Showing papers in "Physica Status Solidi (a) in 1995"

Low‐temperature electrical conductivity and optical absorption edge of ZnO films prepared by chemical vapour deposition

Abstract: The optical and electrical properties of c-axis-oriented ZnO films prepared by chemical vapour deposition (CVD) in oxygen atmosphere containing water vapour are investigated. The minimum dc resistivity gives 2.20 Ω cm at a film formation temperature of 550°C. The temperature dependence of electrical conductivity for the films indicates the band conduction obeying Arrhenius plots for temperatures between 170 and 297 K, which involves thermionic and thermal field emissions over grain boundaries in the films. The donor levels, Ed, produced by interstitial Zn atoms are between 0.02 and 0.20 eV. It is found that the variable-range hopping conduction occurs in the films at temperatures lower than about 170 K. The films are transparent in the visible range and have sharp ultraviolet absorption edges at 380 nm wavelength. The optical band gap energy of the films lies in a range of 3.19 to 3.23 eV. From the analysis of the Urbach tail at the absorption edge the width of the tail of localized states Ee = 0.09 to 0.10 eV is obtained. It is concluded that the localized levels are situated in a range Ee extending from near the edge of the conduction band into the band gap. Les propriete optiques et electriques des couches ZnO orientees a axe c et preparees par CVD dans l'atmosphere oxygene contenant la vapeur d'eau sont etudiees. La resistivite a c.c. minimale de 2.20 Ω cm est donnee pour une temperature de 550°C de la formation de couche. La dependence de temperature de la conductivite electrique des couches indique la conduction due a bande qui obei aux traces Arrhenius de 170 a 290 K de temperature, et qui implique les emissions thermoionique et au champ thermique a travers des frontieres de borne. Les niveaux de donneur, Ed, produit par les atomes Zn interstitiels sont entre 0.02 et 0.20 eV. Il see trouve que la conduction due a “variable-range hopping” a lieu dans les couches aux temperatures inferieures a 170 K. Les couches sont transparentes dans le domaine visible et ont les bords tranchants d'absorption ultraviolette a une longeure d'onde 380 nm. L'interstice de bande optique d'energie des couches a une portee de 3.19 a 3.23 eV. A partir de l'analyse du bout d'Urbach au bord d'absorption, on obtient une largeur du bout pour les etats localises Ee = 0.09 a 0.10 eV. On conclut que les niveaux localises se situent dans l'etendue de ceux-ci due a la bande aux impuretes a tout au-dessous du bord de la bande a conduction.

New laser properties and spectroscopy of orthorhombic crystals YAlO3:Er3+. Intensity luminescence characteristics, stimulated emission, and full set of squared reduced‐matrix elements |〈α[SL] J| |U(t)||α′[S′ L′] J′〉|2 for Er3+ Ions

Abstract: New laser data on orthorhombic YAlO 3 :Er 3+ crystals are obtained. Stimulated emission in the 4 S 3/2 → 5 I 15/2 channel and cascase lasing of the sequential intermanifold 4 S 3/2 → 4 I 11/2 → 4 I 13/2 transitions are excited at 110 K with Xe-flashlamp pumping. Intensity absorption and luminescence characteristics of Er 3+ ions in the YAlO 3 crystal are experimentally determined and quantitatively analyzed in terms of the known semiempirical method. The intensity spectroscopic parameters Ω t obtained (Ω 2 = 0.95, Ω 4 = 0.58, and Ω 6 = 0.55 (in 10 -2 cm 2 )) nicely describe band-area intensities in the absorption spectrum of the YAl0 3 :Er 3+ crystal in the spectral region below 30000 cm -1 . A full set of reduced-matrix elements for the Er 3+ ions is calculated involving all 41 J-manifolds of the 4f 11 configuration lying in energy up to 97000 cm -1 . Using these data, the earlier reported intensity parameter Ω t for the YAlO 3 :Er 3+ crystal are revised and it is shown that involving highly excited levels of Er 3+ ions into intensity spectroscopic analysis leads to an overestimation of the parameters Ω 1 because of the possible presence of some additional absorption sources in the YAlO 3 host.

Analysis of large dislocation populations in deformed metals

Abstract: The microstructural evolution is followed in pure aluminium and nickel cold-rolled over a large strain range. A number of dislocation configurations are characterized and classified and it is found that dislocation rotation boundaries are the dominant feature which subdivide the grains on a finer and finer scale as the strain is increased. These configurations of dislocation boundaries are analyzed on the basis of the LEDS hypothesis for dislocation structures and agreement is found. The strengthening effect of dislocation boundaries is discussed and equations are suggested for the relationship between flow stress and microstructural parameters

Deformation microstructure and orientation of f.c.c. crystals

Abstract: The effect of crystallographic orientation on the microstructural evolution in f.c.c. metals with medium to high stacking fault energy is analyzed. This analysis is based on a literature review of the behaviour of single crystals and polycrystals supplemented with an experimental study of cold-rolled pure aluminium. It is generally observed that all crystallites subdivide during deformation into cell blocks and cells bounded by rotation dislocation boundaries. In general the boundaries have a macroscopic orientation with respect to the geometry of the specimen. A crystallographic analysis shows that dependent on the crystallographic orientation of the crystal the subdividing boundaries may be nearly parallel to slip planes or they may have a non-crystallographic orientation. This difference is discussed on the basis of an analysis of potential slip planes identified by a Schmid factor analysis

