.