Etching or Stabilization of GaAs(001) under Alkali and Halogen Adsorption O. E. Tereshchenko,* ,†,‡ D. Paget, § K. V. Toropetsky, † V. L. Alperovich, †,‡ S. V. Eremeev, ∥,⊥ A. V. Bakulin, ∥,⊥ S. E. Kulkova, ∥,⊥ B. P. Doyle, #,▽ and S. Nannarone #,○ † Institute of Semiconductor Physics, Novosibirsk, 630090 Russian Federation ‡ Novosibirsk State University, Novosibirsk, 636090 Russian Federation § Physique de la matie ̀ re condense ́ e, Ecole Polytechnique, CNRS, F-91128 Palaiseau, France ∥ Institute of Strength Physics and Materials Science, Tomsk, 636021 Russian Federation ⊥ Tomsk State University, Tomsk, 634050 Russian Federation # Laboratorio Nazionale TASC, INFM-CNR, 34012 Trieste, Italy ▽ University of Johannesburg, Auckland Park 2006, South Africa ○ Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Universita di Modena e Reggio Emilia and CNISM, Via Vignolese 905, 41100 Modena, Italy ABSTRACT: Experimentally and by ab initio calculations it is shown that adsorption of electropositive cesium on the As-rich surface of GaAs(001) and, in a symmetric fashion, adsorption of electronegative iodine on the Ga-rich surface, induce a decrease of the surface stability, thus facilitating surface etching. Conversely, Cs adsorption on the Ga- rich surface and I adsorption on the As-rich surface lead to an increased surface stability. Etching occurs when adsorption-induced charge transfer weakens the backbonds of the top arsenic atoms for the case of Cs on the As-rich β2(2 × 4) surface and the lateral bonds in the topmost surface layer for I on the Ga-rich ζ(4 × 2) surface. The possibilities of reversible transitions between the two reconstructed surfaces and of atomic layer etching with monolayer precision are demonstrated. ■ INTRODUCTION The advanced modifications of epitaxial growth techniques such as migration enhanced and atomic layer epitaxies 1,2 allow one to grow semiconductor structures in which the interface smoothness and the thickness of layers are controlled with the ultimate precision of one monolayer. Along with atomic-layer growth, for modern nanotechnology, it is important to develop techniques of atomic-layer (“digital”) etching, which consist of layer-by-layer removal of a semiconductor with monolayer resolution. For III−V semiconductors, atomic layer etching can be realized by using adsorbates, which selectively react with polar surfaces enriched with elements of III or V groups and thus allow selective removal of surface cations or anions. It was shown in ref 3 that iodine adsorption on the Ga-rich GaAs(001) surface, followed by low-temperature annealing, led to the desorption of gallium iodides GaI x and conversion to the As-rich surface with a (2 × 4) reconstruction. Conversely, the adsorption of cesium on the As-rich GaAs(001)-(2 × 4) surface facilitates low-temperature removal of surface arsenic and back transfer to the Ga-rich (4 × 2) reconstruction. 4 Although alternate depositions of iodine and cesium followed by low-temperature anneals 5 can be used for in situ atomic layer etching of GaAs(001), the mechanisms underlying this opportunity are not clear. Because charge exchange from the electropositive Ga to electronegative As is known to stabilize the polar GaAs(001) surface, 6 it can be anticipated that adsorption of electronegative (halogen) or electropositive (alkaline) elements on anion-rich and cation-rich GaAs(001), respectively, leads to an opposite transfer and therefore to a weakening of the backbonds of the topmost atoms. However, this is an essentially electronic mechanism that could be neither proved nor rejected by the structural data presented in refs 4 and 5 as well as by other previous results on the I/GaAs(001) and Cs/GaAs(001) surfaces. 3,7−11 The present work is aimed at the elucidation of the microscopic mechanisms of the selective interaction of iodine and cesium with As-rich β2(2 × 4) and Ga-rich ζ(4 × 2) reconstructed GaAs(001) surfaces by means of photoemission spectroscopy and ab initio calculations, which yield information on the electronic properties of the surfaces. It is found that the “weakening of backbonds” mechanism is not universal and is responsible for the decrease in surface stability only for the Cs/ Received: November 25, 2011 Revised: March 23, 2012 Published: March 29, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 8535 dx.doi.org/10.1021/jp211360d | J. Phys. Chem. C 2012, 116, 8535−8540