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Atomic structures on a GaAs(001) surface grown by molecular beam epitaxya Department of Physical Electronics, Bashkir State University, Ufa, Russian Federation b Hitachi Advanced Research Laboratory, Hitachi Ltd., Hatoyama, Saitama, Japan c Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai, 980-77, Japan A unique apparatus for in-situ atomic-resolution study of solid state structures grown by molecular beam epitaxy (MBE) is developed, in which a scanning tunneling microscope (STM) is combined with an MBE chamber within the same vacuum system. The utility of the apparatus is demonstrated by examining atomic structures on a molecular-beam-epitaxial GaAs(001) surface over a wide range of [As]/[Ga] ratios. By varying the As surface coverage, the 2×4 — α, β, γ and c(4×4) phases are examined in detail. High-resolution STM images indicate that 2×4 — α, β, and γ phases in the outermost surface layer have essentially the same unit cell consisting of two As dimers and two As dimer vacancies. Using the STM images, reflection high-energy electron diffraction (RHEED) patterns and dynamical RHEED calculations, the existing structural models for the 2×4 phases are analysed and a new model of the As-rich GaAs(001) surface is proposed, found to be consistent with most of the previous observations.
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