Atomic Imaging of Nucleation of Trimethylaluminum on Clean and H2O Functionalized Ge(100) Surfaces

The direct reaction of trimethylaluminum (TMA) on a Ge(100) surface and the effects of monolayer H2O pre-dosing were investigated using ultrahigh vacuum techniques, such as scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). At room temperature (RT), a saturation TMA dose produced 0.8 monolayers (ML) of semi-ordered species on a Ge(100) surface due to the dissociative chemisorption of TMA. STS confirmed the chemisorption of TMA passivated the bandgap states due to dangling bonds. By annealing the TMA-dosed Ge surface, the STM observed coverage of TMA sites decreased to 0.4 ML at 250 °C, and to 0.15 ML at 450 °C. XPS analysis showed that only carbon content was reduced during annealing, while the Al coverage was maintained at 0.15 ML, consistent with the desorption of methyl (–CH3) groups from the TMA adsorbates. Conversely, saturation TMA dosing at RT on the monolayer H2O pre-dosed Ge(100) surface followed by annealing at 200 °C formed a layer of Ge–O–Al bonds with an Al coverage a factor of two greater than the TMA only dosed Ge(100), consistent with Ge–OH activation of TMA chemisorption and Ge–H blocking of CH3 chemisorption. The DFT shows that the reaction of TMA has lower activation energy and is more exothermic on Ge–OH than Ge–H sites. It is proposed that the H2O pre-dosing enhances the concentration of adsorbed Al and forms thermally stable Ge–O–Al bonds along the Ge dimer row which could serve as a nearly ideal atomic layer deposition nucleation layer on Ge(100) surface.