Minimization of fatigue in ferroelectric films

Abstract: On the basis of a quantitative fatigue model, various methods to minimize fatigue in oxide ferroelectric films are discussed. The model attributes fatigue to domain pinning by space charge that is caused by oxygen vacancy entrapment at the interfaces (e.g., electrode-ferroelectric, domain boundaries, etc.). Based on this model, it is demonstrated that fatigue can be minimized by a) using conductive oxide electrodes (e.g., RuO 2 ) that prevent space charge formation at the interfaces, b) by addition of donor dopants (e.g., Nb, La in Pb(Zr x Ti 1-x )O 3 ) that reduce the oxygen vacancy concentration, and c) by utilizing ferroelectric materials (e.g., SrBi 2 (Ta x Nb 1-x ) 2 O 9 ) that have a low intrinsic defect concentration.

Transparent conducting undoped and indium-doped zinc oxide films prepared by spray pyrolysis

Abstract: Undoped and indium-doped ZnO thin films are deposited by the spray pyrolysis process onto glass substrates. The effects of starting solution composition and indium doping on the structural, electrical, and optical properties are investigated. The surface morphology of the deposited films is a strong function of the source compounds. Thus two distinct morphologies with large difference in the average grain size are obtained by using separately zinc chloride and zinc acetate. The polycrystalline nature with no second phases is observed by X-ray diffraction. Indium doping leads to a very low resistivity of about 10 -3 Ω cm. High transparency of the films, more than 85% in the visible range, is observed.

Dislocations in PbTiO3 thin films

Abstract: Using conventional and high resolution transmission electron microscopy, the dislocation structures in PbTiO3 thin films grown epitaxially on [001]-oriented MgO and SrTiO3 single crystals are studied. High resolution cross-sectional images of the films grown on MgO exhibit misfit dislocations with Burgers vectors of 1/2 〈010〉 and line directions of 〈100〉. The latter are identified by imaging the interface in two non-parallel zone axes. Plan-view studies of films grown on SrTiO3 reveal dislocations with Burgers vectors of 〈010〉 lying along 〈100〉 directions in the plane of the film. High resolution microscopy of cross-sectional samples shows dislocations in these films that have the additional half-plane on the substrate side of the film, opposite to what is required to accommodate the lattice misfit at room temperature. The mechanisms for dislocation formation and the role of the dislocations in stress relaxation with respect to the competing mechanism of 90° domain formation are discussed. Mit konventioneller und hochauflosender Transmissionselektronenmikroskopie werden Versetzungs-strukturen in PbTiO3-Filmen, die epitaktisch auf [001]-orientierten MgO- und SrTiO3-Einkristallen aufgewachsen wurden, untersucht. Hochauflosungsbilder von Querschnittsproben der Filme auf MgO zeigen Misfit-Versetzungen mit Burgers-Vektoren von 1/2 〈010〉 und Linienrichtungen von 〈100〉. Die letzteren werden durch eine Abbildung der Grenzflache in zwei nicht parallelen Zonenachsen bestimmt. Untersuchungen von Aufsichtsproben der Filme auf SrTiO3 zeigen Versetzungen mit Burgers-Vektoren von 〈010〉 entlang der 〈100〉 Richtungen der Filmebene. Hochauflosende Mikroskopie an Querschnitts-proben zeigt Versetzungen in diesen Filmen, bei denen sich die eingeschobene Halbebene auf der Substratseite des Films befindet, genau umgekehrt, wie es zum Ausgleich der Gitterfehlpassung bei Raumtemperatur notwendig ware. Die Mechanismen fur die Entstehung der Versetzungen und ihre Rolle beim Abbau von Spannungen unter Berucksichtigung des konkurrierenden Mechanismus, der Bildung von 90° Domanen, werden diskutiert.

Tracer diffusion of titanium in α-iron

Abstract: The tracer diffusion coefficient of 44 Ti in α-Fe is determined by the serial sectioning method in the temperature range of 948 to 1174 K. In paramagnetic α-Fe the diffusion coefficient of Ti shows a linear Arrhenius relationship from the α-γ transformation temperature to about 30 K above the Curie temperature T c = 1043 K. Below 1076 K, the temperature dependence of the diffusion coefficient deviates from the linear Arrhenius relationship due to the magnetic spin ordering. The temperature dependence of the diffusion coefficient D in the whole temperature range of α-Fe can be expressed by (... formule...) where M is the magnetic long-range order parameter. The frequency factor D 0 = (0.21 −0.12 +0.32 ) m 2 s −1 and the activation enthalpy Q = (293.2 ± 8.8) kJ mol −1 describe Ti diffusion in paramagnetic α-Fe and correspond to a normal vacancy diffusion mechanism. Ti diffusion is faster by a factor of about 5 when compared to Fe self-diffusion. This can be understood by a vacancy-solute binding due to the larger atomic radius of Ti. A comparison with the diffusion behaviour of other transition elements in α-Fe confirms the correlation between diffusion velocity and solute atomic radius

