Physical Review B

The variation of the potential of a quantum well is similar to that of a deep trap. In that respect a quantum well can capture and emit carriers in much the same way as a trap. The thermal emission energy from a quantum well is closely related to the appropriate band offset. With that in mind, we have carried out deep-level transient spectroscopy measurements on Schottky-barrier diodes containing one or more pseudomorphic InxGa1-xAs/Al0.2Ga0.8As (0<x≤0.18) quantum wells. The objective was to estimate the conduction-band offset, ΔEc, as a function of x and the resulting strain. From detailed balance between emission and capture, an Arrhenius-type expression was derived to analyze the transient emission data. It is seen that the percentage band offset ΔEc/ΔEg varies from 62% for x=0.07 to 70% at x=0.18. Furthermore, a linear interpolation of the data leads to ΔEc/ΔEg=58% at x=0, which is close to the widely accepted value. Our results support recent theoretical calculations from which a monotonic increase in ΔEc with strain in this heterostructure system is predicted.

Neutron powder diffraction has been used to study the crystal structure and magnetic ordering of YBa2Cu3-xCoxO7+y with x=0.84 and y=0.32. Rietveld refinement of the crystal structure was carried out at 295 K (space group P4/mmm,a=3.888 A&#778,c=11.636 A&#778) and showed in particular that Co substitutes in the Cu(1) position of YBa2Cu3O7-y. Long-range, three-dimensional antiferromagnetism was observed up to a Néel temperature of about 405 K. All five detected magnetic peaks could be indexed on a wave vector ((1/2 1) / 2( 1/2) with h,k,l all odd, indicating a body-centered tetragonal magnetic unit cell with amag=a &surd;2, cmag=2c. At 9 K the proposed magnetic structure has moments parallel to c antiferromagnetic ordering within both Cu and Co layers. Adjacent moments in neighboring Cu layers are antiparallel whereas the small moments found in the Co layers are parallel to their adjacent moments in the neighboring Cu layers. The ordered moment was 0.87muB for the Cu layers and 0.16muB in the Co layers, respectively. With increased temperature the ordered moments in the Co layers decrease faster than those in the Cu layer. At room temperature the ordered moment was [0.65(7)]muB for the Cu layers with no detectable ordered moment in the Co layers.

A comparison between bulk and films of Fe${(\mathrm{CO})}_{5}$ adsorbed on graphite shows that dynamical anomalies which occur at $\sim${}110, $\sim${}115, and $\sim${}180 K have in both forms similar effects on the M\"ossbauer parameters. The origin of these transitions is briefly discussed. In the 2.3-layer film, anomalous clustering of the film results in coexistence of solid bulk and a compressed monolayer; consequently, the film displays a novel behavior of the M\"ossbauer quadrupole splitting.

The Sb-Sb dimer structure of a 2×4 reconstructed Sb/GaAs(001) surface was investigated by core-level photoelectron collection mode of back reflection soft x-ray standing-wave technique. Sb atoms occupy the bridge site forming bonds with two underlying Ga atoms and form symmetric Sb-Sb dimers lining up in the [11¯0] direction. The first-layer Sb atomic plane is estimated to be 1.81±0.02 Å above the second-layer Ga lattice plane. The bond length of the Sb-Sb dimer on a GaAs(001) surface is estimated to be 2.95±0.06 Å, which is longer than the reported Sb-Sb bond length for Sb/GaAs(110) surface.

The growth of Ge on Si(001) at elevated temperatures has been monitored in real time using coaxial impact collision ion scattering spectroscopy in the presence of various constant coverages of surfactant, Bi or Sb, with thermal evaporation of the surfactant being compensated automatically. As the coverage of surfactant increases, the intermixing of Ge and Si is suppressed, the crystalline quality of the resulting Ge film is improved, and the nucleation and growth of macroscopic Ge islands is suppressed.

Angle-resolved core-level photoelectron spectroscopy using a monochromatized synchrotron radiation source was used to study the structure of the Se-treated GaAs(001) surface. Depth profiling was achieved by varying the photoelectron angle with respect to the sample surface. The Se 3d spectrum was found to consist of two components. The higher-kinetic-energy component is assigned as the surface-sensitive component, which is opposite to the conventional assignment based on Ga coordination. The intensity ratios of the two Se components are calculated with a layer attenuation model, and the polar angle dependence shows that the two components cannot simply be assigned as ``surface'' and ``inner'' Se but are two different chemical states that exist at both the first and second Se layers, with only one chemical state existing at the third Se layer. Based on these results, we derived a Ga-vacancy zinc-blende structure where the first Ga layer has a vacancy occupancy of approximately 25%.

