Date of Award

12-1-2018

Degree Name

Doctor of Philosophy

Department

Electrical and Computer Engineering

First Advisor

Ahmed, Shaikh

Abstract

An Atomistic Band Anti-Crossing (ABAC) model is developed to investigate the unusual bowing of energy bandgap (EG) of Highly Mismatched Alloys (HMAs) ZnO1-xSx and its dependence on the material composition, that is, dilute oxygen (O) and sulfur (S). For dilute O in ZnS1-xOx alloy, the energy bandgap decreases as the O composition increases mainly because of a down-shift of the conduction band edge (CBE) that results from the interaction between extended ZnS’s CBE and localized O defect energy state. On the other hand, the reduction of energy bandgap as a function of dilute S composition in ZnO1-xSx alloy is because of an up-shift of the valence band edge (VBE) that results from the interaction between ZnO’s VBE and the localized S defect energy state. The ABAC model is used to analyze the energy bandgap of minority anion alloyed materials and determine the splitting sub-bands E+ and E- that are admixture of extended ZnS’s CBE (ZnO’s VBE) and localized O (S) defect energy state. According to this analysis, the predicted defect energy levels for dilute O and S are about 199 meV below the CBE of ZnS and approximately 190 meV above the VBE of ZnO, respectively. The existence of Band Anti-Crossing (BAC) interaction has been verified and quantified using fractional Γ character, where the alloys’ CBE (VBE) are an admixture of O (S) defect energy and the CBE (VBE) of the ZnS (ZnO) host materials. Furthermore, in this work, a multiscale computational model is employed to determine the effect of the internal spontaneous polarization and piezo-electric fields, which arise from atomicity and strain fields, on the electrical and optical properties of ZnO nanostructures for applications in LEDs, Tunnel Field Effect Transistors (TFETs) and solar cells. For these models, a fully-atomistic 8-band 〖sp〗^3-spin tight-binding basis set is utilized to construct the Hamiltonian of the structure. The relaxed strain energy for alloys is computed by means of the Valence Force Field-Molecular Mechanics (VFF-MM) model using Keating potentials. For LEDs, the efficiency droop is investigated taking into account the influence of diameter-dependent piezoelectric polarization on the electrical and optical characteristics. Recently, polarization-engineered inter-band TFETs constructed of III-nitride heterojunctions have received considerable attention due to their potential in offering high ON/OFF current ratio, low sub-threshold swing (< 60 mV/decade) and higher ON current (I_on~100 µA/µm). Along this line, a theoretical performance projection of ZnO/MgO tunnel nanowires is reported. Finally, a recently reported experimental thin-film solar cell device is investigated, which employs ZnO as window layer, CdS as buffer layer, and Cu(In,Ga)Se_2 CIGS as absorber layer. In our revised design, we make use of dilute oxygen and sulfur alloys (ZnS_x O_(1-x)) instead of CdS as buffer layer due mainly to (i) flexibility of tuning the bandgap, and (ii) toxicity of cadmium Cd material and its compounds. The redesigned solar cell offers a power conversion efficiency of ~22.6%.

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