Diffusion of 63Ni and 114mIn in the γ'-phase Ni3Al

Abstract: The diffusion of 63Ni and 114mIn in the intermetallic L12 phase Ni3Al is measured in the temperature range from 900 to 1200 °C and for compositions between 73.5 and 77 at% Ni in steps of 0.5 at%. The In isotope serves as a substitute for 26Al. The usual serial sectioning method is applied using a precision parallel grinder. There is only a weak concentration dependence of the diffusion coefficient at temperatures > 950 °C but it gets stronger with decreasing temperature. There is a minimum of the diffusion coefficient at 76 and not at 75 at% Ni as may be expected. The diffusion of 63Ni in Ni3Al and in pure Ni is comparable. The same is true for the diffusion of 114mIn in Ni3Al. Very likely the diffusion of 63Ni is by a normal vacancy mechanism and the diffusion of 114mIn by In antisite atoms in the Ni sublattice. The D-values are for In (and probably for Al as well) considerably higher than for Ni. This could be due to a higher entropy term. As a consequence of the considerably larger D-values of In an intersection of the two linear Arrhenius plots for Ni and Al is observed at about 950 °C for all concentrations except Ni76Al24. This fits well to interdiffusion investigations where at higher temperatures Al is found to be the faster component, and Ni at lower temperatures. Die Diffusion von 63Ni and 114mIn in der intermetallischen L12-Phase Ni3Al wird im Temperaturbereich von 900 bis 1200 °C und fur Zusammensetzungen zwischen 73,5 und 77,5 at% Ni in Intervallen von 0,5 at% gemessen. Das 114mIn-Isotop dient als Ersatz fur Al. Die Messungen werden mit der ublichen Schichtenteilungs-Methode mit Hilfe einer Prazisionsschleifmaschine durchgefuhrt. Die Konzentrationsabhangigkeit der Diffusionskoeffizienten ist bei Temperaturen > 950 °C gering und wird mit sinkender Temperatur groser. Es gibt ein Minimum des DK bei 76 und nicht bei 75 at% Ni, wie man vermuten konnte. Die Diffusion von 63Ni in Ni3Al und in reinem Ni ist vergleichbar schnell. Dasselbe gilt fur die Diffusion von 114mIn in Ni3Al. Mit hoher Wahrscheinlichkeit erfolgt die Diffusion von 63Ni uber einen normalen Leerstellenmechanismus und die Diffusion von 114mIn uber In-Antistrukturatome im Ni-Teilgitter. Die D-Werte sind fur In (und vermutlich auch fur Al) erheblich hoher als fur Ni. Dies durfte ein Hinweis auf einen hoheren Entropieterm sein. Als Folge des erheblich groseren D-Terms von In ergibt sich fur alle Zusammensetzungen mit Ausnahme von Ni76Al24 eine Uberschneidung der beiden Arrheniusgeraden fur Ni und In bei etwa 950 °C. Dies entspricht Interdiffusions-Untersuchungen, die bei hoheren Temperaturen eine schnellere Diffusion des Al, bei tieferen dagegen des Ni zeigen.

Electrical and optical characterization of electron beam evaporated In2Se3 thin films

Abstract: Electrical and optical measurements are carried out on In 2 Se 3 polycrystalline thin films obtained by electron beam evaporation and thermally annealed at different temperatures. The temperature dependence of Hall mobility and de conductivity is tentatively explained in terms of Petritz's model where the scattering is due to the grain boundaries of the polycrystallites. The effect of grain sizes on the mobility and the barrier height is also reported. The optical results indicate that the absorption mechanism is due to an indirect transition. The energy gap in the as-deposited samples is about 1.40 eV and increases with thermal annealing

Reflectance anisotropy oscillations during MOCVD and MBE growth of GaAs (001)

Abstract: GaAs (001) surfaces in both metalorganic vapour phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are studied under As stabilisation as well as during growth by reflectance anisotropy spectroscopy (RAS). During MBE growth reflection high energy electron diffraction (RHEED) measurements are performed simultaneously. As a consequence of the oscillating density of steps during island growth, time resolved RAS measurements show oscillations with monolayer periodicity in both epitaxial systems. In order to separate morphology related and surface reconstruction domain related contributions to RAS oscillations an anisotropic effective medium model is applied. In both MBE and MOVPE domains of reduced As coverage close to the island boundaries are found to be responsible for RAS oscillations. In MBE this typically results in oscillations between spectra belonging to (2 x 4) reconstructions of different mean As dimer density. In MOVPE the surface oscillates between states with an increased and a decreased concentration of Ga dimers while c(4 x 4)-like As dimers are still present.

