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n-type Ge/SiGe asymmetric coupled quantum wells represent the building block of a variety of nanoscale quantum devices, including recently proposed designs for a silicon-based THz quantum cascade laser. In this paper, we combine structural and spectroscopic experiments on 20-module superstructures, each featuring two Ge wells coupled through a Ge-rich SiGe tunnel barrier, as a function of the geometry parameters of the design and the P dopant concentration. Through a comparison of THz spectroscopic data with numerical calculations of intersubband optical absorption resonances, we demonstrated that it is possible to tune, by design, the energy and the spatial overlap of quantum confined subbands in the conduction band of the heterostructures. The high structural/interface quality of the samples and the control achieved on subband hybridization are promising starting points towards a working electrically pumped light-emitting device.

Regular surface undulations, called cross-hatch patterns, appearing at the free surface of lattice-mismatched heteroepitaxial films are a key signature of plastic relaxation. Here we show that the dynamics of cross-hatch formation is accurately described by a continuum model based on strain-mediated surface diffusion, provided that a realistic distribution of dislocations is considered. We demonstrate quantitative agreement between our time-dependent simulations and dedicated atomic force microscopy experiments on ${\mathrm{Si}}_{0.92}{\mathrm{Ge}}_{0.08}$ films grown on Si(001) at various thicknesses, finally shedding light on the origin and on the dynamical behavior of a widely investigated pattern, first observed more than half a century ago.

By combining scanning probe microscopy with Raman and x-ray photoelectron spectroscopies, we investigate the evolution of CVD-grown graphene/Ge(001) as a function of the deposition temperature in close proximity to the Ge melting point, highlighting an abrupt change of the graphene's quality, morphology, electronic properties and growth mode at 930 degrees. We attribute this discontinuity to the incomplete surface melting of the Ge substrate and show how incomplete melting explains a variety of diverse and long-debated peculiar features of the graphene/Ge(001), including the characteristic nanostructuring of the Ge substrate induced by graphene overgrowth. We find that the quasi-liquid Ge layer formed close to 930 degrees is fundamental to obtain high-quality graphene, while a temperature decrease of 10 degrees already results in a wrinkled and defective graphene film., in press

We provide an in-depth characterization of the dislocation distribution in partially relaxed Si0.92Ge0.08/Si(001) films. This is achieved by an innovative and general method, combining two state-of-the-art characterization techniques through suitable modeling. After having inferred the dislocation positions from transmission-electron-microscopy images, we theoretically reproduce scanning-x-ray-diffraction-microscopy tilt maps measured on the very same region of the sample. We obtain a nearly perfect match between model predictions and experimental data. As a result, we claim that it is possible to establish a local, direct correlation between the dislocations revealed by the transmission-electron-microscopy analysis and the measured lattice tilt distribution.

In this paper we present an atomic force microscopy and X-ray photoemission spectroscopy study of the composition and shape evolution of self-assembled Ge/Si(001) islands upon capping with Si. We found that the islands undergo a reverse Straski-Krastanov shape evolution, with a progressive Si-enrichment of both the wetting layer and the islands. We demonstrate that the island shape evolves at constant volume with silicon atom incorporation occurring in the absence of lateral diffusion of Ge and Si atoms from the wetting layer to the islands themselves. (C) 2008 Elsevier Ltd. All rights reserved.

We have investigated the response of 3D Bi 2 Sr 2 CaCu 2 O 8 vortex structures to a weak perturbation induced by 2D Fe pinning structures acting on one extremity of vortex lines. The pinning patterns were nano-engineered at the sample surface by means of either a Bitter decoration of the vortex lattice or electron-beam lithography. The commensurability conditions between 2D rigid pinning potentials and 3D elastic structures with short-range positional and long-range orientational correlation have been experimentally determined. When the 2D potential is a replica of the nonperiodic vortex structure an amplification of its interaction with the vortex structure takes place. This effect is detected only for the first matching field, becoming negligible for other matching fields. On the other hand, a periodic 2D perturbation is shown to transform the nonperiodic Bragg glass-like structure into an Abrikosov crystal with an effective Debye–Waller factor.

