The temperature and pressure dependences of the Raman frequencies of some lattice and internal modes are calculated using the volume data from the literature through the mode Gr&uuml / neisen parameters. This calculation is performed in different phases of solid nitrogen and benzene. Calculated Raman frequencies are compared with the experimentally measured frequencies for those crystalline systems studied.

Silicon heterojunction solar cells, a-Si:H/c-Si, are promising technology for future photovoltaic systems. An a-Si:H/c-Si heterojunction solar cell combines the advantages of single crystalline silicon photovoltaic with thin-film technologies. This thesis reports a detailed survey of heterojunction silicon solar cells with p-type wafer fabricated by magnetron sputtering and Plasma Enhanced Chemical Vapor Deposition (PECVD) techniques at low processing temperature. In the first part of this study, magnetron sputtering method was employed to fabricate a-Si:H thin films and then a-Si:H/c-Si solar cells. Amorphous silicon (a-Si:H) films were grown on glass in order to perform electrical and optical characterizations. The J-V characteristics of the silicon heterojunction solar cells were analyzed as a function of a-Si:H properties. It was shown that a-Si thin films with well-behaved chemical and electronic properties could be fabricated by the magnetron sputtering. Hydrogenation of the grown film could be achieved by H2 introduction into the chamber during the sputtering. In spite of the good film properties, fabricated solar cells had poor photovoltaic parameters with a low rectification characteristic. This low device performance was caused by high resistivity and low doping concentration in the sputtered film. The second part of the thesis is dedicated to heterojunction solar cells fabricated by PECVD. In this part a systematic study of various PECVD processing parameters were carried out to optimize the a-Si:H(n) emitter properties for the a-Si:H(n)/c-Si(p) solar cell applications. In the next stage, a thin optimized a-Si:H(i) buffer layer was included on the emitter side and on the rear side of the c-Si(p) to improve the surface passivation. Insertion of an a-Si:H(i) buffer layer yielded higher high open circuit voltage (Voc) with lower fill factor. It was shown that high Voc is due to the efficient surface passivation by the front/rear intrinsic layer which was also confirmed by the measurement of high effective lifetime for photo-generated carriers. Low fill factor on the other hand is caused by increasing resistivity of the solar cells by inserting low conductivity a-Si:H(i) layers.

The study aimed to identify deterioration problems, repair and conservation needs of andesites on the walls of the Ankara Castle. Decay forms of walls were documented by visual examination. Samples taken from the surface of the weathered andesites were examined for their basic physical, mechanical compositional and minerological properties. The bulk density and total porosity were determined as basic physical properties. The mechanical properties were expressed as ultrasonic velocity and modulus of elasticity (Emod). Compositional and mineralogical properties were determined by optical microscopy and XRD analyses. Soluble salt content of the andesite samples was determined by spot tests of anions and electrical conductivity measurements. Findings were evaluated in terms of the long-term weathering behaviour of andesites under the effect of the prevailing climate, air pollution problems of Ankara, dampness problems of the structure, previous repairs with incompatible cement mortars. The surfaces of Ankara Castle andesite blocks were heavily weathered. The results were compared with the physical and mechanical properties of fresh andesites from G&ouml / lbaSi-Ankara quarry. The surface of the andesite blocks at the Ankara Castle, had low bulk density and high porosity, low ultrasonic velocity and low Emod values. Thin section and XRD analyses supported those results by revealing the presence of physical and chemical weathering on feldspars and other main minerals of andesite, as well as the presence of amorphous minerals at the surface.

In this thesis, assuming that higher spatial dimensions existed only during the inflationary prematter phases of the universe, we construct a (1+D)-dimensional (D&gt / 3), nonsingular, homogeneous and isotropic Friedmann model for dimensional reduction. In this model, dimensional reduction occurs in the form of a phase transition that follows from a purely thermodynamical consideration that the universe heats up during the inflationary prematter phases. When the temperature reaches its Planck value Tpl,D, which is taken as the maximum attainable physical temperature, the phase of the universe changes from one prematter era with D space dimensions to another prematter era with ( D-1) space dimensions where T_pl,D is higher. In this way, inflation gets another chance to continue in the lower dimension and the reduction process stops when we reach D=3 ordinary space dimensions. As a specific model, we investigate the evolution of a (1+4)-dimensional universe and see that dimensional reduction occurs when a critical length parameter l_4,3 reaches the Planck length of the lower dimension. Although the predictions of our model for the cosmological parameters are beyond the ranges accepted by recent measurements for closed geometry, for a broad range of initial conditions they are within the acceptable ranges for open geometry

