LOCALIZED SURFACE PLASMON MEDIATED PROPERTIES OF Ag AND Ag/ZnO MULTILAYER THIN FILMS

Abstract

Most commonly used material for transparent conducting oxide as transparent electrodes in optoelectronic industry is indium tin oxide (ITO) due to its low resistivity (~1x10-4 Ω cm), high optical transmittance and well-developed fabrication technique. However scarcity in supply of indium and the growing demand for ITO make the resulting fabrication costs prohibitive for future industry. Earlier efforts to improve the conductivity of transparent electrodes focused on increasing the carrier concentration by controlling the deposition process to create intrinsic donors such as lattice defects as well as intrinsic doping into TCO materials. There is an inherent limitation in the increment of electrical conductivity which can be obtained by increasing the carrier concentration because of ionized scattering. Moreover, increased carrier concentration increases the free carrier absorption and decrease the optical transparency. Due to these disadvantages arising from heavy carrier concentrations, oxide/metal/oxide and TCO-metal-TCO multilayer systems have also been studied for improving the conductivity. These systems have very low resistivity, comparable optical transparency in the visible range, lower thickness than single layered TCO films. It can also be used for potential application in light trapping for solar cells. Several reports have been published on extraordinary optical transmission and electrical conductivity of metaloxide/metal/metaloxide multilayer thin films controlled by Localized Surface Plasmon Resonance generated in intermediate metal-island or semi continuous film. Motivated by these reports, work of this thesis has been started to study the optical and electrical properties of multilayer ZnO/Ag/ZnO/glass thin films deposited by low cost Spray Pyrolysis and Thermal evaporation technique. Crystal structure and morphology of all these films have been characterized using XRD, AFM and FESEM. Optical properties of these films have been characterized using UV-visible spectroscopy. Optical transmissions of these films have been simulated theoretically with the help of Effective Medium Maxwell Garnet Theory and Transfer Matrix method. Main feature of these films is the ability to tune the optical transmission band by manipulating the geometrical property of the metal island films sandwiched between two dielectric layers. Electrical properties of these films are also comparable to traditional Transparent Conducting Oxide (TCO) like Indium Tin Oxide ii (ITO), Al:ZnO.. In contrast to the earlier reports these films have high optical transmission in the region 600 nm 900 nm. These films could be used as transparent electrodes for orange (600 nm) or near IR (800 nm) Organic Light Emitting Diode. Another potential use of these films may be as conducting optical filter which can be used for resonator mirrors of semiconductor diode laser in the desired wavelength. Optical properties of these films have been further manipulated by substrate heating as well as post deposition annealing. Post deposited annealed silver island thin films can also be used as UV pass optical filter at 320 nm for He-Cd laser. Localized Surface Plasmons of silver islands also have the unique ability to trap the light of the intense visible region of solar spectra efficiently for solar cells. In this thesis possibility of trapping light by using silver island on top of n-Al:ZnO/p-Si heterostructure has also been explored. Using silver islands photocurrent of this thin film heterostructure has been enhanced significantly. Moreover silver island films could also be used for refractive index based sensor applications.