We make a comparative analysis of the response of the three-dimensional vortex lattice in Bi-2 Sr-2 CaCu2O8 and NbSe2 to the presence of square arrays of pinning sites localized at one extremity of the vortex crystals. The absence of the hexagonal to square symmetry transformation and the induction of a distorted hexagonal symmetry in the vortex lattice of NbSe2 contrast to the observed symmetry change in Bi2Sr2CaCu2O8. The dissimilar response in both cases is explained taking into account the vortex structure solidification mechanisms in both materials: The plastic response in the case of the layered material Bi2Sr2CaCu2O8 is suggested to be a result of the simultaneity of solidification and coupling of bidimensional pancake vortices whereas a viscous solidification is the responsible for the three-dimensional elastic response in NbSe2.

The effect of the deposition rate on the size, density, and uniformity of Ge islands grown on Si(100) is investigated. Upon changing the growth rate from 4 to 110 ML/min the island density increases by one order of magnitude and the strained dome base decreases from 84 to 55 nm. A narrowing of the island size distribution was also observed. We discuss these experimental findings by taking into account island-island interaction effects. (C) 2003 American Institute of Physics.

Display Omitted Argon and oxygen plasma processes are used for controlled disruption of ferritin.XPS detection of Fe in ferritin film after plasma process was enhanced by ~10.XPS reveals the oxidation state of Fe in the ferritin core after the plasma etching. Ferritin is the main iron storage protein in living systems. Recent studies demonstrated that the chemical species present in the ferritin core and their chemical coordination can be different between physiological and pathological ferritin proteins. Therefore it is of paramount importance revealing the presence of metals other than iron and their oxidation state in the protein core.In this work we investigated and compared the use of argon and oxygen plasma processes for the erosion of the protein shell surrounding the mineral core, which is necessary for the chemical analysis of the ferritin core by surface sensitive technique such as X-ray photoelectron spectroscopy. We find that both the proposed etching processes are effective in the disruption of the protein shell and in the exposure of the Fe inside the core of the molecules. Neither oxygen nor argon plasma induced significant artifacts in the samples nor produce dramatic roughening in the surface allowing for a controlled thinning of the ferritin film. In the plasma treated samples X-ray photoelectron spectroscopy detected the Fe(III) oxidation state of metal present inside the ferritin cavity.

Photoemission spectroscopies and atomic force microscopy (AFM) were performed on C-60 evaporated on the monohydride Si (100):H-(2 x 1) surface at different temperatures from room temperature up to 900 degrees C. X-ray photoelectron spectroscopy has revealed that the interaction is with the hydrogenated surface only for temperatures below 250 degrees C. Above this temperature the hydrogen starts to desorb and C-60 interacts directly with the clean silicon surface. The bond between C-60 and the Si(100):H-(2 x 1) surface is van der Waals' in character and is estimated to be of the same order of magnitude or even weaker than the C-60-C-60 interaction. AFM analysis was performed on multilayer samples grown in the range between room temperature and 200 degrees C. Room-temperature samples exhibit small Cs, islands. The order of the islands improves strongly when the temperature of the substrate is increased up to 200 degrees C. At this temperature the islands are all ordered with the hexagonal shape typical of C60 growth along the (111) direction and a lateral size of about 3000 Angstrom. (C) 2000 Elsevier Science B.V. All rights reserved.

We report on the realization of high-mobility two-dimensional electron gas based on modulation-doped Si/SiGe heterostructure grown directly on thin SiGe-On-Insulator (SGOI) substrate. The samples were grown by using low-pressure chemical vapor deposition. A pregrowth procedure for the cleaning of the SGOI surface that preserves the integrity and the composition of the substrate was developed. An electron mobility as high as 10(5)cm(2)V(-1) s(-1) at T = 0.4 K and 2000 cm(2)V(-1) s(-1) at T = 300 K was obtained. (C) 2007 Elsevier B.V. All rights reserved.

In this paper, we have experimentally and numerically studied the nonradiative intersubband (ISB) relaxation in $n$-type Ge/SiGe quantum well (QW) systems. Relaxation times have been probed by means of pump-probe experiments. An energy balance model has been used to interpret the experimental differential transmission spectra and to assess the relevance in the nonradiative relaxation dynamics of both electron and lattice temperature as well as of the carrier density. The comparison between experimental data and theoretical simulation allowed us to calibrate the interaction parameters which describe the electron-optical phonon scattering in two-dimensional (2D) Ge systems. Characteristic relaxation times has been calculated and compared with those of GaAs QWs as a function of the 2D electron density, of the subband energy separation, and of the lattice and electronic temperature. We found that ISB relaxation times for the Ge/SiGe systems are generally shorter than that previously calculated when the electron distribution was neglected. Nonetheless, our main result is that the relaxation time in Ge/SiGe QW systems is longer than 10 ps, also for transition energies above the Ge optical phonon energy, up to 300 K. Furthermore, we obtained that the relaxation times are at least one order of magnitude longer than in GaAs-based systems.