CdS thin films were deposited by thermal evaporation method onto glass substrates without any doping. As a result of the structural and electrical investigation it was found that CdS thin films were of the polycrystalline structure and n-type / and of the transmission analysis optical band gap was found to be around 2.4 eV. Temperature dependent conductivity measurements were carried out in the range of 180 K &ndash / 400 K. The dominant conduction mechanism is identified as tunnelling between 180 K &ndash / 230 K and thermionic emission between 270 K and 400 K. To produce Schottky devices, CdS thin films were deposited onto the tin-oxide and indium-tin-oxide coated glasses, by the same method. Gold, platinum, carbon and gold paste were used as metal front contact in these devices. The area of these contacts were about...... Temperature dependent current-voltage measurements between 200 K and 350 K, room temperature current-voltage measurements, capacitance-voltage measurement in the frequency range 1 kHz &ndash / 1 MHz and photoresponse measurements were carried out for the characterization of these diodes. Ideality factor of the produced Schottky devices were found to be at least 1.5, at room temperature. Dominant current transport mechanism in the diodes with gold contacts was determined to be tunnelling from the temperature dependent current voltage analysis. Donor concentration was calculated to be about ........ from the voltage dependent capacitance measurement.

Nanowire heterostructures play a crucial role in nanoscale electronics, i.e., one-dimensional electronics derives benefits from the growth of heterostructures along the nanowire axis. We use first-principles plane-wave calculations within density functional theory with the localized density approximation (LDA) to get information about the structural and electronic properties of bare and hydrogen passivated GaAs/AlAs nanowire heterostructures. We also take into account the reconstruction of the nanowire surfaces. Modeled nanowire heterostructures are constructed using bulk atomic positions along [001] and [111] direction of zinc-blende structures and cutting out wires from this GaAs/AlAs heterostructure crystal with a diameter of 1 nm. We study for the effects of the surface passivation on the band gap and the band offsets for the planar GaAs/AlAs bulk heterostructure system and GaAs/AlAs nanowire heterostructure system. It is possible to control the potential that carriers feel in semiconductor heterostructures. For the planar lattice-matched heterostructures, the macroscopic average of potential of the two materials is constant far from the interface and there is a discontinuity at the interface depending on the composition of the heterostructure. In order to obtain the valence band offset in the heterostructure system, the shift in the macroscopic potential at the interface and the difference between the valence band maximum values of the two constituents must be added. In nanoscale heterostructures, the potential profile presents a more complex picture. The results indicate that while the discontinuity remains close to the planar limit right at the interface, there are fluctuations on the average potential profile beyond the interface developed by the inhomogeneous surface termination, that is, there are variations of the band edges beyond the interface. We report a first-principles study of the electronic properties of surface dangling-bond (SDB) states in hydrogen passivated GaAs/AlAs nanowire heterostructures with a diameter of 1 nm, where the SDB is defined as the defect due to an incomplete passivation of a surface atom. The charge transition levels of SDB states serve as a common energy reference level, such that charge transition level value for group III and V atoms is a constant value and a periodic table atomic property. We have carried out first-principles electronic structure and total energy calculations of aluminum nanowires for a series of different diameters ranging from 3 Angtrom-10 Angstrom, which is cut out from a slab of ideal bulk structure along the [001] direction. First-principles calculations of aluminum nanowires have been carried out within the density-functional theory. We use the norm-conserving pseudopotentials that are shown to yield successful results for ultrathin nanowire regime. Our results show that the number of bands crossing the Fermi level decreases with decreasing wire diameter and all wires studied are metallic.

