Physics

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Browsing Physics by Author "Abebe, Mekonnen"

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    Electrical Conduction in Amorphous Trinitro - Nine - Fluorenone and Selenium
    (Addis Ababa University, 1992-06) Abebe, Mekonnen
    The theories of electronic structures in amorphous semiconductors are reviewed. The effectiveness of the study of electrical conductions particularly the SCLC, TSC and transient photoconductivity, in providing a wealth of information on electronic properties of disordered solids also discussed. Analytic equations describing the above three conduction properties are also developed. Experimental study of the SCLC, TSC and transient photoconductivity is made fora_TNF and a-Se. Distinctly from the models that are based on the assum~ tions of discrete or uniform distributions of traps in energy, our experimental works on a~TNF and a-Se indicate the existence of energetically dispersed charge carriers trapping states. The observed space charge limited mode of conduction and its temperature sensitivity is found to be best describable interms of a smoothly varying distributions. The Gaussian distribution seems more realistic for the interpretation of experimental results in a~TNF and a~e. Both hopping and multiple trapping models are used in interpreting the transient photo-conductivity experimental results for a-TNF and a.,.Se respectively. Drift mobilities of charge carriers are seen to exhibit an activated nature and also found to be field dependent in a manner similar to the poole~Frenkel mechanism. For an explanation of the dispersive hole transport in a-Se interms of the multiple trapping model, the theoretical predictions developed on the basis of a Gaussian distribution is found to agree with experimental results more satisfactorily. The study of thermally stimulated current characteristics in a-TNF near the glass transition temperature, Tg, renders an activation energy which is significantly different from results of low temperature TSC analysis. Rather, it is nearly identical to the zero field extrapolated activation energy computed from transient photo conductivity experiments. There by indicates this energy parameters to contain the disorder induced term
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    Optical and Electronic Correlation Effects in Semiconductor Nanostructures with Emphasis on Silicon Nanostructure
    (Addis Ababa University, 2011-04) Abebe, Mekonnen
    In this thesis we theoretically investigated static and dynamic correlation effects in semiconductor nanostructures. Structural inhomogneity and confinement quantization effects make this class of materials exhibit exotic properties under different experimental conditions. Time dependent coupling of the local states under time dependent perturbations and the fluctuation of the proximity couplings in the length scale of tunneling give rise to dynamical correlations that put their signature in the optical and transport properties. Based on the Orbital Free Local Density Approximation (OF-LDA), the exchange and correlation contribution to the orbital energy and the size dependent nature of the threshold for optical absorption are studied. The effects of dynamical correlations between the states in a single nanostructure and the effects of dynamical correlations between the states in proximity coupled configurations are investigated with the Non Equilibrium Green Function Formalism (NEGF) as the theoretical frame work. The theoretical formulation of the non equilibrium dynamical analysis is used to describe the coherent regime optical and transport phenomena in semiconductor nanostructures. The results demonstrate that the presence of strong coupling between electronic states in semiconductor nanostructures leads to the formation of d states. First we have shown the emergence of collapse and revival phenomenon during the coherent regime absorption in a single dot. We presented qualitative and quantitative description of the Rabi oscillation. The dependence on the mean photon number and on the strength of the couplings between electronic states is demonstrated. We found that similar phenomenon in a wide range of many body system could be satisfactorily described assuming entanglement of the lower levels. Secondly, we theoretically investigated correlation effects in tunnel coupled nanostructures. The quantum blockade phenomenon in wide experimental samples is v explored. The appearance of double dot structures as a time shared entangler is predicted. It is also found that the presence of a transparent boundary between quantum dots lead to the formation of local entangled states that are too difficult for experimental demonstration. Lastly the possible technological value of a double dot –in series configuration is presented. Its operation as electron-spin polarization is discussed and its potential for application as IR detector is proposed