Master of Science Dissartation
Electrical control of optical properties in multilayer metal-oxide-semiconductor structures
Wydział Fizyki UW
This work presents the development of an electro-optic light modulator in the form of multilayer metal-oxide-semiconductor (MOS) structure and the research on how the electrically controlled process of accumulation layers formation at semiconductor-dielectric interface affects the optical properties of the structure.
In the proposed photonic device, indium tin oxide (ITO) was used as the active material, supplemented with silica layers and silver layers, the latter serving as electrodes. These structures were fabricated using physical vapor deposition with electron beam (e-beam PVD) and were characterized by scanning electron microscope (SEM), atomic force microscope (AFM), spectroscopic ellipsometry and Hall effect measurements. In order to perform modelling of light propagation through the modulator transfer matrix method (TMM) was used.
The research work was divided into three parts. First, process optimization of the ITO layers deposition was performed. An original procedure was developed using oxygen plasma, which made it possible to obtain ITO layers with high carrier concentration (7,9·10^20 cm^-3), low surface roughness (RMS = 1,75 nm) and at the same time high transmittance reaching over 70 %. Moreover, the need to heat the substrate to high temperatures (> 100 ° C) was eliminated, which allowed to avoid problems with agglomeration of the silver layers.
Subsequently, the focus was on the creation and characterization of a multilayer structure intended to act as a light modulator. The technical difficulties encountered were discussed and a number of device geometries were proposed and tested. Finally, an efficient multilayer was obtained, with a capacitance of approx. 500 nF, for which it was possible to study changes of the optical properties under the influence of external voltage. It has been shown that in the structure not only accumulation layers are formed, but also depleted layers are to be obtained at the semiconductor-dielectric interface. On the basis of the ellipsometric model, it was calculated that the change in the value of the refractive index of the active layer under the 2.5 V of external voltage reaches Δn = 0.25 for a wavelength of 1500 nm.
Finally, the work was supplemented with the results of numerical modeling using the multicellular MOS structure, in which it would be possible to intensify the effect of the refractive index change by adding successive layers. To the best of the author's knowledge, it is the only experimental demonstration of this type of device in Poland.