""\\"We present a synergic experimental and theoretical investigation of the plastic relaxation onset in low-temperature Ge growth on Si(001). High-resolution transmission electron microscopy reveals that misfit is released by pairs of coupled 60° dislocations. Atomic resolution proved to be key in distinguishing pairs from single 90°dislocations because of the revealed small intra-pair dislocation distance (even less than 1 nm). By exploiting dislocation theory and molecular dynamics simulations, we demonstrate that the observed pairing naturally occurs as a result of the mutual interactions between the two dislocations. In particular, analytical models show that the stress field arising in a thin film when a dislocation segment lies at the interface with the substrate determines the most favored nucleation site for a new (complementary) dislocation that leads, after migration, to the coupling with the first in a stable position. At the growth temperature, further motion or recombination due to atomic scale effects is excluded by classical molecular dynamics simulations. A clear picture of the early stages in the strain relaxation emerges, gliding out of the interface and\\\\\\\/or short-range climbing (as typically produced by annealing or higher temperature steps, but not taking place under the present growth conditions) being required to transform pairs into a single edge dislocations. The present results also offer answers to the long-held puzzling question about the mechanism originating 90° dislocation in high mismatch Ge\\\\\\\/Si systems. \\"""

""We report on the experimental and theoretical investigation of the relaxation in Ge-rich SiGe\\\/Ge\\\/Si heterostructures. The experimental x-ray diffraction data are interpreted with the help of a model including both edge and 60 degrees misfit dislocations in the calculated x-ray scattering intensity. Our results show that highly positionally correlated edge dislocations dominate in the relaxation of the compressive strain at the Ge\\\/Si interface, while a smaller tensile strain at the SiGe\\\/Ge interfaces released by uncorrelated\\\/little correlated 60 degrees dislocations.""

""We present an energy-balance model of the electronic intersubband relaxation in optically excited n-type Ge\\\/SiGe quantum wells with absorption resonance in the THz range. To this aim, the energy relaxation rates of the electron system due to the interactions with both non-polar optical and acoustic phonons are calculated. The time dependence of the relative dierential transmission is also evaluated and compared with experimental data from recent pump-probe measurements. The energy relaxation rates due to acoustic and optical phonon are investigated for different electron temperatures, set by the pump beam intensity. We \\ffind that the relaxation dynamics strongly depends on the intersubband energy spacing when this is close to the optical phonon energy. Finally, our results evidence that in this material system the time dependence of the depolarization shift may have a strong influence on the relative differential transmission signal.""

We measured the non-radiative intersubband relaxation time in n-type modulation-doped Ge/SiGe multi-quantum wells of different thickness by means of degenerate pump-probe experiments. The photon energy was tuned to be resonant with the lowest conduction band intersubband transition energy (14-29 meV), as measured by terahertz absorption spectroscopy and in agreement with band structure calculations. Temperature-independent lifetimes in excess of 30 ps were observed. © 2011 American Institute of Physics.

We report an extensive study of strained $\text{Ge}/{\text{Si}}_{0.2}{\text{Ge}}_{0.8}$ multiquantum wells grown by ultrahigh-vacuum chemical-vapor deposition. The microstructural properties of the samples were characterized by transmission electron microscopy and Raman spectroscopy. Their electronic properties have been investigated by means of infrared absorption measurements. Both interband and intersubband transitions were analyzed. Intersubband absorption energies were found in the 20--50 meV range, depending on the quantum well width. Interband and intersubband transition energies have been successfully described by means of both a $\mathbit{k}\ensuremath{\cdot}\mathbit{p}$ approach and a tight-binding model. In particular, we found a conduction-band offset between the L edges of 124 meV, well suited for the development of optoelectronic devices operating in the terahertz range. We also found that the energy difference between the ${\ensuremath{\Delta}}_{2}$ minima in the barrier and the L minima in the well is only $\ensuremath{\sim}40\text{ }\text{meV}$. This explains the observed ineffectiveness of the transfer doping in the strained heterostructures considered.