We have performed linear and circular x-ray-dichroism measurements at the L2,3 edges of Ni in as-deposited and annealed layers on an Fe(001) substrate. The linear dichroism results suggest that the magnetic dipole term is small in these Ni layers. Upon annealing, the intermixing of Ni and Fe causes the Ni orbital moment to increase to twice the value for bulk fcc Ni. The possible origins of this increase are discussed. Fundamental importance is attributed to the higher localization of the Ni 3d states, due to the ``impurity'' nature intrinsic to Ni atoms diluted in Fe.

Reconstructed surfaces on Sb/GaAs(001) have been investigated in situ by reflection high-energy electron diffraction (RHEED) and core-level photoelectron spectroscopy by heating a sample prepared by depositing Sb on an As-terminated GaAs(001) surface at room temperature (RT). Before Sb desorption, the halo RHEED pattern changes into 1×4, 1×3, and 2×4 patterns from RT to 560 °C, which is in contrast to Sb/GaAs(110) that shows only a 1×1 pattern. It is found that the GaAs surface with a 2×4 pattern is terminated by a monolayer of Sb, and that these superstructure changes are caused by As atom desorption followed by Sb atom substitution.

Iron was substituted for copper in YBa2(Cu1-xFex)3O7- for 0x0.33. Superconducting transition temperatures and lattice parameters have been determined as a function of Fe content. The orthorhombic distortion a0=2(b-a)/(a+b) decreased to zero near x=0.03. Beyond x=0.03 the structure appears tetragonal. For compositions 0x0.15 the superconducting critical temperature Tc decreased smoothly from Tc=90 K to below 4.2 K. Transmission electron microscopy (TEM) revealed that the spacing between the twin boundaries decreased from 200 nm for x=0 to 20 nm for x=0.02 before the twins disappeared for x>=0.03. The sharp streaks in TEM diffraction patterns were interpreted as due to the twin boundary layers whose thickness varied from 1 nm for the pure to 2 nm for the Fe (x=0.02) containing specimen. A tweed structure in the TEM image and associated diffuse streaks along the 110 directions in electron diffraction pattern appeared for x>0.015. This observation suggests that the structure consists of fine (<5 nm) orthorhombic domains, each domain having the twinlike crystallographic relation with its neighbors. Rietveld refinement of powder neutron diffraction for x=0.10 and 0.167 determined that Fe substituted primarily on the Cu chain site, and for every two atoms of Fe substituted, approximately one extra oxygen is incorporated in the Cu-O plane.

In order to understand the electronic structure of the misfit-layer compound (SnS)1.17NbS2 we carried out an ab initio band-structure calculation of the closely related commensurate compound (SnS)1.20NbS2. The band structure is compared with calculations for NbS2 and for hypothetical SnS with structure and interatomic distances as in (SnS)1.20NbS2. The calculations show that the electronic structure is approximately a superposition of the electronic structures of the two components NbS2 and SnS, with a small charge transfer from the SnS layer to the NbS2 layer. The interlayer bonding between SnS and NbS2 is dominated by covalent interactions. X-ray and ultraviolet photoelectron spectra were obtained for the valence bands. The observed spectra are in good agreement with the band-structure calculations.

In this paper we present polarization-, temperature-, and thickness-dependent x-ray-absorption measurements on extremely thin Dy overlayers. For Dy overlayers deposited at room temperature on Cu(110) and (magnetic) Ni(110) we observe strong linear dichroism. We deduce that the nature of the dichroic effects is predominantly electrostatic. When Dy is evaporated onto Ni(110) and kept at 50 K almost no dichroism is observed. Deposition of Dy on Cu(110) at 50 K, on the other hand, yields strong dichroism. We discuss these effects in greater detail and formulate a model for the overlayer system that seeks to explain the observed differences. Finally, a strong decrease of dichroism is observed as the Dy layer thickness increases, a phenomenon which is independent of the substrate. We propose a mechanism to explain this that involves counteracting contributions to the dichroism from both the overlayer-substrate and surface-vacuum interfaces. We illustrate this with an experiment involving a Ho probing layer.

