Abstract for 1999 American Vacuum Society National Symposium
Seattle, WA  October 1999

Title:  Photoelectron Diffraction of GaSe bilayer grown on Si(111) 

Authors: Shuang Meng, Physics Dept., Univ. of Washington
         Brett R. Schroeder, Physics Dept., Univ. of Washington
         Aaron Bostwick, Physics Dept., Univ. of Washington
         Eli Rotenberg, Advanced Light Source, Lawrence Berkeley Nat'l Lab
         Fumio S. Ohuchi, Dept. of Mat. Sci. & Eng., Univ. of Washington
	 Marjorie A. Olmstead, Physics Dept., Univ. of Washington 

Abstract:

 Initial nucleation of GaSe on Si(111)7x7 results in formation of
a pseudomorphic bilayer, which passivates the substrate dangling bonds and
serves as the interface for further film growth. Component-resolved
photoelectron diffraction (PED) and low-energy electron diffraction show
this bilayer is oriented in a single domain, with the Ga-Se bond aligned
with the substrate Si-Si bond.  Combining scanned-angle and scanned-energy
PED with theoretical calculations reveals the Ga and Se atomic positions.
Ga sits directly atop surface Si, and the resultant interface bonding
leaves Se lone-pair states on the surface.  This makes the Si(111):GaSe
surface highly resistant to contamination, and even additional GaSe does
not stick for T@sub substrate@@>=@550@degree@C.  The measured PED patterns
of Ga and Se 3d states show strong forward focusing along Ga-Se bond as
well as diffraction rings from in-plane Se-Se scattering.   The Ga-Se bond
angle is between that for layered GaSe and cubic Ga@sub 2@Se@sub 3@. PED
also shows strong (>20%) angular variation of the Ga3d spin-orbit
branching
ratio, while the Se 3d branching ratio is constant within 5%. This is
likely associated with differences in photoionization matrix element and
propogation of 3/2 and 5/2 states for the different local environments.
* Supported by NSFDMR9801302