3D DuMond diagrams of multi-crystal Bragg-case synchrotron topography. I. Flat sample

The 3D representation of the DuMond diagram is used to explain the dimensional features of X-ray topographs obtained by multi-crystal configuration with a synchrotron beam. Symmetric Bragg-case reflections are considered for a flat double-crystal monochromator and a flat sample. Two ways of sample alignment are taken into account. They are referred to as σ–σ and σ–π geometries, where the diffraction plane of the sample is parallel and perpendicular, respectively, to the vertical diffraction plane of the monochromator (σ polarization). It is shown that the shape of the sample image is closely connected to the shape the diffraction domain common to monochromator and sample assumes in the 3D DuMond diagram. An experiment is reported for the less commonly used σ–π topography, showing how the lattice mismatch and its lateral homogeneity are determined in samples made by epilayer and substrate. Received: 31 July 2000 / Accepted: 25 January 2001 / Published online: 3 May 2001     

Structure and dynamics of clean and adsorbate-covered crystal surfaces studied by surface X-ray diffraction

Received: 24 March 1998/Accepted: 21 August 1998     

Curved crystal analysis using a triple-crystal X-ray spectrometer

A Bragg crystal spectrometer using cylindrically curved crystals is designed to focus a divergent X-ray beam. The spectral resolution and precision of focus is determined by the quality of the concavely curved crystal. The analysis of the quality of the crystal was done using a triplecrystal X-ray spectrometer, comparing parallel and antiparallel X-ray scans of the third curved crystal, determining flaws resulting from the bend. Topics discussed include the derivation of the crystal's quality, conditions of alignment for the Bragg reflection, and X-ray beam width contributions to the FWHM. Organic and inorganic crystals with varying values of Bragg angles are used.     

Chemical-state specificity in surface structure determination

In the pursuit of an understanding of complex surfaces, the problem of obtaining quantitative structural information about local adsorbate geometry is especially difficult. Conventional diffraction methods rely on long-range order of the adsorbed species, rarely present in complex coadsorption systems. Elementally specific local structural probes can help, but ultimately one also requires chemical-state specificity. This can be achieved in structural methods that involve detection of photoelectrons through the well-known ‘chemical shifts’ in core-level photoelectron binding energies; specific methods of this type are scanned-energy mode photoelectron diffraction (PhD) and normal-incidence X-ray standing waves (NISXW). Recent examples of chemical-shift PhD and NIXSW applications to complex coadsorption systems and to larger molecular adsorbates demonstrate this potential. Received: 28 April 2000 / Accepted: 6 June 2000 / Published online: 7 March 2001     

Influence of Zn doping on electrooptical properties and structure parameters of lithium niobate crystals

We report the dependence of the unit cell parameters and the EO coefficients on Zn doping in optical-damage-resistant LiNbO₃:Zn crystals. Both properties depend in a non-monotonic manner on the Zn content. This is accounted for by different types of Zn ion incorporation into the lattice depending on the Zn concentration in the melt. Extrema observed in the concentration dependence of the EO coefficients at about 2–3 and 6.4 mol.% Zn correlate with an unusual concentration dependence of the unit cell parameters a and c. The low-concentration anomalies may be accounted for by a decrease of the Li vacancy concentration due to the Zn incorporation into Li sites. Anomalies at high concentrations are obviously due to a partial incorporation of Zn ions on Nb sites, which is reflected in the structure data. Anomalies in the concentration dependence of other optical properties at about 6–7 mol.% Zn reported recently are obviously related to a change in the localization of the Zn ions. The combination of high EO coefficients with a reduced optical damage for these concentrations make these crystals attractive for applications as Q-switching or electrooptical modulation. Received: 16 Nowember 1998 / Revised version: 8 December 1998 / Published online: 24 March 1999     

Structural and optical properties of nanophase zinc oxide

Nanophase zinc oxide samples were synthesized by a two-step solid-state reaction method. The phase structure and microstructure were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The vibrational Raman spectra were compared with those from the bulk and their grain size dependence was also examined. Their photoelectric behavior was studied by X-ray photoelectron spectroscopy (XPS). The peaks at 1044.5 and 1021.4 eV were recorded as corresponding to the respective binding energies of Zn 2p_1/2 and Zn 2p_3/2, and the photoelectron spectrum of O 1s in the as-prepared powder was located at 531.2 eV. A strong visible emission centered at 580 nm was clearly observed in the nanosized zinc oxide at room temperature. Photoluminescence (PL) spectra were investigated as a function of grain size after different heat treatments. The origin of the luminescence is attributed to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band. Received: 30 June 2001 / Accepted: 20 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: mszhang@nju.edu.cn     

Study of the growth of fullerene-carbonized epitaxial SiC thin films by synchrotron radiation