We have performed ab initio pseudopotential plane-wave calculations of both the atomic and electronic structure of the Ta (100) and (110) surfaces. The atomic relaxation of Ta (110) is calculated and our results for the structure of Ta (100) agree well with previous experimental reports. We find that the topmost interlayer separations contract relative to their bulk values by (12+/-1)% and (2+/-1)% for Ta (100) and (110), respectively. The changes in the second and deeper interlayer separations differ in sign. The calculated surface formation and relaxation energies are 1.92 eV/atom and -0.17 eV/atom for the relaxed Ta (100) surface and 1.18 eV/atom and -0.03 eV/atom for the relaxed (110) surface. We have also calculated the work functions for both surfaces and find values of 4.0+/-0.1 eV for (100) and 4.9+/-0.1 eV for (110), which are both in good agreement with experiment. Our study of the respective surface states and their variation as a function of the relaxation indicates that, in contradiction to a previous report, the surface states do not provide the driving force for the relaxations. We report our calculation of the [110] zone boundary phonon frequencies for bulk Ta. Lastly, this work demonstrates that the pseudopotential method can be applied accurately and efficiently even in the case of a column V transition metal such as Ta.

The structure of empty electronic levels in Xe on Ag(110) is probed by angle-resolved inverse photoemission. The observed dispersion indicates wetting behavior of a Xe film in (111) orientation on the Ag(110) surface. If the Ag(110) surface is precovered by a metallic potassium layer, the spectra indicate formation of a nonwetting Xe film. The data of the present experiment cast new light on the controversial interpretation of Xe optical spectra and promote the general understanding of wetting and nonwetting adsorption behavior.

A seven-layer slab was used to model the ideal GaAs(110) surface. Two coverages, Θt=1/2 and 1 monolayer (ML) of Na atoms on the surface were studied. The total-energy calculations with the self-consistent pseudopotential method was used to determine the positions of Na atoms on the ideal GaAs(110) surface. We find that the charge transfer is from the Na to the Ga atoms. The shift of the Fermi level with respect to the valence-band maximum from coverage of 1/2 to 1 ML is 0.33 eV downward. This result agrees reasonably with the measured value of 0.2 eV for the n-type GaAs(110) surface.

The unoccupied level structure of K and Na adsorbed on a Ag(110) surface has been probed by inverse photoemission. A peak is observed initially ~3 eV above EF, which shifts in energy towards the Fermi level as a function of increasing coverage. The dispersion of this feature is consistent with a pz character of the observed empty state. For CTHETA>0.3 new structures appear in the spectra indicating the formation of a metallic layer. Energy position, coverage dependence, and dispersion of the unoccupied level are in striking agreement with predictions made on the footing of previous ultraviolet photoemission spectroscopy, electron energy-loss spectroscopy, and theoretical results.

Recent observations on various low-index metallic surfaces of an extraordinary thermal attenuation of diffracted peak intensities suggest that anharmonic effects are important at high temperatures, though such observations have also been interpreted in terms of disordering processes (premelting and roughening). In order to address this problem, we have carried out an extensive molecular-dynamics investigation of the evolution in temperature of the (100) and (110) surfaces of Ni, with the interactions between atoms described using embedded-atom method potentials. We observe both surfaces to suffer an anomalously large thermal expansion, with a concomitant rapid increase of the mean-square amplitudes of vibration; the latter are found in both cases to be larger in the plane of the surface than out of the plane. An analysis of our simulated low-energy electron-diffraction intensities supports the experimental interpretation of the measured ones in terms of enhanced mean-square displacements. Indeed, anharmonicities are found to appear at temperatures well below the onset of disordering, which occurs at 1200 and 1000 K for the (100) and (110) surfaces, respectively (via in-plane diffusion and adatom-vacancy formation). The onset of disordering proceeds differently on the two surfaces: while it involves essentially only the outermost layer on the (100) surface, both the first and second layer participate in the process on the (110) surface. We also calculate the phonon spectra at high-symmetry points of the surface Brillouin zone, which we find to be in good agreement with experimental values at room temperature. We predict, further, that the temperature-dependent frequency shifts are not significantly softened in comparison with bulk phonons, which suggests that surface phonons are affected by anharmonicity in a nontrivial way. Several of the observed properties can be understood in terms of the larger displacements that surface atoms can afford, because of their reduced coordination compared to bulk atoms.

