Showing papers in "Physica Status Solidi (a) in 1996"

Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 (NBT-KBT) system: A structural and electrical study

Abstract: The study of the Na0.5Bi0.5TiO3–K0.5Bio0.5TiO3 (NBT-KBT) system was carried out using X-ray diffraction and dielectric measurements. The limits of rhombohedral and orthorhombic solid solutions were determined, as well as the evolution of their lattice parameters as a function of composition and temperature. Dielectric permittivity measurements in a wide frequency range between 20 and 800°C showed that all materials are ferroelectric at room temperature and exhibit a diffuse, probably second-order phase transition from the ferroelectric to the paraelectric state. Several peculiar compositions showed the best piezoelectric characteristics for this type of ceramic materials. L'etude du systeme Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 (NBT-KBT) a ete realisee par diffraction X et mesures dielectriques. Les limites des solutions solides de symetrie rhomboedrique et orthorhombique ainsi que les evolutions des parametres de maille en fonction de la composition et de la temperature ont ete determinees. Les mesures de permittivite dielectrique effectuees dans une large gamme de frequence entre 20 et 800°C ont montre que tous ces materiaux sont ferroelectriques a la temperature ambiante. Ils presentent une transition de phase diffuse, probablement du deuxieme ordre de l'etat ferroelectrique vers l'etat paraelectrique. Quelques compositions particulieres presentent des caracteristiques piezoelectriques dignes d'interět.

Electrical and Optical Properties of Copper Oxide Films Prepared by Reactive RF Magnetron Sputtering

Abstract: Copper oxide thin films were prepared by reactive rf magnetron sputtering of a pure copper target in an oxygen-argon atmosphere. The phases of the deposited films strongly depend on the oxygen content in the sputtering gas. X-ray diffraction studies show that by controlling the oxygen partial pressure single phase Cu2O and CuO can be obtained. The resistivity of the Cu2O film in the present study is 43 Ω cm. The optical constants were evaluated from transmission and reflection measurements.

Boron-Doped Homoepitaxial Diamond Layers: Fabrication, Characterization, and Electronic Applications

Abstract: For electronic applications, a series of boron-doped homoepitaxial diamond layers of high crystal quality have been grown on (100)-cut diamond substrates using the microwave plasma CVD method. The B-concentration varies from 3 × 1017 to 3 × 1020 atoms/cm3. The layers are selectively grown into the shape of Hall bars using a sputtered SiO2 mask. The diamond substrates are 3×3×1 mm3 in size. Gold wires are bonded to ohmic contacts on the Hall bars formed by an electron beam evaporated Mo/Pt/Au sandwich annealed at 950 °C for 30 min. Electrical characterization is performed in eight samples over the temperature range from 100 to 1300 K. The results are discussed in detail. For diamond layers grown on synthetic nitrogen-doped (type Ib) substrates, current-voltage characteristics of diode type can be observed in the temperature range from 360 to 900°C. Green electroluminescent light is emitted from the p-n junction area. High current Schott-ky diodes are also fabricated which consist of a gold contact to a moderately B-doped homoepitaxial layer grown on a synthetic heavily boron-doped (type IIb) substrate. Fur elektronische Anwendungen wird eine Serie von Bor-dotierten homoepitaktischen Dia-maritschichten hoher kristalliner Gute auf (100)-geschnittenen Diamantsubstraten mittels der Mikrowellenplasma-unterstutzten CVD-Methode aufgewachsen. Die B-Konzentration variiert von 3×1017 bis 3 × 1020 Atome/cm3. Die Schichten werden selektiv mittels gesputterter SiO2-Masken als Hallkreuze gewachsen. Die Diamantsubstrate sind 3×3×1 mm3 gros. Golddrahte werden an ohmsche Kontakte gebondet. Diese werden als Mo/Pt/Au-Schichtpakete mittels Elektronenstrahl-Aufdampfen und halbstundigem Tempern bei 950°C hergestellt. Die elektrische Charakterisierung erfolgt bei acht Proben uber den Temperaturbereich von 100 bis 1300 K. Die Ergebnisse werden eingehend diskutiert. Auf synthetischen Stickstoff-dotierten (Typ Ib) Substraten werden Strom/ Spannungs-Charakteristiken vom Diodentyp im Temperaturbereich von 360 bis 900°C beobachtet. Grumes Elektrolumineszenzlicht wird von der p-n-Kontaktflache emittiert. Hochstrom-Schottkydio-den werden ebenfalls hergestellt. Sie bestehen aus einem Goldkontakt zu einer masig B-dotierten homoepitaktischen Schicht, die auf einem synthetischen, hochdotierten (Typ IIb) Substrat aufgewachsen wurde.

