Surface reconstructions and phase transitions on the GaAs(111)B surface

The (111)B surface of GaAs has been investigated using scanning tunneling microscopy (STM) and a number of different reconstructions have been found at different surface stoichiometries. In accordance with electron diffraction studies, we find the series (2 × 2), (1 × 1)_LT, ( ) and (1 × 1)_HT with increasing annealing temperature, corresponding to decreasing surface As concentration. The (1 × 1)_LT is of particular interest, since it only occurs in a narrow temperature window between the two more established reconstructions, the (2 × 2) and the ( ). We find the (1 × 1)_LT to take the form of a mixture of the local structures of both the (2 × 2) and ( ) phases, rather than having a distinct structure. This is behaviour consistent with a kinetically limited system, dominated by the supply of As adatoms to the surface, and may be an example of a continuous phase transition. Above the (1 × 1)_LT transition, atomic resolution images of the ( ) surface reveal only a three-fold symmetry of the hexagonal structural units, brought about by inequivalent surface bonding due to the 23.4° rotation of the surface unit cell relative to the substrate. This is responsible for the disorder found in the ( ) reconstruction, since the structure may form in one of two domains. At lower surface As concentration, the (1 × 1)_HT surface adopts a structure combining small domains of a 19.1° structure and random disorder. There is no apparent similarity between the (1 × 1)_LT and (1 × 1)_HT structures, which may be due to our measurements being conducted at room temperature and without an As flux to control the surface As concentration.     

GaAs(0 0 1) surface reconstructions: geometries, chemical bonding and optical properties

We re-examine the GaAs(0 0 1) surface by means of first-principles calculations based on a real-space multigrid method. The c(4×4),(2×4) and (4×2) surface reconstructions minimize the surface energy for anion-rich, stoichiometric and cation-rich surfaces, respectively. Structural models proposed in the literature to explain the Ga-rich GaAs(0 0 1) (4×6) surface are dismissed on energetic grounds. The electronic properties of the novel ζ(4×2) structure are discussed in detail. We calculate the reflectance anisotropy of the energetically most favoured surfaces. A strong influence of the surface geometry on the optical anisotropy is found.     

Well-ordered (1 0 0) InAs surfaces using wet chemical treatments

Atomic ordering of HCl-isopropanol (HCl-iPA) treated and vacuum annealed (1 0 0) InAs surfaces was studied by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and reflectance anisotropy spectroscopy (RAS). On the as-treated surface, a diffused (1 × 1) pattern is observed, which successively evolves to the β₂(2 × 4)/c(2 × 8) and (4 × 2)/c(8 × 2) ones after annealing to 330 °C and 410 °C, respectively. At the intermediate temperature of 370 °C, an ₂(2 × 4)/(4 × 2) mixed reconstruction is observed. Reflectance anisotropy spectra are compared with those of the corresponding reconstructions observed after As-decapping and found to be quite similar. Therefore we conclude that high-quality (1 0 0) InAs surfaces can be obtained by wet chemical treatment in an easy, inexpensive and practical way.     

Large scale atomic ordering on uncovered GaAs(0 0 1) after InAs monolayer capping: Atomic structure of the (12 × 6) reconstruction

A well ordered c(8 × 2)-InAs monolayer is grown by molecular beam epitaxy (MBE) on a GaAs(0 0 1) substrate. After slow sublimation of this monolayer up to 560 °C, a homogeneously (n × 6) reconstructed GaAs surface is obtained. This surface is studied by scanning tunneling microscopy (STM) in UHV. This shows that it is well-ordered on a large scale with 200 nm long As dimer rows along and is also locally (12 × 6) reconstructed, the cell structure is proposed. We believe that this surface organization results from the specific As/Ga (0.7) surface atomic ratio obtained after the InAs monolayer growth and sublimation cycle.     

Hydrogen-induced missing-row reconstructions of Pd(110) studied by scanning tunneling microscopy

The effect of hydrogen adsorption on the Pd(110) surface structure at room temperature has been studied by scanning tunneling microscopy. Depending on the partial pressure of hydrogen two different reconstructions of Pd(110) have been observed: a (1 × 3) phase at hydrogen pressures in the 10⁻⁹ mbar range and an additional (1 × 2) phase at p_H2 ≥ 5 × 10⁻⁸ mbar. Both reconstructions are found to be of the missing-row type. The evolution of the surface reconstructions has been followed in situ.     