Technological high strain deformation of ‘wavy glide’ metals and LEDS

Abstract: The evolution of dislocation structures in technological high strain deformation of «wavy glide» metals is studied by the examples of pure aluminium, nickel, copper and hot deformed stainless steel, and compared with the earlier observations by Langford and Cohen on drawn iron wire and Wert et al. on rolled aluminium alloys including precipitates. The various dislocation cell structures in all of these (in contrast to «planar glide» metals which deform via planar arrays and/or Taylor lattices) develop in agreement with the LEDS theory to the effect of practically proving its basic tenet, the LEDS principle. Namely, dislocation rotation boundaries substantially obeying Frank's formula, i.e. LEDS's, are the dominant feature throughout. The stronger, longer and sharper of these delineate volume elements, dubbed «cell blocks» (CBs). The number of active slip systems in the CBs always falls short of the five required by the Taylor criterion, and their selection and/or relative intensitv of operation differs from onc to the next. The CBs in turn are subdivided into ordinary dislocation cells. In the early stages of rolling, believed to represent workhardening stage III, the structure is of large plate-like arrangements of roughly equiaxed cell blocks subdivided into equi'axed dislocation cells. These plates are inclined about ± (40° ± 10°) to the rolling plane. With straining, the rotation angles increase and the scale of the structure shrinks but such that the CB size shrinks faster than the cells, eventually down to n bamboo structure. This evolution proves the dislocations forming the structure to be quite mobile, in agreement with the LEDS theory. Next, in what is believed to be the onset of stage IV, intersecting slip distorts the roughly parallel CB boundaries into S-shapes (S-bands) and thereby rotates them towards parallelity with the rolling plane into finely spaced lamellar walls subdivided by n bamboo structure of cell walls. Finally, roughly equiaxed subgrains form, no longer subdivided by dislocation cells, considered to mark stage V. Rotation angle/axis determinations reveal overlapping rotation angle distributions of CB and cell walls which extend to much larger than expected angles, and that the structure is formed of dislocations from the active slip systems. Throughout, the structure evolves in accord with the LEDS hypothesis, in that the energy per unit length of dislocation line decreases successively. Although the structures could not have been predicted, they can serve for future modelling

Magnetization and magnetostriction in terbium-dysprosium-iron alloys

Abstract: Some results of magnetostriction and magnetization measurements are presented on a small representative sample of four specimens from an original study of over one hundred specimens of the Tb-Dy-Fe alloy which we have measured in recent years. The results of these investigations show a progressive improvement in strain amplitude, Δλ, and strain derivative, dλ/dH, of the material. The highest strain amplitude observed to date is Δγ = 2.2 × 10 −3 and the highest strain derivative is (dλ/dH) max = 0.312 × 10 −6 A −1 m. These improvements result mostly from the reduction of the volume fraction of second phase material and an increase in the fractional occupancy of domains oriented initially at right angles to the unique axis of the cylindrical specimens. The initial domain alignment is a result of thermomagnetic treatment of the finished specimens in which the materials are subjected to a magnetic field normal to the axis of the specimens

Retention and imprint properties of ferroelectric thin films

Abstract: The retention and imprint properties of ferroelectric Pb(Zr 0.53 Ti 0.47 )O 3 (PZT) and SrBl 2 Ta 2 O 9 (SBT) thin films are studied. A simple characterization technique which consists of four measurement pulses is used to differentiate the retention and imprint effects. The high concentration of oxygen vacancies in PZT capacitors results in longer screening lengths and higher depolarization fields that worsen retention properties. A c-axis preferred oriented SBT thin film deposited on Pt(100)/MgO(100) substrate contains a higher concentration of 180° domains and results in better retention properties due to the minimum ferroelastic effect. Ferroelectric SBT thin films with symmetric P-E hysteresis loops are chosen to study the imprint effect. The imprint process changes the symmetry of P-E hysteresis loop, which indicates the imprint process builds up an internal field within the ferroelectric thin films. Electrical charges trapped at electrode-ferroelectric interfaces during the imprint stress are responsible for internal field build-up and screen the spontaneous polarization internally.

Exchange interaction between electron subsystems in iron-based F.C.C. alloys doped by nitrogen or carbon

