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.