 Thin SiC films have been deposited on silicon(0 0 1) substrates by fullerene-carbonization. Using synchrotron radiation X-ray diffraction pole figure measurements have been employed in order to study the texture of the layers. It is qualitatively shown that the films contain epitaxially aligned β-SiC crystallites with the same orientation as the underlying substrate and their twins of first and second order. The orientational spread of the epitaxial crystallites in terms of tilt against and rotation around the substrate normal is smaller than 3°. The formation of twins as a growth defect plays a major role which is even more pronounced at a higher substrate temperature. Furthermore, an additional preferred orientation has been identified which can only be explained by a non-cubic SiC phase. The portion of these crystallites in the film can be considerably reduced by an increase of the deposition temperature. Received: 18 April 1996/Accepted: 12 August 1996     

Inelastic X-ray scattering: new possibilities for Raman spectroscopy

Inelastic X-ray scattering with meV resolution has recently become available for studies of dynamical properties and elementary excitations in solids. Contrary to Raman spectroscopy at visible wavelengths, which in first order is limited to the Brillouin-zone center, the wave vectors in hard X-ray Raman scattering are very large, and the crystal-momentum transfer to elementary excitations, whose energies may range from a few meV up to several eV, can be tuned continuously across the whole Brillouin zone. This paper reviews new and unique possibilities offered by X-ray Raman spectroscopy for crystalline solids, such as phonon-dispersion measurements (GaN), the determination of phonon self-energies (isotopically mixed diamond), and resonance effects and studies of electronic excitations (copper oxides). Received: 19 October 2001 / Accepted: 12 December 2001 / Published online: 27 March 2002 / Published online: 27 March 2002 RID="*" ID="*"Present address: Agilent Technologies Deutschland GmbH, Herrenberger Str. 130, 71034 B?blingen, Germany     

A titanium-rich (111) surface of SrTiO3 single crystals by thermal annealing

A very smooth (111) titanium-rich surface was obtained by annealing SrTiO₃ single crystals in UHV. The surface structure was investigated with SEM, STM and LEED and was found to consist of terraces, interrupted by multi-unit-cell high steps, obviously a result of step bunching. The surface unit cell, is enlarged as a result of our particular preparation procedure. Received: 18 December 1996/Accepted: 18 December 1996     

Photoluminescence investigations on the nanoscale phase transition in Pb(Mg1/3Nb2/3)O3

The temperature dependence of the photoluminescence spectra of the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃ has been reported. The temperature behaviours of the 1.57, 1.67 and 1.73 eV bands indicate a phase transition at 110 K. This is attributed to a structural phase transition in the charged nanoshell. Analysis of the temperature dependence of 1.67 eV band intensity with a thermal quenching model indicated the existence of a phonon mode at 1153 cm⁻¹. This mode is identified in the Raman spectra measurement. The intensity of the 1.73 eV band showing an anomalous behaviour at 210 K is attributed to a transition from a crystalline phase to an amorphous phase in the charged nanoshell.     

The role of carrier mobility in holographic recording in LiNbO3

We investigate the role of carrier mobility in holographic recording in LiNbO₃ crystals. Both normal holographic recording (single wavelength, single trap) and two-center recording are considered, and the differences between the performances of the two methods are explained. We show that increasing mobility by using stoichiometric crystals or by doping with Mg does not improve sensitivity considerably, but does reduce M/# by at least one order of magnitude. Received: 22 February 2001 / Revised version: 5 March 2001 / Published online: 27 April 2001     

Carbonaceous phases by IR laser-induced decomposition of 3-butyn-2-one

2 laser radiation in the gas phase results in the deposition of carbonaceous films composed of a graphitic and monohydrogenated carbon, in which H is dominantly bonded to sp³ sites. The films incorporate oxygen in the topmost layers. The technique is suitable for the preparation of thin C-based films at low substrate temperatures. Received: 23 July 1997/Accepted: 18 November 1997     

Heteroepitaxial growth of oxides on sapphire induced by laser radiation in the solid–liquid interface

2 O₃, Fe₂O₃ and MnO₂ on sapphire from an aqueous solutions of either CrO₃, FeCl₃, or KMnO₄, respectively, under laser irradiation of the interface sapphire/liquid. The interface is exposed through the sapphire substrate to the radiation of a copper vapor laser (wavelength of 510 nm). The etching of sapphire is accompanied by the deposition of oxide films, which are shown to grow epitaxially on the sapphire substrate, while the deposition of the polycrystalline oxide film occurs on a glass substrate under the same experimental conditions. Similarly, the epitaxial growth of cubic Fe₂O₃ and orthorhombic MnO₂ is observed, though their crystallographic structure is different from the hexagonal structure of sapphire. Received: 26 June 1997/Accepted: 7 July 1997     

Photorefractive properties of LiNbO3:Zn crystals related to the defect structure

LiNbO₃:Zn single crystals and powders were studied by precise X-ray diffraction methods. Structural refinement yielded new models of the intrinsic defect structure valid for different Zn concentration ranges. For concentrations up to 7 at. % in the crystal, Zn ions incorporate onto Li sites; at higher concentrations Zn ions are found on both Li and Nb sites. Photorefractive properties of LiNbO₃:Zn are discussed in the context of the deduced defect models. A smooth increase in the photoconductivity up to 7 at. % Zn is accounted for by a decrease of Nb antisites. Steeper growth of the photoconductivity at higher Zn concentrations is related to vanishing Li vacancies. Received: 17 November 2000 / Revised version: 23 January 2001 / Published online: 20 April 2001     

