Sb Overlayers Research Articles

Structure of Sb monolayers on Ge(111)2×1: A combined study using core-level photoemission, x-ray standing waves, and surface extended x-ray absorption fine structure

The atomic stucture of an annealed monolayer of Sb on the Ge(111)-(2×1) surface has been studied with core-level photoemission, x-ray standing waves (XSW), and surface extended x-ray absorption fine structure (SEXAFS). Phase analysis of the SEXAFS data taken over the Sb L3 edge using bulk Sb and GaSb as standards determines the bond lengths within the first Sb shell to be 2.87±0.03 Å and 2.67±0.03 Å for the Sb–Sb and Sb–Ge bonds, respectively. The XSW data for (111) planes, taken in the backreflection diffraction geometry, place the Sb layer 2.60±0.05 Å above the Ge(111) surface. A relatively high coherent fraction (0.85) indicates small disorder or buckling in the Sb overlayer. The Sb overlayer is found to quench the clean surface shifted components of the Ge 3d core-level spectrum, which indicates an ideal termination of the Ge(111) surface with all dangling bonds saturated by the Sb adatoms. Lack of chemically shifted components in both the Ge 3d and the Sb 4d core level spectra indicates a single bonding site for the Sb atoms. Although it is not possible to differentiate between Sb trimers [as found for the Sb/Si(111) interface] or zigzag chains (common for Sb/III–V interfaces) on the Ge surface, substitutional geometries, in which Sb replaces the topmost layer of the Ge surface bilayer [as found for As/Ge(111)], can be ruled out.

Fermi-level variation on GaAs(110) surface with Sb overlayer studied with a photoelectron microscope

Bare and Sb covered GaAs(110) surfaces were studied with a photoelectron microscope. For the cleaved surfaces, maximum band bendings of 0.85 and 0.50 eV was observed for n- and p-type GaAs, respectively. For n-type, evaporation of Sb reduces the band bending from 0.85 to 0.55 eV. Annealing p-GaAs at 350 °C almost restored the flatband condition for an initially unpinned area. A reduction of band bending was observed for an initially heavily pinned area. This suggests that the cleavage defects originate partly from surface strain which can be removed by relaxing the strain with the formation of an ordered Sb overlayer. Sb desorbs from the GaAs surfaces when annealed at 560 °C. The Fermi level on this surface is at 0.35 eV above the valence-band maximum regardless of initial band bending conditions. The new surface is attributed to the destructive process of Sb desorption.

X-ray standing-wave study of monolayers of Sb on GaAs(110).

The x-ray standing-wave technique has been used to determine the geometrical structure of Sb on GaAs(110). Using the back reflection diffraction geometry from (220) and (400) planes, we find the average vertical distance of Sb atoms from the GaAs surface to be 2.27\ifmmode\pm\else\textpm\fi{}0.05 \AA{} in agreement with theoretical calculations for the epitaxial continued layer strucuture. Other models of buckled zigzag chains are found inconsistent with our results. Additionally, our data indicate less than 10% disorder in the Sb overlayer.

Strain and the two-dimensional electronic structure of monolayers of Bi/InAs(110) and Bi/GaAs(110)

The two‐dimensional band structure of monolayers of the Bi/InAs(110)(1×1) and Bi/GaAs(110)(1×1) systems have been measured using the technique of angle‐resolved photoemission spectroscopy with a synchrotron light source. Measurements of the dispersion and bandwidth of the uppermost occupied electronic state nearest the Fermi level revealed that the bandwidth was significantly smaller for the Bi/InAs(110) system. A comparison to available theoretical calculations of Sb overlayers on III–V semiconductor surfaces implies that the differences between the two Bi systems is greater than would be predicted by a standard theoretical analysis. The authors interpret the results as indicating the importance of strain in these systems.

Quenching the surface electronic structure of P-containing III–V semiconductors via ordered (1×1) Sb overlayers: a PL2,3VV Auger line shape analysis

We report experimental P L2,3VV Auger line shapes of as-cleaved GaP(110) and InP(110) surfaces and of the relative Sb p(1 × 1) interfaces along with calculated slab-resolved densities of states (DOS). The joint experimental and theoretical analysis allows us to disentangle the surface-specific P L2,3VV contribution from the total signal of the as-cleaved surfaces.

Orientational ordering and domain-wall formation in Sb overlayers on GaAs(110).