Abstract: The effect of nitrogen and carbon on the electronic and magnetic properties of the iron-base Fe56Cr18Ni16Mn10 3d-alloy with f.c.c. lattice (austenite) is studied by means of the magnetic resonance. It is shown that the electron state density on the Fermi surface in the nitrogen austenite is one order of magnitude higher than that in the carbon one. Based on measurements of the temperature dependence of the g-factor the existence of superparamagnetic clusters is stated, the latter being obviously clusters of substitutional solutes. Signs and values of the exchange interaction between conduction electrons and other magnetic subsystems are obtained from the analysis of the temperature dependence of the paramagnetic susceptibilities. The exchange interaction between conduction electrons and isolated localised spins has antiferromagnetic character, while conduction electron—cluster interaction is ferromagnetic. The size of the clusters is markedly higher in the carbon austenite. This difference in structural homogeneity can be used for an explanation of the higher thermodynamic stability of the nitrogen austenite as compared to the carbon one. Der Einflus von Stickstoff und Kohlenstoff auf die magnetischen Eigenschaften der Fe56Cr18Ni16Mn10 Eisenlegierung mit k.f.z. Gitter (Austenit) wird mit Hilfe der magnetischen Resonanz untersucht. Es wird gezeigt, das die Dichte der Elektronenzustande auf der Fermi-Oberflache im Stickstoffaustenit um eine Grosenordnung hoher als die im Kohlenstoffaustenit ist. Aus den Messungen der Temperatur-abhangigkeit des g-Faktors wird auf die Existenz von superparamagnetischen Clustern, d. h. Clustern von gelosten, substitutionellen Atomen, geschlossen. Vorzeichen und Werte der Austauschwechselwirkung zwischen freien Elektronen und anderen magnetischen Systemen werden aus der Analyse der Temperaturabhangigkeit der paramagnetischen Suszeptibilitat erhalten. Die Austauschwechselwirkung zwischen freien Elektronen und isolierten, lokalisierten Spins hat antiferromagnetischen Charakter, wahrend die Wechselwirkung zwischen freien Elektronen und Clustern ferromagnetisch ist. Die Grose der Cluster ist merklich hoher im Kohlenstoffaustenit. Dieser Unterschied in der strukturellen Homogenitat kann zur Erklarung der hoheren thermodynamischen Stabilitat des Stickstoffaustenits im Vergleich zum Kohlenstoffaustenit herangezogen werden.

Self-organized dislocation structures (SODS) in fatigued metals

Abstract: An attempt to introduce a thermodynamic classification of dislocation structures produced by cyclic deformation is made. A fundamental division is proposed for all types of dislocation structures into two classes: 1. those in equilibrium and 2. strongly non-equilibrium self-organized dislocation structures («SODS»). It is shown that the concept of low energy dislocation structures («LEDS») introduced by Kuhlmann-Wilsdorf can be applied to dislocation structures of the first type, while the concept of SODS is useful in situations connected with the onset of fatigue instabilities (e.g., the formation of the ladder structure of persistent slip bands and dislocation rearrangements accompanying strain bursts during cyclic deformation, e.g., ramp loading). It is stressed that the issue of dislocation patterning in fatigued metals should be considered a part of the general problem of nonlinear dynamical systems functioning in far-from-equilibrium conditions. In this sense the classification proposed has common features between «physico-chemical» behavior of dipolar and multipolar dislocation populations in fatigued metallic alloys, on the one hand, and such essentially dipolar systems as ferrofluids, magnetic bubble materials, and phospholipid monolayer domains, on the other hand

Non-stoichiometry of the crystal lattice of antimony telluride

Abstract: A single crystal of antimony telluride, grown from elements of semiconductor purity by means of the Bridgman method, is characterised by the departure from stoichiometry as determined from energy dispersive X-ray spectroscopy, by the concentration of free charge carriers p as obtained from the interpretation of the reflectivity spectra in the plasma resonance frequency range, by its density d, and by lattice parameters a and c. The investigated crystal of stoichiometry Sb 2 Te 2.948 has the following values: p = 6.714 x 10 19 cm -3 , d = (6.50 ± 0.01) g cm -3 , a = 0.42643(5) nm, c = 3.0427(4) nm. The departure from stoichiometry is explained using a model taking into account the existence of antisite Sb' Te defects as well as tellurium and antimony vacancies in the crystal lattice: this model in connection with the experimental data allows us to calculate the concentration of Sb' Te defects and to determine their formation energy E AS = (0.32 ± 0.01) eV.

Microcracks observed in epitaxial thin films of YBa2 Cu3O7–δ and GdBa2Cu3O7–δ†

Abstract: Microcracking of epitaxially c-oriented YBa2Cu3O7–δ and GdBa2Cu3O7–δ thin films is observed to preferentially occur on buffered, R-cut sapphire substrates of 3 in diameter in comparison to smaller substrates and other substrate materials such as MgO, SrTiO3, and LaAlO3. The area density and the crystallographic appearance of the crack pattern can vary considerably across a given surface, from sample to sample, and in dependence on the temperature treatment. These dependencies indicate the crack behaviour to be seriously affected by the microstructure. The latter is studied by optical, electron, and atomic force microscopy. Cross-sectioning TEM reveals cracks to locally separate the HTSC film from the buffer layer. Optical microscopy using transmitted polarized light is found to be a straightforward, sensitive survey method to observe the microcracking behaviour. Mikrorisse von epitaktisch c-orientierten YBa2Cu3O7–δ- und GdBa2Cu3O7–δ-Dunnschichten zeigen sich bevorzugt auf mit Pufferschicht versehenen, R-orientierten Saphirsubstraten von 3 in Durchmesser im Vergleich zu kleineren Substraten und anderem Substratmaterial wie MgO, SrTiO3 und LaAlO3. Die Flachendichte und das kristallographische Bild der Risse kann betrachtlich variieren, und zwar langs einer gegebenen Oberflache, von Probe zu Probe sowie in Abhangigkeit von der Temperaturbehandlung. Diese Abhangigkeiten deuten auf einen starken Einflus der Mikrostruktur. Letztere wird mittels optischer, Elektronen- und Rastersondenmikroskopie untersucht. Die TEM-Querschnittsabbildung zeigt Risse, welche die HTSL-Schicht lokal von der Pufferschicht trennen. Die Lichtmikroskopie im polarisierten Durchlicht erweist sich als eine rationelle, empfindliche Ubersichtsmethode zur Beobachtung des Risverhaltens.

