A general study of electroabsorption on semi-insulating solids

The electroabsorption technique has been used to study the properties of a wide range of semi - insulating materials including CdTerCl single crystals, solid solutions o f CdS-ZnS, amorphous silicon and polymerized diacetylene Langmuir films. The small change in a sample's optical transmission consequent upon the application of large electric fields has been carefully recorded using phase-sensitive detection equipment. Measurements on CdTe:Cl samples have established an experimentalprocedure to cope with the complications of photoconductivity and space charge effects. The electro absorption spectra in this material are interpreted in terms of the Franz-Keldysh effect, thus allowing the calculation o f the reduced effective mass. The result is in good agreement with published data obtained using other methods. In solid solutions of CdS-ZnS, the observed exciton electroabsorption has been analysed within the context of the Stark theory. This enables the lowest bound exciton states to be located precisely and results the determination of the reduced effective mass of the exciton which is found tovary in early with the band gap as predicted theoretically. Electro absorption spectra for a-Si in dicate that only transitionsinvolving localized states respond to electric field modulat .on. These spectra, which are preparation dependent, have also been interpreted in terms of the Stark effect ; An interesting result obtained for glow dis-charge produced samples has been interpreted in terms of an effective change in the mobility edge with temperature. The final material discussed in the thesis is the diacetylenemolecule; the films were prepared using the Langmuir trough technique developed in Durham. Interpretation of the electro absorption data for the red polymer form has led to our calculating the static polarizability change between the excited and ground states in this molecule. The thesis concludes with a summary of possible applications ofthe electroabsorption technique and suggestions as to how the presentwork could be extended

An exact evaluation of the Casimir energy in two planar models

The method of images is used to calculate the Casimir energy in Euclidean space with Dirichlet boundary conditions for two planar models, namely: i. the non-relativistic Landau problem for a charged particle of mass m for which - irrespective of the sign of the charge - the energy is negative, and ii. the model of a real, massive, noninteracting relativistic scalar field theory in 2 + 1 dimensions, for which the Casimir energy density is non-negative and is expressed in terms of the Lerch transcendent xxx and the polylogarithm xxx with 0 < xxx < 1 and n = 2, 3.Comment: 5 pages, 2 figures,IMFP2009 conference,to appear in forthcoming AIP Conf.Proc.1150 Request:There are three mathematical symbols denoted by xxx in the abstract below which are otherwise present in the abstract of the submission.Could you please include them so that the abstract below is complet

Effective Action and Schwinger Pair Production in Strong QED

Some field theoretical aspects, such as the effective action and Schwinger pair production, are critically reviewed in strong QED. The difference of the boundary conditions on the solutions of the field equation is discussed to result in the effective action both in the Coulomb and time-dependent gauge. Finally, the apparent spin-statistics inversion is also discussed, where the WKB action for bosons (fermions) works well for fermion (boson) pair-production rate.Comment: LaTex 6 pages, no fingure; Invited Talk at IMFP 2009, Malaysia, Jan. 12-16, 2009; typos in signs of eqs and one reference correcte

Highly efficient preparation of ZnO nanorods decorated reduced graphene oxide nanocomposites

A one-step method for the synthesis of zinc oxide/reduced graphene oxide (ZnO/rGO) nanocomposites by a hydrothermal technique is reported. This simple method involves a hydrothermal treatment of a solution comprising graphene oxide (GO), Zn(CH3COO)2.2H2O, NaOH and NH3.H2O. The concentration of GO as a starting material plays an important role in the density distribution of ZnO nanorods on the rGO sheets and on the percentage of the formation of ZnO/rGO nanocomposites. The resulting rod-like ZnO nanoparticles formed on the rGO sheets, in high density, has a potential in the gas sensing application

Optical constants and electronic transition in hydrogenated silicon (Si:H) thin films deposited by layer-by-layer (LBL) deposition technique