Spectroscopy and Origin of Radiation Centers and Scintillation in PbWO4 Single Crystals

Abstract: The results of studies on the spectroscopic properties and the origin of luminescent radiation centers in PbWO4 single crystals are presented in this paper. Investigation of the excitation spectra of luminescence bands and polarization measurements confirm the previously observed two green bands and establish a connection of these bands with transitions within centers based on the WO3 group. The energy level scheme for radiation centers is proposed. [Russian Text Ignored]

On the Mechanisms of Bias Enhanced Nucleation of Diamond

Abstract: The mechanisms of bias enhanced nucleation (BEN) of diamond on silicon substrates are investigated by means of electrical measurements, OES investigations, and macroscopic as well as local characterization of BEN films. Based on electrical investigations we propose that the reproducibility and homogeneity problems often found in BEN are caused by diamond coated surfaces in the vicinity of the substrate. We further show that the nucleation enhancement does not rely on changes in gas phase chemistry but rather on direct bombardment of the surface by ions with a mean energy of about 15 to 20 eV. The influence of bias voltage, carbon concentration in the gas phase, and substrate temperature on the nucleation process is investigated. Finally, the films obtained by BEN are investigated by FTIR and stress measurements. A nucleation sequence (SiC formation, deposition of a hydrocarbon film, formation of diamond and simultaneous development of stress) is established and compared with SEM images of the films. The role of each step of the sequence is discussed in detail.

Dielectric behaviour of nano‐TiO2 bulks

Abstract: Two kinds of nano-TiO 2 samples, including nano-anatase and nano-rutile, were prepared by the hydrothermal method and the gas condensation method, respectively. Results of dielectric measurements are as follows: 1. Nano-TiO 2 bulk samples (anatase and rutile) exhibit much higher dielectric constant than coarse-grain samples in the low measuring frequency range. 2. For both nano-anatase and nano-rutile, one peak appears in the spectrum of dielectric constant versus temperature, and it is located in the measuring temperature range of -150 to 150 °C for nano-anatase and in the measuring temperature range of 25 to 450 °C for nano-rutile. 3. For both nano-anatase and nano-rutile, in the spectrum of dielectric loss versus temperature there exists one peak. The mechanisms of the above-mentioned phenomena were discussed in detail.

Electrical and Optical Properties of Cr2O3 Films Prepared by Chemical Vapour Deposition

Abstract: Polycrystalline Cr2O3 films are prepared by normal pressure chemical vapour deposition (CVD), and electrical and optical properties of the films are investigated. The films are transparent from 800 to 1000 nm wavelength, and absorbing for wavelengths shorter than 800 nm. The optical band gap energy is Eopt = 2.98 to 3.09 eV for films formed at different substrate temperatures. The electrical conductivity σ of the films is from 1 × 10−2 to 2.5 × 10−3 S cm−1 at 500 K. The films are p-type semiconducting. At temperatures higher than 500 K. the conduction is due to small polaron hopping. For temperatures lower than 500 K, a T−1/4 dependence of σT1/2 is found, which is attributed to variable-range hopping. Les couches minces de Cr2O3 sont preparees par CVD a pression normale, et on etudie les proprietes electriques et optiques des couches minces. Les couches sont transparentes de 800 a 1000 nm de longueur d'onde, et absorbantes a longueur d'onde moin que 800 nm. La bande optique d'energie interdite des couches est de 2.98 a 3.09 eV pour celles deposees a temperatures differentes de substrat. La conductivite electrique des couches s'etend de 1 × 10−2 a 2,5 × 10−3 S cm−1 a 500 K. Les couches sont semiconducteurs de type-p. Aux temperatures superieures a 500 K, la conduction est due a “small polaron hopping”. Aux temperatures inferieures a 500 K, une dependence de T−1/4 de σT1/2 se trouve, ce qui est attribuee a “variable-range hopping”.