Real-space scanning tunneling microscope images of Sb thin films on GaAs(110) show disordered surface structures at 300 K, but an ordered overlayer occurs when annealed. The annealed incommensurate Sb thin film shows grains with distinct superstructure orientations. Within each Sb grain, the superstructure exhibits 25-\AA{} periodicity and alignment either 40\ifmmode^\circ\else\textdegree\fi{} or 140\ifmmode^\circ\else\textdegree\fi{} with respect to [11\ifmmode\bar\else\textasciimacron\fi{}0] of the GaAs substrate. Striped domain walls with a mean separation of 110\ifmmode\pm\else\textpm\fi{}7 \AA{} are formed in each grain. The superstructure can be explained in terms of a Moir\'e effect in which the Sb overlayer is rotated by \ifmmode\pm\else\textpm\fi{}7\ifmmode^\circ\else\textdegree\fi{} with respect to the substrate [11\ifmmode\bar\else\textasciimacron\fi{}0] direction.

The interaction of Sb overlayers with Si(001)

Medium-energy ion scattering was used to study the temperature dependence of the saturation coverage, the layer morphology and reordering of the substrate after adsorption of Sb 4 tetramers and Sb 1 monomers on Si(001). Depositions at room temperature result in formation of Sb clusters on top of a dissociatively chemisorbed Sb layer. The clusters desorb at temperatures exceeding 420 K. For depositions at substrate temperatures above 570 K the Sb coverage saturates at 0.7 to 0.9 monolayer. The saturation coverage depends on substrate temperature and absorbing species, i.e., Sb 4 or dissociated Sb 4. The highest coverages are obtained with a beam of dissociated Sb 4. These dependencies are explained by the observation of a decreasing binding energy of the chemisorbed Sb with increasing coverage. The Si directly underneath the Sb layer is structurally bulk-like.

Experimental and theoretical surface component of the PL 2,3VV Auger lineshape in GaP(110): the use of ordered (1 × 1) Sb overlayers

An analysis of the theoretical and experimental surface contribution of the PL 2,3VV Auger lineshape in GaP(110) is performed. From the theoretical point of view the lineshapes are calculated within a single-particle framework from a slab-by-slab tight-binding calculation of the ideal and real relaxed surfaces. The experimental information is extracted by comparing the Auger feature of as-cleaved GaP(110) and of the GaP(110) interfaced with one ordered monolayer of Sb. This allows the LVV surface component to be isolated. A comparison with theory, showing strong similarities in the position of the surface-related features is discussed.

Resolving the Surface Contribution kof the P L2, 3 VV Auger Lineshape of GaP (110) via Use of (1 × 1) Sb Overlayers

An experimental and theoretical study of the band-like P L2, 3 VV Auger lineshape for GaP(110) is reported (kinetic energy ≈120 eV). By deposition of one monolayer of Sb (isoelectronic to P) we determine the surface-related Auger lineshape as the difference between the two spectra measured from the as-cleaved surface and the GaP/Sb interface. Theoretical calculations based on a single particle approach including different structural coordination models (i.e. ideally truncated surface, real relaxed surface, ordered Sb overlayer) are performed, which allows to isolate the surface contribution of the L2, 3 VV Auger profile from the bulk related lineshape. Moreover the results of the joint analysis are discussed giving excellent agreement between calculations and measurements and pointing toward an interpretation of the P L VV lineshape in terms of surface structural probe.

Observation and LEED I–V analysis of the Ge(111)-1×1-Sb system

When antimony is adsorbed at submonolayer coverage on a cleaved Ge(111) surface, the clean (2 × 1) surface transforms to a (1 × 1) structure. A dynamical low energy electron diffraction (LEED) intensity anlysis was performed to study this system. The results suggest that Sb adatoms replace the outermost Ge atoms, resulting in a well-ordered bulk-like (1 × 1) surface. This system, like that of a Sb overlayer on Si(111) surfaces, demonstrates that adatoms can remove a surface reconstruction.

Growth mode of Bi and Sb layers on GaAs(110) and InP(110)

Abstract The growth mode of thin Bi and Sb overlayers on GaAs(110) and InP(110) substrates was studied in ultra high vacuum (UHV) for room temperature (RT) deposition and subsequent annealing. Analysis was performed by means of LEED, Raman spectroscopy and Auger electron spectroscopy (AES). For Bi deposited on GaAs(110), three different modifications are observed with increasing coverage. The first two monolayers grow epitaxially. On top of this layer Bi grows in a pseudocubic orientation. At coverages above 8 ML, islands of the bulk-like D 3d crystal structure appear. Upon annealing the epitaxial Bi layer and the bulk-like Bi modification are stable up to 575 K, the highest annealing temperature investigated, whereas the pseudocubic Bi modification desorbs around 540 K leaving an epitaxial monolayer behind. For Bi on InP(110), only two modifications are observed. An epitaxial monolayer is formed at first, but directly followed by facets of Bi islands of the D 3d structure, with the hexagonal axis parallel to the [001] direction in the InP(110) surface. For large coverages these facets are thermally stable. For Sb deposition on GaAs(110) and InP(110), three Sb modifications have been reported previously: with increasing coverage an epitaxial monolayer, amorphous Sb and the crystalline D 3d Sb have been observed. Upon annealing the amorphous Sb is transformed into crystallites of a pseudocubic modification around 400 K. The crystallites desorb above 475 K, whereas the epitaxial and the D 3d Sb modifications are stable up to at least 575 K.

