Multi-Mode Scanning Probe Microscopy on Ultra-Thin CaF2
Films Grown on Si(111)
Andreas Klust, Aaron Bostwick, Taisuke Ohta, and Marjorie Olmstead
Department of Physics, University of Washington
Box 351560, Seattle, WA 98195-1560, USA
CaF2 is a promising candidate for epitaxial insulating films on Si for
optoelectronic applications
because of its low lattice mismatch with Si (0.5%) and large band gap
(12.4 eV). In addition, the
strongly dissimilar ionic/covalent bonding character in CaF2 allows it
to serves as a model system
both to study heteroepitaxy of two dissimilar materials and to study
the influence of bonding
character and electronic structure on scanning probe microscopy. Here,
we present an investigation
of ultra-thin (1-3 molecular layers) CaF2 films using both non-contact
atomic force microscopy
(ncAFM) and scanning tunneling microscopy (STM). On the one hand, the
extremely
large band-gap of CaF2 makes STM measurements difficult; stable imaging
is not possible for
films thicker than 3-4 molecular layers. On the other hand, STM gives
complementary information
to that obtained with ncAFM. For instance, the contrast during STM
imaging depends strongly
on the polarity of the bias voltage and the film thickness. In
particular, the apparent “high” side
of molecular-height steps changes with tunneling polarity. Non-contact
AFM is used to clarify
this behavior to separate electronic and topographic contributions to
the STM images. Atomicallyresolved
ncAFM images of the CaF/Si interface layer will be shown and compared
with similar
published data obtained from bulk CaF2(111) crystals [1]. The atomic
structure of the CaF/Si interface
layer is practically identical to the surface structure of bulk
CaF2(111), while the electronic
structure differs. We discuss the influence of the different electronic
structure on atomic resolution
ncAFM.
[1] A. S. Foster, C. Barth, A. L. Shluger, and M. Reichling, Phys. Rev.
Lett. 86 (2001) 2373.