In this letter we present the experimental evidence of intersubband absorption in the conduction band of compressively strained germanium quantum wells bounded by Ge-rich SiGe barriers. The measured absorption energies are in the terahertz range and are interpreted by means of tight binding calculations which include self-consistent band-bending and depolarization effects. From the comparison of experimental and numerical results a conduction band offset along the L line of about 120 meV has been estimated for the studied heterostructures.

A set of recent results concerning lateral and vertical ordering of Ge islands grown on Si(001) is reviewed. Experimental data generated by chemical vapour deposition and analysed by atomic force microscopy and photoelectron spectroscopy are compared with computer simulations and modelling based on atomistic approaches and continuum theory. In particular, we show that it is possible to probe experimentally the detailed strain field generated by buried Ge islands at the surface of the Si capping layer. The observed arrangement of small Ge islands grown over the capping layer is demonstrated to be very close to the one predicted by a simple model where the local chemical potential is inferred from the strain field at the atomic scale, as given by Tersoff- potential molecular dynamics simulations. Moreover, we review recent experimental evidence for lateral ordering, triggered by partial Si capping, in the first layer of Ge islands on Si(001). Theoretical support is given by showing that when two islands lie in close proximity the elastic field is likely to generate a flow of atoms leading to an effective gliding motion along opposite directions of both islands, eventually stopped by the presence of further neighbouring islands.

In this paper we present a study on the influence of the number and the thickness of silicon spacer layer on the optical properties of single- and multi-layers of self assembled Ge/Si (001) islands performed by means of cathodoluminescence spectroscopy, high resolution X-ray diffraction and transmission electron microscopy. In single-layer sample, we do not evidence dependence of the island no-phonon emission peak position on the silicon cap-layer thickness. In multi-layer samples having a thin (33 nm) silicon spacer layer the no-phonon emission energy value progressively blue-shifts for an increasing number of island layers. This is interpreted as an enhanced intermixing driven by the strain interaction existing between island layers. On the contrary, island emission energy position is independent on the number of layers in the sample series having a thicker spacer layer (60 nm). These findings are consistent with the X-ray diffraction observation that islands belonging to different layers have the same composition. As a consequence we can conclude that multilayers with 60-nm spaced islands layer are more homogeneous and ordered.

In this paper, we show that lateral arrangement of Ge/Si(00 1) self-assembled islands in a square array oriented along the [100]-[0 10] directions can be obtained through the lateral displacement of the islands themselves. We found that when the deposited islands are exposed to an external silicon flux, the impinging silicon atoms induce Ge-Si intermixing resulting in island shape transformation from domes to large pyramids. This transformation leads to an array of closely spaced islands interacting elastically among themselves through the substrate. By means of atomistic simulations we demonstrate that this elastic repulsion drives a net flux of Ge and Si atoms from one side to the other side of the islands, leading to a lateral displacement of the whole island. This displacement ends when the two islands are sufficiently far away, or when another island is approached during the motion. In a dense ensemble of islands, this mechanism drives the tendency to order observed experimentally. (C) 2006 Elsevier Ltd. All rights reserved.

(Received 4 October 2005; published 16 March 2006)We provide a direct experimental proof and the related modeling of the role played by Si overgrowth inpromoting the lateral ordering of Ge islands grown by chemical vapor deposition on Si(001). Thedeposition of silicon induces a shape transformation, from domes to truncated pyramids with a larger base,generating an array of closely spaced interacting islands. By modeling, we show that the resulting gradientin the chemical potential across the island should be the driving force for a selective flow of both Ge and Siatoms at the surface and, in turn, to a real motion of the dots, favoring the lateral order.

We report on the tailoring of the stress field distribution at the surface in a multilayered $\mathrm{Ge}∕\mathrm{Si}$ system in order to have a spontaneous ordering of the self-assembled islands in multi-island clusters. These clusters are ordered along [100]-[010] directions and arranged in a simple planar lattice. We investigate the evolution, from nucleation to ripening, of the Ge islands inside the clusters for two different stress modulations at the surface. The control over the stress at the surface has been obtained by tuning the last Si spacer layer thickness in a stacked-island multilayered structure. An appropriate selection of this spacer layer thickness allowed us to modulate the stress in two different limits of strong and weak island-island vertical interaction. We demonstrate that the stress modulation can be used to influence the island growth dynamics in order to induce an ordered spatial position of islands inside the cluster, to reduce the island size and to narrow the island size distribution.