Initial stages of praseodymium growth on Si(111): morphology and electronic structure

An STM study of the Pr---Si(111)-(7 × 7) interface reveals a much more heterogeneous growth morphology than is suggested by diffraction techniques and spectroscopies which average over the surface. Deposition of 1 ML followed by annealing at 650°C gives the most orderly growth, but this falls short of 2D epitaxy. At high temperatures (> 1000°C), the small amount of rare earth remaining on the surface stabilises some novel Si(111) surface reconstructions.     

Metastable (1 × 2) and (1 × 3) reconstructions of Pd(110)

By adsorption and subsequent reduction of oxygen on Pd(110), metastable (1 × 2) and (1 × 3) reconstructed surfaces have been produced. Oxygen was not present after the reduction but a small amount of residual hydrogen (< 0.15 monolayers) remained. However this is not the origin of the reconstruction as adsorption of this amount of hydrogen on the clean surface did not cause reconstruction. The structures were stable up to ˜ 370 K, and at higher temperatures they reverted to (1 × 1). These results are compared with Rh(110) where similar reconstructions have been found.     

STM and LEED observations of erbium silicide nanostructures grown on Si(1 0 0) surface: atomic-scale understandings

In this work, erbium silicide is grown on the Si(1 0 0) surface by depositing Er onto the substrate and annealing at 600–700 °C. Many nanowires of Er silicide are formed with lengths in the range 10–100 nm. The formation and evolution of this nanostructure are investigated at atomic scale directly with scanning tunneling microscopy and low-energy electron diffraction. The direction of these nanowires is found perpendicular to that of Si dimer rows. It is shown that Er coverage and annealing temperature have an effect on the formation of nanowires. On the surface between nanowires, new (5×2) and c(5×4) reconstructions are observed, giving an implication to understand the growth behaviors of Er silicide on Si(1 0 0) surface.     

Surface structure evolution during Sb adsorption on Si(1 1 1)–In(4×1) reconstruction

The Sb adsorption process on the Si(1 1 1)–In(4×1) surface phase was studied in the temperature range 200–400 °C. The formation of a Si(1 1 1)–InSb (2×2) structure was observed between 0.5 and 0.7 ML of Sb. This reconstruction decomposes when the Sb coverage approaches 1 ML and Sb atoms rearrange to and (2×1) reconstructions; released In atoms agglomerate into islands of irregular shapes. During the phase transition process from InSb(2×2) to Sb (θ_Sb>0.7 ML), we observed the formation of a metastable (4×2) structure. Possible atomic arrangements of the InSb(2×2) and metastable (4×2) phases were discussed.     

Atomic structure of β-SiC(100) surfaces: an ab initio study

We examine several different reconstructions of the β-SiC(100) surface by the ab initio Car-Parrinello method. Our results confirm that the lowest energy c(2 × 2) reconstructed surface consists of triply bonded carbon dimers in a bridging position between neighboring underlying silicon dimers. Added hydrogen atoms bond to the carbon dimers, resulting in a lengthened double-bonded dimer, and a larger separation for the underlying silicon dimers, although those Si bonds do not disappear. The most stable structure found for the (3 × 2) reconstructed surface with a 1/3 monolayer excess of silicon is an alternate dimer row structure rather than the added dimer row model proposed by others. The energetics of various surface reactions that may be involved in growth of SiC are discussed.     

Dynamic observations of the formation of thin Cu layers on clean and hydrogen-terminated Si(1 1 1) surfaces

The growth of Cu on the clean and hydrogen-terminated Si(1 1 1) surfaces is studied in situ by low-energy electron microscopy (LEEM). The dependence of the growth of the “5×5” layer on the clean Si(1 1 1) 7×7 surface upon the deposition temperature is investigated by combining LEEM with LEED. After completion of the “5×5” layer not only the regular-shaped three-dimensional islands reported before are observed but also irregular shaped more two-dimensional islands. On the hydrogen-terminated Si(1 1 1) surface the formation of the “5×5” structure is suppressed and nano-scale islands form preferentially at the step edges and domain boundaries. This is attributed to the enhancement of the surface migration of Cu atoms by the elimination of the surface dangling bonds.     

InAs wetting layer evolution on GaAs(0 0 1)

The evolution of two-dimensional (2D) strained InAs wetting layers on GaAs(0 0 1), grown at different temperatures by molecular beam epitaxy, was studied by in situ high-resolution scanning tunneling microscopy. At low growth temperature (400 °C), the substrate exhibits a well-defined GaAs(0 0 1)-c(4 × 4) structure. For a disorientation of 0.7°, InAs grows in the step-flow mode and forms an unalloyed wetting layer mainly along steps, but also in part on the terrace. The wetting layer displays some local c(4 × 6) reconstruction, for which a model is proposed. 1.2 monolayer (ML) InAs deposition induces the formation of 3D islands. At a higher temperature (460 °C), the wetting layer is obviously alloyed even at low InAs coverage. The critical thickness of the wetting layer for the 2D-to-3D transition is shifted to 1.50 ML in this case presumably since the strain is reduced by alloying.     

