The authors use scanning tunneling microscopy to study the initial stage of oxidation for H2O exposed Si(100). Following H2O dissociation and saturation of the surface with Cl, a mild anneal allows the oxygen to insert into the Si dimer bonds. Bridge-bonded oxygen atoms appear as a dark spot in the center of the dimer. The density of these “split dimer” defects correlates with the c-type defect density on the clean surface. These results also show how to produce nearly defect-free halogen-terminated Si(100).
Spontaneous desorption of Cl, Br, and I from n-and p-type Si͑100͒-͑2 ϫ 1͒ was studied with scanning tunneling microscopy at temperatures of 620-800 K where conventional thermal bond breaking should be negligible. The activation energies and prefactors determined from Arrhenius plots indicate a novel reaction pathway that is initiated by the capture of electrons which have been excited by phonon processes into Si-halogen antibonding states. This configuration is on a repulsive potential energy surface, and it is sufficiently long lived that desorption can occur, constituting phonon-activated electron-stimulated desorption. Surprisingly, the Arrhenius plots for differently doped samples crossed and, above a critical temperature, the reaction with the largest activation energy had the highest rate. This is explained by large entropy changes associated with the multiphonon nature of the electronic excitation. For Cl desorption from p-type Si, these entropy changes amounted to 34k B . They were 19k B , 13k B , and 8k B for Br desorption from p-type, lightly doped n-type, and heavily doped n-type Si, respectively. The desorption rates for I were nearly three orders of magnitude larger than the rates observed for Cl and Br. Here, the Si-I antibonding states overlap the conduction-band minimum, so that conduction-band electrons with this energy can be captured by the Si-I antibonding states. Together, these results reveal that a complex relationship exists between phonons and electronic excitations during chemical reactions at surfaces.
Adsorbed halogen atoms on Si͑100͒-͑2ϫ1͒ can induce roughening at temperatures where material removal ͑etching͒ is minimal. Variable temperature scanning tunneling microscopy was used to follow roughening at 700-750 K for surfaces with 0.1-0.99 ML of Cl. Dimer vacancies and Si adatoms were observed at short times, and at longer times the progression toward a state of dynamic equilibrium was traced. Once dynamic equilibrium was reached, the appearance of individual pits and regrowth islands changed but their densities and mean sizes did not. The results show that the roughness depends nonlinearly on Cl coverage with surfaces having 0.3 ML being nearly ten times rougher than those with 0.1 ML. The importance of Cl-free dimers is stressed, and the role of Cl as an impediment for vacancy and adatom diffusion is demonstrated. Roughening is attributed mainly to adsorbate-adsorbate repulsive interactions.
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