A scanning tunneling microscopy and density functional theory study of halogen dynamics and etching reaction on the silicon(100) surfaces

Halogen adsorption, dynamics and etching reaction on Si(100) 2 x 1 surfaces have been studied by real time Scanning Tunneling Microscopy and Density Functional Theory calculations. Due to terrace crowding and electrostatic interactions there are significant repulsions between halogen adsorbates which render the atomically smooth halogenated Si(100)-2 x 1 surfaces unstable. To reduce repulsions, these surfaces roughen spontaneously at elevated temperature by forming pits and islands without silicon halide desorption. Current halogen etching mechanism based on the observation of surface pits and islands may not be relevant due to this roughening process. The strength of these steric repulsions scales with the size of halogen atoms, i.e. bigger atoms have stronger repulsions. These repulsions also play an important role in determining the equilibrium morphology. We compared surface morphologies of bromine and chlorine passivated surfaces prepared with identical procedures and found that chlorinated surface has near-square pits, less islands and smoother steps while brominated surface has long and narrow pits, more islands and rougher steps. Within the Steric Interaction model, we show that these differences result from the different amplitudes of repulsions between chlorine and bromine. The roughening process also occurs on submonolayer chlorinated surfaces and is coupled with formation of chlorine patches. The attractive interactions between bare dimers and high energy of chlorine-bare dimer boundary offset the repulsion and allow the chlorine patch formation. The repulsions within the patch induce local roughening. However, the roughening process become frustrated as the surface coverage approaches a complete monolayer due to the increased diffusion barrier of silicon and halogen species on these coverages. Meta-stable intermediates for etching reaction were identified. Their behaviors point to the importance of gas phase halogen source in etching reaction. The availability of gas phase halogen determines whether etching or roughening reaction dominates when halogenated surfaces are annealed. Long range coupling of dangling bond states exists on the bare Si(100) 2 x 1 surface and gives birth to the observed band structures. Adsorbates remove dangling bonds states and disrupt such long-range interactions. In such cases, even though the local structures remain similar, electronic structures are much different and may gain new reactivity.