Morphology and composition of Au films on Si(100)

Spot profile analysis low-energy electron diffraction, low-energy ion scattering and Auger electron spectroscopy were employed to study the morphology and composition of Au films on Si(100). After annealing, two distinct surface reconstructions were observed: a two-domain c(8×2) phase and a four-domain incommensurate (5×3.2)R5.7° phase. During the transition from the c(8×2) to the (5×3.2)R5.7° phase, the subsurface composition changes drastically from Au-rich to Si-rich, whereas the outermost layer composition remains almost constant (about 65 at.% Au). Detailed information concerning the domain structure for the two phases is subtracted from the profiles of the LEED spots.     

The mesoscopic and microscopic structural consequences from decomposition and desorption of ultrathin oxide layers on Si(100) studied by scanning tunneling microscopy

The spatially inhomogeneous decomposition and desorption reaction of oxide layers with coverage 1-0.3 monolayers (ML) from a silicon (100) surface has been studied using scanning tunneling microscopy (STM). After desorption, microscopic changes to the (2 × 1) reconstruction produce two variations on the dimer row reconstruction with decreased surface atom density. A (2 × n) vacancy chain reconstruction and a c(4 × 4) incomplete row reconstruction were observed; a structure for the latter is proposed. Both reconstructions are metastable, reforming the (2 × 1) reconstruction upon heating. At greater length scales during desorption from an initial 1.0 ML coverage, the mesoscopic changes to the surface structure include pitting and roughening, with up to a measured 20 fold increase in the edge density as compared to the clean Si(100) surface.

These structural changes suggest a reaction mechanism involving a substantial rearrangement of the substrate silicon. From an initial 1.0 ML oxygen coverage, using measured void size distributions at total desorption levels of 13% and below — before voids have begun to coalesce — the evolution of void sizes during initial desorption can be followed. A mechanism for desorption is proposed in which silicon atoms must diffuse from adjacent clean surface area to the oxide boundary, producing a reactive complex from which SiO is desorbed. Void growth rates derived from two rate limiting cases for this desorption reaction mechanism can be compared to measured results. We show that the measured void area evolution is consistent with a reaction mechanism where the rate limiting step for monolayer desorption is the promotion of a silicon atom in a lattice site to a mobile monomer within the void.     

Rotational epitaxy of a hexagonal layered material on a square substrate: PbI2 on InSb(001)

PbI₂ has been adsorbed on the clean InSb(001)-(4 × 1) reconstructed surface and on the InSb(001)-(1 × 3)-Pb lead covered reconstructed surface. On the clean surface epitaxial growth occurred with the unit mesh of the layered PbI₂ aligning exactly with both the substrate [110] and [1 0] directions. On desorption a reaction occurred between the last layer of PbI₂, and the substrate, forming a series of structures which finished with a well-formed (1 × 3)-Pb structure in which the surface is depleted/enriched in In/Sb compared to the clean (4 × 1). The Pb in this structure is thought to partially replace surface In. Epitaxial adsorption also occurred on the (1 × 3)-Pb surface generating a single, well-formed structure with the hexagonal net of the PbI₂ aligned with just the [1 0] substrate direction. The structures and reactions are discussed and a row matching model is proposed to explain the single epitaxial orientation of PbI₂ on the (1 × 3)-Pb surface.     

Boron and indium substitution in GaAs(0 0 1) surfaces: Density-functional supercell calculations of the surface stability

Using the density-functional method and surface supercells the surface formation energies are calculated for the most stable GaAs(0 0 1) surface reconstructions without and with up to four indium or/and boron substitutions. Optimal conditions for the growth of the alloys are derived from calculated surface stability diagrams. The incorporation of indium into GaAs without phase separation is possible under strong As-rich conditions and medium to In-rich conditions. Less As-rich conditions can lead to the formation of an InAs phase. Ga-rich conditions give an InGa phase. Isovalent boron incorporation into GaAs without phase separation is possible under strong arsenic and reduced boron exposure. A BAs phase can be formed under more B-rich conditions. More Ga-rich conditions lead to the boron substitution in arsenic positions. The formed boron dimers can be a starting point for the formation of a boron phase. A true antisite boron substitution is less probable. Using the suitable growth conditions obtained for the ternary alloys it is energetically more favourable to incorporate both indium and boron (formation of BInGaAs) than to incorporate only one of the two elements (In or B). The antisite boron incorporation is not favoured in combination with isovalent boron or indium.     