The self-patterning of the strain field that arises in the growth of stacked multilayers of heteroepitaxial islands, together with the capability of tuning the island size by acting on the deposition temperature, are here exploited to obtain self-organization, resulting in well-ordered clusters composed of regularly disposed, nanosized islands. Our results show that the island spatial distribution can be tuned from a random one to a well-ordered square lattice of island clusters, and that the number of islands inside each cluster can be selected. Moreover, due to the dipole repulsive interaction between adjacent islands, the islands themselves arrange in an ordered fashion inside a single cluster along the same [010]-[100] crystalline directions of the long-range cluster ordering. (C) 2003 American Institute of Physics.

The shape evolution and the effect of deposition temperature on the strain status of Ge/Si(100) islands have been investigated in the deposition temperature range 450-750 degreesC. The island crystallographic structure was investigated by transmission electron microscopy. The analysis of the Moire patterns reveals that the island lattice deformation and the elastic energy per unit volume stored in the islands decrease with increasing island size in quantitative agreement with theoretical results. Furthermore, we evidenced that the effective mismatch E between the silicon substrate and the island epilayer decreases upon increasing the deposition temperature. This misfit reduction is fully accounted by the amount of SiGe intermixing in the epilayer. (C) 2003 Elsevier Science B.V. All rights reserved.

We have been able to observe with single particle resolution the interface between two structural symmetries that cannot be interconnected by a continuous transition. By means of an engineered 2D potential that pins the extremity of vortex strings an square symmetry was imposed at the surface of a 3D vortex solid. Using the Bitter decoration technique and on account of the continuous vortex symmetry we visualize how the induced structure transforms along the vortex direction before changing into the expected hexagonal structure at a finite distance from the surface., 10 pages, 5 figures

The shape evolution and the effect of deposition temperature on size and composition of chemical vapor deposition grown Ge/Si(100) islands have been investigated in the deposition temperature range 450-850 degreesC. It is found that the increase of the growth temperature above 600 degreesC entails a strong island enlargement due to an increased Si/Ge intermixing. The crystallographic structure of the islands was investigated by transmission electron microscopy. The analysis of the resulting Moire pattern reveals that the island lattice deformation decreases with increasing island size and that the effective mismatch epsilon between the silicon substrate and the epilayer decreases with increasing deposition temperature. The island nucleation size, the mean size of coherent islands and the critical size for the insertion of misfit dislocations have been found to scale as epsilon(-2), epsilon(-2), and epsilon(-1), respectively. The agreement of our experimental scaling results with the predictions of theoretical calculation performed for homogeneous heterostructures suggests that, although the Si distribution inside the islands is not homogeneous, the island growth is driven by the mean effective strain. (C) 2002 American Institute of Physics.

We have provided direct evidence for the presence of considerable Si-Ge intermixing in strained and unstrained Ge quantum dots deposited on Si(001) and Si(111). The local structure around Ce was probed by using Ge K-edge X-ray absorption spectroscopy; complementary evidence for intermixing was provided by AFM and STM studies. These results implied that the strain energy in the dots was reduced by Si atoms diffusing into the dots, resulting in a modified form of Stranski-Krastanov growth. (C) 2000 Elsevier Science B.V. All rights reserved.

The interaction between C-60 molecules and the Si(100) surface and the preparation of silicon-carbide thin films by thermal reaction of C-60 molecules with the Si(100) surface have been investigated using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, reflection high-energy electron diffraction and atomic force microscopy measurements. The effects of annealing temperature and C-60 coverage on the SiC formation will be discussed. It is found that the C-60 molecules bond covalently with silicon, and the number of bonds increase upon increasing the annealing temperature. Annealing at T greater than or equal to 830 degrees C entails the formation of stoichiometric silicon carbide clusters that coalesce to form a continuous SiC layer when the C-60 coverage is greater than one monolayer. Deep pits acting as silicon diffusion channels are present with dimensions that increase with the amounts of C-60. The interaction of C-60 With the SiC surface was also investigated. It is found that a similar covalent interaction is present in the two systems C-60/Si and C-60/SiC. (C) 2000 Elsevier Science B.V. All rights reserved.

Valence band photoemission has been carried out on Ge(100) from below room temperature to 1173 K. The c(4 x 2) --> 2 x 1 phase transition is accompanied by the shift of a back bond derived surface state. The high temperature 2 x 1-->1 x 1 transition and a further transition at similar to 1075 K are evident as discontinuities in the emission intensity at the Fermi level and in the attenuation of a bulk and a surface electronic state. (C) 1998 Elsevier Science B.V. All rights reserved.