Stillinger-Weber potentials for III-V compound semiconductors and their application to the critical thickness calculation for InAs/GaAs

Abstract: Parameters in the Stillinger-Weber interatomic potentials are obtained for III–V compounds from the cohesive energy, the lattice constant, and the elastic properties. The obtained potentials coincide with the Keating potentials for small distortion (<1%) but are more accurate for larger distortion. Using the SW potentials, the critical thickness of misfit dislocation nucleation is calculated for InAs/(001)GaAs. The critical thickness is 5 ML for a 60° dislocation and 2 ML for an edge dislocation. These thicknesses are smaller than those calculated using the Keating potentials. Results obtained by the classical elastic theory do not agree with the present results.

Defect chemistry analysis of the defect structure in Mg-doped LiNbO3 crystals

Abstract: on the basis of the Li vacancy model of intrinsic defects in LiNbO3 crystal, the defect structure formed in LiNbO 3 :Mg varying with the Mg content are proposed. The Li 2 O and Nb 2 O 5 contents, the Li/Nb ratio in the crystal, and the density of the crystal are calculated by using the defect structure. The calculated values are in good agreement with the reported experimental results.

Nucleation and Growth of Heteroepitaxial Diamond Films on Silicon

Abstract: The introduction of the bias pretreatment process for in-situ nucleation of oriented diamond crystallites on silicon was an important step towards heteroepitaxial deposition of single crystalline diamond films. However, the quality of these films is still limited mainly due to a significant orientational spread of the epitaxial grains around the perfect heteroepitaxial orientation. A further improvement of these films requires an extensive description of the film texture immediately after the nucleation, of its development with increasing film thickness, and an improved insight into the mechanism responsible for the nucleation process. In this report our recent and new results on the heteroepitaxial deposition of diamond on Si(001) and Si(111) and the subsequent textural characterization by X-ray pole figure measurements are reviewed. The possibilities and limits for improving the film quality by an appropriate textured growth step are discussed. Furthermore, optical measurements inside the plasma during the biasing step and nucleation experiments using patterned substrates indicate that the high field strength above the surface and the resulting acceleration of ions play a dominant role in the formation of diamond nuclei. Die Einfuhrung der in-situ-Bekeimung zur Erzeugung orientierter Diamantkristallite auf Silizium durch Anlegen einer zusatzlichen Gleichspannung zum Mikrowellenplasma stellte einen wichtigen Schritt dar in Richtung heteroepitaktische Abscheidung von einkristallinen Diamantschichten. Die Qualitat dieser Filme bleibt allerdings weiterhin begrenzt durch eine deutliche Streuung der orientierten Kristallite um die perfekte Orientierung. Grundlage fur eine weitere Verbesserung dieser Filme ist eine umfassende Beschreibung der Filmtextur nach der Bekeimung, ihrer Veranderung mit wachsender Schichtdicke sowie ein tieferes Verstandnis der Mechanismen, die dem Bekeimungsprozes zugrunde liegen. Im folgenden Bericht werden bisherige und aktuelle Ergebnisse unserer Arbeit zur heteroepitaktischen Abscheidung von Diamant auf Si(001) und Si(111) und zur nachfolgenden Texturcharakterisierung mit Rontgenpolfiguren zusammengefast. Montglichkeiten und Grenzen der Schichtoptimierung durch einen entsprechenden texturierten Wachstumsprozes werden aufgezeigt. Zusatzliche optische Messungen im Plasma sowie Abscheidungsexperimente auf strukturierten Substraten belegen den entscheidenden Einflus der hohen Feldstarke im Plasma uber dem Substrat und der daraus resultierenden beschleunigten Teilchen fur den Bekeimungsvorgang.

