As-rich GaAs(001) surfaces observed during As4-irradition by scanning   tunneling microscopy
  As-rich GaAs (001) surfaces are successfully observed during As4-irradition by a system in which scanning tunneling microscopy (STM) and molecular beam epitaxy can be performed simultaneously. With a substrate temperature of 440 C and an As4 partial pressure of 2x10-6 torr, reflection high energy electron diffraction patterns and reflectance anisotropy spectra confirm a c(4x4) As-stabilized surface. STM images clearly show alteration of the surface reconstructions while scanning. It is postulated that continual attachment / detachment of As molecules to and from the surface produces the observed dynamic behavior. 
Shiro Tsukamoto, Markus Pristovsek, Bradford G. Orr, Akihiro Ohtake,   Gavin R. Bell, and Nobuyuki Koguchi