Mosaic dislocation structures in aluminium crystals deformed in multiple slip at 0.5 to 0.8TM

Abstract: Deformation microstructures are characterised in high purity aluminium crystals deformed in multiple slip by channel die compression up to strains of 1.5, temperatures of 473 to 773 K, and strain rates of 10 −1 to 10 −3 s −1 . Four single crystal orientations are examined: three orientations corresponding to the stable rolling texture components of polycrystalline f.c.c. metals, C {112} , S {421} , and B {110} and one recrystallisation component {001} . The deformation microstructures are investigated by different techniques over a wide range of scales and the local orientations have been measured by EBSD (electron back scattered diffraction) and by CBED (convergent beam electron diffraction). The deformation microstructures are subdivided by dislocation boundaries which bound cell blocks oriented at ± 30° to ± 55° with respect to the rolling direction. The dislocation boundaries are regularly spaced in two families creating a regular structure of cell blocks whose size is very strain rate and temperature dependent. Such cell blocks are also characterised by lattice rotations of alternating sign (often about the transverse axis). For all crystals, the misorientations between adjacent blocks are of the order of 5° to 10° at e = 0.2. At strains of the order of unity, certain orientations, e.g. C and S, develop local regions of very large misorientations whereas in the B orientation the misorientations do not exceed 10°. The microstructural evolution is analysed within the framework of the LEDS hypothesis and compared to that observed at room temperature. The stability of the mosaic dislocation wall pattern is confirmed by post-deformation anneals

Optical absorption studies in molybdenum trioxide thin films

Abstract: Thin films of molybdenum trioxide are prepared by the electron beam evaporation technique onto glass substrates maintained at temperatures in the range 303 to 523 K. The effect of substrate temperature and oxygen partial pressure on the structure and optical properties of MoO 3 films is investigated. The as-deposited films show α-β mixed phase. Both the films formed at 523 K in an oxygen partial pressure of 26.6 × 10 −3 Pa and the films formed at 523 K and heat treated in air exhibit the orthorhombic α-phase with optical band gap 3.25 and 3.2 eV, respectively

TEM/HREM visualization of nm‐scale coherent InAs islands (quantum dots) in a GaAs matrix

Abstract: The electron microscope determination (high resolution HREM and conventional diffraction contrast TEM) of the structure (geometry, size, shape, etc.) of nm-scale objects is of great interest for the creation of novel semiconducting materials of reduced dimensions as, e.g., quantum dots (QDs) and quantum wires. HREM contrast simulations based on molecular dynamics structure modelling are applied to check the visualization of coherently strained nm-scale InAs islands embedded in a GaAs matrix. Being of pyramidal shape, InAs islands always seem to be truncated owing to lower In content on top of the pyramid and to the high level of strains around the island. Optimum imaging conditions are analysed to reveal shape and size of such objects. Die Untersuchung der Struktur (Geometric, Grose, Form usw.) von Objekten im Nanometerbereich mittels Elektronenmikroskopie (Hochauflosungsabbildung HREM und konventioneller Beugungskontrast TEM) hat eine besondere Bedeutung fur die Konstruktion neuer Halbleitermaterialien mit reduzierten Dimensionen, wie z. B. Quantenpunkte (QDs) und Quantendrahte. HREM Kontrastsimulationen auf der Grundlage molekulardynamisch relaxierter Strukturmodelle werden diskutiert, um die Abbildungen koharent gedehnter InAs Inseln von Nanometergrose, die in einer GaAs Matrix eingeschlossen sind, zu analysieren. Die InAs Inseln zeigen stets den Kontrast von Pyramiden, deren Spitze gekappt scheint, was eindeutig auf den geringeren zu durchstrahlenden In-Anteil in der Pyramidenspitze und das starke Verzerrungsfeld um die QDs herum zuruckzufuhren ist. Es werden optimale Abbildungsbedingungen vorgeschlagen, um dennoch Grose und Form der QDs analysieren zu konnen.

High-resolution transmission electron microscopy of the planar defect structure of hexagonal boron nitride

Abstract: Basal plane stacking disorder, delamination cracks, misorientation bands and low angle (0001) twist boundaries are observed by high resolution transmission electron microscopy in small, highly defective, boron nitride inclusions introduced unintentionally in silicon nitridesilicon carbide composites during the fabrication process. The delamination cracks are produced as a consequence of the magnitude of the thermal stresses introduced during cooling from the consolidation temperature and the layered nature of the hexagonal boron nitride crystal structure. Detailed consideration of the misorientation bands shows that, while the majority can be safely described as kink bands, there are a number of examples where the geometry is strikingly similar to the geometry of deformation twinning in hexagonal boron nitride. Stapelfehlordnung parallel zur Basisflache, Delaminationsrisse, Fehlordnungsbander und Kleinwinkelkorngrenzen ((0001) twist-Grenzen) werden mittels HRTEM in kleinen, defektreichen Bornitrid-Einschlussen beobachtet, die unbeabsichtigt wahrend des Herstellungsprozesses in Siliziumnitrid/Siliziumkarbid-Verbunden gebildet werden. Die Delaminationsrisse entstehen infolge der thermischen Spannungen, die wahrend der Abkuhlung von der Verfestigungstemperatur entstehen und von der Schichtstruktur der hexagonalen Bornitridkristalle herruhren. Detaillierte Untersuchungen der Fehlordnungsbander zeigen, das die Mehrzahl mit Sicherheit als Kinkbander beschrieben werden kann. Einige jedoch weisen eine Geometrie auf, die ahnlich der Geometrie von Deformationszwillingen in Bornitrid ist.

