Fractional Stoichiometry of the GaAs(001)c(4×4)Surface: AnIn-SituX-Ray Scattering Study

Abstract

Synchrotron x-ray diffraction analysis of GaAs(OOl) epilayers grown in situ by molecular-beam epitaxy shows clear evidence of As-As dimers on top of the outermost As layer. These dimers are ordered in a variable way between two structures which both show a c(4x4) symmetry but have different As content. Both structures have twofold symmetry although the unit cell is square.PACS numbers: 61.10.Jv, 68.35.Bs, 68.55.Bd Compound-semiconductor surfaces are fundamentally different from their elemental counterparts because they have an additional composition degree of freedom. GaAs(OOl) is a good example with a sequence of surface symmetries starting with the most As-rich c(4x4) and ending with the 4x6 on the Ga-rich side. 1 ' 2 Change of symmetry is certainly an indication of change of structure, but it is entirely possible that several structures have the same symmetry. For example, total-energy calculations performed on a 2x4 surface cell 3 have demonstrated that two structures with As coverages equal to 0.5 and 0.75, respectively, were equally stable; the latter being primarily observed by scanning tunneling microscopy, 4 together with small patches of other arrangements. Here we show that the c(4x4) state of GaAs(OOl) adjusts its structure between two extremes to accommodate different amounts of As.The c(4x4) is the only reported GaAs(OOl) reconstruction exhibiting an apparent fourfold symmetry which seems to disregard the "native" twofold symmetry induced by the dangling bond orientation on this polar surface; it actually corresponds to an excess of As and may be considered a chemisorbed-type phase produced by chemisorption of either (As)4 or (As)2 molecules. Previous work by LEED in conjunction with molecularbeam epitaxy and later by reflection high-energy electron diffraction on the c(4x4) GaAs(OOl) surface 5 " 7 has not identified a structure. The only model proposed so far comes from photoemission studies which clearly identify As-As covalent bonding not present in the bulk and suggest the presence of As-As dimers. 8 " 10 While LEED or reflection high-energy electron diffraction are useful for surveying surface symmetry, they are difficult to apply for quantitative evaluations because of multiple scattering. On the contrary, in the case of grazing incidence xray diffraction, 11 the diffracted intensities can be interpreted in the simple framework of the kinematical theory and concerning clean semiconductor surfaces, reliable results have been obtained for Si(ll 1)7x7, 1213 Ge(001)2xl, 14 InSb(lll)2x2, 15 and GaSb(lll)-2x2. 16 The present results on GaAs(001)c(4x4) are the first dealings with a GaAs reconstructed surface obtained by grazing incidence x-ray diffraction. The presence of As-As dimers is confirmed with a dimer bond length of 2.59 ±0.06 A, significantly longer than the value for bulk As. Moreover, direct inspection of the x-ray data demonstrates that the true symmetry of the reconstructed surface is not fourfold but belongs to the ram 2 twodimensional symmetry group, in agreement with the bonding s...

Anisotropic diffusion of In adatoms on pseudomorphicInxGa1−xAsfilms: First-p

“…19,20 The latter reconstruction is present on the GaAs substrate when depositing under As-rich conditions. 21,22 The next challenging question concerns the In diffusivity on the WL before the critical thickness is reached, i.e., for Ͻ c .…”

Atomic and electronic structures of GaAs (001) surface

“…Finally, the structure of the γ phase also occurring for As-rich surfaces was found to be a mixture of the β phase and с (4 х 2) phase depending on the actual growth conditions [12,7]. First-principles calculations were performed for several (4 х 4) structures with a varying number of surface dimers [16] and a three-dimer structure [18] was shown to have the lowest surface energy. The calculated As dimer length in the β2 phase [16] was 2.45 and 2.50 Å, which agrees well with experiment (2.2 -2.9 Å) [18,19].…”

“…First-principles calculations were performed for several (4 х 4) structures with a varying number of surface dimers [16] and a three-dimer structure [18] was shown to have the lowest surface energy. The calculated As dimer length in the β2 phase [16] was 2.45 and 2.50 Å, which agrees well with experiment (2.2 -2.9 Å) [18,19]. The As dimer length obtained in [19] was 2.53 and 2.55 Å.…”

Atomic structures on a GaAs(001) surface grown by molecular beam epitaxy

“…, ×ÑÕÑàÎÇÍÕÓÑÐÐÂâ ÔÒÇÍÕÓÑÔÍÑÒËâ [26 ë 28], ËÊÏÇÓÇÐËÇ ÓÂÃÑÕÞ ÄÞØÑAE [25], AEË×ÓÂÍÙËâ ÓÇÐÕÅÇÐÑÄÔÍËØ ÎÖÚÇÌ [29] Ë ÓÇÐÕÅÇÐÑÄÔÍËØ ×ÑÕÑàÎÇÍÕÓÑÐÑÄ [30], AEË××ÇÓÇÐÙËÂÎßÐÂâ ÑÕÓÂÉÂÕÇÎßÐÂâ ÔÒÇÍÕÓÑÔÍÑÒËâ [31], ÔÒÇÍÕÓÑÔÍÑÒËâ ØÂÓÂÍ-ÕÇÓËÔÕËÚÇÔÍËØ ÒÑÕÇÓß àÐÇÓÅËË àÎÇÍÕÓÑÐÑÄ [32], ËÊÏÇÓÇÐËÇ ËÐÕÇÐÔËÄÐÑÔÕË ÄÕÑÓËÚÐÞØ àÎÇÍÕÓÑÐÑÄ [33], ÓÂÔÔÇâÐËÇ ËÑÐÑÄ ÔÓÇAEÐËØ àÐÇÓÅËÌ [34] Ë ³´® [35 ë 43]. ±ÓË àÕÑÏ ÐÂËÃÑÎÇÇ ËÐÕÇÐÔËÄÐÑ ËÔÔÎÇAEÑÄÂÐÐÞÏË ÑÍÂÊÂÎËÔß ÑÃÑÅÂ-ÜÇÐÐÞÇ As ×ÂÊÞ 2  4 Ë c4  4, ÕÂÍ ÍÂÍ ÖÔÎÑÄËÇ ÑÃÑÅÂÜÇÐËâ As âÄÎâÇÕÔâ ÔÖÜÇÔÕÄÇÐÐÞÏ AEÎâ ÒÓÑÙÇÔÔ ®¿.¯ÇÔÍÑÎßÍÑ ÒÑÊÉÇ AEÎâ ÓÇÛÇÐËâ ÄÑÒÓÑÔÂ Ñ ÒÓÇAE-ÒÑÚÕËÕÇÎßÐÑÔÕË ÕÑÌ ËÎË ËÐÑÌ ÒÑÄÇÓØÐÑÔÕÐÑÌ ÔÕÓÖÍÕÖÓÞ c ÕÑÚÍË ÊÓÇÐËâ ÏËÐËÏËÊÂÙËË ÒÑÄÇÓØÐÑÔÕÐÑÌ àÐÇÓÅËË ÔËÔÕÇÏÞ ÃÞÎË ËÔÒÑÎßÊÑÄÂÐÞ ÕÇÑÓÇÕËÚÇÔÍËÇ ÓÂÔÚÇÕÞ àÎÇÍÕÓÑÐÐÑÌ ÔÕÓÖÍÕÖÓÞ ÒÑÄÇÓØÐÑÔÕË GaAs(001) [44 ë 48].…”