Temperature- and angle-dependent surface-extended x-ray-absorption fine-structure (SEXAFS) measurements on the system (2×3)N/Cu(110) are reported. Three inequivalent adsorption sites of the N atoms within the surface unit cell are identified. The successful separation of their contributions in the present SEXAFS analysis allows for a determination of N-Cu bond lengths not only within the surface plane but also to the second Cu layer. The temperature-dependent measurements between 60 and 300 K enable us to separate structural from dynamic effects in the SEXAFS amplitude, leading to a reliable determination of coordination numbers. A strong anisotropy is found in the adsorbate vibrations parallel versus perpendicular to the surface. The relative motion of the N atom relative to the first three Cu neighbor shells is characterized by means of the corresponding Einstein temperatures. These indicate that the N atom is primarily bonded only to four Cu atoms, and not to five as previously suggested. Anharmonic corrections are found necessary for a reliable distance determination of the N-Cu bonds to the second layer. The structural parameters are compared to the ones predicted by the pseudo-(100)-c(2×2)-N reconstruction.

We have measured magnetic circular x-ray dichroism (MCXD) in a remanently magnetized Ni(110) single crystal, with the light in grazing incidence. Measuring the x-ray absorption as a function of the applied magnetic field, we could draw a hysteresis curve similar to one obtained with a magneto-optical Kerr effect on the same crystal. Spectra taken with linearly polarized light show clear variations as the angle of incidence of the light on the sample is changed. The possible origins for this angular-dependent absorption and its consequences for MCXD measurements taken in grazing incidence are discussed.

The phonon spectrum of the Ag(110) surface along the direction is studied by He inelastic scattering with time-of-flight detection. The Rayleigh mode and two surface vibrational resonances are described up to the surface Brillouin-zone boundary. The dispersion curves of the resonances show the avoided crossing behavior predicted but never observed for the (110) surfaces of fcc metals. Surface lattice-dynamics calculations using a simple central nearest-neighbor force-constant model are performed and found to be in close agreement with the experiment.

We report experimental evidence for strong 3{ital d}{r arrow}4{ital f} x-ray-absorption dichroism in rare-earth ions on a nonmagnetic substrate: Dy on Si(111)7{times}7. The results are discussed by evaluating the role of the crystal-field splitting of the ground state as an alternative to the Zeeman splitting underlying the magnetic x-ray dichroism theory.

The in-plane structure of the Si(111):As-1×1 surface has been studied under ultrahigh vacuum using x-ray standing waves produced by a substrate silicon crystal in the grazing-angle diffraction geometry. The observed As fluorescence profiles are consistent with models indicating a bulklike surface with threefold-coordinated As atoms in the first layer. Evidence indicating that the As atoms occupy these high-symmetry sites with little disorder is obtained from investigation of the dependence of fluorescence profiles on the displacement and order parameters.

The valency changeover in Sm overlayers on Si(111)7×7 is probed with high accuracy using x-ray-absorption spectroscopy at the Sm M4,5 edges. The valency of Sm has been studied as a function of the layer thickness in the chemisorption regime, and compared with the results obtained for epitaxially grown samarium silicide layers.

The relaxed geometries and electronic properties of the hydrogenated phases of the Si(111)-7\ifmmode\times\else\texttimes\fi{}7 surface are studied using first-principles molecular dynamics. A monohydride phase, with one H per dangling bond adsorbed on the bare surface, is found to be energetically favorable. Another phase where 43 hydrogens saturate the dangling bonds created by the removal of the adatoms from the clean surface is found to be nearly equivalent energetically. Experimental scanning tunneling microscopy and differential-reflectance characteristics of the hydrogenated surfaces agree well with the calculated features. \textcopyright{} 1996 The American Physical Society.

In this paper, surface diffusion of Ge adatoms on the In-stabilized moderate temperature phase of Ge(111) was studied with a room-temperature scanning tunneling microscope and it has been found that, in addition to diffusion of individual adatoms neighboring to some defects, the majority of the moving adatoms forms strings or closed loops consisting of segments lying along directions. The mean lifetime of Ge adatoms on Ge(111) has been obtained, from which the activation energy barrier has been determined to be 0.83 +/- 0.02 eV. This experimental result of this quantity is in excellent agreement with its theoretical value for clean Ge(111) surfaces, thus showing not only that the energy barrier obtained here is a characteristic of clean Ge(111) surfaces, but also that there is no complicated collective motion involved in surface diffusion of Ge adatoms.

Surfaces of Sn growing on InSb\{111\}A,B have been studied by using the reflection high-energy electron-diffraction intensity oscillation technique. The surfaces proceed in the formation of a bilayered lattice in the whole range of film thickness. However, the geometry of the outermost surface layer is quite different in both systems: The growing surface on InSb(111)A smoothens with the same period as the lattice formation, whereas on InSb(111)B, below and above 6 ML of Sn, smooth surfaces emerge every period of monolayer and bilayer, respectively. The monolayer-period change in surface geometry is ascribed to Sb segregation on the growing surface.