We report on symmetry changes of the bulk hexagonal vortex lattice induced by commensurate and incommensurate engineered 2D square pinning structures of Fe dots, deposited on one surface of Bi2Sr2CaCu2O8 single crystals. (C) 2004 Elsevier B.V. All rights reserved.

The density of states of the lowest unoccupied molecular orbital band as a function of K doping was investigated by yield spectroscopy excited with visible and near-uv photons. The Fermi level position was determined at each composition with high precision. The superconducting phase ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ exhibits a clear metallic edge, although the Fermi level is positioned at low state density [N(${\mathit{E}}_{\mathit{f}}$)=0.8\ifmmode\pm\else\textpm\fi{}0.2 states/eV molecule] past the band maximum. The ${\mathrm{K}}_{4}$${\mathrm{C}}_{60}$ phase is insulating. Correlation effects are necessary to explain the data as a function of doping.

The valence band states of a-Si1-xCx:H have been studied by combining photoemission and Auger spectroscopies. It was possible to establish the relative positions of the onsets and of the maxima of the local electronic-state density at C and Si atoms. It was also found that heteronuclear Si-C bonds are strongly favoured.

The density of occupied states at the valence band edge and in the pseudo-gap is investigated by using Total yield spectroscopy. It is found that close to the valence band edge the yield spectra follow a (h-nu - E(v))3/(h-nu)2 dependence. A large increase of gap state density with increasing carbon content is also found.

It is shown that the local density of states in a-Si 1− x C x :H can be investigated by combining information from Auger effect, which is site specific, with those from photoemission spectra. In the diluted C regime, it is found that the C centered states with dominant s-like character are localized and lie below the bottom of the dominant Si-Si valence band. The onset of the C centered states with dominant p-like character is placed below the top of the Si valence band. This last result shows that carbon atoms act as strong scattering centers for holes.

In this article we present a quantitative study of the influence of the number and the thickness of the silicon spacer layer on the optical and structural properties of single and multilayers of self-assembled Ge/Si(001) islands. By means of cathodoluminescence spectroscopy, high resolution x-ray diffraction, and transmission electron microscopy, we will show that the island composition and strain status of single-layer samples do not depend on the silicon cap-layer thickness. On the contrary, we found that the strain interaction existing between island layers separated by a silicon spacer layer 33 nm thick (i.e., similar to 3 times the mean island height), enhances the SiGe intermixing during the deposition of subsequent layers bringing to a composition inhomogeneity of the intermixed islands of about 18% along the stack. This inhomogeneity is not present in samples having a thicker spacer layer (60 nm) although the strain interaction between different layers is strong enough to drive the island stacking along the vertical direction. We conclude that the latter spacer layer thickness allows us to obtain a highly ordered three-dimensional superlattice of an island having a homogeneous size and composition along the stack itself. (c) 2007 American Institute of Physics.

a-Si:H/a-Si 1− x C x multi-quantum-well structures have been grown and investigated. The density of defects at the interface is measured by photothermal deflection spectroscopy and correlated to the decrease of photoconductivity observed with decreasing silicon layer thickness.

a-Si:H/a-Si1−xCx:H multi-quantum-well structures have been grown and investigated. Optical transmission and electroabsorption measurements show that the optical gap increases with decreasing a-Si:H layer thickness. This behaviour is explained in terms of quantum confinement effects. An increase of the Urbach tail and a decrease of the photoconductivity were also observed with decreasing silicon thickness. Both effects have been attributed to the observed increase of the gap state density.

Valence-band photoemission at a photon energy of 32 eV has been carried out on Ge(100) from below room temperature to 1173 K. The c(4X2)-->2x1 phase transition is accompanied by a shifting of a back-bond-derived surface state. The high-temperature 2x1-->1x1 transition is apparent in the discontinuity in the measured emission intensity of both a bulk and a surface electronic state. A further discontinuity occurs in both of these features and of the Fermi level intensity at higher temperature, approximately 1075 K, indicating the presence of a further reversible phase transition whose nature is discussed.

Valence band and Ge 3d core level photoemissions have been carried out on Gd(110) at room and high temeprature. Structures in the room-temperature valence band spectrum are interpreted in terms of bulk interband transitions and a surface state. Major changes in the valence band take place at similar to 800 K, at which temperature, the centroids of the core level spectra start to move to a lower binding energy, and their widths start to increase more quickly. (C) 1998 Elsevier Science B.V. All rights reserved.