Growth mode of Bi and Sb layers on GaAs(110) and InP(110)

The growth mode of thin Bi and Sb overlayers on GaAs(110) and InP(110) substrates was studied in ultra high vacuum (UHV) for room temperature (RT) deposition and subsequent annealing. Analysis was performed by means of LEED, Raman spectroscopy and Auger electron spectroscopy (AES). For Bi deposited on GaAs(110), three different modifications are observed with increasing coverage. The first two monolayers grow epitaxially. On top of this layer Bi grows in a pseudocubic orientation. At coverages above 8 ML, islands of the bulk-like D3d crystal structure appear. Upon annealing the epitaxial Bi layer and the bulk-like Bi modification are stable up to 575 K, the highest annealing temperature investigated, whereas the pseudocubic Bi modification desorbs around 540 K leaving an epitaxial monolayer behind. For Bi on InP(110), only two modifications are observed. An epitaxial monolayer is formed at first, but directly followed by facets of Bi islands of the D3d structure, with the hexagonal axis parallel to the [001] direction in the InP(110) surface. For large coverages these facets are thermally stable. For Sb deposition on GaAs(110) and InP(110), three Sb modifications have been reported previously: with increasing coverage an epitaxial monolayer, amorphous Sb and the crystalline D3d Sb have been observed. Upon annealing the amorphous Sb is transformed into crystallites of a pseudocubic modification around 400 K. The crystallites desorb above 475 K, whereas the epitaxial and the D3d Sb modifications are stable up to at least 575 K.

Surface extended x‐ray adsorption fine structure studies of the Si(001) 2×1–Sb interface

Surface extended x‐ray adsorption fine structure (SEXAFS) has been used to investigate the structure of Sb on the Si(001) 2×1 surface. The coverage of Sb which remains after annealing thick layers at 375 °C, previously reported to be one monolayer (ML), is found in this work to form a disordered overlayer with three dimensional Sb clusters. This finding is concluded from the Sb L3 absorption spectra which are similar for this coverage to that of bulk Sb. After a 550 °C anneal, Auger electron spectroscopy, and scanning tunneling microscopy (STM) show that about 1 ML of Sb remains. Phase and amplitude analysis of the Sb L3 edge SEXAFS shows that the remaining Sb atoms occupy a modified bridge site with a Si–Sb bond length of 2.63±0.04 Å. The Sb atoms form dimers with a Sb–Sb bond length of 2.91±0.04 Å, which is almost identical to the bulk Sb–Sb bond length of 2.90 Å. The Sb atoms lie 1.74±0.06 Å above the Si(001) surface. STM confirms the dimer structure of the Sb overlayer.

Long-range ordering of Sb multilayers on GaAs(110): Evolution of resonant inverse photoemission.

Temperature- and coverage-dependent ordering of Sb multilayers on GaAs(110) has been investigated with use of inverse-photoemission spectroscopy (IPES) and low-energy electron diffraction. IPES studies show that an empty state at ${\mathit{E}}_{\mathit{F}}$+0.3 eV resonates for incident electron energies of ${\mathit{E}}_{\mathit{i}}$=16.2 eV because the radiative decay of a plasmon within the Sb film (\ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathit{p}}$=15.9 eV) competes with conventional inverse photoemission. The resonance is first observed at \ensuremath{\sim}3.5 monolayers (ML) as the empty-state feature shifts from \ensuremath{\sim}0.4 eV at 3 ML to \ensuremath{\sim}0.3 eV at 3.5 ML for Sb overlayers grown on GaAs(110) at 300 K and annealed to 475 K. The empty structure is associated with long-range ordering of the Sb multilayers rather than Sb-As antibonding states at the interface. The thin film structure and the thickness of the film play critical roles in determining the empty-state resonance. Structural studies for 20-\AA{} Sb films show ringlike diffraction patterns with the coexistence of (1\ifmmode\times\else\texttimes\fi{}1) structures and fractional-order spots adjacent to integer-order spots along [11] or [11\ifmmode\bar\else\textasciimacron\fi{}]. The ringlike structures are attributed to hexagonal basal planes of individual Sb crystallites that are randomly rotated and oriented parallel to the (110) surface. Two equivalent quasihexagonal domains are observed during the initial growth stages. These results suggest a correlation between the resonance and growth phases consisting of long-range-ordered (110) domains with Sb crystallites in between.