Spatially resolved dynamics of the TiO2(1 1 0) surface reconstruction

We use low-energy electron microscopy to image the reversible transformation of the TiO₂(1 1 0) surface between a high-temperature 1 × 1 structure and a low-temperature 1 × 2 structure. The reconstruction dynamics are novel: 1 × 2 bands nucleated during cooling at the steps of the starting 1 × 1 surface and then grew laterally from the steps. The transformation kinetics are dominated by mass flow from the surface to the bulk, a process that facilitates converting the high-density 1 × 1 phase to the lower-density 1 × 2 phase. We have also imaged how the 1 × 1 surface reconstructs to 1 × 2 phase after sufficient oxygen is removed from the crystal’s bulk during vacuum annealing. 1 × 2 bands also nucleated and grew laterally from the initial 1 × 1-surface’s steps. However, because this isothermal 1 × 1-to-1 × 2 transition occurs largely by mass redistribution on the surface, the steps of the initial 1 × 1 surface and final 1 × 2 surface are offset. We propose models of mass redistribution during the 1 × 1/1 × 2 phase transition to explain this effect. We conclude that the phase transition is first-order because it always occurred by the nucleation and growth of discrete phases. Finally, we show that quenching can roughen TiO₂’s surface by forming pits and that changing temperature causes step motion on 1 × 2 surfaces.     

In situ measurements of InAs and InP (0 0 1) surface stress changes induced by surface reconstruction transitions

The anisotropic surface stress changes associated with the transition between different surface reconstructions of InAs and InP (0 0 1) surfaces are measured in situ and in real time in a molecular beam epitaxy (MBE) system. Reflectivity anisotropy of the surface measured at 1.96 eV, together with reflection high energy electron diffraction (RHEED) pattern, are used in order to identify the surface reconstructions, and the monitoring of the substrate curvature evolution to determine the variations in surface stress. Our results show the important contribution to the surface stress of the dimers present in these reconstructed surfaces. Furthermore, we provide for the first time quantitative values of the surface stress changes due to the transition between surface reconstructions for these III-V semiconductors compounds. We obtain values for these changes up to 0.7 Nm⁻¹, that is, of the same magnitude as the stress induced by deposition of one monolayer during growth of lattice-mismatched III-V semiconductor heteroepitaxial systems. This points out the great importance of surface stress evolution in this kind of processes.     

Morphological anisotropy during migration enhanced epitaxy of GaAs(0 0 1)

The GaAs(0 0 1) surface morphology and structure during growth by migration enhanced epitaxy (MEE) has been studied by reflection high energy electron diffraction and scanning tunneling microscopy. Changes induced by varying the incident As/Ga flux ratio, growth temperature and the total amount of material deposited in each cycle have been studied and the results compared with GaAs(0 0 1) growth by conventional molecular beam epitaxy (MBE). Comparison of the surface morphology at the end of the Ga and As cycles indicates no clear evidence for any enhancement in the Ga adatom diffusion length during the Ga cycle. However, the morphological anisotropy of the growth front does change significantly and it is proposed that this changing anisotropy during MEE enables Ga adatom diffusion along both azimuths. The surface anisotropy during MEE growth is found to increase with the Ga/As ratio. Although there is a clear correlation between composition and morphology, we have also found that highly ordered and flat surfaces are not necessarily an indication of stoichiometric material. We also attempt to clarify a recent controversy on the structure of the c(4 × 4) reconstruction by studying the surface structure at the end of the As cycle as a function of the As/Ga ratio.     

Atomic structures of GaAs(100)-(2 × 4) reconstructions

Atomic structures of the As-rich GaAs(100)-(2 × 4) reconstructions based on converged first-principles total-energy calculations are reported. All geometries are characterized by similar structural elements such as As dimers with a length of about 2.5 Å, dimer vacancies, and a nearly planar configuration of the three-fold coordinated second-layer Ga atoms leading to a steepening of the dimer block. For an As coverage of θ = 3/4 we find the two-dimer β2 phase to be energetically preferred over the three-dimer β phase. A structure with partial replacement of As by Ga in the uppermost layer corresponding to an As coverage of is found to be slightly less favourable than the phase of GaAs(100). Geometry parameters are given for all structures and compared with the available experimental results.