Determinants of surface atomic geometry: The CuCl(110) test case

Abstract

The atomic geometry of the CuCKl 10)-(1 x 1) surface is determined by dynamical analysis of lowenergy electron-diffraction intensities. This surface undergoes a relaxation characterized by a -30° Cu-Cl surface bond rotation, a 0.15 A contraction of the top-to-second layer distance, and a 0.4 A horizontal displacement of CI relative to Cu. The relaxation is consistent with the "universal" structure deduced from the analysis of cleavage surfaces of tetrahedrally coordinated III-V and II-VI compounds, thereby revealing that this feature of the structure does not depend significantly on the ionicity of the compound.PACS numbers: 61.l4.Hg, 68.35.BsFor decades the study of the dependence of surface atomic geometries on parameters characteristic of the bulk chemical bonding [1,2] (e.g., crystal class, lattice parameters, bonding character) has been a major topic in surface science. Historically, the cleavage faces of tetrahedrally coordinated compound semiconductors have been a fertile ground for the proposition and testing of such structure-bonding relationships because of their reproducible surface composition and extensive structure studies [3-5]. In recent years, however, the validity of earlier experimentally established scaling rules [6] has been questioned by a series of model predictions of the surface structures of the cleavage faces of zinc-blendestructure compounds [7]. The case of CuCl(llO) emerged from these predictions as a crucial test case in which the scaling rules failed completely [7]. Our purpose in this paper is to report a structure analysis of CuClO 10) revealing that it is compatible with the experimentally established scaling rules and hence constitutes a critical piece of experimental evidence that atomic size and surface topology rather than bonding ionicity are the dominant determinants of the surface atomic geometries of the (110) cleavage surfaces of zinc-blende-structure compound semiconductors. Zinc-blende (110) surfaces experience significant (-1A) relaxation of surface atomic positions from their bulk positions. Because of the reproducible 1:1 stoichiometry of these surfaces, the questions of the dominant driving force of the relaxations and their correlation with bulk structural and energetic parameters may be posed precisely. The earliest quantitative relaxation model predicted a link between relaxation and ionicity based on the small-molecule chemistry of the threefold-coordinated surface species [8]. According to this model, the surface anion and cation of mostly covalent III-V compounds would assume pyramidal p-like and planar 5"p 2 -like bonding configurations, leading to the observed bond rotation, whereas their counterparts on the more ionic II-VI compounds would not. Yet, all geometries subsequently determined appeared to be largely independent of the ionicity of the compound, exhibiting an approximately constant bond rotation angle of 29° ±3° [4,5]. Moreover, atomic displacements were found to scale in such a way that all materials exhibit the same basic structure when dimensi...

Defect-induced nonpolar-to-polar transition at the surface of chalcopyrite semiconductors

“…We generalized the usual coordinates used to define such relaxations 5 by defining separate rotation angles for the two distinct surface cations with respect to an average position of the two surface anions, obtaining Cu ϭ36.1°and In ϭ38.7°. These surface rotation angles are larger than in typical II-VI and III-V compounds (25°Ͻ Ͻ33°) and approach those seen in experimental studies of the CuCl ͑110͒ surface ͑41.3°and 44.1°, respectively.͒ 24 These large rotation angles reflect the greater ionic character in CIS relative to the III-Vs and II-VIs. Bonds in the second atomic layer of the CIS ͑110͒ slab rotate by about Ϫ6°while those in the third layer rotate from the plane of the slab by less than 1°.…”

Ab initioprediction of GaN (101¯0) and (110) anomalous surface relaxation

“…15 However, the I-VII noble-metal halides ͑e.g., CuCl͒, which are the most ionic group of tetrahedrally coordinated compounds, have very large ͑110͒ surface rotation angles ͑ϳ41°-44°͒. 16 Thus ionicity cannot be a general case of small surface rotations, though it does appear to be important 16 for surface bond contraction. Another possibility would be the large size mismatch between the Ga and N atoms, e.g., the difference of 0.56 Å in their Pauling 17 covalent radii.…”

Low-energy positron diffraction from CdTe(110):mA minimum-variance R-factor analysis

“…Second, beginning in 1987, Kasowski and co-workers 5-7 revived an earlier notion 8-10 that the zinc-blende ͑110͒ surface structures should depend sensitively on ionicity and at high ionicities collapse to slightly relaxed bulklike structures. Although contradicted by several early experimental results 11,12 and subsequent structural studies, 13,14 this notion of ionicityinduced structural collapse seems to live on. Thus the present study of highly ionic ͑Phillips ionicity 15 f i ϭ0.72͒ CdTe͑110͒ by LEPD was undertaken to provide another independent experimental test of its validity.…”