Vincent P. LaBella

School of NanoSciences and NanoEngineering,
University at Albany-SUNY, Albany, NY 12302

 The GaAs(001)-(2×4) reconstructed surface is one of the most technologically important single crystal surfaces.  It is utilized as the starting surface for fabricating lasers found in CD players using epitaxial techniques such as molecular beam epitaxy (MBE).  During MBE, layers of atoms are deposited onto a single crystal surface under ultra high vacuum conditions. Consequently, over the past 30 years, state-of-the-art techniques from both theory and experiment have been utilized to investigate its surface structure.  Surprisingly, the atomic structure of the GaAs(001)-(2x4) had not been determined until recently. This talk will review some of the history and present the recent results that have helped to end a 30 year controversy in the atomic structure of the (2x4) phase.  Specifically high resolution STM images in conjunction with first principles density functional theory will be presented which resolve all the atomic features of the 2x4 unit cell.  It has also been demonstrated that this surface spontaneously forms one-atomic-layer-high islands when it is annealed above 525 C.  It will be shown that these structures are in equilibrium and can be described by the celebrated 2D Ising model. STM images of the microscopic domain structure on a scale comparable to the gas's constituents give never-before-seen insight into the 2D Ising model. It has been determined that these single level 2D islands are a pre-roughening process which is followed by a KT-like roughening.  STM images of these structures as well as the coupling energies for a complete Hamiltonian of this surface will be presented.  Implications of theseresults on the growth of GaAs will also be discussed.