HgCdTe (MCT) infrared (IR) photodetectors have been used for various military and civilian applications including thermal imaging, medical imaging, and astronomy. These detectors are commonly fabricated on MCT layers grown on a foreign substrate epitaxially using delicate growth techniques such as Molecular Beam Epitaxy (MBE). The crystal quality of epitaxial layers grown on a substrate critically depends on the quality of the substrate. One of the stringent requirements in choosing the substrate material is the lattice match between grown layer and the underlying substrate. With perfectly matching lattice structure, CdZnTe has been the major substrate material for the MCT growth. The production of defect free single crystal CdZnTe bulk crystal has then attracted great attention among the research and industrial community of MCT based photodetectors. This thesis focuses on the growth and characterization of Cd1-xZnxTe crystals with the main objective of obtaining high-quality, CZT bulk crystal with large crystal sizes. To be compatible with the subsequent MCT growth, we aimed to obtain Cd0.96Zn0.04Te crystals with (211) crystal surface orientation. CdZnTe bulk crystal growths were performed in three-zone vertical Bridgman furnace by a high temperature melt process called &ldquo / Modified Bridgman Technique&rdquo / . Difficulties in both growth and characterization are presented and discussed in this report. Characterization of the grown CdZnTe crystal was performed to determine the crystallographic orientation, crystal quality, Zn distribution, IR transmission, resistivity, polarity, etch pit density, and surface properties. For this purpose, electron microscopy with analytical diagnostic tools like EDS and EBSD, XRD, optical transmission spectroscopy, and electrical measurement systems have been employed. We demonstrated the successful growth of single crystal CZT crystals using our simple Bridgman furnace. Physical properties of the grown crystal were very promising and encouraging for future applications. Crystal pieces having sizes larger than 5 x 5 mm2 with uniform Zn distribution and (211) surface orientation were obtained. IR transmission of nearly 60% which is as good as that of the commercial substrates was achieved. The electrical resistivity was much better (higher) than generally accepted values. However, the XRD results indicated the presence of defects and/or micro grains in the bulk crystal. These structures seemed to have prevented obtaining good FWHM values, which are the measure of crystal quality, in the XRD analysis.

In this thesis, the recently discovered Soft Gamma Ray Repeaters SGR J1833-0832, SWIFT J1822.3-1606 and SWIFT J1834.9-0846 are analysed using the archival Swift, RXTE, Chandra and XMM-Newton observations. The period fluctuations and timing noise properties of these sources are investigated. Spectral characteristics and long term frequency evolution of these sources are presented. Investigation for timing noise structure of these magnetars has shown a correlation between first frequency derivative of the spin frequency and torque noise strength.

The scalar tensor theory of gravitation is constructed in D dimensions in all possible geometries of spacetime. In Riemannian geometry, theory of gravitation involves a spacetime metric g with a torsion-free, metric compatible connection structure. If the geometry is non-Riemannian, then the gauge theory of gravitation can be constructed with a spacetime metric g and a connection structure with torsion. In non-Riemannian theory, connections may be metric compatible or non-metric compatible. It is shown that theory of gravitation which involves non-metric compatible connection and torsion, can be rewritten in terms of torsion-free theory. It is also shown that scalar tensor theory can be reformulated in Einstein frame by applying a conformal transformation. By adding an antisymmetric axion field, the axi-dilaton theory is studied in Riemannian and non-Riemannian geometries. Motion of massive test particles is examined in all these geometries. The static, spherically symmetric and stationary, Kerr-type axially symmetric solutions of the scalar tensor and axi-dilaton theories are presented. As an application, the geodesic elliptical orbits based on a torsion-free connection and the autoparallel orbits based on a connection with a torsion, are examined in Kerr Brans-Dicke geometry. Perihelion shift of the elliptical orbit is calculated in both cases and the results are compared.

Passivation of the crystalline silicon (c-Si) wafer surface and decreasing the number of interface defects are basic requirements for development of high efficiency a-Si:H/c-Si heterojunction solar cells. Surface passivation is generally achieved by development of detailed silicon wafer cleaning processes and the optimization of PECVD parameters for the deposition of intrinsic hydrogenated amorphous silicon layer. a-Si:H layers are grown in UHV-PECVD system. Solar cells were deposited on the p type Cz-silicon substrates in the structure of Al front contact/a-Si:H(n)/a-Si:H(i)/c-Si(p)/Al back contact. Solar cell parameters were determined under standard test conditions namely, using 1000 W/m2, AM 1.5G illumination at 25 oC. Growth of (i) a-Si:H, films on the clean wafer surface was investigated as a function of substrate temperature, RF power density, gas flow rate, hydrogen dilution ratio and deposition time and was characterized using SEM, HRTEM, AFM, SE, ATR-FTIR and I/V measurements. Structural properties of the films deposited on silicon wafer surface are directly effective on the solar cell efficiency. Morphological characterization of the grown films on the crystalline surface was found to be very complex depending on the deposition parameters and may even change during the deposition time. At 225 oC substrate temperature, at the beginning of the deposition, (i) a-Si:H films was found grown in epitaxial structure, followed by a simultaneous growth of crystalline and amorphous structure, and finally transforming to complete amorphous structure. Despite this complex structure, an efficiency of 9.2% for solar cells with total area of 72 cm2 was achieved. In this cell structure, TCO and back surface passivation do not exist. In the