Parallel excitation pathways in ultrafast interferometric pump-probe correlation measurements of hot-electron lifetimes in metals

Hot electron (E-E_Fermi=0.75 to 1.55 eV) lifetimes for cesiated Cu(100) and Cu(111) surfaces are measured via interferometric time-resolved two-photon photoemission with a 19-fs intensity FWHM mode locked Ti:sapphire laser at 1.55 eV. The data are analyzed using the optical Bloch equations and a laser pulse characterized in situ via surface second-harmonic generation interferometric autocorrelation. It is found that the retrieved hot-electron lifetimes are unphysically fast, and have a strong dependence on the temperature of the sample and the polarization of the laser. A simple explanation for the data is that the measured signal consists of contributions from transitions through both virtual and real intermediate states. Received: 26 July 2000 / Accepted: 8 September 2000 / Published online: 12 October 2000     

Structural and optical characterization of Er3+/Yb3+-doped LiNbO3

We report the dependence of the unit-cell parameters and the extraordinary and ordinary refractive indices of Er³⁺/Yb³⁺-codoped LiNbO₃ crystals. Both properties depend in a non-monotonic manner on the Er³⁺/Yb³⁺ content. A singularity was observed at concentrations of 1.1-1.2 mol. % in the crystal (0.6-0.7 mol. % in the melt). In the same way the Er and Yb concentration influences the periodically poled lithium niobate formation. The observed behavior of refractive indices and unit-cell parameters of Er³⁺/Yb³⁺-codoped LiNbO₃ crystals could be explained in terms of the RE³⁺-ion concentration affecting the Li-vacancy concentration and the RE³⁺-ion positions in the crystal. Received: 21 May 2001 / Revised version: 22 August 2001 / Published online: 23 October 2001     

Absorption gratings in ferroelectric Bi4Ti3O12

4 Ti₃O₁₂. Basic properties of absorption gratings in this crystal are investigated with beam-coupling experiments. Depending on the grating spacing, wavelength, and intensity of the writing beams, an absorption grating in our sample can be either in phase or shifted by 180° with respect to the light pattern. The formation mechanism of absorption grating is discussed and proposed to originate from a shallow-trap effect. Received: 10 February 1997     

Microstructural investigations of Y2O3 thin films deposited by laser ablation on MgO

The epitaxial relationships and the microstructure of thin Y₂O₃ film, in situ grown by laser ablation at 700 °C under 0.5 mbar oxygen pressure on MgO substrate, are studied by X-ray and HRTEM investigations. X-ray φ-scanexperiments show that Y₂O₃ exhibits two different crystallographic growing directions <111> and <100> on (100) single-crystalline MgO substrate. The <111> growing direction appears with four different epitaxial relationships where the <110> Y₂O₃ in-plane direction is parallel to the <110> in-plane direction of MgO. The <100> growing direction appears with one epitaxial relationship, cube Y₂O₃ on cube MgO. The HRTEM experiments show the columnar aspect of the Y₂O₃ thin film. Both (111) and (100) Y₂O₃-oriented grains are observed in good agreement with the X-ray experiments. Received: 16 May 2000 / Accepted: 22 May 2000 / Published online: 13 September 2000     

Electronic structure of light emitting centers in Er doped Si

Ab-initio calculations are carried out for the Er-related electrically active centers in Si. Our proposed microscopic model is consistent with photoluminescence measurements on Si:Er and Si:Er:O samples. For isolated Er, the tetrahedral interstitial site is the stable configuration, being related to the photoluminescence lines in Si:Er. Several configurations containing oxygen and fluorine atoms, surrounding the Er impurities, are proposed to simulate the effects of co-implantation. The results suggest that six oxygen atoms around substitutional Er can stabilize the center, which can be related to the strong photoluminescence lines in Si:Er:O samples. On the other hand, no configuration containing fluorine atoms could explain the stronger photoluminescence lines resulting from fluorine co-implantation. Received: 9 September 2002 / Accepted: 12 September 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +55-11/3091-5585, E-mail: jjusto@lme.usp.br     

Fermi-level engineering of BaTiO3 by alkali codoping: increasing the near-infrared absorption by rhodium

Using electron paramagnetic resonance, optical absorption, and fast spectroscopy of light-induced absorption changes, it is shown that codoping BaTiO₃:Rh with Na_Ba acceptors raises the charge state of Rh³⁺ to Rh⁴⁺. Subsequent oxidation under high oxygen pressures can lower the Fermi level to Rh^4+/5+, leading to increased infrared absorption. The light-induced charge transfer in such specimens is characterised by “one center” behaviour. Received: 18 November 1998 / Revised version: 23 December 1998 / Published online: 7 April 1999