New surface atomic structures for column V overlayers on the (110) surfaces of III–V compound semiconductors

Two new minimum-energy surface structures have been identified for p(1×1) overlayers of Sb on the (110) surface of III–V compound semiconductors using a tight-binding total-energy formalism previously developed for these systems. The first is the ‘‘epitaxical on top structure’’ (EOTS) in which Sb zig–zag chains are commensurate with, and on top of, the Ga–As unreconstructed surface zig–zag chains. This structure differs from the previously found ‘‘epitaxical continued layer structure’’ (ECLS) by virtue of the registry of the Sb chains ‘‘on top of ’’ rather than ‘‘in between’’ the substrate Ga–As chains. Like the ECLS, the EOTS is compatible with both scanning tunneling microscopy (STM) and photoemission data. The second new structure is the ‘‘epitaxical overlapping chain structure’’ (EOCS) in which the Sb chains are 180° out-of-phase with, and on top of, the Ga–As substrate chains. This structure is, however, incompatible with both low-energy electron diffraction and STM data for GaAs(110)–p(1×1)-Sb. Computations for GaP(110)–p(1×1)-Sb and InSb(110)–p(1×1)-Sb reveal that the energy difference between the EOTS and ECLS is approximately constant over this range of overlayer/substrate lattice mismatch.

Thermal stability and Schottky barrier of Sb overlayers on GaAs(110) and InP(110)

The adsorption of thin Sb overlayers on GaAs(110) and InP(110) and their behavior during thermal annealing were studied with Raman spectroscopy, soft x-ray photoelectron spectroscopy, Auger electron spectroscopy, and low-energy electron diffraction. When deposited at room temperature, Sb forms a fairly well ordered monolayer with amorphous Sb on top. Upon annealing, the Sb forms crystalline islands which desorb at temperatures above 475 K leaving a well ordered monolayer of Sb behind. On InP(110) this monolayer is stable only up to around 500 K, whereas on GaAs(110) it remains stable to annealing temperatures above 580 K. For this well ordered Sb monolayer, the Schottky barriers are found also to be different for GaAs and InP. On GaAs, the barrier is low on both p- and n-type substrates, whereas on InP we find a low barrier on n-type and a barrier of 0.6 eV on p-type substrates. The differences in Schottky barriers and thermal stability of the ordered Sb monolayer leads to the conclusion that Sb bonds differently to GaAs and InP.

Photoemission spectroscopy of ordered overlayers on GaP (110)

Interfaces between UHV-cleaved GaP (110) and overlayers of Sn, Sb, and Bi have been studied using soft X-ray photoemission spectroscopy. The substrate core level attentuation indicates that these three interfaces exhibit Stranski-Krastanov growth. The overlayer core level spectra (Sn 4d, Sb 4d, and Bi 5d) exhibit two components for coverages up to one monolayer, indicating that the adatoms form a zigzag chain overlayer as has been reported at the Sb/GaAs interface. The Ga 3d and P 2p surface components disappear as the first monolayer develops, and the remaining bulk components are used to determine various lineshape parameters of these core levels. The Schottky barrier formation at these interfaces has also been examined. The band bending at these interfaces occurs chiefly for coverages exceeding one monolayer, when islands are forming on the ordered layer. For all three of these interfaces the final surface Fermi level position is in the neighborhood of 1.0eV above the valence band maximum.

New Surface Atomic Structures for III-V(110)-p(1x1)-Sb(1ML):Chemical Bonding and Electronic Structure

ABSTRACTTight-binding total energy computations are used to examine the chemical bonding and electronic structure for two new minimum-energy surface atomic structures for p(lxl) overlayers of Sb on III-V(110) surfaces. The bonding in each of these structures is unique, having no analog in either the bulk or small molecule coordination chemistry of these materials, and is a phenomenon uniquely associated with the constrained epitaxical growth of the Sb overlayer.

Surface core level shifts on InP(110): Use of Sb overlayers

The surface core level shift of the P2p from a clean InP(110) surface has been studied by use of the Sb overlayer. It is shown that the nonreactive Sb overlayer removes the surface core level shift and does not introduce any other component. The unique possibility to separately characterize the bulk component allows a precise determination of the surface core level shift. We show that the surface P2p core level is shifted to lower binding energy by −0.29 eV, while an opposite and nearly equal in magnitude +0.30 eV shift is established for the In4d.

Surface core level shifts on InP(110) use of Sb overlayers

The surface core level shift of the P2p from a clean InP(110) surface has been studied by use of the Sb overlayer. It is shown that the nonreactive Sb overlayer removes the surface core level shift and does not introduce any other component. The unique possibility to separately characterize the bulk component allows a precise determination of the surface core level shift. We show that the surface P2p core level is shifted to lower binding energy by -0.29 eV, while an opposite and nearly equal in magnitude +0.30 eV shift is established for the In4d.