Investigation of the relationship between reflectance difference spectroscopy and surface structure using grazing incidence X-ray scattering

Abstract: In this work, in-situ grazing incidence X-ray scattering is used to correlate surface structures observed during organometallic vapor phase growth of GaAs with in-situ optical measurements using reflectance difference spectroscopy (RDS). Our observations of several reconstructions confirm that RDS signals vary among the different surfaces present under vapor phase epitaxy conditions, as also occurs under ultra-high vacuum growth conditions. In addition, the simultaneous observation of intensity oscillations in the X-ray scattering signal and the reflectance difference signal indicates that both of these techniques can be used to monitor layer-by-layer growth processes under some conditions.

Analysis of synchronous phase, pump power, and pump wavelength dependent complex PR spectra from GaAs MBE structures

Abstract: A quantitative full-fitting analysis of photoreflectance (PR) spectra in the region of the fundamental energy gap E 0 from MBE n-GaAs layer structures (n = 5 x 10 15 to 10 17 cm -3 ) on n-GaAs is performed. They exhibit a complex spectral behaviour consisting of Franz-Keldysh oscillations, and excitonic structure near E 0 , and for high-energetic interference oscillations (LEIO). Spectra are measured using pump beam wavelengths of 488 and 633 nm with modulation frequencies between 167 Hz and 1.42 kHz and pump power densities from 2 x 10 -3 to 2 W/cm 2 . Amplitudes and lineshapes of the three components are shown to vary strongly with changes in the excitation conditions and the phase of the synchronously detected signal which allows to investigate the effect of partial modulation and temporal behaviour of the field modulation mechanisms.

Anomalous slip in high-purity niobium and tantalum single crystals

Abstract: High-purity niobium and tantalum single crystals having orientations in the central part of the stereographic triangle and a residual resistance ratio RRR of better than 5500:1 are deformed in tension, compression, and by cyclic deformation at temperatures below ambient. The deformation behaviour is studied by observation of the slip-line morphology, Laue X-ray backreflection, X-ray topography, and transmission electon microscopy (TEM). Anomalous (011) slip is observed at temperatures below about 200 K for the niobium crystals and at liquid nitrogen temperatures for the tantalum crystals, respectively. The influence of the specimen purity on the appearance of the anomalous slip is investigated by etching experiments: for example, etching in an acid solution subsequently suppresses anomalous slip in the tantalum specimens owing to the influence of hydrogen interstitials. Prestraining of the specimens has different effects on the appearance of the anomalous slip, depending on the temperature and the mode of the predeformation: anomalous slip is generally suppressed by prestraining unless the specimens are predeformed into stage I of the three-stage work-hardening curve at the appropriate temperatures. The TEM investigations reveal that the dislocation arrangement consists of regular networks of screw dislocations with primary and conjugate Burgers vectors, lying in the anomalous slip plane. The X-ray topography investigations demonstrate that twist walls of alternating signs are built up during anomalous slip. The separation of two adjacent twist walls of alternating signs corresponds to the separation of the anomalous slip lines on the crystal surfaces. From the angle of twist the density of the excess dislocations in the twist walls is estimated to be about 3% of the total dislocation density. Hochreine, mittelorientierte Niob- und Tantaleinkristalle mit einem Restwiderstandsverhaltnis von besser als 5500:1 werden im Zugversuch, Druckversuch und durch Wechselverformung bei Temperaturen unterhalb von Raumtemperatur verformt. Das Verformungsverhalten wird mit Hilfe von Gleitlinienbeobachtungen, Laue-Ruckstrahlaufnahmen, Berg-Barrett Rontgentopographie und Durch-strahlungselektronenmikroskopie untersucht. Bei den Niobeinkristallen wird bei Verformungstemperaturen kleiner als etwa 200 K die sogenannte anomale Gleitung auf (011) Ebenen gefunden. Dagegen tritt sie bei den Tantaleinkristallen erst nach Verformung bei Stickstofftemperatur auf. Der Einflus der Probenreinheit auf das Auftreten von anomaler Gleitung wird mit Hilfe von Atzexperimenten untersucht. Infolge des Einflusses von Wasserstoff-Zwischengitteratomen wird die anomale Gleitung bei Tantal nach Atzen in einer Saurelosung unterdruckt. Vorverformen von Proben wirkt sich auf verschiedene Weise auf das Auftreten der anomalen Gleitung aus, abhangig von der Temperatur und der Art des Verformens. Im allgemeinen wird die anomale Gleitung bei vorverformten Kristallen unterdruckt, sofern die Proben nicht bei der „richtigen” Temperatur bis in Stufe I der Verfestigungskurve vorverformt wurden. Die Untersuchungen mittels Durchstrahlungselek tronenmik roskopie ergeben, das die Versetzungsanordnung aus regelmasigen Netzwerken von Schraubenversetzungen mit primaren und konjugierten Burgers-Vektoren besteht, die in anomalen (011) Ebenen liegen. Die Berg-Barrett-Rontgentopographie-Untersuchungen zeigen, das wahrend der anomalen Gleitung Drillwande mit alternierenden Vorzeichen aufgebaut werden. Dabei entspricht der Abstand zweier benachbarter Drillwande dem Abstand der anomalen Gleitlinien auf der Kristalloberflache. Aus der Grose des Drillwinkels wird die Dichte der uberschussigen Versetzungen in den Drillwanden auf etwa 3% der Gesamtversetzungsdichte geschatzt.

