No description available.

Gallium nitride and its alloys are direct band gap semiconductors with a wide variety of applications. Of particular importance are light emitting diodes and laser diodes. Due to the lack of suitable lattice-matched substrates, epitaxial layers contain a high density of defects such as dislocations. To reduce their number and to design a device with desired specifications, multilayered systems with varying composition (and thus material properties) are grown. Theoretical modelling is a useful tool for gaining understanding of various phenomena and materials properties. The scope of the present work is wide. It ranges from a continuum theory of dislocations treated within the linear elasticity theory, connects the continuum and atomistic level modelling for the case of the critical thickness of thin epitaxial layers, and covers some issues of simulating the electronic structure of III-nitride alloys by means of the first principle methods. The first part of this work discusses several topics involving dislocation theory. The objectives were: (i) to apply general elasticity approaches known from the literature to the specific case of wurtzite materials, (ii) to extend and summarise theoretical studies of the critical thickness in heteroepitaxy. Subsequently, (iii) to develop an improved geometrical model for threading dislocation density reduction during the growth of thick GaN films. The second part of this thesis employs first principles techniques (iv) to investigate the electronic structure of binary compounds (GaN, AlN, InN) and correlate these with experimentally available N K-edge electron energy loss near edge structure (ELNES) data, (v) to apply the special quasi-random structures method to ternary III-nitride wurtzite alloys aiming to develop a methodology for modelling wurtzite alloys and to get quantitative agreement with experimental N K-edge ELNES structures, and (vi) to theoretically study strain effects on ELNES spectra.

This thesis employs experimental and theoretical methods to characterise carbon nitride solids and proposes a generalstructural model for amorphous carbon nitride (a-C:N). It finds that a-C:N deposited by several methods is essentially identical, with similar bonding environments for carbon and nitrogen atoms. Using evidence from several techniques, the saturation of nitrogen in an sp2 carbon matrix is discussed. The experimental studies on a range of carbon nitride solids show no evidence for a crystalline form of carbon nitride. In addition to the experimental characterisation of a-C:N, ab initio molecular dynamics were used to investigate bonding and structure in carbon nitride. These simulations show that the most common form of nitrogen bonding was three-fold sites with a lone pair of electrons. Two-fold nitrogen sites were also found in agreement with experimental findings. An increase of nitrogen in a-C:N decreases the sp3-carbon fraction, but this is not localised on the nitrogen and the effect is most severe at high densities. A simulation of a low density/high nitrogen content network shows that the nitrogen saturation seen experimentally may be due to the formation of N2 dimers and C-N molecules which are easily driven out of the structure. The ab initio simulations also explore the nature of charged nitrogen and carbon sites in a-C:N. An analysis based on Wannier Function centres provided further information about the bonding and allowed for a detailed classification of these sites. The removal of electrons from the networks caused structural changes that could explain the two-state conductivity in ta-C:N memory devices. Finally, a theoretical study of the electron energy-loss near-edge structure (ELNES) calculated using multiple scattering theory is presented. The calculated ELNES of diamond, graphite and boron, silicon and carbon nitride structures compare well to experiment and supports the experimental finding that no crystalline carbon nitride had (or has) been produced. These ELNES calculations will however, provide a means of identifying crystalline beta-C3N4 should it be synthesised.

<p>Two new methods for calculating the double differential scattering cross-section (DDSCS) in electron energy loss spectroscopy (EELS) have been developed, allowing for simulations of sample geometries which have been unavailable to earlier methods of calculation. The new methods concerns the calculations of the <em>thickness function</em> of the DDSCS. Earlier programs have used an analytic approximation of a sum over the lattice vectors of the sample that is valid for samples with parallel entrance and exit surfaces.The first of the new methods carries out the sum explicitly, first identifying the unit cells illuminated by the electron beam, which are the ones needed to be summed over. The second uses an approach with Fourier transforms, yielding a final expression containing the <em>shape amplitude</em>, the Fourier transform of the <em>shape function</em> defining the shape of the electron beam inside the sample. Approximating the shape with a polyhedron, one can quickly calculate the shape amplitude as sums over it’s faces and edges. The first method gives fast calculations for small samples or beams, when the number of illuminated unit cells is small. The second is more efficient in the case of large beams or samples, as the number of faces and edges of the polyhedron used in the calculation of the shape amplitude does not need to be increased much for large beams. A simulation of the DDSCS for magnetite has been performed, yielding diffraction patterns for the L₃ edge of the three Fe atoms in its basis.</p>

Two new methods for calculating the double differential scattering cross-section (DDSCS) in electron energy loss spectroscopy (EELS) have been developed, allowing for simulations of sample geometries which have been unavailable to earlier methods of calculation. The new methods concerns the calculations of the thickness function of the DDSCS. Earlier programs have used an analytic approximation of a sum over the lattice vectors of the sample that is valid for samples with parallel entrance and exit surfaces.The first of the new methods carries out the sum explicitly, first identifying the unit cells illuminated by the electron beam, which are the ones needed to be summed over. The second uses an approach with Fourier transforms, yielding a final expression containing the shape amplitude, the Fourier transform of the shape function defining the shape of the electron beam inside the sample. Approximating the shape with a polyhedron, one can quickly calculate the shape amplitude as sums over it’s faces and edges. The first method gives fast calculations for small samples or beams, when the number of illuminated unit cells is small. The second is more efficient in the case of large beams or samples, as the number of faces and edges of the polyhedron used in the calculation of the shape amplitude does not need to be increased much for large beams. A simulation of the DDSCS for magnetite has been performed, yielding diffraction patterns for the L3 edge of the three Fe atoms in its basis.