Formation and properties of SrBi2Ta2O9 thin films

Abstract: Layered structure SrBi 2 Ta 2 O 9 thin films were grown on Pt/Ti/Si/SiO 2 substrates by laser ablation. The films were deposited at temperatures ranging from 500 to 750 °C and characterized for their phase formation, morphology, surface composition, and ferroelectric properties. Although crystalline phase formation was observed at temperatures as low as 500°C, well defined saturated hysteresis loops were observed only in films deposited at temperatures of 700°C or above. The transition in ferroelectric properties between 650 and 700°C was associated with a change in orientation and grain size. The orientation of the films changed from highly c-axis oriented at 650 °C to randomly polycrystalline at 700 °C, while the grain size of the films increased from an average value of 80 nm (at 650 °C) to 160 nm (at 700 °C). An understanding of process-structure relationships is required in order to fabricate high quality films at lower temperatures.

The Relationship between Resistivity and Boron Doping Concentration of Single and Polycrystalline Diamond

Abstract: Temperature dependent resistivity and Hall measurements have been carried out on in situ boron doped polycrystalline diamond films. The temperature dependence of the resistivity can be described by a two band conduction model with two conduction mechanisms working parallel. A fair agreement between the depth distribution of the boron concentration determined from spreading resistance and secondary ion mass spectroscopy is found. Room temperature resistivities and activation energies from various sources are compared with the present work. With the aid of these curves the doping and hole concentrations can be estimated from room temperature resistivity measurements.

Effects of local non‐equilibrium solute diffusion on rapid solidification of alloys

Abstract: A conceptual foundation for the study of local non-equilibrium solute diffusion under rapid solidification conditions is proposed. The model takes into account the relaxation to local equilibrium of the solute flux and incorporates two diffusion speeds, V Db , the bulk liquid diffusion speed, and V Di , the interface diffusive speed, as the most important parameters governing the solute concentration in the liquid phase and solute partitioning. The analysis of the model predicts complete solute trapping and the transition to a purely thermally controlled solidification, which occur abruptly when the interface velocity V equals the bulk liquid diffusion speed V Db . The abrupt change in the solidification mechanism is described by the velocity dependent effective diffusion coefficient D * = D(1 - V 2 /V Db 2 ) and the generalized partition coefficient K * . If V > V Db , then D * = 0 and K * = 1. This implies an undisturbed diffusion field in the liquid (diffusionless solidification) and complete solute trapping at V > V Db . The interface diffusion speed V Di governs solute redistribution at a relatively low interface velocity V ∼ V Di < V Db . The influence of the local non-equilibrium solute transport on the temperature field and interface stability under rapid solidification conditions is also discussed.

Evaluation of stress gradients ??ij(z) from their discrete laplace transforms ??ij(τk) obtained by x-ray diffraction performed in the scattered vector mode

Abstract: Due to the absorption, X-ray diffraction analysis of residual stress gradients yields only exponentially weighted averages (“Laplace profiles”) of the strains and stresses, eψφ(τ) and σij(τ), with respect to the penetration depth τ of the X-rays. However, because the main interest is focussed on the actual depth profiles of the stresses, σij(z), the τ-profiles have to be inverted into those of the (original) z-space by suitable methods of the inverse Laplace transform. For discrete profiles σij(τk) obtained in the scattering vector mode, where τ is adjusted by rotating the sample around the normal of the reflecting lattice planes, Nh, the calculations may be performed numerically by the methods of orthogonal polynomials. It will be shown on examples of simulated profiles that the use of Jacobi polynomials gives the best results, because they offer the maximum flexibility with regard to the selection of the discrete values τk of the penetration depth. Aufgrund der Absorption liefert die rontgenographische Analyse von Eigenspannungsgradienten lediglich exponentiell gewichtete Mittelwerte (“Laplace-Profile”) der Dehnungen und Spannungen eψφ(τ) bzw. σij(τ) bezuglich der Eindringtiefe τ der Strahlung. Da man jedoch vorrangig an den tatsachlichen Tiefenverlaufen sij(z) der Spannungen interessiert ist, mussen die τ-Profile mittels geeigneter Methoden der inversen Laplace-Transformation in solche des (originalen) z-Raumes umgewandelt werden. Fur diskrete Profile σij(τk), die im Streuvektor-Mesmodus ermittelt wurden, bei dem die Einstellung von τ durch Drehung der Probe um die Normale Nh der reflektierenden Netzebenen erfolgt, lassen sich die Berechnungen numerisch mit den Methoden der orthogonalen Polynome durchfuhren. Anhand von simulierten Profilen wird gezeigt, das der Einsatz von Jacobi-Polynomen die besten Ergebnisse liefert, da sie die grostmogliche Flexibilitat bei der Festlegung der diskreten Werte τk fur die Eindringtiefe bieten.

