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Novel reactions on halogen-terminated SI(100)
Trenhaile, Brent Reid
Doctoral Committee Chair(s)
Weaver, John H.
Department of Study
"The (100) surface of Si is an ideal surface on which to study fundamental
processes because of its relative simplicity and its rich literature. In my research,
I have sought to elucidate novel surface reactions on halogen-terminated Si(1OO).
I have extensively utilized room-temperature and low-temperature scanning
tunneling microscopes, as well as the variable-temperature instrument available in
the Center of Microanalysis of Materials at the University of Illinois at Urbana Champaign,
to get insights into atomic-level processes. My work has led to a new
model for desorption of atoms from surfaces, has shown that halogens prevent hut
formation during growth of Ge on Si(l00), and has demonstrated an innovative
way that halogens can be used to investigate oxidation of Si(l00).
Spontaneous desorption of Cl, Br, and I from n- and p-type Si(l00)-(2xl)
was studied 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 that 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 phononactivated
electron-stimulated-desorption. The desorption rates for I were nearly
three orders of magnitude larger than the rates 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.
The consequences of Ge deposition on Br-terminated Si(l00) were studied
at ambient temperature after annealing at 650 K. One monolayer of Br was
sufficient to prevent the formation of Ge huts beyond the critical thickness of 3
ML. This was possible because Br acts as a surfactant whose presence lowered
the diffusivity of Ge adatoms. Hindered mobility was manifest at low coverage
through the formation of short Ge chains. Further deposition resulted in the
extension and connection of the Ge chains and gave rise to the buildup of
Finally, the initial stage of oxidation was studied for H20-exposed Si(l00)
in the presence of Cl. Following H20 dissociation and saturation of the surface
with Cl, a mild anneal allowed oxygen atoms to insert into Si dimer bonds. It was
demonstrated that Cl allowed the bridge-bonded oxygen atoms to be imaged as a
dark spot in the center of the dimer. The density of these ""split dimer"" defects
correlated with the c-type defect density on the clean surface. These results also
showed how to produce nearly defect free halogen-terminated Si(l00)."