Es werden die physikalischen Grundlagen zur inelastischen Streuung mittelschneller Elektronen im Hinblick auf die Untersuchung des Energieverlustes beschrieben. Die instrumentellen Grundlagen der Energieverlust-Spektroskopie unter besonderer Berücksichtigung des Einsatzes in Transmissionselektronenmikroskopen werden erläutert. Der Einfluß des erfaßsten Streuwinkelbereichs wird diskutiert. Es werden Möglichkeiten zur Auswertung von Energieverlustmessungen im Bereich der Interbandübergangs- und Plasmonanregungen sowie im Bereich der Anregung von tieferliegenden (Rumpf-)Zuständen angegeben. Zur Anwendung der Elektronen-Energieverlust-Spektroskopie werden einige Beispiele angeführt. Von Messungen an ionengestützt abgeschiedenen Kohlenstoff- und Kohlenstoff-Stickstoff-Schichten werden Aussagen zur elektronischen und atomaren Struktur abgeleitet. Diese Ergebnisse werden unter Berücksichtigung relevanter Strukturmodelle und Abscheideparameter diskutiert. Aus Untersuchungen von Bornitridschichten wird eine vertikale Schichtung von kubischem Bornitrid über hexagonal koordiniertem verifiziert. Die Streuphase des bei der Ionisation des Al-1s-Zustandes entstehenden Sekundärelektrons bei der Rückstreuung an den nächsten Nachbarn wird durch Untersuchung der kantenfernen Feinstruktur bestimmt. Weitere Untersuchungen kantennaher Feinstrukturen an einer amorphen SiCrAl-Schicht sowie an Kohlenstoffschichten werden vorgestellt. Mögliche Einflüsse kovalenter Bindungen auf die Ergebnisse werden dabei diskutiert. Schließlich werden räumlich hochauflösende Energieverlustmessungen vorgestellt, die zum Nachweis etwa 2 nm dicker Vanadiumoxidschichten auf Rutilkristalliten führten.

No description available.

This thesis employs experimental and theoretical methods to characterise carbon nitride solids and proposes a generalstructural model for amorphous carbon nitride (a-C:N). It finds that a-C:N deposited by several methods is essentially identical, with similar bonding environments for carbon and nitrogen atoms. Using evidence from several techniques, the saturation of nitrogen in an sp2 carbon matrix is discussed. The experimental studies on a range of carbon nitride solids show no evidence for a crystalline form of carbon nitride. In addition to the experimental characterisation of a-C:N, ab initio molecular dynamics were used to investigate bonding and structure in carbon nitride. These simulations show that the most common form of nitrogen bonding was three-fold sites with a lone pair of electrons. Two-fold nitrogen sites were also found in agreement with experimental findings. An increase of nitrogen in a-C:N decreases the sp3-carbon fraction, but this is not localised on the nitrogen and the effect is most severe at high densities. A simulation of a low density/high nitrogen content network shows that the nitrogen saturation seen experimentally may be due to the formation of N2 dimers and C-N molecules which are easily driven out of the structure. The ab initio simulations also explore the nature of charged nitrogen and carbon sites in a-C:N. An analysis based on Wannier Function centres provided further information about the bonding and allowed for a detailed classification of these sites. The removal of electrons from the networks caused structural changes that could explain the two-state conductivity in ta-C:N memory devices. Finally, a theoretical study of the electron energy-loss near-edge structure (ELNES) calculated using multiple scattering theory is presented. The calculated ELNES of diamond, graphite and boron, silicon and carbon nitride structures compare well to experiment and supports the experimental finding that no crystalline carbon nitride had (or has) been produced. These ELNES calculations will however, provide a means of identifying crystalline beta-C3N4 should it be synthesised.

Chemnitz, Techn. Univ., Diss., 1998.

Es werden die physikalischen Grundlagen zur inelastischen Streuung mittelschneller Elektronen im Hinblick auf die Untersuchung des Energieverlustes beschrieben. Die instrumentellen Grundlagen der Energieverlust-Spektroskopie unter besonderer Berücksichtigung des Einsatzes in Transmissionselektronenmikroskopen werden erläutert. Der Einfluß des erfaßsten Streuwinkelbereichs wird diskutiert. Es werden Möglichkeiten zur Auswertung von Energieverlustmessungen im Bereich der Interbandübergangs- und Plasmonanregungen sowie im Bereich der Anregung von tieferliegenden (Rumpf-)Zuständen angegeben. Zur Anwendung der Elektronen-Energieverlust-Spektroskopie werden einige Beispiele angeführt. Von Messungen an ionengestützt abgeschiedenen Kohlenstoff- und Kohlenstoff-Stickstoff-Schichten werden Aussagen zur elektronischen und atomaren Struktur abgeleitet. Diese Ergebnisse werden unter Berücksichtigung relevanter Strukturmodelle und Abscheideparameter diskutiert. Aus Untersuchungen von Bornitridschichten wird eine vertikale Schichtung von kubischem Bornitrid über hexagonal koordiniertem verifiziert. Die Streuphase des bei der Ionisation des Al-1s-Zustandes entstehenden Sekundärelektrons bei der Rückstreuung an den nächsten Nachbarn wird durch Untersuchung der kantenfernen Feinstruktur bestimmt. Weitere Untersuchungen kantennaher Feinstrukturen an einer amorphen SiCrAl-Schicht sowie an Kohlenstoffschichten werden vorgestellt. Mögliche Einflüsse kovalenter Bindungen auf die Ergebnisse werden dabei diskutiert. Schließlich werden räumlich hochauflösende Energieverlustmessungen vorgestellt, die zum Nachweis etwa 2 nm dicker Vanadiumoxidschichten auf Rutilkristalliten führten.