High-efficient stimulated-raman scattering in ferroelectric and ferroelastic orthorhombic Gd2(MoO4)3 crystals

Abstract: New nonlinear properties of the orthorhombic ferroelectric and ferroelastic Gd2(MoO4)3 crystals, the ability for efficient (up to 70%) generation of multiple (up to fourth) Stokes emission and creation of filament optical channeling, are observed. Gd2(MoO4)3 crystals with Ln3+ ions are supported to be also good candidates for lasers with self-stimulated Raman scattering. For KY(WO4)2, KGd(WO4)2, and NaLa(MoO4)2 crystals new Stokes wavelengths were excited, too. Die nichtlinearen Eigenschaften orthorhombischer ferroelektrischer und ferroelastischer Gd2(MoO4)3 Kristalle wurden untersucht, die bis zu 70% der eingestrahlten Energie in mehrere Stokeslinien konvertieren (bis zur vierten Ordnung) und die Ausbildung eines optischen Kanals durch Selbstfokussierung und Beugung zeigen. Ln3+ dotierte Gd2(MoO4)3 Kristalle sollten sich daher gut fur den Einsatz in Lasern mit selbst-stimulierter Raman-Streuung eignen. Weiterhin wurden in KY(WO4)2, KGd(WO4)2 und NaLa(MoO4)2 neue Stokeswellenlangen angeregt.

Positron lifetime studies of free volume hole size distribution in glassy polycarbonate and polystyrene

Abstract: Positron lifetime spectroscopy has been applied to estimate the free volume hole size distribution in polycarbonate (PC) and in polystyrene (PS) at room temperature. The hole radius density distributions are determined from the ortho-positronium lifetime distributions obtained via a Laplace inversion of the measured positron lifetime spectra. The hole volume density distributions and the number density distributions of holes are estimated from the hole radius density distributions. All of the distributions may be well approximated by single Gaussians. The hole radius and the hole number density distributions have centres (r f ) and (v fn ) at 0.29 nm and 0.1 nm 3 in PC, and 0.28 nm and 0.09 nm 3 in PS. The FWHM of the corresponding distributions are 0.042 nm and 0.040 nm 3 (PC), and 0.039 nm and 0.34 nm 3 (PS), respectively. Both, the shape and the width of the distributions correlate well with the free volume theory of Bueche. The possible influence of different effects, such as a nonspherical shape of holes and a scanning of several holes by positronium before annihilation, on the mean hole size and the hole size distribution as detected by the positron lifetime method is finally discussed.

Recent Developments in Heteroepitaxial Nucleation and Growth of Diamond on Silicon

Abstract: After the successful controlled heteroepitaxial nucleation of diamond on 3C-SiC epitaxial films and on bare silicon wafers, respectively, obtained by research groups in the USA and Germany in 1992, considerable technical progress and scientifical understanding in the area of heteroepitaxy of CVD diamond have been achieved. The decisive role of ion bombardment for the bias-enhanced epitaxial diamond nucleation is clearly demonstrated. Cross-sectional high resolution electron microscopic (XREM) investigations show the direct epitaxy of diamond on silicon with a periodic 3-to-2 registry of {111} atom planes of the epitaxial diamond—silicon interface. Although the epitaxial growth is of Volmer-Weber type with slight misorientation, i.e., polycrystalline with epitaxial orientation of individual crystallites, the crystallographical and physical properties of these films represent a significant improvement over the usual randomly oriented films, owing to the reduction of the grain boundary angle distribution width to a few degrees. This paper summarizes results obtained recently in our group in the area of heteroepitaxy of diamond on silicon and discusses problems still to be resolved.

