Paper TF-TuA11
Surface Reactions of TiCl₄ and Al(CH₃)₃ on GaAs(100)

Tuesday, October 19, 2010, 5:20 pm, Room Ruidoso

III-V materials have been considered as alternative channel materials for future high-speed low-power digital logic applications mainly because of their higher electron mobility compared to silicon. However, the integration of III-V materials into device structures faces multiple challenges due to the lack of a high quality native oxide and the high density of traps at metal-oxide interface, resulting in Fermi level pinning.

Most of the recent work on GaAs passivation has been done using atomic layer deposition (ALD) method. ALD offers precise control of film thickness, low processing temperatures, and excellent conformality. Additionally, a self-cleaning effect has been reported during Al₂O₃, HfO₂, and TiO₂ ALD on GaAs leading to the reduction or removal of GaAs oxides. Understanding the surface reactions involved in this self-cleaning effect is important in order to improve the GaAs-oxide interface, while depositing high-k dielectric films using ALD.

In this study, the ALD of trimethylaluminum (TMA, Al(CH₃)₃) and titanium tetrachloride (TiCl₄) on hydrofluoric acid (HF) treated GaAs (100) samples was conducted to investigate the similarities and differences in the surface chemistry of these precursors. After aqueous HF etching and air re-oxidation, the oxides were composed of As₂O₃ and Ga₂O. Annealing the sample desorbed volatile compounds such as As₂O₃ from the surface resulting in an As-rich (2.7:1, As:Ga ratio) surface. The precursors, TiCl₄ and TMA, reacted with As and Ga oxides and completely removing them from the surface. This is the first time that a TiCl₄ self-cleaning effect on GaAs is reported. TMA, as expected, produced a film of Al₂O₃ (2.3 monolayers thick) on top of the GaAs surface when the reaction was performed at 170°C.

In contrast to TMA, the oxygen and titanium levels remained below the XPS detection limits with TiCl₄ exposures in the temperature range from 170°C to 230°C, while a 0.03 monolayer-thick film was deposited at temperatures ranging from 89°C to 170°C. XPS results showed that in the higher temperature range, TiCl₄ reacted with the GaAs oxides on the surface producing only volatile compounds, leading to a clean, sharp GaAs surface. The proposed mechanism consists of two processes: TiO₂ formation on the surface and in-situ etching of these TiO₂ islands by chlorine atoms from breakdown of the precursor. This novel GaAs oxide cleaning method produced an oxygen-free surface, which has potential applications for GaAs integration in microelectronics and optoelectronics devices.