Optical constants derived from optical transmission (T) and reflectance (R) spectra in the wavelength range of 220 to 2200 nm are presented in this paper for hydrogenated silicon (Si:H) thin films deposited by plasma enhanced chemical vapor deposition (PECVD) using the layer-by-layer (LBL) deposition technique. The films were deposited on quartz substrate by decomposition of SiH4 and H2 gases at flow-rate of 5 sccm and 20 sccm, respectively. The substrate temperature, deposition pressure and deposition rate are 100°C, 0.8 mbar and 2.8 nm/s, respectively. The as-prepared films were annealed in nitrogen for one hour at annealing temperatures of 400°C, 600°C, 800°C and 1000°C. The as-prepared film thickness of 301 nm decreased to 260 nm when samples were annealed at 1000°C. The refractive indices (~ 3.0 to 3.4) of annealed films were determined from the interference fringes of transmission spectrum following Manifacier and Davies methods. The electronic transition from valence band to conduction band in these films are characterized from the optical energy gap; EG (~1.64 to 2.41 eV), the dispersion energy; Ed (~26.4 to 34.0 eV) and the oscillator strength; Eo (~2.8 to 3.2 eV). It is interesting to note that EG is lowest for the films annealed at temperature of 600°C which has the lowest hydrogen content, CH in the film. Evidence of the presence of nanocrystallites formed in amorphous matrix is also observed for the films annealed at temperatures above 600°C

Effects of rf power on structural properties of Nc-Si:H thin films deposited by layer-by-layer (LbL) deposition technique

The effects of rf power on the structural properties of hydrogenated nanocrystalline silicon (nc-Si:H) thin films deposited using layer-by-layer (LbL) deposition technique in a home-built plasma enhanced chemical vapor deposition (PECVD) system were investigated. The properties of the films were characterized by X-ray diffraction (XRD), micro-Raman scattering spectroscopy, high resolution transmission electron microscope (HRTEM) and Fourier transform infrared (FTIR) spectroscopy. The results showed that the films consisted of different size of Si crystallites embedded within an amorphous matrix and the growth of these crystallites was suppressed at higher rf powers. The crystalline volume fraction of the films was optimum at the rf power of 60 W and contained both small and big crystallites with diameters of 3.7 nm and 120 nm, respectively. The hydrogen content increased with increasing rf power and enhanced the structural disorder of the amorphous matrix thus decreasing the crystalline volume fraction of the films. Correlation of crystalline volume fraction, hydrogen content and structure disorder of the films under the effect of rf power is discussed

A novel nanoporous silicon prepared using electrochemical deposition technique for light emitting diode applications

The electroluminescence (EL) studies on the aluminum-doped nanoporous silicon (NPSi:Al) have been conducted. Nanoporous silicon (NPSi) layers have been prepared by anodically etching the unpolished p-type Si[100] wafer with surface resistivity of 4-8 Ωcm-1 in hydrofluoric solution at 1:1 ratio of ethanol. Aluminum (Al) was doped on NPSi using cathodic electrodeposition composed of aluminum chloride (AlCl3) and ethanol electrolyte. A diode structure has been fabricated comprising a semi-transparent Au/NPSi:Al/p-Si/Al ohmic contact electrode showing rectification on a forward bias I-V curve. EL from NPSi and NPSi:Al/p-Si has also been observed using the diode structure. The NPSi:Al/p-Si device shows increasing EL quantum efficiency at about 30%, and blue-shift EL spectra are observed. Possible reasons for the enhancement will be discussed. © World Scientific Publishing Company

Optical characteristics of ZnSxSe1−x thin films prepared by electron beam evaporation

The optical transmission measurements are used to determine various optical constants and properties of ZnSxSe1-x thin films prepared by electron beam evaporation onto glass substrates at 60 °C. The dispersion of the complex refractive index, the complex dielectric function and the absorption coefficient is studied in the transparent region of the spectrum and compared with the theoretical results calculated based on the model dielectric function. The fundamental optical energy gap is estimated by fitting the absorption coefficient data in the high absorption region to the direct transition expression. The variation of the energy gap with the composition in the film is investigated and compared with the results reported previously by other workers. The shift in the energy gap caused by the uniaxial stress inside the film and the grain size effect is estimated. © 2005 Elsevier B.V. All rights reserved