The Effect of Stationary Uv Excitation on the Optical Behavior of Electrochemically Self-assembled Semiconductor Nanowires

THE EFFECT OF STATIONARY UV EXCITATION ON THE OPTICAL BEHAVIOR OF ELECTROCHEMICALLY SELF-ASSEMBLED SEMICONDUCTOR NANOWIRES By Rajesh A. Katkar, M.S. A Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Virginia Commonwealth University. Virginia Commonwealth University, 2006 Major Director: Dr. Gregory B. Tait Associate Professor, Electrical and Computer Engineering In this work, we investigate the optical response of the semiconductor quantum wire array when excited by stationary UV light. The array is synthesized by selectively electro-depositing the semiconductor material in electrochemically self-assembled porous alumina templates. Our studies are based on the optical behavioral changes in CdS, ZnO, ZnSe and CdSe quantum wires of 50-, 25and 10-nm diameters. We use a set of generalized Bloch equations to solve the interband polarization function of the semiconductors derived within the Hartree-Fock approximation, and theoretically model the UV excitation effect on the quantum wires. The solutions which xiii consider the effects of screening, Coulomb interaction between the carriers and many body effects on excitons are generated for a quasi-equilibrium regime using a devised accelerated fixed point method. The solution technique is developed in Mathematica to iteratively solve this complex set of equations. The optical constants generated for individual quantum wires are incorporated into a finite-element electromagnetic wave simulator, HFSS, to investigate the full behavior of the array of wires. Theoretically calculated values of the dielectric permittivity of the un-excited quantum wires are shown to decrease progressively as the wire diameter reduces. We perform the experimental analysis using a pump-probe excitation scheme incorporated in a sensitive Michelson interferometer in a homodyne setup. We measure extremely small changes in the phase shift between the interfering IR probe beams and hence measure the refractive index changes caused by the UV pump. While the decreasing filling factor acts to reduce the optical activity in narrower wire arrays, the shifting of the DOS function with additional quantum confinement serves to increase it. These competing effects give rise to the size-dependent non-monotonic optical activity experimentally observed in ZnO, CdS and ZnSe nanowire arrays. The simulation results show a rapid increase in the changes in effective permittivity values of the individual quantum wires as diameter decreases. The substantial changes observed in the refractive index for the whole thin film array at intermediate wire diameter sizes may be suitable for optical phase shifting, intensity modulation and switching applications in integrated optical devices. CHAPTER