Quantitative strain mapping using high‐resolution electron microscopy

Abstract: Lattice images as obtained by high-resolution electron microscopy are used to quantify strain and strain profiles in semiconductor layer structures Inherent parts of the analysis are the definition of spatial resolution and detection limit on the basis of a quantification of noise As an application the tetragonal strain in pseudomorphic InxAl1–xAs heterostructures is analyzed The detection limit is 28% strain at a spatial resolution of 04 × 04 nm2 using a 2σ-criterion If resolution along the heterostructure is reduced by averaging, even 02% strain can be detected at a spatial resolution of 04 × 35 nm2 Mit hochauflosender Transmissionselektronenmikroskopie erzeugte Gitterabbildungen werden benutzt, um in Halbleiterschichtstrukturen Gitterverzerrungen und Verzerrungsprofile zu quantifieren Zu dem Verfahren gehort die Definition der Ortsauflosung und der Nachweisgrenze basierend auf der Quantifizierung des Rauschens Als Anwendung wird die tetragonale Verzerrung in einer pseudomorphen InxAl1–xAs-Heterostruktur ausgewertet Hier ist die Nachweisgrenze 2,8% Verzerrung bei einer Ortsauflosung von 0,4 × 0,4 nm2, wenn das 2σ-Kriterium benutzt wird Bei einer Reduktion der Ortsauflosung durch Mittelung langs der Heterostruktur ist es sogar moglich, eine Verzerrung von 0,2% bei einer Ortsauflosung von 0,4 × 35 nm2 nachzuweisen

Lorentz microscopy of giant magnetoresistive AuCo alloys

Abstract: The magnetic domain structure of melt-spun giant magnetoresistant Au71.6Co28.4 samples in as-prepared and annealed states is determined by means of Lorentz microscopy using the JEOL-1000 atomic resolution microscope (ARM) in a modified diffraction mode. The interpretation of the domain structure is done by comparing magnetic contrasts observed in Fresnel mode with simulated contrasts using a geometrical algorithm based on the Lorentz equation. Cobalt precipitates 50 to 100 nm in diameter are found in a single domain state with a curling-like magnetization, that reduces their magnetic stray fields. Larger precipitates (≈ 200 nm) are multidomain particles. Die magnetische Domanenstruktur von schmelzgesponnenen und ausgelagerten Au71.6Co28.4 Proben mit gigantischem Magnetowiderstand wird mit Hilfe der Lorentzmikroskopie bestimmt. Die dazu notwendigen Experimente werden in dem JEOL-1000 Mikroskop (ARM) mit atomarem Auflosungsvermogen durchgefuhrt. Die Interpretation der Domanenstruktur wird durch Vergleich mit Kontrastsimulationen vorgenommen. Der dazu verwendete geometrische Algorithmus basiert auf der Anwendung der Lorentzkraftgleichung. Es kann nachgewiesen werden, das Kobaltausscheidungen mit Durchmessern von 50 bis 100 nm im Einbereichsteilchenzustand vorliegen, der durch Curling der Magnetisierung und reduzierte Streufelder ausgezeichnet ist. Grosere Teilchen (≈ 200 nm) sind Multidomanenteilchen.

Anisotropy of the refractive index and absorption coefficient in the layer plane of gallium telluride single crystals

Abstract: Refractive index and absorption coefficient of GaTe for light polarized in the optical axis directions in the layer plane are determined from transmission measurements. The refractive index is determined in the wavelength range from 0.7 to 25 μm and the absorption coefficient in the range of energies from 1.6 to 2 eV at room temperature and at 30 K. The optical constants are found to be anisotropic in the layer plane. The refractive index dispersion is interpreted through a Phillips-Van Vechten model and the Penn gaps for each direction are found to be E pg (⊥b) = 3.37 eV and E pg (∥b) = 3.58 eV. In the long-wavelength region a polar phonon contribution is also taken into account. The absorption edge is found to be very slightly anisotropic, which is shown to be coherent with the Se p z -like character of the valence band.