Microstructure Evolution and Defect Incorporation in Highly Oriented and Textured CVD Diamond Films

Abstract: A series of highly oriented and textured microwave CVD diamond films, where only the deposition time was varied, was deposited on silicon wafers in order to follow the evolution of the microstructure and defect content with film thickness. SEM, XRD, Raman spectroscopy, luminescence measurements, and ESR were applied to determine the microstructure and the defect density. It is shown that the growth of highly oriented and textured film proceeds in two stages. The first is characterized by marked changes of the microstructure (size, orientation, and shape of the grains) which directly influence the generation of defects. The second stage, which begins as soon as the film surface is covered with {100} tiles, leads to minor changes in the microstructure and defect incorporation. The defect-related part of the Raman spectrum has been decomposed into two contributions, the D-band which mainly responds to grain boundary defects, and the G-band which is more sensitive to volume defects. ESR measures volume defects, grain boundaries provide only a minor contribution. In this respect, ESR and Raman G-band are similar. {111} growth sectors produce defect densities which are three to four times higher than their {100} counterparts. The thermal conductivity is shown to depend not only on the grain boundary density but also on bulk defects. Optical transparency in the visible range is strongly affected by surface roughness, however, using a new method, it has been possible to determine an absorption coefficient for the second stage which is consistent with recently published literature data.

Optical Investigation of Er:Ca2Al2SiO7 and Yb:Ca2Al2SiO7 for Laser Applications in the Near Infrared

Abstract: Single crystals of the aluminosilicate Ca 2 Al 2 SiO 7 (CAS) activated with Er 3+ or Yb 3+ were grown by the floating zone method for several dopant contents. The crystals of good optical quality, were investigated using optical spectroscopy with the aim to assess their potential interest as laser materials in the 1.5 μm (Er:CAS) and 1 μm (Yb:CAS) range, respectively. In both cases, absorption, stimulated emission, and gain cross sections were determined. Radiative and experimental lifetimes were also calculated. In the two compounds, as the two levels involved in the transitions are the ground and first excited states, radiation trapping and energy migration occur. For Er:CAS, emission around 1.55 μm (in the middle of the eye-safe range) may be obtained for a reasonable value of the population inversion ratio β (β = 0.4). The Judd-Ofelt parameters are significantly higher than those of Er:YAG and the nonradiative relaxation of the excited states located above 4 I 13/2 is fast (phonon energy cut-off around 1020 cm -1 ) ensuring efficient population of the 4 I 13/2 emitting level. For Yb:CAS, the absorption and emission cross sections are higher than for Er:CAS. Positive gain cross section is possible for very low β values, for instance β = 0.04 at 1045 nm. Taking into account the pump saturation intensity and the β value, it appears that this compound should be one of the best ytterbium activated laser crystal (if one excepts the apatite-type hosts).

Optical and electrical properties of vanadium pentoxide thin films

Abstract: Electrochromic vanadium pentoxide (V 2 O 5 ) thin films are prepared by the physical vapour deposition method using an electron beam gun. The transmission spectra of V 2 O 5 thin films are measured and studied before and after UV irradiation. It is observed that the optical transmission minimum which occurs at 320 nm does not change with irradiation. The measurement of optical band gaps of V 2 O 5 thin films before and after irradiation shows that the band gap increases due to irradiation. The electrical conductivity of the films as a function of time of irradiation is found to decrease rapidly and reaches a steady value with time when the irradiation is cut off. V 2 O 5 thin films can be used for storing optical information and for development of optical memory switching devices. Activation energies for different thicknesses of thin films are reported.

