The microscopic structure of the Sb stabilized GaAs(100)-(2 3 4) surfaces is investigated combining reflectance anisotropy spectroscopy with first-principles total energy minimization and tight-binding calculations of optical properties. We show that the model accepted so far, containing three Sb dimers in the outermost layer, is not a stable surface geometry. Our results reveal a coexistence of Sb and Ga dimers on the Sb-stabilized ͑2 3 4͒ surface. [S0031-9007(96)01679-1] PACS numbers: 68.35.Bs, 68.35.Md, 78.30.Fs, 78.66.Fd Understanding of the surface atomic structures and their modification through deposition of foreign atoms is crucial for both science and technology. In particular, the adsorption of group-V elements on (100) faces of III-V compound semiconductors is technologically important for the III-V heteroepitaxy [1]. Especially the adsorption of Sb on GaAs(100) has attracted interest due to the formation of an exceptionally abrupt interface [1,2] and the possible use of Sb as surfactant for metal growth on GaAs(100) [3,4]. Soft x-ray photoemission spectroscopy (SXPS), Auger electron spectroscopy (AES), and electron diffraction [LEED and reflection high-energy electron diffraction (RHEED), respectively] studies have shown that by Sb deposition on GaAs(100) and subsequent annealing a well-ordered ͑2 3 4͒ reconstruction is formed even at temperatures much higher than for the clean, As-terminated surface, due to stabilization by Sb dimers [1,5,6].Based on SXPS data and on the ͑2 3 4͒ RHEED pattern a first structural model for the Sb-induced reconstruction was suggested [1]. This model, based on three Sb dimers per surface unit cell, was constructed in analogy to the so-called GaAs(100)b͑2 3 4͒ structure, widely used in the past to describe As-terminated GaAs(100)-͑2 3 4͒ surfaces [7,8]. For the clean GaAs(100)-͑2 3 4͒ surface, however, recent experimental and theoretical studies have demonstrated that the equilibrium structure contains only two As dimers per unit cell in the topmost atomic layer, denoted as b2͑2 3 4͒ structure [9-12]. As shown in Ref. [9], the electrostatic interaction between the negatively charged surface As dimers destabilizes the three-dimer compared to the two-dimer structure. The same argument should hold if the As dimers are substituted by another group-V element like Sb, thus questioning the validity of the suggested structure for the Sb-stabilized ͑2 3 4͒ GaAs(100) surface. Meanwhile, x-ray standing wave (XSW) experiments [13] confirmed the existence of Sb dimers on the surface; however, conclusive results about the validity of the three-Sb-dimer model [1] were not obtained.The present study is aimed at the determination of the atomic structure of the Sb-stabilized GaAs(100)-͑2 3 4͒ surface, using a novel approach to investigate microscopic surface properties based on the surface optical response. We apply reflectance anisotropy spectroscopy (RAS), ab initio total-energy minimization (TE), and tightbinding (TB) calculations of optical properties to study the precise surface ge...

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In this study we report on the microscopic structure of GaSb͑001͒ c(2ϫ6) surfaces prepared by Sb decapping. Molecular beam epitaxy grown GaSb͑001͒ layers capped with a protective Sb layer were transferred through the atmosphere into an UHV-analysis system and investigated by scanning tunneling microscopy ͑STM͒ and low energy electron diffraction ͑LEED͒. After thermal desorption of the capping layer clear c(2ϫ6) LEED patterns were observed. STM images show flat surface areas with a rowlike, somewhat disordered structure. High-resolution images resolve individual Sb dimers on the surface. The surface is covered by an incomplete layer of dimerized Sb, adsorbed on a complete second layer of Sb, which is also dimerized in that regions not covered by the fractional Sb top layer.

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