The study of ferroelectric LiNbO<SUB>3</SUB> and LiTaO<SUB>3</SUB> doped with transition metals involves the characterisation of LiNbO<SUB>3</SUB>/LiTaO<SUB>3</SUB> solid solutions in the systems Li<SUB>2</SUB>O-Nb<SUB>2</SUB>O<SUB>5</SUB>-M<SUB>x</SUB>O<SUB>y</SUB> and Li<SUB>2</SUB>O-Ta<SUB>2</SUB>O<SUB>5</SUB>-M<SUB>x</SUB>O<SUB>y</SUB> (M = Cr, Mn, Fe, Co, Ni, Cu). Compounds were made by solid state reaction at temperatures between 1000 and 1500°C, depending on the system studied. The emphasis of this work is on the characterisation of the defect structure of LiNbO<SUB>3</SUB>/LiTaO<SUB>3</SUB> solid solutions using phase diagram determination, X-ray and neutron powder diffraction, EXAFS, ESR and optical spectroscopy. The valence of the incorporated cations was studied by magnetic measurements. The electrical properties of these materials have been investigated using AC impedance spectroscopy. It can be shown that the physical properties of LiNbO<SUB>3</SUB> and LiTaO<SUB>3</SUB> depend strongly on the defects in the structure which can be controlled by purposeful doping with other cations. Structural refinements of the X-ray and neutron powder diffraction data have shown that the defects arising from nonstoichiometry are accommodated by vacancies created on the Li site. This affects the structure when doped with third cations, giving rise to complex substitution mechanisms Spectroscopic studies have shown that the dopants (Cr<SUP>3+</SUP>, Mn<SUP>2+</SUP>/Mn<SUP>3+</SUP>, Co<SUP>2+</SUP>, Ni<SUP>2+</SUP>, Cu<SUP>+</SUP>, Cu<SUP>2+</SUP>) are shifted from the central octahedral position towards the adjacent empty octahedron. The electrical properties of LiNbO<SUB>3</SUB> and LiTaO<SUB>3</SUB>, measured by AC impedance spectroscopy, depend strongly on the dopant content: the conductivity generally rises, whereas the activation energy for the electrical conductivity drops with increasing dopant concentration. The microstructure of electroceramics can also be probed by AC impedance spectroscopy and it was shown that the texture of all samples was bad due to poor sintering of the pellets. Longer sintering times did not improve the quality of the ceramics which exhibit a large pore size distribution. An important aspect for future work would be the quality improvement of LiNbO<SUB>3</SUB>/LiTaO<SUB>3</SUB> based ceramics.

The thesis mainly probes into the effects that the structure below multi quantum layer as regards the efficiency of luminescence of blue light LED in the different number layers and make further comparison. In the article, the students separately make analysis and comparison to the single quantum well , five multi quantum wells , ten multi quantum wells and thirty multi quantum wells. And discovered that different number layers of quantum well will occur different Phase Separation and Strain in the film. So the article mainly focuses on : (1.)Phase Separation in various of quantum well, it occurs different In-rich reaction and (2.)Different Strain levels which occurs different dislocation reaction. The two mechanisms will be discussed in detail with the effects of luminescence reaction of LED. According to the results of experiment, We found that it is easier to form V-shape defects and dislocation with the increasing indium content. Under the high indium content, the density of In-rich will increase obviously and spread to the GaN barrier, then the original structure of quantum well will be destroyed and descend the efficacy of luminescence. In the thicker GaInN quantum well, it will induce larger energy of strain inside the film, So the defect density will increase due to release the strain energy. It was also discovered the intensity of luminescence descend after measuring by PL. When grow different number layers , it was discovered that higher quantum layer will produce the roughness surfaces when using AFM . So the higher quantum layers will make greater influence in the efficacy of luminescence. By experiment, we found that the five to ten quantim wells will have the better photo characteristic.

Abstract The defect structure of CuMX2 (M&#x00BA;In, Ga and X&#x00BA;S, Se, Te) was investigated. The defect concentrations were derived as a function of nonmolecularity( X) and nonstoichiometry( Y), and the carrier concentrations were calculated quantitatively using a theoretical defect model, which could be used to select the proper region of stoichiometry for device designs with specified carrier concentrations. The compensation reaction between dopants and intrinsic defects in CuMX2, compound semiconductors is quantitatively illustrated. If for a small increment of temperature the Fermi level EF is shifted by £GEF and the concentration of free majority carriers is increased by £Gn, then the ratio£Gn /£GEFis a measure of the defect-level concentration within£GEF. The electrical and optical properties of CuMX2 have been investigated using various types of electrically active intrinsic defects caused by deviations from the ideal stoichiometry of the compound. The defect complexes are formed by reactions among the impurity and native defects and/or among the native defects during crystal growth or during processing, and play a significant role in determining the characteristics and performance of devices by affecting lifetimes, degradation and breakdown etc.

No description available.

Mesoscopic structures in liquids have an impact on the diffusion dynamics of the constituting molecules. Smectic 8CB liquid crystals on silicon wafers show the formation of mesoscopic structures on the μm scale at a film thickness of 200 nm. Depending on the kind of substrate (thermally grown or native SiOx), we observed the formation of focal conic domains (FCDs) and a new type of terraced holes, respectively. Dynamics are described via single perylene diimide tracer molecule tracking of translational diffusion and in the case of FCDs by a combination of translation and rotation detected via fluorescence correlation spectroscopy. Tailoring perylene diimide molecules such that the optical transition dipole moment follows the liquid crystal director allows mapping out FCDs and investigating the dynamics within a single FCD.

No description available.