Graphitization Effects on Diamond Surfaces and the Diamond/Graphite Interface

Abstract: Graphitic layers have previously been conjectured to play an active role in diamond nucleation by Lambrecht et al. and may also be involved in a mechanism for homoepitaxial diamond growth since the surfaces of diamond may partially graphitize under high-temperature conditions typical of growth processes. Recent molecular dynamics simulations of the diamond {111} surface, briefly reviewed and discussed here, indicate a progressive graphitization with increasing temperature which is strongly facilitated by any kind of surface perturbation or roughness such as step-like adsorbates. Here we show specifically that also twin boundaries promote graphitization. The process of debonding of the surface layer which is a simple displacive motion of the outer layer is also shown to be closely related to the delamination of the tetrahedrally bonded icosahedral C100 molecule into two concentric C20 and (fullerene-like) C80 fragments. In contrast, the tetrahedrally bonded icosahedral C300 molecule which contains one more concentric shell, does not spontaneously graphitize into a bucky onion (consisting of concentric C80 and C240 fullerenes) although the latter has lower energy. Progressive graphitization at a surface towards deeper layers before the top layer is delaminated can occur under certain conditions and then may lead to graphite/diamond prism plane interfaces similar to those previously investigated in connection with nucleation. The structural stability of the prism plane interface between graphite and diamond is re-investigated here. While the initial calculations with a classical potential underestimated the interface energy, the structural stability of the models previously presented is confirmed by the present quantum mechanical simulations.

The influence of additional doping on the spectroscopic and scintillation parameters of PbWO4 crystals

Abstract: The structural peculiarities and the influence of additional doping of PbWO 4 crystals with Nb, Bi, Yb, and Mg ions have been investigated. It is shown that even a small amount of specific impurities leads to an important redistribution of the structural point defects, the radiating centres, and their properties in the crystals. The data have been used for further optimization of PbWO 4 scintillator development for electromagnetic calorimeter (ECAL) of compact muon solenoid (CMS) at CERN.

The phase diagram of domain walls in narrow magnetic strips

Abstract: A magnetic strip with rectangular cross section and infinite extension in the third, easy direction is used as a vehicle to explore numerical micromagnetic algorithms. The manifold of (meta-)stable domain wall solutions is calculated as a function of the thickness of the strip and of the relative strength of the magnetic anisotropy. With improved algorithms it became possible to calculate the complete phase diagram of domain walls including all first- and second-order phase transitions between the walls. We took care to make sure that the obtained results were independent of the chosen discretization. Anhand eines magnetischen Streifens mit rechteckigem Querschnitt und unendlicher Ausdehnung in der dritten, leichten Richtung werden verschiedene numerische Verfahren im Mikromagnetismus untersucht. Die Vielfalt der (meta-)stabilen Domanenwandkonfigurationen wird als Funktion der Dicke des Streifens und der relativen Starke der Anisotropie berechnet. Verbesserte Algorithmen erlaubten die Berechnung eines kompletten Phasendiagramms von Wanden einschlieslich aller Stabilitatsgrenzen und Ubergange zwischen diesen Wanden. Besonderer Wert wurde darauf gelegt, das die Ergebnisse unabhangig von der gewahlten Diskretisierung waren.

Structure, Composition, Dielectric, and AC Conduction Studies on Tin Selenide Films

Abstract: Al-Snse-Al thin film capacitors were fabricated onto well cleaned glass substrates by thermal evaporation under a pressure of 2 mPa. Multiple beam interferometry (MBI) was used to measure the thicknesses of the SnSe films. The composition and structure of Sn x Se 1-x films were analysed using Rutherford backscattering spectrometry and X-ray diffractogram, respectively. The capacitor samples were stabilised by aging and annealing. The variations of the dielectric constant and loss as a function of frequency at different temperatures were observed and the results are discussed. AC conduction studies reveal that the conduction mechanism is due to hopping of holes. The activation energies were also calculated.

Defect‐Induced Optical Absorption in CVD Diamond Films

Abstract: Photothermal deflection spectroscopy (PDS) was used to study defect-induced optical absorption in CVD diamond films. PDS has shown very high sensitivity, allowing to measure absorbance values down to 10 -5 level. Moreover, PDS is significantly less sensitive to elastic light scattering than optical transmission measurements. Based on PDS data, it is shown that undoped polycrystalline and homoepitaxial diamond films exhibit a very characteristic subgap absorption, which is ascribed to electronic transitions between π - π* states, appearing in the gap and due to the presence of amorphous carbon. The incorporation of N (and H and O) atoms in diamond films was studied. The optical absorption of N-doped films is compared with the optical absorption due to single substitutional N in Ib synthetic diamond. The incorporation of Li by in-situ doping with organo-metallic compounds during film growth is discussed and the observed transitions are tentatively explained. Finally. a mathematical model for the characteristic subgap absorption is described and the experimentally obtained optical absorption coefficients are deconvoluted to obtain parameters for the description of the density of gap states.

X‐ray diffraction studies on YBa2Cu3O7‐δ with Co substitution

Abstract: Ceramic samples of YBa 2 (Cu 1-y Co y ) 3 O 7-δ with y = 0.00 to 0.08 were synthesized. These samples are free of impurity phases according to X-ray diffraction. The critical temperature T c decreases with the increase of Co content. The orthorhombic-to-tetragonal transition occurs in the range of 0.02 < y < 0.04. Structural parameters, including the copper-oxygen bond lengths, vary smoothly with y within each phase but exhibit different behavior in the region from orthorhombic to tetragonal phase. These observations are consistent with a model in which superconducting behavior is controlled by charge transfer between the Cu(2)-O 2 sheets and Cu(1)-O chains, which act as reservoirs of charge.

The low-temperature instability of the decagonal phase in Al-Ni-Fe

Abstract: The Al-Ni-Fe decagonal quasicrystalline phase is thermodynamically stable between about 847 and 930 °C. The features of its transformation below 847 °C into three neighboring crystalline phases (Al 13 (Fe, Ni) 4 , Al 3 Ni 2 (Fe), and Al 3 Ni(Fe)) are reported.

Optical characterization of europium and cerium in strontium thiogallate thin films and powders

Abstract: Luminescent properties of Eu 2+ or Ce 3+ doped SrGa 2 S 4 powder samples are investigated over the temperature range 30 to 300 K. Up to 300 K the lifetime remains constant (τ 0 = 480 and 24 ns for Eu 2+ and Ce 3+ , respectively) showing that the probability of non-radiative transitions is negligible at room temperature. The decay of thin films prepared by reactive radio-frequency magnetron sputtering deviates strongly from an exponential one as a consequence of energy transfer to defects. Codoping of Ce 3+ with Na + considerably increases the luminescence efficiency.

Elastic and plastic relaxation in slightly undulated misfitting epitaxial layers ‐ A quantitative approach by three‐dimensional finite element calculations

Abstract: We quantitatively calculate with three-dimensional finite elements the elastic relaxation by surface undulations (consisting of a two-dimensional periodic array of troughs and ridges) of growing heteroepitaxial layers on a low misfitting substrate. Our calculations can generally be applied and we use Si 0.97 Ge 0.03 on Si as an example. The geometrical shapes of the undulations that are introduced into the three-dimensional finite element models are obtained from transmission electron microscopy and atomic force microscopy. Our calculations show that ridges are correlated with a relaxation of the lattice towards the bulk lattice constant (of SiGe), whereas the troughs represent regions of increased strain. The lattice distortion in the SiGe layer is transformed into distortions with opposite sign in the substrate below the interface (i.e. for SiGe/Si: tensile strains at the ridges in the layer are transformed into compressional strain in the substrate below the ridges). The calculations show that it is the free relaxation of the ridges, which makes the elastic relaxation, whereas the substrate does not contribute to elastic strain relaxation. The plastic relaxation starts in the troughs by dislocation nucleation and glide, due to the enhanced shear stresses there. We discuss the pertaining mechanisms with due consideration of the actual layer geometry and strain distribution. Especially, we quantify shear stresses in the substrate, that drive misfit dislocations from the (chemical) interface into the substrate as frequently observed.